Patent Publication Number: US-2017349220-A1

Title: Different material joining structure and different material joining method

Description:
TECHNICAL FIELD 
     The present invention relates to a different material joining structure and a different material joining method for joining different kinds of metals. 
     BACKGROUND ART 
     For example, Patent Document 1 discloses, as shown in  FIG. 8A , that a rivet  4  is used to join a joint face of an aluminum roof panel  1  to a joint face of a side roof rail  2  made of steel. A structural adhesive  3  having an electric insulating property is pasted between the joint faces of the aluminum roof panel  1  and the steel side roof rail  2 . 
     Further, Patent Document 2 discloses, as shown in  FIG. 8B , that, a rivet  6  for joining different materials is made to penetrate a steel panel  5  to join, and then, the rivet  6  for joining different materials is spot-welded on a panel  7  made of an aluminum alloy material, to join the panels  5 ,  7  made of different materials together. 
     PRIOR ART DOCUMENT 
     Patent Document 
     Patent Document 1: Japanese Patent Application Publication No. 2005-119577 
     Patent Document 2: Japanese Patent Application Publication No. 2010-207898 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     However, in the joining structure disclosed in Patent Document 1, when the aluminum roof panel  1  is joined to the steel side roof rail  2 , the rivet  4  needs to penetrate the aluminum roof panel  1  and the side roof rail  2 . In recent years, iron members are made to have higher strength in accordance with the increasing demand for weight reduction, causing the rivet  4  to have difficulty in penetrating the iron members. Consequently, it becomes difficult to join the aluminum roof panel  1  to the side roof rail  2  by the rivet  4 . Specifically, for example, it is difficult for the rivet to penetrate through high-strength iron materials having tensile strength of 980 MPa or more. 
     Further, in the joining structure disclosed in Patent Document 1, on a vehicle body assembly line, separate steps are needed for applying the structural adhesive  3  and for welding the rivet  4 , respectively. Therefore, as compared with an assembly step using conventional spot welding, assembly time increases and manufacturing cost soars due to additional new facility investment (for example, a structural adhesive coating device). 
     Still further, in the joining structure disclosed in Patent Document 2, the steel panel  5  needs drilling a prepared hole through which the rivet for joining different materials penetrates. This lowers production efficiency and water may leak through the prepared hole, or the water and moisture may pass through the prepared hole, to form rust on the contact faces between the rivet for joining different materials and the panel  7  made of an aluminum alloy material. 
     The purpose of the present invention is to provide a different material joining structure and a different material joining method which can be used in accordance with iron members having a high strength and requires no drilling for a prepared hole. 
     Means for Solving the Problems 
     To solve the above problems, the present invention provides a different material joining structure having: a first panel made of a first metal material; a second panel made of a second metal material that has a higher conductivity than that of the first panel, is different from the first metal material, and at least a part of which faces the first panel; and a rivet for joining the first panel and the second panel, wherein the rivet is made of the first material, and includes a head portion that is embedded in the second panel in a non-penetrating manner and a bottom portion that abuts on the first panel, and wherein a nugget portion as a joint portion is disposed between the first panel and the bottom portion, the nugget portion being formed by resistance welding the rivet to the first portion with the rivet disposed between the first panel and the second panel. 
     According to the present invention, the head portion of the rivet is embedded in the second panel (for example, by mechanical fastening such as calking) in a non-penetrating manner in advance. Then, the second panel, the rivet and the first panel are overlaid from top down in that order between a pair of electrodes used for resistance welding, and the pair of electrodes is energized in a state that the rivet is disposed between the second panel and the first panel for resistance welding. At that time, the second panel has a higher electrical conductivity than that of the first panel, to make an electric resistance the highest between the rivet and first panel having a low electrical conductivity, so as to generate heat to form the nugget portion as the joint portion. Therefore, the present invention allows the rivet which is mechanically fastened to the second panel to firmly join to the first panel made of the same kind of metal material as the rivet by resistance welding. Consequently, in the present invention, the first panel is firmly joined to the second panel via the rivet. 
     In the present invention, the rivet does not need to penetrate as the prior art disclosed in Patent Document 1, and can be used, for example, with the first panel made of a high strength material, specifically, a high strength material made of iron having a tensile strength of 980 MPa or more. 
     Further, in the present invention, the rivet is mechanically fastened to the second panel and a through hole (prepared hole) is not formed in the second panel, because the through hole (prepared hole) is unnecessary. Accordingly, in the present invention, water does not enter through a prepared hole so that galvanic corrosion (electrochemical corrosion) can be prevented even without a separate sealing material. This reduces the manufacturing cost. 
     Still further, in the present invention, the rivet can be mechanically fastened to the second panel in advance, on a line separate from the vehicle body assembly line. Accordingly, the rivet only has to be resistance welded to the first panel on the vehicle body assembly line, which increases productivity and requires no new facility investment (for example, a structural adhesive coating device), so that the cost associated with facility investment is avoided. 
     Further, in the present invention, the rivet includes a locking portion that, before the head portion is embedded in the second panel, protrudes in an axial direction from an peripheral edge of the head portion, and then is pressed to the second panel to spread in a direction orthogonal to an axial direction so as to be locked in the second panel, and, after the head portion is embedded in the second panel, the locking portion is made flush with the head portion. 
     According to the present invention, in the initial state where the rivet is not embedded in the second panel, the rivet includes the locking portion protruding in the axial direction from the peripheral edge of the head portion. In the state where the rivet is pressed to fasten the second panel, the locking portion is deformed to spread in the direction orthogonal to the axial direction, to form a flat face by the head portion and the locking portion. The joined face of the second panel which faces the flat face formed by the head portion and the locking portion is also made flat. As a result, in the present embodiment, the current during resistance welding stably flows through the nugget portion as a joint portion, allowing for forming the nugget portion stably. In other words, the fastened faces (joined faces) of the rivet and the second panel are made flat, respectively, to form a stable power supply path from the electrodes during resistance welding, so that welding defects are avoided to secure the stable nugget portion. 
     Still further, in the present invention, before the head portion is embedded in the second panel, the locking portion includes: a first side that extends from an outer peripheral face of a shaft arranged between the head portion and the bottom portion in an axial direction, and a second side that extends radially to incline downward from an extended end of the first side toward the head portion in the axial direction, and the locking portion is formed in a substantially triangular shape in cross-section by the first side, the second side and a boundary line between the locking portion and the head portion. 
     According to the present invention, the locking portion is formed in a substantially triangular shape in cross-section by the first side, the second side and the boundary line. When the rivet is pressed to join to the second panel, the locking portion is deformed to spread in the direction orthogonal to the axial direction, to form the flat face by the head portion and the locking portion. 
     Yet further, in the present invention, after the locking portion is joined to the second panel, respective contact faces of the second panel and the locking portion are made flat. 
     According to the present invention, the contact faces of the locking portion of the rivet and the second panel are made flat, respectively, to form a stable power supply path from the electrodes during resistance welding, so that welding defects are avoided to secure the stable nugget portion. 
     Yet further, in the present invention, the second panel includes an annular protrusion formed by pressing the rivet to the second panel, and, after the locking portion is embedded in the second panel, the minimum inner diameter of the protrusion is set to be smaller than the maximum outer diameter of the locking portion. 
     According to the present invention, the minimum inner diameter (D 1 ) of the protrusion is set to be smaller than the maximum outer diameter (D 2 ) of the locking portion (D 1 &lt;D 2 ), to allow for deforming the locking portion outward while spreading it so as to join (fasten) the rivet stably to the second panel. 
     Yet further, in the present invention, the first panel includes plate members and an outer diameter of the bottom portion is set to be larger than an outer diameter of the shaft. 
     According to the present invention, the bottom portion constituting the rivet has a larger diameter than the shaft, to allow for enlarging a contact area between the bottom portion of the rivet and the first panel at the time of resistance welding, as compared with a case of the bottom portion having the same diameter as the shaft. Accordingly, current density flowing during resistance welding is decreased to allow heat to be generated at a position closer to a boundary or boundaries between the plate members constituting the first panel. As a result, the rivet and the plate members constituting the first panel are joined simultaneously. 
     Yet further, in the present invention, the resistance welding is spot welding. 
     Spot welding needs a relatively short time (takt) for welding among several kinds of resistance welding and has a high welding stability, and therefore, according to the present invention, productivity is improved. Further, spot welding has been used conventionally on the common vehicle body assembly line, so that a new welding facility is not necessary and a new facility investment is avoided. 
     Yet further, in the present invention, plating is applied on an external face of the first panel. 
     According to the present invention, plating is applied on the external face of the first panel to achieve antirust effect of the steel plates constituting the first panel. If the rivet is made to penetrate the steel plates as in the prior art, the plated layer may be peeled off, which needs to be repaired, to reduce productivity. In the present embodiment, the plated layer formed on the external face suffers under little influence from the rivet, allowing for improving productivity. 
     Yet further, in the present invention, plating is applied on an external face of the rivet. 
     According to the present invention, plating is applied on the external face of the rivet to prevent galvanic corrosion (electrochemical corrosion), that is, rusting between the second panel and the rivet which are formed of different kinds of metals. Further, the rivet is made of the first panel, to prevent deterioration of rust-prevention property due to contact between different kinds of metals. 
     Yet further, in the present invention, a vehicle includes: a pair of right and left roof side rails that extends in a longitudinal direction of the vehicle at upper vehicle body sides; and a pair of right and left side outer panels that covers vehicle outer sides of the respective roof side rails to form design faces of the vehicle body sides, wherein the first panel is formed of each roof side rail, and the second panel is formed of each side outer panel. 
     According to the present invention, the side outer panel which is a larger-sized part than other body parts can be made of aluminum, aluminum-magnesium alloy, or the like having a higher conductivity than that of iron, leading to weight reduction of the vehicle body. Further, the roof side rails as vehicle body frame members can be formed of a high-tensile steel plate, leading to a high strength and a weight reduction of the vehicle body. 
     According to a different material joining method of the present invention, a head portion of a rivet is embedded in a second panel in a non-penetrating manner in advance by, a mechanical fastening method such as calking. Then, a bottom portion of the rivet is abutted on a face of the first panel that faces the second panel. While the abutting state is kept, resistance welding is performed in a state that the rivet is disposed between the first panel and the second panel, so that a nugget portion is formed between the first panel and the bottom portion. As a result, in the different material joining method of the present invention, the first panel is firmly joined to the second panel via the rivet. 
     Effect of the Invention 
     The present invention provides a different material joining structure and a different material joining method which can be used in accordance with iron members having a high strength and requires no drilling for a prepared hole. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a side view of a vehicle body side of a vehicle having a different material joining structure according to an embodiment of the present invention;  FIG. 1B  is an end view taken along a line II-II in  FIG. 1A ; 
         FIG. 2  is a partially enlarged end view of an A-portion as a first joint portion in  FIG. 1B ; 
         FIG. 3A  is a perspective view of a rivet in an initial shape;  FIG. 3B  is a cross-sectional view taken along a line in  FIG. 3A ; 
         FIGS. 4A to 4C  are cross-sectional views of a fastening step in which a head portion of the rivet is calked so as to fasten the rivet to an side outer panel; 
         FIGS. 5A to 5C  are views of steps of a different material joining method according to the present embodiment; 
         FIG. 6A  is an explanatory view of a rivet of a comparative example being spot-welded in which a diameter of a shaft is the same as that of a bottom portion;  FIG. 6B  is an explanatory view of a rivet according to the present embodiment being spot-welded in which a diameter of the bottom portion is larger than that of the shaft; 
         FIG. 7A  is an explanatory view of a case in which plating is applied on an external face of the rivet; FIG. 7 B is an explanatory view of a case in which plating is applied on an external face of a roof side rail;  FIG. 7C  is an explanatory view of a case in which plating is applied on both external faces of the rivet and the roof side rail; and 
         FIGS. 8A and 8B  are cross-sectional views of respective joining structures according to a prior art. 
     
    
    
     EMBODIMENTS FOR CARRYING OUT THE INVENTION 
     Next, a description will be given of an embodiment of the present invention with reference to the accompanying drawings appropriately.  FIG. 1A  is a side view of a vehicle body side of a vehicle having a different material joining structure according to the embodiment of the present invention,  FIG. 1B  is an end view taken along a line II-II in  FIG. 1A , and  FIG. 2  is a partially enlarged end view of an A-portion as a first joint portion in  FIG. 1B . 
     As shown in  FIGS. 1A and 1B , a vehicle  10  includes a pair of right and left roof side rails  12 ,  12  which is supported by a pair of right and left center pillars  11 ,  11  and extends in a longitudinal direction of the vehicle  10  at upper vehicle body sides, and a pair of right and left side outer panels  14 ,  14  which covers vehicle outer sides of the respective roof side rails  12  to form design faces of the vehicle body sides. 
     Note that  FIGS. 1A and 1B  only show the roof side rail  12  and the side outer panel  14  on the left side and do not show them on the right side. 
     The roof side rail  12  is made of a metal material such as steel in a hollow shape to serve as a “first panel”. Note that plating with zinc or the like is preferably applied on an an external face of the roof side rail  12  (see  FIGS. 7B and 7C  to be described later). 
     Each roof side rail  12  includes plate members which are overlaid along an upper/lower direction of the vehicle or in a substantially vertical direction, one of which being a side rail inner  16  arranged inside a vehicle compartment, the other of which being a side rail stiffener  18  positioned between the side rail inner  16  and the side outer panel  14  and arranged more outside the vehicle compartment than the side rail inner  16 . 
     Each side outer panel  14  is made of, for example, aluminum or an aluminum-magnesium alloy and is formed of a different metal material having a higher conductivity than the roof side rail  12 , to serve as a “second panel”. 
     A roof panel  20  is arranged at an upper portion of the vehicle  10  which extends in the longitudinal direction of the vehicle  10  so as to be joined to, and supported by, the pair of right and left roof side rails  12 ,  12 . 
     As shown in  FIG. 1B , a first joint portion  22  is arranged between an inner end  14   a  in the vehicle width direction of the side outer panel  14  located on an upper side and an inner end  12   a  in the vehicle width direction of the roof side rail  12  located on a lower side. A rivet (rivet for joining different materials)  24  (see  FIG. 2 ) is disposed in the first joint portion  22  for joining the side outer panel  14  to the roof side rail  12 . 
     A second joint portion  26  is arranged between an outer end in the vehicle width direction of the side outer panel  14  and an outer end  12   b  in the vehicle width direction of the roof side rail  12 . Another rivet  24  is disposed in the second joint portion  26  for joining the side outer panel  14  to the roof side rail  12 . 
     In the first joint portion  22  and the second joint portion  26 , the respective rivets  24  have the same shape. Note that, in the first joint portion  22  and the second joint portion  26 , the three plates which are the side outer panel  14 , the side rail stiffener  18  and the side rail inner  16  are, from top down, overlaid so as to be joined in one piece. 
     The rivet  24  is made of the same metal material such as iron as the roof side rail  12  which serves as a first panel. Further, as shown in  FIGS. 7A and 7C  to be described later, plating is preferably applied on an external face of the rivet  24 . 
     As shown in  FIG. 2 , the rivet  24  formed to have a substantially cylindrical shape generally in a state of the roof side rail  12  being joined to the side outer panel  14 . The rivet  24  includes a head portion  28  positioned on the upper side, a bottom portion  30  positioned on the lower side, and a shaft  32  arranged between the head portion  28  and the bottom portion  30 . 
     In the joining state, the outer diameter of the head portion  28  positioned on the upper side of the rivet  24  is set larger than that of the shaft  32  positioned under the head portion  28 . Further, an annular flange  33  is formed at the bottom portion  30  so as to be continuous to the shaft  32  and to extend radially outward from the outer peripheral face at the lower end of the shaft  32 . The outer diameter of the annular flange  33  is set larger than those of the head portion  28  and the shaft  32 . 
     The side outer panel  14  includes an annular protrusion  34  which is formed, as described later, by pressing the rivet  24  toward the side outer panel  14  in a state that the head portion  28  of the rivet  24  is in contact with a face of the side outer panel  14  which faces the roof side rail  12 . The protrusion  34  is formed by an annular expanded portion which expands radially inward (toward the rivet  24 ). The annular expanded portion is formed to have a chevron shape in cross-section. The annular expanded portion surrounds the upper side of the outer peripheral face of the shaft  32  at a skirt  28   a  on the outer periphery of the head portion  28  and the boundary portion between the head portion  28  and the shaft  32 . 
     The head portion  28  of the rivet  24  is calked in a non-penetrating manner to join a face of the side outer panel  14  which faces the roof side rail  12 . A nugget portion  36  as a joint portion is formed between the side outer panel  14  and the bottom portion  30  of the rivet  24  by spot-welding, as described later, in a state that the rivet  24  is disposed between the side outer panel  14  and the roof side rail  12 . The rivet  24  is firmly fixed to the roof side rail  12  by the nugget portion  36 . 
     Here, the initial shape of the rivet  24  will be described, in a state that the side outer panel  14  is not joined to the roof side rail  12  and the rivet  24  is not calked to the side outer panel  14 . 
       FIG. 3A  is a perspective view of the rivet in the initial shape, and  FIG. 3B  is a cross-sectional view taken along a line III-III in  FIG. 3A . 
     As shown in  FIG. 3A , in the initial shape, the rivet  24  basically includes the head portion  28  at the upper side, the bottom portion  30  at the lower side and the shaft  32  arranged between the head portion  28  and the bottom portion  30 . Further, the rivet  24  includes an annular locking portion  38  which protrudes upward in the axial direction from the peripheral edge of the head portion  28 . 
     The locking portion  38 , in the cross-sectional shape shown in  FIG. 3B , includes a first side  40  which extends upward in the axial direction of the shaft  32  from the outer peripheral face of the shaft  32  arranged between the head portion  28  and the bottom portion  30 , and a second side  42  which extends radially to incline downward (slopes downward) toward the head portion  28  from the extended end (upper end) of the first side  40 . In this case, the locking portion  38  is formed in a substantially triangular shape having acute angles in cross-section by the first side  40 , the second side  42  and a boundary line  44  which forms the boundary between the locking portion  38  and the head portion  28 . 
     The locking portion  38  is pressed to the side outer panel  14  so as to be calked, and then, spreads in the direction orthogonal to the axial direction (radially outward) to be locked by the protrusion  34  of the side outer panel  14 . 
     Further, after the locking portion  38  is pressed to be calked in the side outer panel  14 , respective contact faces of the side outer panel  14  and the locking portion  38  are made flat (see  FIG. 5A  to be described later). Thus, after the head portion  28  is calked to join the side outer panel  14 , the locking portion  38  is formed to be flush with the face of the head portion  28 . 
     As shown in  FIG. 2 , after the rivet  24  is calked and joined to the side outer panel  14 , the minimum inner diameter D 1  of the protrusion  34  surrounding the outer periphery of the locking portion  38  is set to be smaller than the maximum outer diameter D 2  of the locking portion  38  (D 1 &lt;D 2 ). 
     The vehicle body side port having the different material joining structure according to the present embodiment is basically constructed as described above. Next, advantageous effects of the structure will be described.  FIGS. 4A to 4C  are cross-sectional views of a fastening step in which the head portion of the rivet is calked to fasten the side outer panel, and  FIGS. 5A to 5C  are views of steps of a different material joining method according to the present embodiment. 
     In the assembly step of the vehicle  10 , joining the first joint portion  22  and the second joint portion  26  on the vehicle body side by spot welding (resistance welding) will be described. Note that the first joint portion  22  and the second joint portion  26  are the same in that they are joined via the rivets  24 , respectively. Therefore, the first joint portion  22  will be described, in which the roof side rail  12  including the two plates (side rail inner  16  and the side rail stiffener  18 ) is joined to the single side outer panel  14 . 
     First of all, a step (see  FIG. 5A ) will be described, in which the locking portion  38  arranged on the head portion  28  of the rivet  24  is calked in a non-penetrating manner so as to fasten to the face of the side outer panel  14  which faces the roof side rail  12 . 
     The single side outer panel  14  is set on a die  52  formed with a circular recess  50  recessed downward, and the head portion  28  of the rivet  24  is placed on the upper face of the side outer panel  14  to face the side outer panel  14  such that the rivet  24  is positioned above the recess  50  of the die  52 . As shown in  FIG. 4A , in such an arrangement, the rivet  24  is pressed from above with a predetermined pressing force F by a punch (not shown). This pressure force F makes, in the initial state, the locking portion  38  in a triangle shape having acute angles in cross-section is gradually pushed into the side outer panel  14  from top down (toward the die  52 ). Further, the side outer panel  14  is deformed along the recessed shape of the die  52 . 
     Note that the inner diameter of the recess  50  in the die  52  is set to be larger than the outer diameters of the locking portion  38  of the rivet  24 , the head portion  28  and the shaft  32 . Further, the “initial state” indicates a state in which the side outer panel  14  pressed to be deformed is not in contact with an inner bottom face  54  of the recess  50  in the die  52 . 
     Then, as shown in  FIG. 4B , after the side outer panel  14  contacts (abuts on) the inner bottom face  54  of the recess  50  in the die  52 , the side outer panel  14  which is deformed by the pressing force F from the punch (not shown) is forced to deform toward the direction (downward) pushed by the pressing force. However, the side outer panel  14  does not have a space to deform downward because the side outer panel  14  abuts on the inner bottom face  54 , so that the side outer panel  14  is deformed outward (toward directions shown by arrows). At that time, the locking portion  38  of the rivet  24  is deformed to follow the outward deformation of the side outer panel  14 . 
     As shown in  FIG. 4C , the side outer panel  14  is deformed to be filled into the recess  50  of the die  52  by the pressing force from the punch, and the annular protrusion  34  is formed which protrudes inward so as to surround the locking portion  38  of the rivet  24  and the lower end of the shaft  32 . Further, the locking portion  38  of the rivet  24  follows the outward deformation of the side outer panel  14 , allowing the locking portion  38  to extend outward so as to be deformed to protrude radially outward whose direction is substantially orthogonal to the axis of the shaft  32 . 
     After the fastening step is completed in which the rivet  24  is calked to the side outer panel  14  as shown in  FIGS. 4A to 4C , the bottom portion  30  of the rivet  24  is brought into contact with the face of the roof side rail  12  which faces the side outer panel  14  (see  FIG. 5B ). 
     Finally, while the bottom portion  30  of the rivet  24  is maintained to contact the roof side rail  12 , the roof side rail  12  is spot-welded to the side outer panel  14  in a state of having the rivet  24  therebetween (see  FIG. 5C ). 
     In other words, a predetermined current is flown through a pair of electrodes  56   a ,  56   b  facing to each other along the vertical direction in a state that the roof side rail  12  and the side outer panel  14  are respectively pressed and held by the electrodes  56   a ,  56   b . In this way, the electrodes  56   a ,  56   b  are energized to form the nugget portion  36  as a joint between the roof side rail  12  and the bottom portion  30  of the rivet  24 . The nugget portion  36  is formed largely over the two overlaid members of the side rail stiffener  18  and the side rail inner  16 , and the rivet  24 , to weld integrally and join the side rail stiffener  18 , the side rail inner  16  and the bottom portion  30  of the rivet  24  firmly. 
     Note that, in the present embodiment, spot welding is used as an example of resistance welding, but, for example, seam welding may be used for joining. 
     In the present embodiment, the head portion  28  of the rivet  24  is calked in a non-penetrating manner to the side outer panel  14  in advance for mechanical fastening ( FIG. 5A ). Then, the side outer panel  14 , the rivet  24 , and the roof side rail  12  are overlaid from top down in that order between the pair of electrodes  56   a ,  56   b  to be used for resistance welding. The pair of electrodes  56   a ,  56   b  is energized for resistance welding in a state that the rivet  24  is disposed between the side outer panel  14  and the roof side rail  12 . At that time, the side outer panel  14  has a higher electrical conductivity than that of the roof side rail  12 , to make an electric resistance the highest between the rivet  24  and the roof side rail  12  having a low electrical conductivity, so as to generate heat to form the nugget portion  36  as the joint. 
     Therefore, in the present embodiment, the rivet  24  mechanically fastened to the side outer panel  14  can be firmly joined by resistance welding to the roof side rail  12  made of the same kind of metal material as the rivet  24 . As a result, in the present embodiment, the side outer panel  14  is firmly joined to the roof side rail  12  via the rivet  24 . 
     In the present embodiment, the rivet  24  does not need to penetrate as with the prior art disclosed in Patent Document 1, and can be used, for example, with the roof side rail  12  made of a high strength material, specifically, a high strength material made of iron having tensile strength of 980 MPa or more. 
     Further, in this embodiment, the rivet  24  is mechanically fastened to the side outer panel  14  and a through hole (prepared hole) is not formed in the side outer panel  14 , because the through hole (prepared hole) is unnecessary. Accordingly, in the present embodiment, water does not enter through a prepared hole so that galvanic corrosion (electrochemical corrosion) can be prevented even without a separate sealing material. This reduces the manufacturing cost. 
     Still further, in the present embodiment, the rivet  24  can be mechanically fastened to the side outer panel  14  in advance, on a line separate from the vehicle body assembly line. Accordingly, the rivet  24  only has to be resistance welded to the roof side rail  12  on the vehicle body assembly line, which increases productivity and requires no new facility investment (for example, a structural adhesive coating device), so that the cost associated with facility investment is avoided. 
     Yet further, in the present embodiment, in the initial state where the rivet  24  is not joined to the side outer panel  14 , the rivet  24  includes the locking portion  38  protruding in the axial direction from the peripheral edge of the head portion  28 . In the state where the rivet  24  is pressed to fasten the side outer panel  14 , the locking portion  38  is deformed to spread in the direction orthogonal to the axial direction of the head portion  28 , to form the flat face by the head portion  28  and the locking portion  38  (see  FIG. 4C ). The joined face of the side outer panel  14  which faces the flat face formed by the head portion  28  and the locking portion  38  is also made flat. As a result, in the present embodiment, the current during resistance welding stably flows through the nugget portion  36  as a joint, allowing for forming the nugget portion  36  stably. In other words, the fastened faces (joined faces) of the rivet and the second panel are made flat, respectively, to form a stable power supply path from the electrodes during resistance welding, so that welding defects are avoided to secure the stable nugget portion  36 . 
     Furthermore, in the present embodiment, the locking portion  38  is formed in a substantially triangular shape in cross-section by the first side  40 , the second side  42  and the boundary line  44  (see  FIG. 3B ). When the rivet  24  is pressed to join to the side outer panel  14 , the locking portion  38  is deformed to spread in the direction orthogonal to the axial direction of the head portion  28 , to form the flat face by the head portion  28  and the locking portion  38 . 
     Furthermore, in the present embodiment, the contact faces of the locking portion  38  of the rivet  24  and the side outer panel  14  are made flat, respectively, to form a stable power supply path from the electrodes during resistance welding, so that welding defects are avoided to secure the stable nugget portion  36 . 
     Furthermore, in the present embodiment, the minimum inner diameter (D 1 ) of the protrusion  34  is set to be smaller than the maximum outer diameter (D 2 ) of the locking portion  38  (D 1 &lt;D 2 ), to allow for deforming the locking portion  38  while spreading it outward so s to join (fasten) the rivet  24  stably to the side outer panel  14 . Note that, in  FIG. 2 , the relationship between the minimum inner diameter D 1  and the maximum outer diameter D 2  is conveniently illustrated, in which the rivet  24  is disposed between, and joined to, the roof side rail  12  and the side outer panel  14 . The relationship is unchanged after the rivet  24  is calked and fastened to the side outer panel  14 . 
     Furthermore, in the present embodiment, the bottom portion  30  constituting the rivet  24  has a larger diameter than the shaft  32 , to allow for enlarging a contact area between the bottom portion  30  of the rivet  24  and the roof side rail  12  at the time of resistance welding (see  FIG. 6B ), as compared with the rivet R (see  FIG. 6A ) in the comparative example of the bottom portion and the shaft having the same diameters so as to have the same diameter as the shaft  32  of the rivet  24  according to the present embodiment. In addition, in the rivet R according to the comparative example, as shown in  FIG. 6A , a nugget portion N is formed between the lower end face of the rivet R and the roof side rail  12 . In contrast, in the present embodiment, the current density flowing during resistance welding is decreased, to allow the nugget portion  36  to be formed at a position closer to the side rail inner  16  and the side rail stiffener  18  constituting the roof side rail  12 , as compared with the rivet R according to the comparative example. As a result, in the present embodiment, the rivet  24 , and the side rail inner  16  and the side rail stiffener  18  constituting the roof side rail  12  are joined simultaneously. 
     Furthermore, in the present embodiment, spot welding requires relatively short time (takt) for welding among several kinds of resistance welding and has high welding stability, and therefore productivity is improved. Further, spot welding has been used conventionally on the common vehicle body assembly line, so that a new welding facility is not necessary and new facility investment is avoided. 
       FIG. 7A  is an explanatory view of a case in which plating is applied on the external face of the rivet,  FIG. 7B  is an explanatory view of a case in which plating is applied on the external face of the roof side rail, and  FIG. 7C  is an explanatory view of a case in which plating is applied on the external faces of the rivet and the roof side rail. 
     Furthermore, in the present embodiment, plating P (see  FIG. 7B ) is applied on the external face of the roof side rail  12  to achieve antirust effect of the steel plates. If the rivet is made to penetrate the steel plates as in the prior art, the plated layer may be peeled off, which needs to be repaired, to reduce productivity. In the present embodiment, the plated layer (plating P) formed on the external face suffers under little influence from the rivet, allowing for improving productivity. 
     Furthermore, the plating P (see  FIG. 7A ) is applied on the external face of the rivet  24  to prevent galvanic corrosion, that is, rusting between the side outer panel  14  and the rivet  24  which are formed of different kinds of metals. Further, the rivet  24  is made of the same metal material as the roof side rail  12 , to prevent deterioration of rust-prevention property due to contact between different kinds of metals. Note that, as shown in  FIG. 7C , rust-prevention effect is further increased by applying the plating P on the respective external faces of the rivet  24  and the roof side rail  12 . 
     Furthermore, in the present embodiment, the side outer panel  14  which is a larger-sized part than other body parts can be made of aluminum, an aluminum alloy, magnesium or the like having a higher conductivity than that of iron, leading to weight reduction of the vehicle body. Still further, the roof side rails  12  as vehicle body frame members can be formed of a high-tensile steel plate, leading to high strength and weight reduction of the vehicle body. 
     DESCRIPTION OF REFERENCE NUMERALS 
       10  vehicle body  12  roof side rail (first panel)  14  side outer panel (second panel)  24  rivet (rivet for joining different materials)  28  head portion  30  bottom portion  32  shaft  34  protrusion  36  nugget portion  38  locking portion  40  first side  42  second side  44  boundary line P plating