Abstract:
A seam welding device sandwiches between a first roller electrode and a second roller electrode a laminate, which is formed by laminating a plurality of workpieces and disposing the thinnest workpiece, which has the smallest thickness among the workpieces, on the outside, to carry out seam welding. The second roller electrode, which is in contact with the thinnest workpiece, is disposed further along the direction of welding progress than the first roller electrode.

Description:
TECHNICAL FIELD 
     The present invention relates to a seam welding method and a seam welding device for performing seam welding with respect to a stacked assembly, which is formed by stacking a plurality of workpieces (includes a plurality of workpieces stacked), wherein among the workpieces, a thinnest workpiece having a minimum thickness is arranged on an outermost side of the stacked assembly. 
     BACKGROUND ART 
     Seam welding is widely known as a technique for joining metal plates together (for example, see Japanese Laid-Open Patent Publication No. 2007-167896). Using seam welding, after stacked metal plates (stacked assembly) are sandwiched between a pair of roller electrodes, a current is applied between the roller electrodes. More specifically, within the stacked assembly, a current pathway is formed along the stacking direction. A current that flows out from the plus electrode is conducted successively through the metal plate in contact with the plus electrode, contact sites between the metal plates, and the metal plate in contact with the minus electrode, whereupon the current reaches the minus electrode. 
     During application of current, resistance heating (Joule heat) is generated at the contact sites between the metal plates. Thus, melting takes place at such sites. 
     Thereafter, by the stacked assembly being moved relative to the pair of roller electrodes, the current pathway also moves, so that ultimately, the sites at which resistance heating occurs in the stacked assembly move as well. That is, the current moves away from the sites that have been melted prior to such movement, and therefore, resistance heating of the sites is completed. As a result, the temperature at the sites decreases, whereby the sites become solidified and acquire a solid phase. Such solidified sites are referred to primarily as nuggets. 
     On the other hand, at the sites corresponding to the newly formed current pathway, in the same manner as described above, the contact sites between the metal plates undergo melting. Thereafter, by sequentially repeating the above-described phenomenon, the metal plates are joined continuously to each other. 
     SUMMARY OF INVENTION 
     Incidentally, as the stacked assembly, a structure exists in which plural metal plates of different thicknesses are stacked. In addition, in the case that the workpiece (thinnest workpiece) with the smallest thickness is seam welded while being stacked on the outermost side of the stacked assembly, a situation occurs in which nuggets do not grow sufficiently between the thinnest workpiece and the other workpiece adjacent to the thinnest workpiece. The reason therefor is assumed to be that adequate resistance heating does not occur, due to the fact that the specific resistance is minimal as a result of the thickness of the thinnest workpiece being smallest. Although it can be considered to increase the current value so that nuggets grow sufficiently large in the vicinity of the thinnest workpiece, in this case, a defect is brought about in that so-called spatter (welding debris), a phenomenon that the workpiece is melted and undergoes scattering, is easily caused. 
     The present invention has been devised taking into consideration the aforementioned problems, and has the object of providing a seam welding method and a seam welding device, which enable nuggets of sufficient size to be formed between a thinnest workpiece arranged on an outermost side of a stacked assembly and a workpiece adjacent to the thinnest workpiece, while also dispelling any concerns over generation of welding debris. 
     [1] A seam welding method according to the present invention is characterized by performing seam welding by sandwiching a stacked assembly between a pair of roller electrodes, the stacked assembly comprising a plurality of workpieces stacked, together with arranging a thinnest workpiece having a smallest thickness on an outermost side, wherein, in a state in which one of the roller electrodes in contact with the thinnest workpiece is arranged more forwardly in the welding direction than the other roller electrode, supply of current is carried out between the pair of roller electrodes while the pair of roller electrodes is moved relatively with respect to the stacked assembly. 
     In accordance with the seam welding method according to the present invention, since the one roller electrode in contact with the thinnest workpiece is disposed more forwardly in the welding direction than the other roller electrode, in the stacked assembly, a current pathway, which is inclined in a direction opposite to the welding direction, is formed from the one roller electrode toward the other roller electrode. When this is done, in the current pathway, at a certain point in time T 1 , resistance heating occurs at a contact site (first contact site) between the thinnest workpiece and the workpiece adjacent to the thinnest workpiece. In addition, at a point in time T 2  at which the pair of roller electrodes is moved relatively with respect to the stacked assembly, a second site adjacent to the first site in the welding direction is heated, together with a third site adjacent to the other roller electrode side of the first site being heated. At this time, the first site, which already has been heated, is increased in heat further by the second site and the third site, and therefore, a nugget (well-proportioned nugget) of a sufficient size is formed at the first site. Consequently, a joint is obtained which is superior in bonding strength. 
     Further, in the case that the one roller electrode is arranged more forwardly in the welding direction than the other roller electrode, in comparison with a situation in which the two roller electrodes are aligned at the same position in the welding direction, the contact area of the respective roller electrodes with respect to the stacked assembly becomes larger, together with the contact area between the workpieces becoming larger. Consequently, since the current density of the current pathway is comparatively small, even in the case that the current value flowing between the pair of roller electrodes is increased, the occurrence of welding debris can suitably be suppressed. 
     [2] In the aforementioned seam welding method, there may be carried out a calculating step of calculating a ratio of a thickness of the stacked assembly with respect to the thickness of the thinnest workpiece, and a setting step of setting an angle of inclination along the welding direction of a line segment that passes through axes of rotation of the respective roller electrodes with respect to a line segment along a stacking direction of the stacked assembly, depending on the ratio calculated in the calculating step. 
     According to such a method, since the angle of inclination is set corresponding to the ratio of the thickness of the stacked assembly with respect to the thickness of the thinnest workpiece, nuggets that are sufficiently large in size can efficiently be formed in the stacked assembly. 
     [3] In the aforementioned seam welding method, in the setting step, the angle of inclination may be set to 5° or less. According to such a method, since the angle of inclination is set to 5° or less, it is possible to prevent the pair of roller electrodes from becoming excessively distanced from one another. Consequently, nuggets of sufficient size can be formed more effectively in the stacked assembly. 
     [4] A seam welding device according to the present invention is characterized by performing seam welding by sandwiching a stacked assembly between a pair of roller electrodes, the stacked assembly comprising a plurality of workpieces stacked, together with arranging a thinnest workpiece having a smallest thickness on an outermost side, wherein one of the roller electrodes in contact with the thinnest workpiece is arranged more forwardly in the welding direction than the other roller electrode. 
     In accordance with the seam welding device according to the present invention, since the one roller electrode in contact with the thinnest workpiece is disposed more forwardly in the welding direction than the other roller electrode, the same effects as those of the aforementioned seam welding method can be offered. 
     [5] In the aforementioned seam welding device, there may be provided a ratio calculating unit configured to calculate a ratio of a thickness of the stacked assembly with respect to the thickness of the thinnest workpiece, and an inclination angle setting unit configured to set an angle of inclination along the welding direction of a line segment that passes through axes of rotation of the respective roller electrodes with respect to a line segment along a stacking direction of the stacked assembly, based on the ratio calculated by the ratio calculating unit. 
     According to such a device, since the angle of inclination is set based on the ratio that is calculated by the ratio calculating unit, nuggets that are sufficiently large in size can efficiently be formed in the stacked assembly. 
     [6] In the aforementioned seam welding device, the inclination angle setting unit may set the angle of inclination to 5° or less. According to such a device, since the angle of inclination is set to 5° or less, it is possible to prevent the pair of roller electrodes from becoming excessively distanced from one another. Consequently, nuggets of sufficient size can be formed more effectively in the stacked assembly. 
     As has been described above, according to the present invention, since the one roller electrode in contact with the thinnest workpiece is disposed more forwardly in the welding direction than the other roller electrode, nuggets of sufficient size can be formed in the stacked assembly between the thinnest workpiece arranged on the outermost side and a workpiece that lies adjacent to the thinnest workpiece. In addition, since the contact area between the workpieces and the contact area of the respective roller electrodes with respect to the stacked assembly can be made larger, any concerns over generation of welding debris can be dispensed with. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an overall side view in outline form of a seam welding device according to an embodiment of the present invention; 
         FIG. 2  is a perspective view of a seam welding machine shown in  FIG. 1 ; 
         FIG. 3  is a schematic partial front view of the seam welding machine; 
         FIG. 4  is a flowchart for describing a seam welding method according to an embodiment of the present invention; 
         FIG. 5  is an explanatory drawing for describing a state in which a site within a stacked assembly is sandwiched by a first roller electrode and a second roller electrode, and the site is warped; 
         FIG. 6  is a schematic explanatory drawing for describing a contact area of the first roller electrode with respect to a first workpiece, and a contact area of the second roller electrode with respect to a third workpiece; 
         FIG. 7A  is an explanatory diagram showing a heated site of the stacked assembly at a time T 1 ; 
         FIG. 7B  is an explanatory diagram showing a heated site at a time T 2  that is advanced beyond time T 1 ; 
         FIG. 7C  is an explanatory diagram showing a heated site at a time T 3  that is advanced beyond time T 2 ; 
         FIG. 8  is a schematic view of a seam welding device according to a first exemplary embodiment of the present invention; 
         FIG. 9  is a cross-sectional view taken along line IX-IX of  FIG. 8 ; 
         FIG. 10  is a graph showing experimental results of the exemplary embodiment of  FIG. 8 ; and 
         FIG. 11  is an explanatory drawing for describing a seam welding method according to a modified example of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In relation to a seam welding method and a seam welding device that implements such a method according to the present invention, preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. 
     As shown in  FIGS. 1 and 2 , a seam welding device  10  according to an embodiment of the present invention performs seam welding on a stacked assembly  100  that serves as an object to be welded, and is equipped with a multi-jointed articulated robot  12 , a seam welding machine  16  supported on a distal end arm  14  of the articulated robot  12 , an input unit  17 , and a control unit  18 . 
     First, a description will be given concerning the stacked assembly  100 . The stacked assembly  100  according to the present invention is a member that is used as a door opening portion of an automobile, which is formed by stacking three workpieces (metal plates) W 1 , W 2 , W 3 . 
     The workpiece W 1 , for example, is made up from high-tensile steel such as JAC590, JAC780 or JAC980 (high-performance high-tensile steel sheets defined according to the Japan Iron and Steel Federation Standard), and forms one outermost layer (outermost surface) of the stacked assembly  100 . The workpiece W 2  is made up from the same material (high-tensile material) as the aforementioned workpiece W 1 , and forms a middle layer of the stacked assembly  100 . The workpiece W 3 , for example, is made up from JAC270 (a so-called mild steel, which is a high-performance steel sheet for press-forming defined according to the Japan Iron and Steel Federation Standard), and forms the other outermost layer (outermost surface) of the stacked assembly  100 . 
     Therefore, in the stacked assembly  100  of the present embodiment, in comparison with the workpiece W 1  and the workpiece W 2 , which are high-tensile steel materials, the workpiece W 3 , which is a mild steel material, has a characteristic of being relatively difficult to generate heat therein, due to the fact that the specific resistance thereof is low and the thermal conductivity thereof is high. 
     As understood from  FIG. 3 , the thickness of the workpiece W 1  and the thickness of the workpiece W 2  are set to D 1  (e.g., roughly 1 mm to roughly 2 mm), whereas the thickness of the workpiece W 3  is set to a smaller dimension D 2  (e.g., roughly 0.5 mm to roughly 0.7 mm) than the thickness D 1 . Stated otherwise, the workpiece W 3  is the thinnest workpiece of the stacked assembly  100 . The thickness of the workpiece W 1  and the thickness of the workpiece W 2  need not be the same and may differ from each other. 
     The articulated robot  12  is configured as a so-called industrial robot. Under the action of the control unit  18 , the articulated robot  12  can move the seam welding machine  16  in an arbitrary posture and at an arbitrary position (see  FIG. 1 ). 
     The seam welding machine  16  includes a guide rail  20 , which is fixed through a mount  19  with respect to the distal end arm  14  of the articulated robot  12 , a first drive mechanism  22  and a second drive mechanism  24 , which are disposed on the guide rail  20 , a first roller electrode  26 , which is disposed on the first drive mechanism  22 , and a second roller electrode  28 , which is disposed on the second drive mechanism  24 . 
     The guide rail  20  is configured in a rectangular parallelepiped shape, and substantially in a central part in the widthwise direction (the direction of the arrow Z in  FIG. 2 ) of a surface on an opposite side from the side on which the distal end arm  14  is positioned, a protrusion  30  is formed that spans across the entire length thereof. 
     The first drive mechanism  22  includes a first moving table  32  disposed movably with respect to the guide rail  20  along the direction of extension (the direction of the arrow Y) of the guide rail  20 , and a first rotary shaft  34  disposed on the first moving table  32  and to which the first roller electrode  26  is fixed. 
     A recess  36 , which engages slidably with the protrusion  30  of the guide rail  20 , is formed on the first moving table  32 . By a first cylinder and a first rod (not illustrated), for example, which are provided on the guide rail  20 , the first moving table  32  is capable of moving with respect to the guide rail  20  along the direction of extension of the guide rail  20 . 
     The first rotary shaft  34  extends along the thickness direction (the direction perpendicular to the direction of the arrow Y and the direction of the arrow Z) of the guide rail  20 , and is capable of being rotated by a non-illustrated first rotary motor that is disposed on the first moving table  32 . 
     The second drive mechanism  24  includes a second moving table  38  disposed movably with respect to the guide rail  20  along the direction of extension (the direction of the arrow Y) of the guide rail  20 , a third moving table  40  disposed movably with respect to the second moving table  38  along the widthwise direction (the direction of the arrow Z in  FIG. 2 ) of the guide rail  20 , and a second rotary shaft  42  disposed on the third moving table  40  and to which the second roller electrode  28  is fixed. 
     A recess  44 , which engages slidably with the protrusion  30  of the guide rail  20 , is formed on the second moving table  38 . By a second cylinder and a second rod (not illustrated), for example, which are provided on the guide rail  20 , the second moving table  38  is capable of moving with respect to the guide rail  20  along the direction of extension of the guide rail  20 . On a surface on an opposite side from the surface on which the recess  44  is formed in the second moving table  38 , a protrusion  46  is formed that spans across the entire length of the guide rail  20 . 
     A recess  48 , which engages slidably with the protrusion  46  of the second moving table  38 , is formed on the third moving table  40 . By a third cylinder and a third rod (not illustrated), for example, which are provided on the second moving table  38 , the third moving table  40  is capable of moving with respect to the second moving table  38  along the widthwise direction (the direction of the arrow Z) of the guide rail  20 . 
     The second rotary shaft  42  is capable of being rotated by a non-illustrated second rotary motor that is disposed on the third moving table  40  in a state of being arranged parallel to the first rotary shaft  34 . 
     Each of the first roller electrode  26  and the second roller electrode  28  is formed in a disc shape. As understood from  FIGS. 2 and 3 , an outer circumferential surface of the first roller electrode  26  contacts one surface of the stacked assembly  100  (workpiece W 1 ), and an outer circumferential surface of the second roller electrode  28  contacts the other surface of the stacked assembly  100  (workpiece W 3 ). The first roller electrode  26  and the second roller electrode  28  may be formed with the same structure, and may be formed such that the two electrodes differ in a dimension in the diametrical direction or in the widthwise direction. 
     The seam welding machine  16  includes a welding power source unit  50 , a first lead wire (power line)  52  that electrically connects the first roller electrode  26  and a negative electrode of the welding power source unit  50 , and a second lead wire (power line)  54  that electrically connects the second roller electrode  28  and a positive electrode of the welding power source unit  50  (see  FIGS. 1 and 3 ). 
     The welding power source unit  50  is constituted to include an AC power source and a welding transformer, etc., and performs supply of current between the first roller electrode  26  and the second roller electrode  28  through the first lead wire  52  and the second lead wire  54 . 
     The input unit  17  is capable of inputting information with respect to the control unit  18 , such as the plate thicknesses of the respective workpieces W 1  to W 3  that make up the stacked assembly  100 . 
     The control unit  18  includes a robot controller  58 , a plate thickness ratio calculating unit  59 , an inclination angle setting unit  60 , and a welding machine controller  62 . The robot controller  58  controls driving of the articulated robot  12 . 
     The plate thickness ratio calculating unit  59  calculates the ratio (plate thickness ratio R=D 2 /D 0 ) of the thickness D 0  of the stacked assembly  100  with respect to the thickness D 2  of the workpiece (thinnest workpiece) W 3 , based on the thickness information, etc., of the respective workpieces W 1  to W 3 , which is input from the input unit  17 . 
     The inclination angle setting unit  60  sets the angle of inclination θ of a line segment B along the welding direction with respect to a line segment A along the stacking direction (the direction of the arrow Y) of the stacked assembly  100 , based on the plate thickness ratio R calculated by the plate thickness ratio calculating unit  59  (see  FIG. 3 ). The line segment B passes through the axis of rotation Ax 1  of the first roller electrode  26  and the axis of rotation Ax 2  of the second roller electrode  28 . More specifically, the inclination angle setting unit  60  sets the angle of inclination θ to become larger as the plate thickness ratio R becomes larger. By setting the angle of inclination θ in this manner, the second roller electrode  28  is arranged more forwardly in the welding direction than the first roller electrode  26 . 
     According to the present embodiment, the inclination angle setting unit  60  sets the angle of inclination θ to less than or equal to 7°, and more preferably, to less than or equal to 5°. By setting the angle of inclination in this manner, excessive separation between the first roller electrode  26  and the second roller electrode  28  can be suppressed. 
     Further, in the case that the plate thickness ratio R is greater than 5, the inclination angle setting unit  60  preferably sets the angle of inclination θ within a range of 3°≦θ≦5°, and in the case that the plate thickness ratio R is less than or equal to 3, the inclination angle setting unit  60  preferably sets the angle of inclination θ to be approximately 1°. 
     This is because, if the angle of inclination θ is too small, it becomes difficult for the contact site between the workpiece W 2  and the workpiece W 3  to be sufficiently heated, whereas if the angle of inclination θ is too large, the current pathway formed in the stacked assembly  100  becomes excessively long (the interval between the first roller electrode  26  and the second roller electrode  28  is too wide), and the welding current value needed to perform seam welding becomes excessively large. 
     The welding machine controller  62  controls the first through third cylinders, the first and second rotary motors, and the welding power source unit  50 . 
     The seam welding device  10  according to the present embodiment is constructed basically as has been described above. Next, effects and advantages of the seam welding device  10  will be described in relation to a seam welding method according to the embodiment. 
     First, the plate thickness ratio calculating unit  59  calculates the plate thickness ratio R by obtaining information of the thickness dimensions of the respective workpieces W 1  to W 3  (step S 1  of  FIG. 4 ). The thickness dimensions of the respective workpieces W 1  to W 3  may be obtained by the operator making an input to the input unit  17 , or may be obtained from workpiece information (information of the thickness dimensions of each of the workpieces) stored in advance in a storage unit or the like of the control unit  18 . 
     Next, the inclination angle setting unit  60  sets the angle of inclination θ based on the calculated thickness ratio R (step S 2 ). More specifically, the inclination angle setting unit  60  sets the angle of inclination θ to become larger as the plate thickness ratio R becomes larger. At this time, the inclination angle setting unit  60  sets the angle of inclination θ within a range of 3°≦θ≦5°, for example. 
     Next, the relative positions of the first roller electrode  26  and the second roller electrode  28  are adjusted so as to bring about the set angle of inclination θ (step S 3 ). More specifically, by controlling the pressure in the third cylinder, the welding machine controller  62  moves the third moving table  40  in the direction of the arrow Z, whereby the second roller electrode  28  is moved more forwardly in the welding direction than the first roller electrode  26 , and the angle of inclination θ is placed at the set value. 
     Thereafter, the stacked assembly  100  is sandwiched and gripped between the first roller electrode  26  and the second roller electrode  28  (step S 4 ). More specifically, at first, the robot controller  58  controls the articulated robot  12 , moves the seam welding machine  16  to the vicinity of a welding starting point of the stacked assembly  100 , and adjusts the posture of the seam welding machine  16  to position the first roller electrode  26  on the side of the workpiece W 1  and to position the second roller electrode  28  on the side of the workpiece W 3 . In addition, by controlling the pressures in the first and second cylinders, the welding machine controller  62  causes the first roller electrode  26  and the second roller electrode  28  to approach one another mutually. Consequently, the outer circumferential surface of the first roller electrode  26  contacts one surface of the workpiece W 1 , together with the outer circumferential surface of the second roller electrode  28  contacting the other surface of the workpiece W 3 . 
     As a result, the stacked assembly  100  is pressed and sandwiched by the first roller electrode  26  and the second roller electrode  28 . Part of the stacked assembly  100  which has been sandwiched is warped corresponding to the shapes of the roller electrodes  26 ,  28  (see  FIG. 5 ). Therefore, compared to the case where the roller electrodes  26 ,  28  are aligned in the welding direction, a contact area S 1  of the first electrode  26  on the workpiece W 1  increases along the welding direction and the widthwise direction of the first roller electrode  26 , and a contact area S 2  of the second roller electrode  26  on the workpiece W 3  also increases along the opposite direction of the welding direction and the widthwise direction of the second roller electrode  28  (see  FIG. 6 ). 
     Subsequently, seam welding is carried out by supplying current while the first roller electrode  26  and the second roller electrode  28  undergo rotation (rolling) (step S 5 ). More specifically, the robot controller  58  controls the articulated robot  12 , and while the seam welding machine  16  is moved, the welding machine controller  62  rotates the first roller electrode  26  by driving the first rotary motor, and rotates the second roller electrode  28  by driving the second rotary motor. At roughly the same time, the welding machine controller  62  carries out supply of current between the first roller electrode  26  and the second roller electrode  28  by driving the welding power source unit  50 . 
     Upon doing so, in the stacked assembly  100 , an inclined current pathway, which is inclined in a direction opposite to the welding direction, is formed from the second roller electrode  28  toward the first roller electrode  26 . Therefore, as shown in  FIG. 7A , at a certain point in time T 1 , in the current pathway, resistance heating takes place at a contact site (first site) H 1   a  between the workpiece W 2  and the workpiece W 3 , at a site H 1   b  in the workpiece W 2  slightly more rearwardly than the contact site H 1   a , and at a contact site H 1   c  between the workpiece W 1  and the workpiece W 2  slightly more rearwardly than the site H 1   b.    
     In addition, as shown in  FIG. 7B , at a point in time T 2  after time T 1 , since the first roller electrode  26  and the second roller electrode  28  are moved slightly in the welding direction with respect to the stacked assembly  100 , resistance heating then takes place at a contact site (second site) H 2   a , which is shifted slightly in the welding direction from the contact site H 1   a , at a site H 2   b  (third site), which is shifted slightly in the welding direction from the site H 1   b , and at a contact site H 2   c , which is shifted slightly in the welding direction from the contact site H 1   c . At this time, the contact site H 1   a , which has already been heated, is subjected to further heating by the contact site H 2   a  and the contact site H 2   b.    
     In addition, as shown in  FIG. 7C , at a point in time T 3  after time T 2 , similar to the case of time T 2 , resistance heating then takes place at a contact site H 3   a , which is shifted slightly in the welding direction from the contact site H 2   a , at a contact site H 3   b , which is shifted slightly in the welding direction from the site H 2   b , and at a contact site H 3   c , which is shifted slightly in the welding direction from the contact site H 2   c.    
     At this time, since the contact site H 3   c  is a contact site between the workpiece W 1  and the workpiece W 2 , which are greater in thickness and greater in specific resistance than the workpiece W 3 , sufficient resistance heating takes place. 
     In the foregoing manner, by performing seam welding in a state in which the second roller electrode  28  is arranged more forwardly than the first roller electrode  26 , since the contact site H 1   a  between the workpiece (thinnest workpiece) W 3  and the workpiece W 2  for which inadequate heating is easy to occur are sufficiently heated and melted, nuggets that are sufficiently large in size can be formed at the contact site between the workpiece W 2  and the workpiece W 3 . 
     Further, according to the present embodiment, because the nuggets that are formed at the contact site H 3   c  grow up to the contact site between the workpiece W 2  and the workpiece W 3 , nuggets which are well-proportioned as a whole can be obtained. Consequently, this leads to the workpiece W 1  and the workpiece W 2  being bonded together firmly, as well as the workpiece W 2  and the workpiece W 3  being bonded together firmly. 
     Thereafter, at a point in time that completion of welding by the first roller electrode  26  and the second roller electrode  28  is reached, seam welding is brought to an end (step S 6 ). More specifically, the welding machine controller  62  controls the welding power source unit  50  and stops current between the first roller electrode  26  and the second roller electrode  28 . Further, by adjusting the pressures inside the first and second cylinders, the first roller electrode  26  and the second roller electrode  28  are separated away from the stacked assembly  100 . At this time, the current cycle of seam welding is brought to an end. 
     According to the present embodiment, since the second roller electrode  28  in contact with the workpiece (thinnest workpiece) W 3  is disposed more forwardly in the welding direction than the first roller electrode  26 , in the stacked assembly  100 , a current pathway, which is inclined in a direction opposite to the welding direction, is formed from the second roller electrode  28  toward the first roller electrode  26 . When this is done, in the current pathway, at a certain point in time T 1 , resistance heating occurs at the contact site (first contact site) H 1   a  between the workpiece W 3  and the workpiece W 2  adjacent to the workpiece W 3 . In addition, at the point in time T 2  that the first roller electrode  26  and the second roller electrode  28  are moved relatively with respect to the stacked assembly  100 , the second site H 2   a  adjacent to the first site H 1   a  in the welding direction is heated, together with the third site H 2   b  adjacent to the first roller electrode  26  side of the first site H 1   a  being heated. At this time, the first site H 1   a , which already has been heated, is increased in heat further by the second site H 2   a  and the third site H 2   b , and therefore, a nugget (well-proportioned nugget) of a sufficient size is formed at the first site H 1   a . Consequently, a joint is obtained which is superior in bonding strength. 
     Further, according to the present embodiment, compared to a situation in which the positions of the first roller electrode  26  and the second roller electrode  28  in the welding direction are aligned, the contact area S 1  of the first roller electrode  26  with respect to the workpiece W 1 , and the contact area S 2  of the second roller electrode  28  with respect to the workpiece W 3  can be increased. Consequently, since the current density of the current pathway is comparatively small, even in the case that the current value flowing between the first roller electrode  26  and the second roller electrode  28  is increased, the occurrence of welding debris can suitably be suppressed. 
     Furthermore, according to the present embodiment, since the angle of inclination θ is set corresponding to the ratio R of the thickness D 0  of the stacked assembly  100  with respect to the thickness D 2  of the workpiece W 3 , nuggets that are sufficiently large in size can efficiently be formed in the stacked assembly  100 . In addition, since the angle of inclination θ is set to 5° or less, excessive separation between the first roller electrode  26  and the second roller electrode  28  can be suppressed. Consequently, nuggets of sufficient size can be formed more effectively in the stacked assembly  100 . 
     The present invention will be described in greater detail by presenting the following exemplary embodiment according to the present invention. 
     First Example 
     With the present exemplary embodiment, as shown in  FIGS. 8 and 9 , using a seam welding device  200  having the same structure as the above-described seam welding device  10 , after seam welding was performed on a stacked assembly  202  as an object to be welded, a shear strength test was performed. 
     As the stacked assembly  202 , there were stacked in this order a workpiece W 10  (JAC270F) with a plate thickness of 0.65 mm, a workpiece W 20  (JSC590R) with a plate thickness of 1.80 mm, and a workpiece W 30  (JSC590R) with a plate thickness of 1.40 mm. More specifically, the plate thickness R of the stacked assembly  202  was greater than 5 at roughly 5.9, as determined from the equation R=(1.40+1.80+0.65)/0.65. 
     With the seam welding device  200  according to the present exemplary embodiment, a first roller electrode  204  was arranged on the side of the workpiece (thinnest workpiece) W 10 , and a second roller electrode  206  was arranged on the side of the workpiece W 30 . 
     For each of the first roller electrode  204  and the second roller electrode  206 , water-cooled roller electrodes constituted from a copper chromium alloy (CrCu) were used. Further, a thickness t 1  of the first roller electrode  204  was set to 10 mm, and the radius of curvature r 1  of the outer circumferential surface thereof was set to 15 mm. A thickness t 2  of the second roller electrode  206  was set to 10 mm, and the radius of curvature r 2  of the outer circumferential surface thereof was set to 100 mm. 
     As welding conditions for the present exemplary embodiment, the welding speed was set to 4 m/min, the applied pressure was 450 kgf, a current-supplying cycle with an energization time of 6 msec and a rest time 6 msec was used, and the up-slope time was set to 150 msec. 
     Test results according to the present exemplary embodiment are shown in  FIG. 10 . In  FIG. 10 , the welding current is shown on the horizontal axis, and the angle of inclination is shown on the vertical axis. In  FIG. 10 , ⊚ indicates a base material fractured sample in which debris was not generated, ◯ indicates an interfacial fractured sample in which debris was not generated, Δ indicates a base material fractured sample in which debris was generated, and X indicates a sample in which peeling occurred without generation of debris. More specifically, the portions indicated by ⊚ and ◯ imply that the heat input balance of the respective workpieces W 10 , W 20 , W 30  was favorable. 
     According to the test results, in the case that the second roller electrode  206  was disposed more forwardly than the first roller electrode  204  (i.e., in the case that the angle of inclination was −1 degrees and −3 degrees), compared to a case in which the angle of inclination is zero degrees, the welding current width became narrow with the sample ◯. 
     In the case that the angle of inclination was +1 degrees, compared to the angle of inclination being zero degrees, although the welding current width for the sample ◯ was the same, the region for the sample ◯ was shifted toward the high current side. In the case that the angle of inclination was +3 degrees, compared to the angle of inclination being +1 degrees, the region for the sample ◯ was shifted further toward the high current side, while in part, the results of the sample ⊚ could be obtained. In the case that the angle of inclination was +5 degrees, compared to the angle of inclination being +3 degrees, the welding current width for the sample ⊚ was widened. In the case that the angle of inclination was +7 degrees, the results for samples ◯ and ⊚ could not be obtained. 
     In the foregoing manner, in the case that the plate thickness ratio R is greater than 5, rather than with the angle of inclination being zero degrees, it was proven that weldability was improved when the angle of inclination resided within a range of 0°≦θ≦5°. Further, it was proven that weldability was further improved when the angle of inclination resided within the range of 3°≦θ≦5°. 
     The present invention is not limited to the embodiments described above, and it is a matter of course that various additional or modified structures could be adopted therein without deviating from the essential gist of the present invention. 
     For example, the seam welding method according to the present invention is not limited to the example of performing seam welding with respect to a stacked assembly  100  that is made up by stacking three workpieces W 1 , W 2 , W 3 . For example, as shown in  FIG. 11 , the same effects and advantages of the aforementioned embodiment can be offered, even in the case that seam welding is carried out with respect to a stacked assembly  102  that is constituted by stacking two workpieces W 1  and W 3 . The same holds true for a case in which seam welding is carried out with respect to a stacked assembly that is constituted by stacking four or more workpieces.