Laser Cladding

Laser Cladding

Laser Cladding

Ehsan Toyserkani Amir Khajepour Stephen Corbin

CRC PR E S S Boca Raton London New York Washington, D.C.

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Dedications

Ehsan Toyserkani To my father and mother, who taught me to learn. To my beloved wife and son, Homeyra and Ali, who have truly brought joy in my life.

Amir Khajepour To my son and wife for the joy and comfort you have given to my life, and my family for your support.

Stephen Corbin To the memory of my sister, Pamela Corbin, who was an enthusiastic and passionate teacher for over 20 years. Pamela inspired my own interest in the field of education which ultimately led to my academic career.

© 2005 by CRC Press LLC

Preface

Laser cladding by powder injection has received significant attention in recent years due to its unique features and capabilities in various industries involved in metallic coating, high-value components repair, prototyping, and low-volume manufacturing. This emerging laser material processing technique is an interdisciplinary technology utilizing laser technology, computer-aided design and manufacturing (CAD/CAM), robotics, sensors and control, and powder metallurgy and rapid solidification. Further development of this technique depends on enhancement of the technologies involved and understanding the interconnections among these technologies and the process quality. A good comprehension of the underlying physics of the laser cladding process is key in the development of the process as a reliable coating and manufacturing technology. In addition to a good grasp of feedback control and automation, a strong knowledge of material science, heat transfer, and fluid dynamics is essential to a successful development of an automated laser cladding system. The intent of this book is to address the lack of a comprehensive book dealing with the dierent aspects of laser cladding. The authors have used the results of their own research and experience in the past few years along with the findings of many other researchers in the preparation of this book. The book provides a solid and detailed description of laser cladding in modeling, materials, and control and can be used in both academia and industry. The book begins with a review of the applications of laser cladding, and continues with physical descriptions of the process and the parameters involved, process modeling and control, process applications, and the physical metallurgy of alloying and solidification during laser cladding. We illustrate the general principles of the technique with several case studies based on a number of important common laser cladding applications. Extensive references to the current literature have also been provided to guide the reader to further information on desired topics. We are very much indebted to many colleagues and students for their help in the preparation of this book. Special thanks go to Professors David Weckman, Walter Duley and Jan Huissoon of the University of Waterloo for their valuable comments. We are grateful to Dr. Steen Nowotny of the Fraunhofer Institute for Material and Beam Technology, Dr. Lijue Xue of the Integrated Manufacturing Technologies Institute (IMTI) of National Research Council of Canada (NRC), Dr. Frank Arcella of AeroMet Cooperation, Dr. Joohyun Choi of University of Missouri at Rolla, Mr. David Gill of Sandia National Laboratories, and Mr. Michael Kardos of Optomec Inc. for providing us © 2005 by CRC Press LLC

with copyright permissions for several photos used throughout the book. We would like to express our sincere appreciation and gratitude to Dr. Hamid Niazmand for providing us with coaxial model analysis, Mr. Ian Fraser and the Safety O!ce of the University of Waterloo for allowing us to use their safety materials, Ms. Sarah Mask for editing the manuscript, Ms. Ji-Hyun Kim for drawing the figures, and Ms. Lori Brown for administrative support. Thanks are especially due for the financial support of Materials and Manufacturing of Ontario (MMO) and the National Sciences and Engineering Research Council of Canada (NSERC). Finally, thanks to our families, who make it all worthwhile. Ehsan Toyserkani, Amir Khajepour, Stephen Corbin Waterloo, Ont., Canada, 2004

© 2005 by CRC Press LLC

About the Authors

Dr. Ehsan Toyserkani received his Ph.D in mechanical engineering from the University of Waterloo, Ontario, Canada, in 2003. His early research and industrial interests were on design of mechatronics systems. This interest was expanded to include the development of intelligent controllers for laser cladding technology during his Ph.D program. He was awarded postdoctoral fellowships from the National Sciences and Engineering Research Council of Canada (NSERC) and the Canadian Space Agency (CSA) to conduct projects related to laser cladding technology. In 2004, he joined the Mechatronics Engineering Group of the Department of Mechanical Engineering at the University of Waterloo as an Assistant Professor.

Dr. Amir Khajepour received his Ph.D in mechanical engineering from the University of Waterloo in Ontario, Canada in 4996. In 4997, he joined the Department of Mechanical Engineering, University of Waterloo, where he is currently a Professor in Mechatronics Engineering. The thrust of Dr. Khajepour’s research is in modeling and control of dynamic systems with focus on automated laser cladding, ultra high-speed robotics, and advanced vehicle systems. His extensive research collaboration with industry has resulted in many new technologies, patents, and journal publications.

Dr. Stephen Francis Corbin received his B.Eng and M.Sc. degrees in metallurgical engineering from Dalhousie University in 4986/87 and his Ph.D. in materials engineering from McMaster University in 4993. Following four years as a product and process development specialist with the Westaim Corporation in Fort Saskatchewan, Alberta, Dr. Corbin joined the University of Waterloo, Ontario, Canada in 4997 where he is currently an Associate Professor in the Materials Engineering and Processing Group within the Department of Mechanical Engineering. He has taught introductory materials engineering courses as well as senior and graduate courses in process and physical metallurgy and materials characterization. His areas of expertise include: powder metallurgy, laser processing, solders and brazes, porous materials and metal/ceramic composites.

© 2005 by CRC Press LLC

Contents

1 Introduction 4.4 What is Laser Cladding? 4.2 Dierent Names, Same Technology 4.3 Why Laser Cladding? 4.4 History of Laser Cladding 4.5 Applications and Market Opportunities 4.5.4 Coating 4.5.2 Parts Repair and Refurbishment 4.5.3 Rapid Prototyping and Tooling 4.6 Future Direction of Laser Cladding Technology 4.7 Looking Ahead 2 Background and Basic Overview 2.4 Laser Material Techniques 2.2 Dierences Between Laser Cladding, Alloying and Glazing 2.3 Dierent Methods of Laser Cladding 2.3.4 Two-Step Laser Cladding (Pre-placed Laser Cladding) 2.3.2 One-Step Laser Cladding 2.4 Clad Dimensional Characteristics 2.5 Important Parameters in Laser Cladding by Powder Injection 2.5.4 Dilution 2.5.2 Wetting Angle and Interfacial Free Energies 2.5.3 Laser Pulse Shaping 2.6 Combined Parameters 2.6.4 Aspect Ratio 2.6.2 Combined Energy and Powder Densities’ Parameters 2.7 Comparison Between Laser Cladding and Other Metallic Coating Techniques 2.8 Comparison Between Laser Cladding and Other Prototyping Techniques 3 Laser Cladding Equipment 3.4 Lasers 3.4.4 Laser Types 3.4.2 Laser Beam Characteristics © 2005 by CRC Press LLC

3.4.3 3.2

3.3

Types of Lasers and Laser Beam Characteristics in Laser Cladding Process Powder Feeders and Powder Delivery Nozzles 3.2.4 Powder Feeder Types 3.2.2 Applications of Powder Feeders to Laser Cladding 3.2.3 Nozzles Positioning Devices 3.3.4 CAD/CAM System for Trajectory Generation

4 Laser Cladding Process Modeling 4.4 Physics of the Process 4.2 Governing Equations 4.2.4 Essential Boundary Conditions 4.3 Laser Cladding Models in Literature 4.3.4 Steady-State Models 4.3.2 Dynamic Models 4.4 Lumped Models 4.5 Analytical Modeling 4.6 Numerical Modeling — A Case Study 4.6.4 Thermal Mathematical Model 4.6.2 Solution Algorithm 4.6.3 Numerical Parameters 4.6.4 Numerical Results 4.6.5 Experimental and Numerical Analysis 4.6.6 Comparison Between Numerical and Experimental Results 4.7 Flow Field Modeling at the Exit of Coaxial Nozzle 4.7.4 Laminar Model 4.8 Experimental-Based Modeling Techniques 4.8.4 Stochastic Analysis 4.8.2 Artificial Neural Network Modeling 5 Control of Laser Cladding Process 5.4 Sensors 5.2 Closed-Loop Control of Laser Cladding 5.3 Closed-Loop Control of Laser Cladding, An Example 5.3.4 Equipment and Configuration 5.3.2 Optical CCD-based Detector 5.3.3 Control Strategy 5.3.4 Closed-Loop vs. Open-Loop 5.3.5 Application of the Developed Controller to Fabrication of Two Simple Components 5.4 Application of Knowledge-Based Control to Laser Cladding 5.4.4 Fuzzy Logic Controller © 2005 by CRC Press LLC

6 Physical Metallurgy and Material Systems of Laser Cladding 6.4 Cladability 6.4.4 Processing Parameter Considerations 6.4.2 Metallurgical Considerations 6.2 Solidification Conditions Encounter in Laser Cladding 6.2.4 Process Conditions 6.2.2 Constitutional Supercooling 6.2.3 Rapid Solidification 6.2.4 Microstructure Maps 6.2.5 Microstructural Scale 6.3 Material Systems Used in Laser Cladding 6.3.4 High Temperature Alloys 6.3.2 Composites 7 Safety 7.4 Laser Classification 7.2 Laser Hazards 7.2.4 Eye Hazards 7.2.2 Collateral Radiation 7.2.3 Electrical Hazards 7.2.4 Chemical Hazards 7.2.5 Fire Hazards 7.2.6 Explosion Hazards 7.2.7 Eye Protection 7.3 Powder Hazards References

© 2005 by CRC Press LLC

1 Introduction

With the rapid growth of laser applications and the reduced cost of laser systems, laser material processing has gained increased importance in a variety of industries. Automotive, aerospace, navy, defense, and many other sectors are widely adapting laser technology for welding, cutting, and hardening. Among the applications of laser technology, laser cladding has received significant attention in recent years due to its diversified potential for material processing such as metallic coating, high-value components repair, prototyping, and even low-volume manufacturing.

1.1

What is Laser Cladding?

Laser cladding is an interdisciplinary technology utilizing laser technology, computer-aided design and manufacturing (CAD/CAM), robotics, sensors and control, and powder metallurgy. Laser cladding utilizes a laser heat source to deposit a thin layer of a desired metal on a moving substrate. The deposited material can be transferred to the substrate by several methods: powder injection, pre-placed powder on the substrate, or by wire feeding. Among these methods, laser cladding by powder injection has been demonstrated to be most eective. In this process, the laser beam melts the powder particles and a thin layer of the moving substrate to deposit a layer of the powder particles on the substrate. A great variety of materials can be deposited on a substrate using laser cladding by powder injection to form a layer with thicknesses ranging from 0.05 to 2 mm and widths as narrow as 0.4 mm. In addition to metallic coating applications, the laser cladding process oers a revolutionary layered manufacturing and prototyping technique. Integration of the laser cladding technology with a three-dimensional CAD solid model, which is sliced into many layers, provides the ability to fabricate complex components without intermediate steps. The development of the laser cladding technology depends on enhancement of the technologies involved. Understanding the interconnections between the involved technologies and the process quality is a major step for the development of laser cladding. However, numerous interactions between the © 2005 by CRC Press LLC

technologies involved in laser cladding not only increase the complexity of the process but also increase the number of process parameters.

1.2

Dierent Names, Same Technology

A survey of the literature indicates that many names have been given to laser cladding technology based on its highly diversified applications. For example, in the coating applications, in addition to “laser cladding”, researchers use the terms “laser coating” [4, 2], “laser powder deposition” [3, 4] or “laser surfacing” [5, 6, 7]. In the rapid prototyping or layered manufacturing applications, numerous names have been used. In prototyping by pre-placed powder laser cladding, the technology is called “selective laser sintering of metals” (SLSM) [8, 9], or “direct metal laser sintering” [40]. In the powder injection laser cladding process, which is the focus of this book, a wide variety of names have been used as outlined below. • At Sandia National Laboratories, a process has been developed for rapid prototyping, which is called “Laser Engineered Net ShapingTM ” R ) [44, 42, 43]. (LENS° • At the University of Michigan in Ann Arbor, the developed process is called “direct metal deposition” (DMDTM ). This process incorporates features of laser cladding, CAD/CAM package, vision and control system [44, 45]. The University of Missouri at Rolla also uses the same name for the process [46, 47]. • “Laser direct casting” (LDC) is used at the University of Liverpool to describe the process, in which a coaxial nozzle is utilized to produce 3D components from a selection of metal powders [48]. • The Integrated Manufacturing Technologies Institute (IMTI) of National Research Council of Canada (NRC) uses the term “laser consolidation” for this process [49, 20, 24, 22]. • The term “laser powder fusion” (LPF) is used by some industries that are involved in turbine blade repair [23]. • At the Laser Aided Manufacturing Processes Laboratory at the University of Missouri at Rolla and at Swiss Federal Institute of Technology the process is named “laser metal forming” (LMF) [24, 25]. • “Directed light fabrication” (DLF) is the name used at Los Alamos National Laboratory in which a process has been developed for free forming and prototyping [26, 27, 28]. © 2005 by CRC Press LLC

• “Laser powder deposition” (LPD) has been used by several research groups in China and England [29, 30]. • At the University of Waterloo, an automated laser cladding technology has been developed for coating and prototyping, which is named “automated laser powder deposition” (ALPD) [34]. • At Stanford University, Carnegie Mellon University, and Penn State University, the process is called “solid free-form fabrication” or “shape deposition manufacturing” [32]. • “Laser rapid forming” (LRF) is the name used by a research group at Shanghai Jiaotong University [33]. This name can be confused with another laser-based technology, laser bending, which is also called laser rapid forming. • Finally, “laser additive manufacture” (LAMSM ) is the acronym used by AeroMet Corporation of Eden Prairie at Minnesota, fully owned subsidiary of MTS Systems Corporation [34]. Despite the variety of names, in practice all of the terms describe technology that share several common features: deposition of thin layers of powder particles melted by a laser heat source on a substrate. The process will be termed “laser cladding” throughout this book.

1.3

Why Laser Cladding?

Laser cladding oers many advantages over conventional coating processes such as arc welding and plasma spraying. The laser cladding technique can produce a much better coating, with minimal dilution, minimal distortion, and better surface quality. There are also a number of advantages to use this technique as a rapid prototyping technique. Rapid prototyping can be used to produce a mechanical component in a layer-by-layer fashion, which enables the fabrication of part with features that may be unique to laser cladding prototyping, such as a homogeneous structure, enhanced mechanical properties, and one-step production of complex geometries. Parts fabricated using the technique are near net shape, but will generally require final machining. They also have good grain structure, and have properties similar to, or even better than the intrinsic materials. Both pre-placed and powder injection laser cladding oer these features; however, laser cladding by powder injection has fewer material limitations than pre-placed in which it does not require secondary placing powder laminating operations and it can be used to repair parts as well as to fabricate them. © 2005 by CRC Press LLC

Due to its additive nature, laser cladding can be applied in a variety of ways to parts, tools and advanced manufacturing to overcome the limitations of existing metal fabrication technologies. This results in a number of benefits as follows: 4. Reduction of production time: The length of time required to build a prototype is a problem for new product development. In many cases, both prototype and production tooling are needed; therefore, the length of time to produce a prototype and the necessary tooling can be several months. The laser cladding process can reduce this time by fabricating tools and main prototypes directly from the CAD solid model [44]. 2. Enhancement of thermal control: The laser cladding process oers a well-controlled heat-treated zone due to the nature of the laser beam. A high-power laser beam is well confined and tense and, as a result, the rapid heating and cooling that occur in the process have little eect of heat on the base material. Therefore, the original properties of the base material are aected to a limited extent only. In addition, this thermal zone can be monitored and optimized during the process, which can significantly improve the quality of the tools produced. Laser cladding also oers a controllable energy over the surface of the desired tool to control the rate of solidification, which is the main parameter in the formation of microstructure and mechanical properties [35]. 3. Parts repair: Current tool repair technology relies on destructive, high-temperature welding processes. In addition, machining errors or last-minute engineering changes can aect on-time delivery of tooling, and potentially impact the introduction date of a new product. Laser cladding can be applied as a safe technology to repair tooling, especially on critical contacting surfaces. Laser cladding increases tool life and in many cases can save a high-value tool that would otherwise need to be replaced [20, 36]. 4. Production of a functionally graded part: In conventional metallic fabrication, it is di!cult to produce a part from dierent materials layers. Laser cladding oers a method to produce functionally graded parts by injecting dierent materials during the fabrication of the parts. It is also possible to produce desired alloys by injection of dierent powders through various nozzles around the process zone [37, 38]. 5. Production of smart structure: In conventional metallic fabrication methods, embedding objects into the tools is impossible due to the nature of manufacturing. Laser cladding, with its additive nature, oers the ability to create “smart structure” by embedding objects such as sensors and magnets during fabrication. Encapsulating these objects reduces the potential for damage or failure from temperature and environmental conditions [39]. © 2005 by CRC Press LLC

Despite its obvious benefits, laser cladding is not yet widely utilized in metallic coating or prototyping applications. While laser cladding clearly offers a number of advantages over conventional fabrication technologies, the process can also have some drawbacks. Due to disturbances in the process, the clad quality may vary significantly. Variations of the quality may even be observed between processing cycles performed using the same operating conditions. This poor reproducibility arises from the high sensitivity of laser cladding to small changes in the operating parameters such as laser power, beam velocity and powder feed rate, as well as to process disturbances such as variations in absorptivity. Finding an optimal set of parameters experimentally and using them in an open-loop laser cladding process may not result in a good quality clad due to random or periodic disturbances in the system. Therefore, development of an intelligent closed-loop control system is essential for overcoming the eects of disturbances in the process. High investment cost, low e!ciency of the laser sources, and lack of control over the cladding process are disadvantages of the use of this technology in coating and prototyping processes. However, with continued technological developments in high-power diode lasers (HPDL), fiber lasers, and sophisticated knowledge-based controllers, the laser cladding process shows a great industrial potential for use in metallic coating and prototyping applications.

1.4

History of Laser Cladding

The invention of the first working laser by Maiman [40] in the 4960’s was a breakthrough in science. Immediately after this invention, scientists claimed that the laser was the answer to a multitude of scientific problems that might not have been even known during those years. These problems had arisen in many areas resulting in lasers being adapted to many technologies to dissolve the problems with their unique features. One of the areas that benefited from the lasers technology was material processing, which was rapidly developing in the 4970’s when the e!ciencies and power of commercial lasers became higher and higher. The development of high-power gas lasers (e.g., CO2 lasers) in 4975 made laser welding, cutting and metal hardening possible in that decade. Among the laser material processing technologies, laser cladding was used by Gnanamuthu at Rockwell International Corporation in Thousand Oaks of California in the late 4970’s [44]. A pre-placed laser cladding method was used to investigate the feasibility of the process in applying dense ceramic cladding to metallic workpieces. At about the same time, several research groups around the world began projects to develop apparatus and systems for development and improvement of the process. Among these groups, the project conducted by William M. © 2005 by CRC Press LLC

Steen first at Imperial College of University of London, England and subsequently at Liverpool University, where he moved in April 4988, had a great impact on the development of laser cladding technology [6, 42]. He along with Vijitha Weerasinghe introduced laser cladding by powder injection to academia and conducted a number of projects to evaluate the developed process [43, 44, 45, 46]. The other research group, led by Jyoti Mazumder, at the University of Illinois at Urbana-Champaign, Urbana, USA, contributed many fundamental principles to this area in the 4980’s. Mazumder’s group not only developed models for this process and studied the mechanism of the process [47, 48, 49], but they also applied the technology to many metals and ceramics to investigate their potential for cladability, and also wear and corrosion resistance [50, 54, 52, 53, 54]. A review of the literature shows that the number of papers and patents related to this technology increased significantly in the 4980’s. These papers disclosed the devices for enhancement of the technology, such as development of powder feeders, cooling systems, hemispherical reflecting device for re-absorbing the reflected light, etc. [55]. The applications of the technology for wear and corrosion resistant alloys were also reported by many research groups in the 4980’s [56, 57]. The features of this technology received attention from industry in the 4980’s as well [58]. Laser cladding was identified as a process with a significant edge over the conventional processes for wear and corrosion resistant coating. The research projects being conducted by dierent industries were even ahead of academic projects. The first reported use of the laser cladding by industry was the hard-facing of Nimonic turbine blade interlock shrouds for the RB244 jet engine at Rolls Royce in 4984. In 4983, at Pratt and Whitney, the nickel-base alloy turbines of JT8 and JT9 engines were hard-faced using preplaced laser cladding [59]. Hard-facing of turbine blade shroud tips and znotches by laser cladding continued to gain acceptance by dierent companies. The technology was being accepted by leading engine manufacturers such as General Electric, Pratt & Whitney, Allied Signal, Rolls-Royce, Allison, Solar and MTU. From a commercialization point of view, several companies, such as Avco Everett Metalworking Lasers Inc. and United Technologies Industrial Lasers Inc. were established in the 4980’s to address the needs of industry for metallic coating and repair in North America and Europe. In the automotive industry, laser cladding technology was transferred to the market for the engine valve seat coating by some European and Asian automotive companies, such as Fiat [58], Toyota [60], and Mercedes Benz. In the component repair market, laser cladding brought a huge amount of consideration in the 4980’s. Laser cladding was successfully utilized for rebuilding and coating of the H-dimension (airfoil section thickness) of worn turbine vanes, the tip of the turbine blades, and turbine bolts [58, 64, 36, 62, 63]. A number of dierent companies and research groups around the world have utilized laser cladding technology for turbine blade repair, including Human Corporation and Gorham Technologies in the USA, Starrag in Switzerland, © 2005 by CRC Press LLC

Sultzer in the Netherlands, SIFCO Turbine Components in Ireland, and many others. Another application of laser cladding, rapid prototyping or layered manufacturing, received a great deal of attention in the 4990’s and continues to be explored in the new millennium. In 4986, a new process for prototyping complex parts called “stereolithography” was patented. This process used ultraviolet lasers to selectively cure photo polymer materials. In 4988, the first commercial stereolithography machine was sold and a new industry in rapid prototyping was established. Stereolithography gave product developers the ability to quickly and accurately visualize, iterate, optimize, and fabricate new designs directly from a three-dimensional CAD solid model. Although most commercial systems used polymers and photopolymers, the industry was looking for features to rapidly fabricate the metallic prototypes that could be directly used in real machines. Global competition was also forcing product manufacturers to look for new ways to reduce new product design time and manufacturing costs. In response to these demands, eorts began to utilize laser cladding technology for the development of machines for direct metal prototyping. At the University of Illinois at Urbana, the Mazumder research group extended their project to the development of systems for rapid prototyping, which was later called “direct metal deposition” (DMDTM ) [64]. The group examined building parts in one and two dimensions, taking into consideration both the time and cost involved in the process compared with traditional methods. Due to the success of their project, the research group moved to the University of Michigan to conduct their research with more emphasis on automotive manufacturing. In the late 4990’s, the developed technology was licensed to Precision Optical Manufacturing Inc. (POM Inc.) at Plymouth, Michigan to supply molds and dies fabricated by the developed technique to the automotive industry of the Michigan area. Many other research and development groups initiated projects to develop methods for prototyping metallic parts based on laser cladding by powder injection. Among them, Sandia National Laboratories, which is a multi-program laboratory operated by the Lockheed Martin Corporation, was funded by the Departments of Energy of the US government to conduct research for development of laser near shape fabrication methods. The University of Liverpool research group, led by Steen, also began projects for laser direct manufacturing, which contributed extensively to this field [65, 66]. In the late 4990’s, a research group under the leadership of Xue and Islam of the Integrated Manufacturing Technologies Institute (IMTI) of National Research Council (NRC) of Canada developed apparatus and methods for layered manufacturing called “laser consolidation” [49, 20, 24, 22]. Their achievements had a great impact in this field due to the unique surface quality of the parts produced using their technique. In the last few years, a research group at the University of Waterloo conducted research for the development of an intelligent laser cladding apparatus. © 2005 by CRC Press LLC

The main focus of the research is in the direction of development of intelligent modeling techniques and knowledge-based controllers for the process. These knowledge-based controllers will be eventually used in an autonomous laser cladding machine, which can not only deposit a wide range of alloys, but can also make the complex shapes without the need for the presence of specialists [34, 67, 68]. The flexibility of laser cladding is beginning to be recognized by many industries and research groups. The potential of this technology is great as research groups continue to contribute to its growth through research programs and training of students in laser cladding techniques technology.

1.5

Applications and Market Opportunities

As mentioned earlier, laser cladding has several diverse applications. In the following sections, various attempts by research groups and industry to adapt the process to dierent applications are explained.

1.5.1

Coating

Coating results in deposition of a thin layer of material (e.g., metals and ceramics) onto the surface of a selected material. This changes the surface properties of the substrate to those of the deposited material. The substrate becomes a composite material exhibiting properties generally not achievable through the use of the substrate material alone. The coating provides a durable, corrosion-resistant layer, and the core material provides the load bearing capability. A number of dierent types of metals, such as chromium, titanium, nickel, copper, and cadmium, can be used in the metallic coating process. There are many coating deposition techniques available. However, selecting the best depends on many parameters such as size, metallurgy of the substrate, adaptability of the coating material to the technique intended, level of adhesion required, and availability and cost of the equipment. Although laser cladding has the potential for utilization in dierent industrial divisions for metallic coating, its application to metallic coating is limited due to the high cost and the low process speed. However, with improvement of laser e!ciency, reduction in the cost of lasers, and the development of new generation of lasers such as high-power diode and fiber lasers, there is a strong potential for laser cladding to be widely used for coating applications in several major industries. Another indication of the potential of laser cladding for coating of a variety of materials is the increase in the number of published papers and reports concerning the technology in the recent years. Based on © 2005 by CRC Press LLC

information from the Compendex search engine, the number of papers dealing with applications of laser cladding to coating of dierent materials has risen from 620 papers in the 4990’s to more than 750 papers from 2000 to 2004. The majority of published papers related to the metallic coating by laser cladding address the use of several major materials in aerospace, medical, and automotive industries. Titanium-based alloys [69, 70, 74, 72], nickel-based superalloys [73, 74, 75, 76, 77, 78], and cobalt-based alloys [79, 80, 84] are some of the important alloys that are deposited on dierent substrates such as unalloyed steels, alloyed steels, hardened steels, stainless steels, aluminum alloys, cast irons, and nickel or cobalt-based alloys. POM Inc. in Michigan performed the deposition of wear-resistant and high-temperature materials (e.g., cobalt-based Stellite and nickel-based alloys) onto tool surfaces that are exposed to harsh high temperature, thermal shock environmental conditions, to increase their lives [82]. Recently, the biocermics coating on titanium alloys was also performed by laser cladding; the coated parts are then used in orthopedic implants with a calcium phosphate layer in order to promote the growth of the bone when the implant is inserted in the body [83]. Laser cladding along with other laser surface treatment methods has also been examined for the production of glassy metallic layers, which provide superior resistance against wear and corrosion [84]. The most leading metallic coatings market for laser cladding is the coating of commercial aircraft gas turbines. The world original equipment manufacturer’s (OEM) market for coatings used in commercial aircraft gas turbine sections is estimated to be approximately $460 million per annum, based on the information released by Gorham technologies. In response to demands for the development of a higher e!ciency, lower cost industrial gas turbine engine, high-strength, high-temperature capability materials, such as nickelbased superalloys, are coated on the turbine bodies to meet the needs of the hot gas path components. The shroud interlock between turbine blades has been also hardfaced with Triballoy to reduce the wear due to sliding between blades during the warm up and cold down the engine. Laser cladding has been recently used in this sector to deposit the mentioned materials on the spacecraft components. With recent technological improvements in the new generation of lasers, it is expected that laser cladding technology will take on an increasingly important role in this market. In addition, laser cladding also has several other coating applications for industrial parts to produce surfaces, that are resistant to abrasive, erosive and adhesive wear; wet corrosion; and high temperature oxidation and corrosion. Some of the products that have received metallic coating by laser cladding are: • Shafts used in drilling tools • Engine valve seats © 2005 by CRC Press LLC

FIGURE 1.1 Coating of oil drilling tools by laser cladding (Source: Courtesy of Fraunhofer Institute for Material and Beam Technology, Germany [85]).

• Tools hardfacing • Hydraulics pump components • Molds Figure 4.4 shows one of the applications of laser cladding for coating of oil drilling tools, which are subjected to significant wear in their operation [85].

1.5.2

Parts Repair and Refurbishment

A major application of laser cladding is in the repair and refurbishment of high-value components such as tools, turbine blades and military components. Laser cladding can be used to rescue high-value components which are overmachined due to the errors in design or machining process. These engineering or machining errors can easily jeopardize the entire eort of the design and manufacturing of high-value tools or components. Conventional methods use welding to retrieve these damaged components; however, these methods are usually destructive due to the highly distributed temperature over the area of repair. This thermal destruction causes a low mechanical quality, crack, porosity and very short life of the component. Laser cladding can provide a permanent structural repair and refurbishment on many alloys (e.g., aluminum alloys) that are generally considered unweldable by conventional methods. The success of the laser cladding technology in this © 2005 by CRC Press LLC

area is due to the small heat zone, rapid solidification, increased cleanness, lower dilution, and increased controllability over the depth of heat-aected zone. An example of the repair by laser cladding of a shell made from high strength aluminum alloys (i.e., unweldable 7075/7475 aluminum alloys) is shown in Figure 4.2. This shell is used in undersea weapon components, which sustain wear and damage as a result of handling, operation and the corrosive nature of the saltwater environment. Laser cladding has been applied to repair of such components. The results are promising such that the repairs are permanent. They stop corrosion and increase structural integrity [86].

FIGURE 1.2 Repair by laser cladding of a shell made from high-strength aluminum alloys (Source: Courtesy of the Naval Undersea Warfare Center (NUWC), USA).

One of the other areas, in which laser cladding plays an important role is turbine blades repair and refurbishment. Turbine blades are under very high thermal and mechanical stresses (i.e., centrifugal force and thermal gradient) in an aggressive environment. The blades usually suer a variety of damage during operation, such as creep, life cycle fatigue, hot corrosion, impact of external particles on their surfaces, etc. Therefore, in terms of maintenance requirements, manufacturing di!culties and costs, the blades are the most critical item of today’s gas turbines. As a result, blade manufacturing and maintenance companies are looking for repair technologies that not only repair the defected blades superiorly, but also maintain the original mechanical and © 2005 by CRC Press LLC

metallurgical features of the repaired components. The low heat input property of laser cladding is the most unique characteristic of this technology that makes it highly attractive for jet engine components repair applications, in which metal depositions are required to be applied to superalloys. These superalloys are highly susceptible to strength loss and physical distortion when exposed to excessive temperature variations. Conventional repair techniques, such as tungsten inert gas, metal inert gas, plasma and electron beam welding, usually cause a large amount of heat during weld metal deposition which results in large temperature increases in the body of the component. The temperature increases above certain limits cause the base alloy to be weakened. This weakening along with component distortion can cause irreversible damage to the part. In contrast to conventional weld repair, the laser cladding process transfers heat only to localized areas, typically using a 0.5 mm diameter laser beam. As a result, heat inputs are at least one order of magnitude lower than the heat input incurred during conventional welding, which results in reduced residual stresses and distortion and a substantially smaller heat-aected zone [87]. An even greater repair market potential exists for the application of laser cladding in turbine engines. Advanced gas turbines, are now being fitted with single crystal and directionality solidified components in order to achieve maximum thermal e!ciencies by operating the engines at higher turbine inlet temperatures. In the fabrication and repair of such engines, laser cladding is rapidly being recognized as a critical and essential technology. In some cases, it is also recognized as the only repair technology because the oriented airfoil castings are highly susceptible to re-crystallization when subjected to the intense heat induced during conventional weld repair [36, 20, 62]. Figure 4.3 shows the application of laser cladding to repair a tip of turbine blade performed in Sulzer Elbar, which is one of the constitute companies that form Sulzer Turbomachinery Services [88]. The turbine blade is made from precipitation-hardened CC-superalloy Inconel 738. The size of the repair and refurbishment market is immense. However, there seems to be no concrete data that defines the size of this market. One of the key aspects of this market is aircraft engine components maintenance. Aircraft engine maintenance accounts for 30 percent of the total cost of aircraft maintenance, which is a good indication of the size of available market for the technology. The global market of the repair of aircraft engine turbines and compressor blades, used in civil and military applications, has been estimated to be about 4.2 billion dollars per annum [89].

1.5.3

Rapid Prototyping and Tooling

A new and major application of laser cladding is in rapid prototyping (RP) and rapid tooling (RT) markets for rapid fabrication of complex components and tools. Production of tools such as cutting tools, dies and molds, which have traditionally been fabricated by highly-skilled tool and mold makers © 2005 by CRC Press LLC

FIGURE 1.3 Application of laser cladding to repair of a tip of rotor blade made of precipitationhardened CC-superalloy Inconel 738 (Source: Courtesy of Sulzer Elbar, The Netherlands).

using CNC and electrical discharge machining, has always suered from cost problems and slow turnaround times for manufacturers. If tools are fabricated late, market opportunities will be missed, which is often the death knell for a new product. As a result, rapid tooling has received significant attentions in recent years from manufacturers looking for technologies that are able to produce high-value tools and components with high integrity, high density, and good surface quality, at low manufacturing costs with a short manufacturing time [90]. The market for RP and RT is significantly large with a significant annual growth. The worldwide tooling market is estimated in the tens of billions of dollars per annum. The market size for RP was approximately $800 million in 2002, when 4.5 million parts were produced by the available units in the market [94]. The applications of RP in North America are categorized within the following sectors: 25 percent consumer products, 24 percent automotive, 44 percent business machines, 44 percent medical, 8 percent academic, 8 percent aerospace, 5 percent government and 8 percent other. However, many of the customers for RP units are looking for a reliable metallic prototyping machine that will not only be intelligent enough to prototype the components without the need for highly qualified personnel, but that will also be robust enough to produce the components with high quality [92]. Laser cladding technology has the potential to address the current gap for metallic rapid prototyping. Laser cladding has demonstrated promising capabilities for tools and components fabrication. A recent survey by the National © 2005 by CRC Press LLC

Center for Manufacturing Science (NCMS) revealed that laser cladding could reduce the time of die production by 40 percent, if the process is controllable over the dimensions of the product. It has been reported that for the production of surgical tools, the technology can reduce 62 steps into 7 steps [44]. In recent years, researchers have been working on enhancement of laser cladding to construct prototypes and production tooling, even for precision metal parts made from dierent commercial alloys such as H43 tool steel, 346 and 304 stainless steels, nickel-based superalloys (e.g., Inconel 625, 690, 748, 2024), aluminum, composite, and titanium-based material (e.g., Ti-6Al4V). The feature of the technology provides the functionally graded material deposition capability, which are applicable in many aerospace components in which a light weight but hard external surface are requested.

FIGURE 1.4 R (Source: Courtesy of Sandia National LaboratoFabrication of a blade by LENS° ries).

R ) [42], developed at Sandia Laser Engineering Net ShapingTM (LENS ° National Laboratories, is one of the rapid metal forming processes that has demonstrated the feasibility of laser cladding to produce near-net shape metal parts. The system utilizes a CAD solid model to build an object one layer at a time. The solid model is sliced into a series of layers that are subsequently used to generate the motion to deposit each layer of material. These layers are then deposited in a subsequent fashion to build the entire part. R technology. Also, Figure 4.4 shows the fabrication of a blade by the LENS° R to Figure 4.5 shows a special housing which has been fabricated by LENS° reduce the secondary machining time. © 2005 by CRC Press LLC

FIGURE 1.5 R A housing part fabricated by hybrid fabrication method including the LENS ° technology (Source: Courtesy of Sandia National Laboratories).

Among the materials used for part manufacturing and prototyping, titaniumbased alloys have received significant attention due to their use in the aerospace industry. Many eorts have been made to obtain the required processing parameters to produce high-quality titanium alloy material, in terms of mechanical and metallurgical properties. One of the premiere companies in this field is AeroMet Corporation, which has developed the “laser additive manufacturing” (LAMSM ) technology. Their results demonstrate the capability of producing shaped structures (e.g., ribbed structural) from Ti, Ti-6Al-4V and other alloys such as Ti-5Al-2.5Sn, including extra low interstitial (ELI) grades. Figure 4.6 shows several components made by LAMSM , which were then machined to meet the required dimensions and tolerances. The required production time and the quality of products are significantly better than those for forging methods. AeroMet’s current production facility includes an 48 kW CO2 laser and a five-axis manipulation capable of producing component sizes within 3 × 3 × 1 m. Figure 4.7 shows the AeroMet equipment for performing LAMSM . Another key player in components manufacturing by laser cladding is the IMTI-NRC in Canada. As mentioned earlier, the technology used by the NRC is called “laser consolidation”. This technology can produce metallurgically sound parts from IN-625, 346L Stellite 6, and M4 without porosity and crack. The samples showed an excellent dimensional accuracy and surface finish. The produced parts from LC IN-625 alloy and 346L stainless steel were shown to have stronger strength than the respective cast and even wrought materials [24, 49]. Figures 4.8 and 4.9 depict some of the parts fabricated by the NRC. Figure 4.8 depicts the components of a robot fabricated by laser consolidation to evaluate the potential of the process as a rapid functional prototype manufacturing process for the production of structural components using Ti6Al-4V alloy. The figure also indicates the steps of production. These parts © 2005 by CRC Press LLC

FIGURE 1.6 Dierent components fabricated by laser forming technology (Source: Courtesy of AeroMet Corporation, USA).

FIGURE 1.7 AeroMet equipment including a 48 kW CO2 laser for performing laser forming (Source: Courtesy of AeroMet Corporation, USA).

© 2005 by CRC Press LLC

were used in the Advanced Robotic Mechatronics System (ARMS) project, which was initiated by MD Robotics and supported by the Canadian Space Agency (CSA) [22].

a)

b)

c)

FIGURE 1.8 A robot’s joint, fabricated by laser consolidation made from Ti-6Al-4V: a) original interface on the flat substrate, b) fabrication of housing on the interface, c) joint after final machining (Source: Courtesy of the NRC’s Integrated Manufacturing Technologies Institute).

Figure 4.9 depicts two industrial parts: Figure 4.9a shows a part made from wear-resistant Stellite 6; Figure 4.9b shows a part which is a half-part of a complex flextensional transducer shell made from IN-625 [24]. The produced shell has an excellent dimensional accuracy and wall integrity. Direct metal deposition (DMD) is being developed to fabricate molds and dies as well as for parts repair using the laser cladding technology. The wide application of DMD in the aerospace and medical fields is due to its large potential cost savings. Koch et al. [93] demonstrated the application of laseraided DMD to generate components with a dimensional accuracy of 0.25 mm using a closed-loop control of process parameters. The dimensional accuracy of their part depends on the uniformity and repeatability of the process [44]. The commercialization of this technique, which incorporates a closed-loop system, has been successful and a machine called DMD5000 has been introduced to the market by POM Inc., Michigan. This machine provides features for direct metal fabrication. The DMD5000 machine is an integration of a 5-kW CO2 laser integrated with a flying optic, a gantry robot plus a XY table CAD/CAM system, and several special powder feeders. The machine has a workspace with size of 1.5 × 0.5 × 0.45 m. Laser cladding technology has been under investigation at the University of Waterloo with more emphasis on the development of an automated machine for this technology. The developed machine has been used for production of complex parts made from H43 and nickel-based superalloy. The developed © 2005 by CRC Press LLC

a)

b)

FIGURE 1.9 Two fabricated parts by laser consolidation: a) a component made from Stellite 6, b) a complex flextensional transducer shell made from IN-625 (Source: Courtesy of the NRC’s Integrated Manufacturing Technologies Institute).

technology integrates vision-based detectors and knowledge-based controller with conventional laser cladding equipment to control the process parameters such as laser power, process speed, and powder feed rates, which enables control of the clad geometry in real-time. Figure 4.40 shows a logo made from H43, which was fabricated using the system developed at the University of Waterloo.

FIGURE 1.10 A logo fabricated from H43 tool steel at the Mechanical Engineering Department of the University of Waterloo.

© 2005 by CRC Press LLC

1.6

Future Direction of Laser Cladding Technology

Laser cladding technology is in the early stages of commercialization and will oer a revolutionary new manufacturing technique to the industry in the new millennium. Due to its promising features, many industries keep their eyes on the technology. Research groups and companies involved in the development of the technology should canalize their eorts in several main directions to resolve the current shortcomings of this technology. The main focus of research eorts should be the development of autonomous machines for the process, which can not only deposit a wide range of alloys, but can also make complex shapes without the need for the presence of specialists. The development of an automated machine may not be possible without string collaborations between the researchers with dierent disciplines. The development of a knowledgebased controller for such a machine needs expertise of scientists with control, automation, and also metallurgical backgrounds. Additional research eorts should focus on increasing the speed of the process as the current processing speed is slow compared to the competitive techniques such as plasma and thermal spraying. Laser cladding applications to the aircraft industry and the eects of laser cladding on substrate also require further investigation. The investigation of laser cladding and eects on the applications requires an understanding of the process and the relationship between laser energy, process speed, powder feed rate, and mechanical and metallurgical properties. Therefore, eorts should be dedicated to modeling of the process. In the turbine blade repair market, customers are looking for a machine that has the capability to repair the turbine blade in-situ without removing the blades from the rotor. To do this, it is necessary to develop a sophisticated positioning device that will provide a su!cient maneuverability around the rotor. The capability of cladding on an inclined surface is also essential for in-situ turbine blade repair. In the recent years, micro- and nanotechnology have received a significant attention. Scientists say any technology can have applications in micro- and nanotechnology, if it can be miniaturized in the right fashion. Having said that, laser cladding has a great potential to benefit these cutting-edge technologies [94, 95], as laser cladding techniques can provide a revolutionary technique for maskless micro structure fabrication. In recent years, Optomec Inc., a company in the USA, has developed a process called M3 D, which makes it possible to coat dierent substrates in the range of 40 to 50 µm. The 40 µm deposition makes it attractive particularly for space-limited applications in the electronic technology. The concepts of their work are based on the pre-placed laser cladding process; however, the Optomec Inc.’s technology utilizes the spray of liquid droplets mixed either with metals or organic ele© 2005 by CRC Press LLC

ments. Post-heat treatment is then performed by the laser to sinter the metal particles. Figures 4.44 and 4.42 show two samples made using the Optomec Inc.’s technology. Figure 4.44 shows silver lines with 50 µm width deposited over a 500 µm step of a micro-mirror. Figure 4.42 shows a tapered spiral GPS antenna. Due to the fascinating features of this technology, such as deposition on a non-planar micro substrate with 40 µm coating width, it is anticipated that many research groups will concentrate on this field in the Micro Electro Mechanical Systems (MEMS) prototyping development.

FIGURE 1.11 Silver lines with 50-micron width deposited over a 500-micron step by M 3 D (Source: Courtesy of Optomec Inc., USA).

Recently, the NRC of Canada successfully tested laser cladding technology for the use in fabrication of space-related structures, such as the main parts of a robotic arm. These tests resulted in the production of robotic arm components with excellent mechanical properties, which is a significant indication of the potential of laser cladding technology for in-space manufacturing facilities [22]. In June 2003, the NASA Marshall Space Flight Center brought a variety of national engineering and manufacturing specialists to Huntsville, Alabama, for a workshop in “In-Space Manufacturing of Space Transportation Infrastructure”. This workshop addressed strategies for developing a robust in-space transportation infrastructure that may eventually include permanent refueling stations and maintenance platforms in space, as well as cargo vehicles that haul supplies across the shipping lanes of space. A discussion of the characteristics of laser cladding technology identified its potential to lead inspace manufacturing processes, as it would allow space dwellers and explorers to quickly design and produce replacement parts in the space. Therefore, it is anticipated that the laser cladding process will play an important role in the © 2005 by CRC Press LLC

FIGURE 1.12 Tapered spiral GPS antenna by M 3 D technology (Source: Courtesy of Optomec Inc., USA).

development of possible in-space manufacturing methods in the near future. Last but not least, market development should be taken into consideration as a very important issue for exploring the technology by the manufacturing sectors. A number of industries still consider the laser a “fancy” device. This results in a large technological barrier between conventional and advanced manufacturing experts. This barrier has to be brought down through a close dialog between these two groups along with end-users. It is important to expect users to request appropriate features for a particular process; therefore, a partnership between the conventional and advanced manufacturing experts should be conducted to satisfy end-users. The authors believe that breakthroughs in laser cladding will require a hybridization of advanced and conventional techniques, which may not be possible without a close interaction between conventional and advanced specialists on one side and end-users on the other side. Education of highly qualified people in this area can minimize the current technological gap between the advanced and conventional manufacturing sectors. Unfortunately, current laser material processing programs in universities are very limited, most likely due to the lack of high-power laser in institutes. In Canada, just a few universities are equipped with high-power lasers; therefore, the opportunities for education in the field of laser material processing are very limited. In addition, the field is multidisciplinary and requires training in a number of dierent scientific fields, such as laser, optics, automation, control, robotics and material science. © 2005 by CRC Press LLC

1.7

Looking Ahead

This book consists of seven chapters. An introduction to the potential applications of laser cladding technology in industry was explained in Chapter 4. Chapter 2 reviews the background of laser cladding. In Chapter 3, the equipment used in laser cladding (i.e., laser, positioning device, and powder feeder and nozzles) will be explained. In Chapter 4, the physics of the process will be discussed and dierent modeling approaches for the process will be presented. The application of experimental-based modeling techniques to laser cladding including stochastic and artificial neural network techniques will be also addressed in Chapter 4. Chapter 5 addresses the control aspect of the technology, as well as the design, simulation and implementation of several classical and fuzzy controllers applied to laser cladding. Chapter 6 explains the mechanical and metallurgical characteristics of laser cladding for dierent metallic alloys. Also, the eects of process parameters on the clad bead quality and a methodology for evaluation of clad bead quality using the combined parameters will be developed in this chapter. Chapter 7 ends the book by explaining safety issues related to the laser and powder materials.

© 2005 by CRC Press LLC

2 Background and Basic Overview

In this chapter, a basic overview on laser cladding is presented. The laser cladding classification, laser cladding process parameters, and comparison of laser cladding process with the competitive techniques are some of the topics addressed in this chapter.

2.1

Laser Material Techniques

A laser beam provides unique characteristics for material processing. The electromagnetic radiation of a laser beam is absorbed by the surface of opaque materials (e.g., metals). The interaction time between the laser and material leads to dierent processes as shown in Figure 2.4. The relative velocity of the laser beam with respect to the substrate causes the thermal cycle in the surface layer. The figure also shows a schematic representation of the physical phenomena that occur during various laser material processes. These processes are due to dierent combinations of absorption, heat conduction, melting, powder addition, and rapid solidification. However, the common physical phenomenon of all laser material processing techniques is rapid solidification, which causes a superior and fine metallurgical structure. Laser cladding is one of the important types of laser material processing, in which a laser beam irradiates powder particles and the surface of the substrate moved by a positioning device. As a result of additive powder particles, a thin layer called a “clad” is produced on the substrate.

2.2

Dierences Between Laser Cladding, Alloying and Glazing

Adding powder to the melt pool may create three dierent products depending on the type and the amount of material added. Figure 2.2 depicts schematic cross sections of the coating-substrate couple for these three processes. These © 2005 by CRC Press LLC

Absorption v Laser Beam

Heat Conduction

Rapid solidification v Laser Beam

Transformation Hardening & Annealing

v Laser Beam

Adding Powder Melting Rapid Solidification

Remelting, Welding & Shock Hardening

Rapid Solidification

Alloying, glazing, cladding

FIGURE 2.1 Schematic of physical phenomena during dierent laser material processing techniques.

are classified as laser cladding, glazing, and alloying. Included on the right is a schematic of the compositional profile across the coating-substrate couple. In laser alloying, a small amount of powder is fed into the melt pool. As such, homogeneous mixing throughout the melt region may be obtained [96]. Laser cladding resembles laser alloying except that dilution by the substrate is kept to a minimum and more addition of material to the surface is required [35]. In laser glazing, a metallic glass coating is deposited in order to provide an environmentally eective surface in terms of wear and corrosion [97]. The principle advantage of laser glazing is that it alters microstructures without changing the composition [97, 98]. Using laser cladding, the following advantages can be obtained compared to other surface material processing [99, 400, 404]: 4. Reduced dilution, which is the mixing percentage of the substrate to the clad region (compared to laser alloying) © 2005 by CRC Press LLC

2. Improved wear resistance of a part 3. Reduced thermal distortion 4. Reduced porosity, particularly in laser cladding by powder injection 5. Improved controllability of the process 6. Reduced post-cladding machining time and cost

Laser Alloying

A+ B

A 100%

Clad Composition

B 100%

B

A 100%

Clad Composition

B 100%

A

B Laser Glazing

Laser Cladding

A

A

A+B

FIGURE 2.2 Dierent microstructures of laser alloying, glazing, and cladding.

2.3

Dierent Methods of Laser Cladding

Basically, there are two dierent techniques for laser cladding as follows: 4. Two-step process (pre-placed laser cladding) 2. One-step process In the two-step process, the first stage consists of a layer of coating material being placed before laser irradiating. It is then melted with the substrate material by the laser beam in the second stage (see Figure 2.3a) [402]. In the one-step process, an additive material is fed into the melt pool. The additive material may be supplied in the following dierent forms: © 2005 by CRC Press LLC

Laser Beam Paste

Clad Substrate b1) Laser Beam

Clad

Laser Beam

Inert Gas

Placed Powder

Powder & Inert Gas

Clad Substrate

Substrate b2)

a)

Laser Beam

Wire Clad Substrate b3)

FIGURE 2.3 Dierent methods of laser cladding: a) two-step laser cladding, b) one-step laser cladding, including b4: paste laser cladding, b2: powder injection laser cladding, b3: wire feeding laser cladding.

© 2005 by CRC Press LLC

• Powder injection • Wire feeding • Paste Figure 2.3b depicts one-step laser cladding techniques where an inert gas shrouds the laser material interaction zone to prevent oxidation of the surface at the high processing temperature. Powder injection cladding is a more robust method than wire and paste cladding, because there is no direct contact with the melt pool, and the laser beam can pass through the stream of powder particles instead of being obstructed by the wire or paste. In the following, we briefly address several issues involved in two-step laser cladding; then we concentrate on laser cladding by powder injection for the rest of this chapter and the book.

2.3.1

Two-Step Laser Cladding (Pre-placed Laser Cladding)

Pre-placed laser cladding is a simple method used for coating and prototyping. Several issues are involved in this process. The pre-placed powder particles on the substrate must have not only enough bonding to the substrate, but also enough cohesion to each other. It is necessary to prevent the powder particles on the substrate from removing due to the gas flow during the melting in the second step of the process. To overcome this problem, the powder is usually mixed with a chemical binder to ensure its cohesion with the substrate during the process. The side eect of a chemical binder is porosity in the clad due to its evaporation during the process. In the second step of the process the following phenomena occur: 4. Creation of a melt pool in the top surface of the pre-placed powder due to the radiation of laser beam 2. Expansion of melt pool to the interface with the substrate due to the heat conduction 3. Penetration of heat to the substrate causing a fusion bond The control of heat is a very important issue in this method to prevent the high dilution. Dilution is considered to be the mixing percentage of the substrate to the clad region. This problem is one of the important shortcomings of two-step laser cladding that usually limits the process only to single track cladding. Powell et al. [402] developed a theoretical model and analysis technique for pre-placed laser cladding. Their theoretical calculation resulted in a plot, which shows the eect of interaction time on the position of the melt pool front. This result is shown in Figure 2.4. The results indicate that the dilution increases with increasing interaction time (decreasing the process speed). © 2005 by CRC Press LLC

Melt Pool Front Position

Original Powder Surface

0

50W

Depth of Melting (mm)

0.4

100W (3.18x107 W/ m2)

0.8

Cladding Powder

1.2 1.6

200W 400W

2.0 2000W

800W

2.4

Substrate

Increasing Dilution

2.8 0

0.1

0.2

0.3

0.4

0.5

Time (s)

FIGURE 2.4 Displacement of melted front with respect to interaction time at various average laser powers, when the beam radius is 4 mm [402].

2.3.2

One-Step Laser Cladding

As they are shown in Figures 2.3b4 to 2.3b3, one-step laser cladding can be categorized into three methods: powder injection, wire feeding, and paste laser cladding. The common feature of all three methods is the feeding of deposited material in the presence of the laser. 2.3.2.1

Laser Cladding by Powder Injection

In laser cladding by powder injection, powder particles are fed into the heat zone to produce a layer of clad as shown in Figure 2.3b2. We will address this process in depth throughout this book. 2.3.2.2

Laser Cladding by Wire Feeding

In laser cladding by wire feeding, the main idea is to use wire instead of powder as shown in Figure 2.3b3. The wire is usually fed through a ceramic drum containing the desired material wire. Due to the nature of feeding mechanisms, it is essential to use a wire that has been straightened and stored without plastic deformation to ensure stable transport without vibration [403, 404]. Compared to laser cladding by powder injection, it has been claimed that laser cladding by wire feeding has some special advantages [405]. One of its most important advantages is its adaptation to the cladding position. Metal wires © 2005 by CRC Press LLC

are cheaper than metal powders, and also wire feeding wastes less material than powder feeding. In contrast, low surface quality, low bonding strength, porosity, cracks and drop transfer are some problems of wire cladding. The melted liquid at the end of the wire does not flow smoothly and continuously onto the workpiece, which is called drop transfer phenomena. Kim et al. [404] conducted research to find the best process parameters that prevent the drop transfer phenomena. They showed that by selecting correct wire feeding direction and position, the splashing of molten drop for laser cladding with wire feeding can be solved. In this case, wire can be plunged into the melt pool and be melted by the heat of the molten metal. However, a successful process strongly depends on the process parameters, and in the presence of disturbances the quality of clad drops dramatically. 2.3.2.3

Laser Cladding by Paste

In laser cladding by paste, a stream of paste-bound material is deposited on a point of the substrate that is usually a little bit ahead of the laser beam [406], as shown in Figure 2.3b4. The paste consists of the hardfacing powder with a suitable binder. However, the binder must be dried in a short period of time while the hardfacing material in a compact form is still kept; otherwise powder particles are blown away by the shielding gas.

Paste Track Before Laser Processing

Laser Beam

Laser Beam

Laser Beam

Paste Volume

Paste Volume Clad

Clad

Lost Paste Substrate

Ideal Situation

a)

Substrate

Substrate

High Process Speed

Low Process Speed

and/or

and/or

Low Paste Volume

High Paste Volume

b)

c)

FIGURE 2.5 Eect of process speed and paste feed rate on the quality of clad [406].

For this process, a special paste feeding system should be designed. Lugscheider et al. [406] designed and implemented a paste feeding system along with a cooling system to protect the paste from thermal emissions from the process zone. The shape of paste on the substrate is controlled by paste feed rate and substrate speed. To have a good clad quality, it is essential to optimize these parameters. A poor paste supply or too high process speed causes high dilu© 2005 by CRC Press LLC

tion and low track height if the laser energy is kept constant. An oversupply of paste on the substrate increases pores formation since evaporation of the binder is inhibited, and it increases the loss of hardfacing material. Figure 2.5 shows the eect of substrate traverse speed and paste feed rate on the quality of the final clad layer. High porosity, extreme sensitivity of the process to disturbances, and di!culties in paste feeding mechanism are troublesome conditions of paste laser cladding.

2.4

Clad Dimensional Characteristics

Several parameters are associated with the clad geometry, which are shown in Figure 2.6. In this figure, h is the clad height, w is the clad width,  is the angle of wetting, and b is the clad depth representing the thickness of substrate melted during the cladding and added to the clad region.

w Clad Bead

θ

h

b Substrate

FIGURE 2.6 A typical cross section of a clad bead.

2.5

Important Parameters in Laser Cladding by Powder Injection

A large variety of operating parameters and physical phenomena determine the quality of laser cladding. Figure 2.7 summarizes these parameters grouped © 2005 by CRC Press LLC

as inputs, processes, and outputs. Generally, the inputs or operating parameters are the laser, motion device, powder feeder set points, and also the material and ambient properties. The outputs of the process which represent the clad quality are the geometry, microstructure, cracks, porosity, surface roughness, residual stresses and dilution [407, 35, 408, 409, 440]. In the following, some of the major associated parameters with the process are explained.

Inputs Laser

Motion Device

Average power Spot size Wave length Pulsed/CW Beam profile Laser pulse shaping

Material

Relative velocity Relative acceleration System accuracy

Powder Feeder

Substrate geometry Composition Metallurgical, thermo physical & optical properties Powder size

Powder feed rate Inert gas flow rate Nozzle specification Powder stream profile

Surface tension

Processes Physical phenomena

Outputs Clad quality

Absorption

Geometry

Conduction

Microstructure

Diffusion

Hardness

Melt pool dynamics

Cracks

Fluid convection

Pores

Gas/melt pool interaction Laser attenuation by powder Rapid solidification

Residual stresses Surface roughness Microstructure Dilution

Ambient Properties Preheating Shield gas velocity Kind of shield gas

FIGURE 2.7 Inputs, outputs and process parameters of laser cladding by powder injection.

2.5.1

Dilution

One of the properties of the clad layer is called dilution. Dilution has two definitions: geometrical and metallurgical [407]. The geometrical definition of dilution is illustrated in Figure 2.6. According to the specified parameters in the figure, the dilution is b (2.4) dilution = h+b where b is the thickness of substrate that was melted during the cladding process [mm], and h is the height of the clad bead [mm]. © 2005 by CRC Press LLC

Alternatively, dilution may be defined as the percentage of the total volume of the surface layer contributed by melting of the substrate [407]. According to the composition, dilution is defined as dilution =

c (Xc+s  Xc ) s (Xs  Xc+s ) + c (Xc+s  Xc )

(2.2)

where c is the density of melted powder alloy [kg/m3 ], s is density of substrate material [kg/m3 ], Xc+s is weight percent of element X in the total surface of the clad region [%], Xc is the weight percent of element X in the powder alloy [%], and Xs is the weight percent of element X in the substrate [%]. Of interest is the fact that dilution increases with increasing laser power but decreases with increasing travel speed.

2.5.2

Wetting Angle and Interfacial Free Energies

In laser cladding, either pre-placed or powder injection, wetting angle and interfacial free energies are important parameters that indicate the quality of the clad. In general, three types of clad cross section may be produced by laser cladding as shown in Figure 2.8. These cross sections represent the amount of dilution, corresponding wetting angle , and interfacial free energies  [J/m2 ]. Three interfacial energies for laser cladding can be considered as solid-liquid interfacial free energy  SL , solid-vapor interfacial energy  SV , and liquid-vapor interfacial energy  LV .

γ LV γ SL

θ

a)

γ LV γ SV

γ SL

θ

γ SV

b)

γ SL

θ

γ LV γ SV

c)

FIGURE 2.8 Laser cladding cross sections, associated wetting angle and interfacial free energies: a) high dilution, well wetting, b) ideal clad, c) no dilution, non-wetting.

A balance between the mentioned energies governs the shape of the clad bead. This balance is expressed by  SV   SL =  LV cos()

(2.3)

The liquid will wet the substrate as cos() $ 1 or equivalently, if  SV  SL > © 2005 by CRC Press LLC

 LV , which corresponds to Figure 2.8b. A large positive spreading factor S =  SV   SL   LV causes spreading, whereas a lower number causes a non-wetting system as shown in Figure 2.8c. When the laser energy is high, dilution increases and wetting angle decreases as shown in Figure 2.8a. In laser cladding, oxidation is a serious problem at higher processing temperatures, which are required for melting the metals. The oxidation causes a low quality clad as shown in Figure 2.8c. This is due to the poor wetting of an oxide substrate by a liquid metal and also much lower surface energies of metal oxides.

2.5.3

Laser Pulse Shaping

The laser beam can be in the form of continuous wave (CW) or pulsed wave. In the form of pulse wave, several parameters associated with shape of pulses are defined: laser pulse energy E, laser pulse width (laser pulse duration) W , laser pulse frequency (laser pulse repetition rate) F , average power Pl and duty cycle C. These parameters which are shown in Figure 2.9 can be expressed by C = FW

(2.4)

Pl = EF

(2.5)

1-FW F

Energy (J)

W

E

0

Time (s)

1

FIGURE 2.9 Laser pulse shaping, including pulse width W , pulse energy E and pulse frequency

F.

© 2005 by CRC Press LLC

2.6

Combined Parameters

Due to the numerous parameters involved in the process, it is essential to use the combined parameters to address the clad quality of the process by finding the correlation between the combined parameters and the clad quality. Dierent eective parameters have been reported, which are categorized for continuous and pulsed laser energy. In the following, these combined parameters are introduced.

2.6.1

Aspect Ratio

Aspect ratio AR is a ratio between width and height of the clad represented by w AR = h

2.6.2

Combined Energy and Powder Densities’ Parameters

There are dierent approaches to show the correlation of the clad aspect ratio and the eective energy. However, they can be categorized into two types of laser beam waves: continuous wave (CW) and pulse wave. 2.6.2.1

Combined Parameters for Continuous Wave (CW) Laser Beam

Steen et al. [42] showed that the beginning of non-wetting (i.e., discontinuous clad track) is correlated with combined parameter 2rPlwU , and the melting through of the substrate is correlated with combined parameter PUw where, Pw is the absorbed energy to the substrate laser power [W], U is process speed [mm/s], and rl is the laser spot radius on the substrate surface [mm]. They also showed that combined parameter 2Pwm˙Url is correlated with the aspect ratio, where m ˙ is the powder feed rate [g/s] [444]. In addition, they showed that a term 2rPlwm˙ has a maximum value before dilution occurs (e.g., 2500 J/(gmm) for Colmonoy Wallex PC6) and also combined parameter 2rPlwU represents the minimum energy required for cladding before the track starts to be discontinuous (e.g., 22 J/mm2 for Colmonoy alloy). They have arrived at a general plot which shows the correlation of these combined parameters and feasibility of cladding process as shown in Figure 2.40. The figure shows the hatched area that the cladding process is feasible. For continuous wave, Wu et al. [442] introduced two combined parameters for laser cladding by powder injection, which can be fitted to the process to present the critical states. These two combined parameters result in a simpler interpretation of the cladding process. They are called specific energy © 2005 by CRC Press LLC

Aspect Ratio

Power Per Spot Diameter (W/mm)

(e.g., 5 for Colmonoy Alloy)

Dilution Problem 1000

pw 2mrl

Porosity Problem

(e.g., 2500 J/(gmm) for Colmonoy Alloy) 500

le ib as e F

on gi Re

Pw 2rlU

(e.g., 22 J/mm for Colmonoy Alloy)

Non-feasible Cladding Region 0

0.1

0.2

0.3

0.4

0.5

Powder Feed Rate (g/s)

FIGURE 2.10 Correlation of aspect ratio, combined parameters, power per spot diameter, and powder feed rate with feasibility of laser cladding by powder injection [42].

Especif ic [J/mm2 ] and powder density G [g/dm2 ], which are expressed by Especif ic =

Pw 2Url

(2.6)

m ˙ (2.7) 2U rl where Pw is the laser power on the substrate [W], U is the process speed ˙ [mm/s], rl is the radius of the laser beam on the substrate [mm], and m is the powder feed rate [g/min]. Figure 2.44 shows the correlation between Especif ic and G [g/m2 ] and the critical situation in the laser cladding process of Cobalt-based alloy on an A3 steel substrate [442]. G=

2.6.2.2

Combined Parameters for Pulsed Wave Laser Beam

Two combined parameters, eective energy density Eef f [J/mm2 ] and eective powder deposition density #eff [g/mm2 ], are defined for a pulsed wave laser beam [3, 4, 443] and are expressed by Eef f = © 2005 by CRC Press LLC

EF Aef f

(2.8)

2.5

Fine Condition

50

2.0

40

1.5

30

1.0 Critical Condition 0.5

20

10

Single Track Height (mm)

Specific Energy (J/ mm2)

60

0 10

30

50

70

90

110

Powder Density (g/ m2) Correlation between specific energy and powder density Correlation between powder density and single-track height

FIGURE 2.11 Correlation between specific energy Especif ic [J/mm2 ] and powder density G [g/m2 ] and their eect on single track height for Co-based alloy on an A3 steel substrate [442].

#ef f =

mF ˙ W Aeff

(2.9)

where Aeff is the eective area per second which is irradiated by the laser beam and powder stream [mm2 /s], E is the pulse laser energy [J], F is the laser pulse frequency [Hz], W is the laser pulse width [s] and m ˙ is the powder feed rate [g/s]. This is determined not only by the substrate velocity but also by the pulse characteristics of the laser beam. Referring to Figure 2.42 and performing the geometrical analysis, the following equation is obtained for the eective area per second as

Aeff =

; h 1F W 2 A r + 2U r  2F Url  A l F A ? l A A A =

rl2 F + 2Url W F

© 2005 by CRC Press LLC

1F W 2F yU

 rl2 ( 2  sin1 for rl > for

³ ´i

1F W 2F

rl 5

y rl

U

1F W 2F U

(2.40)

where

y=

s

rl2



µ

1  FW U 2F

¶2

(2.11)

U is the process speed [mm/s], and rl is the laser spot radius on the substrate [mm]. Equations (2.10) and (2.11) are derived based on subtracting Ac , which is the half uncovered area of a rectangle created by two successive laser pulses (See Figure 2.12), from the total area covered by the pulses per second.

WU

1 − FW U F

rl

Ac = area uncovered by two succesive laser pulses FIGURE 2.12 A schematic of the aective area of cladding created by succesive laser pulses.

Inherent in Equation (2.9) is the assumption that when the pulse is o, no powder is deposited on the substrate due to the absence of the energy provided by the laser pulses. This aspect of the process is introduced through the inclusion of the duty cycle C = F W in Equation (2.9). Using these two combined parameters, a general plot can be obtained for iron-aluminide coating on mild steel or H13 [3, 4, 113] that presents the corresponding clad quality based on eective energy density and eective powder deposition density as shown in Figure 2.13 © 2005 by CRC Press LLC

160

Effective Energy Density (J/ mm2)

140

Good Quality Clads

120

Roughness, Some Pores & Cracks

100

80

Brittle Clads 60

No Cladding

40

20

1

1.25

1.5

1.75

Effective Powder Deposition Density (g/

2

2.25 x 10-3

mm2)

FIGURE 2.13 Correlation between eective energy density Eef f and eective powder deposition density # ef f for iron-aluminide coating on mild steel.

2.7

Comparison Between Laser Cladding and Other Metallic Coating Techniques

The application of laser cladding should compete with several major coating techniques such as thermal spray, welding, chemical vapor deposition (CVD), and physical vapor deposition (PVD). The thermal spray can be categorized into three methods, which are combustion torch (e.g., flame-spray, high-velocity oxy fuel, and detonation gun), electric (wire) arc, and plasma arc. In addition, PVD can be categorized into ion plating and ion implantation. CVD is also categorized into: sputtering, ion plating, plasma-enhanced CVD, low-pressure CVD, laser-enhanced CVD, active reactive evaporation, ion beam and laser evaporation [114]. Table 2.1 compares several major features of these coating techniques to provide the advantages and disadvantages of these processes for metallic and non-metallic coating applications. As it is listed in Table 2.1, laser cladding creates a very strong bond with low dilution, where a very low heat-aected zone (HAZ) is produced in the substrate. However, the investment cost and © 2005 by CRC Press LLC

TABLE 2.1

Comparison between laser cladding and other coating techniques. Feature Laser Welding Thermal CVD PVD Cladding spray Bonding strength Dilution Coating materials Coating thickness

Repeatability Heat-aected zone (HAZ) Controllability Cost

High High Metals, ceramics 50 µm to 2 mm

High High Metals

Low Nil Metals, ceramics 0.05 µm to 20 µm

Low Nil Metals, ceramics 0.05 µm to 10 µm

Moderate

Moderate Nil Metals, ceramics 50 µm to several mm Moderate

Moderate to high Low

High

High

High

High

Very low

Very low

Moderate to high High

Low

Moderate

Moderate

Moderate

Moderate to high High

Moderate to high High

1 to several mm

maintenance cost of the laser cladding machine are currently high, which are disadvantages of this process. It is anticipated that because of the fast growing of the new generation of lasers such as high-power diode and fiber lasers, which oer higher e!ciency and lower maintenance cost, laser cladding technology will play an important role in the metallic coating market in the near future.

2.8

Comparison Between Laser Cladding and Other Prototyping Techniques

There are about 20 methods of rapid prototyping. Some of the major rapid prototyping techniques are: stereolithography apparatus (SLA), fused deposition manufacturing (FDM), selective laser sintering (SLS), 3D printing, and laser cladding-based prototyping (i.e, DMD, LENS, ALPD, laser consolidation, etc.). Table 2.2 compares the features of these techniques. In this table, one of the features is called “support structure”, which refers to parts to place in the unsupported geometries during fabrication, such as the supporting part for fabrication of the top portions of a part with the shape of the letter “T”. These supports are usually calculated and added to the part by the system’s software and may be formed of the same material as the part, or from an entirely © 2005 by CRC Press LLC

TABLE 2.2

Comparison between laser cladding-based and other prototyping techniques. Feature Laser SLA FDM SLS 3D cladding Printing (e.g., DM D, ALPD, etc.)

Dimensional accuracy Prototype’s material

Moderate

Moderate

Metals and ceramics

Prototype’s quality

High

Support structure Cost of machine

Not required High (still under R&D)

Polymers and photopolymers Low to moderate (e.g., fragile) Required Relatively Moderate

Moderate to high Filament ABS plastic High

Required Moderate

Low Polymers, metals and ceramics Low (e.g., Porosity and cracks) Not Required Moderate

Low to Moderate Hard plastic, runner Low (e.g., fragile)

Required Low

dierent material. Support structures are either mechanically removed or dissolved away in secondary operations before the part can be used. As Table 2.2 indicates, prototyping techniques, which are performed based on laser cladding technology, have superior features for metallic rapid prototyping. However, the cost of this technology is still high due to the need of highly qualified personnel and the cost of laser systems. The development of an autonomous system for performing the laser cladding process without any need to expert personnel is under research and development in several research groups and industry. It is anticipated that providing a fully intelligent laser cladding apparatus will overcome the shortcomings of this technology.

© 2005 by CRC Press LLC

3 Laser Cladding Equipment

The laser cladding process requires the following equipment: a laser, a powder feeder along with delivery nozzles, and a positioning device equipped with CAD/CAM software. It is essential to understand the construction of these devices and their performance under dierent working conditions for the laser cladding process to be successful. This chapter provides a comprehensive comparison of available lasers, powder feeders, and nozzles to demonstrate their potential and suitability for use in laser cladding technology. The chapter also includes a brief review of available positioning devices and CAD/CAM systems suitable for this process.

3.1

Lasers

In the early 1960s, an enormous contribution was made to technology with invention of the first working laser. The word “laser” stands for light amplification by the stimulated emission of radiation. Miaman [40] invented the first ruby laser, which was the result of considerable discovery of Einstein [115], who demonstrated that lasing action should be possible. In general, the light emitted by lasers is dierent from the ordinary light sources such as incandescent bulbs, fluorescent light, and high-intensity arc lamps. Laser light has the following characteristics: • Highly monochromic. All regular light sources emit light (e.g., incandescent and fluorescent light) of many dierent wavelengths. Ordinary colored light consists of a broad range of wavelengths covering a particular portion of the visible-light spectrum. The beam of a laser, on the other hand, consists of an extremely narrow range of wavelengths within one single color portion of the spectrum meaning that it consists of light of almost a single wavelength. This nearly “monochromatic” or “single-colored” property is unique to laser light. • Highly coherent. The light waves within a highly collimated laser beam may be defined as a source of coherent light, unlike other regular light sources. This characteristic leads to a constant phase dierence in two or more waves over time. Two waves are said to be in phase if © 2005 by CRC Press LLC

their crests and troughs meet at the same place and at the same time, whereas the waves are out of phase if the crests of one wave meet the troughs of another. • Highly directional. All conventional light sources emit light in all directions, and it always diverges more rapidly than a laser beam. Directionality is the characteristic of laser light that causes it to travel in a single direction within a narrow cone of divergence. However, perfectly parallel beams of directional light (i.e., collimated light) cannot be produced even by a laser. In some applications, optical systems are employed with lasers to improve the directionality of the output beam. • Sharply focused. For laser light, the focused spot can be very small; for example, an intensity of 1017 W/cm2 is readily obtained by a laser, which is incredibly higher than any energy source (e.g., an oxyacetylene flame has an intensity of only about 103 W/cm2 ). In order to explain how a laser works, it is necessary to explain the following three processes by which the atom can move from one energy state to another: 1. Absorption. If the atom is placed in an electromagnetic field that is resonating at frequency f , the atom can absorb an amount of energy hf as represented by (3.1) hf = Ex  E0 and move to the higher energy state. In the equation, Ex is the higher level of energy and E0 is the ground level of energy for an atom. Figure 3.1a shows the atom in its ground and then in a higher level of energy. 2. Spontaneous emission. After a time, the atom will move of its own accord to its ground state, emitting a photon of energy hf in the process. This process, shown in Figure 3.1b, is called spontaneous emission because the event is not triggered by any outside influence. Usually, the mean life of excited atoms before spontaneous emission occurs is about 108 s. However, for some excited states, this means the life could be as much as 105 times longer; this longer state is called metastable. The light produced by the spontaneous emission of an atom is neither monochromatic and directional, nor coherent. 3. Stimulated emission. In this step, the atom is again in its excited state, but this time radiation with a frequency of f is present. A photon of energy of hf can stimulate the atom to move to its ground state, during the process, the atom emits an additional photon, whose energy is also hf. This process, shown in Figure 3.1c, is called stimulated emission because the event is triggered by the external photon. The emitted photon is in every way identical to the stimulating photon. The waves associated with the photons have the same energy, phase, © 2005 by CRC Press LLC

polarization, and direction of travel. Therefore, stimulated emission produces light that is monochromatic, directional, and coherent; this light appears as the output beam of a laser-stimulated emission for a single atom.

Ex a) Absorption

b) Spontaneous emission

c) Stimulated emission

Ex

hf

None E0

E0

Ex

Ex hf

None E0

E0

Ex

Ex

E0

E0

hf

Radiation

Matter

Matter

hf hf

Radiation

FIGURE 3.1 Interaction of radiation and matter in a) absorption, b) spontaneous emission, and c) stimulated emission.

In practice, generation of laser is subject to the interaction of a large number of atoms in the excitation field. Ludwing Boltzmann’s theorem shows that the number of atoms in a state of higher energy Nx is a function of the number of atoms in their ground state N0 and their corresponding energies, as represented by Nx = N0 e(Ex E0 )/T (3.2) where  is Boltzmann’s constant, E0 is energy of ground state, Ex is energy of atoms in a higher state, and T is thermal equilibrium temperature. This equation indicates that Nx < N0 because Ex > E0 . As a result, there are fewer atoms in the excited state than in the ground state. If a flood of atoms with photons of energy Ex E0 is generated, as shown in Figure 3.2a, photons will disappear via absorption by ground state atoms. Einstein showed that the © 2005 by CRC Press LLC

probabilities per atom for these two processes are identical. Therefore, because there are more atoms in the ground state, the net eect will be the absorption of photons. However, to produce laser light, the number of emitted photons should be more than absorbed photons. To accomplish this, a situation in which stimulated emission dominates should be occurred. The direct way to cause this is to begin with more atoms in the excited state than in the ground state, as shown in Figure 3.2b. This phenomenon is called population inversion. However, such a population inversion is not consistent with thermal equilibrium. Therefore, it is necessary to consider appropriate ways to improve the population inversion phenomenon in any laser type.

a)

Ex

Ex

Eo

Eo b)

FIGURE 3.2 a) equilibiruim distribution of atoms between the ground state E0 and excited state Ex , b) inverted population.

3.1.1

Laser Types

The numerous laser types can be categorized based on physical and operating parameters, which are involved in the laser beam generation. There are several ways to classify laser types; however, the most common way is to classify them based on their physical state of the active material. According to this criterion, lasers can be categorized as follows: • Gas lasers • Excimer lasers • Solid-state lasers • Semiconductor lasers • Liquid dye lasers • Fiber lasers © 2005 by CRC Press LLC

These classes of lasers can provide dierent wavelengths from  1 mm to  1 nm. Output powers cover even greater range of values. For continuous wave (CW) lasers, typical powers range from a few mW, used for signal sources; to tens of kW, used for material processing; and to a few MW, used in some military applications. In pulsed lasers, peak power can be much greater than in CW lasers. It can reach values as high as 1PW (1015 W). The pulse duration can vary from a ms level, typical of lasers operating in the so-called free-running regime (i.e., without any Q-switching or mode-locking elements in the cavity), to about 10 fs for some mode-locked lasers. In the following sections, the construction of the above-mentioned classes of laser will be briefly explained and their potential applications to the laser cladding process will be addressed. 3.1.1.1

Gas Lasers

Gas lasers utilize a gas or gas mixture as the active medium and may be operated in either CW or pulsed modes. Gas lasers are grouped into four categories according to the type of gas used: neutral-atom gas, ionized gas, and molecular gas. Excitation is usually achieved by applying current through the gas. Neutral-atom gas lasers employ electrically-neutral gas atoms as the active medium. The HeNe laser is the most common neutral-atom gas laser. Ion lasers contain ionized gas molecules as their active medium. The most common lasers of this group are the argon and krypton gas lasers. Some lasers, such as helium-cadmium (HeCd), include metal ions in a gas. CO2 is, by now, the most common molecular laser, but several other molecular gases are employed as well, such as CO, HE, and OF. Figure 3.3 depicts the basic construction of a CO2 laser with dierent sources of excitation: RF and DC. As seen, dierent sources of excitation can be embedded in the gas tubes. The wavelength of a CO2 laser is 10.24 µm and output power of the commercial CO2 lasers can be even more than 45 kW. The optical e!ciency of this type of laser is about 40 percent and their wall plug e!ciency is about 20 percent. These e!ciencies are strong functions of temperature. Regardless of low e!ciency, CO2 lasers have a better beam quality and focusability than other types of lasers with the same power. CO2 lasers also have the advantages of being very well absorbed by organics, glass and ceramic materials and are relatively color independent. As a result, selecting a CO2 laser is a trade-o between economical issues and the performance of the laser in dierent industrial applications. Although the high maintenance cost and low wall plug e!ciency are two restrictions for applications of CO2 lasers, this laser has been widely adopted for usage in industrial applications such as welding, cutting, cladding, and processing of glass, ceramics, and organic (e.g., polymer textile, paper, tissue material, and food) materials.

© 2005 by CRC Press LLC

Cathode

Anode

Laser Beam

Discharge

Partially Mirror Mirror

Gas In

Gas Out a)

Mirror Electrode Laser Beam Uniform Discharge

Partially Mirror

Electrode

Gas Out

Gas In b)

FIGURE 3.3 A schematic of CO2 laser with a) DC excitation, b) RF excitation.

3.1.1.2

Excimer Lasers

Excimer stands for “excited dimer”. The principle of operation of an excimer laser is a chemical reaction. The excimer laser is very often dedicated to the generation of a single wavelength. Each molecule of the active medium of an excimer laser is composed of an inert gas atom and a halogen gas atom. Among others, these include krypton fluoride (KrF), xenon fluoride (XeF), argon chloride (ArCl), argon fluoride (ArF), krypton chloride (KrCl), and xenon chloride (XeCl). The rare-gas halide (compound made from a halogen) laser, which emits in the ultraviolet wavelength (126 to 558 nm), operates on electronic transitions of molecules with repulsive ground states, until a diatomic (having two atoms within one molecule) occurs. In general, the excimer laser is generated by combination of two identical atoms or molecules, one of which is excited and the other is at a ground state. For this laser, excitation can be accomplished by E-beam or electric discharge. Figure 3.4 shows the construction of an excimer laser, including electrodes, supplying and storing electrical lines, mirrors, lenses and © 2005 by CRC Press LLC

a chamber for chemical reaction. U0

Laser Beam

FIGURE 3.4 A schematic of an excimer laser.

Typical average output powers of excimer lasers range from less than 1 W up to approximately 700 W. This is two orders of magnitude less than traditional Nd:YAG or CO2 lasers, which operate in the infrared part of the spectrum. The high intensity beam of an excimer laser is the product of pulse energy with 10 to 1000 mJ and the pulse duration of approximately around 10ns. Excimer lasers are widely used in medical technology as well as micromachining, as they provide the ultimate method for skiving, ablation, and micromachining of flex circuits, plastics, and ceramics. With these lasers, the ability to control depth in microns provides an easy and cost eective method for removing excess material, exposing leads and pads, removing oxide coatings, and providing controlled depth cavities. 3.1.1.3

Solid-State Lasers

Solid-State (SS) lasers use a solid crystalline material as the lasing medium, and are optically pumped. These lasers have lasing material distributed in a solid matrix (e.g., the ruby). Solid-state lasers use a pumping source to excite the atoms and supply energy to the crystal rods; typical pump-sources can be flash lamps or diode lasers. Figure 3.5 shows a typical construction of a solid-state laser along with pumping source. © 2005 by CRC Press LLC

The first laser, invented in 1960, was a solid-state laser [40]. It used a synthetic ruby rod (chromium-doped aluminum oxide) with mirrors on both ends (one semitransparent) pumped with a helical xenon flash lamp surrounding the rod. The lamp was similar to those used for indoor and high speed photography. The intense flash of blue-white light raised some of the chromium atoms in the matrix (the aluminum oxide is just for structure and is inert as far as the laser process is concerned) to an upper energy state from which they could participate in stimulated emissions.

Pump Radiation

Pump Radiation

Pump Cavity

Laser Beam

Pump Radiation

Partially Reflecting Mirror

Pump Radiation

Solid State Laser Rod

Mirror

Pump Cavity

FIGURE 3.5 A typical construction of a solid-state laser along with pumping source.

Modern solid-state lasers are not too dierent from the original prototype. The majority of modern solid-state lasers use neodymium (Nd) doped materials such as Nd:YAG (yttrium aluminum garnet, which is Y3 A15 O12 ), Nd:YVO4 , Nd:Glass, and others. These materials have a much lower lasing threshold than ruby as well as other desirable physical and optical properties. The strongest output wavelength of neodymium-doped lasers is approximately 1064 nm which is close to IR (infrared), and it is totally invisible. The exact wavelength of the strongest lasing lines depends on the actual host material. In addition to Nd:YAG and Nd:YVO4 at 1064 nm, there © 2005 by CRC Press LLC

are types of solid-state lasers that lase at slightly shorter wavelengths such as Nd:LSB at 1062 nm, Nd:Glass at 1060 nm, Nd:YLF at 1053 nm, and Nd:NiNbO3 (neodymium-doped lithium niobate) at 1092 nm. Other materials include holmium-doped YAG (Ho:YAG) or Ho:YLF, which provide laser light at approximately 2060 and 2100 nm, respectively. Among the above-mentioned materials used as the main crystal in solidstate lasers, Nd:YAG and Nd:YVO4 are becoming increasingly important for high-power lasers (e.g., 4 kW at 1064 nm). Solid-state lasers can be pulsed, CW, or quasi-CW. In a pulsed solid-state laser, Q-switching (Q stands for quality) is used to stabilize and boost peak power output by preventing the laser cavity from resonating (e.g., one of the mirrors is blocked or forced to be misaligned by a mechanical mechanism) until the population inversion is built up fully. CW solid-state lasers may use xenon or krypton arc lamps or other sources of intense broad spectrum light. However, the trend today is toward the use of arrays of high-power laser diodes for pumping. These can be designed to have a wavelength that matches an absorption band in neodymium (around 800 nm), making for very e!cient excitation. The diode pumped technique is rapidly taking over due to their higher e!ciency than flash one. This results in lower power consumption and heat dissipation, reduction in size, as well as an increase in reliability and decrease in maintenance. This type of laser is further discussed in the next section. Quasi-CW solid-state lasers are actually pulsed lasers but operating with a pulse repetition rate (PRR) that is high enough to appear to be continuous. 3.1.1.4

Semiconductor Lasers

Semiconductor lasers, which are also called diode lasers, are not solid-state lasers. These electronic devices are generally very small and use a low amount of power. They may be built into larger arrays such as the writing head in some laser printers or compact disc players. Figure 3.6 shows the construction of a diode laser. Some of the properties of diode lasers include wide spectrum band (2-20 nm), large beam divergence (up to 40 half-angle), non-symmetrical beam distribution (2.5-6 times dierence in beam divergence in the two orthogonal axes), and lower energy intensity per area. Diode lasers use nearly microscopic chips of gallium-arsenide or other exotic semiconductors to generate coherent light in a very small package of laser. These materials are based on semiconductors of group III-V components. The energy level dierences between the conduction and valence band electrons in these semiconductors are what provide the mechanism for laser action. Population inversion, as a result of electron transitions from the valency band to the conduction band of a doped semiconductor, is achieved by forward biasing the p-n junctions. Spontaneous emission and stimulated emission occur when electrons in the conduction band recombine with the holes in © 2005 by CRC Press LLC

the valency band. The optical cavity in a diode laser is formed by splitting two opposite facets of the semiconductor wafer to form a Farby-Perot lasing cavity [116].

Metal Contact

Active Region Voltage Bias

P N Laser Beam Heat Sink Metal Contact

FIGURE 3.6 A schematic of a diode laser.

The active element in a semi-conductor laser is a solid-state device not all that dierent from an LED. The first type of this laser was developed quite early in the history of lasers but they became widely available and more economical in the early 1980s. Today, there are various diode lasers in terms of output power. The most common types, found in popular devices like CD players and laser pointers, have a maximum output in the 3 to 5 mW range. The new generation of high-power diode laser (HPDL) can produce 4 kW. The high-power terminology is used for CW diode lasers with output power in excess of 0.5 W. Diode lasers have several disadvantages such as poor beam coherence and symmetry. These disadvantages can be overcome by a diode-pumped solidstate laser such as a diode-pumped Nd:YAG laser as well as the use of optical fiber beam delivery. Diode-pumped Nd:YAG lasers are an integration of crystal and diode pumping unit, as it was mentioned in the last section. A schematic of a typical type © 2005 by CRC Press LLC

of this laser is shown in Figure 3.7. In this laser, the p-polarized diode light, which is transmitted into the rod with low loss on the surface of the rod, is used to pump the YAG rod. The diode light can be sent to the rod through three dierent orientations. The laser crystal is mounted inside a flow-tube whose outer surface has AR (anti reflective) and HR (high reflective) coatings for the diode wavelength [117]. The diode pumped Nd:YAG is an established tool for micro-cutting applications; however, there are several disadvantages of diode pumped solid-state lasers such as low wall plug e!ciency, high running costs, and poor thermal stability.

Diode Laser (Pumping Source)

Cylindrical Lens Flow Tube

YAG Rod

High Reflective Coating

Cooling Water LD Light

FIGURE 3.7 A schematic of a diode-pumped Nd:YAG laser, which has three source of diode pumping [117].

3.1.1.5

Liquid Dye Lasers

Dye lasers are unique due to the use of liquid as the lasing medium. Depending on the particular dye used, the output laser beam can be at a wide range of wavelengths spanning the visible spectrum and beyond. Commercial dye lasers are often pumped by other lasers. For example, rhodamine-B, a common dye used in dye lasers for the red region, is often © 2005 by CRC Press LLC

pumped with an argon ion laser at 514 nm for CW operation or with a doubled YAG laser at 532 nm when pulsed. An intensive flash lamp can also be used as a pump source. Figure 3.8 shows a schematic of a liquid dye laser. The most useful feature of dye lasers is their tunability. The lasing wavelength for a given liquid may be varied over a wide range. Taking advantage of the broad fluorescent linewidths (50-100 nm) available in organic dyes, a diraction grating can be used as a wavelength-dispersive optical element in the laser cavity to perform selective tuning. Such tuning can yield extremely narrow linewidths. The hazards of dye lasers are relatively moderate. Some of the organic dye materials used in this type of laser are toxic, and a high voltage power supply (low current but a large energy storage capacitor) is required to fire the flash lamp.

Lens Pump Beam

Laser Output

Grating Dye Cell

Output Coupler

Beam Expander

FIGURE 3.8 A schematic of liquid dye laser.

3.1.1.6

Fiber Lasers

For the past decade, rare-earth-doped fibers have received widespread attention for their applications as laser sources and amplifiers. With wall-plug e!ciencies greater than 20 percent, a huge increase in the output power of fiber lasers has been reported in recent years. The new development in fiber lasers is high-power output, which works at eye-safe wavelengths. In addition, advances in ultrashort pulsed fiber lasers, based on photonic crystal or holey fibers, have opened up an entirely new set of applications in sensing, materials processing and biomedical sciences. A fiber laser for producing very short pulses is formed by placing a laser fiber in a resonant cavity. The fiber laser is formed of two dierent types of fibers, which are joined in series. They are a gain fiber, which contains the laser gain medium, and a pulse shaping fiber, which uses the phenomenon of solution pulse shaping to shorten the pulses. An initially formed pulse is recirculated many times in the resonator. On each pass, the pulse is both © 2005 by CRC Press LLC

amplified and shortened until it reaches steady-state. The zero dispersion wavelength of the pulse shaping fiber is chosen to be slightly less than the laser wavelength. The fiber is pumped by a continuous source, particularly CW laser diodes [118]. Figure 3.9 shows a schematic construction of a typical fiber laser. Medium Lens

Resonant Cavity

Pump Source

Laser Beam Fiber Optic

FIGURE 3.9 A schematic construction of a typical fiber laser.

The first 2 kW continuous-wave fiber laser has been produced and immediately used in automotive applications in 2003 [119]. The spot size of this 2-kW laser is 50 µm giving a power density of 100 MW/cm2 . The size of the unit is only 110 × 60 × 118 cm, including the power supply and air-cooling system. This new high-power laser is seen as a replacement for solid-state Nd:YAG or CO2 lasers because of the scalable power and a beam quality that is up to ten times better. Investigations show that the single-mode fiber laser is an e!cient, reliable and compact solution for micro-machining. Fiber lasers are more easily integrated into industrial processes in comparison with conventional lasers for a number of reasons: standard wall plug operation and high electrical e!ciency, no water cooling required, single mode fiber delivery line, high quality focusable beam, high repetition rate, optimized pulse duration, exceptionally high reliability, and maintenance-free operation.

3.1.2

Laser Beam Characteristics

Laser beam characteristics play an important role in laser material processing including laser cladding. There are many parameters that indicate the quality of a laser beam. Several important laser beam parameters are beam parameter © 2005 by CRC Press LLC

r

z r (z )

θ

r0l

I (r )

z0

FIGURE 3.10 Laser beam geometry.

product (BPP), laser beam mode, energy distribution over the beam spot area, polarization, and focusability. The beam parameter product (BPP) is important because it provides an indication of the focused beam size and the focal depth. It is represented by BP P =

r0l  2

(3.3)

where r0l is the beam spot radius in the waist of the laser beam and  is the far-field full divergence angle, as shown in Figure 3.10. The argument is that reducing the divergence by using a beam expander would increase the beam spot size. Based on Figure 3.10 and Equation (3.3), it can be concluded that a low divergence angle produces a smaller focused spot and greater depth of focus. The laser energy can be distributed in a uniform or Gaussian form over the laser beam spot area. However, generation of Gaussian energy intensity is easier than the uniform energy intensity. In order to achieve a good beam quality, it is necessary to resonate the beam in a resonator. In the resonator, the distribution of the amplitude and phases of the electromagnetic field can be reproduced due to the repeated reflections between mirrors [116]. These specific field shapes produced in the resonator are known as transverse electromagnetic modes (TEM) of a passive resonator. Transverse electromagnetic modes in polar coordinates, which are also called Gaussian-Laguerre modes, are demonstrated by TEMpl . The subscript p indicates the number of nodes of zero intensity transverse to the beam axis in radial direction, and the subscript l indicates the number of nodes of zero intensity transverse to the beam axis in tangential direction. The intensity distribution Ipl (r, *) of a TEMpl mode can be represented by ¸2 · 2r2 M 2 l p 2r2 M 2 2r2 M 2 2 Ipl (r, *) = I0 ( ) ( ) cos (l*) exp( ) (3.4) L l rl2 rl2 rl2 where I0 is the intensity scale factor [W/m2 ], rl is the radius of the laser beam profile, M 2 is the beam quality factor (based on the ISO 11146), and Lpl is the © 2005 by CRC Press LLC

generalized Laguerre polynomial of order p and index l [120]. The intensity scale factor I0 is expressed based on node number and the average power Pl [W] as ( 2r2 M 2 Pl l=0 r2 (3.5) I0 = 4r2 Ml 2 p! P l = 1, 2, 3, 4, ... r2 (p+l) l l

Based on dierent TEM, various beam energy intensities are available. Figure 3.11 shows several TEMs with Gaussian energy intensities. The other important parameters for a laser beam are the beam propagation factor and the quality factor. The radius of a radially symmetric laser beam varies along the propagation axis, which can be expressed by 2 + 4 2 (z  z0 )2 rl (z)2 = r0l

(3.6)

where r0l is the beam radius of the waist [m], z0 is the waist location with respect to an arbitrary coordinate along the propagation axis [m], and  is the far-field divergence angle [rad]. Figure 3.10 shows the denoted parameters. The propagation can also be described by the beam propagation factor Q, or the quality factor M 2 , which are related as M2 =

nrol  1 = Q 2

(3.7)

where  is the laser wavelength in the used medium [m], and n is the index of reflection. The propagation factor k is then defined as k=

1 2  2 M nrol 

(3.8)

If k = M 2 = 1, the beam is Gaussian; if M 2 > 1, the beam is not Gaussian. However, all of the standard Gaussian propagation formulas may be used with appropriate correction factors (see ISO 11146). In most cases, a laser application requires a laser beam with low divergence emitted in fundamental Gaussian mode (TEM00 ). This is not guaranteed for every laser and is unlikely for especially high-power laser systems because the emission may be multimode or may be changed based on the life of laser systems. As a result, the beam quality should be measured by the available measurement devices such as laser beam analyzer (LBA), slit, knife-edge, and CCD-based instrumentation [121]. The other important parameters of laser beams are reflectivity and polarization. The values of absorptivity and reflectivity are related by the following equations: ½ 1A (for opaque materials) R= (3.9) 1AT (for transparent materials) where R is reflectivity, A is absorptivity, and T is transmissivity. The reflectivity R for normal angles of incidence from air to opaque materials with perfect flat and clean surface is derived by © 2005 by CRC Press LLC

TEM

cross-section

distribution I0

M2 = 2

TEM00

I0

M2 = 2

TEM10

I0

M2 = 2

I0

M2 = 2

TEM11

TEM01

I0

M2 = 2

TEM01*

I0

TEM02

FIGURE 3.11 Several TEM modes with Gaussian energy intensity. © 2005 by CRC Press LLC

M2 = 2

TABLE 3.1

Optical properties of several materials for 1.06 micron light wavelength in room temperature. Materials k n Al 8.50 1.75 Cu 6.93 0.15 Fe 4.44 3.81 Ni 5.26 2.62 Pb 5.40 1.41 Ti 4.00 3.80 Zn 3.48 2.88 Glass 0.10 0.50

R = [(1  n)2 + k2 ]/[(1 + n)2 + k2 ]

(3.10)

where n is the refraction coe!cient and k is the extinction coe!cient of material. The absorptivity, A, of an opaque metal surface can be obtained by A = 1  R = 4n/[(n + 1)2 + k2 ]

(3.11)

Table 3.1 lists the optical properties of several materials for the light radiation with 1.06 µm wavelength. It has to be considered that the optical properties will change with temperature and light wavelength. Photons with shorter wavelengths are easier to be absorbed by the materials than photons with longer wavelengths. Therefore, R normally decreases as wavelength becomes shorter. When temperature rises, there will be an increase in the photon population. Therefore, the probability of interaction between the electrons and material increases causing a decrease in the reflectivity and an increase in the absorptivity. Of interest is the fact that the reflectivity is a function of light polarization and angle of incidence. Light can be described as an electromagnetic wave that propagates through a sinusoidal oscillation of an electric field. The direction in which the electric field oscillates as it propagates is known as the polarization. A laser is defined as “polarized”, if 90% or more of its energy is in a given polarization state such as linear, circular, or elliptical. In general, a laser pulse injects polarized electrons, whose spins have a definite orientation determined by the laser’s polarization. The desired polarization state is generated by a combination of dierent optic systems. Figure 3.12 shows examples of two polarization states: linear and circular. In circular state, the electromagnetic wave propagates as a function of time and rotates around a reference line as shown in the figure. Drude [122] showed a variation in reflectivity with both angle of incidence and plane of polarization. If the plane of polarization is in the plane of incidence, the beam is called a p-ray. If the beam has a plane of polarization © 2005 by CRC Press LLC

y y

z x

E

z x

E

E

Ey E

Ey

a)

Ex

b) Ex

FIGURE 3.12 a) Linear polarization, b) circular polarization.

which is normal angles to the plane of incidence, it is called s-ray as shown in Figure 3.13. The corresponding reflectivities for these two types of polarized beams can be obtained by Rp =

[n  (1/ cos *i )]2 + k2 [n + (1/ cos *i )]2 + k2

(3.12)

(n  cos *i )2 + k2 (n + cos *i )2 + k2

(3.13)

Rs =

where Rp is the reflectivity of a p-ray beam, Rs the reflectivity of an s-ray beam, *i is the incident angle, n is the refraction coe!cient, k is the material extinction coe!cient. In general, p-rays are more easily absorbed by materials than s-rays.

3.1.3

Types of Lasers and Laser Beam Characteristics in Laser Cladding Process

In the laser cladding process, it is essential to provide appropriate power density and interaction time between the laser beam and the material. Figure 3.14 shows the range of the power density and interaction time for various laser material processing techniques. As it is seen, the laser cladding process requires a power density from 70 to 100 W/mm2 and an interaction time of 0.01 to 1 second; any laser intended for use in the laser cladding process should provide this level of power density. In addition, the beam quality is a key factor for a successful laser cladding as © 2005 by CRC Press LLC

ϕincident

ϕ reflection

ϕtransmit a)

ϕ reflection

ϕincident

ϕtransmit b)

FIGURE 3.13 a) p-ray b) s-ray.

will be explained in the next section. The selected laser should provide the appropriate beam quality. Another important issue for any laser material processing is the light reflection from the surface of metals. The reflection is strongly a function of laser wavelength and it varies from metal to metal. Figure 3.15 shows the wavelength dependency of several metals’ reflection factor. It is also important to consider the contribution of temperature in reflectivity. As the temperature of the process zone rises, an increase in absorptivity occurs, which indicates the potential of more energy absorption by the material [101]. 3.1.3.1

Types of Lasers Used in Laser Cladding Process

As described in Section 3.1.1, there are many laser systems in the market. However, CO2 lasers, lamp-pumped Nd:YAG lasers, diode-pumped Nd:YAG lasers, and high-power diode lasers (HPDL) are most commonly used in the laser cladding process. There is no report on the use of liquid dye lasers in the laser cladding process. This laser is not widely used in laser material industry due to its low power capacity. To the best knowledge of the authors, there is no report on the use of fiber lasers in the laser cladding process. However, fiber lasers can be adapted to the process due to its high beam quality, cost eectiveness, and e!ciency in near future. There seems to be only a few reports about the application of excimer lasers to the laser cladding process. Panagopoulos et al. [123] carried out a coating of copper on mild steel by a KrF excimer laser with a wavelength of 248 nm. The power density per pulse was varied between 150 and 430 MW/cm2 and © 2005 by CRC Press LLC

Power Density (W/ mm2)

10

8

Shock Hardening

VAPORIZATION

Drilling 6

10

Glazing Cutting MELTING Ablation

10

4

Welding

Magnetic Domain Control

Melting Alloying

Bending

2

10

Transformation Hardening HEATING

1

Cladding LCVD

Stereolithography 10-8

10

-6

-4

10

10-2

1

100

Interaction Time (s)

FIGURE 3.14 Power density and interaction time for various laser material processing [101].

the pulse frequency was 10 Hz. Except for this work, research groups and industry have not utilized the excimer laser as a source of energy for the laser cladding process due to its low average power. Although the peak power of the excimer laser per pulse is high, this high power per pulse can vaporize the powder particles. Excimer lasers, on the other hand, have the potential for use in coating of micro-devices (e.g., MEMS). Table 3.2 summarizes characteristics of these four types of lasers which have been widely used in laser cladding. Both pulsed and continuous wave lasers have been used in laser cladding; however, with pulsed lasers, it is necessary to maintain the peak power of each pulse in a limited range. Pulses with high peak power energy (even those with low average power) can vaporize the powder particles prior to reaching the process zone. There are major performance dierences between Nd:YAG, HPDL, and CO2 lasers. Nd:YAG and HPDL light are emitted at wavelengths of 1.024 and 0.85 µm, respectively, which are in the near infrared, while CO2 light is emitted at 10.6 µm. The material interactions at these wavelengths dier. © 2005 by CRC Press LLC

1

Cu

Al

0.9

Reflectivity

0.8 0.7

Ni

0.6 0.5

W

0.4 0.3

Si

0.2 0.1 0

200

300

400

500

600

700

800

900

1000

7000

8000

9000

Wavelength (nm) a) Cu

1

Reflectivity

Al 0.8

Au Steel

0.6

0 1000

2000

3000

4000

5000

6000

Wavelength (nm) b)

FIGURE 3.15 Correlation of reflectivity and beam wavelength for dierent materials in two dierent wavelength ranges, a) from 200 to 1000 nm, b) from 1000 to 9000 nm. TABLE 3.2

Characteristics of common lasers used in laser cladding. Characteristics Wavelength [ µm] E!ciency [%] Maximum power [kW ] Average power density

CO2

Nd:YAG

Nd:YAG

lamp-pumped

diode-pump ed

HPDL

10.64

1.06

1.06

0.65-0.94

5-10

1-4

10-12

30-50

10

45 6...8

10

4 5...7

10

5 6...9

10

6 3...5

2 [W /cm ]

Service period [hour] Beam parameter product (BPP)[mm × mrad] Fiber coupling © 2005 by CRC Press LLC

1000-2000

200

5000-10000

5000-10000

12

25-45

12

100-1000

No

Yes

Yes

Yes

Metals are more reflective at 10 µm than at 1 µm as shown in Figure 3.15; as a result, Nd:YAG and HPDL are more e!cient than a CO2 laser for metallic processing. Aluminum is relatively highly reflective compared to the CO2 beam, whereas a beam from a Nd:YAG or HPDL laser is almost perfectly absorbed. On the other hand, most carbon steels and stainless steels absorb CO2 and Nd:YAG beams very much the same. HPDL in comparison to CO2 and Nd:YAG lasers has the shorter wavelength and thus higher absorption of the direct diode laser. Figure 3.15 shows the reflection factor as a factor of wavelength for several metals. A CO2 laser can provide a very high power such as 45 kW. Commercial Nd:YAG lasers are available with powers up to 4 kW (continuous) and pulsed Nd:YAG lasers with lower average powers (e.g., 1.5 kW) but have much higher pulse peak power. That is due to the cooling of the solid rod of solid-state lasers, which is a di!cult task [116]. As a result, the solid-state lasers have problems with high average powers. In contrast, CO2 lasers do not have a serious problem with thermal lensing; therefore, they can be fabricated in high power capacity with a good beam quality [116]. CO2 laser beams are focused to smaller spots and they are more symmetrical, which improves the clad width. A 1-kW CO2 laser can be focused to a 100 µm spot, whereas a 1-kW Nd:YAG is generally used with fiber optics for beam delivery and cannot be focused smaller than 400 µm. A HPDL laser provides a wide beam distribution and has a low beam quality. As a result, HPDL can be used only for coating in which a lower energy per area is required. HPDL lasers in today’s market cannot be used for high melting temperature materials. It is reported that a HPDL laser can be used for laser cladding; however, it is applicable to a limited number of materials and coating thicknesses [124]. Another important issue in selecting a laser is the beam delivery. It is impossible to transport the CO2 beam through a fiber optic cable due to its wavelength (i.e., 10.6 µm). As a result, the maneuverability of a motion system along with a CO2 laser is very limited. Although, a flying optic can be integrated into a CO2 laser to provide an extra degree of freedom, its usage in fabrication of complex parts with laser cladding is still limited. In addition, the flying lenses are very sensitive to powder intrusion into the moving lenses. Nd:YAG and HPDL lasers, on the other hand, can be run through a fiber optic cable and as a result, can be connected to the end eector of a robot with any degree of freedom. CO2 lasers generally produce either a dot-mode (TEM00 ) or a ring-mode (TEM01 ) beam, which can focus down to either a single point or a very tiny ring. Nd:YAG lasers can produce a multi-mode beam (i.e., TEM02 , TEM11 , TEM01 , TEM22 , etc.). The time constant for a CO2 laser is very high compared to Nd:YAG and HPDL. Therefore, a CO2 laser is not appropriate, when the power needs to be changed in a short period of time. This weakness can be overcome by the integration of a fast shutter system into the CO2 laser. Diode-pumped Nd:YAG lasers have a very impressive e!ciency. For small © 2005 by CRC Press LLC

(less than 10 W) lasers, total e!ciency is usually greater than 50 percent. Diode-pump Nd:YAG lasers with average power above 4 kW are being introduced to the market. High-power diode lasers HPDL are particularly compact and at the same time, highly e!cient. The development of lasers with an output power of over 1 kW opened a gate towards the use of diode lasers in laser cladding processes. HPDL lasers have been used for generating and repairing molds and motor parts. With HPDL, it is necessary to use a standard lens to achieve an appropriate working distance from the focus point. This distance provides enough space for the cladding modules (powder and inert gas nozzles). There is, however, a very high risk that the protective glass and the lens quickly become dirty or even damaged by the powder particles. It is also not possible to process surfaces with complex shapes (e.g., crankshafts). As a result of this shortcoming, researchers have undertaken projects to develop an appropriate lensing system. There are several claims about the higher dilution between the substrate and the clad layer when a HPDL is used [125, 126]. This is mostly due to the higher energy absorbed in the case of HPDL. However, using HPDL for laser cladding provides the user with a unique line source that produces clads with a controllable width without scanning many times over the surface. CO2 and Nd:YAG lasers have a smaller spot size such that the laser must be scanned over the coated area several times. The shorter wavelength of the HPDL allows for higher absorption into the material being coated so that a higher process speed can be achieved. Figure 3.16 shows typical cross sections of clad and the substrate region (Stellite 6 on steel) performed by HPDL and CO2 lasers [125]. The wavelength of HPDL laser beam was 0.94 µm. The experiments for both cases performed at process speed of 900 mm/min, and the maximum clad rate was 0.5 kg/hr. The diameter of CO2 laser beam was 4.7 mm on the substrate, and the diode beam cross section on the substrate was 4.5 × 4 mm×mm. The laser average power in CO2 experiment was 3900 W and in the HPDL was 1400 W. As seen in the figure, the cross section of the clad region for HPDL case is 1.9 mm2 , and for CO2 laser is 2.1 mm2 . The microstructures of both samples show a fine-dendritic structure, which are metallurgically bonded to the substrate. The power of CO2 laser should be set to 3.9 kW to produce the same clad as produced by the 1.4 kW power of HPDL. Table 3.3 lists the types of lasers currently used by researchers/organizations involved in the laser cladding process. 3.1.3.2

Laser Beam Characteristics in Laser Cladding

Since the processing zone in laser cladding is usually positioned below the focal point, a larger distance between the optical system and the workpiece is available, which facilitates the protection of the optical system. In general, in laser cladding, it is preferred to have a larger focal distance due to reduction of the sensitivity of the spot dimensions to the changes in beam characteristics © 2005 by CRC Press LLC

TABLE 3.3

Laser types of organizations/research groups involved in the laser cladding process. Organization Type of Laser Application Material Ref. used Fraunhofer-Institut fur Lasertechnik, Germany GE Aircraft Engines, USA

HPDL, CO2

coating, prototyping

Co and Febased alloy, SS 304

[125, 127]

CO2

Ni alloy

[128]

Laser X. Co., Japan

CO2

repairing of engine turbine blades coating

[110]

University of Waterloo, Canada

Nd:YAG (pulsed) lamp pumped, HPDL Nd:YAG(CW) lamp pumped

prototyping, coating

Cr-Ni based materials, Stellite 6 Fe-Al, H13

coating

SS 304

[130]

Nd:YAG(pulsed and CW) lamp pumped CO2

prototyping

SS 316L, IN625

[19, 21]

prototyping, blade repairing

SS, Stellite 6, superalloy, CMSX-4

[103, 36].

Nd:YAG (CW) lamp pumped CO2

prototyping

[28]

prototyping

Inconel alloy 690 Al, H13

CO2

coating

Co, Al, Ni

CO2

coating

[77]

CO2 Nd:YAG (CW) diode pumped

coating prototyping

CO2

low volume manufacturing prototyping, coating,

Ni-Al-Cr-Hf on Ni Stellite 6 SS 316, SS 304L, H13, IN718,IN 600 Ti, Ti-6Al-4V, Ti-5Al-2.5Sn H13, Cobased material H13, Ti-based alloy, Copper Ti- Ni, Ceramics

[82]

Ishikawajima-Harima heavy Industries Co., Japan National Research Council of Canada Swiss Federal Institute of Technology, Switzerland Los Alamos National laboratory, USA University of Illinois, USA DRL Institute, Germany Illinois University, USA Westinghouse, USA Sandia National Laboratories, USA AeroMet tion, USA

Corpora-

POM Inc., USA

CO2

University of Michigan, USA South Dakota school of Mines & Technology, USA

CO2 , Nd:YAG diode pumped Nd: YAG (CW) lamp pumped

© 2005 by CRC Press LLC

prototyping, coating prototyping, coating

[129, 68]

[64, 14] [131]

[55] [13]

[34]

[14] [132]

HPDL Laser

CO2 Laser

a)

b)

FIGURE 3.16 Cross section of a single track at process speed of 900 mm/min using dierent laser sources, a) HPDL laser at 1400 W, b) CO2 laser at 3900 W (Source: Courtesy of Fraunhofer Institute for Material and Beam Technology, Germany [125]).

and also reduction of the peak power intensity in the spot point, which can cause plasma formation [108]. In pre-placed laser cladding, a circular laser spot with uniform power distribution seems to be more suitable than a Gaussian beam. The main reason for this is the need to transfer a homogeneous energy on the pre-placed powder layer. If a Gaussian beam were used, it would cause non-homogeneous distribution of energy, which may cause plasma formation or even an unexpected clad width. In laser cladding by powder injection, a Gaussian beam may result in better bead quality, dilution, and homogeneity over the clad microstructure. In the case of lateral nozzle, the powder particles usually have a Gaussian distribution, which is compatible with laser power distribution as shown in Figure 3.17. A TEM00 has been used in many reports dealing with laser cladding [133, 36, 134]. However, Schneider [135] claimed that a laser spot perpendicular to the direction of cladding with a homogeneous distribution provides a uniform temperature distribution over the melt pool. Also, Weerasinghe et al. [44] used a TEM01 mode beam in their experiments and they arrived at a uniform heating eect. There may be cases in which the other mode shapes are preferred, especially when producing a thin wall clad. This line shape beam laser can be generated using two cylindrical mirrors or a segmented mirror [107]. Frenk et al. [136] showed that cladding using far-infrared radiation (e.g., CO2 ) should be done with linearly polarized beams at angles of some 70 to 80 degree. In this way, the transmitted energy can be improved by a factor of 3 to 4. © 2005 by CRC Press LLC

Laser Beam

Powder Stream

Substrate

FIGURE 3.17 A Gaussian laser energy distribution versus a Gaussian powder particles distribution.

In laser cladding, it is also possible to use a rectangular spot with a uniform power intensity, as generated by a diode laser [125]. Such a spot can also be generated using a two-dimensional beam integrator. An alternative to the use of integrating optics for achieving a uniform temperature profile over the width of the track is the use of scanning optics. High-power lasers can become instable when run for a long period in the cladding process. The laser’s properties can be influenced by the process itself, causing the process to fail or be unsatisfactory. Therefore, monitoring and control of laser beam parameters is an important task in the laser cladding processes [137, 138].

3.2

Powder Feeders and Powder Delivery Nozzles

Powder feeders are among the most important pieces of equipment in a number of industrial applications involving powder conveyance, such as thermal spraying, laser cladding and advanced materials processing. As dierent powders have very distinct sizes, shapes, and other physical and mechanical properties, it is nearly impossible to convey each type of them with a steady-state flow using a single feeder machine. With decreasing powder grain size (e.g., ultra-fine powder with size of less than 15 µm), the flowability of the powder is decreased, which causes problems in the powder transporting. Flowability also dramatically decreases with sticky and cohesive powders. Void factor (i.e., ratio of the space of air to that of solid) also plays an important role © 2005 by CRC Press LLC

in flowability of powder. For these reasons, dierent powder feeders are required for each type of powder. For example, the required powder feed rate for thermal spraying can be relatively large, whereas the required powder feed rate for prototyping by laser cladding is relatively small. Therefore, a powder feeder machine needs to be carefully controlled in order to ensure that a stable powder stream with a desired feed rate is generated. Selection of a suitable powder feeder is a vital factor for a successful laser cladding process. A powder feeder should provide a continuous and uniform powder stream with high accuracy in terms of flow rate at a desired feed rate. It is crucial to control the feed rate in real-time with minimum time constant. Also, in a laser cladding process, particular attention has been given to minimizing pulsations and agglomerations in the powder stream. Unfortunately, the current powder feeders in the market cannot provide a low time constant (e.g., 0.5 second) and low powder feed rate at high precision (e.g., 0.1 g/min), which are two important parameters in the laser cladding technology. For this reason, special powder feeders with dierent control strategies have been designed and introduced [139, 140, 141, 142]. Also, researchers are developing feeders for ultra-fine powders to arrive at a continuous stream with low feed rate. These powder feeders are vibrationbased or pressure-assisted feeders, which can even be used in direct-write deposition [143].

3.2.1

Powder Feeder Types

There are many types of powder feeders used in industry. In general, powder feeders can be categorized into the following classes based on dierent principles of operation: • Gravity-based • Mechanical wheel • Fluidized-bed • Vibrating In some powder feeders, a combination of the above methods is used to arrive at a better stability in the powder stream. In all types of powder feeders, a carrier gas should be supplied to transport the powder particles from the starting point to the desired location. A brief explanation of the above powder feeders is provided in the following sections. 3.2.1.1

Gravity-Based Powder Feeder

The principle of operation of gravity-based powder feeders is similar to a simple sand clock. The powder feeder machine essentially consists of a load cell based electronic weighing mechanism and an orifice. Due to the weight, © 2005 by CRC Press LLC

the material flows from hopper to the orifice if the powder particles have the required flowability. By reducing or increasing the area of the orifice, the amount of powder delivered to the nozzle decreases or increases. Figure 3.18 shows a schematic of a gravity-based powder feeder.

Powder Container

g

FIGURE 3.18 A schematic of gravity-based powder feeder.

In order to increase the controllability of gravity-based powder feeders, different devices such as a metering wheel can be integrated into the powder feeder. Also, a back pressure can be supplied on the powder funnel to increase the stability of the powder stream, which can be aected by the change in the height of powders in the funnel. Adding the external component for the measurement of powder is an essential device for obtaining a feed rate with high precision. One of these devices can be a rotating disk with holes around it as shown in Figure 3.19. The feeder machine consists of a powder container from which powder flows by gravity into a slot on a rotating disk. The powder is transported to a suction unit by a gas stream. The dimensions of the slot and the speed of the disk control the volumetric powder feed rate [139]. The other idea for integration of a metering wheel into a gravity-based powder feeder is shown in Figure 3.20. The size of holes around the rotating shaft and the angular velocity of the shaft determine the powder feed rate. The other design can be an integration of a lobe gear with the gravitybased powder feeder as shown in Figure 3.21. This design is not suitable for an application requiring the low powder feed rate.

© 2005 by CRC Press LLC

Powder Container

Gas

Rotating Powder Slot To Powder Inlet

To Powder Nozzle

Powder Pick-up

FIGURE 3.19 A typical gravity-based powder feeder with a rotating wheel for metering.

Back Pressure

Gas

Metering Wheel

FIGURE 3.20 A typical gravity-based powder feeder with a metering wheel. © 2005 by CRC Press LLC

Powder Container

Gas

Metering Wheel

FIGURE 3.21 A typical gravity-based powder feeder with a lobe gear.

3.2.1.2

Mechanical Wheel Powder Feeder

Mechanical wheel powder feeders are also known as screw powder feeders. Mechanical wheel feeders handle a wide range of powders with dierent mesh sizes. They do not seal against an uncontrolled flow of fine powders and normally operate with zero or low-pressure dierential between outlet and inlet. A typical mechanical wheel feeder has a pitch with dierent diameter ratio or a rotor which can grab powder particles from the storage area. There are many screw configurations that can be used to promote uniform flow with dierent feed rates. Figure 3.22 depicts two types of the configuration of mechanical wheel powder feeders. One disadvantage of this type of powder feeder is the interaction of moving parts and abrasive powder particles, which cause rapid wear in the wheel. This can result in variations in coating quality and also increase maintenance costs. 3.2.1.3

Fluidized-Bed Powder Feeder

A fluidized powder feeder operates based on fluidics principle, which eliminates the need for mechanically moving parts to deliver powder. The fluidics © 2005 by CRC Press LLC

Powder Container

Wheel

FIGURE 3.22 A schematic of mechanical wheel powder feeder.

powder feed delivery principle provides a continuous, non-pulsating feed of powder, thereby insuring the user optimum process control and improved coating quality. Another benefit is reduced maintenance and replacement part cost. The system is designed so that a predetermined quantity of gas is delivered to a closed hopper containing powder. The hopper is constructed so that the gas is passed through a filter located at the bottom of the unit, where it is diused through the powder, causing the powder to enter into the gas and therefore become fluidized. A powder pickup tube is positioned above the fluidizing gas inlet allowing the fluidized media to be delivered under a shed on the pickup tube through a number of controlled apertures and then to a carrier area where it is propelled by the carrier gas to the feed hose. Figure 3.23 shows the construction of a fluidized-bed powder feeder. 3.2.1.4

Vibratory-Based Powder Feeder

A vibratory feeder, which is also called a vibratory tray feeder or oscillating feeder, consists of a shallow flat-bottomed tray. As powder flows from the hopper outlet onto the tray, an external drive vibrates the tray, throwing the powder down to control the powder feed rate into the process. A vibratorybased powder feeder can feed most powders from at least 8 g/min to 2000 g/min with ±1% precision. In order to increase the precision, the vibratory powder feeder can consist of a vibrating tray with a number of plates set on a specified angle. Having these plates, the flowing of powder bulk can be controlled. Figure 3.24 shows the construction of a vibratory powder feeder. © 2005 by CRC Press LLC

Back Pressure

Gas In Carrier Gas

FIGURE 3.23 A typical fluidized-bed powder feeder.

Feeder Vibrating

FIGURE 3.24 A schematic of a vibratory-based powder feeder.

© 2005 by CRC Press LLC

TABLE 3.4

Powder feeder types of organizations/research groups involved in the laser cladding process. Organization Type of Type of Used Ref. applicapowder material tions feeder University of Waterloo, Canada University of Waterloo, Canada

Coating

Fluidized bed

Prototyping

NRC, Canada

Prototyping

Gravity-based powder feeder along with metering wheel Fluidized bed

University of Michigan, USA

Prototyping

Sandia, USA

Prototyping

University of Missouri at Rolla, USA

Prototyping

University of Liverpool, UK

Coating

3.2.2

Gravity-based powder feeder along with metering wheel Fluidized bed Mechanical wheel powder feeder Mechanical wheel powder feeder

H13, IronAluminide H13, Ni-based alloys

[3]

316 SS, IN625 H13, Ti-based alloys

[19]

Ti-based alloys, SS H13

—

Al, 316 SS

[144]

—

[14]

[141]

Applications of Powder Feeders to Laser Cladding

So far, dierent types of powder feeders have been used in the laser cladding process. However, it is hard to say which type of powder feeder is more suitable for this process. Due to the wide range of applications of laser cladding, dierent powders, with dierent mesh sizes at various powder feed rates are required for the process. Many research groups, which are developing the laser cladding apparatus, have designed and manufactured their own powder feeder which suits their applications. As it was mentioned, it is impossible to convey every powder with a steadystate flow using a single feeder machine. As a result, various types of powder feeders have been developed for laser cladding to provide the smooth and steady flow in the required flow rates. Table 3.4 lists several types of powder feeders, which are being used in laser cladding by dierent research groups and organizations. © 2005 by CRC Press LLC

3.2.3

Nozzles

In laser cladding by powder injection, the powder delivery nozzle can have dierent configurations as • Coaxial • Lateral Basic layouts of these two nozzles are shown in Figure 3.25. The coaxial supply of powder can be integrated with the optical system [145, 146]. One of the advantages of a coaxial nozzle is its independence from the direction of motion; however, experimental work has shown that its powder e!ciency, which is the ratio between the deposited powder on the substrate and the delivered powder by the powder feeder in a specified period, is significantly less than that of the lateral nozzle [145]. In both types, the powder can be preheated when it passes through the nozzle to increase e!ciency. Several forms of nozzles have been invented based on the above two mentioned nozzles. Islam et al. [20] invented a multiple nozzle processing head for manufacturing and repairing of turbine blades or compressor components. Jeantette et al. [139] invented a coaxial nozzle which is used for producing complex shapes. Their developed nozzle has been licensed to Sandia Corporation. Keicher et al. [147] invented a multiple beam and nozzle system to increase the deposition rate. Their developed nozzle and laser processing head has been currently licensed to Optomec Design Company. The interactions of powder particles, the laser beam and the inert gas with the melt pool are important parameters for arriving at a good quality clad. The interactions of powder particles with dierent surfaces in the process zone may result in dierent impact phenomena as [148] • Solid particles to solid surface impact causing a ricochet • Solid particles or liquid particles to liquid surface of melt pool causing catchment • Liquid particles to solid surfaces causing catchment The adhesion behavior of powder particles on solid or liquid surfaces surrounded by turbulent streams have been carried out by Zimon [149]. The type of nozzle, the angle of powder stream with respect to a reference line, the powder profile in the process zone, and powder stream diameter in the melt pool area will influence the interaction of powder particles with surfaces. An appropriate nozzle is the one that provides the minimum solid particles with solid surfaces. Minimizing impact between the solid particles and solid surfaces increases the powder catchment e!ciency. © 2005 by CRC Press LLC

Laser Beam

Shield Gas

Laser Beam

Shield Gas

Shield Gas

Powder Flow

Shield Gas

Lateral Nozzle

Shaping Gas

Clad Bead

Substrate

Clad Bead

a)

Substrate b)

FIGURE 3.25 a) Coaxial nozzle, b) lateral nozzle.

3.2.3.1

Lateral Nozzle and Powder Profile Quality

The powder delivery system plays an important role in the clad quality. Regardless of the type of the nozzle, knowledge of the intersection of powder stream and laser beam, diameter of powder stream on the workpiece, stability of powder feed rate, homogeneous shape of powder profile and velocity of powder particles are crucial to a successful process. In order to address the eect of nozzle diameters on the above-mentioned parameters, a simple measurement test rig was developed to take pictures of the powder stream. The images were then processed to find the profile characteristics of the powder stream in terms of the distance from the tip of the nozzle. It was found that the profile of the powder stream can be approximated by a parabolic equation as d = z 2 + d0

(3.14)

where d is the profile diameter at any z [mm],  is the powder profile quality coe!cient [1/mm], z is the distance of desired point from the nozzle tip [mm], and d0 is the nozzle diameter [mm]. In the study, several nozzles of PRAXAIR with serial number of TWEP2250 with diameters of 0.8, 1, 1.2, 1.4 and 1.8 mm were used. Figure 3.26 shows the identified parameter corresponding to each nozzle. As it is seen in the table,  is valid for a specific range, which represents the range of stable powder stream. © 2005 by CRC Press LLC

d0 d = profile diameter at any z (mm) d0 = nozzle diameter (mm) z = distance from the nozzle tip (mm)

z d

Powder Shield gas d 0 = 0.8 mm Feed rate Feed rate Valid λ 3 (1/mm) for (g/min) (m /s)

d 0 = 1 mm

λ

d 0 = 1.2 mm

λ

d 0 = 1.4 mm d 0 = 1.8 mm

λ

λ

(1/mm)

Valid for

(1/mm)

Valid for

(1/mm)

Valid for

(1/mm)

Valid for

1

1.56e-5

9.03e-3

z D/Us where T2 T1 is known as the equilibrium freezing range for the alloy. Another form of Equation (6.3) is (Gc /Us ) >

T2  T1 D

(6.4)

The left side of Equation (6.4) describes the important processing conditions present during solidification while the right side describes the material parameters of importance. It should be further noted that the magnitude of the equilibrium freezing range is determined by the equilibrium partition coe!cient defined as Cs (6.5) ke = Cl where Cs and Cl are the equilibrium solidus and liquidus compositions at T1 (see Figure 6.31). When the value of ke approaches unity this corresponds to solidus and liquidus lines that are close together, resulting in small freezing ranges. From Equation (6.4), it can be stated that high temperature gradients and slow solidification rates will be more likely to avoid constitutional supercooling © 2005 by CRC Press LLC

Interface

Co / ke Solid

Wt% A

Liquid

Co D /Us

Position, x a)

Interface

D / Us

Gr

Gc

Temperature

T2 TL Solid

Liquid

T1

Position, x b)

FIGURE 6.32 a) Compositional gradient, b) temperature gradient at the solid/liquid interface during steady-state solidification.

and favor a planar growth condition for a given alloy system. Alternatively, alloys with small freezing ranges will be more likely to solidify with a planar front under moderate values of Gr and Us . Applying these concepts to laser cladding, it can be stated by inspection of Figures 6.29 and 6.30 that at the base of the clad layer, region B, (Gr /Us ) is at a maximum and therefore solidification by a planar front would be favored in this location. On the other hand, (Gr /Us ) are at their lowest at the surface, region C. Therefore, a transition from planar solidification to dendritic growth from the clad deposit near the substrate interface to the clad surface would be expected. Figure 6.30 also illustrates that the change in the ratio of (Gr /Us ) per unit length of the melt pool depth is more significant at higher laser processing speeds. © 2005 by CRC Press LLC

An excellent example of how these changing (Gr /Us ) values alter the microstructure in a clad deposit is given by Vilar [249] for a Stellite 6 clad deposit. At the base of the clad near the substrate surface, a microstructurally featureless zone indicates a region of plane front solidification. This type of zone has been observed by many researchers [247]. The microstructure then transforms to a cellular structure and then finally a fully developed dendritic structure with secondary dendrite arms.

6.2.3

Rapid Solidification

Many complex engineering alloys have a significant equilibrium freezing range and therefore dendritic growth is often the dominant solidification process and therefore most common microstructural feature observed in the clad deposit. This is due to the di!culty in meeting the criterion of Equation (6.4) for a planar form when [T2 T1 ] is relatively large. However, with increasing solidification rates the compositions in the solid and liquid at the tip of the dendrites no longer follow their equilibrium values. This is because the atoms do not have the time to diuse and maintain local equilibrium at the solid/liquid interface. One primary way in which to describe this eect is through the use of the solidification rate dependent partition coe!cient kv [250] (6.6) kv = [ke + ao Us /D]/[1 + ao Us /D] where ao is a characteristic value of the thickness of the interface and the other parameters are as defined above. As the solidification rate Us increases, kv approaches unity. Kurz et al. [247] have applied non-equilibrium solidification modelling to an Al-4%Cu alloy to determine the concentrations in the solid and liquid (i.e., CS and CL , respectively) at the dendrite tip as a function of temperature for high solidification rates. Figure 6.33 plots these results. Also included are the equilibrium solidus and liquidus lines. The fine dashed line shows the values of CS and CL as a function of temperature. At low undercoolings (and therefore low solidification rates), the non-equilibrium values follow closely the equilibrium lines. However, with increasing undercoolings, they deviate significantly. Also plotted in the figure are the eective solidus and liquidus lines at a solidification rate of Va , which Kurz and Trivadi [247] define as the absolute stability point. Clearly, the value of kv is significantly closer to unity than the equilibrium partition coe!cient and therefore the effective freezing range of the alloy is considerably reduced. According to the criterion of Equation (6.4), this could lead to the re-establishment of a stable, planar interface at high solidification rates. This phenomenon has been modelled by Kurz and Trivadi [247], the results of which, for Al-4%Cu, are presented in Figure 6.34. Clearly the type of microstructure that develops depends on both the solidification rate and temperature gradient. A region of planar growth (P ) exists at both low and high growth rates but does depend on the temperature gradient at low values © 2005 by CRC Press LLC

FIGURE 6.33  Calculated compositions in the solid and liquid (CS and CL , respectively) and eective solidus and liquidus lines for an Al-Cu alloy as a result of rapid solidification. Va is the solidification rate corresponding to the absolute stability point. Solid lines indicate the equilibrium solidus and liquidus lines (Source: Reprinted from [247], courtesy of ASME).

of Us . At intermediate growth rates, non-planar growth, in the form of cells or dendrites, exists (see Figure 6.34b). Included in Figure 6.34a are the laser surface remelting traces of Figure 6.30. Based on these traces, one would predict a planar structure near the substrate interface followed by a cellular region and then dendritic structure. At very high rates the planar solidification front can also be replaced by a banded structure. This type of transition has been observed in Al-Cu [251] and Al-Fe [246] binary systems.

6.2.4

Microstructure Maps

From the above discussion, it is clear that both the laser cladding processing conditions, which determine the values of Gr and Us with the melt pool, and the alloy composition, which determines the magnitude of the freezing range (or the values of ke and/or kv ), will determine the type of microstructure formed in the clad layer during solidification. The case studies of Al-Nb, © 2005 by CRC Press LLC

a)

b)

FIGURE 6.34 Mode of solidification and the resultant microstructure as a function of the temperature gradient Gr and solidification rate Us , a) including laser conditions, b) including type of non-planar growth. P-planar, C(F) and C-cellular, D-dendrites and B-banding (Source: Reprinted from [247], courtesy of ASME).

© 2005 by CRC Press LLC

Us

Al-Ni and Al-Fe given in Sections 6.2.2.1 and 6.2.2.2 illustrate that this microstructure can have a large impact on the substrate/clad bond strength, and thus the success of the cladding process. It would, therefore, be extremely useful to be able to predict what microstructures would develop under certain processing conditions so that the optimum microstructure could be selected by the appropriate choice of laser processing parameters. Several researchers [204, 246, 251, 252] have developed microstructural maps with the aim of providing this type of information. An example of such a map is given in Figure 6.35 for an Al rich binary Al-Fe alloy. Similar maps have also been developed for Al-Cu alloys [251] and Ag-Cu [252]. The solid lines in the figure indicate results of experimental observations, whereas the dashed lines indicate calculated values from solidification models.

FIGURE 6.35 The type of microstructure observed (solid lines) and predicted (dashed lines) in an Al-Fe binary alloy as a function of solidification rate and Fe content (Source: Reprinted from [246], courtesy of Elsevier).

At low Fe contents and slow solidification rates, primary dendrites of Al with interdendritic regions of equilibrium eutectic (Al-Al3 Fe) are the preferred microstructure. As the solidification rate increases, a similar microstructure develops except that the equilibrium (or stable) eutectic is replaced by a metastable Al-Al6 Fe eutectic. The formation of metastable phases during rapid solidification has been observed in other alloy systems [240, 253, 254, © 2005 by CRC Press LLC

255]. With a further increase in the solidification rate, the condition for absolute stability of the solid/liquid interface is approached with the formation of a banded and then plane front structure. With an increase in the Fe content, the sequence of microstructural development as a function of solidification rate changes. In the slow to moderate range, the preferred microstructure changes from primary dendrites of Al with interdendritic Al-Al3 Fe (or Al-Al6 Fe), to an all eutectic Al-Al3 Fe (or Al-Al6 Fe) structure and then finally to primary dendrites of Al3 Fe with interdendritic Al-Al3 Fe (or Al-Al6 Fe). At very high solidification rates, the microstructure that forms becomes insensitive to the Fe content where banding or a plane front becomes the preferred structure. What this diagram points out is that with a careful control over the process parameters, including dilution by the substrate (or clad composition), a range of microstructures for the clad can be selected. In the particular case of the problems encountered by Ellis et al. [244] for an Al cladding on Fe, the problems encountered with the formation of brittle Al3 Fe dendrites at the surface may be avoidable if the level of Fe in the cladding (and thus dilution) could be kept to a minimum. Microstructure maps similar to Figure 6.35 are not yet available for more complex ternary and higher order alloy systems. However, the work of Kurz and others points out the tremendous value of these maps and the importance of developing them for other systems.

6.2.5

Microstructural Scale

Another consequence of rapid solidification is that it causes a refinement in the microstructure of the as-cast material. In the case of cellular or dendritic growth, this can be understood by referring to Figure 6.32. As the solid grows it must reject solute into the liquid ahead of the solid/liquid interface. With the formation of a cellular or dendritic structure, solute can be more e!ciently rejected in lateral directions as well as in front of the tip. As the solidification rate increases, the need to reject this solute more eectively increases and the dendrite tip radius becomes smaller. This results in a refinement in the dendrite spacing and overall microstructure. An example of how the laser processing speed can be used to refine the clad microstructure (through an increase in solidification rate) is given in Figure 6.36 for the cobalt-based Stellite 6 alloy [80]. At a slow scanning speed of 1.67 mm/s, the secondary dendrite arm spacing s is in the range of 2.5 to 6 µm, whereas at a scanning speed of 167 mm/s, s is reduced to 0.5 to 0.8 µm. Because the solidification rate varies from near zero to a maximum at the clad surface, it is expected that the microstructure of the clad should also be refined across its height. An example of this refinement is given in Figure 6.37 for an Al-Si-Ni clad material placed onto an Al-Si substrate [256]. In this case, solidification of primary dendrites of Al3 Ni2 and/or Al3 Ni (the white phase in the figure) with interdendritic Al-Al3 Ni are the dominant microstructural features. A clear refinement in the scale of the dendrites is visible in going © 2005 by CRC Press LLC

FIGURE 6.36 Microstructure of a Co-based Stellite 6 clad deposit at a) a laser scanning speed of 1.67 mm/s, b) a laser scanning speed of 167 mm/s (Source: Reprinted from [80], courtesy of Elsevier).

from the bottom to the top of the clad. Gilgien et al. [246] have constructed a microstructure map for Al-Fe which includes the dendrite tip radius (or in the regions where eutectic forms, the interphase spacing). A clear correlation exists between solidification rate and microstructural scale (see Figure 6.38). This diagram also illustrates the great potential that laser cladding has in producing nanoscaled microstructures. These ultra-fine microstructures, the finest of which will be located at the clad surface, are in large part the reasons for the high hardness values or increased wear resistance so often reported in laser cladding research [35, 239, 247, 249, 253, 254, 256, 257]. An additional consequence of rapid solidification is the development of extended solid solubility. This can be understood with reference to Figure 6.33. © 2005 by CRC Press LLC

FIGURE 6.37 Microstructure of an Al-Si-Ni clad on an Al-Si substrate showing the scale of the primary Ni aluminide dendrites (white phase) as a function of the clad depth (Source: Reprinted from [256], courtesy of Elsevier).

© 2005 by CRC Press LLC

Us FIGURE 6.38 Scale of the solidified microstructure of laser remelted Al-Fe as a function of solidification rate and Fe content (Source: Reprinted from [246], courtesy of Elsevier).

As the solidus line shifts to the right of the equilibrium case, more solute remains in solid solution within the primary phase. This phenomenon has been investigated by a number of researchers in systems including Ag-Cu [252], NbAl [241], and Ni-based alloys [51, 49, 160]. Solubility increases can be as high as several percent. This increased solubility will lead to higher levels of solid solution strengthening and contributed to increase hardness and strength in the clad material. Accompanying the non-equilibrium solute content is the formation of metastable phases such as the Al6 Fe based eutectic illustrated in Figure 6.35. The formation of metastable phases have been observed in Ni-based, Cobased, and Fe-based alloys mostly in the form of metal carbides and/or borides [253, 254, 255]. Again, these metastable phases will have an impact of the mechanical properties of the clad deposit.

6.3

Material Systems Used in Laser Cladding

There has been a rapid increase in research activities related to laser cladding, growing from a handful of published journal papers in the 1980’s to over 60 in the year 2003. A wide range of materials have been studied, and it © 2005 by CRC Press LLC

can be safely said that the feasibility of laser cladding using every major class of metallic alloy as well as ceramics, glasses and intermetallics has been investigated. Alloy systems that have been laser clad include Ni, Co, Nb, Ti, Al, and Cu-based alloys as well a full range of steels including tool and stainless steels. However, over the past several years, two general material systems have enjoyed particular attention and will be the focus of this section. By its nature, the major objective of laser cladding is to create a high performance surface with enhanced properties including wear resistance, corrosion resistance, and/or oxidation resistance. Traditionally, materials with these high performance attributes are high temperature materials, which are expensive and di!cult to process in bulk form. Therefore, creating a coating of these materials on less expensive, easily processed base materials is where laser cladding oers the best advantages.

6.3.1

High Temperature Alloys

Over the span of the last 20 years there has been a major emphasis on the laser cladding of high temperature Ni, Co and/or Fe-based alloys. These alloys are generally expensive and di!cult to process by conventional means but oer superior properties. In the majority of cases, these alloys are clad onto relatively less expensive low carbon or low alloy steels [75, 112, 77, 81, 253, 254, 255, 109, 112, 79]. The main alloying elements in most of the clad alloys include Ni, Co, Cr and Fe with smaller amounts of W and Mo as well as C, which forms metal carbides with the alloy ingredient. Generally, these alloys exhibit good cladability with low carbon steel substrates. This can be qualitatively understood in light of the discussion of Section 6.2.2 by realizing that Fe is very soluble in Ni, Co, and Cr in the solid-state near the melt pool temperatures with no intermetallic phases forming between the constituents during solidification. However, this general statement can be complicated by complex alloy compositions and with the presence of C, B and W . Metal carbides can form during solidification and brittle, intermetallic compounds can precipitate in the solid-state during cooling. Therefore, hot and cold cracking can take place in the deposit/substrate during laser cladding [258]. Often pre- and/or post-heat treatments are required to avoid these problems. However, a detailed discussion of these problems is beyond the scope of this text. Another common application of laser processing using high temperature alloys is the cladding of Ni-based substrates with Ni-based cladding containing rare earth elements such as Hf [77, 259, 260, 261]. In these cases the high temperature oxidation resistance of Ni-based superalloys can be improved by the rare earth additions. In these cases, since the clad composition is only slightly dierent than the substrate, cladability is generally very good.

© 2005 by CRC Press LLC

6.3.2

Composites

Since a primary application of laser cladding is to create a hard wear-resistant surface, it is not surprising that a significant research eort is being made to laser clad metal/ceramic composite coatings onto metal substrates [262, 263, 264, 265, 266, 267, 268, 269, 270]. Metal/ceramic composite (MCC) materials are relatively di!cult to process by conventional means such as powder metallurgy or ingot metallurgy, particularly when high ceramic volume fractions are required [271]. These composites also tend to have low ductility and fracture toughness, making them unsuitable in bulk form for many applications [272]. However, by virtue of their ceramic content they exhibit high hardness and excellent wear resistance. Therefore, their use in laser cladding is an ideal application [262, 263, 264, 265, 266, 267, 268, 269, 270]. Generally, there have been two approaches to the cladding of MCCs. In the first, ceramic particles, (sometime with metallic binders) are mixed with metal particles and added to the melt pool either by pre-placement on the substrate or by powder injection [262, 263, 264]. In these investigations, the goal is to limit the reaction between the ceramic particles and the metallic melt pool. WC particles are the ceramic particles of choice in this approach because they seem to be resistant to dissolution into the melt pool. The problems encountered during this process are similar to those encountered in more conventional ingot metallurgy techniques for producing MCCs [272, 273]. The ceramic particles must wet the liquid metal in order to be incorporated into the melt pool. However, there must not be an extensive deleterious reaction taking place between the particle and alloying elements with the metallic liquid. In addition, during solidification particle pushing can lead to inhomogeneous distributions of the ceramic particles within the metal matrix. These problems are only just beginning to be evaluated in laser cladding [263]. In cases where TiC and SiC are used as the ceramic particle, reaction does take place with the metal powder [71, 265, 266]. For example, when SiC and Ni are combined, nickel-silicide compounds form [265] and for TiC and Al, titanium-aluminide compounds form [71]. The reactions that take place between the ceramic particles and melt pool are not necessarily negative for either cladability or wear resistance. In fact, in a second approach, ceramic reinforcement is developed by in-situ reactions between the powder ingredients within the molten zone. [265, 266, 267, 268, 269, 270]. In some cases, ceramic particles are added to the clad deposit and undergo partial or complete dissolution into the melt pool and re-precipitate during solidification [265, 274]. In other cases elemental powders are added to the melt pool and then react to form the reinforcing phase. Examples of this include the addition of Mo, Si and C powders which form MoSi2 and SiC particles [267], the addition of Ni-based alloy and Zr powder to form ZrC particles [269], and Ni-based alloy, graphite and Ti powder to form TiC particles [270]. The advantages of this approach include the formation of fine ceramic particles that are well bonded to the matrix. As Figure 6.39 indicates, © 2005 by CRC Press LLC

a large increase in hardness can result from the in-situ formation of the laser clad MCCs. This fairly new approach to laser cladding shows great potential for the creation of novel materials with unique properties.

a)

b)

FIGURE 6.39 Vickers microhardness readings across clad/substrate couples: a)in-situ formation of ZrC MCC clad on a mild steel substrate, b) in-situ formation of MoSi2 and SiC MCC clad on a commercially pure Al substrate (Source: preprinted from [269] and [267], respectively, courtesy of Elsevier).

© 2005 by CRC Press LLC

7 Safety

Laser material processing is hazardous if all the safety guidelines are not followed. Safety in working with high-power lasers should be paramount because of the serious damage they could impose. In the following, the safety issues related to the laser material processing are discussed .

7.1

Laser Classification

All lasers are classified by the manufacturer and labelled with the appropriate warning labels. Any modification of an existing laser or an unclassified laser must be classified by a laser safety o!cer prior to use. The following criteria are used to classify lasers: 1. Wavelength. If the laser is designed to emit multiple wavelengths the classification is based on the most hazardous wavelength. 2. For continuous wave (CW) or repetitively pulsed lasers, the average power output [W] and limiting exposure time inherent in the design are considered. 3. For pulsed lasers, the total energy per pulse [J], pulse duration, pulse repetition frequency and emergent beam radiant exposure are considered. Class 1 Lasers These are lasers that are not hazardous for continuous viewing or are designed in such a way that prevent human access to laser radiation. These consist of low power lasers or higher power embedded lasers (i.e., laser printers). Class 2 Visible Lasers (400 to 700 nm) W This

chapter is reprinted from the University of Waterloo safety guidelines with permission.

© 2005 by CRC Press LLC

Lasers emitting visible light that because of normal human aversion responses, do not normally present a hazard but would if viewed directly for extended periods of time (i.e., many conventional light sources). Class 3A Lasers that normally would not cause injury to the eye if viewed momentarily but would present a hazard if viewed using collecting optics (fiber optics loupe or telescope). Class 3B Lasers that present an eye and skin hazard if viewed directly. This includes both intrabeam viewing and specular reflections. Class 3B lasers do not produce a hazardous diuse reflection except when viewed at close proximity. Class 4 Lasers Lasers that present an eye hazard from direct and specular reflections. In addition, such lasers may be fire hazards and produce skin burns. In this class, CO2 beams are much safer. The wavelength of a CO2 beam is 10.6 µm, a wavelength that is strongly absorbed by water. Since more than 70% of the human body consists of water, an unfocused or reflected CO2 beam does not penetrate beyond the first few layers of skin. More importantly, this wavelength does not penetrate the eye if a scattered beam is observed. Processing can usually be directly observed with minimal precautions. Lasersafety eye wear is normally su!cient to meet the guidelines. YAG beams have a 1.06 µm wavelength — just below the visible deep-red. This wavelength deeply penetrates the body. If you get hit by a YAG beam, there is more damage than just a surface burn. Worse, the 1.06 µm wavelength is focused by the eye just like “normal” light. A beam scattered from a process will be focused to a point in the eye, probably destroying the spot where it was focused. As such, all YAG workstations must be sealed o and light-tight during processing.

7.2 7.2.1

Laser Hazards Eye Hazards

The potential for injury to the dierent structures of the eye (Figure 7.1) depends upon which structure absorbs the energy. Laser radiation may damage the cornea, lens or retina depending on the wavelength, intensity of the radiation and the absorption characteristics of dierent eye tissues. Ocular Image Wavelengths between 400 nm and 1400 nm are transmitted through the curved cornea and lens and focused on the retina. Intrabeam viewing of a © 2005 by CRC Press LLC

Retina Iris Cornea

V itreous Lens

Fovea

Macula

Optic Disc A queous

Optic Nerve

FIGURE 7.1 Structures of the eye.

point source of light (see Figure 7.2a) produces a very small spot on the retina resulting in a greatly increased power density and an increased chance of damage. A large source of light such as a diuse reflection of a laser beam produces light called extended source which enters the eye at a large angle. An extended source produces a relatively large image on the retina (see Figure 7.2b); energy is not concentrated on a small area of the retina as in a point source. Details of Irradiation Eects on Eyes Cornea absorbs all UV light which produces a photokeratitis (weld). Ultraviolet -B+C (100 — 315 nm) The surface of the flash by a photochemical process causes a denaturation of proteins in the cornea. This is a temporary condition because the corneal tissues regenerate very quickly. Ultraviolet -A ( 315 — 400 nm) The cornea, lens, and aqueous humour allow ultraviolet radiation of 315 — 400 nm wavelengths of which the principal absorber is the lens. Photochemical processes denature proteins in the lens resulting in the formation of cataracts. Visible light and Infrared-A (400 — 1400 nm) The cornea, lens, and vitreous fluid are transparent to electromagnetic radiation of these wavelengths. Damage to the retinal tissue occurs by absorption © 2005 by CRC Press LLC

Extended Source

Point Source a) (a)

b) (b)

FIGURE 7.2 a) Point source viewing, b) Extended source viewing.

of light and its conversion to heat by the melanin granules in the pigmented epithelium or by photochemical action to the photoreceptor. The focusing eects of the cornea and lens will increase the irradiance on the retina by up to 100,000 times. For visible light (400 to 700 nm), the aversion reflex which takes 0.25 s may reduce exposure causing the subject to turn away from a bright light source. However, this will not occur if the intensity of the laser is great enough to produce damage in less than 0.25 s or when light of 700 — 1400 nm (near infrared) is used since the human eye is insensitive to these wavelengths. Infrared-B and Infrared-C (1400 to 106 nm) Corneal tissue will absorb light with a wavelength longer than 1400 nm. Damage to the cornea results from the absorption of energy by tears and tissue water causing a temperature rise and subsequent denaturation of protein in the corneal surface. Wavelengths from 1400 to 3000 nm penetrate deeper and may lead to the development of cataracts resulting from the heating of proteins in the lens. The critical temperature for damage is not much above normal body temperature (w 370 ). Laser Radiation Eects on Skin Skin eects are generally considered of secondary importance except for highpower infrared lasers. However, with the increased use of lasers emitting in the ultraviolet spectral region, skin eects have assumed greater importance. Erythema (sunburn), skin cancer and accelerated skin aging are produced by emissions in the 200 to 280 nm range. Increased pigmentation results from exposure to light with wavelengths of 280 to 400 nm. Photosensitization has resulted from the skin being exposed to light from 310 to 700 nm. Lasers emitting radiation in the visible and infrared regions produce eects that vary from a mild reddening to blisters and charring. These conditions are usually repairable or reversible; however, depigmentation, ulceration, and scarring of © 2005 by CRC Press LLC

CIE Band

U V-C

100

UV-B

280

UV-A

315

400

Photokeratitis

Visible

700

Retinal Burns

Erythema

IR-C

3000 (nm)

1400

C orneal Burns

Cataracts

Cataracts Adverse Effects

IR-B

IR-A

Colour Vision Night Vision Degradation Thermal Skin Burns

FIGURE 7.3 Interaction of optical radiation and various tissues.

the skin, and damage to underlying organs may occur from extremely highpower lasers. Summary of Wavelengths of Light and their Eects on Tissues A summary of interaction of optical radiation and various tissues is shown in Figure 7.3. The wavelengths are divided into bands as defined by the International Commission on Illumination (CIE).

7.2.2

Collateral Radiation

Radiation other than that associated with the primary laser beam is called collateral radiation. For example, x-rays, UV, plasma, and radio frequency emissions are collateral radiation. Ionizing Radiation X-rays could be produced from two main sources in the laser laboratories. One is high-voltage vacuum tubes of laser power supplies, such as rectifiers, thyratrons and crowbars, and the other is electric-discharge lasers. Any power supplies that require more than 15 kilovolts (keV) may produce enough x-rays to cause a health hazard. Interaction between x-rays and human tissue may cause a serious disease such as leukemia or other cancers, or permanent genetic eects which may show up in future generations. UV and Visible UV and visible radiation may be generated by laser discharge tubes and pump lamps. The levels produced may exceed the maximum permissible exposure (MPE) and thus cause skin and eye damage. © 2005 by CRC Press LLC

Plasma Emissions Interactions between very high-power laser beams and target materials may in some instances produce plasmas. The plasma generated may contain hazardous UV emissions.

7.2.3

Electrical Hazards

The most lethal hazard associated with lasers is the high voltage electrical systems required to power lasers. Several deaths have occurred when commonly accepted safety practices were not followed by operators working with high voltage sections of laser systems. Safety Guidelines 1. Do not wear rings, watches or other metallic apparel when working with electrical equipment. 2. Do not handle electrical equipment when hands or feet are wet or when standing on a wet floor. 3. When working with high voltages, regard all floors as conductive and grounded. 4. Be familiar with electrocution rescue procedures and emergency first aid. 5. Prior to working on electrical equipment, de-energize the power source. Lock and tag the disconnect switch. 6. Check that each capacitor is discharged, shorted and grounded prior to working in the area of the capacitors. 7. Use shock preventing shields, power supply enclosures, and shielded leads in all experimental or temporary high-voltage circuits.

7.2.4

Chemical Hazards

Many dyes used as lasing media are toxic, carcinogenic, corrosive or cause of a fire hazard. All chemicals must be accompanied by a material safety data sheet (MSDS). The MSDS will supply appropriate information pertaining to the toxicity, personal protective equipment and storage of chemicals. Various gases are exhausted by lasers and produced by targets. Proper ventilation is required to reduce the exposure levels of the products or exhausts below standard exposure limits. Cryogenic fluids are used in cooling systems of certain lasers. As these materials evaporate, they replace the oxygen in the air. Adequate ventilation must be ensured. Cryogenic fluids are potentially explosive when ice collects in valves or connectors that are not specifically designed for use with cryogenic © 2005 by CRC Press LLC

fluids. Condensation of oxygen in liquid nitrogen presents a serious explosion hazard if the liquid oxygen comes in contact with any organic material. Although the quantities of liquid nitrogen that are used are small, protective clothing and face shields must be used to prevent freeze burns to the skin and eyes. Compressed gases used in lasers present serious health and safety hazards. Problems may arise when working with unsecured cylinders, cylinders of hazardous materials not maintained in ventilated enclosures, and gases of dierent categories (toxins, corrosives, flammable, oxidizers) stored together.

7.2.5

Fire Hazards

Class 4 lasers represent a fire hazard. Depending on construction material beam enclosures, barriers, stops and wiring are all potentially flammable if exposed to high-beam irradiance for more than a few seconds.

7.2.6

Explosion Hazards

High-pressure arc lamps, filament lamps, and capacitors may explode violently if they fail during operation. These components are to be enclosed in a housing that will withstand the maximum explosive force that may be produced. Laser targets and some optical components also may shatter if heat cannot be dissipated quickly enough. Consequently, care must be used to provide adequate mechanical shielding when exposing brittle materials to high intensity lasers.

7.2.7

Eye Protection

The following is an account written by a researcher who sustained permanent eye damage viewing the reflected light of a Class 4 neodymium YAG laser emitting a 10-ns pulse of 6 mJ radiation at 1064 nm. “When the beam struck my eye, I heard a distinct popping sound caused by a laser-induced explosion at the back of my eyeball. My vision was obscured almost immediately by streams of blood floating in the vitreous humour. It was like viewing the world through a round fish bowl full of glycerol into which a quart of blood and a handful of black pepper have been partially mixed.” Dr. C.D. Decker. The researcher had eye protection available but failed to wear it. Eye protection is required, and its use is enforced by the supervisor when engineering controls may fail to eliminate potential exposure in excess of the applicable MPE. Laser radiation is generated both by systems producing discrete wavelengths and by tunable laser systems producing a variety of wavelengths. For © 2005 by CRC Press LLC

this reason it is impractical to select a single eye protection filter that will provide su!cient protection from all hazardous laser radiation. Therefore, it is important to choose an eye protection specific for the wavelength and power of a particular laser. Laser Protective Eyewear Requirements 1. Laser protective eyewear is to be available and worn by all personnel within the nominal hazard zone (NHZ) of Class 3B and Class 4 lasers where the exposures above the maximum permissible exposure (MPE) can occur. 2. The attenuation factor optical density of the laser protective eyewear at each laser wavelength shall be specified by a laser safety o!cer. 3. All laser protective eyewear shall be clearly labelled with the optical density and the wavelength for which protection is aorded. 4. Laser protective eyewear shall be inspected for damage prior to use. The use of beam attenuators to align visible lasers will reduce laser beam intensities to a level that will allow the operator to align the beam without personal protective equipment. Laser alignment cards for ultraviolet and infrared radiation allow operators to locate the beam during alignment procedures.

7.3

Powder Hazards

It is important to consult material safety data sheets (supplied with materials), and applicable national and health regulations before using any powder or spray materials. Unusual sensitivity to powder materials is seen in some individuals that may result in health hazards. In general, a suitable exhaust system is essential in laser material processing to avoid the toxic eects of powder and fumes that are generated by the process. In addition to the above-mentioned hazards, some materials are inherently dangerous to our health. For example, beryllium and tellurium are very harmful to the respiratory system, and the fumes of cadmium and chromium alloys are extremely hazardous. Similarly, the fumes of nickel components are potentiality hazardous. Before using any material, proper measures need to be considered by consulting an industrial hygienist.

© 2005 by CRC Press LLC

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