Patent Publication Number: US-2023150053-A1

Title: Friction stir welding method and friction stir welding apparatus

Description:
This application is based on and claims the benefit of priority from Japanese Patent Application No. 2021-186224, filed on 16 Nov. 2021, the content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a method and apparatus for welding workpieces of aluminum materials or the like to each other by friction stir welding. 
     Related Art 
     When friction stir welding workpieces to each other, from a viewpoint of welding strength or the like, it is desirable to eliminate a gap between the workpieces as much as possible and to weld the workpieces to each other in a state in which the workpieces have been brough into close contact with each other as much as possible. Therefore, for example, the workpieces may be flattened by machining before welding, may have surfaces pressed with a pneumatically or hydraulically driven jig before welding, or may be temporarily welded to each other by spot-welding before main welding. 
     Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2007-038253 
     SUMMARY OF THE INVENTION 
     However, flattening workpieces before welding results in an increase in labor and man-hours required prior to friction stir welding. 
     In a case of temporarily welding workpieces by spot-welding in a state where the workpieces are being pressed by means of a surface (hereinafter referred to as “surface-pressed”, “surface-pressing”, etc.), it is necessary to temporarily weld the workpieces outside a surface-pressing member, for avoiding interference with the surface-pressing member that, spreads in a planar, shape. Therefore, a certain region cannot be temporarily welded, which may cause a region to remain without being temporarily welded with a sufficient density. Furthermore, temporary welding can be performed only outside the surface-pressing member, and hence there is concern that temporary welding can be performed only at a position where workpieces are not sufficiently in close contact with each other. As described above, there is concern that a coupling force in temporary welding becomes insufficient and that a temporarily welded workpiece is separated to be turned up due to stirring in a main welding step (friction stir welding). The disadvantages described above are noticeable especially in a case where workpieces are large because a large surface-pressing member is used accordingly. 
     The present invention has been achieved in view of the circumstances described above and it is an object of the present invention to friction stir weld workpieces to each other efficiently firmly while reducing labor and man-hours. 
     The present inventors have made the present invention based on the findings that firm temporary welding of workpieces can be efficiently achieved by a procedure in which the workpieces are pressed against a receiving table by means of a plurality of spring pins spaced apart from each other, and the workpiece are temporarily welded to each other in a state of being pressed, at locations different from the positions of the spring pins. The present invention provides a friction stir welding method according to the following aspects features (1) to (4), and a friction stir welding apparatus according to the following aspect (5). 
     (1) A friction stir welding method for friction stir welding workpieces to each other, the method including: 
     a fixing step of fixing the workpieces to a receiving table by pressing the workpieces against the receiving table by means of a plurality of spring pins that are spaced apart from one another;
 
a temporary welding step of spot-welding the workpieces to each other by friction stir welding the workpieces at locations different from positions of the spring pins in a state where the workpieces are fixed; and
 
a main welding step of line-welding the spot-welded workpieces to each other by friction stir welding.
 
     According to the aspect (1) above, while the workpieces are pressed by means of the plurality of spring pins, the workpieces are temporarily welded to each other (spot-welded) at the locations different from the positions of the spring pins. Therefore, a certain region that cannot be temporarily welded is smaller, in comparison with a case of temporarily welding the workpieces to each other outside a surface-pressing part to avoid interference with the surface-pressing part, while pressing the workpieces with the surface-pressing part that spreads in a planar shape. This feature makes it easy to temporarily weld respective portions of the workpieces with a sufficient density. Furthermore, in comparison with a case where temporary welding can be performed only outside the surface-pressing part that spreads in a planar shape, the workpieces can be easily temporarily welded at an appropriate position where the workpieces are sufficiently in full contact with each other. As a result, firm temporary welding can be efficiently achieved, and even by way of temporary welding at a level similar to spot-welding, the workpieces can be inhibited from being turned up due to friction stirring in the main welding step. As a result, the workpieces can be friction stir welded to each other efficiently firmly. 
     Further, according to the aspect (1), prior to the main welding step, the workpieces only need to be pressed against the receiving table by means of the plurality of spring pins and temporarily welded (spot-welded), and hence labor and man-hours required prior to the main welding step are reduced, for example, in comparison with a case where the workpieces are flattened by machining. 
     As described above, according to aspect (1), the workpieces can be friction stir welded to each other efficiently firmly, while reducing the labor and man-hours. 
     (2) In the friction stir welding method according to aspect (1), the temporary welding step includes performing the spot-welding at a corner of an outer edge of a first workpiece as one of the workpieces, and the main welding step includes performing the line-welding along the outer edge. 
     According to aspect (2) above, temporary welding (spot-welding) is performed in a corner portion that is likely to be turned up in a case of performing the friction stir welding (line-welding) along the outer edge of the workpiece. Therefore, also in this respect, the workpiece can be efficiently inhibited from being turned up. Further, the corner is hard to press by means of the spring pin and is not very suitable for placing the spring pin, and hence the corner portion can be effectively used in performing the temporary welding. 
     (3) In the friction stir welding method according to aspect (1) or (2), the workpieces include a first workpiece made of an expanded material and a second workpiece made of a die-cast material, and the fixing step includes pressing the first workpiece against the second workpiece by means of the plurality of spring pins. Examples of the expanded material include a press material, an extruded material and the like processed by rolling or forging during manufacturing. 
     According to aspect (3) above, the first workpiece of the expanded material in which flatness is easy to obtain is pressed against the second workpiece of the die-cast material in which flatness is hard to obtain. Therefore, the second workpiece in which flatness is hard to obtain is held between the first workpiece in which flatness is easy to obtain and the receiving table or the like and is easily corrected to be flattened. Specifically, also in the present invention in which a plurality of locations of the workpieces are discretely pressed by means of the plurality of spring pins, the second workpiece in which flatness is hard to obtain can be surface-pressed by the first workpiece in which flatness is easy to obtain. Therefore, also in this respect, temporary welding can be efficiently firmly performed. 
     (4) In the friction stir welding method according to any one of aspects (1) to (3), the workpieces include a first workpiece and a second workpiece, 
     the second workpiece has an opening that opens in a predetermined direction, and a stepped portion that is recessed in a direction opposite to the predetermined direction and is provided outside an inner peripheral surface of the opening, the stepped portion having a bottom surface continuous with an inner peripheral surface of the opening, the fixing step includes fitting an outer edge portion of the first workpiece into the stepped portion of the second workpiece, and pressing the first workpiece against the second workpiece in the direction opposite to the predetermined direction by means of the plurality of spring pins,
 
the temporary welding step includes spot-welding the first workpiece and the second workpiece in a butted portion including a side surface of the outer edge portion of the first workpiece and a portion of the second workpiece that faces the side surface, and
 
the main welding step includes line-welding the first workpiece to the second workpiece in an overlap portion in which the bottom surface of the stepped portion and the first workpiece overlap with each other in the predetermined direction.
 
     According to aspect (4) above, the first workpiece can be positioned with respect to the second workpiece by fitting the outer, edge portion of the first workpiece into the stepped portion of the second workpiece, and also in this respect, the temporary welding can be more efficiently performed. Further, the main welding step includes performing the line-welding in the overlap portion, and hence welding strength can be more easily obtained in comparison with a case of performing the line-welding in the butted portion. 
     (5) A friction stir welding apparatus for friction stir welding workpieces to each other, the apparatus comprising: a plate including spring pins mounted at a plurality of locations that are spaced apart from one another, and having insertion holes in a plurality of locations that are different from and do not interfere with the plurality of locations, the plate being configured to fix the workpieces to a receiving table by pressing the workpieces against the receiving table by means of the plurality of spring pins; 
     a temporary welding tool that spot-welds the workpieces to each other by friction stir welding in the insertion holes; and
 
a main welding tool that line-welds the spot-welded workpieces to each other by friction stir welding.
 
     According to aspect (5) above, effects similar to those of the method of aspect (1) can be obtained. 
     As described above, according to present invention, workpieces can be friction stir welded to each other efficiently firmly, while reducing labor and man-hours. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a front cross-sectional view showing a friction stir welding apparatus of a first embodiment; 
         FIG.  2    is a front cross-sectional view showing a first half of a fixing step; 
         FIG.  3    is a front cross-sectional view showing a second half of the fixing step; 
         FIG.  4    is a plan view showing completion of the fixing step; 
         FIG.  5    is a front cross-sectional view showing a fixing step of a modification; 
         FIG.  6    is a front cross-sectional view showing the fixing step of the present embodiment; 
         FIG.  7    is a front cross-sectional view showing a temporary welding step; 
         FIG.  8    is a plan view showing completion of the temporary welding step; 
         FIG.  9    is a front cross-sectional view showing a main welding step; 
         FIG.  10    is a plan view showing the main welding step; 
         FIG.  11    is a perspective view showing a modification; 
         FIG.  12    is a front cross-sectional view showing a fixing step of a second embodiment; and 
         FIG.  13    is a front cross-sectional view showing a main welding step. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the present invention is not limited to the following embodiments and can be appropriately modified and implemented within a range that does not depart from the spirit of the present invention. 
     First Embodiment 
       FIG.  1    is a front cross-sectional view showing a friction stir welding apparatus  50  of the present embodiment. The friction stir welding apparatus  50  includes a receiving table  19 , a plate  15 , a support stand  17 , and a plurality of spring pins  10 . 
     The receiving table  19  is a table on which a first workpiece W 1  and a second workpiece W 2  that are to be friction stir welded to each other are placed with a surface length direction of the workpieces being parallel to a horizontal direction. The plate  15  is mounted on the support stand  17  and is thereby disposed above the receiving table  19  such that the surface length direction of the plate  15  is parallel to the horizontal direction. 
     The spring pins  10  are mounted to the plate  15  at a plurality of locations that are spaced apart from one another in the horizontal direction. Specifically, each spring pin  10  includes a pin  12  and a spring  11 . Each pin  12  is supported by the plate  15  such that the pin  12  can be displaced in an up-down direction. The spring  11  is provided around the pin  12 , supported by the plate  15 , and urges the pin  12  downward (toward that receiving table  19 ). The plate  15  downwardly presses, by means of the plurality of spring pins  10 , the workpieces W 1  and W 2  at a plurality of locations spaced apart from one another in the horizontal direction, thereby fixing the workpieces W 1  and W 2  to the receiving table  19  while pressing the workpieces W 1  and W 2  against the receiving table  19 . 
     The plate  15  has insertion holes  16  at a plurality of locations that do not interfere with the spring pins  10  (i.e., locations that do not overlap with the spring pins  10  in a planar view). Each insertion hole  16  is a hole through which a temporary welding tool  20  for temporarily welding the workpieces W 1  and W 2  to each other is inserted from above in a downward direction. 
     A friction stir welding method of the present embodiment includes a fixing step S 1 , a temporary welding step S 2  and a main welding step S 3 , which are described below. 
       FIG.  2    is a front cross-sectional view showing a first half of the fixing step S 1 . In the first half of the fixing step S 1 , a worker or the like sets the second workpiece W 2  on the receiving table  19  and sets the first workpiece W 1  on the second workpiece W 2 . The first workpiece W 1  is made of an expanded material, and the second workpiece W 2  is made of a die-cast material. Therefore, it is easier to obtain flatness in the first workpiece W 1  than in the second workpiece W 2 . This is because in the die-cast material, curves such as warpage and undulation and a change in plate thickness (nonuniformity) are likely to be generated by contraction of the material due to heat lowering after molding, whereas the expanded material (press material, extruded material) is flatly stretched and molded and is therefore easy to ensure flatness. 
       FIG.  3    is a front cross-sectional view showing a second half of the fixing step S 1 . In the second half of the fixing step S 1 , the worker or the like mounts the plate  15  on the support stand  17 , thereby placing the plate  15  above the workpieces W 1  and W 2 . As a result, the first workpiece W 1  and the second workpiece W 2  are fixed to the receiving table  19  while being pressed against the receiving table  15  by means of the plurality of spring pins  10  distributed and arranged in the plate  15 . 
       FIG.  4    is a plan view showing an example at completion of the fixing step S 1 . For example, as shown in  FIG.  4   , the plurality of spring pins  10  are disposed in portions other than corners Ec of an outer edge E of the first workpiece W 1 . However, in place of or in addition to these portions, the spring pin  10  may be disposed in a portion other than the outer edge E in the first workpiece W 1 . The number of spring pins  10  may be arbitrarily set, for example, to five or more, 10 or more, 20 or more, or the like in accordance with a shape, size or the like of the workpieces W 1  and W 2 . 
     If there is a gap between the first workpiece W 1  and the second workpiece W 2  due to warpages or undulations of the respective workpieces W 1  and W 2 , welding quality deteriorates. If there is a gap between the second workpiece W 2  and the receiving table  19 , in the portion with the gap, the workpieces W 1  and W 2  cannot be pressed against the receiving table  19 , which is also a factor of the deterioration of the welding quality. In this respect, according to the present embodiment, the first workpiece W 1  (expanded material) in which flatness is easy to obtain is pressed against the second workpiece W 2  (die-cast material) in which flatness is hard to obtain. Therefore, the first workpiece W 1  can be efficiently fitted along the second workpiece W 2 , and the second workpiece W 2  can be efficiently fitted along the receiving table  19 . Hereinafter, the details will be described. 
       FIG.  5    shows a modification in which not only the second workpiece W 2  but also the first workpiece W 1  are made of a die-cast material. In  FIG.  5   , a curve or the like of each workpiece W 1 , W 2  is exaggerated for visibility. Thus, according to the modification, in not only the second workpiece W 2  but also the first workpiece W 1 , a curve such as warpage or undulation and a change in plate thickness (nonuniformity) are likely to be generated. Therefore, depending on the portion, for example, the gap between the workpieces W 1  and W 2  becomes large due to overlap of a curve or the like of the first workpiece W 1  with a curve or the like of the second workpiece W 2  in a direction reverse to a direction of the curve or the like of the first workpiece. For such a reason, for example, the first workpiece W 1  cannot, be efficiently fitted along the second workpiece W 2 . 
     In a case where the workpieces W 1  and W 2  are strongly pressed by means of the spring pins  10  until the curves on the first workpiece W 1  and the second workpiece W 2  are eliminated, an output load required for the spring pins  10  and a withstand load required for the receiving table  10  increase. 
     In this respect, according to the present embodiment, as shown in  FIG.  6   , the first workpiece W 1  (expanded material) in which flatness is easy to obtain is pressed against the second workpiece W 2  (die-cast material) in which flatness is hard to obtain by means of the plurality of spring pins  10 . Therefore, the second workpiece W 2  in which flatness is hard to obtain is held between the first workpiece W 1  in which flatness is easy to obtain and the receiving table  19  and is easily corrected and flattened. Specifically, also in the present embodiment in which a plurality of locations on the workpieces W 1  and W 2  are discretely pressed by means of the plurality of spring pins  10 , the second workpiece W 2  in which flatness is hard to obtain can be surface-pressed by the first workpiece W 1  in which flatness is easy to obtain. Therefore, the first workpiece W 1  can be efficiently fitted along the second workpiece W 2 , and the second workpiece W 2  can be efficiently fitted along the receiving table  19 . 
       FIG.  7    is a front cross-sectional view showing the temporary welding step S 2 . The friction stir welding apparatus  50  further includes a movable body such as an arm, and the temporary welding tool  20 . The temporary welding tool  20  protrudes downward from a head  20   h  of the movable body and is rotated and driven around an axis (around a vertical line) by a driver such as a motor. The temporary welding tool  20  includes a shoulder  25  that protrudes downward from a lower end of the head  20   h,  and a probe  26  that protrudes downward from a lower end of the shoulder  25 . The friction stir welding apparatus  50  is capable of moving the temporary welding tool  20  in the vertical and horizontal directions by controlling the movable body (arm or the like). 
     In the temporary welding step S 2 , the movable body moves the temporary welding tool  20  to a position directly above the insertion hole  16 , and then, lovers the temporary welding tool  20  therefrom. Thereby, the temporary welding tool  20  is inserted into the insertion hole  16 , and the probe  26  is pressed against the first workpiece W 1  and the second workpiece W 2 . Thereby, the first workpiece W 1  is spot-welded to the second workpiece W 2 , to provide a temporarily welded portion Tj. 
     The spot-welding (temporary welding) may be performed sequentially by inserting one temporary welding tool  20  into a plurality of insertion holes  16  sequentially. Alternatively, in a case where the friction stir welding apparatus  50  includes a plurality of temporary welding tools  20 , a plurality of locations may be simultaneously temporarily welded with the plurality of temporary welding tools  20 . 
       FIG.  8    is a plan view showing an example at completion of the temporary welding step S 2 . Each insertion hole  16  is provided at positions that overlaps with the corners Ec of the outer edge E of the first workpiece W 1  in the plate  15 . Therefore, in the temporary welding step S 2 , the corners Ec of the outer edge E of the first workpiece W 1  are spot-welded to the second workpiece W 2 . Thereby, the temporarily welded portions Tj are provided at the corner Ec. 
       FIG.  9    is a front cross-sectional view showing the main welding step S 3 . The friction stir welding apparatus  50  further includes a main welding tool  30 . The main welding tool  30  may be the same tool as the temporary welding tool  20  or may be another tool.  FIG.  9    shows the main welding tool  30  that is different from the temporary welding tool  20 , and a probe  36  that is longer than the probe  26  of the temporary welding tool  20 . The main welding tool  30  also protrudes downward from a head  30   h  of a movable body in the same manner as the temporary welding tool  20  and is rotated and driven around an axis (around a vertical line) by a driver such as a motor. The friction stir welding apparatus  50  is capable of moving the main welding tool  30  in the vertical and horizontal directions by controlling the movable body. 
     In the main welding step S 3 , the worker or the like removes the plate  15  from the support stand  17 , thereby releasing the workpieces W 1  and W 2  from pressing by the plurality of spring pins  10 . Thereafter, the friction stir welding apparatus  50  line-welds the spot-welded workpieces W 1  and W 2  to each other by the friction stir welding using the main welding tool  30 . Specifically, the friction stir welding apparatus  50  moves the main welding tool  30  to a position directly above a desired location inside the outer edge E of the first workpiece W 1 , and then, lowers main welding tool  30  therefrom. Thereby, the probe  36  is pressed onto an upper surface of the first workpiece W 1  and is made to penetrate the first workpiece W 1  to reach the second workpiece W 2 . 
       FIG.  10    is a plan view showing the main welding step S 3 . The friction stir welding apparatus  50  moves the main welding tool  30  along the outer edge E of the first workpiece W 1  from the state where the probe  36  has reached the second workpiece W 2 , thereby line-welding a portion of the first workpiece inside the outer edge E of the first workpiece W 1  to the second workpiece W 2 . Thereby, a main welded portion Mj is provided along the outer edge E of the first workpiece W 1 . 
       FIG.  11    is a plan view showing workpieces W 1  and W 2  according to a modification of the present embodiment. As can be seen, the first workpiece W 1  may be butted against the second workpiece W 2  in a horizontal direction side. Thereafter, a temporarily welded portion Tj may be provided in a portion including a corner Ec; of an outer edge E of the first workpiece W 1  and a portion of the second workpiece W 2  that faces the corner Ec, and main welding may be performed along the outer edge E of the first workpiece W 1 . 
     The effects of the present embodiment summarized as follows. According to the present embodiment, while pressing the workpieces W 1  and W 2  downward by means of the plurality of spring pins  10  that are spaced apart from one another, the workpieces W 1  and W 2  are temporarily welded (spot-welded) to each other at a plurality of locations (insertion holes  16 ) different from the positions of the spring pins  10 . Therefore, a certain region that cannot be temporarily welded is smaller, in comparison with a case of temporarily welding the workpieces W 1  and W 2  to each other outside a surface-pressing part to avoid interference with the surface-pressing part, while pressing the workpieces W 1  and W 2  downward by means of the surface-pressing part that spreads in a planar shape in the horizontal direction. This feature makes it easy to temporarily weld respective portions of the workpieces W 1  and W 2  with a sufficient density. Furthermore, in comparison with a case where temporary welding can be performed only outside the surface-pressing part that spreads in the planar shape, the workpieces W 1  and W 2  can be more easily temporarily welded at an appropriate position where the workpieces are sufficiently in close contact with each other. In the manner described above, firm temporary welding can be efficiently achieved, and even by way of the temporarily welded portion Tj at a spot-welding level, the first workpiece W 1  can be inhibited from being turned up due to friction stirring using the main welding step S 3 . As a result, the workpieces W 1  and W 2  can be friction stir welded to each other efficiently firmly. 
     Additionally, the fixing step S 1  includes pressing the first workpiece W 1  (expanded material) against the second workpiece W 2  (die-cast material; by means of the plurality of spring pins  10 . Therefore, the second workpiece W 2  in which flatness is hard to obtain can be surface-pressed by the first workpiece W 1  in which flatness is easy to obtain. Therefore, also in this respect, firm temporary welding can be efficiently achieved. 
     Furthermore, in the temporary welding step S 2 , the temporary welding (spot-welding) is performed at the corner Ec that is likely to be turned up in a case of performing the friction stir welding (line-welding) along the outer edge E of the first workpiece W 1 . Therefore, also in this respect, the first workpiece W 1  can be efficiently inhibited from being turned up. Additionally, the corner Ec is hard to press by means of the spring pin  10  and is not very suitable for placing the spring pin  10 , and hence the corner portion can be effectively used in performing the temporary welding. 
     Further, according to the present embodiment, prior to the main welding step S 3 , the workpieces W 1  and W 2  only need to be pressed against, the receiving table  19  by means of the plurality of spring pins  10  and temporarily welded (spot-welded), and hence labor and man-hours required prior to the main welding step S 3  are reduced, for example, in comparison with a case of flattening the workpieces W 1  and W 2  by machining. Furthermore, in the subsequent main welding step S 3 , the temporarily welded portion Tj inhibits the first workpiece W 1  from being turned up, and hence a need to decelerate a moving speed of the main welding tool  30  can be reduced. As a result, also in the main welding step S 3 , the labor and man-hours can be reduced. 
     As described above, according to the present embodiment, the workpieces W 1  and W 2  can be friction stir welded to each other efficiently firmly, while reducing the labor and man-hours. 
     Second Embodiment 
     Next, a second embodiment will be described. As to the present embodiment, components and functions that are different from those of the first embodiment will be mainly described on the basis of the first embodiment, and a description of the same or similar components and functions to those of the first embodiment will be omitted as appropriate. 
       FIG.  12    is a front cross-sectional view showing a fixing step S 1 . A second workpiece W 2  has an opening W 2   a  and a stepped portion W 2   b.  The opening W 2   a  is a portion that opens upward. Specifically, the opening W 2   a  corresponds to a concave portion that opens upward in  FIG.  12   . However, the opening W 2   a  may be a through hole or the like penetrating the second workpiece W 2  in the vertical direction. The stepped portion W 2   b  is recessed downward outside an inner peripheral surface of the opening W 2   a  in the second workpiece W 2  and has a bottom surface continuous with the inner peripheral surface of the opening W 2   a.    
     In the fixing step S 1 , a worker or the like places the second workpiece W 2  on a receiving table  19  and places a first workpiece W 1  on the second workpiece W 2 . At this time, an outer edge portion W 1   b  of the first workpiece W 1  is fitted in the stepped portion W 2   b  of the second workpiece W 2 . 
     Hereinafter, a portion including a side surface of the outer edge portion W 1   b  of the first workpiece W 1  and a portion of the second workpiece W 2  that, faces the side surface is referred to as “a butted portion Bp”. Also, a portion in which the first workpiece W 1  and the bottom surface of the stepped portion W 2   b  overlap with each other in the vertical direction is referred to as “an overlap portion Op”. 
     The worker or the like places the first workpiece W 1  on the second workpiece W 2  and then mounts a plate  15  (not shown in  FIG.  12   ) on a support stand  17 . Thereby, the first workpiece W 1  and the second workpiece W 2  are fixed to the receiving table  19  while being pressed against the receiving table  19  by means of a plurality of spring pins  10  distributed and arranged in the plate  15 . 
       FIG.  13    is a front cross-sectional view showing a main welding step S 3 . Insertion holes  16  (not shown in  FIG.  13   ) are provided in the plate  15  at a plurality of locations directly above the butted portion Bp. In a temporary welding step S 2 , a friction stir welding apparatus  50 , with a temporary welding tool  20  (not shown in  FIG.  13   ) inserted into an insertion hole  16 , spot-welds the first workpiece W 1  to the second workpiece W 2  in the butted portion Bp, to provide a temporarily welded portion Tj. 
     In the main welding step S 3 , the worker or the like removes the plate  15  from the support stand  17 . Thereafter, the friction stir welding apparatus  50  moves a main welding tool  30  to a position directly above a desired location in the overlap portion Op, and then, lowers the main welding tool  30  therefrom. Thereby, a probe  36  is pressed onto an upper surface of the first workpiece W 1  and is made to penetrate the first workpiece W 1  to reach the second workpiece W 2 . From this state, the friction stir welding apparatus  50  moves the main welding tool  30  along the outer edge portion W 1   b  of the first workpiece W 1 , to line-weld the outer edge portion W 1   b  of the first workpiece W 1  to a bottom portion of the stepped portion W 2   b  of the second workpiece W 2 . Thereby, a main welded portion Mj is provided along the outer edge portion W 1   b  of the first, workpiece W 1 . 
     According to the present embodiment, by fitting the outer edge portion W 1   b  of the first workpiece W 1  into stepped portion W 2   b  of the second workpiece, the first workpiece W 1  can be positioned to the second workpiece W 2 , and also in this respect, the temporary welding can be more efficiently performed. Also, in the main welding step S 3 , the line-welding is performed in the overlap portion Op, and hence a welding strength can be more easily obtained in comparison with a case of performing the line-welding in the butted portion Bp. 
     Another Embodiment 
     The above embodiments may be, for example, modified and implemented as follows. While in the first embodiment, the corner Ec of the outer edge E of the first workpiece W 1  is temporarily welded to the second workpiece W 2 , additionally or alternatively, a portion other than the corner Ec in the first workpiece may be temporarily welded to the second workpiece W 2 . 
     EXPLANATION OF REFERENCE NUMERALS 
       10 : Spring pin 
       15 : Plate 
       16 : Insertion hole 
       19 : Receiving table 
       20 : Temporary welding tool 
       30 : Main welding tool 
       50 : Friction stir welding apparatus 
     Bp: Butted portion 
     E: Outer edge of a first workpiece 
     Ec: Corner of the outer edge of the first workpiece 
     Op: Overlap portion 
     S 2 : Fixing step 
     S 2 : Temporary welding step 
     S 3 : Main welding step 
     W 1 : First workpiece 
     W 2   b : Outer edge portion of the first workpiece 
     W 2 : Second workpiece 
     W 2   a : Opening of the second workpiece 
     W 2   b : Stepped portion of the second workpiece