Patent Publication Number: US-2010108742-A1

Title: Fracture resistant friction stir welding tools

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of U.S. Ser. No. 11/868,262 filed Oct. 5, 2007, which claims the benefit of U.S. provisional patent application No. 60/893,246 filed on Mar. 6, 2007, each of which is incorporated by reference herein in their entireties for all purposes. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to friction stir welding tools and, more particularly, the present disclosure relates to friction stir welding tools having fracture resistant/stress reducing features. 
     BACKGROUND 
     The friction stir welding (FSW) process is a solid-state based joining process, which makes it possible to weld a wide variety of materials (e.g., aluminum, copper, stainless steel) to themselves and to weld various combinations (e.g., aluminum alloys 6xxx/5xxx, 2xxx/7xxx) to each other. The process is based on plunging a rotating friction stir welding tool into the joining area. The rotating friction stir welding tool heats the workpiece(s) by friction, and thus the material becomes plasticized and flows around the axis of the tool due to shear caused by the rotating tool. 
     Conventional friction stir welding tools typically include a threaded pin, a shank and a shoulder having an engaging surface. The shank is for gripping in a chuck or collet of a friction stir welding machine so that tool can be rotated. While the tool is rotating, the pin is pressed and plunged into the joint area between the workpiece(s) which is/are to be welded. Friction between the workpiece(s) and pin causes the material of the workpiece(s) to become heated to its softening temperature and thus becomes plasticized. Pressure between the pin and the plasticized workpiece(s) causes the pin to be plunged into the workpiece(s). Friction between the pin and the workpiece(s) may cause plasticized workpiece material to flow about and around the axis of the pin allowing welding to occur without melting. 
     SUMMARY 
     In view of the foregoing, a broad objective of the present disclosure is to produce improved friction stir welding tools. A related objective is to increase the fracture resistance of friction stir welding tools, such as when the tools are under cyclic fatigue loading during welding. A further related objective is to decrease the failure rate of friction stir welding tools that include an internal tension member. Another objective is to facilitate friction stir welding at higher operational speed and temperatures to facilitate welding of thick and/or strong and/or hard alloys and other materials. 
     In addressing one or more of the above objectives, a friction stir welding tool comprising a hollow body interconnected with, but decoupled from, an internal tension member may be used to eliminate or reduce the transfer of torsion forces from the pin to the tension member. In one embodiment, the tension member is decoupled from the body and/or pin of the friction stir welding tool via one or more decoupling members. The decoupling member may act as a swivel to restrict, and in some instances eliminate, the transfer of torsion forces from the body/pin of the friction stir welding tool. In one embodiment, the decoupling member comprises a thrust bearing (e.g., thrust ball-bearing; a high temperature thrust bearing material) located at or near a distal end of the tool body. Other decoupling members or materials may be used, such as various other bearing types (e.g., oil bearings, hydraulically driven bearings). Lubricants, such as dry lubricating powders (e.g., molybdenum-containing powders) may be applied between the tension member and the internal bore of the body/pin of the friction stir welding tool, thereby facilitating rotational and axial movement of the tension rod relative to the pin along a common axis. 
     In one embodiment, one or more spring members may be utilized to provide an axial force (e.g., a spring force) relative to the tension member, thereby axially tensioning the tension member and thus compressing the pin of the friction stir welding tool. In one embodiment, the spring members may also dampen tension variations experienced by the tension member due to interactions with the pin and/or due to temperature variations. The spring members may comprise one or more springs (e.g., disk springs) and may thus act as a bellows. 
     In some instances, hoop-type stresses induced in the pin by the shoulders of the internal tension member may be reduced by utilizing a non-linear interface/transition between the pin and the tension member shoulder. In one embodiment, the tension member shoulder includes at least one rounded portion for engagement with a corresponding rounded portion of the pin. In one embodiment, both the tension member shoulders and the corresponding internal pin shoulders include rounder portions with a gap therebetween. Thus, hoop-type stresses at the pin and tension member shoulder interfaces may be reduced. 
     In some instances, hoop stresses may be reduced by utilizing a pin having a larger diameter middle portion relative to the diameter of the base portion of the pin. In one embodiment, the pin diameter progressively decreases from the middle portion of the pin toward the base portion of the pin. Thus, the middle portion may be a bulging portion with increased surface area, thereby inducing a stress distribution in this region, which may reduce tension-type hoop stresses. This tapered diameter concept (e.g., larger middle diameter progressing to smaller base diameter) may also intensify the compression loading at the base of the pin, thereby reducing tensile stresses in this region. In other instances, a pin having a constant diameter from a middle portion to a base portion may be used (e.g., with high-strength tension members, described below). 
     In some instances, the tension member and the pin may comprise differing materials. In one approach, the tension member may employ metal alloys. The metal alloys may include fastener alloys and/or superalloys. In one embodiment, the metal alloy is a cobalt-based alloy. In another embodiment, the metal alloy is a steel-based alloy. In another approach, the tension member may comprise composite materials. In one embodiment, the composite materials include ceramics. The ceramics may include, for example, tungsten-based ceramics and materials including organic or carbon fibers. Since the tensile strengths of these materials may be significantly greater than the pin material (e.g., not less than about 500,000 ksi for a composite material compared to about 220 ksi for the pin material), the compression forces applied to the pin via the composite tension member may be significantly greater than the forces applied to the pin via the use of a tension member that is made of the same material as the pin. In turn, pin diameter may be decreased, and more durable pins may be produced. Smaller diameter pins may also afford higher welding speed of travel. Furthermore, the composite materials may have a higher temperature resistance, thus facilitating operation of the friction stir welding tool at higher temperatures. 
     The tension member may thus comprise bundles of composite type materials (e.g., a plurality of fibers), bars and/or rods and end-anchored cylinders that are produced (e.g., preformed, adhesively bonded, molded, cured, machined) with interconnection features that may be utilized to interconnect the tension member to the pin (e.g., via the rounded portions, described above) and/or the body of the friction stir welding tool. With respect to ceramic tension members, the ceramics may be anchored to the tool via any suitable anchor, such as complementary mechanical features (e.g., hooks/holes, dimples/recesses, tongue/groove) or via chemical bonding (e.g., superadhesives). In one embodiment, coolants may be provided to one or more of the tension member and/or pin during welding to assist in maintaining the integrity of those components. 
     In one embodiment, a composite tension member comprises a plurality of high-strength fibers (e.g., organic or carbon fibers) capable of twisting or rotational movement along a common axis within the bore of the body and/or pin of the friction stir welding tool as the tool operates. In this embodiment, the above-referenced decoupling member may not be needed as the plurality of fibers will eliminate or reduce the risk of breaking the torsion member due to transfer of torsion forces from the pin to the tension member. 
     In some instances, irrespective of the use of a monolithic pin (e.g., when utilizing a conventional friction stir welding tool) or a hollow pin (e.g., when utilizing a friction stir welding tool comprising a tension member), fracture resistance may be increased by utilizing a pin that includes at least one threadless band, which is located at the “base” of the pin next to the shoulder of the tool. The use of a threadless band may reduce stress-rising effects from the threads of the pin. This threadless band may be positioned about the pin at strategic locations to reduce pin failure at high fracture prone areas. In one embodiment, a threadless band is positioned proximal a shoulder portion of the tool, near the transition between the pin and the shoulder (e.g., at the base of the pin, next to the tool shoulder). In one embodiment, the threadless band has a width of at least about 2 mm. In one embodiment, the threadless band has a width of not greater than bout 8 mm. 
     In some instances, irrespective of the use of a monolithic pin (e.g., when utilizing a conventional friction stir welding tool) or a hollow pin (e.g., when utilizing a friction stir welding tool comprising a tension member), fracture resistance may be increased via threads that have a relatively high radius to depth ratio (r/d). The use of relatively high radius to depth ratios may reduce stress rising effects of the threads. In one embodiment, the radius to depth ratio is constant over the surface of the pin. In another embodiment, the radius to depth ratio progressively increases (e.g., linearly increases; exponentially increases) from a first portion of the pin toward a second portion of the pin. In one embodiment, the radius to depth ratio progressively increases from a middle portion of the pin toward a base portion of the pin. 
     In another approach, the pin may include threaded segments and bare portions. For example, the pin may include a plurality of segmented regions, some of which include threads and some of which do not include threads (e.g., bare portions or threadless band). The threaded segments may be spaced about the surface of the pin, with the bare portions separating the threaded segments from one another. In one embodiment, the pin includes three separate threaded segments spaced about the surface of the pin and separated by three bare portions. In one embodiment, the pin includes four separate threaded segments spaced about the surface of the pin and separated by four bare portions. In one embodiment, the threaded segments are spaced equidistance from one another, separated by bare portions. Each of the threaded segments may include the same thread pattern/orientation as the other threaded segments, or one or more of the threaded segments may include differing thread patterns. Hence, a first threaded segment may include a first thread pattern, and a second threaded segment may include a second thread pattern, the second thread pattern being different than the first thread pattern. In one embodiment, conventional uni-directional threads may be used for one or more of the threaded segments. In another embodiment, r-threads (e.g., left-hand, right-hand, horizontal) may be used for one or more of the threaded segments. One or more of the threaded segments may include one or more other surface features, such as dimples, intermittent grooves, or localized multi-faceted walls, to name a few. The bare portions are generally substantially bare of features (e.g., are substantially smooth) and can have a radius or round contour similar to the adjacent threaded sections or flat. The bare portions are approximately spaced every 90° to 120° apart. The use of threaded segments and bare portions may reduce the force(s) (e.g., Fz and Fx) and torque on the pin during welding, and may facilitate improved control over flow, fill-up and consolidation of the plasticized region about the pin. Extended pin lifetime may further be witnessed. 
     In one embodiment, the pin includes four threaded segments spaced equidistance from one another separated by bare portions. A first one and third one of these threaded segments may include a first threaded pattern (e.g., a right-hand pattern) and a second one and a fourth one of these threaded segments may include a second threaded pattern (e.g., a left-hand pattern). The first and third threaded segments may be on opposing sides of the pin and adjacent to bare portions. Likewise, the second and fourth threaded segments may be on the other opposing sides of the pin and adjacent bare portions. 
     In one embodiment, a friction stir welding tool generally includes a body, a pin, a tool shoulder, a tension member and, optionally, an end assembly. The body may define a cavity for receiving at least a portion of a tension member. The body may include a shank/grip for engagement with a chuck or collet of a friction stir welding machine. The end assembly comprises one or more of the above-described decoupling members and/or spring members. A distal end portion of the tension member may be interconnected with the end assembly (e.g. via a mechanical interface), which upon loading the tension member under tension may provide axial compressive force onto the tool&#39;s pin. A proximal end portion of the tension member may be interconnected with the pin (e.g., via transitions) and thus the pin may be axially compressed due to engagement of the tension member with the end assembly. Hence, cyclic tensile stresses due to bending moments on the pin as it rotates may be reduced. The tension member may comprise one or more of the above-described tension member related features (e.g., non-linear shoulder for interfacing with the pin). The pin may comprise one or more of the above-described pin-related features (e.g., linear tapered pin, bulging middle portion, segregated threaded portions, and non-linear internal transition for interfacing with the non-linear shoulder of a tension member). In one embodiment, a proximal end of the pin is contiguous with the working surface of the shoulder portion of the pin and shoulder. The tool shoulder portion may include a scrolled working surface for engaging at least one surface of the workpiece(s) to prevent plasticized material from flowing out of the plasticized region formed about and around the pin. 
     Various benefits may be evidenced via the inventive friction stir welding tools. For instance, the friction stir welding tools may be capable of welding materials that generally cannot be welded using conventional friction stir welding techniques. Materials requiring high weld temperatures and/or high toughness and/or high strengths may be welded using the improved friction stir welding tools. The friction stir welding tools may also facilitate welding of thicker sections of materials (e.g., a thickness of at least about 43 millimeters with a 7085 alloy). The friction stir welding tools may also facilitate faster welding speed, thereby increasing productivity and producing stronger welds due to the lowered heat inputs required per linear length. The friction stir welding tools may be utilized with numerous alloys and with numerous material thicknesses, thus reducing the number and types of apparatus required to complete welding operations. Tool life may be significantly extended, such as when welding tougher and stronger materials and/or thick sections of materials. Thus, the friction stir welding tools may be more cost effective. 
     As may be appreciated, various ones of the inventive features provided above may be combined in various manners to yield various friction stir welding tools. These inventive features may be utilized with conventional anvil-based tools, or with bobbin-type tools. Fixed and self-adjusting bobbin tools with multiple shoulders may be employed with any of the above-described features for simultaneously welding multiple parallel walls. Furthermore, the above inventive concepts do not generally require a redesign of the tool shoulder and/or compression sleeve. Hence, the tool shoulder may be any of a suitable configuration, such as a smooth configuration or a scrolled configuration with concentric rings or spiraled ridges, to name a few. These and other aspects, advantages, and novel features of the disclosure are set forth in part in the description that follows and will become apparent to those skilled in the art upon examination of the following description and figures, or may be learned by practicing the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1   a  is a perspective view illustrating one embodiment of a friction stir welding tool useful in accordance with the present disclosure; 
         FIG. 1   b  is a close-up, perspective view of the pin of the friction stir welding tool of  FIG. 1   a;    
         FIG. 1   c  is a cross-sectional side view of the friction stir welding tool of  FIG. 1   a;    
         FIG. 1   d  is a close-up, cross-sectional view of the interface between the tension member shoulder and the internal pin shoulder of  FIG. 1   c;    
         FIG. 1   e  is a perspective view of the tension member of  FIGS. 1   a - 1   d;    
         FIG. 1   f  is an exploded view of the end assembly of the friction stir welding tool of  FIGS. 1   a  and  1   c;    
         FIG. 1   g  is a side view of the friction stir welding tool of  FIGS. 1   a  and  1   c;    
         FIG. 1   h  is a side view of the pin of the friction stir welding tool of  FIGS. 1   a - 1   d  and  1   f - 1   g;    
         FIG. 1   i  is a close-up, cross-sectional view of the pin of the friction stir welding tool of  FIGS. 1   a - 1   d  and  1   f - 1   h;    
         FIG. 1   j  is an illustration of the threaded radius to depth dimensions; 
         FIG. 2   a  is a first side view of another embodiment of a pin useful with a friction stir welding tool; 
         FIG. 2   b  is a second side view of the pin of  FIG. 2   a;    
         FIG. 2   c  is a bottom view from the proximal end of the pin of  FIGS. 2   a - 2   b;    
         FIG. 3   a  is a side view of one embodiment of a friction stir welding tool having a transitioning shoulder assembly; 
         FIG. 3   b  is a cross-sectional, side view of the friction stir welding tool of  FIG. 3   a;    
         FIG. 4  is a cross-sectional side view of a bobbin-type friction stir welding tool; 
         FIG. 5  is a cross-sectional, side view of a case for transporting a friction stir welding tool; 
         FIG. 6  is a cross-sectional side view of one embodiment of a friction stir welding tool having a monolithic body; 
         FIG. 7  is a cross-sectional side view of one embodiment of a friction stir welding tool having a tapered tool shoulder; 
         FIG. 8  is a cross-sectional side view of one embodiment of a friction stir welding tool having a monolithic body and a tapered tool shoulder; 
         FIG. 9  is a side view of one embodiment of a friction stir welding tool having monolithic body with a straight tapered pin; and 
         FIG. 10  are side and cross-section views of another embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the accompanying drawings, which at least assist in illustrating various pertinent embodiments of the present disclosure. For this application, monolithic is defined to describe a component that is made or formed into or from a single item and not from multiple parts; integral is defined as consisting or composed of parts that together constitute a component; follow is defined as having a cavity, gap, or space within, nest is defined as fitting snuggly together or within another or one another; and steady state condition is defined as thermal and mechanical stresses have stabilized and there are no significant variations of same over time. 
     The present disclosure can be illustrated in many embodiments including those shown in  FIGS. 1   c  and  10 . For convenience, the detailed disclosure will profile the embodiment  10  illustrated in  FIG. 1   c . Common features between embodiment  10  and embodiment  100  shown in  FIG. 10  are the same. It should be understood that the description (including torsional load path and stresses) that follows for embodiment  10  is also applicable to embodiment  100  and other embodiments contemplated but not shown herein. 
     Referring now to  FIGS. 1   a ,  1   c , and  1   e , one embodiment of a friction stir welding tool  10  comprises a body  20  interconnected with a pin portion  30 , a tool shoulder  40 , a tension member  50 , and an end assembly  60 . The tension member  50  has a length L 1  and can be disposed within an internal bore  21  of the body  20  having length L 1  and extends therethrough. The tension member  50  is interconnected to the pin portion  30  via transitions  41  disposed near the proximal end  80  of the pin portion  30 , as described in further detail below with respect to  FIG. 1   d . The end assembly  60  interconnects with and puts the tension member  50  in tension relative to body  20 , as described in further detail below, thereby creating a closed-loop torsional load path or circuit. The end assembly  60  may include at least one decoupling member  62 , described in further detail below, that facilitates decoupling of one end of the tension member  50  from the portion of the friction stir body  20  that directly cooperates with the drive system (not shown) of the friction stir welding machine (not shown) that induces the rotational speed (defined herein as input rotational speed and used synonymously with input torque) on to body  20  of the friction stir welding tool  10 . The decoupling member  62  breaks or disengages the closed-loop circuit to relieve torsional load on the tension member  50 . 
     One embodiment of a friction stir welding tool body  20  includes a friction stir welding machine drive system interface  24 , such as grip portion as shown in  FIG. 1   a , capable of cooperation with a friction stir welding machine drive system (not shown) to apply an input rotational speed onto the friction stir welding tool body  20 . The pin portion  30 , which is adjacent and rigidly coupled to the friction stir welding machine drive system interface  24 , will rotate at the same rotational speed or torque as the input rotational speed at steady state conditions prior to initiation of the friction stir welding operation. However, after pin portion  30  is plunged into a joint to be welded, there is torsional resistance on the pin, which is caused by the shear stresses between the plasticized material and the pin as a result the rotational speed (defined herein as output rotational speed and used synonymously with output torque) of the pin portion  30  can decrease as a result of resistance of the joint. Therefore, the output rotational speed can be less than the input rotational speed as the pin portion  30  plasticizes the material in the joint to be friction stir welded. 
     Now turning to  FIG. 1   e , one embodiment of the tension member  50  includes a proximal end portion  52  and a distal end  54 . As disclosed above, proximal end  52  can be interconnected or fixedly coupled to the pin portion  30  to induce a compressive load thereon. The proximal end  52  rotates at substantially the same rotational speed as the pin portion  30  before, during, and after the friction stir welding operation. Distal end  54  can be operably connected to, via end assembly  60 , with distal end  25  of body  20 , which is located in close proximity to the friction stir welding machine drive system interface  24  (see  FIG. 1   c ). Prior to disengagement distal end  54  has substantially the same rotational speed as the friction stir welding machine drive system interface  24 . During the friction stir welding (FSW) operation when the output rotational speed is less than the input rotational speed, an angular displacement of the distal end  54  relative to the proximal end  24  may occur, which induces a torsional stress within tension member  50 . This occurs because distal end  54  rotates at the input rotational speed and the proximal end  52  rotates at the output rotational speed, which may be different during FSW operation. A decoupling member  62  can be independently and operatively connected to the distal end  54  of the tension member  50  and the friction stir welding machine drive system interface  24  to decouple the distal end  54 , for example, from body  20  in proximity to the source of input rotational speed. Other physical embodiments that result in decoupling the tension member  50  from the input source are contemplated herein. One such embodiment is decoupling member  62  capable of relative movement or slip to decouple the distal end  54  of the tension member  50  from body  20  in proximity to the friction stir welding machine drive system interface  24  when a predetermined torsional value or stress is exceeded, for example, at a decoupling member interface  43 ,  45  ( FIG. 1   c ) with either the decoupling retainer  63  or distal end  25  of body  20 , respectively. The predetermined torque value or stress can be determined by a normal force and a coefficient of friction at the decoupling member interface  43 ,  45 . Thereby, the torsional stress within the tension member  50  caused by the angular displacement is reduced or eliminated when the decoupling member  62  effectively decouples or disengages the distal end  54  of the tension member  54  from the friction stir weld machine drive interface  24 . 
     The physical interaction of the above components can be described in terms of torsional load path. As illustrated in  FIGS. 1   c  and  1   f , the above embodiment illustrates a torque release mechanism (decoupling member  62 ) that is not in the direct load path between the input drive source (friction stir welding machine drive system interface  24 ) and the output work tool (pin portion  30 ). This embodiment allows for flexibility in locating the torque release mechanism away from spatial constraints associated between the input drive source and the output work tool. For example, the torsional load path starts at the friction stir welding machine drive system interface  24  that is operably connected to the friction stir weld drive system (not shown) and rotates the entire tool  10  at a predetermined input rotational speed or torque when the tool  10  is not under load (no load mode). The three above named features rotate in unison until the pin portion  30  plunges into the joint to be welded and encounters resistance from the joint (load mode). Since the distal end  25  of body  20  is in close proximity to the friction stir welding machine drive system interface  24 , distal end  25  of body  30  rotates at substantially the same rotational speed and load conditions as friction stir welding machine drive system interface  24 . The torsional load realized by these features is negligible at steady state conditions prior to commencement of the friction stir welding operation (no load mode). When the pin portion  30  plunges into the joint, the rotational speed of the pin portion  30  decreases while the rotational speed of the other above named features stays substantially the same. This action creates a torsional load path that travels from the friction stir welding machine drive system interface  24  to the pin portion  30 . (Note that the input drive source is between the torque release mechanism and the output work tool.) This results in an angular displacement between the proximate end  52  and distal end  54 , which results in a torsional stress. The torsional load path travels from the pin portion  30  to the proximate end  52  of tension member  50  and continues to run the entire length of the tension member  50  to distal end  54 , which is operably connected to the friction stir welding machine drive system interface  24  through the decoupling member  62 , thereby completing the load path at the decoupling interfaces  43 ,  45 . The intimate relationship of the components of the end assembly  60 , discussed in detail below, results in no relative movement or slip therebetween while conditions are below the predetermined torque or stress value. Once the torque or stress value exceeds the predetermined value, the decoupling member  62  will slip or decouple at either decoupling interface  43  or  45  and interrupt or break the load path. 
     Now turning to  FIGS. 1   a  and  1   c , one embodiment of body  20  generally comprises a monolithic member having an axial bore  21  having inner diameters ID 1  and ID 2  extending through the longitudinal axis A for an entire length L 1  of the body  20  for receiving the tension member  50 . Body  20  further includes proximal end  23  and distal end  25 . The body  20  generally further includes friction stir welding machine drive system interface  24 , such as a grip portion in the form of a cutout of the outer diameter, for facilitating grip of the friction stir welding tool  10  by a corresponding chuck or collet of a friction stir welding tool machine (not shown) having a drive system to induce the input rotational speed or torque. The body  20  may be made of any suitable material, such as, for example, cobalt or carbon-based steels. The body  20  further generally includes at least one set of complementary engaging features  22  (such as external threads) for receiving the complementary engaging features  42  (such as internal threads) of the tool shoulder  40  for facilitating interconnection of the tool shoulder  40  with the body  20 . The pin portion  30  may be a portion of the monolithic body  20 , as shown in  FIG. 1   c , at the proximal end  23  of body  20 . In other embodiments, the pin may be a separate component that is interconnected to the body  20  via complementary engaging features to form an integral body/pin component. The dimensions of the body  20 , pin portion  30 , tool shoulder  40  and tension member  50  are generally application specific, and are dependent upon, for example, thickness, hardness and strength of the materials to be welded. The decoupling member  62  is disposed between the distal end  25  of the body  20  and the distal end  54  of the tension rod  50 , wherein the decoupling member  62  inhibits or counters relative rotational or torsional movement along the common axis A of the tension member  50  with respect to the body  20  when an applied torque is below a predetermined torque value. 
     Referring now to  FIGS. 1   h  and  1   i , pin portion  30  generally comprises a plurality of external threaded segments or longitudinal portions  32  (hereinafter referred to as threaded sections  32 ) separated from one another by bare portions or threadless sections  34 . The bare portions  34  are generally substantially bare of features (e.g., are substantially smooth) and can have a radius or round contour similar to the adjacent threaded sections or flat. The bare portions  34  are approximately space every 90° to 120° apart. The threaded segments  32  are located about the outer surface  43  of the pin portion  30 . In the illustrated embodiment, the threaded segments  32  comprise right-hand threads. However, other threaded configurations may be utilized. For example, one or more of the threaded segments  32  may comprise a left-handed and/or a horizontal threaded portion, such as illustrated and described below with respect to  FIGS. 2   a - 2   c , or a combination thereof. The number, and size/dimensions of the threads and threaded segments  32  is generally application specific. 
     Now turning to  FIG. 1   j , the threads of the threaded portions  32  generally comprise a high radius (R) to depth (D) ratio. In one embodiment, the radius to depth ratio is constant throughout the threaded portions  32 . In another embodiment, the radius to depth ratio is different for various threads of the threaded portions  32 . In one embodiment, a first threaded portion comprises a first radius to depth ratio, and a second thread portion comprises a second radius to depth ratio, the second radius to depth ratio being different than the first radius to depth ratio. In one embodiment, the radius to depth ratio of at least some of the threads progressively increases as the threads proceed from a middle portion of the pin portion  30  towards the distal end  81  of the pin portion  30 . In one embodiment, the radius to depth ratio linearly progressively decreases. In another embodiment, the radius to depth ratio non-linearly progressively decreases (e.g., exponentially progressively decreases). The use of relatively high radius to depth ratios and/or progressively changing radius to depth ratios may reduce stress rising effects of the thread on the pin portion  30 , which may extend tool life. The radius to depth ratio is generally application specific. 
     Referring now to  FIGS. 1   c ,  1   d , and  1   e  as noted above, transitions  41  may be utilized to interconnect the tension member  50  to pin portion  30  of the body  20  of the friction stir welding tool  10 . In one embodiment, and with reference to  FIG. 1   d , the transitions may comprise non-linear and complementary engaging surfaces of the pin portion  30  and the tension member  50 . In the illustrated embodiment, the transitions comprise complementary engaging portions  33 ,  53 . Thus, a smooth (e.g., non-abrupt) interface may be facilitated. One embodiment of the engaging portions  33 ,  53  are formed by difference diameters (ID 1 , ID 2 ) of internal bore  21  and (OD 1 , OD 2 ) of tension member  50 , respectively. For example, ID 1  is smaller than adjacent ID 2 , wherein engaging portion  33  is formed at the step or shoulder between the inner diameters (ID 1 , ID 2 ), and OD 2  of proximal end  52  is larger than OD 1  of base portion  56 , wherein engaging portion  53  is formed at step or shoulder  51 . In a particular embodiment, the complementary engaging surfaces of at least one of the pin portion  30  and the tension member  50  comprise, for example rounded engaging surfaces  33 ,  53  that do not completely match, but leave one or more gaps G so as to decrease the likelihood that the tension member  50  will “nest” or seat within the pin portion  30 . These gaps G may be provided by rounding the surface of the complementary rounded portions  33 ,  53  such that negative angles (A) are created, wherein at least a portion of the complementary engaging surfaces on the pin portion  30  and tension member  50  are slanted relative to the neutral axis of the pin portion  30 . These non-linear complementary engaging surfaces may reduce hoop stresses in the pin portion  30  due to the compressive force. 
     Referring now to  FIGS. 1   a ,  1   b ,  1   c , and  1   i  the pin portion  30  may also include a threadless band  36  located near a distal end  81  of the pin portion  30 . The threadless band  36  may extend about the entire perimeter of the pin portion  30  having a diameter  38  ( FIG. 1   c ). The threadless band  36  comprises a width (w) that may vary or may be constant about the perimeter of the pin portion  30  ( FIG. 1   i ). In one embodiment, the width (w) of the threadless band  36  is at least 2 mm. In a related embodiment, the width (w) of the threadless band  36  may be not greater than 8 mm. The threadless band  36  is generally located next to the proximal end  82  of the tool shoulder  40  so as to facilitate transitioning between the welding effects from the threaded segments  32  of the pin portion  30  and the welding effects from the working surface  44  of the tool shoulder  40 . Thus, the threadless band  36  may facilitate reduction in stress-rising effects. 
     Referring now to  FIGS. 1   c ,  1   h , and  1   i , the pin portion  30  may comprise varying diameters to facilitate stress reduction in the pin portion  30 . In particular, and with reference to  FIGS. 1   h  and  1   i , the pin portion  30  may include a tip portion  31  with outer thread diameter D 1  or plurality of outer threaded diameters D 1   n , a middle portion  35  with outer thread diameter D 2  or plurality of outer threaded diameters D 2   n , and a base portion  37  with outer thread diameter D 3  or plurality of outer threaded diameters D 3 . The outer diameter of the threads may progressively decrease as the outer threads; for example, proceed from the middle portion  35  towards the proximal end  80  of the pin portion  30  with outer diameter D 4 , wherein D 2  is greater than D 4 . In a related embodiment, the outer diameter of the threads may progressively decrease as the outer threads proceed from the middle portion  35  towards the distal end  81  of the pin (i.e., toward threadless band  36 ) with outer diameter D 5 , wherein D 2  is greater than D 5 . Thus, the pin portion  30  may comprise a bulged profile with a depression  47  near threadless band  36  as a result of the diametrical differences. This bulged profile may facilitate reduction in hoop stresses in the pin portion  30  by increasing the cross-sectional area in the middle portion  35  of the pin portion  30 . In particular, the bulge portion may reduce hoop stress and yield through plastic deformation in region  39  ( FIG. 1   h ) of pin portion  30 . 
     In yet another embodiment, one or more other surface features, such as dimples, intermittent grooves, or localized multi-faceted walls, to name a few, instead of the threaded segments. 
     Referring now to  FIGS. 1   a  and  1   c , the tool shoulder  40  generally is interconnected with the body  20  of the tool  10  via complementary engaging features  22 ,  42 . Such features may include, for example, male (external)/female (internal) threads. The tool shoulder  40  may be any suitable shoulder useful in a friction stir welding tool setting. For example, the tool shoulder  40  may be of a smooth configuration or of a scroll configuration with concentric rings and/or spiraled ridges, to name a few. A bottom portion of the tool shoulder  40  generally comprises a working surface  44 , which acts to engage work pieces at the start of welding and during welding contain the plasticized material formed about and around the pin, directly underneath the working surface  44 . Various working surfaces  44  are known in the art and any of such surfaces may be employed with the tool shoulder  40  of the friction stir welding tool  10 . 
     Referring now to  FIGS. 1   a ,  1   c ,  1   d  and  1   e , the tension member  50  is generally designed to snugly fit within the chamber of the body  20  of the friction stir welding tool  10  such that tension member  50  and body  20  share a common longitudinal axis A. A snuggly fit is defined herein as the outer diameter(s) OD of tension member  50  is slightly smaller than inner diameter(s) ID of internal bore  21  of body  20 . As discussed above, the tension member  50  is also generally designed to apply compression (e.g., axially compressive forces) to the pin portion  30 . In the illustrated embodiment, the tension member  50  comprises a rod configuration, the rod having a base portion  56 , a proximal end portion  52  and a distal end portion  54 . The proximal end portion  52  comprises a tension member shoulder  51  and/or a corresponding complementary engaging surface  53  for engaging with a complementary engaging surface  33  of the pin portion  30 , as described above. The distal end portion  54  generally comprises an engagement portion  55  for engaging with at least one member of the end assembly  60 . In the illustrated embodiment, the engagement portion  55  comprises a recess for engagement with a split collar  66  of the end assembly  60  (discussed in further detail below). One embodiment of recess can be a convex shape, however any shape is acceptable. Another embodiment of the engagement portion  55  can include projections (not shown) that are received into openings (not shown) in split collar  66 . Any complimentary features of the split collar  66  and engagement portion  55  that retains the split collar  66  to the tension member  50  and that does not interfere with the insertion and sliding of the tension member  50  into and through internal bore  21  is acceptable. For example, engagement portion  55  can include a spring loaded protrusion (such a ball) that can be depressed into the tension member  50  to allow it to enter and move freely through the internal bore  21  of body  20  and then extend sufficiently outward in a radial direction as it emerges or exits the internal bore  21  to engage a receiving member or opening of split collar  66 . Thus, when the tension member  50  is interconnected with the other portions of the tool  10 , as discussed in further detail below, at least one member of the end assembly  60  engages the engagement portion  55  of the tension member  50  and, in conjunction with other members of the end assembly  60 , applies an axial tensile load on the tension member  50 , the axial tensile force generally comprising a force vector oriented towards the distal end portion  54  of the tension member  50 . As an axial tensile load is applied to the distal end  54  of the tension member  50 , engaging features  53  of tension member shoulder  51  induce a force on the surface of the internal bore  21  in proximity of engaging feature  33 . Thus, compression forces are realized at the pin portion  30  of the tool  10  via engagement of the tension member shoulder  51  with internal portions of the pin portion  30 , which will reduce the mechanical assembly stress component and thereby, reduce the alternating tensile stress range during operation by starting with a lower minimum stress than would have been present without the induction of the compressive forces or loads. In turn, the pin portion  30  may be axially compressed during operation of the friction stir welding tool  10 , which may reduce tensile stresses incurred by the pin portion  30  during operation of the friction stir welding tool  10 . 
     The tension member  50  may comprise materials similar to those utilized for the body  20 , the pin portion  30  and/or the tool shoulder  40 , or materials differing from those components. In one embodiment, the tension member  50  comprises a high tensile strength material. In one embodiment, the tension member  50  comprises a metal alloy such as a fastener alloy and/or a superalloy. In a particular embodiment, the metal alloy may be a cobalt-based alloy. In another embodiment, the metal alloy may be a steel-based alloy. In another embodiment, the tension member  50  may comprise a composite material, such as a ceramic. The ceramic material may be, for example, a tungsten-based ceramic material. In another embodiment, the composite may comprise one or more bundles of ceramic organic or carbon fibers. With respect to ceramic materials, it may be appreciated that a recessed engagement surface, such as engagement portion  55 , may not be readily attained due to difficulties arising in machining ceramic parts. Thus, in one embodiment of a tension member  50  comprising a ceramic material, the tension member  50  includes an anchor for anchoring the tension member  50  to at least one other portion of the tool  10 , such as a body portion  20  or a pin portion  30 . The anchor may be a mechanical fastener or a chemical fastener. In one embodiment, the anchor comprises complementary fastening features, such as hooks/holes, dimples/recesses and/or a tongue-groove arrangement, to name a few, a first one of which is utilized on the tension member  50 , and a second one of which is utilized on at least one of the body  20 , pin portion  30 , and end assembly  60 . In one embodiment, a chemical fastener is used, such as a high bond strength adhesive (e.g., a high temperature, super adhesive). In some instances, the tension member  50  generally comprises a monolithic body. However, in other instances, the tension member  50  may comprise separate components. For example, the tension member  50  may comprise a separate distal end portion and/or a separate proximal end portion for interconnection with the base portion of the tension member  50 . 
     Referring now to  FIGS. 1   f  and  1   g , the end assembly  60  is generally utilized to achieve at least one of, and sometimes both of, the following: (i) axially tension the tension member  50  and (ii) decouple the tension member  50  from the body  20  and/or pin portion  30  of the friction stir welding tool  10 . In the illustrated embodiment, the end assembly  60  comprises a decoupling member  62  and a decoupling retainer  63  for retaining the decoupling member  62 . As discussed above, the decoupling member  62  facilitates decoupling of the tension member  50  from the body  20  of the friction stir welding tool  10 . Thus, transfer of torque and/or other undesired forces from the base  20  and/or pin portion  30  to the tension member  50  may be restricted and/or eliminated. The decoupling member  62  may be, for example, a thrust bearing, such as a thrust ball-bearing and/or high temperature thrust bearing. In another embodiment, the decoupling member  62  may comprise different types of bearings, such as oil bearings and hydraulically-driven bearings. In one embodiment the rotational or torsional displacement of the distal end  54  relative to the proximal end  52  may be up to 15° prior to decoupling at a predetermined torque value. In another approach, the decoupling member  62  and its retainer may be absent from the end assembly  60 , such as when the tension member  50  comprises one or more bundles of fibers that are capable of twisting during operation of the tool, hence reducing stress effects from the pin portion  30  and/or body  20  in the tension member  50 . 
     Also, lubricants (such as a dry lubricating powder) may be applied between the tension member  50  and the internal bore of the body  20  and/or pin portion  30  of the tool  10 , thereby facilitating movement (e.g., radial movement) of the tension member  50  relative to the body  20  and/or pin portion  30  of the tool  10 . In one embodiment, the dry lubricating powder is a molybdenum-containing powder. 
     The end assembly  60  may also and/or alternatively include one or more spring members  64 . Spring members  64  can be selected based on a spring constant (k) that yields the desired spring force to apply a tensile load on the tension member  50 . In one embodiment, the spring members  64  include one or more springs, such as Belleville disk springs, that preload the tension member  50  with a designed tensile load when the end assembly  60  is engaged with the tension member  50 . The spring members  64  may thus act to preload the tension member  50  with a desired force F in an axial direction relative to the pin portion  30 . Also, a pneumatic drive system (not shown) can be adapted to the tool  10  to work in combination with or in place of the spring members  64 . Thus, the pin portion  30  may be compressed, and reduced mechanical tensile stresses may be realized, as described above, which reduces the alternating stress range. 
     The spring members  64  may be utilized to dampen tension variations experienced by the tension member  50  due to interactions with the pin portion  30  and/or body  20  of the tool  10 . The spring members  64  may further be utilized to dampen tension variations experienced by the tension member  50  due to temperature fluctuations during operation of the friction stir welding tool  10 . Thus, the spring members  64  may act not only to provide the desired axial compression of the pin portion  30 , but also to dampen tension variations experienced by the tension member  50 . In the illustrated embodiment, the spring members  64  comprise disk springs that provide both dampening and compressing actions relative to tension member  50 . It will be appreciated that, in other embodiments, separate components may be utilized to provide tensile loading to the tension member  50  and dampen tensile stress variations experienced by the tension member  50 . 
     The end assembly  60  may include a collar  66  for engaging an engagement portion  55  of the tension member  50 . The collar  66  may be, for example, a split collar having set screws  68  to facilitate engagement of the collar  66  with the engagement portion  55  of the tension member  50 . A washer  65  may be utilized between the spring members  64  and the collar  66  so as to facilitate assembly of the end assembly  60 . Once the decoupling member  62 , spring members  64  and/or collar  66  are assembled and mounted to the tension member  50 , a spring force F may be affected in the axial direction, as illustrated in  FIG. 1   g . To protect the distal end portion  83  of the end assembly  60 , a retainer  67  may be interconnected with the collar  66 . 
     The end assembly  60  may facilitate one or more functions with respect to the tension member  50 . By way of primary example, the end assembly  60  may act to decouple the tension member  50  from the body  20  of the tool  10 . By way of secondary example, the end assembly  60  may act to provide a tensile force with respect to the tension member  50 , thereby compressing at least a portion of the pin portion  30  of the tool  10 . By way of tertiary example, the end assembly  60  may facilitate dampening of the tension member  50  due to variations experienced by the tension member  50  from interactions with the pin portion  30  and/or body  20  of the tool  10 , or due to temperature variations experienced by the tension member  50  during operation of the friction stir welding tool  10 . 
     Another embodiment of pin portion  30  is shown in  FIG. 9  to include a taper  900  as a result of the other diameters (D 1   n , D 2   n , D 3   n , and D 5   n , all shown in  FIG. 1   h ) reducing linearly from D 5  (or proximal end  81 ) to D 4  (distal end  80 ). The linear reduction can be constant (straight taper as shown in  FIG. 9 ) or vary (not shown). 
     As noted above, the pin portion  30  may include one or more threaded segments  32  for facilitating operation of friction stir welding tool  10 . Each segment includes a predetermined length with a distal end and a proximal end that are directly adjacent to the respective a proximal end and a distal end of an adjacent segments or end of threadless band  36 . For example, the end of threadless band  36  is directly adjacent to the distal end  37   d  of the threaded segment  37 , the proximal end  37   p  of threaded segment  37  is directly adjacent to the distal end  35   d  of the threaded segment  35 , and the proximal end  35   p  of threaded segment  35  is directly adjacent to the distal end  31   d  of the threaded segment  31 . In another approach, one or more of the threaded segments  32  may comprise differing thread orientations relative to other threaded segments  32 . In a particular embodiment, and with reference to  FIGS. 2   a - 2   c , a pin  230  may comprise a plurality of alternating threaded segments  232   a ,  232   b . In the illustrated embodiment, the pin  230  comprises a first set of threaded segments  232   a  and a second set of threaded segments  232   b . In the illustrated embodiment, the first set of threaded segments  232   a  comprises right-handed threads. The second set of threaded segments  232   b  comprises left-handed threads. Thus, the pin  230  comprises a first set of threaded portions comprising a first thread orientation, and a second set of thread segments, comprising a second thread orientation. Bare portions  234  are included between the threaded segments  232   a ,  232   b . In the illustrated embodiment, the threaded portions  232   a ,  232   b  are spaced equidistance from one another, and the bare portions  234  are also thus spaced equidistant from one another, approximately 90° on center as shown in  FIG. 2   c . In the illustrated embodiment, the first thread segments  232   a  are separated from each other by bare portion  234  and adjacent second threaded segments  232   b  on either side of the first threaded segments  232   a . Likewise, the second threaded segments  232   b  are separated from the first threaded segments  232   a  via adjacent bare portions and first threaded segments  232   a  on either side of the second threaded segments  232   b . While left-handed/right-handed threaded orientations are illustrated, other thread orientations may be utilized, such as horizontal thread orientations. Further, the threads may include various other surface features, such as dimples, intermittent grooves, and localized multi-faceted flaps, to name a few. The use of varying thread orientations may facilitate more efficient mixing of plasticized regions about the pin  20 / 230  during operation of the friction stir welding tool  10 . In turn, the forces and torque witnessed by the pin  20 / 230  during welding operations may be reduced. Improved control over flow, fill-up and consolidation of the plasticized regions about the pin  20 / 230  may also be witnessed, as well as improved pin life. 
     In one embodiment of pin portion  30 , the outer diameters of the threaded segments are substantial constant along their respective lengths. 
     In another embodiment of pin portion  30 , the outer diameters of the threaded segments are not substantial constant along their respective lengths. 
     In another embodiment of pin portion  30  (shown in  FIG. 1   h ), the outer diameters D 1   n  of the threaded segment  31  increases from it proximal end  31   p  to the distal end  31   d ; the outer diameters D 2   n  of the threaded segment  35  increases from its proximal end  35   p  to a predetermined point P 1  along a predetermined length along its length L 4  and then decreases from the predetermined point P 1  to its distal end  35   d ; and the outer diameters D 2   n  of the threaded segment  35  decreases from its proximal end  37   p  to its distal end  37   d , whereby at the point where the ends of the adjacent threaded segments intersect, the outer diameters of the threaded sections are substantially the same. In other words, the outer diameter D 1  of the distal end  37   d  of the threaded portion  31  is substantially equal to the outer diameter D 2  of the proximal end  35   p  of the threaded end  35 , and the outer diameter D 1  of the distal end  35   d  of the threaded end  35  is substantially equal to the outer diameter D 3  of the proximal end  3 ′ 7   p  of the threaded end  37 . 
     In another embodiment of pin portion  30  ( FIG. 1   h ), the plurality of threaded segments  32  circumscribe the outer surface  34  of the pin portion  30  for a portion of the length L 2  of the pin portion  30  and at least two thread-less longitudinal sections  34  span the entire length L 2  of the pin portion  30  that form equidistance spaces S between the plurality of threaded segments  32 . 
     In another embodiment of pin portion  30 , at least one threaded segment  32  is left-handed threads and another threaded segment  32  is right-handed threads ( FIGS. 2   a - 2   c ). 
     In another embodiment of pin portion  30 , all the threaded segments  32  are all either left-handed threads or all right-handed. 
     In another embodiment of pin portion  30 , at least one segment ( 31 ,  35 , or  37 ) comprises at least one outer diameter therein (D 1   n , D 2   n , or D 3   n ) that increases at a linear rate from proximal to distal ends, which is defined as the segment diameters along the segment length (L 3 , L 4 , or L 5 ) increases or decrease at a constant or linear rate (positive or negative), for example 1 mm diameter increase for every 1 mm length of segment. 
     In another embodiment of pin portion  30 , at least one segment ( 31 ,  35 , or  37 ) comprises at least one outer diameter therein (D 1   n , D 2   n , or D 3   n ) that increases at a linear rate from proximal to distal ends, which is defined as the segment diameters along the segment length (L 3 , L 4 , or L 5 ) increases or decrease at a non-constant or nonlinear or exponential rate, for example 1 mm diameter increase for the first 1 mm length of segment and when an increase or decrease in diameter that is not a 1 mm diameter increase for the subsequent 1 mm length of segment. 
     In another embodiment of pin portion  30 , at least one segment ( 31 ,  35 , or  37 ) comprises outer diameters (D 1   n , D 2   n , or D 3   n ) that increase at a linear rate ( FIG. 9 ) and at least one outer diameter of the outer diameters increase at a non-linear rate. 
     Referring now to  FIG. 1   c , as illustrated, the tool shoulder  40  generally comprises a monolithic member. However, the tool shoulder  40  may comprise separate components. In one approach, and as described in further detail below, the tool shoulder  40  comprises a first shoulder portion for interconnection with the body  20  of the friction stir welding tool  10 . The tool shoulder  40  may further include a second shoulder portion interconnected to the first shoulder portion near the proximal end of the first shoulder portion and overlaying such first shoulder portion. A second shoulder portion may thus have a working surface proximal a distal end  81  of the pin portion  30  of the friction stir welding tool  10 . In turn, a transitioning portion of the first shoulder portion may protrude through the working surface of the second shoulder portion to provide a transition between the pin portion  30  and the working surface of the second shoulder portion. As described below, this transitioning portion may smooth the flow of plasticized material by providing a non-abrupt change in the interface between the tool shoulder  40  and the pin portion  30 . 
     For example, and with reference to  FIGS. 3   a  and  3   b , a friction stir welding tool  300  may comprise a body  20 , a pin portion  30 , a tension member  50 , and an end assembly  60 , as described above. The friction stir welding tool  300  may further comprise a tool shoulder comprising a first shoulder portion  340  and a second shoulder portion  342 . The first shoulder portion  340  may be interconnected to the body  20  via complementary engaging features  22 ,  345  of the body  20  and first shoulder portion  340 , respectively. A second shoulder portion  342  may be interconnected with the first shoulder portion  340 , overlaying an outer surface  347  of the first shoulder portion  340 . The first shoulder portion  340  and second shoulder portion  342  may be interconnected via complementary engaging features  343 ,  344  of the first shoulder portion  340  and second shoulder portion  342 , respectively. The first shoulder portion  340  may comprise a non-threaded portion  346  having a smooth transitioning surface that protrudes through the working surface  348  of the second shoulder portion  342 , thereby facilitating a smooth transition between the pin portion  30  and the working surface  348  of the second shoulder portion  342 . Thus, the transition between the tool shoulder  340 ,  342  and the pin portion  30  may be more gradual (e.g., smoother), thus restricting, and in some instances preventing, the formation of un-bonded discontinuities along the advancing sides of the welds by smoothing the flow of plasticized material at this turbulent point of the friction stir welding tool  10 . 
     Although in many of the illustrated embodiments, the tool shoulder  40  is illustrated as a separate piece, the tool shoulder  40  may be integral with the body  20  and/or pin portion  30  of the friction stir welding tool, as illustrated in  FIG. 6 . Hence, in one embodiment, the friction stir welding tool  600  comprises a monolithic structure  610  with the body  620 , pin  630  and tool shoulder  640  all being integral with one another. In this embodiment, fabrication processes may be simplified and fabrication costs may be reduced. 
     Furthermore, the tool shoulder may comprise a substantially planar working face, as illustrated in  FIGS. 1   d ,  3   a , and  3   b , or may comprise a non-planar working face. For example, and with reference to  FIG. 7 , a friction stir welding tool  700  may comprise a body  20  and pin portion  30 , such as described above. The friction stir welding tool  700  may further comprise a tool shoulder  740  having a non-planar working surface, such as the tapered working face  744  illustrated in  FIG. 7 . The tapered working face  744  generally comprises an inner edges  745  and outer edges  747 . The height (“h”) of the outer surface  746  of the tapered working surface generally progressively decreases from the inner edge  745  toward the outer edges  747 . In one embodiment, the height of the outer surface  746  linearly progressively decreases from the inner edges  745  to the outer edges  747 . In one embodiment, the height of the outer surface  746  generally non-linearly progressively decreases (e.g., exponentially) from the inner edges  745  to the outer edges  747 . Friction stir welding tools utilizing this tapered tool shoulder approach may be employed with a non-integral tool shoulder, as illustrated in  FIG. 7 , or may be employed with an integral tool shoulder, an embodiment of which is illustrated in  FIG. 8 . In the illustrated embodiment of  FIG. 8 , the friction stir welding tool  800  comprises a monolithic structure  810  with the body  820 , pin  830  and tool shoulder  840  all being integral with one another. 
     Although many of the above-described features have generally been described in relation to conventional anvil-based friction stir welding tools, bobbin-type tools may also be employed. Such bobbin-type tools may employ various ones of the concepts/embodiments described above. One embodiment of a bobbin-type tool employing an end assembly comprising a decoupling member and a spring member is illustrated in  FIG. 4 . In the illustrated embodiment, the bobbin-type tool  400  comprises a threaded pin  430 , a plurality of tool shoulders  440  interconnected with the threaded pin  430 , and a tension member  450  contained within the threaded pin  430 . An end assembly  460  is employed at one end of the tension member  450  to provide tension to the tension member  450  and facilitate decoupling of the tension member  450  from the threaded pin  430 . The tension member  450  is further mounted to the threaded pin  430  via a physical connector  470  such as a bolt/washer assembly. The end assembly  460  may include any of the features described above with reference to end assembly  60  of the anvil-type tool, such as a decoupling member  62 , a retaining ring  63 , spring members  64 , washer  65  and collar  66 . The threaded pin  430  may also include many of the features described above with respect to the pin portion  30  of the anvil-type friction stir welding tool  10 , such a high radius to depth ratios and alternating/varying thread orientations, to name two. The tension member may include any of the features described above with reference to engagement portion  55 . 
       FIG. 10  is an illustration of another embodiment  100  having the decoupling member  62  in close proximity to distal end  52  of tension rod  50  instead of being in close proximity to proximate end  54  ( FIG. 1   c ), and a multi-shoulder  40  arrangement having shoulder retainer  102  and split collar  104 . As discussed above, the other reference numbers illustrated in  FIG. 10  are common with the features in previously disclosed embodiments. 
     A storage/transportation container may be utilized to store and/or transport any of the friction stir welding tools. One embodiment of a suitable container is illustrated in  FIG. 5 . In the illustrated embodiment, the container  500  comprises a first portion  520  interconnectable with a second portion  530  (e.g., via complementary male and female threads  540 ). The first portion  520  is adapted to receive a first portion of the friction stir welding tool  10 , and the second portion  530  of the storage/transportation container is adapted to receive the remaining other portions of the friction stir welding tool  10 . The internal dimensions of the container  500  may be tailored to the outer dimensions of the friction stir welding tool  10  to provide a snug fit of the friction stir welding tool  10  within the container  500  when the first portion  520  is engaged with the second portion  530 . Various types of padding may be employed within the storage container  500 . Thus, the friction stir welding tool  10  may be protected during transportation and/or shipment. 
     Example of Assembly of One Embodiment Illustrated in FIGS. 1 c  and  1   f    
     B. Assemble shoulder  40  to body  20 /pin portion  30  assembly (unless the body/pin/shoulder are monolithic  FIGS. 6 and 8 ); 
     C. Insert distal end  54  of tension member  50  into internal bore  21  of body  20  at proximate end  23  of body  20 ; 
     D. Axially slide tension member  50  within internal bore  21  until the complimentary engaging features  33 ,  53  of tension member  50  and body  20 , respectively, engage; 
     E. Slide decoupling member  62  onto tension member  50  and position decoupling member  62  directly adjacent and in contact with distal end  25  of body  20 ; 
     F. Slide decoupling retainer  63  onto tension member  50  and position over decoupling member  62  and adjacent distal end  25  of body  20 ; 
     G. Slide one or more spring members  64  onto tension member  50  and position at least one spring member  64  directly adjacent and in contact with decoupling retainer  63  (note that the number of springs will influence the compressive stresses induced onto pin portion  30 , add as many or as little as necessary to achieve the desired compressive stress condition in the pin portion  30 ); 
     H. Slide washer  65  onto tension member  50  and position directly adjacent and in contact with at least one spring member  64 ; 
     I. Position a split collar  66  on to distal end  54  of the tension member  50  and insert and loosely secure screws  68  into complimentary threaded holes of split collar  66 ; 
     J. Axially push with a press, washer  65  inward toward the spring members  64  to depress the spring members  64  sufficient to expose engagement portion  55  of the tension member  50 ; 
     K. Position a split collar  66  to seat within engagement portion  55  of the tension member  50 ; 
     L. Tighten screws  68  to secure split collar  66  to the tension member  55 ; 
     M. Connect a retainer  67  with the collar  66  to inhibit relative axial movement between collar  66  and distal end  54  of tension member and loosening of the screws from the split color  66 ; and 
     N. Attach assembled friction stir welding tool to friction stir welding equipment. 
     Optionally, apply lubricant as discussed above, and apply additional axial tension during the friction stir welding operation to increase the compressive stresses in pin portion  30 . 
     While various embodiments of the present disclosure have been described in detail, it is apparent that modifications and adaptations of those embodiments may occur to those skilled in the art. However, it is to be expressly understood that such modifications and adaptations are within the spirit and scope of the present disclosure.