Abstract:
Disclosed are a resistance welding method for welding a stack of three or more works and a device therefor. A stack is formed in such that the thinnest work having the smallest thickness among the works is placed in the outermost position. The stack is held between a first welding electrode and a second welding electrode. A pressing member is brought into contact with a location of the thinnest work different from the location with which the first welding electrode is in contact, and the pressing member is caused to press the stack from the thinnest work side. In a state that the pressing force exerted on the stack by the first welding electrode and the pressing member is balanced with the pressing force exerted on the stack by the second welding electrode, current is applied between the first welding electrode and the second welding electrode.

Description:
TECHNICAL FIELD  
       [0001]    The present invention relates to a resistance welding method and a resistance welding apparatus (device) for resistance-welding a stacked assembly of three or more workpieces including a thinnest workpiece, which is disposed on an outermost side of the stacked assembly. 
       BACKGROUND ART  
       [0002]    One known process of joining a plurality of metal sheets together is a resistance welding process in which the metal sheets are stacked in a stacked assembly. Then, after the stacked assembly is gripped and pressed by a set of welding electrodes, an electric current is passed between the welding electrodes to melt a region of the metal sheets near contact surfaces thereof. When solidified, the melted region is turned into a solid phase, which is referred to as a nugget. In certain cases, three or more metal sheets are joined together by the resistance welding process. 
         [0003]    Thicknesses of metal sheets or workpieces to be resistance-welded may not necessarily be identical to each other, but differ from each other in most cases. Therefore, the metal sheets tend to include a workpiece, the thickness of which is the smallest (hereinafter also referred to as a “thinnest workpiece”). 
         [0004]    If the stacked assembly of metal sheets, including the thinnest workpiece disposed on an outermost side of the stacked assembly, is resistance-welded while pressing forces, which are applied to the stacked assembly by a pair of respective welding electrodes, are held in substantial equilibrium with each other, then the nugget that is formed between the thinnest workpiece and the workpiece adjacent thereto may not grow sufficiently. The reason for insufficient growth of the nugget is considered to be based on the fact that, if the stacked assembly comprises three stacked workpieces, then since the contact resistance between the thinnest workpiece and the adjacent workpiece is reduced due to ends of the thinnest workpiece flexing away from the adjacent workpiece, a sufficient amount of Joule heat is not generated between the thinnest workpiece and the adjacent workpiece, as disclosed in Japanese Patent No. 3894545. 
         [0005]    The electric current, which is passed between the welding electrodes, may be increased in order to cause the nugget near the thinnest workpiece to grow sufficiently, thereby increasing the amount of Joule heat generated by the thinnest workpiece. However, such an increased amount of electric current tends to flow into the thicker workpieces, unduly melting the workpieces and producing scattered metal particles, thereby resulting in sputtering. 
         [0006]    It may be considered effective to increase the energization time over which electric current is passed between the welding electrodes. However, it is not easy for the thinnest workpiece to be made to generate a sufficient amount of Joule heat, even with an increased energization time. The increased energization time also leads to a reduction in welding efficiency because the welding time is increased. 
         [0007]    The present applicant has proposed in Japanese Patent No. 3894545 that the pressing force applied by the welding electrode, which is pressed against the thinnest workpiece, should be made smaller than the pressing force applied by the other welding electrode. By adjusting the pressing forces applied by the welding electrodes to the stacked assembly in this manner, it is possible to bring the amount of Joule heat generated at the interface between the workpieces into substantial equilibrium. Consequently, it is possible to allow the nugget between the thinnest workpiece and the adjacent workpiece to grow to a size which is substantially the same as the size of the nugget between the adjacent workpiece and the other workpiece. 
       SUMMARY OF INVENTION  
       [0008]    A general object of the present invention is to provide a resistance welding method, which is capable of further growing a nugget between a thinnest workpiece on an outermost side of a stacked assembly and a workpiece disposed adjacent to the thinnest workpiece. 
         [0009]    A principal object of the present invention is to provide a resistance welding method, which avoids the tendency to cause sputtering. 
         [0010]    Another object of the present invention is to provide a resistance welding apparatus, which is capable of further growing a nugget, as described above. 
         [0011]    Still another object of the present invention is to provide a resistance welding apparatus, which avoids the tendency to cause sputtering. 
         [0012]    According to an aspect of the present invention, there is provided a resistance welding method for resistance-welding a stacked assembly of at least three workpieces, including a thinnest workpiece of smallest thickness disposed on an outermost side of the stacked assembly, comprising the steps of: 
         [0013]    gripping the stacked assembly with a first welding electrode and a second welding electrode, holding a pressing member in abutment against an area of the thinnest workpiece, which differs from an area against which the first welding electrode is held in abutment, and causing the pressing member to press the stacked assembly from the side of the thinnest workpiece; and 
         [0014]    passing an electric current between the first welding electrode and the second welding electrode, while pressing forces applied from the first welding electrode and the pressing member to the stacked assembly and a pressing force applied from the second welding electrode to the stacked assembly are held in equilibrium with each other. 
         [0015]    Since the sum of the pressing forces applied from the first welding electrode and the pressing member to the stacked assembly is held in equilibrium with the pressing force applied from the second welding electrode to the stacked assembly, the pressing force applied from the first welding electrode is smaller than the pressing force applied from the second welding electrode. Therefore, between the first welding electrode and the second welding electrode, which substantially faces toward the first welding electrode, the active range of the pressing forces grows progressively wider from the first welding electrode toward the second welding electrode. Therefore, a force acting on the interface between the thinnest workpiece and the workpiece adjacent thereto is smaller than the force acting on the interface between the remaining workpieces. 
         [0016]    Because of the above distribution of pressing forces, the area of the thinnest workpiece, which contacts the workpiece adjacent thereto, is smaller than the area of the remaining workpieces that are in contact with each other. Consequently, the contact resistance at the interface between the thinnest workpiece and the workpiece adjacent thereto is increased, thereby increasing the generated amount of Joule heat. As a result, a joint strength between the thinnest workpiece and the workpiece adjacent thereto is achieved. 
         [0017]    In addition, since the thinnest workpiece is pressed by the pressing member, the thinnest workpiece is prevented from becoming spaced from the workpiece adjacent thereto. Therefore, the softened melted region is prevented from being scattered as sputter from a region where the thinnest workpiece and the workpiece adjacent thereto might otherwise be spaced from each other. 
         [0018]    The pressing member may comprise an auxiliary electrode, which is opposite in polarity to the first welding electrode, such that when electric current is passed between the first welding electrode and the second welding electrode, either a branched electric current directed from the first welding electrode toward the auxiliary electrode, or a branched electric current directed from the auxiliary electrode toward the first welding electrode is produced. 
         [0019]    Since the electric current directed from the first welding electrode toward the auxiliary electrode or the electric current directed in the opposite direction flows through the thinnest workpiece, the electric current sufficiently heats the interface between the thinnest workpiece and the workpiece adjacent thereto. As a consequence, a sufficiently sized nugget is grown at the interface, thereby providing a joined product having an excellent joint strength. 
         [0020]    According to another aspect of the present invention, there also is provided a resistance welding apparatus for resistance-welding a stacked assembly of at least three workpieces, including a thinnest workpiece of smallest thickness disposed on an outermost side of the stacked assembly, comprising: 
         [0021]    a welding gun including:
       a first welding electrode that abuts against the thinnest workpiece;   a second welding electrode that grips the stacked assembly in coaction with the first welding electrode; and   a pressing member, which abuts against an area of the thinnest workpiece that differs from an area against which the first welding electrode is held in abutment, and which presses the stacked assembly from the side of the thinnest workpiece;       
 
         [0025]    a pressing mechanism, which applies pressing forces for pressing the stacked assembly against the pressing member; and 
         [0026]    control means for controlling the pressing mechanism, 
         [0027]    wherein when an electric current is passed between the first welding electrode and the second welding electrode, the control means holds pressing forces, which are applied from the first welding electrode and the pressing member to the stacked assembly, and a pressing force applied from the second welding electrode to the stacked assembly, in equilibrium with each other. 
         [0028]    With the above arrangement, the pressing forces applied from the first welding electrode and the second welding electrode are distributed such that the acting range thereof grows progressively greater from the first welding electrode (thinnest workpiece) toward the second welding electrode. As a result, the contact resistance at the interface between the thinnest workpiece and the workpiece adjacent thereto is increased. Thus, the joint strength of the thinnest workpiece and the workpiece adjacent thereto is increased. 
         [0029]    If the welding gun is supported on a robot, then the pressing mechanism should preferably be mounted on the welding gun. Inasmuch as reactive forces from the stacked assembly can be absorbed by the welding gun, reactive forces are prevented from acting on the robot. Therefore, the robot does not need to be significantly rigid. Stated otherwise, the robot may be reduced in size and hence facility investments may be reduced. 
         [0030]    The pressing member may comprise an auxiliary electrode, which is opposite in polarity to the first welding electrode, such that when electric current is passed between the first welding electrode and the second welding electrode, either a branched electric current directed from the first welding electrode toward the auxiliary electrode, or a branched electric current directed from the auxiliary electrode toward the first welding electrode is produced. As described above, since the interface between the thinnest workpiece and the workpiece adjacent thereto is sufficiently heated due to the electric current directed from the first welding electrode toward the auxiliary electrode or the electric current flowing in the opposite direction, a sufficiently sized nugget is grown at the interface, thereby providing a joined product having excellent joint strength. 
         [0031]    According to the present invention, as described above, the first welding electrode and the second welding electrode grip the stacked assembly therebetween, and the thinnest workpiece, which is disposed on the outermost side of the stacked assembly, is pressed by the pressing member, during which time the stacked assembly is resistance-welded. Therefore, the pressing forces applied to the stacked assembly are distributed such that the acting range thereof grows progressively greater from the first welding electrode toward the second welding electrode. 
         [0032]    Since the pressing forces are distributed, the area of contact at the interface between the thinnest workpiece and the workpiece adjacent thereto is reduced, resulting in an increase in contact resistance at the interface. Therefore, a sufficient amount of Joule heat, which is capable of heating the interface, is generated, thereby allowing a nugget of sufficient size to be grown at the interface. The thinnest workpiece and the workpiece adjacent thereto are thus joined to each other with a sufficient joint strength. 
         [0033]    Stated otherwise, a sufficient joint strength is maintained between the thinnest workpiece and the workpiece adjacent thereto. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0034]      FIG. 1  is an enlarged view of a central portion of a resistance welding apparatus according to a first embodiment of the present invention; 
           [0035]      FIG. 2  is a vertical cross-sectional view showing the manner in which a stacked assembly to be welded is sandwiched by a lower tip, an upper tip, and a pressing rod (pressing member); 
           [0036]      FIG. 3  is a front elevational view showing the manner in which an appropriate surface pressure distribution is developed between a workpiece positioned on an uppermost side of the stacked assembly and a workpiece positioned directly therebelow; 
           [0037]      FIG. 4  is a vertical cross-sectional view showing the manner in which the stacked assembly is sandwiched by only the lower tip and the upper tip; 
           [0038]      FIG. 5  is a vertical cross-sectional view showing the manner in which the resistance welding apparatus, from the state shown in  FIG. 2 , begins to pass an electric current, which flows from the upper tip toward the lower tip; 
           [0039]      FIG. 6  is an enlarged view of a central portion of a resistance welding apparatus according to a modification of the first embodiment of the present invention; 
           [0040]      FIG. 7  is an enlarged view of a central portion of a resistance welding apparatus according to another modification of the first embodiment of the present invention; 
           [0041]      FIG. 8  is an enlarged perspective view, partially in transverse cross-section, showing a central portion of a resistance welding apparatus according to a second embodiment of the present invention; 
           [0042]      FIG. 9  is a vertical cross-sectional view showing the manner in which a stacked assembly is sandwiched by a first electrode tip, a second electrode tip, and an auxiliary electrode; 
           [0043]      FIG. 10  is a vertical cross-sectional view showing the manner in which the resistance welding apparatus, from the state shown in  FIG. 9 , begins to pass an electric current, which flows from the upper tip toward the lower tip; 
           [0044]      FIG. 11  is a vertical cross-sectional view showing the manner in which the resistance welding apparatus continues to pass electric current from the state shown in  FIG. 10 ; 
           [0045]      FIG. 12  is a vertical cross-sectional view showing the manner in which only the auxiliary electrode is lifted from the stacked assembly, and electric current is continuously passed so as to flow from the upper tip toward the lower tip; 
           [0046]      FIG. 13  is a vertical cross-sectional view showing the manner in which the upper tip also is lifted from the stacked assembly continuously from the state shown in  FIG. 12 , in order to finish the resistance welding process to pass the electric current; 
           [0047]      FIG. 14  is a vertical cross-sectional view showing the manner in which the resistance welding apparatus passes an electric current from the lower tip and a current branching electrode to the upper tip, conversely to the state shown  FIG. 10 ; and 
           [0048]      FIG. 15  is a vertical cross-sectional view showing the manner in which an electric current, which is directed from the first electrode tip toward the current branching electrode, flows to a workpiece, which is positioned on the uppermost side of the stacked assembly, and another workpiece positioned directly therebelow. 
       
    
    
     DESCRIPTION OF EMBODIMENTS  
       [0049]    Resistance welding methods according to embodiments of the present invention will be described in detail below with reference to the accompanying drawings, in connection with resistance welding apparatus that carry out the resistance welding methods. 
         [0050]      FIG. 1  is an enlarged view of a central portion of a resistance welding apparatus  10  according to a first embodiment of the present invention. The resistance welding apparatus  10  comprises a robot having an arm (both not shown) and a welding gun  14  supported on a wrist  12  of the arm. 
         [0051]    The welding gun  14  is a so-called C-type welding gun, including a substantially C-shaped fixed arm  30  disposed below a main gun body  24 . A lower tip  32  that serves as a second welding electrode is mounted on the lower tip end of the fixed arm  30  in facing relation to the main gun body  24 . The lower tip  32  extends toward the main gun body  24 . 
         [0052]    The main gun body  24  houses a ball screw mechanism (not shown). The ball screw mechanism includes a ball screw, which serves to vertically displace a connecting rod  34  (in the direction indicated by the arrow Y 2  or the arrow Y 1  in  FIG. 1 ). The connecting rod  34  projects from the main gun body  24  and extends toward the lower tip  32 . The balls screw is rotated about its axis by a servomotor (not shown) of the ball screw mechanism. 
         [0053]    An upper tip  38  that serves as a first welding electrode is mounted by a stay  36  on the tip end of the connecting rod  34  in facing relation to the lower tip  32 . A cylinder mechanism  42  that serves as a pressing mechanism is supported on the stay  36  by a bridge  40 . The cylinder mechanism  42  has a cylinder tube  44  from which there projects a pressing rod  46  serving as a pressing member parallel to the upper tip  38 . According to the first embodiment, both the pressing mechanism (cylinder mechanism  42 ) and the pressing member (pressing rod  46 ) are mounted on the welding gun  14 . 
         [0054]    A stacked assembly  48  to be welded will be described below. The stacked assembly  48  comprises three metal sheets  50 ,  52 ,  54 , which are stacked successively from below in this order. The metal sheets  50  and  52  have a thickness D 1  (e.g., in the range from about 1 mm to about 2 mm), and the metal sheet  54  has a thickness D 2  (e.g., in the range from about 0.5 mm to about 0.7 mm), which is smaller than the thickness D 1 . In other words, the metal sheets  50  and  52  are identical in thickness to each other, and the metal sheet  54  is thinner than the metal sheets  50  and  52 . The metal sheet  54  is the thinnest of the workpieces  50 ,  52 ,  54 . 
         [0055]    The metal sheets  50  and  52  are made of so-called high-tension steel according to JAC590, JAC780, or JAC980 (high-performance high-tensile-strength sheet steel specified by the Japan Iron and Steel Federation Standards), for example, and the metal sheet  54  is made of so-called mild steel according to JAC270 (high-performance sheet steel for drawing specified by the Japan Iron and Steel Federation Standards). The metal sheets  50  and  52  may be of one metal type or may be of different metal types. 
         [0056]    Alternatively, all of the metal sheets  50 ,  52 ,  54  may be made of mild steel, or only the metal sheet  50  may be made of high-tension steel while the metal sheets  52  and  54  may be made of mild steel. 
         [0057]    The metal sheets  50 ,  52 ,  54  are not limited to the above steel materials, but may be of any materials insofar as the metal sheets can be resistance-welded. 
         [0058]    The lower tip  32  and the upper tip  38  grip the stacked assembly  48  therebetween and pass an electric current through the stacked assembly  48 . The lower tip  32  is connected electrically to the negative pole of a power supply  56 , and the upper tip  38  is electrically connected to the positive pole of the power supply  56 . According to the first embodiment, therefore, an electric current flows from the upper tip  38  toward the lower tip  32 . 
         [0059]    As described later, the upper tip  38  and the pressing rod  46  are spaced from each other by a distance Z 1 , which is set to a value for providing an appropriate surface pressure distribution between the thinnest workpiece  54  and the metal sheet  52  directly below the thinnest workpiece  54 . 
         [0060]    The servomotor of the ball screw mechanism, the cylinder mechanism  42 , and the power supply  56  are electrically connected to a gun controller  58 , which serves as a control means. Therefore, the servomotor, the cylinder mechanism  42 , and the power supply  56  are operated or energized and de-energized under the control of the gun controller  58 . 
         [0061]    The resistance welding apparatus  10  according to the first embodiment is basically constructed as described above. Operations and advantages of the resistance welding apparatus  10  will be described below in relation to a resistance welding method according to the first embodiment. 
         [0062]    For resistance-welding the stacked assembly  48 , or stated otherwise, for joining the metal sheets  50  and  52  to each other and joining the metal sheets  52  and  54  to each other, the robot moves the wrist  12 , i.e., the welding gun  14 , in order to position the stacked assembly  48  between the lower tip  32  and the upper tip  38 . 
         [0063]    After the main gun body  24  has been moved to a predetermined position, the gun controller  58  is operated to energize the servomotor of the ball screw mechanism, which starts to rotate the ball screw. The upper tip  38  and the pressing rod  46  are lowered toward the stacked assembly  48  along the direction indicated by the arrow Y 1 . As a result, the stacked assembly  48  is gripped between the lower tip  32  and the upper tip  38 . 
         [0064]    The gun controller  58  also actuates the cylinder mechanism  42 . The pressing rod  46  further projects along the direction indicated by the arrow Y 1 . At the same time or almost at the same time that the stacked assembly  48  is gripped between the lower tip  32  and the upper tip  38 , or before or after the stacked assembly  48  has been gripped between the lower tip  32  and the upper tip  38 , the pressing rod  46  abuts against the metal sheet  54 .  FIG. 2  shows in vertical cross section the lower tip  32 , the upper tip  38 , the pressing rod  46 , and the stacked assembly  48  at this time. 
         [0065]    As shown in  FIG. 3 , the distance Z 1  between the upper tip  38  and the pressing rod  46  is set to a value such that the surface pressure, which acts on the interface between the thinnest workpiece  54  and the metal sheet  52 , is greatest in an area where the stacked assembly  48  is pressed by the upper tip  38 , and is second greatest in an area where the stacked assembly  48  is pressed by the pressing rod  46 . Stated otherwise, the interface includes an area where the surface pressure is lower than the surface pressure applied by the upper tip  38  and the surface pressure applied by the pressing rod  46 . As a result, a distribution of pressing forces as shown in  FIG. 2  is developed. 
         [0066]    The distribution of pressing forces will be described in detail below. 
         [0067]    The gun controller  58  controls the rotational force of the servomotor, which rotates the ball screw of the ball screw mechanism, and the thrusting force of the cylinder mechanism  42 , such that the sum (F 1 +F 2 ) of the pressing force F 1 , which is applied from the upper tip  38  to the metal sheet  54 , and the pressing force F 2 , which is applied from the pressing rod  46  to the metal sheet  54 , is held in equilibrium with the pressing force F 3 , which is applied from the lower tip  32  to the metal sheet  50 . Therefore, the pressing force (F 1 +F 2 ) that acts on the stacked assembly  48  along the direction indicated by the arrow Y 1  and the pressing force (F 3 ) that acts on the stacked assembly  48  along the direction indicated by the arrow Y 2  are substantially equal to each other. 
         [0068]    At this time, F 1 &lt;F 3 . Consequently, the forces that the stacked assembly  48  receive from the lower tip  32  and the upper tip  38  are distributed such that the acting range of the forces grows progressively wider or greater in a direction from the upper tip  38  toward the lower tip  32 , as shown in  FIG. 2 . Therefore, the force acting on the interface between the metal sheets  52  and  54  is smaller than the force acting on the interface between the metal sheets  50  and  52 . The above distribution of pressing forces will not be developed if an area is not formed where the surface pressure is lower than the surface pressure applied by the upper tip  38  and the surface pressure applied by the pressing rod  46 , because the distance Z 1  is too small. 
         [0069]      FIG. 4  shows a distribution of forces that the stacked assembly  48  receive from the lower tip  32  and the upper tip  38  if the pressing rod  46  is not used and F 1 =F 3 . As shown in  FIG. 4 , the forces that the stacked assembly  48  receive from the lower tip  32  and the upper tip  38  remain constant along the direction from the upper tip  38  toward the lower tip  32 . Stated otherwise, the force acting on the interface between the metal sheets  52  and  54  is equal to the force acting on the interface between the metal sheets  50  and  52 . 
         [0070]    In  FIGS. 2 and 4 , the acting range of the force on the interface between the metal sheets  52  and  54  is indicated by the thick solid line. As can be seen from  FIGS. 2 and 4 , the acting range of the force on the interface between the metal sheets  52  and  54  is smaller when F 1 &lt;F 3  than when F 1 =F 3 . This means that the area of the metal sheet  54  that is pressed against the metal sheet  52  is smaller when F 1 &lt;F 3  than when F 1 =F 3 , or stated otherwise, the area of the metal sheet  54  that contacts the metal sheet  52  is smaller when F 1 &lt;F 3  than when F 1 =F 3 . 
         [0071]    Since the pressing forces from the upper tip  38  toward the lower tip  32  are distributed so as to reduce the area of the metal sheet  54  that is in contact with the metal sheet  52 , the stacked assembly  48  develops a reactive force, which is directed from the stacked assembly  48  toward the upper tip  38 . According to the first embodiment, the reactive force is borne by the pressing rod  46 . 
         [0072]    As described above, the cylinder mechanism  42  including the pressing rod  46  is supported by the bridge  40  on the connecting rod  34 , which is coupled to the ball screw mechanism housed in the main gun body  24 . Therefore, the reactive force borne by the pressing rod  46  is absorbed by the main gun body  24  (welding gun  14 ). 
         [0073]    The reactive force from the stacked assembly  48  is thereby prevented from acting on the robot. Accordingly, the robot does not need to have a large rigidity. Stated otherwise, the robot may be of a reduced size and thus facility investments can be reduced. 
         [0074]    Then, the gun controller  58  sends a control signal to the power supply  56  for initiating supply of electric current. As shown in  FIGS. 2 and 4 , an electric current i starts to flow along the direction from the upper tip  38  toward the lower tip  32 , because the upper tip  38  and the lower tip  32  are connected respectively to positive and negative poles of the power supply  56 , as described above. Based on the electric current i, the interface between the metal sheets  50  and  52  and the interface between the metal sheets  52  and  54  are heated by Joule heat. 
         [0075]    As described above, the area of the metal sheet  54  that contacts the metal sheet  52 , as shown in  FIG. 2 , is smaller than the area of the metal sheet  54  that contacts the metal sheet  52 , as shown in  FIG. 4 . Therefore, the contact resistance and current density at the interface between the metal sheets  52  and  54  is greater in  FIG. 2  than in  FIG. 4 , or stated otherwise, is greater when F 1 &lt;F 3  than when F 1 =F 3 . Consequently, the amount of Joule heat, i.e., the amount of generated heat, is greater when F 1 &lt;F 3  than when F 1 =F 3 . Therefore, when F 1 &lt;F 3 , as shown in  FIG. 5 , a heated region  60 , which is generated at the interface between the metal sheets  50  and  52 , and a heated region  62 , which is generated at the interface between the metal sheets  52  and  54 , grow substantially the same in size. 
         [0076]    The interface between the metal sheets  50  and  52  as well as the interface between the metal sheets  52  and  54  are heated by the heated regions  60  and  62 , and begin to melt when the temperature thereof rises sufficiently. As a result, nuggets  64 ,  66  are formed respectively between the metal sheets  50  and  52 , and between the metal sheets  52  and  54 . 
         [0077]    As described above, inasmuch as the heated region  60  formed at the interface between the metal sheets  50  and  52 , and the heated region  62  formed at the interface between the metal sheets  52  and  54  are of substantially the same size, the nuggets  64 ,  66  also are of substantially the same size. 
         [0078]    During this time, the metal sheet  54  is pressed against the metal sheet  52  by the pressing rod  46 . Since the metal sheet  54  is pressed in this manner, the metal sheet  54 , which is of low rigidity, is prevented from warping due to electric current passing therethrough (heating thereof), i.e., the metal sheet  54  is prevented from becoming spaced from the metal sheet  52 . Accordingly, the softened melted region is prevented from being scattered as sputter from a region where the metal sheets  54  and  52  might otherwise be spaced from each other. 
         [0079]    After the nuggets  64 ,  66  grow sufficiently upon elapse of a predetermined time, supply of electric current is stopped, and the upper tip  38  is spaced away from the metal sheet  54 . Alternatively, the upper tip  38  may be spaced away from the metal sheet  54  in order to electrically insulate the upper tip  38  from the lower tip  32 . 
         [0080]    The above operation sequence, from the start to the end of the resistance welding process, is performed entirely under the control of the gun controller  58 . 
         [0081]    When supply of electric current is stopped, heating of the metal sheets  50 ,  52 ,  54  also is completed. As time passes, the nuggets  64 ,  66  become cooled and solidified, thereby producing a joined product in which the metal sheets  50  and  52  are joined to each other, and the metal sheets  52  and  54  are joined to each other. 
         [0082]    In the joined product, since, as described above, the nuggets  66  between the metal sheets  52  and  54  grow sufficiently due to a sufficient amount of Joule heat being generated at the interface between the metal sheets  52  and  54 , the joint strength of the metal sheets  50  and  52  as well as the joint strength of the metal sheets  52  and  54  are excellent. 
         [0083]    According to the first embodiment, as described above, the nugget  66 , which is substantially the same in size as the nugget  64  formed between the metal sheets  50  and  52 , can be grown between the metal sheets  52  and  54  while avoiding generation of sputter. Accordingly, a formed product in which the joint strength between the metal sheets  52  and  54  is excellent can be obtained. 
         [0084]    According to the first embodiment, as the pressing force F 2  applied by the pressing rod  46  increases, the nugget  66 , which is formed between the metal sheets  52  and  54 , also increases. However, the size of the nugget  66  tends to become saturated. In other words, even if the pressing force F 2  is increased excessively, it is difficult for the nugget  66  to grow beyond a certain size. If the pressing force F 2  is increased too much, then it is necessary to reduce the pressing force F 1  excessively in order to keep the sum of the pressing forces F 1  and F 2  in equilibrium with the pressing force F 3 . As a result, the nugget  64  formed between the metal sheets  50  and  52  is liable to become small in size. 
         [0085]    Therefore, it is preferable for the difference between the pressing force F 1  applied by the upper tip  38  and the pressing force F 2  applied by the pressing rod  46  to be set to a value by which the nuggets  64 ,  66  can be made as large as possible. 
         [0086]    With the resistance welding apparatus  10  shown in FIG. 
         [0087]      1 , the cylinder mechanism  42  is supported on the connecting rod  34 . However, the cylinder mechanism  42  may be supported on the main gun body  24 , as shown in  FIG. 6 , or may be supported on the fixed arm  30 , as shown in  FIG. 7 . 
         [0088]    At any rate, the cylinder mechanism  42  may be replaced by any of various pressure applying means, such as a spring coil, a servomotor, etc. 
         [0089]    The pressing member may have annular shape surrounding the upper tip  38 , or may be in the form of a plurality of round rods. 
         [0090]    The pressing member may serve as an auxiliary electrode. A second embodiment of the present invention, which incorporates an auxiliary electrode, will be described below. Parts of the second embodiment, which are identical to those shown in  FIGS. 1 through 7 , are denoted by identical reference characters and such features will not be described in detail below. 
         [0091]      FIG. 8  is an enlarged perspective view, partially in transverse cross-section, showing a central portion of a resistance welding apparatus according to a second embodiment of the present invention. Similar to the welding gun of the resistance welding apparatus according to the first embodiment, a welding gun (not shown) of the resistance welding apparatus according to the second embodiment is mounted on the wrist  12  of a non-illustrated robot, and includes a lower tip  32  (second welding electrode), an upper tip  38  (first welding electrode), and an annular auxiliary electrode  68  surrounding the upper tip  38 . Also in the second embodiment, it is assumed that an electric current flows from the upper tip  38  toward the lower tip  32 . 
         [0092]    The upper tip  38  is supported on a main gun body  24 , which includes a displacing mechanism for displacing the auxiliary electrode  68  toward or away from the stacked assembly  48 , e.g., a ball screw mechanism, a cylinder mechanism, or the like. The displacing mechanism is capable of displacing the auxiliary electrode  68  toward or away from the stacked assembly  48  independently of the upper tip  38 . In the second embodiment, the displacing mechanism is mounted on the welding gun. 
         [0093]    According to the second embodiment, the upper tip  38  is electrically connected to a positive pole of the power supply  56 , whereas the lower tip  32  and the auxiliary electrode  68  are electrically connected to a negative pole of the power supply  56 . As can be understood from this fact, although both the upper tip  38  and the auxiliary electrode  68  are held against the metal sheet  54  of the stacked assembly  48 , the upper tip  38  and the auxiliary electrode  68  are opposite in polarity to each other. 
         [0094]    Similar to the first embodiment, in order to distribute pressing forces, the upper tip  38  and the auxiliary electrode  68  are spaced from each other by a distance Z 2 , which is set to a value such that an area (see  FIG. 3 ) is developed where the surface pressure is lower than the surface pressure applied by the upper tip  38  and the surface pressure applied by the auxiliary electrode  68 . The upper tip  38  and the auxiliary electrode  68  are spaced from each other by a certain distance. However, if the distance Z 2  between the upper tip  38  and the auxiliary electrode  68  is too large, then the resistance between the upper tip  38  and the auxiliary electrode  68  becomes so large that it is difficult for a branched electric current i 2  (see  FIG. 2 ) to flow, as will be described later. 
         [0095]    Therefore, the distance Z 2  is set to a value that provides an appropriate surface pressure distribution between the thinnest workpiece  54  and the metal sheet  52 , as well as for making the resistance between the upper tip  38  and the auxiliary electrode  68  of a value that allows a branched electric current i 2  to flow at an appropriate current value. 
         [0096]    The displacing mechanism and the power supply  56  are electrically connected to the gun controller  58 . 
         [0097]    A central portion of the resistance welding apparatus according to the second embodiment is basically constructed as described above. Operations and advantages of the resistance welding apparatus will be described below, in relation to a resistance welding method according to the second embodiment. 
         [0098]    For resistance-welding the stacked assembly  48 , the welding gun  14  is moved so as to position the stacked assembly  48  between the upper tip  38  and the lower tip  32 , similar to the first embodiment. Thereafter, the upper tip  38  and the lower tip  32  are displaced relatively toward each other, thereby gripping the stacked assembly  48  therebetween. 
         [0099]    At the same time or almost at the same time that the stacked assembly  48  is gripped between the upper tip  38  and the lower tip  32 , the auxiliary electrode  68  is held against the metal sheet  54 , in the state shown in vertical cross section in  FIG. 9 . The auxiliary electrode  68  is displaced into abutment against the metal sheet  54  by the displacing mechanism, which displaces the auxiliary electrode  68 . 
         [0100]    The gun controller  58  sets the pressing force F 2 , which is applied from the auxiliary electrode  68  to the metal sheet  54 , such that the sum (F 1 +F 2 ) of the pressing force F 2  and the pressing force F 1 , which is applied by the upper tip  38 , is held in equilibrium with the pressing force F 3 , which is applied by the lower tip  32 . 
         [0101]    According to the second embodiment, as with the first embodiment, it is preferable to set the difference between the pressing force F 1  applied by the upper tip  38  and the pressing force F 2  applied by the auxiliary electrode  68  to a value at which the nugget formed between the metal sheets  52  and  54  can be made as large as possible. 
         [0102]    Then, supply of electric current is initiated. As shown in  FIG. 10 , an electric current i 1  flows from the upper tip  38  toward the lower tip  32  because the upper tip  38  and the lower tip  32  are connected respectively to positive and negative poles of the power supply  56 . The interface between the metal sheets  50  and  52  as well as the interface between the metal sheets  52  and  54  is heated by Joule heat, based on the electric current i 1 , thereby developing heated regions  70  and  72 . 
         [0103]    The auxiliary electrode  68  also is held against the metal sheet  54  and has a negative polarity. Simultaneously with the electric current i 1 , a branched electric current i 2  begins to flow from the upper tip  38  toward the auxiliary electrode  68 . Since the auxiliary electrode  68  has an annular shape, the branched electric current i 2  flows radially. 
         [0104]    According to the second embodiment, as described above, the branched electric current i 2  is generated, which does not flow to the metal sheets  50  and  52 , but flows only to the auxiliary electrode  68 . As a result, the value of the electric current that passes through the metal sheet  54  is greater than in a conventional resistance welding process, which employs only the upper tip  38  and the lower tip  32 . 
         [0105]    Consequently, separate from the heated region  72 , another heated region  74  is developed in the metal sheet  54 . As shown in  FIG. 11 , the heated region  74  heats the interface between the metal sheets  52  and  54  in a radial fashion. The heated region  74  spreads over time and combines integrally with the heated region  72 . 
         [0106]    Thus, heat is transferred from both heated regions  72  and  74 , which are combined integrally with each other, to the interface between the metal sheets  52  and  54 . Similar to the first embodiment, the contact resistance at the interface between the metal sheets  52  and  54  is greater than the contact resistance of the interface between the metal sheets  50  and  52 . Therefore, the temperature of the interface between the metal sheets  52  and  54  increases sufficiently and begins to melt, thereby producing a nugget  76  between the metal sheets  52  and  54 . 
         [0107]    It is possible for the heated region  74  to be made larger in size, since the proportion of the branched electric current i 2  is larger. However, if the proportion of the branched electric current i 2  becomes too large, then since the value of the electric current i 1  is reduced, the heated regions  70  and  72  are reduced in size. Therefore, the size of the nugget  76  tends to become saturated, and the nugget  78  tends to be reduced in size. Therefore, the proportion of the branched electric current i 2  should preferably be set to a value such that the nugget  78  grows sufficiently. 
         [0108]    As described above, the ratio between the electric current i 1  and the branched electric current i 2  can be adjusted, for example, by changing the distance Z 2  (see  FIGS. 8 and 9 ) between the upper tip  38  and the auxiliary electrode  68 . 
         [0109]    The nugget  76  grows over time as long as electric currents continue to be passed through the stacked assembly  48 . Consequently, the nugget  76  can grow sufficiently by continuously passing electric currents over a predetermined time. 
         [0110]    The value of the electric current i 1  that flows through the metal sheets  50  and  52  is smaller than in a conventional resistance welding process. Accordingly, the amount of heat generated by the metal sheets  50  and  52  is prevented from increasing, while the nugget  76  between the metal sheets  52  and  54  grows to a larger size. Therefore, the possibility of sputtering is avoided. 
         [0111]    During this time, the nugget  78  also is formed between the metal sheets  50  and  52  by the electric current i 1 . If the branched electric current i 2  flows continuously, then since the total amount of current i 1  that passes is smaller than if the branched electric current i 2  were stopped, the heated region  70 , and hence the nugget  78 , tend to be slightly reduced in size. 
         [0112]    Therefore, in order for the nugget  78  to grow further, it is preferable for only the auxiliary electrode  68  to be spaced from the metal sheet  54 , and to continue passing electric current from the upper tip  38  toward the lower tip  32 , as shown in  FIG. 12 . Since the value of the electric current i 1  becomes greater as the auxiliary electrode  68  is further spaced from the thinnest workpiece  54 , the total amount of electric current i 1  increases until supply of the electric current is brought to an end. 
         [0113]    In this case, inasmuch as the branched electric current i 2  is eliminated, only the electric current i 1 , which is directed from the upper tip  38  toward the lower tip  32 , flows through the metal sheet  54 . As a result, the heated region  74  (see  FIG. 11 ) disappears. 
         [0114]    On the other hand, the metal sheets  50  and  52  are in the same state as in an ordinary resistance welding process. More specifically, the amount of generated Joule heat increases in the thicker metal sheets  50  and  52 , with the result that the heated region  70  spreads and the temperature thereof becomes higher. The interface between the metal sheets  50  and  52  is heated by the heated region  70 , the temperature of which becomes higher, whereas the region near the interface has a temperature that rises sufficiently and is melted, thereby accelerating growth of the nugget  78 . 
         [0115]    Subsequently, the electric current flows continuously until the nugget  78  grows sufficiently, for example, until the nugget  78  becomes combined integrally with the nugget  76 , as shown in  FIG. 13 . The rate at which the nugget  78  grows with respect to the time during which electric current flows continuously may be confirmed in advance in a resistance welding test, using test pieces or the like. 
         [0116]    The interface between the metal sheets  50  and  52  is preheated by the heated region  70 , which is developed by the electric current i 1  that passes when the nugget  78  grows between the metal sheets  52  and  54 . Therefore, the metal sheets  50  and  52  are fitted together adequately before the nugget  78  is grown, so that sputtering is less likely to be produced. 
         [0117]    According to the second embodiment, as described above, sputtering is prevented from being produced as the nugget  76  grows between the metal sheets  52  and  54 , and as the nugget  78  grows between the metal sheets  50  and  52 . 
         [0118]    After the nugget  78  has grown sufficiently upon elapse of a predetermined time, passage of electric current is stopped, and as shown in  FIG. 13 , the upper tip  38  is spaced away from the metal sheet  54 . Alternatively, the upper tip  38  may be spaced away from the metal sheet  54  in order to electrically insulate the upper tip  38  from the lower tip  32 . 
         [0119]    The above operation sequence, from the start to the end of the resistance welding process, is performed entirely under the control of the gun controller  58 . 
         [0120]    When supply of electric current is stopped, heating of the metal sheets  50  and  52  also is completed. As time passes, the nugget  78  becomes cooled and solidified, thereby joining the metal sheets  50  and  52  to each other. 
         [0121]    As described above, the metal sheets  50  and  52  are joined to each other, and the metal sheets  52  and  54  are joined to each other to thereby produce a joined product. 
         [0122]    In the joined product, since, as described above, the nugget  76  between the metal sheets  52  and  54  is grown sufficiently due to the branched electric current i 2  that flows through the metal sheet  54 , the joint strength of the metal sheets  50  and  52  as well as the joint strength of the metal sheets  52  and  54  are excellent. 
         [0123]    As can be understood from the above, the resistance welding apparatus according to the second embodiment may be constructed by providing the auxiliary electrode  68 , and the displacing mechanism for displacing the auxiliary electrode  68 . The structure of the resistance welding apparatus is not further complicated by providing the auxiliary electrode  68 . 
         [0124]    In the second embodiment, the auxiliary electrode  68  is spaced away from the metal sheet  54  before the upper tip  38  becomes spaced from the metal sheet  54 . However, the auxiliary electrode  68  and the upper tip  38  may be spaced away from the metal sheet  54  simultaneously. 
         [0125]    Furthermore, as shown in  FIG. 14 , an electric current may be supplied and flow from the lower tip  32 , which is held against the metal sheet  50 , to the upper tip  38 , which is held against the metal sheet  54 . In this case as well, the auxiliary electrode  68 , which is held against the metal sheet  54 , is opposite in polarity to the upper tip  38 . More specifically, the lower tip  32  and the auxiliary electrode  68  are electrically connected to the positive pole of the power supply  56 , whereas the upper tip  38  is electrically connected to the negative pole of the power supply  56 . Thus, an electric current i 1  directed from the lower tip  32  toward the upper tip  38 , and a branched electric current i 2  directed from the auxiliary electrode  68  toward the upper tip  38  are produced. 
         [0126]    Moreover, as shown in  FIG. 15 , a branched electric current i 2  may flow not only into the thinnest workpiece  54 , which is held in contact with the upper tip  38 , but also into the metal sheet  52 , which is positioned directly below the thinnest workpiece  54 . 
         [0127]    Instead of spacing the auxiliary electrode  68  from the metal sheet  54 , a switch may be connected between the auxiliary electrode  68  and the power supply  56 . In this case, only an electric current directed from the upper tip  38  toward the auxiliary electrode  68 , or only an electric current directed in the opposite direction may be stopped by turning off the switch. The switch may be connected or turned on in order to produce the heated region  74 . 
         [0128]    In this case, there is no need for a displacing mechanism for displacing the auxiliary electrode  68  separately away from the upper tip  38 . Consequently, the structure of the apparatus and operational control for the apparatus are simplified. 
         [0129]    At any rate, the auxiliary electrode  68  is not limited to the above annular shape. The auxiliary electrode  68  may be in the form of an elongate rod, similar to the upper tip  38  and the lower tip  32 . In this case, the auxiliary electrode  68  may comprise a single electrode or a plurality of electrodes. If the auxiliary electrode  68  comprises a plurality of electrodes, then such electrodes may be brought simultaneously into or out of abutting engagement with the metal sheet  54 . 
         [0130]    The resistance welding apparatus according to the second embodiment may carry out the resistance welding method according to the first embodiment, assuming that the auxiliary electrode  68  and the power supply  56  are electrically insulated from each other. With the arrangement of the resistance welding apparatus according to the second embodiment, it is possible to select whether the resistance welding method according to the second embodiment or the resistance welding method according to the first embodiment is carried out, by selectively passing or not passing an electric current through the auxiliary electrode  68 . 
         [0131]    In the first embodiment and the second embodiment, a C-type welding gun has been illustrated. However, the welding gun may be a so-called X-type welding gun. More specifically, the lower tip  32  and the upper tip  38  may be mounted on a pair of respective chuck fingers, which are openable and closable, wherein the chuck fingers are opened or closed to move the lower tip  32  and the upper tip  38  away from or toward each other. 
         [0132]    The stacked assembly may comprise four or more metal sheets.