Friction Stir Welding and Processing

333 Pages·2007·14.638 MB·English

by  Mishra R.S., Mahoney M.W. (eds.)

Friction Stir Welding and Processing Copyright © 2007 ASM International®
Rajiv S. Mishra, Murray W. Mahoney, editors, p 1-5 All rights reserved.
DOI:10.1361/fswp2007p001 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 retreating side is on the left, where the tool rota-
invented at The Welding Institute (TWI) of the tion is opposite the tool travel direction (parallel
United Kingdom in 1991 as a solid-state joining to the direction of metal flow).
technique and was initially applied to aluminum The tool serves three primary functions, that
alloys (Ref 1, 2). The basic concept of FSW is is, heating of the workpiece, movement of mate-
remarkably simple. A nonconsumable rotating rial to produce the joint, and containment of the
tool with a specially designed pin and shoulder is hot metal beneath the tool shoulder. Heating is
inserted into the abutting edges of sheets or plates created within the workpiece both by friction
to be joined and subsequently traversed along the between the rotating tool pin and shoulder and by
joint line (Fig. 1.1). Figure 1.1 illustrates process severe plastic deformation of the workpiece. The
definitions for the tool and workpiece. Most defi- localized heating softens material around the pin
nitions are self-explanatory, but advancing and and, combined with the tool rotation and transla-
retreating side definitions require a brief expla- tion, leads to movement of material from the
nation. Advancing and retreating side orienta- front to the back of the pin, thus filling the hole in
tions require knowledge of the tool rotation and the tool wake as the tool moves forward. The tool
travel directions. In Fig. 1.1, the FSW tool rotates shoulder restricts metal flow to a level equivalent
in the counterclockwise direction and travels into to the shoulder position, that is, approximately to
the page (or left to right). In Fig. 1.1 the advanc- the initial workpiece top surface.
ing side is on the right, where the tool rotation As a result of the tool action and influence on
direction is the same as the tool travel direction the workpiece, when performed properly, a
(opposite the direction of metal flow), and the 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 dif -
ferent thermomechanical histories and is not
homogeneous. In spite of the local microstruc-
tural inhomogeneity, one of the significant bene-
fits of this solid-state welding technique is the
fully recrystallized, equiaxed, fine grain micro-
structure created in the nugget by the intense
plastic deformation at elevated temperature (Ref
4–7). As is seen within these chapters, the fine
Fig. 1.1 Schematic drawing of friction stir welding grain microstructure produces excellent me-
2 / Friction Stir Welding and Processing
chani cal properties, fatigue properties, enh anced Friction stir welding is considered to be the
formability, and exceptional superplasticity. most significant development in metal joining
Like many new technologies, a new nomen- in decades and, in addition, is a “green” tech-
clature is required to accurately describe obser- nology due to its energy efficiency, environ-
vations. In FSW, new terms are necessary to mental friendliness, and versatility. As com-
adequately describe the postweld microstruc- pared to the conventional welding methods,
tures. The first attempt at classifying friction stir FSW consumes considerably less energy, no
welded microstructures was made by Thread- consumables such as a cover gas or flux are
gill (Ref 8). Figure 1.2 identifies the different used, and no harmful emissions are created dur-
micros tructural zones existing after FSW, and a ing welding, thereby making the process envi-
brief description of the different zones is pre- ronmentally friendly. Further, because FSW
sented. Because the preponderance of work to does not involve the use of filler metal and
date uses these early definitions (with minor because there is no melting, any aluminum alloy
modifications), this reference volume continues can be joined without concern for compatibility
to do so. The system divides the weld zone into of composition or solidification cracking—
distinct regions, asfollows: issues associated with fusion welding. Also,
dissimilar aluminum alloys and composites can
• Unaffected material or parent metal: This is be joined with equal ease (Ref 9–11).
material remote from the weld that has not In contrast to traditional friction welding,
been deformed and that, although it may have which is a welding process limited to small
experienced a thermal cycle from the weld, is axisymmetric parts that can be rotated and
not affected by the heat in terms of micro- pushed against each other to form a joint (Ref
structure or mechanical properties. 12), FSW can be applied to most geometric
• Heat-affected zone:In this region, which lies structural shapes and to various types of joints,
closer to the weld-center, the material has such as butt, lap, T-butt, and fillet shapes (Ref
experienced a thermal cycle that has modified 13). The most convenient joint configurations
the microstructure and/or the mechanical for FSW are butt and lap joints. A simple square
properties. However, there is no plastic defor- butt joint is shown in Fig. 1.3(a). Two plates or
mation occurring in this area. sheets with the same thickness are placed on a
• Thermomechanically affected zone (TMAZ): backing plate and clamped firmly to prevent the
In this region, the FSW tool has plastically abutting joint faces from being forced apart. The
deformed the material, and the heat from the backing plate is required to resist the normal
process will also have exerted some influence forces associated with FSW and the workpiece.
on the material. In the case of aluminum, it is During the initial tool plunge, the lateral forces
possible to obtain significant plastic strain are also fairly large, and extra care is required to
without recrystallization in this region, and ensure that plates in the butt configuration do
there is generally a distinct boundary be - not separate. To accomplish the weld, the rotat-
tween the recrystallized zone (weld nugget) ing tool is plunged into the joint line and tra-
and the deformed zones of the TMAZ. versed along this line, while the shoulder of
• Weld nugget: The fully recrystallized area, the tool is maintained in intimate contact with
sometimes called the stir zone, refers to the the plate surface. Tool position and penetration
zone previously occupied by the tool pin. The depth are maintained by either position control
term stir zone is commonly used in friction or control of the applied normal force. On the
stir processing, where large volumes of mate- other hand, for a lap joint configuration, two
rial are processed. lapped plates or sheets are clamped, and a back-
Fig. 1.2 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 gists needing new tools to locally improve prop-
on the lower plate thickness. A rotating tool is erties, and to all engineers interested in sustain-
vertically plunged through the upper plate and ability, that is, the ability to build structures while
partially into the lower plate and traversed along minimizing the negative impact to our environ-
the desired direction, joining the two plates ment. The dual objectives of this first volume are
(Fig. 1.3d). However, the tool design used for a to provide a ready reference to identify work
butt joint, where the faying surfaces are aligned completed to date and to provide an educational
parallel to the tool rotation axis, would not be tool to understand FSW and how to both use and
optimal for a lap joint, where the faying surfaces apply FSW. Not all process details can be pre-
are normal to the tool rotation axis. The orienta- sented within these pages, and readers are
tion of the faying surfaces with respect to the encouraged to obtain the original references for
tool features is very important and is discussed more details, especially weld parameters and
in detail in Chapter 2. Configurations of other appropriate boundary conditions.
types of joint designs applicable to FSW are To meet these objectives, the book is orga-
also illustrated in Fig. 1.3.Additional key bene- nized to first include a full description of tool
fits of FSW compared to fusion welding are materials and tool designs for both low- and high-
summarized in Table 1.1. temperature metals (Chapter 2). Understanding
This volume is the first comprehensive compi- tools is a natural starting point to successfully use
lation of friction stir welding and friction stir pro- FSW. Chapter 3 provides an introduction to the
cessing data. This handbook should be valuable fundamentals of FSW, including heat generation
to students studying joining and metalworking and metal flow. Although somewhat controver-
practices, to welding engineers challenged to sial at this time, Chapter 3 helps one visualize
improve properties at reduced cost, to metallur- fundamental FSW characteristics and current
Fig. 1.3 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 Environmental benefits Energy benefits
• Solid-phase process • No shielding gas required • Improved materials use (e.g., joining different
• Low distortion • Minimal surface cleaning required thickness) allows reduction in weight
• Good dimensional stability and repeatability • Eliminate grinding wastes • Only 2.5% of the energy needed for a laser
• No loss of alloying elements • Eliminate solvents required for degreasing weld
• Excellent mechanical properties in the joint • Consumable materials saving, such as • Decreased fuel consumption in lightweight
area rugs, wire, or any other gases aircraft, automotive, and ship applications
• Fine recrystallized microstructure • No harmful emissions
• Absence of solidification cracking
• Replace multiple parts joined by fasteners
• Weld all aluminum alloys
• Post-FSW formability
Source: Ref 14
4 / Friction Stir Welding and Processing
metal flow concepts. Because the preponderance practice is highlighted in Chapters 11 and 13.
of work has been performed on aluminum alloys, Chapter 11 illustrates the portability and versatil-
Chapter 4 presents micro structural evolution fol- ity of FSW whereby it can be applied with robots.
lowing FSW as an individual chapter. The ability Further, Chapter 11 discusses current FSW
to weld all aluminum alloys, including the 7xxx mac hine capabilities. Chapter 12 presents an
and metal-matrix composites, introduces new overv iew of friction stir spot welding (FSSW).
issues and benefits. In concert, Chapter 5 pres- The total cycle in FSSW is relatively short, and
ents material prope rties for the common alu- the dynamics of the process are close to the
minum alloys, including the 2xxx, 3xxx, 5xxx, plunge part of FSW. The potential to produce
6xxx,7xxx,AlLi, and metal-matrix composites. solid-state spot welds is generating considerable
Considerable data are available for hardness, interest in the automotive industry. Chapter 13
mechanical properties, fatigue response, and, in summarizes current FSW applications. It is
some cases, fracture toughness and fatigue crack anticipated that the number of applications will
propagation. Chapter 5 provides a ready refer- grow rapidly as fabricators learn the ease of
ence to identify what properties can be expected application and property benefits attributable to
following FSW. Although the database is not as FSW. Chapter 14 presents an outgrowth of FSW,
extensive, Chapter 6 presents microstructure and that is, friction stir processing (FSP). Because of
properties of ferrous and nickel-base alloys. the creation of a fine grain micro structure and the
With the development of high-temperature tool- ability to eliminate casting defects, FSP offers
ing, that is, polycrystalline cubic boron nitride the ability to locally tailor properties within a
tools, FSW is rapidly expanding into the welding structure such that the structure can survive bet-
of high-temperature alloys, and considerable ter in its environment. For example, by applying
growth is anticipated in this area. Chapter 7 con- FSP, local properties can be improved, such as
tinues the theme of high-temperature FSW but abrasion resistance, strength, ductility, fatigue
for titanium alloys. Titanium alloys offer unique life, formability, and superplasticity. Friction stir
difficulties, and although the available data are processing is a growth technology that may be-
limited at this time, there is considerable interest. come as important as FSW. Lastly, FSW and FSP
The challenge to identify long-life tooling to fric- are essentially new technologies not much be-
tion stir weld titanium alloys remains, but early yond their infancy. The growth potential for the
results illustrate the metallurgical potential future can be considerable. Chapter 15 offers the
to apply FSW. Copper alloys (~1000 °C, or authors’ thoughts on technology gaps to be over-
1830 °F) are intermediate in FSW temperature come to accelerate growth as well as some specu-
be tween aluminum alloys (~500 °C, or 930 °F) lation on future opportunities and applications.
and ferrous alloys (~1100 to 1200 °C, or 2010 to Interest and Growth in FSW. The field of
2190 °F). Considerable FSW success has already FSW has seen tremendous growth in the last ten
been demonstrated (Chapter 8), and because of years. Figure 1.4shows the increase in publica-
the intermediate temperature, different high-
temperature flow, and different physical proper-
ties such as thermal conductivity, different
lessons can be learned. Chapter 9 presents post-
FSW corrosion properties of aluminum alloys.
Compared to fusion welds, corrosion sensitivity
following FSW is always equivalent or less.
However, FSW does introduce local heat, creat-
ing 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 weld-
ing practice. Chapter 10 presents results from
computational modeling of FSW. Modeling
helps visualize fundamental behavior and allows
for comparison of flow and temperature response
Fig. 1.4 Significant increase in publications on friction stir
for different weld parameters and boundary con- welding/friction stir processing. This figure is based
ditions without performing costly ex periments on the Institute for Scientific Information Web of Science data-
base and does not include proceedings papers published in The
and subsequent evaluation. The advancement of
Welding Institute international symposiums and TMS annual
FSW out of the laboratory and into commercial meeting symposiums.
Chapter 1: Introduction / 5
tions in this field. This is a summary from the gel, R.A. Spurling, and C.C. Bampton,
Institute for Scientific Information Web of Sci- Scr. Mater.,Vol 36, 1997, p 69
ence database and does not include proceedings. 5. G. Liu, L.E. Murr, C.S. Niou, J.C. Mc-
The first international symposium was held at Clure, and F.R. Vega, Scr. Mater.,Vol 37,
Rockwell Science Center and was organized by 1997, p 355
TWI in 1999. From that time, many sympo- 6. K.V. Jata and S.L. Semiatin, Scr. Mater.,
siums have been organized, including three in Vol 43, 2000, p 743
TMS annual meetings, which have accompany- 7. S. Benavides, Y. Li, L.E. Murr, D.
ing proceedings. Brown, and J.C. McClure, Scr. Mater.,
Vol 41, 1999, p 809
8. P.L. Threadgill, TWI Bull.,March 1997
REFERENCES 9. L.E. Murr, Y. Li, R.D. Flores, and E.A.
Trillo, Mater. Res. Innov., Vol 2, 1998,
1. W.M. Thomas, E.D. Nicholas, J.C. Need- p 150
ham, M.G. Murch, P. Templesmith, and 10. Y. Li, E.A. Trillo, and L.E. Murr, J.
C.J. Dawes, G.B. Patent 9125978.8, Dec Mater. Sci. Lett.,Vol 19, 2000, p 1047
1991 11. Y. Li, L.E. Murr, and J.C. McClure,
2. C. Dawes and W. Thomas, TWI Bull.,Vol Mater. Sci. Eng. A, Vol 271, 1999,
6, Nov/Dec 1995, p 124 p 213
3. B. London, M. Mahoney, B. Bingel, 12. H.B. Cary, Modern Welding Technology,
M. Calabrese, and D. Waldron, in Pro- Prentice Hall
ceedings of the Third Int. Symposium on 13. C.J. Dawes and W.M. Thomas, Weld. J.,
Friction Stir Welding, Sept 27–28, 2001 Vol 75, 1996, p 41
(Kobe, Japan) 14. R.S. Mishra and Z.Y. Ma, Mater. Sci.
4. C.G. Rhodes, M.W. Mahoney, W.H. Bin- Eng. R,Vol 50, 2005, p 1
Friction Stir Welding and Processing Copyright © 2007 ASM International®
Rajiv S. Mishra, Murray W. Mahoney, editors, p 7-35 All rights reserved.
DOI:10.1361/fswp2007p007 www.asminternational.org
CHAPTER 2
Friction Stir Tooling:
Tool Materials and Designs
Christian B. Fuller, Rockwell Scientific Company
FRICTION STIR WELDING AND PRO- chapter uses two sections to examine the evolu-
CESSING (collectively referred to as friction tion of tool material and design since 1991. The
stirring) is not possible without the nonconsum- first section describes tool materials, including
able tool. The tool produces the thermomechani- the material characteristics needed for a tool
cal deformation and workpiece frictional heating material and a listing of published friction stir
necessary for friction stirring. A friction stir tool materials. The second section presents a
welding (FSW) butt joint is schematically illus- history of friction stir welding and processing
trated in Figure 1 in Chapter 1, “Introduction,” tool design, general tool design philosophy, and
and the same steps are necessary for friction stir associated tool topics.
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 2.1 Tool Materials
probe, and shoulder. Contact of the pin with the
workpiece creates frictional and deformational Friction stirring is a thermomechanical defor-
heating and softens the workpiece material; con- mation process where the tool temperature
tacting the shoulder to the workpiece increases approaches the workpiece solidus temperature.
the workpiece heating, expands the zone of soft- Production of a quality friction stir weld requires
ened material, and constrains the deformed mate- the proper tool material selection for the desired
rial. Typically, the tool dwells (or undergoes only application. All friction stir tools contain features
rotational motion) in one place to further increase designed for a specific function. Thus, it is unde-
the volume of deformed material. After the dwell sirable to have a tool that loses dimensional sta-
period has passed, the tool begins the forward tra- bility, the designed features, or worse, fractures.
verse along a predetermined path, creating a fine-
2.1.1 Tool Material Characteristics
grained recrystallized microstructure behind the
tool. Forward motion of the tool produces loads Selecting the correct tool material requires
parallel to the direction of travel, known as trans- knowing which material characteristics are im-
verse load; normal load is the load required for portant for each friction stir application. Many
the tool shoulder to remain in contact with the different material characteristics could be con-
workpiece. sidered important to friction stir, but ranking the
The initial aluminum FSW studies conducted material characteristics (from most to least
at The Welding Institute (TWI) used a cylindri- important) will depend on the workpiece mate-
cal threaded pin and concave shoulder tool rial, expected life of the tool, and the user’s own
machined from tool steel (Ref 2). Since that experiences and preferences. In addition to the
time, tools have advanced to complex asym - physical properties of a material, some practical
metric geometries and exotic tool materials to considerations are included that may dictate the
friction stir higher-temperature materials. This tool material selection.
8 / Friction Stir Welding and Processing
Ambient- and Elevated-Temperature Strength. Tool Reactivity. Tool materials must not
The candidate tool material must be able to with- react with the workpiece or the environment,
stand the compressive loads when the tool first which would change (generally in a negative
makes contact with the workpiece and have suffi- way) the surface properties of the tool. Titanium
cient compressive and shear strength at elevated is well known to be reactive at elevated tempera-
temperature to prevent tool fracture or distortion tures; thus, any reaction of titanium with the tool
for the duration of the friction stir weld. Cur- material will change the tool properties and alter
rently, predicting the required tool strength the joint quality. Environmental reactions of the
requires complex computational simulations, so tool (e.g., oxidation) could change the tool wear
typically, the strength requirements are based on resistance or even produce toxic substances (i.e.,
experience. At a minimum, the candidate tool formation of MoO ). These environmental reac-
3
material should exhibit an elevated- (workpiece tions can be mitigated with cover gases, but these
solidus temperature) temperature compressive can add complexity to the welding system. The
yield strength higher than the expected normal workpiece can also exhibit environmental reac-
forces of the tool. tions; in the case of titanium alloys, a cover gas is
Elevated-Temperature Stability. In addi- needed to prevent workpiece oxidation.
tion to sufficient strength at elevated tempera- Fracture Toughness. Tool fracture tough-
ture, the tool must maintain strength and dimen- ness plays a significant role during the tool
sional stability during the time of use. Creep plunge and dwell. The local stresses and strains
(and creep fatigue) is a consideration for long produced when the tool first touches the work-
weld lengths, where poor creep resistance piece are sufficient to break a tool, even when
would change the tool dimensions during weld- mitigation methods are used (pilot hole, slow
ing. Tool materials that derive their strength plunge speed, and preheating of the workpiece).
from precipitates, work hardening, or transfor- It is generally accepted that the tool plunge and
mation hardening have defined maximum-use dwell periods produce the most damage to a tool
temperatures. Tools used above the maximum- (Ref 4). The friction stir machine spindle run-
use temperatures will, in time, exhibit a de - out (lateral movement during spindle rotation)
crease in mechanical properties. The change in should also be considered when selecting a tool
mechanical properties is due to overaging, ma terial. Low-fracture-toughness tools, for ex -
annealing and recovery of dislocation substruc- ample, ceramics, should only be used in friction
tures, or reversion to a weaker phase. In friction stir machines that contain low spindle runout
stirring, these microstructural changes will (less than 0.0051 mm, or 0.0002 in.) to avoid
weaken the tool and either change the tool shape premature tool fracture.
or fracture the tool. Thermal fatigue strength Coefficient of Thermal Expansion (Bi -
should be considered when the friction stirring metal Tools). Thermal expansion is a consider-
tools are subjected to many heating and cooling ation in multimaterial tools. Large differences in
cycles (e.g., friction stir spot welding or short the coefficient of thermal expansion (CTE)
production welds). However, in most cases, between the pin and shoulder materials lead to
other tool material characteristics will cause either expansion of the shoulder relative to the
failure before thermal fatigue. pin or expansion of the pin relative to the shoul-
Wear Resistance. Excessive tool wear der. Both of these situations increase the stresses
changes the tool shape (normally by removing between the pin and shoulder, thus leading to
tool features), thus changing the weld quality tool failure.
and increasing the probability of defects. In fric- Additional consideration should be made
tion stirring, tool wear can occur by adhesive, when the pin and shoulder are made of one
abrasive, or chemical wear (which is addressed material, while the tool shank (portion of tool
subsequently as reactivity) mechanisms. The within the spindle) is a different material. One
exact wear mechanism depends on the interac- way to mitigate this situation is with a thermal
tion between the workpiece and tool materials barrier designed to prevent heat removal from
and the selected tool parameters. For example, in the tool into the shank. An example of this is
the case of polycrystalline cubic boron nitride used with PCBN tools where a thermal barrier
(PCBN) tools, wear at low tool rotation speed is prevents heat from moving into the tungsten
caused by adhesive wear (also known as scoring, carbide shank (Ref 5). The CTE differences
galling, or seizing), while wear at high tool rota- between the tool and workpiece are not found to
tion speed is caused by abrasive wear (Ref 3). have a significant influence on friction stirring.
Chapter 2: Friction Stir Tooling: Tool Materials and Designs / 9
Machinability. Many friction stir tools are This is because a majority of the published FSW
designed with features that must be machined, literature is on aluminum alloys, which are eas-
ground, or electrodischarged machined into ily friction stirred with tool steels. The advan-
the tool. Any material that cannot be processed tages to using tool steel as friction stir tooling
to the required tool design should not be material include easy availability and machin-
con sidered. ability, low cost, and established material char-
Uniformity in Microstructure and Den- acteristics. References cite AISI H13 (Ref 7, 8,
sity. Tool materials are not useful if there are 10–12, 14–16, 18–20, 24–27) more than any
local variations in microstructure or density. other steels. AISI H13 is a chromium-molybde-
These slight variations produce a weak region num hot-worked air-hardening steel and is
within the tool where premature fracture occurs. known for good elevated-temperature strength,
Powder metallurgical alloys are manufactured thermal fatigue resistance, and wear resistance.
with different densities, so friction stirring tools In addition to friction stir welding aluminum
should only be manufactured from a fully dense alloys, H13 tools have been used to friction stir
grade. weld both oxygen-free copper (Cu-OF) and
Availability of Materials. A tool material phosphorus-deoxidized copper with high resid-
is not useful if a steady supply of tool material ual phosphorus (Cu-DHP) (Ref 25). However,
is not available. This is especially true in a pro- the limited travel speed in Cu-DHP would limit
duction environment, where production specifi- the production use of H13. Another study found
cations dictate the use of a specific material. that tool steel FSW tools could weld 3 mm (0.12
in.) thick copper, but 10 mm (0.4 in.) thick cop-
per filled the tool features and softened the tool
2.1.2 Published Tool Materials
steel, distorting the pin profile (Ref 28). Other
This section considers all of the published tool steels used for FSW tools include oil-
tool materials listed for friction stir welding and hardened 0-1 (Ref 13, 17, 29), D2 (Ref 30),
processing. The listed tool materials should not SKD61 (Ref 23), Orvar Supreme (Ref 31), and
be viewed as an exhaustive list, because many Divar (Ref 32). The maximum-use temperature
papers do not specify the tool material or claim of tool steels depends on the type of tool steel:
the tool materials are proprietary. In instances oil- and water-hardened tool steels can be used
where specific alloys are not cited, effort was up to 500 °C (930 °F); secondary-hardened tool
made to include the class of tool materials used. steels can be used up to 600 °C (1110 °F).
The exception is tool steels, where many papers Nickel- and Cobalt-Base Alloys. High-
cite tool steels but not the specific alloy. Table temperature nickel- and cobalt-base alloys were
2.1 is a summary of the current tool materials developed to have high strength, ductility, creep
used to friction stir the indicated materials and resistance, and corrosion resistance. These
thicknesses. These data are assembled from the alloys derive their strength from precipitates, so
indicated literature sources. the use temperature must be kept below the pre-
Tool Steels. Tool steel is the most common cipitation temperature (typically 600 to 800 °C,
tool material used in friction stirring (Ref 6–26). or 1110 to 1470 °F) to prevent precipitate over-
aging and dissolution. Nickel- and cobalt-base
alloys were initially designed for aircraft engine
components, so much is known about the alloys,
Table 2.1 Summary of current friction stir
and a reasonable supply exists. It is reasonable
welding tool materials
to assume that new alloys will improve the qual-
Thickness
ity and use temperature of nickel- and cobalt-
Alloy mm in. Tool material base alloys, thus providing additional alloys for
Aluminum alloys <12 <0.5 Tool steel, WC-Co friction stirring. Nickel- and cobalt-base alloys
<26 <1.02 MP159 can be difficult to machine, especially for the
Magnesium alloys <6 <0.24 Tool steel, WC
highly alloyed alloys. Several different nickel-
Copper and copper <50 <2.0 Nickel alloys, PCBN(a),
alloys tungsten alloys base alloys have been used to friction stir weld
<11 <0.4 Tool steel
copper alloys, including IN738LC, IN939 (Ref
Titanium alloys <6 <0.24 Tungsten alloys
Stainless steels <6 <0.24 PCBN, tungsten alloys 26), MAR-M-002, Stellite 12, IN-100, PM
Low-alloy steel <10 <0.4 WC, PCBN 3030, Nimonic 90, Inconel 718, Waspalloy (Ref
Nickel alloys <6 <0.24 PCBN
33), and Nimonic 105 (Ref 33, 34). Aluminum
(a) PCBN, polycrystalline cubic boron nitride alloys have been friction stirred with tools made
10 / Friction Stir Welding and Processing
from the cobalt-nickel-base alloy MP 159 (Ref for friction stirring tools: W (Ref 5), W-25%Re
14, 32, 35), which is readily machined. Figure (Ref 33, 39), Densimet (Ref 28, 33, 34, 41, 44,
2.1shows the ultimate tensile strength as a func- 47), and W-1%LaO (Ref 48). Tungsten-rhe-
2
tion of test temperature for selected nickel- and nium has a high operational temperature, but
cobalt-base alloy bars (Ref 36, 37). machining features require grinding (more diffi-
Refractory Metals. The refractory metals cult than conventional machining), and tung-
(tungsten, molybdenum, niobium, and tanta- sten-rhenium has a high cost. Densimet consists
lum) are used for their high-temperature capa- of small spheres of tungsten bound in a matrix
bilities (e.g., light bulb filaments) and high den- containing either nickel-iron or nickel-copper
sities (ballistic projectiles). Many of these combinations (Ref 49). Figure 2.2demonstrates
alloys are produced as a single phase, so that the matrix of Densimet lowers the opera-
strength is maintained to nearly the melting- tional temperatures (relative to other tungsten-
point temperature. Therefore, refractory metals base alloys). However, in contrast to other tung-
are among the strongest alloys between 1000 sten-base alloys (i.e., tungsten-rhenium),
and 1500 °C (1830 and 2730 °F). However, tan- Densimet is readily machined by conventional
talum and niobium have high solubility of oxy- methods and has a lower raw material cost. The
gen at elevated temperatures, which quickly high thermal conductivity of Densimet has been
degrades the ductility. The drawbacks to using cited as a reason to use this material for the
refractory metals include limited material avail- shoulder of FSW tools (Ref 33, 34) used to weld
ability, long lead times, cost, and difficult 50 mm (2 in.) thick copper. Another tungsten-
machining (typically involving grinding base alloy is W-1%LaO (Ref 48), which has
2
processes). Powder processing is the primary the cost and machinability of Densimet but the
production method for refractory alloys. Occa- temperature range of tungsten-rhenium tools.
sionally, partially dense powder-processed The ultimate tensile strength temperature
material is manufactured, which produces a dependence of tungsten (Ref 50), W-27%Re
friction stir tool that easily fractures. Thus, care (Ref 51), Densimet (Ref 49), and W-1%LaO
2
must be taken to ensure that the raw material is (Ref 52) is shown in Fig. 2.2.
fully dense before machining. Friction stir tools were also made from molyb-
Tungsten-base alloys have been used in the denum-base alloys (Ref 4, 33). Cederqvist
friction stirring of copper alloys, nickel- examined four molybdenum-base alloys to fric-
aluminum bronze, titanium alloys, and steels tion stir weld up to 50 mm thick copper plates
(Ref 4, 15, 26, 28, 33, 34, 38–48). The FSW of (Ref 33). However, none of the alloys survived
1018 steel (Ref 4) and ultrahard 0.29C-Mn-Si- the plunge sequence and remained dimension-
Mo-B 500 Brinell steel (Ref 40) caused tool ally unchanged after a 1 m (3 ft) long weld.
wear on tungsten alloy FSW tools. Four tung- Carbides and Metal-Matrix Composites.
sten-base materials have been specifically cited Carbides (or cermets) are commonly used as
Fig. 2.2 Elevated-temperature tensile properties for W, W-
Fig. 2.1 Elevated-temperature tensile properties for select 27%Re, Densimet D175, and W-1%LaO. Source:
2
nickel- and cobalt-base alloys. Source: Ref 36, 37 Ref 49–52
Chapter 2: Friction Stir Tooling: Tool Materials and Designs / 11
machining tools due to superior wear resistance Direct Comparison of Tool Materials.
and reasonable fracture toughness at ambient Only a handful of published studies have exam-
temperatures (especially when compared to ined the effect of different tool materials on
other ceramics). Because they are made for FSW. Midling and Rorvik (Ref 31) examined
machining tools, carbides perform well at ele- how weld heat input changed with different tool
vated temperatures. Friction stirring tools made shoulder materials using 6 mm (0.25 in.) thick
from tungsten carbide are reported to have 7109.50-T79 Al friction stir welds. To perform
smooth and uniform thread surfaces for the FSW this task, they constructed a tool shank made of
of 6061 Al (Ref 10). The superior wear resistance titanium, into which hardened tool steel (Orvar
of WC-Co allows threadless pins to friction stir Supreme) pin and tool shoulder inserts were
weld 5 mm (0.2 in.) thick AC4A (aluminum- placed. Shoulder inserts consisted of Inconel
silicon alloy) + 30 vol% SiC with little wear (Ref 718, Nimonic 105, a zirconia engineering ce -
53). However, severe wear is observed when the rami c, 94%WC + 6%Co, and a Ni (Si,Ti,Cr)
3
tools contain threads. The high-temperature intermetallic. All the metallic tool materials
strength of WC and WC-Co tools was used to behaved similarly to the reference tool steel
weld interstitial-free steel (Ref 23) and carbon except at the slowest welding speed (5 mm · s–1,
S45C steel to 6064 Al (Ref 54, 55). or 0.2 in. · s–1), where all the tool materials exhib-
Metal-matrix composites using TiC as the ited better heat generation than the reference tool
reinforcing phase have also been used as tool steel. However, the zirconia ceramic insert pro-
materials for copper alloys (Ref 26). Both sin- duced 30 to 70% more heat than the reference
tered TiC:Ni:W and hipped TiC:Ni:Mo alloys Orvar Supreme tool steel. The higher heat input
were used to friction stir copper alloys. How- allowed the tool travel speed to increase from 12
ever, both TiC-containing alloys produced brit- to 18 to 30 mm · s–1(0.5 to 0.7 to 1.2 in. · s–1), just
tle tools that fractured during the tool plunge. by changing the tool shoulder material.
Cubic Boron Nitride. Polycrystalline cubic Savolanen et al. (Ref 25) examined how dif-
boron nitride was originally developed for the ferent tool materials were able to friction stir
turning and machining of tool steels, cast irons, weld four different 10 to 11 mm (0.40 to 0.43 in.)
and superalloys. Recently, PCBN has gained thick copper alloys: Cu-OF, Cu-DHP, aluminum
acceptance as a friction stir tool material, espe- bronze, and Cu-25%Ni. The evaluated tool mate-
cially for high-temperature alloys (Ref 3, 5, 26, rials included H13 tool steel, IN738LC, IN939,
33, 40, 44, 56–69). The PCBN was chosen as a IN738LCmod, sintered TiC:Ni:W (2:1:1), hipped
friction stir tool based on its prior success in TiC:Ni:Mo (3:2:1), pure tungsten, and PCBN.
extreme machining applications. The manufac- Tool steel (H13) and nickel-base alloy tools were
turing of PCBN occurs via an ultrahigh-temper- only suitable for Cu-OF and CU-DHP, but the
ature/high-pressure process, where the extreme welding speeds with H13 tools were quite low.
temperatures and pressures limit the size of Both of the TiC-base alloys were too brittle, and
PCBN that can be produced. Only the shoulder the tungsten tools worked for only Cu-OF and
and pin of the tool are produced from PCBN; the Cu-DHP (a tungsten-base alloy was postulated to
shank is made from tungsten carbide, and both produce better results, Ref 25). The PCBN was
are held together by a superalloy locking collar the only tool material to produce quality friction
(Ref 3, 58). The high tool costs (due to the stir welds in all four copper alloys.
extreme manufacturing methods) and the low Cederqvist studied 17 different tool materials
fracture toughness mean that care should be used to friction stir weld 50 mm thick copper (Ref 33),
with PCBN tools. The PCBN tools require a low and the first material evaluations were for use as
eccentricity spindle to minimize tool fracture. the tool pin. Tungsten carbide-cobalt pins pro-
Successful PCBN friction stir welds have been vided the initial welding parameter develop-
made with ferritic steels (Ref 5, 40, 54), dual- ment, but tool life issues (due to large spindle
phase steels (Ref 5, 65), austenitic stainless eccentricities) made this tool material impracti-
steels (Ref 5, 56, 59, 60, 63, 64, 67), type 430 cal for production. Likewise, eccentricity issues
stainless steel (Ref 5), 2507 super duplex stain- caused PCBN, alumino-silicate, and yttria-
less steel (Ref 5), class 40 gray cast iron (Ref 68), stabilized zirconium oxide pins to fail within the
nickel-base alloys (Ref 5), Narloy Z (Ref 5), Ni- plunge or dwell sequence of the friction stir
Al bronze (Ref 5), Invar (Ref 5), copper (Ref 26, welds. A majority of the pins manufactured from
33), sonoston (Ref 61), ultrafine-grained steels refractory metals (four molybdenum-base and
(Ref 62), and nitinol (Ref 44). three tungsten-base) did not have dimensional