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SEPT.OCT.2010.IJPM cover_July_August IJPM cover 9/22/2010 11:28 AM Page 1
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September/October 2010
Focus Issue: PM Titanium
Newsmaker: Paul Beiss, FAPMI
46/5
Powder Metallurgy Titanium—Challenges and Opportunities Metal Powder Injection Molding of Titanium Titanium-Powder-Production Methods Cold Compaction and Sintering of Titanium and Its Alloys Mechanical Properties of Powder Metallurgy Titanium
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FRONT MATTER_ FRONT MATTER 9/22/2010 9:54 AM Page 1
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EDITORIAL REVIEW COMMITTEE P.W. Taubenblat, FAPMI, Chairman I.E. Anderson, FAPMI T. Ando S.G. Caldwell S.C. Deevi D. Dombrowski J.J. Dunkley Z. Fang B.L. Ferguson W. Frazier K. Kulkarni, FAPMI K.S. Kumar T.F. Murphy, FAPMI J.W. Newkirk P.D. Nurthen J.H. Perepezko P.K. Samal D.W. Smith, FAPMI R. Tandon T.A. Tomlin D.T. Whychell, Sr., FAPMI M. Wright, PMT A. Zavaliangos INTERNATIONAL LIAISON COMMITTEE D. Whittaker (UK) Chairman V. Arnhold (Germany) E.C. Barba (Mexico) P. Beiss, FAPMI (Germany) C. Blais (Canada) G.F. Bocchini (Italy) F. Chagnon (Canada) C-L Chu (Taiwan) O. Coube (Europe) H. Danninger, FAPMI (Austria) U. Engström (Sweden) O. Grinder (Sweden) S. Guo (China) F-L Han (China) K.S. Hwang (Taiwan) Y.D. Kim (Korea) G. L’Espérance, FAPMI (Canada) H. Miura (Japan) C.B. Molins (Spain) R.L. Orban (Romania) T.L. Pecanha (Brazil) F. Petzoldt (Germany) G.B. Schaffer (Australia) L. Sigl (Austria) Y. Takeda (Japan) G.S. Upadhyaya (India) Publisher C. James Trombino, CAE
[email protected] Editor-in-Chief Alan Lawley, FAPMI
[email protected] Managing Editor James P. Adams
[email protected] Contributing Editor Peter K. Johnson
[email protected] Advertising Manager Jessica S. Tamasi
[email protected] Copy Editor Donni Magid
[email protected] Production Assistant Dora Schember
[email protected] Graphics Debby Stab
[email protected] President of APMI International Nicholas T. Mares
[email protected] Executive Director/CEO, APMI International C. James Trombino, CAE
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46/5 September/October 2010
2 Editor’s Note 4 Newsmaker ...Paul Beiss, FAPMI 7 Consultants’ Corner James G. Marsden, FAPMI FOCUS: PM Titanium 9 Powder Metallurgy Titanium—Challenges and Opportunities Z.Z. Fang
11 Status of Metal Powder Injection Molding of Titanium Randall M. German
19 Review of Titanium-Powder-Production Methods C.G. McCracken, C. Motchenbacher and D.P. Barbis
29 Cold Compaction and Sintering of Titanium and Its Alloys for Near-Net-Shape or Preform Fabrication M. Qian
45 A Critical Review of Mechanical Properties of Powder Metallurgy Titanium H. Wang, Z.Z. Fang and P. Sun
58 61 62 63 64
DEPARTMENTS PM Industry News in Review Meetings and Conferences APMI Membership Application PM Bookshelf Advertisers’ Index Cover: Titanium powder produced via Armstrong process. Photo courtesy Colin G. McCracken, Reading Alloys Inc.
The International Journal of Powder Metallurgy (ISSN No. 0888-7462) is a professional publication serving the scientific and technological needs and interests of the powder metallurgist and the metal powder producing and consuming industries. Advertising carried in the Journal is selected so as to meet these needs and interests. Unrelated advertising cannot be accepted. Published bimonthly by APMI International, 105 College Road East, Princeton, N.J. 08540-6692 USA. Telephone (609) 4527700. Periodical postage paid at Princeton, New Jersey, and at additional mailing offices. Copyright © 2010 by APMI International. Subscription rates to non-members; USA, Canada and Mexico: $100.00 individuals, $230.00 institutions; overseas: additional $40.00 postage; single issues $55.00. Printed in USA. Postmaster send address changes to the International Journal of Powder Metallurgy, 105 College Road East, Princeton, New Jersey 08540 USA USPS#267-120 ADVERTISING INFORMATION Jessica Tamasi, APMI International 105 College Road East, Princeton, New Jersey 08540-6692 USA Tel: (609) 452-7700 • Fax: (609) 987-8523 • E-mail:
[email protected] FRONT MATTER_ FRONT MATTER 9/22/2010 9:54 AM Page 2
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EDITOR’S NOTE
E
ven a cursory review of the history of titanium powder metallurgy (PM) reveals a chronology permeated with advances and setbacks in the technology. Currently, non-military applications of titanium are limited, a reflection of its high cost, regardless of the process by which it is produced. Since the aerospace industry dominates the titanium market, any PM approach must compete with ingot metallurgy on a cost basis and address the sensitivity of fatigue properties to oxygen and porosity. Achieving low cost and high performance is the key challenge in the PM processing of titanium and its alloys. Where does the technology go from here and what are the future prospects for PM titanium? This “Focus Issue,” coordinated by Zak Fang, University of Utah, addresses these challenges in relation to titanium powder production, compaction and sintering, injection molding, and mechanical properties. Collectively, these reviews reflect a positive outlook for the future of PM titanium. Returning to the “Consultants’ Corner,” Jim Marsden address issues related to the sintering of ferrous materials. Specifically, he rationalizes the absence of a standard for “combined carbon,” and cites reasons for the discoloration of austenitic and martensitic stainless steels sintered in pure dissociated ammonia. “Newsmaker” Paul Beiss, FAPMI, recently retired from RWTH Aachen following a distinguished and productive career embracing teaching and research in powder metallurgy, with a focus on fatigue. Of particular note is his attention to real-world problem solving, a career-long mission that reflects close ties with the PM industry in Europe.
Alan Lawley Editor-in-Chief
In the previous issue of the Journal, I wrote briefly on differentiating between engineering and science, and hence between engineers and scientists. In a nutshell, “scientists seek to understand what is, whereas engineers seek to create what never was.” As such, it is engineering that has a direct influence on our overall standard of living. Given the paucity of media coverage, it is left to the engineering profession to educate the non-technical populace on the influence of engineering in their daily lives and on its many significant accomplishments. How is this being done and who speaks for the profession? For the “materials” discipline, primary advocates include the Minerals, Metals and Materials Society (TMS), ASM International, the American Ceramic Society (ACS), and the Federation of Materials Societies (FMS). And powder metallurgy and particulate materials is receiving increasing exposure on the beltway through the efforts of the Metal Powder Industries Federation (MPIF) in touting their “green” energy-efficient technology. Arguably, across the entire engineering discipline, the leading voice is the National Academy of Engineering (NAE). The goal of their proactive programs is to better prepare America to navigate our technology-dependent society in relation to technological literacy. Examples include: K-12 engineering education; the public understanding of engineering through public and media relations; diversity in the engineering workforce; engineering and health; and the interface between the nation’s economy and engineering innovation. Further insight into the programs and activities of the NAE is available at www.nae.edu.
2
Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
FRONT MATTER_ FRONT MATTER 9/22/2010 9:54 AM Page 3
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Features and Benefits t Improved machining performance/tool life t Stain free t No detrimental effects on mechanical properties
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NEWSMAKER_ NEWSMAKER 9/22/2010 9:56 AM Page 4
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NEWSMAKER
PAUL BEISS, FAPMI
By Peter K. Johnson*
Paul Beiss, FAPMI, recently retired professor at the Institute for Materials Applications in Mechanical Engineering, Aachen, Germany, is an academician who enjoys the real-world, problemsolving side of industrial production. “We educate for industry and are totally dedicated to this mission,” he says. “Sixty-five percent of our faculty have five to 10 years of industrial experience.” His career in powder metallurgy (PM) happened solely by chance. “I was looking for a job in 1979 after receiving my doctorate from the Technical University of Aachen and accepted a position with Sintermetallwerk Krebsöge,” he says. “I had many options but Lothar Albano-Müller, the managing director, offered me the responsibility of commercializing a new vacuum sintering process for making fully dense high-speed steels.” Because his father was a commercial manager of a copper-base alloy firm, he was familiar with metals and technology from an early age. “Before I was 15 I was determined to become an engineer,” he says. “During school vacations I worked in the plant where my father was employed learning about melting, extrusion, and rod and wire drawing.” However, his university studies had not included any PM classes; his PhD thesis covered the thermal extrusion of copper. Nevertheless, he accepted the Krebsöge offer and there he mastered vacuum sintering in a hands-on environment making PM high-speed steel indexable inserts, trimming dies, and gun barrel blocks. He remembers the late PM pioneer Gerhard Zapf, who had retired as managing director, checking his progress during regular weekly visits to the plant. “Dr. Zapf was very curious about my work,” Beiss says. After managing the high-speed steel operation until 1983, he was named technical manager of the conventional PM–parts unit, which had 340 *Contributing editor
4
employees. The plant, which made iron and aluminum PM parts and bearings, had 35 compacting presses up to 250 mt, three mesh-belt sintering furnaces, and one walking-beam furnace. Within five years production at the plant doubled. Beiss was responsible for tooling design and fabrication, quality control, and equipment purchasing. In 1988 he was promoted to vice president of engineering for the entire Krebsöge Group under a new owner, MAAG Gear-Wheel & Machine Co. Ltd., Zurich, Switzerland. MAAG had purchased a majority interest in Krebsöge the previous year and expanded Beiss’s responsibilities to include PM plants in the U.S. and Canada. He remained in this position until 1991 when serious problems emerged at the PM parts operation in Bad Brückenau, Germany, where sales had plunged by 40 percent within the previous year. This plant was an important production center for automotive products such as oil-pump parts; camshaftdrive, crankshaft, and water-pump pulleys; and synchronizer hubs and rings. It had compacting presses up to 1,250 mt and employed 655 workers. Beiss was tapped to fix the problem and return the plant to profitability, a task he accomplished. He stayed at Krebsöge until 1994 when he left in order to begin his academic career at the Institute for Materials Science in Aachen by establishing a PM program. His duties covered teaching mechanical engineering students in materials science and PM, and introducing PM into R&D programs. “I focused on iron and steel,” he says. Currently the university annually graduates about 10 PM diploma engineering students who have been exposed to PM courses. Beiss has devoted a considerable research effort to axial, bending, torsion, and rolling-contactfatigue testing, as evidenced by his department’s 35 fatigue-testing machines, some of which were inijpm
Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
NEWSMAKER_ NEWSMAKER 9/22/2010 9:56 AM Page 5
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NEWSMAKER: PAUL BEISS, FAPMI
house designed and built; other in-house built machines were even sold to PM-industry customers. He has also focused on the machining of green PM parts. Two doctoral students did their theses on machining and green-machining processes, which have been adopted by several industrial companies. His academic career and close ties to the PM industry have given Beiss a strategic platform to examine PM’s technology needs and barriers to growth. “The technology needs more shape capability and more levels in compacting presses and more complex parts with undercuts,” he suggests. As an example, he points to the compacting of cross holes in hard metals, a technique already underway in Europe. Yet another area that needs further development is that of higher-density PM parts capable of replacing forgings. “Many design engineers in Europe look at PM microstructures and will not buy porous parts,” he says.
Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
Beiss has been recognized internationally for his work. In addition to being named an APMI Fellow in 2008, he received the Skaupy Lecture Award from the German Joint Committee on PM and the Ivor Jenkins Award from the Institute of Materials, Minerals and Mining in England. He has authored more than 240 technical papers, about 70 percent of which deal with PM-related topics. Apart from PM, other research areas of his have been grey and vermicular cast iron, fatigue and failure analysis of metallic materials, structure–property relationships, and alloy development in tin-based sliding bearings. Here, the aim is to increase the strength of new alloys to the load-bearing capacity of aluminum–tin bearing alloys to make use of their superior adhesive wear performance. Several patents have been granted on these developments. Paul Beiss plans to continue consulting and lecturing two days a week to undergraduate and graduate students until 2014. ijpm
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NEWSMAKER_ NEWSMAKER 9/22/2010 9:56 AM Page 6
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THE POWDER EXPERTS Rutile Milled, Rutile Extra Fine, Titanium Powder, Ferro Titanium Powder
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CONSULTANTS' CORNER_ CONSULTANTS' CORNER 9/22/2010 9:57 AM Page 7
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CONSULTANTS’ CORNER
JAMES G. MARSDEN, FAPMI* Q A
Why is there no MPIF standard for checking “combined carbon”? I do not believe there is an ASTM standard for this either. I cannot speak for MPIF but I can give you several reasons why standards have not been established by the PM industry. For many of us who have spent several years of our lives studying microstructures and estimating the combined carbon of PM compacts in particular, it is easy to understand why no one has attempted to establish a definitive standard. Several charts have been produced over the years that give some guidelines on estimating combined carbon in iron–carbon alloys, but these are not standards. Probably foremost is the fact that the microstructures of PM steel compacts are not homogeneous and therefore the carbon distribution will vary from particle to particle and area to area throughout the entire cross section of the compact. Another point to consider is that not every parts manufacturer has the equipment and/or the personnel to perform reliable microstructural analysis. To my knowledge, and based on my experience, there are three different methods for checking the carbon content in a PM compact and each one results in a different conclusion. These methods include metallographic analysis, chemical analysis, and gas analysis, sometimes referred to as LECO analysis. Metallographic analysis requires the preparation and microscopic examination of the cross section to be analyzed. It also requires interpretation by a technician with an extensive background in microscopy since it is an estimate and does not give a specific carbon level. The one benefit that it does offer is that one can establish the location of the carbon in cases where there is decarburization or carburization of the compact during sintering. Limitations are that it is only feasible in iron–car-
bon and iron–copper– carbon alloys in which the microstructure consists of ferrite, lamellar pearlite and, in some cases, iron– carbides (hyper eutectoid steel) that form in the grain boundaries. When evaluating low-alloy steels, the presence of molybdenum retards the formation of the carbide platelets during cooling, and this produces a structure of randomly spaced carbide platelets (compared with a lamellar shape in iron–carbon alloys). One can guess at the carbon content but I have always found it not feasible to give an accurate analysis. It must also be remembered that adding alloying elements to the base iron will change the eutectoid composition. Instead of basing the analysis on 0.8 w/o C for the eutectoid, as you would with iron-base powders, it must be based on a eutectoid composition of ~0.6 w/o C. Chemical analysis will give the total carbon and the free carbon; therefore, the combined carbon can be determined by subtracting the free carbon from the total carbon. This method provides a reasonable estimate of what percentage of graphite has gone into solution in the base iron and what percentage has not. However, this method will not establish the location of the carbon and whether or not the compact has been carburized or decarburized during sintering or heat treatment. Gaseous analysis consists of burning a small sample of the compact, in the form of drillings, a very small section, and/or a powder sample. Burning the specimen will produce CO and CO2. A small amount of oxygen is added to these gases and then passed over a catalyst of platinum and silica so that the oxygen combines with the CO and converts it to CO2. Molecular oxygen can then be separated from the carbon by passing the CO2
*Consultant, Furnace & Atmosphere Service Technology, Inc. (F.A.S.T., Inc.), P.O. Box 43, Big Run, Pennsylvania 15715-0043, USA; Phone: 814-427-2228; E-mail:
[email protected] Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
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CONSULTANTS' CORNER_ CONSULTANTS' CORNER 9/22/2010 9:57 AM Page 8
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CONSULTANTS’ CORNER
through an O2 molecular sieve. Once the oxygen is removed the total carbon content can be determined. This method will only give the total carbon, not the combined carbon content. If sampled correctly, it is possible to arrive at some indication of the carbon distribution by following this procedure. First, drillings from several areas at the top and bottom surfaces of the part (to a depth ~1.6 mm) are each analyzed for carbon content. Then, using the same holes, you drill into the core of the part and analyze these drillings for carbon content. It is important to insure that any contaminants, for example, drilling solutions, and quench oil are removed by washing in acetone before burning. This will give an indication of the carbon distribution throughout the cross section of the part, and whether or not the part has been decarburized or carburized during processing. When looking at the total picture it is not difficult to be convinced that it would be virtually impossible to develop a standard that would not only establish the combined carbon content in a compact but also identify the location and percentage of graphite that actually diffused into the base metal. In summary, the variables that must be considered are so numerous that if someone could outline a method that incorporated all these factors I am sure the standards committee would be most interested in listening.
Q
Our trials on stainless steel powders, both austenitic and martensitic grades, are utilized in the density range of 6.0 to 6.5 g/cm3. The sintering temperature was approximately 1,160°C (2,120°F) in an atmosphere of pure dissociated ammonia with a soak time of 40 min. All the parts exhibited a blue/black color after sintering. What went wrong and what is the correct dew point for sintering stainless steel compacts? First let me discuss the dew point for sintering stainless steels in dissociated ammonia. The furnace dew point for this atmosphere should be -29°C (-20°F) or lower. The dew point should be checked by placing a 6.4 mm stainless steel tube down the middle of the furnace into the hot zone (I am assuming this is a continuous belt furnace). Using a diaphragm pump, a filter, and a dew point analyzer, check the dew point in this zone of the furnace under normal operating conditions. I would also recommend checking the dew point of the gas at the dissociator. There should not be
A
8
much difference in dew point between these two locations. However, in relation to the blue/black surface oxide that is forming on the parts, I am sure there will be a considerable difference between the two locations with the dew point in the furnace being much higher than at the dissociator. The formation of the blue/black oxide is a strong indication of either air entering the furnace in the transition zone between the high heat and cooling zone, a water leak in the cooling zone, or that the parts are exiting the furnace at a temperature at which they cannot be handled without gloves. I tend to believe the first or second scenario is actually what is happening. Since you are sintering in an atmosphere of 75 v/o hydrogen/25 v/o nitrogen, there is ample hydrogen to combine with any air ingression into the furnace. However, if the leak is large enough, the oxygen in the air will attack the metal and form a surface oxide resembling the one you are describing. The temperature of the metal, when attacked, will dictate the color of the oxide and the color you describe is typical of this location in the furnace. One way to determine if the oxygen ingress is from air or water is to use an atmosphere of pure nitrogen (if you have the capability). If the dew point goes down to -40°C to -51°C (-40°F to -60°F) there is an air leak. However, if the dew point remains high, there is a water leak. I expect that you will find that an air leak rather than a water leak is causing the problem. If, after conducting the dew point tests, it proves to be an air leak, make sure that all the bolts on the flange area that connect the high-heat and cooling zones are tight, unless they are welded flanges. If that is the case then look for cracks in the welds. Another place the air may be coming from is loose joints in the piping from the dissociator to the furnace. You can determine this by leak testing the joints using a soap/water mixture. Air can be readily sucked into the furnace through loose joints in the line and, if you are injecting the atmosphere in the transition zone, it will attack the metal parts and form the observed blue/black oxide. If there are any questions or if I can be of any further assistance please feel free to contact me @ 814-427-2228. ijpm Readers are invited to send in questions for future issues. Submit your questions to: Consultants’ Corner, APMI International, 105 College Road East, Princeton, NJ 085406692; Fax (609) 987-8523; E-mail:
[email protected] Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
Fang-Intro_Zheng et al 9/22/2010 9:58 AM Page 9
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PM TITANIUM
POWDER METALLURGY TITANIUM—CHALLENGES AND OPPORTUNITIES Z. Zak Fang*
Powder metallurgy (PM) titanium and its alloys are, in many ways, similar to ferrous and other nonferrous metals. There are conventional press-and-sinter manufacturing routes, and advanced processing technologies based on plasma atomization and hot isostatic pressing. The former is the least expensive approach while the latter results in highperformance materials. The difficulties in developing PM titanium and its alloys are, however, significantly more challenging as compared with most other PM materials. Similar to other PM materials, titanium competes with ingot metallurgy by offering a low-cost net- or near-net-shape alternative. The mechanical properties, corrosion resistance, and biocompatibility of titanium alloys are compelling and well established. However, ingot metallurgy titanium and its alloys find limited applications because of cost—more than four times higher than that of steels. Therefore, the PM route could offer an attractive alternative. Civilian applications of titanium are limited, hence the commercial market is significantly smaller (a fraction) of that of other metals. This reflects the high cost of titanium, regardless of the processes by which it is produced. The aerospace industry dominates the marketplace and this forces PM to compete and address challenges including the sensitivity of titanium powder to oxygen and the effect of porosity on fatigue properties. The combined effect of the requirements for low cost and high performance is therefore a trade-off between these entities. The conventional press-and-sinter approach offers low-cost materials with adequate static mechanical properties, while the hot isostatic pressing of prealloyed powders results in mechanical properties equivalent to those of ingot metallurgy materials at a significantly increased cost. How to achieve both low cost and high performance has been, and will remain, the key challenge in the PM processing of titanium and its alloys. Another reality, arguably due to the challenge cited above, is the reality that the market size for PM titanium is small, except for powders and isolated cases of parts manufacturing. Therefore, the question or challenge, phrased in a different way, is where to go from here and what are the future prospects for PM titanium? It is against this backdrop that this “Focus Issue” on PM titanium and its alloys is compiled. Obviously, one such attempt will not address *Associate Professor, University of Utah, Metallurgical Engineering Department, 135 S. 1460 East, Room 412, Salt Lake City, Utah 84112, USA; E-mail:
[email protected] Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
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Fang-Intro_Zheng et al 9/22/2010 9:58 AM Page 10
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POWDER METALLURGY TITANIUM—CHALLENGES AND OPPORTUNITIES
all the concerns and questions regarding PM titanium; rather, it aims to provide a realistic review of the status, challenges, and opportunities. The articles are intended to highlight developments that are uniquely promising. First, a review of powder production methods is coauthored by McCracken that includes a brief introduction to different methods and, more important, insights into the pros and cons, economics, and the markets served by the different powder production methods. Following powder production, Qian takes us through a history of the sintering of titanium and titanium alloy powders. A fundamental understanding, with respect to the issue of titanium oxide on particle surfaces, the effects of vacuum and atmosphere, and interparticle diffusion, are particularly worth noting. Qian also cites a specific example of the commercial production of PM titanium automobile parts, albeit the only viable tonnage commercial production to date. In relation to parts manufacturing, German presents a convincing case that the powder injection molding of titanium is “ready for prime time” from a knowledge and technology-readiness perspective. The key hurdle that continues to prevent the metal injection molding of titanium from rapid growth is the high cost of titanium powder. The final article by Wang et al. focuses on mechanical properties and their dependence on microstructure, porosity, and oxygen content, all of which are unique to PM processing. This critical review, and a comparison of the mechanical properties of PM titanium, with ASTM Standards for ingot metallurgy titanium puts things in perspective, details future challenges and, hopefully, illustrates opportunities. Although the topics covered in this “Focus Issue” are not orchestrated to address any specific industry, the emphasis on low-cost methods for civilian applications is evident. To paraphrase a well-known cliché, the three most important factors in relation to titanium are cost, cost, and cost. The cost of titanium will not decrease until it is more widely used in civilian applications such as automobile parts and consumer products. The counter argument is that, if the cost of titanium is significantly reduced, non-military industries will then adopt titanium. In this chicken-or-egg dilemma, we predict PM technologies will play a crucial role in eventually bringing about a watershed event (products) that will cascade affordable titanium over the entire manufacturing spectrum. ijpm
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Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
Randall M. German_Zheng et al 9/22/2010 11:54 AM Page 11
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PM TITANIUM
STATUS OF METAL POWDER INJECTION MOLDING OF TITANIUM Randall M. German, FAPMI*
INTRODUCTION Titanium production by the metal powder injection molding process (Ti-MIM) has been an area of heated debate. In 1997 Animesh Bose and I wrote a book on MIM/PIM at the request of the Metal Powder Industries Federation.1 As with all such books, reviews were performed on the manuscript to ensure accuracy. The reviewers complained that Ti-MIM was not possible, contrary to the detailed description given in the book. We begged to disagree, so on the front cover of the book we included a micrograph to show the structure possible with Ti-MIM, in 1997. Saying it is “not possible” fails to change the fact that Ti-MIM is technologically well advanced. TITANIUM STATUS Titanium is poised for significant activities by MIM, but early efforts did not properly balance technical sophistication and economics. To make the case, this status report will show that significant progress has been made in powder fabrication and in component fabrication. What often has been missing is the realization that markets for Ti-MIM require high-performance and high-value applications, areas that specify aerospace and medical quality. Demonstration components in Ti-MIM span areas that include the following, several of which are illustrated in Figures 1 to 7: • automotive gearshift knobs • toy components, including “transformer” hinges, train links and wheels • surgical tools, including scalpel holders • rifle and firearm components • watch cases, watch bands, watch clasps • eyeglasses components, even eyeglasses frames • cell phone hinges, knuckles • heat valves • golf clubs, ranging from putters to drivers • implant devices, including chemotherapy pumps • tooth-implant anchors • orthodontic brackets • jet-engine fasteners • cosmetic cases • decorative hardware for luggage and purses
The metal powder injection molding process (MIM or PIM) has been applied to titanium in various forms since the late 1980s. Powder production is well advanced with many offerings, giving a wide choice in particle size, particle shape, purity, and cost. Likewise, component production steps exist with many variants, but a general baseline process is identified here. There is much knowledge on how to fabricate titanium using MIM, but this knowledge is concentrated in the hands of a few fabricators. Against this baseline, emerging innovations can be assessed. Demonstration products have been shown for several applications, including watch cases, toy trains, cell phones, heat valves, dental implants, and medical surgical tools. This article rationalizes the technical and economic factors to show why research efforts in titanium injection molding are focused on reduced interstitial contents.
*Associate Dean of Engineering, Professor of Mechanical Engineering, San Diego State University, 5500 Campanile Drive, San Diego, California 92182-1326, USA; E-mail:
[email protected] Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
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STATUS OF METAL POWDER INJECTION MOLDING OF TITANIUM
Figure 1. Surgical scalpel holder fabricated using Ti-MIM Figure 5. Implant pump fabricated via Ti-MIM
Figure 2. Underside of a Ti-MIM golf putter Figure 6. Clasp on watch band formed using Ti-MIM
Figure 3. Example of a firearm component fabricated by Ti-MIM
Figure 7. Toy component fabricated by Ti-MIM
Figure 4. Cellular-telephone cover formed using Ti-MIM
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Clearly the range of target applications for TiMIM is large, but the technology segregates into three groups: 1) Decorative items where mechanical and other properties are secondary to the marketing advantage—watch cases and golf putters are in this category 2) Mechanical components where mechanical and corrosion properties exceed those possible from a stainless steel—surgical tools that Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
Randall M. German_Zheng et al 9/22/2010 11:54 AM Page 13
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STATUS OF METAL POWDER INJECTION MOLDING OF TITANIUM
can be repeatedly autoclaved without rusting are in this category 3) Demanding applications where titanium is critical to success—biomedical implants and aerospace components are in this category The decorative items are cost sensitive and often only make it to the demonstration phase in Ti-MIM due to cost; the marketing advantage from “titanium” does not justify the added cost. This was seen when some of the first cellular telephone hinges were made using Ti-MIM, but later switched to stainless steel. Also this happened 15 years ago when sunglasses frames and golf clubs were first injection molded from titanium. The marketing infatuation with titanium continues. Remember the titanium golf balls? No one really expected to find a metallic core and it seems marketing took some liberty with the white titania in the coating. However, several successes have occurred in titanium components where mechanical and corrosion properties are dominant, such as lightweight eyeglasses hinges. Now we are on the cusp of seeing Ti-MIM move into taxing applications where the added cost is justified. Accordingly, several groups are at work isolating the powders, processing steps, and property tradeoffs possible. This article provides an update on the status of Ti-MIM, with a prime focus on the tradeoffs being faced as the technology translates into production. As knowledge is gained, Ti-MIM turns its attention to the economics of powder fabrication and processing. TITANIUM POWDER COST FACTORS The thermodynamics of titanium reduction from ores is a well-explored topic and well over 30 research efforts were launched on this topic in the past ten years. A few of those efforts are coming to fruition with new powder options. However, powder cost is not low. Part of the problem can be traced to the 50-fold greater energy required to convert ore to metal for titanium vs. common metals like copper and iron. This alone makes the cost of titanium high. An additional factor is that titanium use is only 0.01% that of steels, so global production of titanium sponge (recently ~220,000 mt at an average price of nearly $13/kg) is over three times the commercial value of iron powder production, and most of the material goes into mill products, castings, and forgings, not into powder metallurgy Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
(PM) components. Even so, over 40 firms sell titanium powder, with prices that range from $30/kg upward.2 Powders required for Ti-MIM are made by several approaches, including gas atomization, hydride– dehydride, plasma atomization, and rotating electrode.3 Figure 8 is a scanning electron micrograph (SEM) of a typical spherical, -45 µm powder used in Ti-MIM. Although there has been much effort to innovate, current production costs ($10/kg powder) are high, and classification of the powder into the small sizes required for MIM results in prices from $40 to $220/kg, depending on the alloy, particle size, and purity level. Indeed, the most popular Ti-MIM powders tend to average ~$120/kg. This powder cost precludes Ti-MIM from being a low-cost option and drives the field toward highervalue applications. Consumer products exist, but tend to be small and highly valued for being lightweight—watch components, cellular-telephone components, eyeglasses components, and such. On the other hand, biomedical applications rely heavily on titanium because of density, strength, corrosion resistance, and biocompatibility. The three product categories—cosmetic, structural, demanding—require different price structures and powder attributes. The demanding applications require control over the impurities and that means every step in the PIM process must be monitored and controlled. The oxygen content in the final product is a reflection of the starting powder purity. To avoid contamination of the powder, the typical decision is to use a larger particle size than is normal in MIM to reduce the surface area and hence contamination, with attendant difficulties in sintering densification. At least
Figure 8. Spherical gas-atomized titanium alloy powder customized for Ti-MIM. SEM
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Randall M. German_Zheng et al 9/22/2010 11:55 AM Page 14
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three titanium powder suppliers are recognized for producing powders with low interstitial levels meeting biomedical standards, and a fourth has announced plans to enter production in 2010. Their powders are priced in the $110 to $220/kg range, depending on purity, quantity, alloying, and particle size. Curiously, gas-atomized cobalt– chromium MIM powder sells at $160/kg. On a volume basis, titanium is lower in cost (density of 8.4 g/cm3 for cobalt–chromium vs. 4.5 g/cm3 for Ti-6Al-4V). Accordingly, titanium should displace cobalt–chromium in several biomedical applications. KEY FEATURES IN TITANIUM PM A few parameters dominate the mechanical properties of titanium when it is fabricated by PM—density, interstitial content, alloying, and microstructure. Residual pores degrade mechanical properties, so full density is desirable. Hot isostatic pressing (HIPing) is a common means to attain full density, since grain growth during the sintering of large particles degrades strength as full density is approached. Without full density the fracture toughness and fatigue strength suffer more than the tensile strength. A demonstration of this for titanium is given in Figure 9. This is a plot of tensile strength, yield strength, and fatigue strength for Ti-6Al-4V fabricated to different sintered-density levels using blended elemental powders.4 Between 94% and 100% of the pore-free density the yield strength increases 30% but fatigue strength increases fourfold. Yet, as illustrated in
Figure 9. Properties vs. sintered density for Ti-6Al-4V fabricated from blended elemental powders using the press-and-sinter PM process, showing property gains from the elimination of pores
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Figure 10, when sintering is used, concurrent grain growth degrades the strength, notwithstanding density gains. Thus, densification via HIPing after sintering is a typical step in Ti-MIM, but this adds to the production expense. The interstitial content has a significant effect on mechanical properties, biocompatibility, and corrosion resistance. For PM, oxygen is the usual focus. Interstitials of carbon, hydrogen, nitrogen, and oxygen increase strength and hardness, but decrease ductility. Typically, alloys are sorted by grade levels that correspond to the oxygen content. As an example of the sensitivity of titanium to oxygen, consider that the ASTM standard for unalloyed (CP or commercially pure) grade-1 titanium requires