Abstract:
A method of repairing a discrepancy in a casting, weldment or other workpiece comprised of a material characterized by thermoplastic properties includes excising the discrepancy by cutting, milling, or otherwise removing workpiece material surrounding the discrepancy so as to remove the discrepancy and form a cylindrical hole in the workpiece. The method further includes placing a cylindrical plug in the cylindrical hole and moving a friction stir welding tool around the circumference of the plug to weld the plug to the workpiece.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to friction stir welding for repair of discrepancies in weldments, castings and other entities and, more particularly, it relates to friction stir welding for repair of a friction stir weld.  
       BACKGROUND OF THE INVENTION  
       [0002]     U.S. Pat. No. 5,460,317 is a classic reference on friction stir welding. In particular, it cites the use of a non-consumable probe comprised of a material which is harder than the workpieces being joined. (The probe should remain hard at elevated temperatures which are sufficient to soften the workpieces.) In a typical application, the method is used for butt welding. The probe is rotated relative to the workpieces and is pressed against the workpieces at the juncture between two workpieces. Friction between the probe and the workpieces generates heat which plasticizes the workpieces and permits the probe to enter.  
         [0003]     The workpieces are not melted by this procedure, but are softened (i.e. plasticized) sufficiently for flow to occur. Oxidation problems are thus avoided. After the probe has been plunged into the joint between the workpieces, and the portions of the workpieces in the immediate vicinity of the probe are plasticized, the probe is translated along the interface between the two workpieces, thereby thoroughly intermingling the two workpieces. It is noted that the plasticized material flows around the axis of the rotating probe whereby thorough mixing occurs.  
         [0004]     This patent also teaches repair of a crack by plunging a rotating probe into the workpiece at the site of a crack, and translating it along the crack. Plasticized material on both sides of the crack is thoroughly intermingled to repair the crack.  
         [0005]     U.S. Pat. No. 5,971,252 presents a process to repair voids in an aluminum alloy, particularly a friction stir weld in an aluminum alloy. For an elongated void, the method includes machining a trough that subsumes the void. A strip of material having the same composition as the alloy to be repaired and having sufficient volume to fill the trough is then placed in the trough. A friction stir welding tool is then traversed longitudinally through the strip whereby the strip fills the trough and is bonded to it. The strip of material does not need to be machined precisely to the shape of the trough, but it has sufficient volume to fill the trough.  
         [0006]     U.S. Pat. No. 5,975,406 teaches a method to repair a void in an aluminum alloy plate, particularly a void resulting from friction stir welding. The method includes machining the void to provide a tapered bore through the plate. Grooves and ridges are formed on the sidewall of the tapered bore. A consumable tapered plug of the same aluminum alloy as the plate is inserted into the bore and is rotated as the plug is pressed into the bore. Friction welding occurs at the interface between the plug and the bore and rotation is continued to produce a strong bond consisting of refined and recrystallized fine metal. The weld is then allowed to cool. Subsequently, portions of the plug (referred to as stabs) on each side of the plate are machined away.  
         [0007]     U.S. Pat. No. 6,213,379 is directed toward friction stir welding in which requirements of the job make it necessary for all equipment to be located on one side of the work piece. Friction stir welding is done by a plug which is held in a chuck turned by a motor. The proximal end of the plug, which is held by the chuck, has a smaller diameter than the distal end of the plug. The plug may taper from its smaller diameter at the proximal end to its larger diameter at the distal end. The plug is inserted from the back side of the workpiece (on the opposite side from the chuck and motor) and it is pulled toward the chuck. This method, friction plug pull welding, is directed toward repair of tanks such as the propellant tanks for the space shuttle.  
         [0008]     U.S. Pat. No. 6,230,957 teaches the use of friction stir welding to repair fusion welds. Passing a friction stir welding tool through a fusion weld converts the cast microstructure to a fine grained, dynamically recrystallized microstructure showing higher strength, ductility, toughness and resistance to weld cracking.  
         [0009]     U.S. Pat. No. 6,422,449 presents a method of repairing a friction stir weld by means of inert gas arc welding.  
         [0010]     U.S. Pat. No. 6,638,641 provides teaching concerning the exact shape of a rotating member for stir welding, and a recess in a workpiece. In particular, conic sections of rotation are employed for both the rotating member and the recess. These may be paraboloids, spheroids, or conoids. The shape of the rotating member may differ slightly from that of the recess to minimize the torque needed to begin the friction welding process.  
         [0011]     The teachings of the aforementioned patents are hereby incorporated into the present application by reference thereto.  
       INTRODUCTION TO THE INVENTION  
       [0012]     The present invention employs friction stir welding and, therefore, is applicable to thermoplastic materials such as metals and thermoplastics. Such materials soften when they are heated. Generally, any material which can be hot forged or hot extruded can be friction stir welded.  
         [0013]     The present invention most particularly contemplates friction stir butt welding of two workpieces on a numerically controlled friction stir welding machine. The machine has a vertical spindle which is turned by a motor, and the spindle has a chuck for a friction welding tool. The machine also has a bed which may be translated in any horizontal direction under numerical control. The workpieces are placed on the bed in position for joining, and are clamped to the bed. A backup anvil may be placed underneath the workpieces where they are to be joined.  
         [0014]     It is noted that direction-dependent terms such as vertical, horizontal, top, bottom, etc. are employed in this patent only in a relative sense. In the preceding paragraph, what is important is that the bed and spindle may be translated relatively to each other under numeric control in any direction.  
         [0015]     The present invention particularly contemplates a situation in which, during a friction welding pass, the tool breaks and remains embedded in the workpiece. This is not uncommon during friction welding due to the very high cyclic stresses which are imposed on the tool. In addition, the present invention contemplates a discrepant portion of a friction stir weld, which needs to be removed and repaired.  
         [0016]     A typical prior art response to this situation is to remove the workpieces from the bed and drill a hole on the backside of the workpieces in line with the tool. A punch is then inserted into the hole and it is pounded to remove the tool. A tapered hole is then formed though the workpieces. A tapered plug comprised of the material of the workpieces is machined, the large end of the plug is then grasped in the chuck on the spindle and the plug is friction welded into the tapered hole. Rotation of the plug is continued until a good metallurgical bond is obtained between the plug and the workpieces. The spindle is then stopped and the chuck is disengaged from the plug.  
         [0017]     The large end of the plug, which had been gripped by the chuck, is then removed by milling flush with the surfaces of the workpieces, and the small end, which protruded through the tapered hole, is also removed by milling.  
         [0018]     As can easily be appreciated, these are time-consuming steps. For example, for milling the ends of the plug, it may be preferable or necessary to employ a milling machine having a spindle with a horizontal axis. This may make it necessary to move the workpieces from one machine to another.  
         [0019]     The method of the present invention avoids these steps and, typically, performs the entire repair without removing the workpieces from the bed of the original friction stir welding machine. The first step, which is preferred, but not mandatory, is to make a small tack weld joining the two workpieces at a point ahead of the point which the tool had reached when it broke. This, preferably, is done with a small (i.e. short) friction stir welding tool. The next step is to employ a hole saw or trephining tool to excise a cylindrical plug which includes the broken tool. Alternatively, the plug can be excised by employing a small diameter milling cutter which, while it is rotated about its axis for cutting, is also moved in an orbital manner around the broken tool. The resulting hole has the shape of a right circular cylinder. It is noted that when the tack weld was made, it was positioned, preferably, so it would be on the circumference of the cylindrical hole. It serves to prevent relative movement of the unwelded portions of the two workpieces adjacent the hole during removal of the cylindrical plug containing the broken tool.  
         [0020]     A machined plug, having the same composition as the workpieces, is then placed in the cylindrical hole. Typically, it would rest on the backup anvil. It is preferred that the plug be tack welded in place, with a short friction stir welding tool, and then a full depth friction welding tool is used to travel in an orbit continuously around the circumference of the plug to weld the plug to each of the workpieces.  
         [0021]     Preferably, after the friction welding tool completes its orbit around the circumference of the plug, then, while still rotating and without being withdrawn from the workpiece(s), the friction stir welding tool proceeds along the interface between the two workpieces to complete the weld.  
         [0022]     It is an objective of the present invention to provide a method for repair of friction stir welds which, in some cases, can be accomplished in the same friction stir welding machine as the one on which the original welding is being carried out, while the workpieces remain held in the same locating/holding fixture.  
         [0023]     It is an additional objective of the present invention to provide a method for repair of friction stir welds which does not require removal of the partially friction stir welded assemblies to a separate station.  
         [0024]     It is another objective of the present invention to provide a method for repairing friction stir welds that does not require precise re-alignment between the locating/holding fixture and the friction stir welding tool.  
         [0025]     It is a further objective of the present invention to provide a method for repair of friction stir welds which employs the same process and system to repair and also weld the workpieces.  
         [0026]     It is yet another objective of the present invention to provide a highly repeatable process for repair of friction stir welds.  
         [0027]     It is also an objective of the present invention to provide a process for friction stir welding which employs the same numerically controlled machine for both repair and welding.  
         [0028]     It is a further objective of the present invention to employ one or more friction tack welds to immobilize a repair plug to facilitate friction stir welding it into place.  
         [0029]     It is an additional objective of the present invention to provide means for repairing a backup anvil after a friction stir weld repair.  
       SUMMARY OF THE INVENTION  
       [0030]     In one aspect, the present invention is a method for repairing a friction stir weld wherein a friction stir weld is being made along a joint between a first workpiece and a second workpiece, and wherein a first friction stir welding tool has broken and become embedded in the workpieces, or a situation in which a discrepant portion (i.e. having voids) has to be removed and repaired. The method includes: excising the broken tool or discrepant portion by cutting, milling or otherwise removing workpiece material surrounding the broken tool or discrepancy so as to remove the broken tool or discrepancy and form a cylindrical hole in the workpieces. A cylindrical plug is then placed in the cylindrical hole, and a second friction stir welding tool is moved around the circumference of the plug to weld the plug to the workpieces.  
         [0031]     In another aspect, the present invention is a method of repairing a discrepant feature in a casting, weldment, forging, or other workpiece comprised of a material characterized by thermoplastic properties. The method includes excising the discrepant feature by cutting, milling, or otherwise removing workpiece material surrounding the discrepant feature so as to remove the discrepant feature and form a cylindrical hole in the workpiece. The method further includes placing a cylindrical plug in the cylindrical hole and moving a friction stir welding tool around the circumference of the plug to weld the plug to the workpiece.  
         [0032]     In an additional aspect, the present invention is a method of repairing a discrepant area in a casting, weldment, or other workpiece comprised of a material characterized by thermoplastic properties. The method includes placing the workpiece on a backup anvil, and excising the discrepancy by cutting, milling, or otherwise removing workpiece material surrounding the discrepancy so as to remove the discrepancy and form a cylindrical hole in the workpiece. A cylindrical plug is then placed in the cylindrical hole and a friction stir welding tool is moved around the circumference of the plug to weld the plug to the workpiece. The method further includes repairing the backup anvil. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]      FIG. 1  is a sketch of two integrally stiffened panels which are juxtaposed for joining by friction stir welding;  
         [0034]      FIG. 2  is a schematic illustration of a tool used for friction stir welding;  
         [0035]      FIG. 3  is an illustration of a partially completed friction stir weld joining the panels shown in  FIG. 1 , and it shows a broken friction stir welding tool;  
         [0036]      FIG. 4  is a close-up of the broken tool embedded in the panels;  
         [0037]      FIG. 5  is a schematic illustration of a small friction stir welding tool making a friction tack weld on the joint between the two panels at a point ahead of the broken tool;  
         [0038]      FIG. 6  is a view showing the panels and a hole left after the broken tool has been excised;  
         [0039]      FIG. 7  is a detail of the hole shown in  FIG. 6 ;  
         [0040]      FIG. 8  is a sketch showing a machined plug being placed in the hole shown in  FIGS. 6 and 7 ;  
         [0041]      FIG. 9  is a view of the plug lying flush in the hole;  
         [0042]      FIG. 10  is an illustration of the first of three friction stir tack welds used to immobilize the plug;  
         [0043]      FIG. 11  is an illustration of the second of three tack welds used to immobilize the plug;  
         [0044]      FIG. 12  is an illustration of the third tack weld;  
         [0045]      FIG. 13  is a schematic illustration of a friction stir welding tool beginning to weld the plug in place;  
         [0046]      FIG. 14  is a schematic illustration showing the friction stir welding tool continuing to weld the plug to the panels;  
         [0047]      FIG. 15  is a schematic illustration of the friction stir welding tool concluding its orbital path around the circumference of the plug;  
         [0048]      FIG. 16  is a view of the friction stir welding tool after it has finished welding the plug and is beginning to recommence the welding pass which was interrupted by the failure of the original tool;  
         [0049]      FIG. 17  is a detail of the friction stir welding tool as it is beginning to recommence the welding pass;  
         [0050]      FIG. 18  is a view of the friction stir welding tool after it has finished welding the integrally stiffened panels;  
         [0051]      FIG. 19  is an illustration of a backup anvil with a hole which was formed when the broken stir welding tool was excised from the panels;  
         [0052]      FIG. 20  is an illustration showing a plug placed in the hole in the anvil;  
         [0053]      FIG. 21  is an illustration of the plug fusion welded to the anvil and machined smooth;  
         [0054]      FIG. 22  is a schematic illustration of an anvil with an annular recess formed during removal of a broken tool; and  
         [0055]      FIG. 23  is an illustration of a ring employed to repair the annular recess shown in  FIG. 22 . 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0056]     The methods of the present invention are applicable to materials having thermoplastic properties, that is to say, materials which soften upon heating.  
         [0057]     Attention is directed to  FIG. 1 , which illustrates workpieces  10  and  12  positioned for friction stir welding along the joint  16 . The methods of the present invention are applied to workpieces  10  and  12  having thermoplastic properties. Workpieces  10  and  12  have integral stiffening members  14 . Integral stiffening members  14  are not relevant to the present invention, but are included in the drawings because they are typical in parts to be joined by friction stir welding. Workpieces  10  and  12  are clamped by known means (not shown) to the bed (not shown) of a numerically controlled friction stir welding machine. A weld is required along joint  16  between first workpiece  10  and second workpiece  12 . The friction stir welding pass is to be started at point  17  and be continued to point  18 .  
         [0058]      FIG. 2  shows the tool  20  employed by a friction stir welding machine. Tool  20  includes a probe  22  which is plunged into the workpiece(s) to perform the friction stir welding. Tool  20  would, typically, be held in the chuck of the friction stir welding machine (not shown) with the probe  22  downward. Tool  20  is caused to rotate and then, while rotating, probe  22  is plunged into the workpieces at point  17  to begin the stir welding pass. A person skilled in the art will recognize that tool  20  operates by heating the workpieces by friction between the probe  22  and the workpieces  10  and  12  to plasticize the workpieces. During friction stir welding, the temperature generally remains below the melting point of the workpieces. For aluminum alloys, the temperature is typically about 550 C. Plasticized material adjacent the probe  22  is caused to flow around probe  22  by the rotation of the probe, thus intermingling material from workpiece  10  and workpiece  12 . A solid state bond is thus produced between workpiece  10  and workpiece  12 .  
         [0059]     Friction stir welding is similar to the extrusion process in that the worked material is heated to its softening point, and then caused to flow. Generally, in neither case, is the material melted.  
         [0060]     Friction stir welding tool  20  also includes a shoulder  24  which constrains the plasticized material from flowing upwards and out of the joining area. Preferably, shoulder  24  has a concave end  26 , which serves to contain plasticized material. It is presently preferred that probe  22  be tapered and include threads  23 . The direction of rotation of tool  20  is opposite to the rotation which would cause probe  22  to screw itself into workpieces  10  and  12  as a self-threading screw. The direction of rotation is such that threads  23  tend to cause plasticized material to move downward, into the workpieces. This type of welding has the advantage, relative to fusion welding, that formation and entrainment of oxides is much less a problem. Also, it generally results in a fine grain structure, and the process tends not to produce voids. For the case illustrated here, friction stir welding tool  20  is a full penetration friction stir welding tool.  
         [0061]     During a normal friction stir welding pass, the probe  22  of tool  20  would, while rotating, be plunged into the workpieces at initial point  17  on the joint  16  between workpiece  10  and workpiece  12 . While the tool  20  continues to rotate, it is moved along the joint  16  toward the final point  18 . As it proceeds, it continues to sustain the adiabatic friction heating within the plasticized region adjacent to the probe, and to cause flow of the plasticized material of workpieces  10  and  12  so that they intermingle. Generally, the surface of the weld produced is fairly smooth due to the shoulder  24  of tool  20 .  
         [0062]     The probe  22  of friction stir welding tool  20  is subjected to very high static and cyclic stresses and elevated temperatures. Consequently, a friction stir welding probe such as probe  22  will sometimes break and will remain embedded in the welded joint between workpieces  10  and  12 . It is a particular purpose of the present invention to recover from that situation.  
         [0063]      FIG. 3  and the magnified view in  FIG. 4  illustrate this condition. These figures show a partially completed friction stir weld,  40  having edges  42 . These figures also show the broken probe  44 .  
         [0064]      FIG. 5  illustrates a step which is an optional part of the present invention. In this optional step, a small (i.e. short) friction stir welding tool  46  is used to make a tack weld  48  (best seen in  FIG. 7 ) on joint  16  ahead of the broken probe  44 .  
         [0065]      FIGS. 6 and 7  illustrate the next step, which is to cut around the perimeter of the broken probe  44  to remove it, and leave a cylindrical hole  52 . This step may be accomplished by a hole saw or a trepanning tool. It may also be done by causing, under numeric control, a small diameter cutting tool to move orbitally around the broken probe  44 . It is noted that this orbital motion is in addition to the rotary cutting motion of the tool.  
         [0066]     The purpose of the tack weld  48  can be understood from  FIG. 7 . It serves to prevent relative separation and movement of workpieces  10  and  12  along joint  16  as hole  52  is cut.  FIG. 7  shows the exit hole  50  from which the small friction stir welding tool  46  was withdrawn after the friction tack weld  48  was completed.  
         [0067]     At this point, it is relevant to note that a backup anvil  80 , shown in phantom in  FIGS. 3 and 6 , is placed underneath joint  16  between work pieces  10  and  12  prior to beginning the friction welding pass.  
         [0068]     Preferably, hole  52  is cut through tack weld  48 , as shown in  FIG. 7 . After hole  52  is cut, a cylindrical plug  54  having about the same diameter as hole  52  and the same thickness as workpieces  10  and  12  along joint  16 , is placed in hole  52 , as shown in  FIG. 8 . Plug  54  rests on anvil  80  and, preferably, is flush with the upper surfaces of the workpieces  10  and  12 , as shown in  FIG. 9 . Preferably, the diameter of plug  54  is equal to or is slightly less than the diameter of hole  52 , whereby the diametral clearance between plug  54  and hole  52  is from 0.0 to 0.05 mm.  
         [0069]      FIGS. 10, 11  and  12  show optional steps which, preferably, are taken to immobilize plug  54  before a full penetration friction stir weld is made to weld plug  54  into hole  52 . The small friction stir welding tool  46  ( FIG. 5 ) is, preferably, employed to make the first tack weld  56  shown in  FIG. 10 . It is then employed to make the second tack weld  56  shown in  FIG. 11 , and then the third tack weld  56  shown in  FIG. 12 . Preferably, these are disposed  120  degrees apart around the circumference of plug  54 , as shown in these figures. The short friction stir welding tool  46  is moved orbitally in the direction  57  indicated in  FIG. 10  as the tack welds are made.  
         [0070]      FIGS. 13, 14  and  15  show a new full penetration friction stir welding tool  20  which is plunged into workpieces  10  and  12  after the tack welds  56  are made. Tool  20  is then, under numeric control, caused to move orbitally around the perimeter  62  of repair plug  54 . After completing the orbital welding pass, the tool  20  is then located, as shown in  FIGS. 16 and 17  at the joint  16 . Tool  20 , while still rotating, is then moved along joint  16  to end point  18  (shown in  FIG. 1 ). The friction stir weld repair is then complete, as shown in  FIG. 18 . The completed weld includes the friction stir weld  40  which was made before the failure of probe  22 , the friction stir welded area  60 , and the continued friction stir weld  64 . An exit hole will remain at point  18 , but that is consistent with the original plan for the friction stir weld, which was to be completed at point  18 . For many applications, no further machining steps will be required for the weldment comprising the first workpiece  10  and the second workpiece  12 .  
         [0071]     The weldment produced by this process will, generally, have a microstructure which reveals the process. For example, the first workpiece  10  and second workpiece  12  have a microstructure characteristic of extrusions or castings. The joint  16 , after friction stir welding, and also the friction stir welded area  60  which incorporates the plug  54  into the workpieces, has a microstructure characteristic of friction stir welding. Typically, the friction stir welded areas will have a finer grain size than portions of workpiece  10  and workpiece  12  which were not altered by friction stir welding.  
         [0072]     It is noted that, at no point in the preceding procedure, was it necessary to move the workpieces to a different machine, or to mount them in a different position on the same machine. All the preceding steps are performed with a minimum of human intervention, being performed, principally, by the numerically controlled friction stir welding machine.  
         [0073]     Some repair may, however, be required for the anvil  80 .  FIG. 19  shows the anvil  80  with a hole  66  which was formed when the broken probe  44  ( FIG. 4 ) was excised from the workpiece(s)  10  and  12 . In  FIG. 20 , a plug  68  is placed in the hole, and in  FIG. 21 , it is welded in place, preferably by gas metal arc welding. It is then machined flat whereby the welded and machined plug  70  is flush with the top surface  69  of anvil pad  82 .  
         [0074]      FIG. 22  shows an anvil  80  designed for replacement of pads. Three pads  82  are shown. These are secured by shoulder bolts  84 . Prior to performing the friction stir welding of joint  16 , anvil  80  would be positioned so pads  82  lie under joint  16 . Individual pads  82  may economically be discarded, rather than repaired.  
         [0075]      FIG. 23  shows repair of an anvil pad  82  having an annular groove  90 . The preferred repair comprises forming a ring  92  to fit into groove  90 . Ring  92  is then pressed into groove  90  to repair anvil pad  82 .  
         [0076]     The repair method described above is not limited to the repair of friction welds interrupted by broken friction stir welding tools. In general, the method can be applied to any localized discrepancy in any thermoplastic material. Generally, materials which can be formed by hot forging, extruding, etc can be repaired by the method of the present invention.  
         [0077]     The method broadly comprises:  
         [0078]     (1) Forming a cylindrical hole which removes the discrepancy.  
         [0079]     (2) Placing a cylindrical plug in the hole, the plug closely fitting inside the hole.  
         [0080]     (3) Optionally tack welding the plug in place.  
         [0081]     (4) Employing a friction stir welding tool to move around the circumference of the plug and weld it to the workpiece.  
         [0082]     (5) Optionally moving the friction stir welding tool to a place in the workpiece where an exit hole is acceptable, or where the exit hole can be repaired without harming the workpiece.  
         [0083]     The presently preferred embodiments having been described above, it is to be understood that the invention may be otherwise embodied within the scope of the following claims.