Abstract:
A method of repairing a hole in a work piece includes mechanically constraining donor plates against a work piece. Constraint plates keep donor plates flush against the work piece during conductive heat resistance welding. By constraining the donor plates, the molten mixture produced by conductive heat resistance welding cannot escape the hole and a weldment substantially free of voids is formed.

Description:
BACKGROUND 
       [0001]    The present disclosure relates to a method of repairing holes in a work piece. More particularly, the present disclosure relates to an improved method of repairing holes in a work piece using conductive heat resistance welding. 
         [0002]    An undesirable defect or hole located in a metallic work piece may be repaired by welding. Welding is a known process for joining materials by causing coalescence. For example, a damaged work piece and adjacent filler material are heated to form a molten pool in the area of the defect. The molten pool is then cooled to solidification thereby forming a weldment. 
         [0003]    It is known to fill a defect or hole in the work piece with a consumable filler plug and sandwich the work piece between sacrificial donor plates to create a stack. The stack is further sandwiched between one or more layers of electrode backing plates, which are then brought into contact with welding electrodes. An electrical current is passed between the electrodes, thereby resistively heating the electrode backing plates and conductively heating the sacrificial donor plates, work piece, and consumable filler plug. The donor plates, work piece, and filler plug coalesce into a liquid pool within the hole, which is then cooled to form a weldment. 
       SUMMARY 
       [0004]    A method of repairing a hole in a work piece includes placing a plug within the hole of the work piece and covering opposing top and bottom open end portions of the hole, as well as a portion of adjacent work piece, with a top donor sheet and a bottom donor sheet, respectively. A portion of the top donor sheet that is located above the hole is covered with at least one top electrode backing plate and a portion of the bottom donor sheet that is located beneath the hole is covered with at least one bottom electrode backing plate. The top electrode backing plate is contacted with a top electrode and the bottom electrode backing plate is contacted with a bottom electrode. Electrical current is transmitted between the top electrode and the bottom electrode to resistively heat the top and the bottom electrode backing plates and conductively heat a portion of the top and the bottom donor sheets, the plug, and the work piece to form a molten mixture that at least partially fills the hole. The top donor sheet and the bottom donor sheet are constrained with constraint plates that force the donor sheets against the work piece to contain the molten mixture within the hole. 
         [0005]    Another method of repairing a hole in a work piece includes preparing a stack that includes at least one top electrode backing plate, a top donor sheet, a work piece having a hole therein and a plug located within the hole, a bottom donor sheet, and at least one bottom electrode backing plate. The electrode backing plates and donor sheets all lie above or beneath the hole in the work piece. The top and the bottom donor sheets are constrained against the work piece. The top and the bottom electrode backing plates are contacted with electrodes. The stack is heated by electrical current to melt the top and the bottom donor sheets, the plug, and the work piece thereby forming a weldment. 
         [0006]    A welding stack for performing repairs includes a work piece having a top surface, a bottom surface, and a hole extending from the top surface to the bottom surface. A plug is located within the hole. A top donor plate is in contact with the top surface of the work piece and a bottom donor plate is in contact with the bottom surface of the work piece, so that the hole is sandwiched between the top donor plate and the bottom donor plate. At least one top constraint plate is in contact with the top donor plate and at least one bottom constraint plate is in contact with the bottom donor plate. The constraint plates force the donor plates against the work piece to facilitate preventing expulsion of molten material from the hole during conductive heat resistance welding. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a perspective view of a work piece having defects. 
           [0008]      FIG. 2  is a cross-sectional view of the work piece and defect taken on line  2 - 2  of  FIG. 1 . 
           [0009]      FIG. 3  is a cross-sectional view of a welding stack according to the prior art. 
           [0010]      FIG. 4  is a cross-sectional view of the welding stack from  FIG. 3  after conductive heat resistance welding showing porosity, expulsion of molten material, and bent donor plates. 
           [0011]      FIG. 5A  is a top view of a welding stack in accordance with an exemplary embodiment of the present disclosure. 
           [0012]      FIG. 5B  is a cross-sectional view of the welding stack taken on line  5 B- 5 B of  FIG. 5A . 
           [0013]      FIG. 6A  is a top view of an alternative embodiment of a welding stack in accordance with another exemplary embodiment of the present disclosure. 
           [0014]      FIG. 6B  is a cross-sectional view of the welding stack taken on line  6 B- 6 B of  FIG. 6A . 
       
    
    
     DETAILED DESCRIPTION 
       [0015]      FIG. 1  is a perspective view of work piece  10  having top side  12 , bottom side  14 , and defects  16 ,  18 , and  20 . Work piece  10  may include any metal, such as but not limited to, aluminum and aluminum alloys. Top side  12  and bottom side  14  are substantially planar and parallel to one another. Defect or hole  16  is substantially cylindrical and extends through work piece  10  from top side  12  to bottom side  14 , such that work piece  10  defines opposing top and bottom open end portions of hole  16 . Defects  18  and  20  are dents or other imperfections located on top side  12  of work piece  10 . 
         [0016]      FIG. 2  is a cross-sectional view of work piece  10  having top side  12 , bottom side  14 , and hole  16  taken on line  2 - 2  of  FIG. 1 . Work piece  10  may be an aluminum flange for use in the aircraft industry, and hole  16  may be formed from insertion of a bolt into work piece  10 . If hole  16  is undesirable it can be filed by conductive heat resistance welding. 
         [0017]      FIG. 3  is a cross-sectional view of prior art welding stack  22  using conductive heat resistance welding as known in the art. Welding stack  22  includes work piece  10 , top side  12 , bottom side  14 , hole  16 , plug  24 , top and bottom donor sheets  26 , top and bottom electrode backing plates  28 , top and bottom electrodes  30 , and molten pool  32 . Welding stack  22  having work piece  10  is constructed to at least partially fill hole  16  by conductive heat resistance welding. 
         [0018]    To form prior art welding stack  22 , work piece  10  having hole  16  is sandwiched between donor sheets  26 A,  26 B, electrode backing plates  28 A,  28 B, and electrodes  30 A,  30 B. Plug  24  is substantially cylindrical and smaller than hole  16  so that plug  24  fits inside of hole  16 . Plug  24  is consumed by the welding process and is formed of the same material as work piece  10 . A top end of hole  16 , as well as top side  12  of work piece  10  adjacent hole  16 , are covered by top donor sheet  26 A. Similarly, a bottom end of hole  16 , as well as bottom side  14  of work piece  10  adjacent hole  16 , are covered by bottom donor sheet  26 B. Donor sheets  26 A,  26 B are consumable by the welding process and are formed of the same material as work piece  10 . Although donor sheets  26 A,  26 B are shown singularly, there can be additional layers of donor sheets in succession. A top side of top donor sheet  26 A is covered by top electrode backing plate  28 A and a bottom side of bottom donor sheet  26 B is covered by bottom electrode backing plate  28 B. Thus, electrode backing plates  28 A,  28 B sandwich donor sheets  26 A,  26 B that sandwich work piece  10  to create stack  22 . Electrode backing plates  28 A,  28 B include substantially circular plates and are located in vertical alignment or centrally above and beneath hole  16 . Electrode backing plates  28 A,  28 B, can be formed of steel or any other material having a higher melting temperature than the melting temperature of donor sheets  26 A,  26 B. Although electrode backing plates  28 A,  28 B are shown singularly, there can be additional layers of electrode backing plates in succession. A top side of top electrode backing plate  28 A is in contact with top electrode  30 A and a bottom side of bottom electrode backing plate  28 B is in contact with bottom electrode  30 B. Molten pool is located in a center of welding stack  22  and encompasses the area of hole  16  as well as a potion of work piece  10 , and donor sheets  26 A,  26 B. 
         [0019]    Electrical current is transmitted through an approximate center of welding stack  22  by electrodes  30 A,  20 B. Electrode backing plates  28 A,  28 B are resistively heated by electrodes  30 A,  30 B and donor sheets  26 A,  26 B are conductively heated by electrode backing plates  28 A,  28 B. The conductive heating of donor sheets  26 A,  26 B also conductively heat plug  24  and work piece  10  adjacent hole  16  so that donor sheets  26 A,  26 B, plug  24 , and work piece  10  form molten pool  32  in the area of hole  16 . When current to electrodes  30 A,  30 B is turned off or electrodes  30 A,  30 B are withdrawn from stack  22 , molten pool  32  cools and solidifies into a weldment. 
         [0020]      FIG. 4  is a cross-sectional view of the welding stack from  FIG. 3  after conductive heat resistance welding showing porosity, expulsion of molten material, and bent donor plates. Depicted in  FIG. 4  are welding stack  22  including work piece  10 , top side  12 , bottom side  14 , hole  16 , top and bottom donor sheets  26 , top and bottom electrode backing plates  28 , and top and bottom electrodes  30 , weldment  34 , voids  36 , and expulsed material  38 . As described above, donor sheets  26 A,  26 B, plug  24 , and work piece  10  adjacent hole  16 , are all conductively heated to form molten pool  32 . When cooled, molten pool  32  forms weldment  34  in the area where hole  16  was located on work piece  10 . 
         [0021]    Located within, and scattered throughout weldment  34 , are voids  36 . Most materials, including aluminum and aluminum alloys, shrink during solidification causing cracking, porosity, or voids  36  in weldment  34 . Voids are highly undesirable as they reduce the strength and other mechanical properties of both weldment  34  and the overall work piece  10 . The area of work piece  10  surrounding weldment  34  is deformed or “sucked in”. Furthermore, donor plates  26  are deformed and curving away from weldment  34 . The deformation of work piece  10 , weldment  34 , and/or donor plates  26  is undesirable as it indicates the presence of unsatisfactory weldment  34 . Expulsed material  38  was part of molten pool  32  that escaped upwardly and outwardly from hole  16  during the welding process and hardened to a bottom surface or a top surface of top donor sheet  26 A and/or top electrode backing plate  28 A. Expulsed material  38  is undesirable as it is a sign of a non-uniform weld. Expulsed material  38  often reduces the volume of molten pool  32  leaving voids  36  within hole  16  thereby creating a satisfactory weldment  34 . There is a need to improve the quality and consistency of weldments  34  formed using prior art stack  22  with conductive heat resistance welding. 
         [0022]      FIG. 5A  is a top view and  FIG. 5B  is a cross-sectional view of welding stack  40  in accordance with an exemplary embodiment of the present disclosure. Welding stack  40  includes work piece  10 , top side  12 , bottom side  14 , hole  16 , plug  24 , donor sheets  26 A,  26 B, electrode backing plates  28 A,  28 B, electrodes  30 A,  30 B, molten pool  32 , and constraint plates  44 A,  44 B. The components of welding stack  40  are arranged similarly to the components of prior art welding stack  22  described above with reference to  FIG. 3 . A difference between prior art welding stack  22  and welding stack  40  in accordance with the present disclosure is the presence of constraint plates  44 A,  44 B. Constraint plates  44 A,  44 B are substantially planar, can include a space or hole for circular electrode backing plates  28 A,  28 B, and are spaced laterally or horizontally from electrode backing plates  28 A,  28 B. Top constraint plate  44 A is in contact with, and substantially covers top donor sheet  26 A. Similarly, bottom constraint plate  44 B is in contact with, and substantially covers bottom donor sheet  26 B. Constraints plates  44 A,  44 B are secured to welding stack  40  by clamps or bolts or the like. 
         [0023]    Constraint plates  44 A,  44 B provide mechanical restraint to donor sheets  26 A,  26 B, respectively, by forcing donor sheets  26 A,  26 B centrally toward work piece  10 . By constraining donor sheets  26  toward work piece  10 , hydrostatic pressure within hole  16  is maintained and molten pool  32  is not expulsed during the welding process. While electrode backing plates  28 A,  28 B provide some force to donor sheets  26 A,  26 B located in substantial vertical alignment directly above hole  16 , constraint plates  44 A,  44 B provide substantial force to donor sheets  26 A,  26 B peripherally or at a location spaced laterally or horizontally away from hole  16 . In the depicted embodiments, a space or recess for electrode backing plates  28 A,  28 B and electrodes  30 A,  30 B is cut out of constraint plates  44 A,  44 B. Constraint plates  44 A,  44 B facilitate preventing cracks and/or deformation of donor sheets  26 A,  26 B during welding. By keeping donor sheets  26 A,  26 B stiff and pressed against work piece  10 , molten pool  32  has no path to escape hole  16  and therefore, the resulting weldment is substantially free of voids. 
         [0024]      FIG. 6A  is a top view and  FIG. 6B  is a cross-sectional view of an alternative embodiment of welding stack  46  in accordance with the present disclosure. Welding stack  46  includes work piece  10 , top side  12 , bottom side  14 , hole  16 , plug  24 , donor sheets  26 A,  26 B, electrode backing plates  28 A,  28 B, electrodes  30 A,  30 B, molten pool  32 , and constraint plates  48 A,  48 B. The components of welding stack  46  are arranged similarly to the components of welding stack  40  described above with reference to  FIGS. 5A &amp; 5B . A difference between welding stack  40  and welding stack  46  is constraint plates  48 A,  48 B. While welding stack  40  includes a single top constraint plate  44 A and a single bottom constraint plate  44 B both having cut-outs, welding stack  46  includes two top constraint plates  48 A and two bottom constraint plates  48 B. Constraint plates  48 A,  48 B are located on either side of, and spaced apart from electrode backing plates  28 A,  28 B and electrodes  30 A,  30 B, respectively. Top constraint plates  48 A are in contact with, and substantially cover top donor sheet  26 A. Similarly, bottom constraint plates  48 B are in contact with, and substantially cover bottom donor sheet  26 B. Constraints plates  48 A and  48 B are secured to welding stack  46  by clamps or bolts or the like. 
         [0025]    As described above, constraint plates  48 A,  48 B provide mechanical restraint to donor sheets  26 A,  26 B by forcing donor sheets  26 A,  26 B toward work piece  10 . The functionality of constraint plates  48 A,  48 B is similar to constraint plates  44 A,  44 B. By keeping donor sheets  26 A,  26 B pressed against work piece  10 , molten pool  32  has no path to escape hole  16  and therefore, the resulting weldment is substantially free of voids. After conductive heat resistance welding is applied to welding stack  40  or  46 , any unconsumed portion of donor sheets  26 A,  26 B, electrode backing plates  28 A,  28 B, electrodes  30 A,  30 B, constraint plates  44 A,  44 B,  48 A,  48 B are removed from work piece  10 . If any excess, extraneous, or undesirable material remains attached to work piece  10  after welding, it can be removed so that the mechanical properties of the repaired work piece  10  similar to the mechanical properties of original work piece. 
         [0026]    Vertical, horizontal, above, beneath, top and bottom have been used through the specification to help define relative directions. Although the present disclosure has been described with reference to exemplary embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the disclosure.