Abstract:
In a friction stir welding method, a pair of plate materials is arranged opposing one another such that the ends thereof are butted together, after which friction stir welding is performed. A first shoulder is arranged at one surface of the pair of plate materials and a second shoulder is arranged at the other surface of the plate materials to sandwich therebetween a joint to be formed by friction stirring of the end parts of the plate materials, and the first shoulder and the second shoulder are rotated, thereby friction stirring the joint of the pair of plate materials. As the joint is friction stirred, the first shoulder and the second shoulder are moved from the other surface toward the one surface of the plate materials, thereby forming at the joint a protruding part protruding from the one surface of the plate materials as the friction stir welding is performed.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a friction stir welding method of friction stirring ends of a pair of plate materials so as to weld the plate materials. 
         [0002]    Priority is claimed on Japanese Patent Application No. 2013-231375, filed Nov. 7, 2013, the content of which is incorporated herein by reference. 
       BACKGROUND ART 
       [0003]    In the related art, in order to realize light weight, high rigidity, and improved smoothness, a hollow member which is formed of aluminum alloy and has a double skin structure (two-face structure) is used in a side structure or a floor structure of a vehicle, an aircraft, or the like, a roof structure of a building, or the like. In addition, as a method of butt-welding metal plate materials (plate-shaped members and portions) such as these hollow members, since a method referred to as friction stir welding has many advantages compared to arc welding (TIG, MIG, or the like) or the like such as an advantage in which welding defects of blow holes, solidification cracking, or the like do not easily occur, friction stir welding is widely used. 
         [0004]    In addition, as a method for performing friction stir welding, there is a method referred to as a bobbin tool type friction welding method. In this bobbin tool type friction welding method, as shown in  FIG. 4 , a rotary shaft  1  and a pin-shaped probe  2  on the tip of the rotary shaft  1  are integrally provided so as to be disposed to be coaxial with each other on an axial line O 1 , and an upper shoulder (first shoulder)  5  and a lower shoulder (second shoulder)  6  are disposed such that a joint (butting portion) S, which is disposed to butt ends  3   a  and  4   a  of a pair of plate materials  3  and  4 , is interposed between the upper shoulder and the lower shoulder. In addition, the rotary shaft  1  and the probe  2  are rotated around the axial line O 1 , and the upper and lower shoulders  5  and  6  rotate at high speed while pressing the joint S between the plate materials  3  and  4  with a predetermined pressing force. 
         [0005]    Accordingly, friction heat occurs on a contact surface between the joint S between the pair of plate materials  3  and  4  and the shoulders  5  and  6 , and the vicinity of the contact surface is plasticized. 
         [0006]    Simultaneously, structures of the ends  3   a  and  4   a  (joint S) of the pair of plate materials  3  and  4  intermix with each other due to stirring action according to high-speed rotation of the probe  2 , and the ends  3   a  and  4   a  of the pair of plate materials  3  and  4  can be strongly welded to each other (for example, refer to PTL 1). 
       CITATION LIST 
     Patent Literature 
       [0007]    [PTL 1] Japanese Unexamined Patent Application Publication No. 2006-88173 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0008]    However, in the friction stir welding method of the related art, since the upper shoulder  5  and the lower shoulder  6 , which are disposed such that the joint S between the pair of plate materials  3  and  4  is interposed therebetween, are rotated at high speed while being pressed with a predetermined pressing force, as shown in  FIG. 5 , outer surfaces (one surface  7 ) and an inner surface (the other surface  8 ) of the plate materials  3  and  4  become recessed. For example, since the plate materials  3  and  4  may be used to be uncoated in a vehicle in which hollow members having a double skin structure are welded, when indentations  9  occur on the outer surface  7  of the plate materials  3  and  4 , a disadvantage such as an external appearance of the vehicle being damaged occurs. 
       Solution to Problem 
       [0009]    According to a first aspect of the present invention, there is provided a friction stir welding method in which friction stir welding is performed in a state where ends of a pair of plate materials are disposed to face each other so as to be butted together, including: a step of disposing a first shoulder on one surface of each of the plate materials and a second shoulder on the other surface of each of the plate materials such that a joint which is welded by friction-stirring the ends of the pair of plate materials is interposed between the first shoulder and the second shoulder; a step of rotating the first shoulder and the second shoulder and friction-stirring the joint between the pair of plate materials; and a step of friction-stirring the joint, moving the first shoulder and the second shoulder in a direction from the other surface of each of the plate materials toward the one surface thereof, and performing friction stir welding while forming a protruding portion protruding from the one surface of each of the plate materials on the joint. 
         [0010]    Moreover, in a second aspect of the present invention, in the friction stir welding method according to the first aspect, preferably, the friction stir welding method further includes a step of removing the protruding portion such that the joint is flush with one surface of each of portions of the plate materials other than the protruding portion. 
         [0011]    Moreover, in a third aspect of the present invention, in the friction stir welding method according to the second aspect, preferably, the plate materials are formed in advance such that a plate thickness of the joint is a predetermined thickness in the state where the protruding portion is removed. 
         [0012]    Moreover, in a fourth aspect of the present invention, in the friction stir welding method according to each aspect, preferably, a contact area between the first shoulder and the one surface of each of the plate materials is smaller than a contact area between the second shoulder and the other surface of each of the plate materials. 
         [0013]    Moreover, in a fifth aspect of the present invention, in the friction stir welding method according to the first to third aspects, a contact area between the second shoulder and the other surface of each of the plate materials may be smaller than a contact area between the first shoulder and the one surface of each of the plate materials. 
         [0014]    Moreover, in a sixth aspect of the present invention, in the friction stir welding method according to each aspect, preferably, the first shoulder and the second shoulder move in the direction from the other surface of each of the plate materials toward the one surface thereof while the one surface of one plate material and the one surface of the other plate material are pressed by a pressing member in a state where the joint is interposed therebetween, and the protruding portion is formed so as to be the joint. 
       Advantageous Effects of Invention 
       [0015]    In a friction stir welding method of the present invention, by disposing a first shoulder and a second shoulder such that a joint (butting portions) which is disposed to butt the ends of a pair of plate materials are interposed between the first shoulder and the second shoulder, rotating the first shoulder and the second shoulder, and moving the first shoulder and the second shoulder in a direction from the other surface of each of the plate materials toward the one surface thereof while friction-stirring the joint between the pair of plate materials plasticized by friction heat, it is possible to perform friction stir welding while forming a protruding portion protruding from the one surface of each of the plate materials on the joint. 
         [0016]    Accordingly, unlike in the related art, since the protruding portion is formed in a state where indentations do not occur on the outer surface (one surface) of each of the plate materials, the protruding portion is cut/removed by grinding if necessary, and the protruding portion becomes flush with (becomes smoothly connected to) the one surface of each of portions of the plate materials the protruding portion. Accordingly, a strong joint is formed, and unlike in the friction stir welding method of the related art, it is possible to eliminate a disadvantage such as indentations remaining and an external appearance being damaged. 
         [0017]    In addition, in the friction stir welding method of the present invention, by forming the plate materials in advance such that a plate thickness of the joint is a predetermined thickness in the state where the protruding portion is removed, it is possible to prevent the plate thickness of the plate material of the joint from being thinner than portions other than the protruding portion due to the removal of the protruding portion, for example. Accordingly, even when the friction stir welding is performed while the protruding portion protruding from the one surface of each of the plate materials is formed so as to be the joint and the protruding portion is removed, it is possible to secure a desired strength and yield strength of the joint between the plate materials. 
         [0018]    In addition, in the friction stir welding method of the present invention, if the contact area between the first shoulder and the one surface of each of the plate materials is smaller than a contact area between the second shoulder and the other surface of each of the plate materials, it is possible to form a small protruding portion corresponding to the difference between the contact areas. Accordingly, when the protruding portion is processed such that the protruding portion is cut off and removed such that there is a flush surface with respect to the one surface of each of the portions of the plate materials other than the protruding portion, it is possible to decrease an amount and a size of a region of the cutting with respect to the protruding portion. 
         [0019]    Moreover, in the friction stir welding method of the present invention, if the contact area between the second shoulder and the other surface of each of the plate materials is smaller than a contact area between the first shoulder and the one surface of each of the plate materials, when the first shoulder and the second shoulder move in the direction from the other surface of each of the plate materials toward the one surface thereof and the protruding portion protruding from the one surface of each of the plate materials is formed so as to be the joint, it is possible to collectively apply a force to the joint between the pair of plate materials from the second shoulder. Accordingly, the protruding portion is easily formed, and it is possible to appropriately form the protruding portion with a small force. 
         [0020]    In addition, in the friction stir welding method of the present invention, the one surface of one plate material and the one surface of the other plate material are pressed by the pressing member in the state where the joint is interposed between the first shoulder and the second shoulder, and in this state, the first shoulder and the second shoulder move in the direction from the other surface of each of the plate materials toward the one surface thereof. Accordingly, a reaction force is secured by the pressing member, it is possible to collectively apply a force to the joint between the pair of plate materials, and it is possible to appropriately form the protruding portion. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0021]      FIG. 1  is a view showing a bobbin tool type friction stir welding device according to an embodiment of the present invention. 
           [0022]      FIG. 2  is a view showing a bobbin type friction stir welding method according to an embodiment of the present invention, (a) is a view when a friction stir welding device in which upper and lower shoulders are fixed to each other is used, and (b) is a view when a friction stir welding device in which a gap between upper and lower shoulders is variable is used. 
           [0023]      FIG. 3  is a view showing a state where a protruding portion formed by the friction stir welding method according to the embodiment of the present invention is removed. 
           [0024]      FIG. 4  is a view showing a bobbin tool type friction stir welding method of the related art. 
           [0025]      FIG. 5  is a view showing a joint (butting portion) of the bobbin tool type friction stir welding method of the related art. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0026]    Hereinafter, with reference to  FIGS. 1 to 3 , a friction stir welding method according to an embodiment of the present invention will be described. Here, in the present embodiment, a plate material according to the present invention is a hollow member which is formed of aluminum alloy or the like and has a double skin structure, and the hollow member is used in a side structure or a floor structure of a vehicle, an aircraft, or the like, a roof structure of a building, or the like. In addition, in the present embodiment, friction stir welding is performed such that metal plate materials (plate-shaped members and portions) forming the pair of hollow members are butted to each other. Moreover, the friction stir welding method of the present invention is not limited to the welding of the hollow members, and may be applied to welding of any plate material as long as it can be welded by the known friction stir welding method of the related art. 
         [0027]    First, an example of a friction welding device used in the friction stir welding method of the present embodiment will be described. As shown in  FIG. 1 , in a friction stir welding device  10  of the present embodiment, one plate material (a plate-shaped portion forming a hollow member)  3  and the other plate material  4  are disposed such that ends  3   a  and  4   a  of the plate materials  3  and  4  are butted to each other, and a joint (butting portions) S between the pair of plate materials  3  and  4  is welded by friction stir welding. 
         [0028]    In addition, the friction stir welding device  10  is configured so as to include a workpiece placement portion  11  on which the plate materials  3  and  4  are disposed, a main body section  12  which is disposed above outer surface (one surface)  7  side of each of the plate materials  3  and  4  with respect to the pair of plate materials  3  and  4  which is a workpiece disposed on the workpiece placement portion  11 , and a tool holding portion  14  which is provided in the main body section  12  and holds a bobbin tool  13  serving as a tool. 
         [0029]    In addition, the friction stir welding device  10  of the present embodiment includes pressing members  15  which support the tool holding portion  14  above the plate materials  3  and  4  and press the outer surface  7  of each of the plate materials  3  and  4  so as to stably press and hold the plate materials  3  and  4 , and load application means  16  for applying a preset load to the pressing members  15  toward the plate materials  3  and  4  disposed on the workpiece placement portion  11 . 
         [0030]    As shown in  FIGS. 1 and 2  ( FIG. 2( a ) : a bobbin tool in which upper and lower shoulders are fixed to each other and  FIG. 2( b ) : a bobbin tool in which a bobbin tool in which a gap between the upper and lower shoulders is variable), the bobbin tool  13  includes an upper shoulder (first shoulder)  5  having a first shoulder surface  5   a  which is disposed on the outer surface  7  side of each of the plate materials  3  and  4  and presses the outer surface  7 , a lower shoulder (second shoulder)  6  having a second shoulder surface  6   a  which is disposed on an inner surface  8  side of each of the plate materials  3  and  4  and presses the inner surface  8 , and a probe  2  which protrudes from the first shoulder surface  5   a  of the upper shoulder  5  and is connected to the lower shoulder  6 . 
         [0031]    In addition, a through-hole which is opened to the first shoulder surface  5   a  is formed on the upper shoulder  5  of the bobbin tool  13 , and the probe  2  is inserted into the through-hole. Accordingly, the probe  2  move forward or backward along the through-hole in a vertical direction T 1 , and it is possible to allow the second shoulder surface  6   a  to approach or to be separated from the first shoulder surface  5   a . That is, the friction stir welding device  10  of the present embodiment is the device in which the gap between the upper and lower shoulders is variable. 
         [0032]    Next, as shown in  FIG. 1 , the tool holding portion  14  is configured so as to include upper shoulder attachment member  20  having an approximately tubular shape in which the upper shoulder  5  of the bobbin tool  13  is attached to the end surface, a lower shoulder attachment shaft (rotary shaft)  1  in which the probe  2  is attached to the tip, and a support tube  21  which is formed in an approximately tubular shape and supports the upper shoulder attachment member  20  and the lower shoulder attachment shaft  1 . In addition, the upper shoulder attachment member  20 , the lower shoulder attachment shaft  1 , and the support tube  21  are disposed so as to be coaxial with one another on a center axial line O 1  and are provided along the vertical direction T 1  in which a direction of the center axial line O 1  faces the plate materials  3  and  4  which are workpieces. 
         [0033]    The upper shoulder attachment member  20  is formed in an approximately tubular shape, the lower shoulder attachment shaft  1  is inserted into a hole of the upper shoulder attachment member  20 , and the lower shoulder attachment shaft  1  is provided such that the base end protrudes from the upper shoulder attachment member  20 . In addition, a key  1   a , which extends along the axial line O 1  direction on a portion which is inserted into the upper shoulder attachment member  20 , is formed in the lower shoulder attachment shaft  1 , and a flat plate-shaped piston  1   b  is provided so as to extend in a radial direction on a portion protruding from the upper shoulder attachment member  20 . 
         [0034]    Meanwhile, a key groove  20   a , with which the key  1   a  provided in the lower shoulder attachment shaft  1  engages, is formed on the upper shoulder attachment member  20  along the center axial line O 1 . Accordingly, the lower shoulder attachment shaft  1  is provided so as to be non-rotatable with respect to the upper shoulder attachment member  20  around the center axial line O 1 , and so as to move forward or backward in the center axial line O 1 . 
         [0035]    The support tube  21  is configured to include an attachment member accommodation portion  21   a  which is opened downward and rotatably accommodates the upper shoulder attachment member  20  around the center axial line O 1 , a cylinder portion  21   b  which moves the lower shoulder attachment shaft  1  forward or backward along the center axial line O 1 , and a shaft-shaped supported portion  21   c  which is supported by the main body section  12  and protrudes from the cylinder portion  21   b  on the center axial line O 1 . 
         [0036]    A bearing portion  21   d , which accommodates a flange  20   b  of the upper shoulder attachment member  20  and rotatably supports the flange  20   b  around the center axial line O 1 , is formed in the attachment member accommodation portion  21   a . In addition, a motor  22 , which rotates the upper shoulder attachment member  20  around the center axial line O 1 , is built in an inner peripheral surface of the attachment member accommodation portion  21   a . Accordingly, the upper shoulder attachment member  20  and the lower shoulder attachment shaft  1 , which are connected to each other due to the engagement between the key  1   a  and the key groove  20   a , can rotate around the center axial line O 1  by driving of the motor  22 . 
         [0037]    The cylinder portion  21   b  is configured to include a piston support portion  21   e  having an approximately tubular shape which supports the piston  1   b , and a tip side support portion  21   f  and a base-end side support portion  21   g  which are provided on openings of a tip side and a base-end side of the piston support portion  21   e  so as to support the lower shoulder attachment shaft  1 . 
         [0038]    In addition, a bearing  21   h  is provided on an outer peripheral surface of the piston  1   b , and due to the bearing  21   h , the piston  1   b  is supported so as to move forward or backward with respect to the piston support portion  21   e  along the center axial line O 1  and so as to be rotatably supported around the center axial line O 1 . In addition, bearings  21   i  and  21   j  are respectively provided in the tip side support portion  21   f  and the base-end side support portion  21   g , and due to the bearings  21   i  and  21   j , the lower shoulder attachment shaft  1  is supported so as to move forward or backward with respect to the piston support portion  21   e  along the center axial line O 1  and so as to be rotatably supported around the center axial line O 1 . 
         [0039]    Moreover, a first hydraulic chamber  21   m  and a second hydraulic chamber  21   n , to which a hydraulic oil is supplied, are formed between the piston  1   b , and the tip side support portion  21   f  and the base-end side support portion  21   g . Moreover, hydraulic oil supply lines  21   p  and  21   q , which penetrate the main body section  12  so as to be connected to the outside and respectively communicate with the first hydraulic chamber  21   m  and the second hydraulic chamber  21   n , are formed in the cylinder portion  21   b.    
         [0040]    Accordingly, a hydraulic pressure is selectively input to the first hydraulic chamber  21   m  or the second hydraulic chamber  21   n  via the hydraulic oil supply lines  21   p  and  21   q  by a hydraulic control device (not shown) provided on the outside, and it is possible to allow the lower shoulder attachment shaft  1  to slide to the tip side or the base-end side along the center axial line O 1 . In addition, pressurizing means  23  is configured of the hydraulic control device, the cylinder portion  21   b , and the piston  1   b , the pressurizing means  23  applies a force to the plate materials  3  and  4  disposed the workpiece placement portion  11  along the center axial line O 1 , the second shoulder surface  6   a  of the lower shoulder  6  is pressed to the plate materials  3  and  4 , and the plate materials  3  and  4  which are disposed on the workpiece placement portion  11  can be pressurized. 
         [0041]    The main body section  12  is attached to a machining device main shaft  10   a . In addition, the main body section  12  is formed in an approximately tubular shape, and is configured so as to include an accommodation portion  25  which is opened toward the lower side close to the outer surface  7  side of the plate materials  3  and  4  disposed in the workpiece placement portion  11  and in which the tool holding portion  14  is accommodated, and a main body support portion  26  which supports the tool holding portion  14  accommodated in the accommodation portion  25  so as to be moved forward or backward in a vertical direction T 1  in which the tool holding portion  14  approaches or is separated from the plate materials  3  and  4 . The main body support portion  26  includes bearings  26   a  and  26   b , and supports the supported portion  21   c  by the bearings  26   a  and  26   b  so as to be movable forward or backward along the center axial line O 1 . 
         [0042]    The pressing members  15  include support members  27  which protrudes from a lower end surface of the attachment member accommodation portion  21   a  of the tool holding portion  14 , rollers  28  which are rotatably supported by the support member  27  and come into contact with the outer surfaces of the plate materials  3  and  4 , and projecting members  29  which are provided so as to be projected from the support member  27  in the radial direction. 
         [0043]    Moreover, the pressing members  15  are disposed in pair on both sides such that the bobbin tool  13  is interposed between the pressing members  15  in a lateral direction T 3  orthogonal to a direction of welding (an inner side direction on a paper surface in  FIG. 1 ) T 2  in which the bobbin tool  13  held by the tool holding portion  14  is scanned. In addition, each of the rollers  28  is provided so as to be rotatable around the rotary shaft disposed along the lateral direction T 3  orthogonal to the direction of welding, and this is, is provided so as to be rolled on the outer surface  7  of each of the plate materials  3  and  4  in the direction of welding T 2  according to scanning of the bobbin tool  13 . 
         [0044]    The load application means  16  is configured so as to include the tool holding portion  14  to which the pressing member  15  is connected and which applies a downward tool holding portion&#39;s weight and a force P 1  of the weight of the held bobbin tool  13 , and a lifting force application portion  30  which is provided between the projecting member  29  and the lower end surface of the main body section  12  and operates an auxiliary force P 2  in the direction along the center axial line O 1 . 
         [0045]    For example, the lifting force application portion  30  is an air cylinder, and it is possible to adjust the force (P 1 ) which is applied from the rollers  28  to the plate materials  3  and  4  by driving of the air cylinder. In addition, the pressing member  15  of the present embodiment is configured such that the support member  27  can be extended or contracted according to extension and contraction of the lifting force application portion  30 , and if the lifting force application portion  30  is extended, it is possible to the main body section  12 , the tool holding portion  14 , the upper shoulder  5 , and the lower shoulder  6  in a direction from the inner surface  8  of each of the plate materials  3  and  4  toward the outer surface  7 . That is, if the lifting force application portion  30  is extended, it is possible to pull the upper shoulder  5  and the lower shoulder  6  upward with respect to the plate materials  3  and  4 . 
         [0046]    In addition, when the ends  3   a  and  4   a  of the pair of plate materials  3  and  4  are welded to each other using the friction stir welding device  10  of the present embodiment having the above-described configuration, the upper shoulder  5  is disposed on the outer surface (one surface)  7  side of each of the plate materials  3  and  4  and the lower shoulder  6  is disposed on the inner surface (the other surface)  8  side of each of the plate materials  3  and  4  such that the joint S, which welds the ends  3   a  and  4   a  of the pair of plate materials  3  and  4  by friction stirring, is interposed between the upper shoulder  5  and the lower shoulder  6 . In addition, the rollers  28  of the pressing members  15  are disposed on the outer surfaces  7  of the plate materials  3  and  4 . In this state, the upward auxiliary force P 2  having predetermined strength is applied to the pair of plate materials  3  and  4  by the lifting force application portion  30 , and the load P 1  from the pressing members  15  is applied to the pair of plate materials  3  and  4 . 
         [0047]    Moreover, the motor  22  is driven, the entire tool holding portion  14  is rotated, and an upward pressing force is applied to the lower shoulder  6  by the pressurizing means  23 . Accordingly, in each of the pair of plate materials  3  and  4 , the inner surface  8  receives the pressing force from the second shoulder surface  6   a , and the force corresponding to the pressing force is similarly applied to the outer surface  7  from the first shoulder surface  5   a . Accordingly, for example, friction heat of approximately 400° C. to  50000 C is generated between the first shoulder surface  5   a  and the second shoulder surface  6   a , that is, between the outer surface  7  and the inner surface  8  of each of the pair of the plate materials  3  and  4 , and according to this friction heat, the joint S between the plate materials  3  and  4  is softened, the softened portion is stirred, and the ends  3   a  and  4   a  of the pair of the plate materials  3  and  4  are welded to each other by friction stirring. 
         [0048]    Meanwhile, in the friction stir welding method of the present embodiment, as shown in  FIG. 2 , the joint S between the pair of plate materials  3  and  4  is friction-stirred by rotating the upper shoulder  5  and the lower shoulder  6 , and the upper shoulder  5  and the lower shoulder  6  is moved in a direction from the inner surface  8  of each of the plate materials  3  and  4  toward the outer surface  7  thereof by driving the lifting force application portion  30 . That is, the upper shoulder  5  and the lower shoulder  6  are pulled upward. Accordingly, a protruding portion  31  protruding upward from the outer surface  7  of each of the plate materials  3  and  4  forms the joint S, and friction stir welding is performed on the protruding portion. 
         [0049]    In addition, in this case, for example, the upper shoulder  5  and the lower shoulder  6  are pulled with a pulling force of approximately 10 kgf to 50 kgf, and the protrusion portion  31  having a protrusion height of 0.3±0.2 mm, preferably, a protrusion height of approximately 0.1 mm to 0.5 mm is formed. 
         [0050]    Moreover, in the present embodiment, as shown in  FIG. 3 , the protruding portion  31  is cut/removed by grinding, and the joint S becomes flush with (smoothly connected to) the outer surface  7  of each of portions of the plate materials  3  and  4  other than the protruding portion. Accordingly, unlike the related art, the welding operation is performed in a state where an improved external appearance is maintained without generating indentations on the outer surface  7  of the joint S between the plate materials  3  and  4 . 
         [0051]    Here, in the friction stir welding method of the present embodiment, as shown in  FIGS. 2 and 3 , the plate materials  3  and  4  are formed such that a plate thickness t 2  of the joint S is larger than a plate thickness t 1  of each of portions of the plate materials  3  and  4  other than the protruding portion in advance, and for example, in the state where the protruding portion  31  is removed, the plate thickness t 2  of the joint S is equal to or more than the plate thickness t 1  of the portion other than the protruding portion. That is, in the present embodiment, the plate materials  3  and  4  are formed in advance such that the plate thickness t 2  of the joint S is a predetermined thickness in the state where the protruding portion  31  is removed. 
         [0052]    Accordingly, during the friction stir welding, the protruding portion  31  is formed by pulling the upper shoulder  5  and the lower shoulder  6  upward, and even when the protruding portion  31  is removed, the plate thickness t 2  of the joint S can be equal to or more than the plate thickness t 1  of each of portions other than the protruding portion, and there is no concern that yield strength of the joint S or the like may decrease. 
         [0053]    In addition, in the present embodiment, by pulling the upper shoulder  5  and the lower shoulder  6  upward while pressing the outer surface  7  of one plate material  3  and the outer surface  7  of the other plate material  4  by the pressing member  15  in the state where the joint S is interposed between the upper shoulder and the lower shoulder, the protruding portion  31  is formed so as to be the joint S. Accordingly, a reaction force is secured by the pressing member  15  during the upward pulling, and a force is collectively applied to the joint S between the plate materials  3  and  4 . Accordingly, it is possible to reliably and suitably form the protruding portion  31  having a desired height and size. 
         [0054]    In addition, a contact area between the upper shoulder  5  and the outer surface of each of the plate materials  3  and  4  may be smaller than a contact area between the lower shoulder  6  and the inner surface  8  of each of the plate materials  3  and  4 . That is, an area of the first shoulder surface  5   a  of the upper shoulder may be smaller than an area of the second shoulder surface  6   a  of the lower shoulder  6 . In this case, the formed protruding portion  31  is relatively small, and when the protruding portion  31  is cut/removed by grinding, labor of the operation decreases. 
         [0055]    Conversely, a contact area between the lower shoulder  6  and the inner surface  8  of each of the plate materials  3  and  4  may be smaller than a contact area between the upper shoulder  5  and the outer surface  7  of each of the plate materials  3  and  4 . That is, the area of the second shoulder surface  6   a  of the lower shoulder  6  may be smaller than the area of the first shoulder surface  5   a  of the upper shoulder. In this case, when the upper shoulder  5  and the lower shoulder  6  are pulled upward, a force is collectively applied from the small lower shoulder  6  to the joint S between the pair of plate materials  3  and  4 . Accordingly, the protruding portion  31  is easily formed, and it is possible to appropriately form the protruding portion  31  with a small force. 
         [0056]    Accordingly, in the friction stir welding method of the present embodiment, by disposing the upper shoulder (first shoulder)  5  and the lower shoulder (second shoulder)  6  such that joint (butting portions) S which is disposed so as to butt the ends  3   a  and  4   a  of the pair of plate materials  3  and  4  is interposed between the upper shoulder and the lower shoulder, rotating the upper shoulder  5  and the lower shoulder  6 , and pulling the upper shoulder  5  and the lower shoulder  6  upward while friction-stirring the joint S between the pair of plate materials  3  and  4  plasticized by friction heat, it is possible to perform the friction stir welding while forming the protruding portion  31  protruding from the outer surface (one surface)  7  of each of the plate materials  3  and  4  on the joint S. 
         [0057]    Accordingly, unlike the related art, since the protruding portion  31  is formed a state where indentations do not occur on the outer surface  7  of each of the plate materials  3  and  4 , the protruding portion  31  is cut/removed by grinding if necessary, and the protruding portion  31  is flush with the one surface  7  of each of portions of the plate materials  3  and  4  other than the protruding portion. Accordingly, a strong joint is formed, and unlike the friction stir welding method of the related art, it is possible to eliminate a disadvantage such as indentations remaining and an external appearance being damaged. 
         [0058]    In addition, in the friction stir welding method of the present invention, by forming the plate materials  3  and  4  in advance such that the plate thickness t 2  of the joint S is equal to or more than the predetermined thickness (t 1 ) in the state where the protruding portion  31  is removed, it is possible to prevent the plate thickness t 2  of each of the plate material  3  and  4  of the joint S from being thinner than portions other than the protruding portion due to the removal of the protruding portion  31 , for example. Accordingly, even when the friction stir welding is performed while the protruding portion  31  protruding from the outer surface  7  of each of the plate materials  3  and  4  is formed so as to be the joint S and the protruding portion  31  is removed, it is possible to secure desired strength and yield strength of the joint S between the plate materials  3  and  4 . 
         [0059]    In addition, in the friction stir welding method of the present invention, if the contact area between the upper shoulder  5  and the outer surface  7  of each of the plate materials  3  and  4  is smaller than the contact area between the lower shoulder  6  and the other surface  8  of each of the plate materials  3  and  4 , it is possible to form a small protruding portion  31  corresponding to the difference between the contact areas. Accordingly, when the protruding portion  31  is processed such that the protruding portion  31  is cut off and removed such that there is a flush surface with respect to the outer surface  7  of each of the portions of the plate materials  3  and  4  other than the protruding portion, it is possible to decrease a processing amount and a region of the cutting. 
         [0060]    Moreover, if the contact area between the lower shoulder  6  and the inner surface  8  of each of the plate materials  3  and  4  is smaller than a contact area between the upper shoulder  5  and the outer surface  7  of each of the plate materials  3  and  4 , when the upper shoulder  5  and the lower shoulder  6  are pulled upward and the protruding portion  31  is formed so as to be the joint S, it is possible to collectively apply a force to the joint S between the pair of plate materials  3  and  4  from the lower shoulder  6 . Accordingly, the protruding portion  31  is easily formed, and it is possible to appropriately form the protruding portion  31  with a small force. 
         [0061]    In addition, the outer surface  7  of one plate material  3  and the outer surface  7  of the other plate material  4  are pressed by the pressing member  15  in the state where the joint S is interposed between the first shoulder and the second shoulder, and in this state, the upper shoulder  5  and the lower shoulder  6  are pulled upward. Accordingly, a reaction force is secured by the pressing member  15 , it is possible to collectively apply a force to the joint S between the pair of plate materials  3  and  4 , and it is possible to appropriately form the protruding portion  31 . 
         [0062]    Hereinbefore, the embodiment of the friction stir welding method according to the present invention is described. However, the present invention is not limited to the embodiment and may be appropriately modified within a scope which does not depart from the gist. 
       INDUSTRIAL APPLICABILITY 
       [0063]    According to a friction stir welding method according to the present invention, it is possible to perform friction stir welding such that hollow members are butted to each other, in which the hollow members are used in a side structure or a floor structure of a vehicle, an aircraft, or the like, a roof structure of a building, or the like, are formed of aluminum alloy, and have a double skin structure, and metal plate materials (plate-shaped members and portions) forming a pair of hollow members are butted to each other. In addition, the friction stir welding method of the present invention is limited to the welding of the hollow members, and may be applied to welding of any plate material as long as it can be welded by the known friction stir welding method of the related art. 
       REFERENCE SIGNS LIST 
       [0000]    
       
         
           
               1 : lower shoulder attachment shaft (rotary shaft) 
               1   a : key 
               1   b : piston 
               2 : probe 
               3 : plate material (one plate material) 
               3   a : end 
               4 : plate material (the other plate material) 
               4   a : end 
               5 : upper shoulder (first shoulder) 
               5   a : first shoulder surface 
               6 : lower shoulder (second shoulder) 
               6   a : second shoulder surface 
               7 : outer surface (one surface) 
               8 : inner surface (the other surface) 
               9 : indentation 
               10 : friction stir welding device 
               10   a : machining device main shaft 
               11 : workpiece placement portion 
               12 : main body section 
               13 : bobbin tool 
               14 : tool holding portion 
               15 : pressing member 
               16 : load application means 
               20 : upper shoulder attachment member 
               20   a : key groove 
               20   b : flange 
               21 : support tube 
               21   a : attachment member accommodation portion 
               21   b : cylinder portion 
               21   c : supported portion 
               21   d : bearing portion 
               21   e : piston support portion 
               21   f : tip side support portion 
               21   g : base-end side support portion 
               21   h : bearing 
               21   i : bearing 
               21   j : bearing 
               21   m : first hydraulic chamber 
               21   n : second hydraulic chamber 
               21   p : hydraulic oil supply line 
               21   q : hydraulic oil supply line 
               22 : motor 
               23 : pressurizing means 
               25 : accommodation portion 
               26 : main body support portion 
               26   a : bearing 
               26   b : bearing 
               27 : support member 
               28 : roller 
               29 : projecting member 
               30 : lifting force application portion 
               31 : protruding portion 
             O 1 : axial line (center axial line) 
             S: joint (butting portion) 
             T 1 : vertical direction 
             T 2 : direction of welding 
             T 3 : lateral direction 
             t 1 : plate thickness 
             t 2 : plate thickness