Friction Stir Welding and Processing

Friction Stir Welding and Processing Rajiv S. Mishra, Murray W. Mahoney, editors, p 1-5 DOI:10.1361/fswp2007p001

Copyright © 2007 ASM International® All rights reserved. www.asminternational.org

CHAPTER 1

Introduction Rajiv S. Mishra, Center for Friction Stir Processing, University of Missouri-Rolla Murray W. Mahoney, Rockwell Scientific Company

FRICTION STIR WELDING (FSW) was invented at The Welding Institute (TWI) of the United Kingdom in 1991 as a solid-state joining technique and was initially applied to aluminum alloys (Ref 1, 2). The basic concept of FSW is remarkably simple. A nonconsumable rotating tool with a specially designed pin and shoulder is inserted into the abutting edges of sheets or plates to be joined and subsequently traversed along the joint line (Fig. 1.1). Figure 1.1 illustrates process definitions for the tool and workpiece. Most definitions are self-explanatory, but advancing and retreating side definitions require a brief explanation. Advancing and retreating side orientations require knowledge of the tool rotation and travel directions. In Fig. 1.1, the FSW tool rotates in the counterclockwise direction and travels into the page (or left to right). In Fig. 1.1 the advancing side is on the right, where the tool rotation direction is the same as the tool travel direction (opposite the direction of metal flow), and the

Fig. 1.1

Schematic drawing of friction stir welding

retreating side is on the left, where the tool rotation is opposite the tool travel direction (parallel to the direction of metal flow). The tool serves three primary functions, that is, heating of the workpiece, movement of material to produce the joint, and containment of the hot metal beneath the tool shoulder. Heating is created within the workpiece both by friction between the rotating tool pin and shoulder and by severe plastic deformation of the workpiece. The localized heating softens material around the pin and, combined with the tool rotation and translation, leads to movement of material from the front to the back of the pin, thus filling the hole in the tool wake as the tool moves forward. The tool shoulder restricts metal flow to a level equivalent to the shoulder position, that is, approximately to the initial workpiece top surface. As a result of the tool action and influence on the workpiece, when performed properly, a solid-state joint is produced, that is, no melting. Because of various geometrical features on the tool, material movement around the pin can be complex, with gradients in strain, temperature, and strain rate (Ref 3). Accordingly, the resulting nugget zone microstructure reflects these different thermomechanical histories and is not homogeneous. In spite of the local microstructural inhomogeneity, one of the significant benefits of this solid-state welding technique is the fully recrystallized, equiaxed, fine grain microstructure created in the nugget by the intense plastic deformation at elevated temperature (Ref 4–7). As is seen within these chapters, the fine grain microstructure produces excellent me-

2 / Friction Stir Welding and Processing

chanical properties, fatigue properties, enhanced formability, and exceptional superplasticity. Like many new technologies, a new nomenclature is required to accurately describe observations. In FSW, new terms are necessary to adequately describe the postweld microstructures. The first attempt at classifying friction stir welded microstructures was made by Threadgill (Ref 8). Figure 1.2 identifies the different microstructural zones existing after FSW, and a brief description of the different zones is presented. Because the preponderance of work to date uses these early definitions (with minor modifications), this reference volume continues to do so. The system divides the weld zone into distinct regions, as follows:

•

•

•

•

Unaffected material or parent metal: This is material remote from the weld that has not been deformed and that, although it may have experienced a thermal cycle from the weld, is not affected by the heat in terms of microstructure or mechanical properties. Heat-affected zone: In this region, which lies closer to the weld-center, the material has experienced a thermal cycle that has modified the microstructure and/or the mechanical properties. However, there is no plastic deformation occurring in this area. Thermomechanically affected zone (TMAZ): In this region, the FSW tool has plastically deformed the material, and the heat from the process will also have exerted some influence on the material. In the case of aluminum, it is possible to obtain significant plastic strain without recrystallization in this region, and there is generally a distinct boundary between the recrystallized zone (weld nugget) and the deformed zones of the TMAZ. Weld nugget: The fully recrystallized area, sometimes called the stir zone, refers to the zone previously occupied by the tool pin. The term stir zone is commonly used in friction stir processing, where large volumes of material are processed.

Fig. 1.2

Friction stir welding is considered to be the most significant development in metal joining in decades and, in addition, is a “green” technology due to its energy efficiency, environmental friendliness, and versatility. As compared to the conventional welding methods, FSW consumes considerably less energy, no consumables such as a cover gas or flux are used, and no harmful emissions are created during welding, thereby making the process environmentally friendly. Further, because FSW does not involve the use of filler metal and because there is no melting, any aluminum alloy can be joined without concern for compatibility of composition or solidification cracking— issues associated with fusion welding. Also, dissimilar aluminum alloys and composites can be joined with equal ease (Ref 9–11). In contrast to traditional friction welding, which is a welding process limited to small axisymmetric parts that can be rotated and pushed against each other to form a joint (Ref 12), FSW can be applied to most geometric structural shapes and to various types of joints, such as butt, lap, T-butt, and fillet shapes (Ref 13). The most convenient joint configurations for FSW are butt and lap joints. A simple square butt joint is shown in Fig. 1.3(a). Two plates or sheets with the same thickness are placed on a backing plate and clamped firmly to prevent the abutting joint faces from being forced apart. The backing plate is required to resist the normal forces associated with FSW and the workpiece. During the initial tool plunge, the lateral forces are also fairly large, and extra care is required to ensure that plates in the butt configuration do not separate. To accomplish the weld, the rotating tool is plunged into the joint line and traversed along this line, while the shoulder of the tool is maintained in intimate contact with the plate surface. Tool position and penetration depth are maintained by either position control or control of the applied normal force. On the other hand, for a lap joint configuration, two lapped plates or sheets are clamped, and a back-

Various microstructural regions in the transverse cross section of a friction stir welded material. A, unaffected material or parent metal; B, heat-affected zone; C, thermomechanically affected zone; D, weld nugget

Chapter 1: Introduction / 3

ing plate may or may not be needed, depending on the lower plate thickness. A rotating tool is vertically plunged through the upper plate and partially into the lower plate and traversed along the desired direction, joining the two plates (Fig. 1.3d). However, the tool design used for a butt joint, where the faying surfaces are aligned parallel to the tool rotation axis, would not be optimal for a lap joint, where the faying surfaces are normal to the tool rotation axis. The orientation of the faying surfaces with respect to the tool features is very important and is discussed in detail in Chapter 2. Configurations of other types of joint designs applicable to FSW are also illustrated in Fig. 1.3. Additional key benefits of FSW compared to fusion welding are summarized in Table 1.1. This volume is the first comprehensive compilation of friction stir welding and friction stir processing data. This handbook should be valuable to students studying joining and metalworking practices, to welding engineers challenged to improve properties at reduced cost, to metallur-

Fig. 1.3

gists needing new tools to locally improve properties, and to all engineers interested in sustainability, that is, the ability to build structures while minimizing the negative impact to our environment. The dual objectives of this first volume are to provide a ready reference to identify work completed to date and to provide an educational tool to understand FSW and how to both use and apply FSW. Not all process details can be presented within these pages, and readers are encouraged to obtain the original references for more details, especially weld parameters and appropriate boundary conditions. To meet these objectives, the book is organized to first include a full description of tool materials and tool designs for both low- and hightemperature metals (Chapter 2). Understanding tools is a natural starting point to successfully use FSW. Chapter 3 provides an introduction to the fundamentals of FSW, including heat generation and metal flow. Although somewhat controversial at this time, Chapter 3 helps one visualize fundamental FSW characteristics and current

Joint configurations for friction stir welding. (a) Square butt. (b) Edge butt. (c) T-butt joint. (d) Lap joint. (e) Multiple lap joint. (f) T-lap joint. (g) Fillet joint. Source: Ref 14

Table 1.1 Key benefits of friction stir welding (FSW) Metallurgical benefits

• Solid-phase process • Low distortion • Good dimensional stability and repeatability • No loss of alloying elements • Excellent mechanical properties in the joint area • Fine recrystallized microstructure • Absence of solidification cracking • Replace multiple parts joined by fasteners • Weld all aluminum alloys • Post-FSW formability Source: Ref 14

Environmental benefits

• No shielding gas required • Minimal surface cleaning required • Eliminate grinding wastes • Eliminate solvents required for degreasing • Consumable materials saving, such as rugs, wire, or any other gases • No harmful emissions

Energy benefits

• Improved materials use (e.g., joining different thickness) allows reduction in weight • Only 2.5% of the energy needed for a laser weld • Decreased fuel consumption in lightweight aircraft, automotive, and ship applications

4 / Friction Stir Welding and Processing

metal flow concepts. Because the preponderance of work has been performed on aluminum alloys, Chapter 4 presents microstructural evolution following FSW as an individual chapter. The ability to weld all aluminum alloys, including the 7xxx and metal-matrix composites, introduces new issues and benefits. In concert, Chapter 5 presents material properties for the common aluminum alloys, including the 2xxx, 3xxx, 5xxx, 6xxx, 7xxx, AlLi, and metal-matrix composites. Considerable data are available for hardness, mechanical properties, fatigue response, and, in some cases, fracture toughness and fatigue crack propagation. Chapter 5 provides a ready reference to identify what properties can be expected following FSW. Although the database is not as extensive, Chapter 6 presents microstructure and properties of ferrous and nickel-base alloys. With the development of high-temperature tooling, that is, polycrystalline cubic boron nitride tools, FSW is rapidly expanding into the welding of high-temperature alloys, and considerable growth is anticipated in this area. Chapter 7 continues the theme of high-temperature FSW but for titanium alloys. Titanium alloys offer unique difficulties, and although the available data are limited at this time, there is considerable interest. The challenge to identify long-life tooling to friction stir weld titanium alloys remains, but early results illustrate the metallurgical potential to apply FSW. Copper alloys (~1000 °C, or 1830 °F) are intermediate in FSW temperature between aluminum alloys (~500 °C, or 930 °F) and ferrous alloys (~1100 to 1200 °C, or 2010 to 2190 °F). Considerable FSW success has already been demonstrated (Chapter 8), and because of the intermediate temperature, different hightemperature flow, and different physical properties such as thermal conductivity, different lessons can be learned. Chapter 9 presents postFSW corrosion properties of aluminum alloys. Compared to fusion welds, corrosion sensitivity following FSW is always equivalent or less. However, FSW does introduce local heat, creating heat-affected zones and potential segregation of second-phase particles at grain boundaries. Corrosion sensitivity following FSW should always be considered, as one would for any welding practice. Chapter 10 presents results from computational modeling of FSW. Modeling helps visualize fundamental behavior and allows for comparison of flow and temperature response for different weld parameters and boundary conditions without performing costly experiments and subsequent evaluation. The advancement of FSW out of the laboratory and into commercial

practice is highlighted in Chapters 11 and 13. Chapter 11 illustrates the portability and versatility of FSW whereby it can be applied with robots. Further, Chapter 11 discusses current FSW machine capabilities. Chapter 12 presents an overview of friction stir spot welding (FSSW). The total cycle in FSSW is relatively short, and the dynamics of the process are close to the plunge part of FSW. The potential to produce solid-state spot welds is generating considerable interest in the automotive industry. Chapter 13 summarizes current FSW applications. It is anticipated that the number of applications will grow rapidly as fabricators learn the ease of application and property benefits attributable to FSW. Chapter 14 presents an outgrowth of FSW, that is, friction stir processing (FSP). Because of the creation of a fine grain microstructure and the ability to eliminate casting defects, FSP offers the ability to locally tailor properties within a structure such that the structure can survive better in its environment. For example, by applying FSP, local properties can be improved, such as abrasion resistance, strength, ductility, fatigue life, formability, and superplasticity. Friction stir processing is a growth technology that may become as important as FSW. Lastly, FSW and FSP are essentially new technologies not much beyond their infancy. The growth potential for the future can be considerable. Chapter 15 offers the authors’ thoughts on technology gaps to be overcome to accelerate growth as well as some speculation on future opportunities and applications. Interest and Growth in FSW. The field of FSW has seen tremendous growth in the last ten years. Figure 1.4 shows the increase in publica-

Fig. 1.4

Significant increase in publications on friction stir welding/friction stir processing. This figure is based on the Institute for Scientific Information Web of Science database and does not include proceedings papers published in The Welding Institute international symposiums and TMS annual meeting symposiums.

Chapter 1: Introduction / 5

tions in this field. This is a summary from the Institute for Scientific Information Web of Science database and does not include proceedings. The first international symposium was held at Rockwell Science Center and was organized by TWI in 1999. From that time, many symposiums have been organized, including three in TMS annual meetings, which have accompanying proceedings. REFERENCES

1. W.M. Thomas, E.D. Nicholas, J.C. Needham, M.G. Murch, P. Templesmith, and C.J. Dawes, G.B. Patent 9125978.8, Dec 1991 2. C. Dawes and W. Thomas, TWI Bull., Vol 6, Nov/Dec 1995, p 124 3. B. London, M. Mahoney, B. Bingel, M. Calabrese, and D. Waldron, in Proceedings of the Third Int. Symposium on Friction Stir Welding, Sept 27–28, 2001 (Kobe, Japan) 4. C.G. Rhodes, M.W. Mahoney, W.H. Bin-

5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

gel, R.A. Spurling, and C.C. Bampton, Scr. Mater., Vol 36, 1997, p 69 G. Liu, L.E. Murr, C.S. Niou, J.C. McClure, and F.R. Vega, Scr. Mater., Vol 37, 1997, p 355 K.V. Jata and S.L. Semiatin, Scr. Mater., Vol 43, 2000, p 743 S. Benavides, Y. Li, L.E. Murr, D. Brown, and J.C. McClure, Scr. Mater., Vol 41, 1999, p 809 P.L. Threadgill, TWI Bull., March 1997 L.E. Murr, Y. Li, R.D. Flores, and E.A. Trillo, Mater. Res. Innov., Vol 2, 1998, p 150 Y. Li, E.A. Trillo, and L.E. Murr, J. Mater. Sci. Lett., Vol 19, 2000, p 1047 Y. Li, L.E. Murr, and J.C. McClure, Mater. Sci. Eng. A, Vol 271, 1999, p 213 H.B. Cary, Modern Welding Technology, Prentice Hall C.J. Dawes and W.M. Thomas, Weld. J., Vol 75, 1996, p 41 R.S. Mishra and Z.Y. Ma, Mater. Sci. Eng. R, Vol 50, 2005, p 1

Friction Stir Welding and Processing Rajiv S. Mishra, Murray W. Mahoney, editors, p 7-35 DOI:10.1361/fswp2007p007

Copyright © 2007 ASM International® All rights reserved. www.asminternational.org

CHAPTER 2

Friction Stir Tooling: Tool Materials and Designs Christian B. Fuller, Rockwell Scientific Company

FRICTION STIR WELDING AND PROCESSING (collectively referred to as friction stirring) is not possible without the nonconsumable tool. The tool produces the thermomechanical deformation and workpiece frictional heating necessary for friction stirring. A friction stir welding (FSW) butt joint is schematically illustrated in Figure 1 in Chapter 1, “Introduction,” and the same steps are necessary for friction stir processing (Ref 1). During the tool plunge, the rotating FSW tool is forced into the workpiece. The friction stirring tool consists of a pin, or probe, and shoulder. Contact of the pin with the workpiece creates frictional and deformational heating and softens the workpiece material; contacting the shoulder to the workpiece increases the workpiece heating, expands the zone of softened material, and constrains the deformed material. Typically, the tool dwells (or undergoes only rotational motion) in one place to further increase the volume of deformed material. After the dwell period has passed, the tool begins the forward traverse along a predetermined path, creating a finegrained recrystallized microstructure behind the tool. Forward motion of the tool produces loads parallel to the direction of travel, known as transverse load; normal load is the load required for the tool shoulder to remain in contact with the workpiece. The initial aluminum FSW studies conducted at The Welding Institute (TWI) used a cylindrical threaded pin and concave shoulder tool machined from tool steel (Ref 2). Since that time, tools have advanced to complex asymmetric geometries and exotic tool materials to friction stir higher-temperature materials. This

chapter uses two sections to examine the evolution of tool material and design since 1991. The first section describes tool materials, including the material characteristics needed for a tool material and a listing of published friction stir tool materials. The second section presents a history of friction stir welding and processing tool design, general tool design philosophy, and associated tool topics.

2.1 Tool Materials Friction stirring is a thermomechanical deformation process where the tool temperature approaches the workpiece solidus temperature. Production of a quality friction stir weld requires the proper tool material selection for the desired application. All friction stir tools contain features designed for a specific function. Thus, it is undesirable to have a tool that loses dimensional stability, the designed features, or worse, fractures.

2.1.1 Tool Material Characteristics Selecting the correct tool material requires knowing which material characteristics are important for each friction stir application. Many different material characteristics could be considered important to friction stir, but ranking the material characteristics (from most to least important) will depend on the workpiece material, expected life of the tool, and the user’s own experiences and preferences. In addition to the physical properties of a material, some practical considerations are included that may dictate the tool material selection.

8 / Friction Stir Welding and Processing

Ambient- and Elevated-Temperature Strength. The candidate tool material must be able to withstand the compressive loads when the tool first makes contact with the workpiece and have sufficient compressive and shear strength at elevated temperature to prevent tool fracture or distortion for the duration of the friction stir weld. Currently, predicting the required tool strength requires complex computational simulations, so typically, the strength requirements are based on experience. At a minimum, the candidate tool material should exhibit an elevated- (workpiece solidus temperature) temperature compressive yield strength higher than the expected normal forces of the tool. Elevated-Temperature Stability. In addition to sufficient strength at elevated temperature, the tool must maintain strength and dimensional stability during the time of use. Creep (and creep fatigue) is a consideration for long weld lengths, where poor creep resistance would change the tool dimensions during welding. Tool materials that derive their strength from precipitates, work hardening, or transformation hardening have defined maximum-use temperatures. Tools used above the maximumuse temperatures will, in time, exhibit a decrease in mechanical properties. The change in mechanical properties is due to overaging, annealing and recovery of dislocation substructures, or reversion to a weaker phase. In friction stirring, these microstructural changes will weaken the tool and either change the tool shape or fracture the tool. Thermal fatigue strength should be considered when the friction stirring tools are subjected to many heating and cooling cycles (e.g., friction stir spot welding or short production welds). However, in most cases, other tool material characteristics will cause failure before thermal fatigue. Wear Resistance. Excessive tool wear changes the tool shape (normally by removing tool features), thus changing the weld quality and increasing the probability of defects. In friction stirring, tool wear can occur by adhesive, abrasive, or chemical wear (which is addressed subsequently as reactivity) mechanisms. The exact wear mechanism depends on the interaction between the workpiece and tool materials and the selected tool parameters. For example, in the case of polycrystalline cubic boron nitride (PCBN) tools, wear at low tool rotation speed is caused by adhesive wear (also known as scoring, galling, or seizing), while wear at high tool rotation speed is caused by abrasive wear (Ref 3).

Tool Reactivity. Tool materials must not react with the workpiece or the environment, which would change (generally in a negative way) the surface properties of the tool. Titanium is well known to be reactive at elevated temperatures; thus, any reaction of titanium with the tool material will change the tool properties and alter the joint quality. Environmental reactions of the tool (e.g., oxidation) could change the tool wear resistance or even produce toxic substances (i.e., formation of MoO3). These environmental reactions can be mitigated with cover gases, but these can add complexity to the welding system. The workpiece can also exhibit environmental reactions; in the case of titanium alloys, a cover gas is needed to prevent workpiece oxidation. Fracture Toughness. Tool fracture toughness plays a significant role during the tool plunge and dwell. The local stresses and strains produced when the tool first touches the workpiece are sufficient to break a tool, even when mitigation methods are used (pilot hole, slow plunge speed, and preheating of the workpiece). It is generally accepted that the tool plunge and dwell periods produce the most damage to a tool (Ref 4). The friction stir machine spindle runout (lateral movement during spindle rotation) should also be considered when selecting a tool material. Low-fracture-toughness tools, for example, ceramics, should only be used in friction stir machines that contain low spindle runout (less than 0.0051 mm, or 0.0002 in.) to avoid premature tool fracture. Coefficient of Thermal Expansion (Bimetal Tools). Thermal expansion is a consideration in multimaterial tools. Large differences in the coefficient of thermal expansion (CTE) between the pin and shoulder materials lead to either expansion of the shoulder relative to the pin or expansion of the pin relative to the shoulder. Both of these situations increase the stresses between the pin and shoulder, thus leading to tool failure. Additional consideration should be made when the pin and shoulder are made of one material, while the tool shank (portion of tool within the spindle) is a different material. One way to mitigate this situation is with a thermal barrier designed to prevent heat removal from the tool into the shank. An example of this is used with PCBN tools where a thermal barrier prevents heat from moving into the tungsten carbide shank (Ref 5). The CTE differences between the tool and workpiece are not found to have a significant influence on friction stirring.

Chapter 2: Friction Stir Tooling: Tool Materials and Designs / 9

Machinability. Many friction stir tools are designed with features that must be machined, ground, or electrodischarged machined into the tool. Any material that cannot be processed to the required tool design should not be considered. Uniformity in Microstructure and Density. Tool materials are not useful if there are local variations in microstructure or density. These slight variations produce a weak region within the tool where premature fracture occurs. Powder metallurgical alloys are manufactured with different densities, so friction stirring tools should only be manufactured from a fully dense grade. Availability of Materials. A tool material is not useful if a steady supply of tool material is not available. This is especially true in a production environment, where production specifications dictate the use of a specific material.

2.1.2 Published Tool Materials This section considers all of the published tool materials listed for friction stir welding and processing. The listed tool materials should not be viewed as an exhaustive list, because many papers do not specify the tool material or claim the tool materials are proprietary. In instances where specific alloys are not cited, effort was made to include the class of tool materials used. The exception is tool steels, where many papers cite tool steels but not the specific alloy. Table 2.1 is a summary of the current tool materials used to friction stir the indicated materials and thicknesses. These data are assembled from the indicated literature sources. Tool Steels. Tool steel is the most common tool material used in friction stirring (Ref 6–26).

Table 2.1 Summary of current friction stir welding tool materials Thickness Alloy

mm

Aluminum alloys