Abstract:
A tool for making simultaneously a plurality of parallel friction stir welds includes at least one shank for holding in a chuck or collet of a friction stir welding machine, a plurality of friction stir welding pins, and friction stir welding shoulders including at least four working surfaces adjacent said ins, the shoulders and pins mounted in axial relationship; dimensions of said friction stir welding pins and shanks corresponding to dimensions and spacings of said friction stir welds.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Application Ser. No. 60/613,925 filed on Sep. 28, 2004, which has the same title as the present application. It is also closely related to the U.S. application entitled “Advanced Friction Stir Welding Tools”, application Ser. No. 11/100,878 Filed on Apr. 7, 2005 and the application entitled “Fracture Resistant Friction Stir Welding Tools”, application Ser. No. 11/133,083 filed on May 19, 2005. The teachings of these applications are incorporated herein by reference thereto. These applications have the same inventors and the same assignee as the present application. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to friction stir welding and, more particularly, the present invention relates to simultaneous friction stir welding of a plurality of parallel joints between components having parallel portions.  
       BACKGROUND OF THE INVENTION  
       [0003]     The Friction Stir Welding (FSW) process is a solid-state based joining process, which makes it possible to weld a wide variety of materials alloys (Aluminum, Copper, Stainless Steel, etc.) to themselves and combinations (e.g. 6xxx/5xxx, 2xxx/7xxx, etc.). The joining is affected by a rotating FSW tool, which is forced into the joining area to heat it by friction and thus “plasticizes” the parts about it. Plasticized material flows around the axis of the rotating FSW tool, and the plasticized regions coalesce into sound metallurgical bonds. The process can be implemented with conventional FSW tools each consisting of a single pin and shoulder that requires backup with an anvil during welding.  FIG. 1  illustrates a prior art friction stir welding tool  10  having a shank  18  that may be held in a chuck or collet of an FSW machine. Shank  18  may have a flat  19  to facilitate the application of torque to FSW tool  10 .  
         [0004]     FSW tool  10  also includes a pin  12  and shoulder  14  having a workpiece engaging surface  16 . Pin  12  may include a thread  13  and flats  15 . FSW tool  10  is rotated in the direction which causes thread  13  on pin  12  to push plasticized material toward the tip of pin  12 . Workpiece engaging surface  16  of shoulder  14  may include a spiral thread  17 . The pitch of spiral thread  17  is such that it tends to move plasticized material inwardly, toward the base of pin  12 , when FSW tool  10  is rotated in the direction which tends to push plasticized material toward the tip of pin  12 .  
         [0005]      FIG. 2  illustrates two plates  111  being butt welded to each other by FSW tool  10 . A backup anvil  11  on the back side of plates  111  is necessary to counteract the forging force exerted by the FSW tool onto the plasticized joint and prevent escape of plasticized material, and produce a smooth surface on the back side. Hence, FSW tools similar to FSW tool  10  have the limitation that they cannot be employed for welds for which it is not possible to access the back side of the components being welded.  
         [0006]     In order to weld components wherein it is not possible to access the back side of the weld to place a backup anvil, bobbin-type tools may be employed. Such tools include two shoulders and a pin between them. The concept for such tools was patented by Kevin Colligan on Dec. 30, 2003 U.S Pat. No. 6,660,075 ( FIG. 3 ). The bobbin-type FSW tool  20  illustrated in  FIG. 3  includes a FSW pin  21  and a pair of shoulders  22 , shoulders  22  including workpiece engaging surfaces  23 . Since the shoulders  22  have the taper angle  24 , they can be integral with pin  21 . In order to impart the forging force to weld workpieces  111  having some tolerance in thickness, the workpiece engaging surfaces  23  are tapered away from workpieces  111  at the taper angle  24  shown in  FIG. 3 .  
         [0007]     Not only does the taper angle  24  enable workpieces having somewhat variable thicknesses to be welded, it also ensures that the necessary forging force is applied to the plasticized region whereby plasticized material is confined to the weld region, and smooth surfaces are produced on the upper and lower surfaces of the weld. The teachings of U.S. Pat. No. 6,660,075 are included herein by reference thereto.  
         [0008]     A more complete drawing of a bobbin-type FSW tool is given in  FIG. 4 . Bobbin-type FSW tool  30  includes a shank  36  and an FSW pin  39 . Pin  39  includes a proximal pin portion  31  on the proximal side of the center  38  of pin  39 , and a distal pin portion  37  on the distal side of the center  38  of pin  39 . Proximal pin poretion  31  and distal pin portion  37  have opposite pitch, and FSW tool  30  is rotated in the direction which tends to cause plasticized material to flow towards the center  38  of pin  39 .  
         [0009]     FSW tool  30  also includes a proximal shoulder  32  having workpiece engaging surface  33  and distal shoulder  34  having workpiece engaging surface  35 . Again, the workpiece engaging surfaces  33  and  35  are tapered to tolerate variations in workpiece thickness and to apply the required forging force to the plasticized material. The bobbin-type FSW tool  30  is described in the copending patent application entitled “Advanced Friction Stir Welding Tools”, application Ser. No. 11/100,878 Filed on Apr. 7, 2005.  
         [0010]     FSW tool  30  includes a tension member  27 , which is placed in tension by nut  28  acting through spring washer  29 . The purpose of tension member  27  is to place pin  39  in compression to prevent fracture of pin  39  due to the combination of severe cyclic torsion and bending moments it experiences during friction stir welding.  
         [0011]      FIG. 5  illustrates the bobbin type FSW tool  30  in position for welding joint  113 , which is one of a pair of joints  113  and  114  needed to produce a rectangular tube from a pair of elongate members, each elongate member having a cross-section shaped like a square bracket, each elongate member corresponding to one half of the cross-section of the rectangular tube. It is noted that bobbin tools of the type taught by Mr. Colligan are capable of welding only one joint at a time.  
         [0012]      FIG. 6  illustrates a prior art FSW tool  50  having superior mechanical properties. It includes an integral shank-pin ensemble with a shoulder  54  threaded onto the shank-pin ensemble. FSW tool  50 , preferably, has a close fit  57  between the shank  53  and the inside of the shoulder  54 . It also has a close fit  58  between the pin  52  and the inside of shoulder  54  near the base of pin  52 , and it has a firm stop  59  between the inside of shoulder  54  and the shank  53 . FSW tool  50  is presented in the copending patent application: “Advanced Friction Stir Welding Tools”, application Ser. No. 11/100,878 Filed on Apr. 7, 2005.  
         [0013]     That application also advances the concept of including an internal tension member to provide compression loading of the pin of a bobbin type FSW tool.  FIG. 7  provides preferred internal detail regarding the prior art bobbin-type FSW tool. Preferably, FSW tool  30  includes a snug fit  42  at the proximal end of proximal shoulder  32 , snug fit  41  at the distal end of proximal shoulder  32 , and firm stop  42 . Likewise, FSW tool  30  includes snug fit  44  at the proximal end of distal shoulder  34 , and the firm stop  45 . Both shoulder  32  and shoulder  34  may be assembled by threading them on from the distal end of FSW tool  30 .  
         [0014]     While the FSW tools described above have a number of desirable features, each is capable of welding only one joint at a time. A need remains for a FSW tool which can make a plurality of welds such as joint  113  and  114  shown in  FIG. 5 .  
       SUMMARY OF THE INVENTION  
       [0015]     In one aspect, the present invention is a friction stir welding tool for simultaneously making a plurality of parallel welds. The friction stir welding tool includes a plurality of friction stir welding modules, each of the friction stir welding modules including at least one friction stir welding pin, and a pair of workpiece engaging surfaces facing the at least one friction stir welding pin. Each of the workpiece engaging surfaces is disposed on a shoulder attached to or integral with the at least one friction stir welding pin, whereby the shoulders and pin(s) rotate in unison. The friction stir welding modules are connected to each other or integrally formed whereby the modules rotate in unison. At least one shank is attached to or integral with at least one of the friction stir welding modules, whereby the shank and the modules rotate in unison. The at least one shank is for engagement with a chuck or collet of a friction stir welding machine to be rotated thereby.  
         [0016]     In another aspect, the present invention is a method of making a plurality of parallel friction stir welds simultaneously to join a pair of workpieces. The method comprises placing the workpieces in juxtaposition, moving the workpieces through a FSW machine, the FSW machine having a FSW tool having a plurality of welding modules, whereby the plurality of FSW welds are produced by the FSW tool.  
         [0017]     In another aspect, the present invention is a weldment comprising two or more parallel friction stir welds made in a single pass. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  is a sketch of a prior art friction stir welding tool;  
         [0019]      FIG. 2  is a schematic illustration of a prior art friction stir welding tool with a backup anvil;  
         [0020]      FIG. 3  is a drawing from an issued United States patent for a bobbin type friction stir welding tool;  
         [0021]      FIG. 4  is an illustration of a prior art bobbin type friction stir welding tool including a tension member for placing the pin in compression;  
         [0022]      FIG. 5  is an illustraton of a prior art bobbin type friction stir welding tool readied for welding one of a pair of parallel joints;  
         [0023]      FIG. 6  is a sectional drawing of a friction stir welding tool having an integral pin-shank ensemble, and a self-locking shoulder threaded onto the integral pin-shank ensemble;  
         [0024]      FIG. 7  is a sectional drawing of a bobbin type friction stir welding tool having an integral pin-shank and a pair of shoulders that self lock to the pin-shank and also having an internal tension member which places the pin in compression;  
         [0025]      FIG. 8  is a sketch of a multi-shouldered fixed bobbin type friction stir welding tool for welding two parallel joints simultaneously;  
         [0026]      FIG. 9  is a sketch of a multi-shouldered bobbin type friction stir welding tool, according to the present invention, for simultaneously welding three parallel joints;  
         [0027]      FIG. 10  is a detail of the multi-shouldered bobbin type friction stir welding tool illustrated in  FIG. 9 ;  
         [0028]      FIG. 11  is an illustration, partly in section, of the multi-shouldered bobbin type friction stir welding tool illustrated in  FIG. 9 , the shoulders being sectioned to show the self-locking feature;  
         [0029]      FIG. 12  is a sketch of a FSW tool according to the present invention readied for for simultaneous welding of two parallel joints;  
         [0030]      FIG. 13  is a sketch of a FSW tool according to the present invention readied for for simultaneous welding of three parallel joints;  
         [0031]      FIG. 14  is an illustration of a system for simultaneous welding of extrusions with multiple parallel walls using a multi-shouldered fixed bobbin tool according to the present invention;  
         [0032]      FIG. 15  is a detail showing the extrusions shown in  FIG. 14  being welded;  
         [0033]      FIG. 16  is a sketch of a mechanical arm having two parallel welds made by the FSW tool of the present invention;  
         [0034]      FIG. 17  is a sketch of an angled mechanical link having two parallel welds made by the FSW tool of the present invention;  
         [0035]      FIG. 18  is an illustration of a coaxial structure having multiple pairs of parallel welds made by the FSW tool of the present invention;  
         [0036]      FIG. 19  is an illustration of a double walled structure made by the FSW tool of the present invention;  
         [0037]      FIG. 20  is an illustration of a multi-width double walled panel made by the FSW tool of the present invention;  
         [0038]      FIG. 21  illustrates optional torque communication features on adjacent pins;  
         [0039]      FIG. 22  illustrates a threaded tension member which is an optional aspect of the present invention; and  
         [0040]      FIG. 23  illustrates a portion of an alternative multi-shouldered fixed bobbin tool having an integral shank-pin ensemble wherein the shoulders are threaded onto the shank-pin ensemble from the shank sides.  
         [0041]      FIG. 24  is a median section of a multi-shouldered fixed bobbin tool that is integrally formed. 
     
    
     NOMENCLATURE  
       [0000]    
       
           10  Prior art FSW tool  
           11  Backup anvil  
           12  Pin  
           13  Threads on pin  12   
           14  Shoulder  
           15  Flat on Pin  
           16  Workpiece engaging surface of shoulder  
           17  Spiral thread on shoulder  
           18  shank  
           19  Flat on shank  
           20  Prior art Fixed bobbin tool  
           21  Pin of tool  20   
           22  Shoulder of tool  20   
           23  Workpiece engaging surface of shoulder  22   
           24  Taper angle of workpiece engaging surface  23   
           27  Tension member  
           28  Nut  
           29  Spring washer  
           30  Prior art bobbin tool  
           31  Proximal pin portion  
           32  Proximal shoulder  
           33  Workpiece engaging surface of shoulder  32   
           34  Distal shoulder  
           35  Workpiece engaging surface of shoulder  34   
           36  Shank of FSW tool  30   
           37  Distal pin portion  
           38  Center of pin  39   
           39  Pin  
           41  Snug fit of proximal shoulder near working face thereof  
           42  Snug fit of proximal shoulder  
           43  Firm stop of proximal shoulder  
           44  Snug fit of distal shoulder  
           45  Firm stop on distal shoulder  
           50  Prior art FSW tool having integral pin and shank  
           52  Pin  
           53  Shank  
           54  Shoulder  
           55  Workpiece engaging surface of shoulder  
           56  Threaded interface between shank and shoulder  
           57  Proximal close fit  
           58  Distal close fit  
           59  Firm stop  
           60  FSW tool for simultaneously welding two joints  
           61  Tension member  
           62  Nut  
           63  Spring washer  
           66  Long shank  
           67  Right handed shoulder  
           68  Left handed shoulder  
           69  Spacer washer  
           70  Multi-shouldered fixed bobbin tool for FSW 3 points  
           71  L.H. Pin portion  
           72  Abutting ends of pins  
           73  R.H. Pin portion  
           75  Long shank  
           76  Tension member  
           77  Nut  
           78  Compression washer  
           82  L.H. Shank  
           83  Spacer washer  
           84  R.H. shank  
           85  Snug fit at proximal end of L.H. shoulder  
           86  Snug fit at distal end of L.H. shoulder  
           87  Firm stop on L.H. shoulder  
           88  Snug fit at distal end of R.H. shoulder  
           89  Snug fit at proximal end of R.H. shoulder  
           90  Firm stop on R.H. shoulder  
           111  Plate being welded  
           112  C-shaped extrusion  
           113  Lower joint to be welded  
           114  Upper joint to be welded  
           122  E-shaped extrusion  
           123  Upper joint to be welded  
           124  Center joint to be welded  
           125  Lower joint to be welded  
           130  Machine for welding extrusions  
           132  Loading conveyor  
           134  Unloading conveyor  
           136  FSW motor  
           138  Upper FSW chuck or collet  
           140  Lower FSW chuck or collet  
           142  Grippers  
           144  Belt  
           150  Mechanical link arm made by present invention  
           152  Upper weld  
           154  Lower weld  
           156  Angle link arm  
           157  Upper weld  
           158  Lower weld  
           160  Cylindrical structure  
           162  Weld in cylindrical structure  
           170  U-shaped member  
           172  Weld in U-shaped member  
           180  Deck plate  
           182  Weld in deck plate  
           192  Tension member  
           194  Upper pin  
           195  Non axisymmetric end of pin  194   
           196  Lower pin  
           197  Mating non axisymmetric end of pin  196   
           202  Threaded tension member  
           204  Upper pin having internal threads  
           205  Planar end of pin  204   
           206  Lower pin having internal threads  
           207  Planar end of pin  206   
           210  Bobbin type FSW tool with shoulders which thread on from shank  
           212  Shank  
           214  First shoulder  
           215  Firm stop on first shoulder  
           216  First pin  
           217  Proximal portion of pin  216   
           218  Distal portion of pin  216   
           219  Second shoulder  
           222  Third shoulder  
           224  Firm stop on third shoulder  
           226  Spacer washer  
           228  Second pin  
           230  Integral multi shouldered fixed bobbin type FSW tool  
           232  Shank  
           234  Upper shoulder  
           236  Working face of shoulder  
           238  Upper pin portion  
           240  Lower pin portion  
           242  Working face of shoulder  
           244  Center shoulder  
           246  Lower shoulder  
       
     
       DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0168]     The present invention teaches the concept of multi-shouldered fixed bobbin tools that afford simultaneous friction stir welding of multiple parallel walls between parts. The term “wall” here can mean a sheet, a plate, a flange or web, a planar portion of an extrusion or rolled product, or a planar portion of a casting, etc.  
         [0169]     In the discussion which follows, directional terms such as “top”, “bottom”, “upper”, “lower”, etc are for reference only. The tools described are for use in any orientation.  
         [0170]      FIG. 8  illustrates a multi-shouldered fixed bobbin tool  60 , according to the present invention. Bobbin tool  60  is for making two parallel welds simultaneously.  
         [0171]     The inventors have realized that in order to FS weld with a multi-shouldered fixed Bobbin tool: 
        a) Multiple parallel walls (e.g. 2-4),     b) Relatively thick walls (2.5 cm), and     c) Tough/strong alloys (e.g. 7085), the tool must be extra strong to resist the severe bending and twisting at its pins during welding. Fortuitously this realization coincided with a recent invention for Advanced Friction Welding Tools”, which was cited above in the Cross Reference to Related Applications.        
 
         [0175]     In order to impart to the proposed multi-shouldered Bobbin tool the required strength to resist the intense cylic bending and twisting during welding of multiple parallel walls, the present invention advances the concept of combining the use of compression loading of the pins, between the shoulders, with the aid of an internal tension member and also the concept of an integral pin/shank ensemble with a self-locking shoulder threaded onto the pin/shank ensemble.  
         [0176]     In regard to the Presently preferred embodiments: 
        a) Each tool will include a threaded tension member ( FIG. 8 ), which runs along the entire length of the tool (i.e. through all pins and shoulders).     b) The proposed tools will be assembled by threading opposing pairs of pin/shanks, with their respective self-locking shoulders already threaded on, and firmly “tightening” them against each other at their abutting noses, to the required level (torque and/or required elongation of the tension member). This will put the opposing pins into compression and the internal threaded rod into tension. During FSW the internal tension member will be “protected” (or “shielded”) from excessive flexing by the compressing pins, the near-perfect forced abutment between the noses of the pins and/or use of torque-sharing locks between the abutting noses of the pins ( FIG. 21 ). Torque can also be shared through roughning of the pins&#39; noses and/or other transitional locking parts placed between the noses of the pins. The compression applied to the pins by the tension member makes it possible to employ ceramics or hard, brittle alloys.     c) Once the required number of pin/shanks, with their respective self-locking shoulders have been threaded onto the tension member and tightened against each other, compression will be applied to the pins by tightening one or two tension nuts at the end(s) of the internal tension member ( FIG. 8 ).     d) Because each pair of opposing pins represents a discrete welding area of two abutting or overlapping walls, the total number (e.g. 2-4) of parallel walls to be welded between parts would be accommodated by varying the number of pin-pairs and their corresponding number of shoulders.     e) By varying the length of the pins of each pair, a bobbin tool according to the present invention can be adjusted for welding parallel walls with different thickness.     f) If the distances between different parallel walls (three or more) are not the same, with the proposed multi-shouldered FSW tool these distances are accommodated by adding or removing spacer-washers.        
 
         [0183]     The FSW tool  60  illustrated in  FIG. 8  is for producing a pair of parallel welds, for example, one weld between the upper pair of plates  112  and one weld between the lower pair of plates  112 . FSW tool  60  includes a long shank  66  which may be held in a chuck or collet of a friction stir welding machine. FSW tool  60  includes two each of shoulders  67  and  68  and two each of pin portions  71  and  73 .  
         [0184]     In the following discussion, it is presumed that FSW tool  60  is to be rotated clockwise, as seen from the lower end of long shank  66 . In that case, both shoulders  67  are right handed shoulders, that is to say, have clockwise internal threads so that friction with workpieces  112  forces the right handed shoulders  67  into firm engagement with the shank-pin ensembles to which they are attached. For example, the lower right hand shoulder  67  is attached to a shank-pin assembly including long shank  66  and the lowest pin  71 . The upper right hand shoulder  67  is attached to a shank pin assembly which includes pin portion  71 .  
         [0185]     Similarly, both of the shoulders  68  are left handed shoulders, that is to say, they have counterclockwise internal threads so that friction with workpieces  112  forces the left handed shoulders  68  into firm engagement with the shank-pin ensembles to which they are attached. These shank-pin ensembles include the pin portions  73 .  
         [0186]     The threads on pin portions  71  are left handed threads, so that plasticized material is urged toward the juncture of pin portion  71  and pin portion  73  when FSW tool  60  is rotated in a clockwise direction as seen from long shank  66 . Likewise, the threads on pin portions  73  are right handed threads so that plasticized material is urged toward the juncture of pin portion  71  and pin portion  73  when FSW tool  60  is so rotated.  
         [0187]     An optional spacer washer  69  may be employed to accommodate variable separation between the workpieces  112 . The shoulders  67  and  68 , the pin portions  73  and  73 , and spacer washer  69  are held in compression by tension member  61 , which, preferably has threaded ends and is placed in tension by nut  62  acting through a spring washer  63 . Spring washer  63  may, for example only, be a Belleville© washer.  
         [0188]      FIGS. 9, 10  and  11  show a FSW tool  70  which is for making three welds simultaneously. FSW tool  70  includes a long shank  66  which, preferably, is integral with the lowest pin  71 . FSW tool  70  is made to be rotated clockwise, as seen from long shank  66 . Preferably, each of the three welding units includes a right handed shoulder  67 , a left handed shoulder  68 , a left handed pin portion  71  and a right handed pin portion  73 .  
         [0189]      FIGS. 10 and 11  show detail of the abutting pin portions  71  and  73 , which meet at abutting junction  72 . The purpose of having the abutting junction  72  rather than making the pin portions  71  and  73  integrally is so that the shoulders  67  and  68  can be assembled by passing them over the pin portions  71  and  73 , respectively. Preferably, the right handed shoulder  67  shown in  FIGS. 10 and 11  is asembled to the shank-pin ensemble comprising pin  71  and right handed shank  84 , prior to the final assembly of FSW tool  70 . Likewise, the left handed shoulder  68  is assembled to the shank-pin ensemble comprising the long shank  75  and pin  73  shown in  FIGS. 10 and 11 , prior to the final assembly of FSW tool  70 . The thread on pin  71  is left handed and the thread on pin  73  is right handed to cause plasticized material to move toward abutting junction  72  when FSW tool  70  is rotated in a clockwise direction, as seen from the long shank  66  seen in  FIG. 9 .  
         [0190]     Each of the shoulders  67  and  68  are assembled to their respective shank-pin ensembles, before the final assembly of FSW tool  70 .  FIG. 10  shows a pair of spacer washers  83  which may be placed between the left handed shank  82  and right handed shank  84 . Final assembly of FSW tool  70  is accomplished by placing all the components, including the shoulders, each assembled to its corresponding shank-pin ensemble, and any spacer washers required onto the tension member  76 , attaching nuts  77  and spring washers  78 , and then lightning nuts  77  to provide the appropriate tension on tension member  76 , and thus the corresponding compression on pin portions  71  and  73 . To ensure torque transmission between shanks  82  and  84  the  83  washers may be designed to lock into each other and to the two shanks.  
         [0191]     Preferably, FSW tool  70  is rotated synchronously at both ends, by rotating long shanks  66  and  75 . Two electric motors, which are connected electrically, may be employed for this purpose, or one electric motor attached to a chuck or collet for one of the long shanks, and gearing to drive a chuck or collet for the other long shank may be employed.  
         [0192]      FIG. 12  illustrates FSW tool  60 , which was shown in  FIG. 8 , positioned to weld joints  113  and  114  between two extrusions  112 . Likewise,  FIG. 13  illustrates FSW tool  70 , which was illustrated in  FIGS. 9, 10  and  11 , being employed to simultaneously weld joints  123 ,  124  and  125  between two extrusions  122 .  
         [0193]      FIG. 14  illustrates a production FSW machine  130  for making a plurality of welds simultaneously, in this case, three welds.  FIG. 15  shows a section cut along the axis of FSW tool  70 , which is included in FSW machine  130 .  FIG. 14  illustrates a loading conveyor  132  and an unloading conveyor  134 .  FIG. 14  also shows a motor  136  which is for rotating the chuck or collet  138  shown in  FIG. 15 . Preferably the chuck or collet  140  at the lower end of FSW tool  70  is also turned by a second motor which has electrical connection to motor  136 , or by shafts and gears driven by motor  136 .  
         [0194]     Preferably, the workpieces, as for example, the extrusions  122 , are held and moved by grippers  142  attached to moving belt  144 . A person skilled in the art will recognize that the motive power for belt  144  must be carefully controlled to obtain a preferred velocity for the welding process, and to prevent breaking of FSW tool  70 .  
         [0195]      FIG. 16  is an illustration of a link arm having joints  152  and  154 , which can be produced in a single pass by a FSW tool such as FSW tool  60 , shown in  FIG. 8 .  
         [0196]      FIG. 17  illustrates an angle link arm having joints  157  and  158  which can likewise be produced in a single pass by a FSW tool such as FSW tool  60 .  
         [0197]      FIG. 18  illustrates a cylindrical double-walled vessel having joints  162 , which can be made by repeated passes of a FSW tool such as FSW tool  60 .  
         [0198]      FIG. 19  illustrates a double walled structure such as a boat hull, vat, tank, etc, having joints  172 , which can be made made by repeated passes of a FSW tool such as FSW tool  60 .  
         [0199]      FIG. 20  illustrates a multi-width panel having joints  182  which can be made by FSW tool  60 .  
         [0200]      FIGS. 21 and 22  illustrate two approaches to enhancing the ability of adjacent pin portions to communicate the torsion required for friction stir welding. In  FIG. 21 , a tension member  192  is employed, which, preferably is not threaded, except at the ends to receive tightening means such as nuts  62 .  
         [0201]     In order for pin portions  194  and  196  to communicate torque between them, pin portion  194  has a non-axisymmetric surface  195 , and pin portion  196  has a complimentary non-axisymmetric surface  197 . When tension in tension member  192  forces pin portion  194  tightly against pin portion  196 , torsion may be communicated between non-axisymmetric surface  195  and non-axisymmetric surface  197 . In the configuration shown in  FIG. 22 , the tension member  202  is threaded, and inside threads are formed in pin portion  204  and  206 . The threads on tension member  202  and the inside threads on pin portion  204  and  206  are employed to force the end  205  of pin portion  204  against the end  207  of pin portion  206 , so that torsion can be communicated between pin portion  204  and pin portion  206 .  
         [0202]      FIG. 23  illustrates an alternative embodiment of the present invention. FSW tool  210  includes a pin  216  having pin portions  217  and  218 . Pin portions  217  and  218  are integrally formed. There is no abutting junction such as abutting junction  72  between pin portions  71  and  73  shown in  FIG. 10 . Accordingly, shoulders  214  and  219  are threaded on from the shank sides, not from the side of the pin portions.  
         [0203]     Shoulder  214  is threaded on over shank  212  and threaded on until firm stop  215  is encountered. Likewise, shoulder  219  is threaded on from below until firm stop  220  is encountered. Likewise, shoulder  222 , lying below spacer washer  226  is threaded on until firm stop  224  is encountered. Pin  228 , like pin  216 , is integrally formed and lacks an abutting junction such as abutting junction  72 .  
         [0204]      FIG. 24  illustrates an embodiment of the present invention which is integrally formed. FSW tool  230  is for making two FSW welds simultaneously. FSW tool  230  includes upper and lower shanks  232  to be held in chucks or collets of a FSW machine. Upper shoulder has working face  236  which is adjacent upper pin portion  238 . Lower pin portion  240  is adjacent working face  242  of shoulder  244 . Shoulder  244  also has a lower working face  236 . Below the lower working face  236  of shoulder  244  is an upper pin portion  238 , which lies above a lower pin portion  240 . Lower shoulder  246  has a working face  242  adjacent lower pin portion  240 .  
         [0205]     Pin portions  238  and  240 , preferably, have opposed threads so that when FSW tool  230  is rotated in an appropriate direction, pin portions  238  and  240  urge material toward the centers of the plates being welded. Likewise when tool  230  is rotated in that direction, threads on working faces  236  and  242  urge material inwardly toward the pin portions  238  and  240 , respectively.  
         [0206]     Although presently preferred and various alternative embodiments of the present invention have been described in considerable detail above with particular reference to the figures, it should be understood that various additional modifications and/or adaptations of the present invention can be made or envisioned by those persons skilled in the relevant art without departing from either the spirit of the instant invention or the scope of the appended claims.