Patent Publication Number: US-2016221110-A1

Title: Resistance spot welding apparatus, composite electrode, and resistance spot welding method

Description:
TECHNICAL FIELD 
     The present invention relates to technologies of resistance spot welding and in particular to a resistance spot welding apparatus for performing welding on a sheet set including a plurality of lapped metal sheets. Furthermore, the present invention relates to a composite electrode and a resistance spot welding method that are used in the resistance spot welding. 
     BACKGROUND ART 
     Transportation machines such as automobiles and industrial machines include a plurality of structural parts. Resistance spot welding (hereinafter also simply referred to as “spot welding”) is utilized in production of structural parts in many cases. 
     In general, spot welding is performed in the following manner. A sheet set is prepared as a workpiece. The sheet set includes a portion in which a plurality of metal sheets are lapped. Next, the sheet set is clamped by a pair of electrodes and the electrodes are pressed against the sheet set. Then, a current is applied across the electrodes while the forces by the pressing of the electrodes are being applied to the sheet set. Thus, in the sheet set, the forces applied by the electrodes bring adjacent metal sheets into contact with each other and the current flows through the contact area and nearby areas. The areas are melted by Joule heating due to the electrical resistance and then solidify to form a nugget. By means of the formation of a nugget, the metal sheets in the sheet set are joined and connected together, whereby structural parts are produced. 
     Examples of the electrodes that may be used include a flat type electrode tip, a DR (double R) type electrode tip, and an SR (single R) type electrode tip. Flat type electrode tips have a columnar shape with a flat end surface. DR type electrode tips have a substantially columnar shape with an end portion projecting in a convex shape in which the end surface is a convex curved surface having a large radius of curvature. SR type electrode tips have a substantially columnar shape with an end surface which is a convex curved surface having a large radius of curvature. 
     In recent years, there has been an increasing trend toward use of light weight structural parts, and thus the metal sheets constituting the sheet set are often high tensile strength steels, so-called high tensile steels. High tensile steels, particularly high tensile steels having a tensile strength of 590-780 MPa Grade or higher grade (hereinafter also referred to as “super high tensile steels”) are less prone to plastic deformation and have a high electrical resistance. 
     According to such material properties as described, in spot welding of a super high tensile steel, the suitable range of the welding current to be applied to the electrodes (hereinafter also referred to as “suitable current range”) tends to be narrower. The term “suitable current range” refers to a range of current values from the minimum current value required to obtain a nominal nugget diameter, which is set according to the design specification, to the maximum current value up to which no expulsion occurs. As the suitable current range expands, the advantages increase in ensuring stable operation of spot welding and achieving the nugget diameter. 
     In addition, when a super high tensile steel is spot welded, enhancement of the joint strength is difficult to achieve. For example, in the case of a base metal (high tensile steel) having a tensile strength exceeding the range of 590 to 780 MPa, the tensile strength in the peeling direction, i.e., the so-called cross tension strength (CTS), which is one of the weld joint strength criteria, tends to decrease rather than increase. 
     Thus, spot welding of super high tensile steels involves the problems of a narrower suitable current range and a decrease in CTS, and therefore there is a requirement for expansion of the suitable current range and increase of the weld joint strength. 
     To expand the suitable current range, one possible technique is to increase the force applied by the electrodes when pressing them against the sheet set and another possible technique is to perform multi-stage current applications when applying the current across the electrodes. However, increasing the applied force has its limitations in association with the stiffness of the apparatus. Also, multi-stage current applications result in increased welding time and thus decreased productivity. Hence, neither of these techniques is practical. 
     To increase the weld joint strength, one possible technique is to perform additional, subsequent heating after formation of the nugget and another possible technique is to enlarge the nugget diameter. Subsequent heating tempers and softens the formed nugget to thereby improve its toughness. As a result, the weld joint strength increases. However, performing subsequent heating results in increase of welding time and thus decrease of productivity. Hence, subsequent heating is not practical. 
     Enlargement of the nugget diameter effectively contributes to increasing the weld joint strength. This is because, as the nugget diameter increases, the weld joint strength increases. To enlarge the nugget diameter, one possible technique is to perform multi-stage applications of the current across the electrodes and another possible technique is to increase the diameter of the electrode end surface. However, the multi-stage current application process is a process in which the nugget growth progresses gradually, which results in increased welding time and thus decreased productivity. Hence, the multi-stage current application process is not practical. 
     Increase of the electrode end diameter poses the following problems. When a flat type electrode tip is employed as the electrode, for example, the extended flat end surface needs to be uniformly contacted with the sheet set. For this reason, extremely high dimensional accuracy is required for the flatness of the electrode end surface. On the other hand, when a DR type electrode tip is employed as the electrode, it is necessary to press the extended convex curved end surface deeply into the sheet set so that it is in contact over the entire area. However, an increased amount of pressing leads to the occurrence of sheet separation to limit the current paths, and therefore enlargement of the nugget diameter is limited. Accordingly, for flat type electrode tips, DR type electrode tips, and the like, simply increasing the electrode end diameter is not deemed to be practical. 
     As opposed to these approaches, a technique to enlarge the nugget diameter from a different point of view is proposed in Japanese Patent Application Publication No. 2012-55896 (Patent Literature 1). Patent Literature 1 discloses a resistance spot welding apparatus including: a pair of main electrodes facing each other so as to hold a sheet set therebetween; and an annular auxiliary electrode disposed so as to surround one of the main electrodes (hereinafter also referred to as “first main electrode” for convenience of description). According to the technique disclosed in Patent Literature 1, the auxiliary electrode has a polarity opposite to the polarity of the first main electrode, and currents are applied across the pair of main electrodes and across the first main electrode and the auxiliary electrode. Accordingly, currents flow between the main electrodes and also between the first main electrode and the auxiliary electrode. 
     In the case where the thickness of the metal sheet with which the first main electrode and the auxiliary electrode are brought into contact is thin, the current flows over a large region in the contact area between the thin metal sheet and an adjoining metal sheet because the contact area is located close to the first main electrode and the auxiliary electrode. Consequently, a nugget having a large nugget diameter is formed, according to Patent Literature 1. 
     However, the technique disclosed in Patent Literature 1 is unable to increase the nugget diameter in the case where the thickness of the metal sheet with which the first main electrode and the auxiliary electrode are brought into contact is large. The reason is that the application range of the current that flows in the contact area cannot be extended because the contact area, which is positioned between the thick metal sheet and an adjoining metal sheet, is located away from the first main electrode and the auxiliary electrode. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent Application Publication No. 2012-55896 
     SUMMARY OF INVENTION 
     Technical Problem 
     As described above, in spot welding of a super high tensile steel, there is a requirement for expansion of the suitable current range and increase of the weld joint strength. However, none of the techniques described above are practical. Furthermore, while enlargement of the nugget diameter is effective at increasing the weld joint strength, even the technique disclosed in Patent Literature 1 cannot achieve sufficient enlargement of the nugget diameter. 
     An object of the present invention is to provide a resistance spot welding apparatus, composite electrode, and resistance spot welding method having the following characteristics: 
     Expansion of the suitable current range in spot welding of a super high tensile steel is achieved; and 
     Increase of the weld joint strength in spot welding of a super high tensile steel is achieved. 
     Solution to Problem 
     A resistance spot welding apparatus according to an embodiment of the present invention is an apparatus for performing resistance spot welding on a sheet set including a plurality of lapped metal sheets, the apparatus including a pair of composite electrodes facing each other so as to hold the sheet set therebetween. The composite electrodes each include: a rod-shaped electrode body having an end surface that is brought into contact with the sheet set and pressed against the sheet set; a rigid body including an electrically conductive material being insulated from the electrode body, the rigid body having a through hole in which the electrode body is inserted and having an end surface that is brought into contact with the sheet set and pressed against the sheet set; and a resilient member coupled to a rear end of the rigid body, the resilient member configured to apply a pressing force to the rigid body as the electrode body and the rigid body are pressed against the sheet set. 
     In the above resistance spot welding apparatus, at least part of the end surface of the rigid body may include an electrically conductive material. 
     In the above resistance spot welding apparatus, the rigid body preferably has a cylindrical shape. The rigid body may be configured such that an inner periphery of the end surface is circular and an outer periphery of the end surface is oval, elliptical, or substantially rectangular. 
     In the above resistance spot welding apparatus, the resilient member may be a compression coil spring or the resilient member may be a cylindrical molded polymeric component. 
     In any of the above resistance spot welding apparatuses, a spacing between an outer periphery of the end surface of the electrode body and an inner periphery of the end surface of the rigid body is preferably at most 7 mm 
     The above resistance spot welding apparatuses each preferably include a cooling mechanism that cools the rigid body. 
     A composite electrode according to an embodiment of the present invention is a composite electrode for use in resistance spot welding of a sheet set including a plurality of lapped metal sheets, the composite electrode including: a rod-shaped electrode body having an end surface that is brought into contact with the sheet set and pressed against the sheet set; a rigid body including an electrically conductive material being insulated from the electrode body, the rigid body having a through hole in which the electrode body is inserted and having an end surface that is brought into contact with the sheet set and pressed against the sheet set; and a resilient member coupled to a rear end of the rigid body, the resilient member configured to apply a pressing force to the rigid body as the electrode body and the rigid body are pressed against the sheet set. 
     In the above composite electrode, at least part of the end surface of the rigid body may include an electrically conductive material. 
     In the above composite electrode, the rigid body preferably has a cylindrical shape. The rigid body may be configured such that an inner periphery of the end surface is circular and an outer periphery of the end surface is oval or substantially rectangular. 
     In the above composite electrode, the resilient member may be a compression coil spring or the resilient member may be a cylindrical molded polymeric component. 
     In any of the above composite electrodes, a spacing between an outer periphery of the end surface of the electrode body and an inner periphery of the end surface of the rigid body is preferably at most 7 mm 
     The above composite electrodes each preferably include a cooling mechanism that cools the rigid body. 
     A resistance spot welding method according to an embodiment of the present invention is a method for performing resistance spot welding on a sheet set including a plurality of lapped metal sheets, the method including a series of steps including a first step, a second step, and a third step. The first step includes: arranging a rod-shaped first electrode body and a rod-shaped second electrode body to face each other with the sheet set interposed therebetween; and arranging a first rigid body including an electrically conductive material and a second rigid body including an electrically conductive material to face each other with the sheet set interposed therebetween, the first rigid body having a through hole in which the first electrode body is inserted and having a rear end to which a first resilient member is coupled, the second rigid body having a through hole in which the second electrode body is inserted and having a rear end to which a second resilient member is coupled. The second step includes applying a force to the sheet set by: pressing the end surface of the first electrode body and the end surface of the second electrode body against the sheet set; and pressing the end surface of the first rigid body and the end surface of the second rigid body against the sheet set while a pressing force from the first resilient member is being applied to the first rigid body and a pressing force from the second resilient member is being applied to the second rigid body. The third step includes applying a current across the first electrode body and the second electrode body while applying the force to the sheet set. 
     Advantageous Effects of Invention 
     A resistance spot welding apparatus, composite electrode, and resistance spot welding method of the present invention have significant advantages such as the following: 
     Expansion of the suitable current range in spot welding of a super high tensile steel can be achieved; and 
     Increase of the weld joint strength in spot welding of a super high tensile steel can be achieved. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional view of a sheet set used as a workpiece to which welding is to be applied; 
         FIG. 2A  is a schematic diagram of a resistance spot welding apparatus according to a first embodiment, showing a state prior to welding; 
         FIG. 2B  is a schematic diagram of the resistance spot welding apparatus according to the first embodiment, showing a state during welding; 
         FIG. 3  is a schematic diagram illustrating a situation in which a nugget is for lied by spot welding using the resistance spot welding apparatus shown in  FIG. 2 ; 
         FIG. 4  is a graph showing the relationships between the electrode-to-rigid body spacing and the maximum nugget diameter and between the electrode-to-rigid body spacing and the suitable current range; 
         FIG. 5A  is a schematic diagram of a resistance spot welding apparatus according to a second embodiment, showing a state prior to welding; 
         FIG. 5B  is a schematic diagram of the resistance spot welding apparatus according to the second embodiment, showing a state during welding; and 
         FIG. 6  is a graph showing the results of a spot welding test in Example. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the resistance spot welding apparatus, composite electrode, and resistance spot welding method of the present invention will be described in detail below. 
     The resistance spot welding apparatus of the present embodiment is utilized to perform spot welding on a sheet set including a plurality of lapped metal sheets. The composite electrode of the present embodiment is mounted to the spot welding apparatus and utilized in spot welding. The resistance spot welding method of the present embodiment is utilized in spot welding using the spot welding apparatus. 
     First Embodiment 
     1. Configuration of the Resistance Spot Welding Apparatus and Composite Electrode 
       FIG. 1  is a cross-sectional view of a sheet set used as a workpiece to which welding is to be applied. As shown in  FIG. 1 , a sheet set  1  used as a workpiece in the present embodiment has a portion in which two metal sheets  2 A,  2 B are lapped over each other. Both of the metal sheets  2 A,  2 B are super high tensile steels having a tensile strength of 590-780 MPa Grade or higher grade. The metal sheets  2 A,  2 B each have a thickness of about 0.5 to 3 mm, and the thicknesses may be the same or different from each other. 
     Depending on the form of the structural part to be manufactured by the spot welding, the sheet set may have a portion in which three or more metal sheets are lapped over each other. The properties of the metal sheets are not limited as long as spot welding can be applied and therefore they may be a high tensile steel having a tensile strength lower than 590 MPa or may be a mild steel. Also, the presence or absence of coating, the type of coating, etc. are not limiting. The plurality of lapped metal sheets may be of the same metal or may be of dissimilar metals. 
       FIGS. 2A and 2B  are schematic diagrams of a resistance spot welding apparatus according to a first embodiment.  FIG. 2A  shows a state prior to welding and  FIG. 2B  shows a state during welding. The spot welding apparatus shown in  FIGS. 2A and 2B  includes a pair of composite electrodes  10 ,  20 . Hereinafter, for convenience of description, one of the composite electrodes  10 ,  20  (the upper composite electrode in  FIGS. 2A and 2B ) is also referred to as the first composite electrode  10 , and the other (the lower composite electrode in  FIGS. 2A and 2B ) is also referred to as the second composite electrode  20 . The first composite electrode  10  and the second composite electrode  20  are configured in the same manner and are arranged to face each other so as to hold the sheet set  1  therebetween. Specifically, the first composite electrode  10  includes a first electrode body  11  and a first rigid body  12 , and the second composite electrode  20  includes a second electrode body  21  and a second rigid body  22 . 
     The first electrode body  11  includes a straight, rod-shaped shank  11   b  and an electrode tip  11   a  attached to an end of the shank  11   b,  forming a rod shape as a whole. The shank  11   b  has a flange portion  11   ba  adjacent the electrode tip  11   a.  The electrode tip  11   a  is a DR type electrode tip. Specifically, the electrode tip  11   a  has a substantially columnar shape with an end portion projecting in a convex shape in which an end surface  11  as is a convex curved surface having a large radius of curvature. The electrode tip  11   a  may be a known electrode tip other than a DR type electrode tip, and therefore a flat type electrode tip, an SR type electrode tip, or the like may be employed. The shank  11   b  is secured at its rear end to a holder  14 . 
     The first rigid body  12  is cylindrically shaped and has a circular through hole  12   b  about the central axis, and the first electrode body  11  is disposed concentrically with the through hole  12   b.  The electrode tip  11   a  and flange portion  11   ba  of the first electrode body  11  are inserted in the first rigid body  12  and are relatively movable along the axial length to an end surface  12   a  of the first rigid body  12 . The first rigid body  12  has, at its rear end portion, a stopper surface  12   c,  with which the flange portion  11   ba  of the first electrode body  11  is brought into contact so as to prevent the first rigid body  12  from detaching from the first electrode body  11 . 
     The first rigid body  12  and the fist electrode body  11  are insulated from each other without being electrically connected. Specifically, an insulator such as an engineering plastic is disposed in the region where the first rigid body  12  and the first electrode body  11  can be directly or indirectly connected to each other. For example, an insulator is disposed in a region where the shank  11   b  slides, among regions of the through hole  12   b  of the first rigid body  12 . 
     A retainer plate  15  is secured to the front end of the holder  14 . A compression coil spring  13 A, employed as a first resilient member  13 , is disposed between the rear end of the first rigid body  12  and the retainer plate  15 . The shank  11   b  of the first electrode body  11  passes through the compression coil spring  13 A (first resilient member  13 ) concentrically therewith. The first rigid body  12  is relatively movable along the shank  11   b.    
     Likewise, the second electrode body  21  includes a straight, rod-shaped shank  21   b  and an electrode tip  21   a  attached to an end of the shank  21   b,  forming a rod shape as a whole. The shank  21   b  has a flange portion  21   ba  adjacent the electrode tip  21   a.  The electrode tip  21   a  is a DR type electrode tip. The shank  21   b  is secured at its rear end to a holder  24 . 
     The second rigid body  22  is cylindrically shaped and has a circular through hole  22   b  about the central axis, and the second electrode body  21  is disposed concentrically with the through hole  22   b.  The electrode tip  21   a  and flange portion  21   ba  of the second electrode body  21  are accommodated in the second rigid body  22  and are relatively movable along the axial length to an end surface  22   a  of the second rigid body  22 . The second rigid body  22  has, at its rear end portion, a stopper surface  22   c,  with which the flange portion  21   ba  of the second electrode body  21  is brought into contact so as to prevent the second rigid body  22  from detaching from the second electrode body  21 . 
     The second rigid body  22  and the second electrode body  21  are insulated from each other without being electrically connected. Specifically, an insulator such as an engineering plastic is disposed in the region where the second rigid body  22  and the second electrode body  21  can be directly or indirectly connected to each other. For example, an insulator is disposed in a region where the shank  21   b  slides, among regions of the through hole  22   b  of the second rigid body  22 . 
     A retainer plate  25  is secured to the front end of the holder  24 . A compression coil spring  23 A, employed as a second resilient member  23 , is disposed between the rear end of the second rigid body  22  and the retainer plate  25 . The shank  21   b  of the second electrode body  21  passes through the compression coil spring  23 A (second resilient member  23 ) concentrically therewith. The second rigid body  22  is relatively movable along the shank  21   b.    
     Examples of the material of the shanks  11   b,    21   b  and electrode tips  11   a,    11   b,  which constitute the first electrode body  11  and the second electrode body  21 , include a copper-chromium alloy, a copper-chromium-zirconium alloy, a copper-beryllium alloy, an aluminum-oxide-dispersion-strengthened copper alloy, and a copper-tungsten alloy. The material of the first electrode body  11  and the second electrode body  21  is not particularly limited as long as they can form electrodes. 
     The first rigid body  12  and the second rigid body  22  are rigid bodies that do not deform under an external force, each including an electrically conductive material such as a metal. The end surfaces  12   a,    22   a  of the first rigid body  12  and the second rigid body  22  may be formed entirely of an electrically conductive material or partially of an electrically conductive material. 
     The material of the first rigid body  12  and the second rigid body  22  is not particularly limited as long as it has a high electrical conductivity, and may be the same as the material of the first electrode body  11  and the second electrode body  21  or may be different. However, the material of the first rigid body  12  and the second rigid body  22  needs to have an electrical conductivity higher than the electrical conductivity of the sheet set (metal sheets) to be welded. This is intended to effectively draw the current that flows through the sheet set during spot welding into the first rigid body  12  and the second rigid body  22  as described in detail later. 
     The holders  14 ,  24  of the thus configured first composite electrode  10  and the second composite electrode  20  are attached to a spot welding gun (not shown). Specifically, the welding gun includes a pair of aims capable of opening and closing operation. The holder  14  of the first composite electrode  10  is attached to an end of one of the arms and the holder  24  of the second composite electrode  20  is attached to an end of the other of the arms. The opening and closing operation of the two arms causes the first composite electrode  10  and the second composite electrode  20  to be moved away from and close to each other. At this time, the first electrode body  11  and the second electrode body  21  are coaxially aligned to face each other and the first rigid body  12  and the second rigid body  22  are also coaxially aligned to face each other. Optionally, one of the pair of arms may be stationary. 
     The first electrode body  11  and the second electrode body  21  are connected to a power supply (not shown). For example, when a DC power supply is used as the power supply, the positive electrode of the power supply is connected to the first electrode body  11  and the negative electrode of the power supply is connected to the second electrode body  21 . The connections to the positive electrode and the negative electrodes may be opposite. The power supply may alternatively be an AC power supply. 
     2. Resistance Spot Welding 
     With reference to  FIG. 2  described above and  FIG. 3  described below, a process of spot welding using the spot welding apparatus of the present embodiment will be described. 
     Firstly, as shown in  FIG. 2A , the sheet set  1  having a portion in which two metal sheets  2 A,  2 B are lapped over each other is prepared as a workpiece. Next, the first electrode body  11  of the first composite electrode  10  and the second electrode body  21  of the second composite electrode  20  are arranged to face each other with the sheet set  1  interposed therebetween, and the corresponding first rigid body  12  and second rigid body  22  are arranged to face each other with the sheet set  1  interposed therebetween. This operation is carried out by movement of the welding gun or by transfer of the sheet set  1 . 
     Next, the operation of closing the two arms of the welding gun is carried out to begin the operation of pressing the first composite electrode  10  and the second composite electrode  20  against the sheet set  1 . The operation causes the holder  14  to move toward the sheet set  1  in the first composite electrode  10  and simultaneously causes the holder  24  to move toward the sheet set  1  in the second composite electrode  20 . Accordingly, in the first composite electrode  10 , the end surface  12   a  of the first rigid body  12  is firstly brought into contact with and pressed against the surface of the metal sheet  2 A of the sheet set  1  so that further movement of the first rigid body  12  is restricted. In the second composite electrode  20 , the end surface  22   a  of the second rigid body  22  is firstly brought into contact with and pressed against the surface of the metal sheet  2 B of the sheet set  1  so that further movement of the second rigid body  22  is restricted. 
     Furthermore, in the first composite electrode  10 , the first electrode body  11  is continuously moved toward the metal sheet  2 A. In this process, the spacing between the first rigid body  12  and the retailer plate  15  gradually decreases, and the first resilient member  13  (compression coil spring  13 A) undergoes compressive deformation. Concurrently, in the second composite electrode  20 , the second electrode body  21  is continuously moved toward the metal sheet  2 B. In this process, the spacing between the second rigid body  22  and the retailer plate  25  gradually decreases, and the second resilient member  23  (compression coil spring  23 A) undergoes compressive deformation. 
     Subsequently, as shown in  FIG. 2B , in the first composite electrode  10 , the end surface  11  as of the first electrode body  11  is brought into contact with and pressed against the surface of the metal sheet  2 A so that further movement of the first electrode body  11  is restricted. Concurrently, in the second composite electrode  20 , the end surface  21  as of the second electrode body  21  is brought into contact with and pressed against the surface of the metal sheet  2 B so that further movement of the second electrode body  21  is restricted. 
     In this manner, the sheet set  1  is clamped by the first electrode body  11  and the second electrode body  21 , which face each other, and by the first rigid body  12  and the second rigid body  22 , which face each other. In this process, pressing forces from the first electrode body  11  and the second electrode body  21  are applied to the sheet set  1 , and pressing forces from the first rigid body  12  and the second rigid body  22  are also applied to the sheet set  1 . 
     Here, it should be noted that a resilient force due to the compressive deformation of the compressively deformed first resilient member  13  acts on the first rigid body  12 , and a resilient force due to the compressive deformation of the compressively deformed second resilient member  23  acts on the second rigid body  22 . As a result, the metal sheets  2 A,  2 B, which constitute the sheet set  1 , are subjected to the application of forces not only at the contact areas with the first electrode body  11  and the second electrode body  21  but also at the surrounding, annular areas (contact areas with the first rigid body  12  and the second rigid boy  22 ), so that the metal sheets  2 A,  2 B are placed in a state of sufficient contact over a large area. Consequently, the occurrence of sheet separation is inhibited. 
     In this state, the power supply is driven and a current is applied across the first electrode body  11  and the second electrode body  21 . 
       FIG. 3  is a schematic diagram illustrating a situation in which a nugget is formed by spot welding using the resistance spot welding apparatus shown in  FIG. 2 . In FIG.  3 , the dashed arrows show the flow of the welding current. 
     As shown in  FIG. 3 , the contact area between the metal sheets  2 A,  2 B is not limited to the area corresponding to the areas in contact with the first electrode body  11  and the second electrode body  21  but extends over a larger area including the surrounding area corresponding to the areas in contact with the first rigid body  12  and the second rigid boy  22 , unlike in cases of conventional spot welding techniques. As a result, when a current is applied across the first electrode body  11  and the second electrode body  21 , the current flows over a large region within the sheet set  1 , i.e., within the metal sheets  2 A,  2 B without causing marked sheet separation. 
     Specifically, the current flows not only simply from the first electrode body  11  to the second electrode body  21 , but also is drawn toward the first rigid body  12  from the first electrode body  11  and then is drawn toward the second rigid body  22 , and finally flows to the second electrode body  21 . This is due to the sufficient contact between the metal sheets  2 A,  2 B at the area corresponding to the areas facing the first rigid body  12  and the second rigid body  22  by virtue of the strong forces from the first rigid body  12  and the second rigid body  22 , and also due to the high electrical conductivities of both the first rigid body  12  and the second rigid body  22 . 
     Typically, expulsion occurs between metal sheets but, in the case where a large current is applied across the electrodes, the contact areas between the electrodes and the metal sheets can become overheated, so that expulsion may occur on the surfaces of the metal sheets. In the present embodiment, by virtue of the first rigid body  12  and the second rigid body  22 , which are both electrically conductive, the current from the first electrode body  11  is partially diverted to the electrically conductive first rigid body  12  or the current from the second electrode body  21  is partially diverted to the second rigid body  22 , and therefore heat generation is inhibited at the contact areas between the electrodes and the metal sheets, and consequently the present embodiment provides the further advantage of inhibiting expulsion on the metal sheets. 
     Thus, because of the strong forces of the first rigid body  12  and the second rigid body  22  applied to the metal sheets  2 A,  2 B, the contact area between the metal sheets  2 A,  2 B is fused over a large area, so that a nugget  3  having a large nugget diameter is formed. 
     With the spot welding of the present embodiment, it is possible to enlarge the nugget diameter and therefore to increase the weld joint strength including the CTS. Moreover, it is possible to expand the suitable current range in association with the enlargement of the nugget diameter. 
     In order to produce the effect of inhibiting sheet separation, an important issue is the spacing between the outer periphery of the end surface  11   aa  of the first electrode body  11  and the inner periphery of the end surface  12   a  of the first rigid body  12  and the spacing between the outer periphery of the end surface  21   aa  of the second electrode body  21  and the inner periphery of the end surface  22   a  of the second rigid body  22 . Hereinafter, the spacings are also collectively referred to as the electrode-rigid body spacing. The electrode-rigid body spacing is preferably as small as possible to the extent that the electrode body and the rigid body are not in contact with each other during welding. If the electrode-rigid body spacing is too large, the effect of inhibiting sheet separation will be reduced and, in addition, the current cannot extend easily. The electrode-rigid body spacing is preferably at most 7 mm. More preferably, the electrode-rigid body spacing is at most 5 mm, and still more preferably at most 3 mm. On the other hand, if the electrode-rigid body spacing is too small, inadvertent contact between the electrode bodies and the rigid bodies occurs to cause conduction during welding, so that the welding current becomes unstable. For this reason, the electrode-rigid body spacing is preferably not less than 0.3 mm for practical purposes. More preferably, the electrode-rigid body spacing is not less than 0.5 mm, and more preferably not less than 1.0 mm 
       FIG. 4  is a graph showing the relationships between the electrode-to-rigid body spacing and the maximum nugget diameter and between the electrode-to-rigid body spacing and the suitable current range. The relationships shown in  FIG. 4  are results from analysis of the influence of the electrode-rigid body spacing on spot welding, conducted using spot welding analysis software SORPAS (a registered trademark of SCSK Corporation). In the analysis, the conditions for extending the current from the electrode bodies toward the rigid bodies were set with various electrode-rigid body spacings. The metal sheets to be welded were hot stamped steel sheets (non-plated) having a tensile strength of 1500 MPa Grade with a thickness t of 1.2 mm. The material of the electrodes and the rigid bodies was a copper-chromium alloy (1 mass % Cr—Cu). The electrode tips of the electrode bodies were SR type electrode tips, each having an outside diameter, including that of the end surface, of 8 mm with the radius of curvature R of the end surface being 80 mm. The force applied by the electrode bodies was 3.43 kN (350 kgf) and the welding time was 16 cycles (frequency: 60 Hz). Different welding currents were used for each of the various electrode-rigid body spacings, and the resulting nugget diameter and the occurrence of expulsion were investigated for each condition. 
     In the investigation, the maximum nugget diameter and the suitable current range were evaluated for each of the electrode-rigid body spacings. The maximum nugget diameter was defined as the largest nugget diameter that can be obtained without causing expulsion. The suitable current range was defined as a range of current values from a current value required to obtain a nugget having a nugget diameter of 4 √t to a maximum current value up to which no expulsion occurs. From  FIG. 4 , it is seen that, starting from the point of the electrode-rigid body spacing of 7 mm, the maximum nugget diameter increases and the suitable current range expands with the decreasing electrode-rigid body spacing. This demonstrates that a preferred electrode-rigid body spacing is at most 7 mm. 
     In the spot welding apparatus of the present embodiment, the first electrode body  11  (the electrode tip  11   a  in particular) is surrounded by the first rigid body  12 . Likewise, the second electrode body  21  (the electrode tip  21   a  in particular) is surrounded by the second rigid body  22 . For this reason, heat generated in spot welding tends to accumulate in the first electrode body  11  and the second electrode body  21 , which can shorten the lives of the electrode tips  11   a,    21   a.  Therefore, it is desired that the first rigid body  12  and the second rigid body  22  be actively cooled to inhibit heat accumulation and that the first electrode body  11  and the second electrode body  21  be indirectly cooled. An example of the cooling structure may be such that a cooling water passage is provided within the first rigid body  12  so that cooling water is circulated through the cooling water passage. Another example of the cooling structure may be such that cooling water is sprayed on the outer peripheral surface of the first rigid body  12 . In the latter case, the cooling water to be used is one containing an anti-rust agent. These cooling structures may also be employed for the second electrode body  21 . 
     Second Embodiment 
       FIGS. 5A and 5B  are schematic diagrams of a resistance spot welding apparatus according to a second embodiment.  FIG. 5A  shows a state prior to welding and  FIG. 5B  shows a state during welding. The spot welding apparatus according to the second embodiment shown in  FIGS. 5A and 5B  are based on the configuration of the spot welding apparatus according to the first embodiment shown in  FIGS. 2A and 2B , and thus redundant descriptions will not be repeated where appropriate. 
     In the second embodiment, the shank  11   b  of the first electrode body  11  does not include the flange portion  11   ba  like the one in the first embodiment described above. Accordingly, the first rigid body  12  does not include the stopper surface  12   c  at its rear end portion like the one in the first embodiment described above. 
     The movable plate  16  is secured to the rear end of the first rigid body  12 , and the retainer plate  15  is secured to the front end of the holder  14 . The shank  11   b  of the first electrode body  11  passes through the movable plate  16  and the retainer plate  15 . A cylindrical molded polymeric component  13 B, employed as the first resilient member  13 , is disposed between the movable plate  16  and the retainer plate  15 . The shank  11   b  of the first electrode body  11  passes through the molded polymeric component  13 B (first resilient member  13 ) concentrically therewith. A plurality of guide bolts  17  are screwed into a peripheral region of the retainer plate  15  so as to pass through a peripheral region of the movable plate  16 . Thus, the first resilient member  13  is sandwiched and retained between the movable plate  16  and the retainer plate  15 . The first rigid body  12 , integrally with the movable plate  16 , is relatively movable along the shank  11   b  by means of the guiding of the guide bolts  17 . 
     The first rigid body  12  and the fist electrode body  11  are insulated from each other without being electrically connected. Specifically, an insulator such as an engineering plastic is disposed in the region where the first rigid body  12  and the first electrode body  11  can be directly or indirectly connected to each other. For example, the movable plate  16 , which can slide against the shank  11  b, is made of an insulating material. 
     Likewise, the shank  21   b  of the second electrode body  21  in the second embodiment does not include the flange portion  21   ba  like the one in the first embodiment described above. Accordingly, the second rigid body  22  does not include the stopper surface  22   c  at its rear end portion like the one in the first embodiment described above. 
     The movable plate  26  is secured to the rear end of the second rigid body  22 , and the retainer plate  25  is secured to the front end of the holder  24 . The shank  21   b  of the second electrode body  21  passes through the movable plate  26  and the retainer plate  25 . A cylindrical molded polymeric component  23 B, employed as the second resilient member  23 , is disposed between the movable plate  26  and the retainer plate  25 . The shank  21   b  of the second electrode body  21  passes through the molded polymeric component  23 B (second resilient member  23 ) concentrically therewith. A plurality of guide bolts  27  are screwed into a peripheral region of the retainer plate  25  so as to pass through a peripheral region of the movable plate  26 . Thus, the second resilient member  23  is sandwiched and retained between the movable plate  26  and the retainer plate  25 . The second rigid body  22 , integrally with the movable plate  26 , is relatively movable along the shank  21   b  by means of the guiding of the guide bolts  27 . 
     The second rigid body  22  and the second electrode body  21  are insulated from each other without being electrically connected. Specifically, an insulator such as an engineering plastic is disposed in the region where the second rigid body  22  and the second electrode body  21  can be directly or indirectly connected to each other. For example, the movable plate  26 , which can slide against the shank  21   b,  is made of an insulating material. 
     Examples of the material of the first resilient member  13  and the second resilient member  23  include a material having excellent durability and suitable resiliency such as a polyurethane resin. 
     During welding using the spot welding apparatus configured as described above, pressing forces are applied to the first rigid body  12  and the second rigid body  22  from the compressively deformed first resilient member  13  and second resilient member  23 , i.e., the molded polymeric components  13 B,  23 B. This situation is the same as that in the first embodiment described above. Therefore, the second embodiment also produces advantageous effects similar to those of the first embodiment described above. 
     EXAMPLES 
     To verify the advantages of the present invention, a welding test was conducted in which spot welding was performed using a spot welding apparatus according to the first embodiment as shown in  FIG. 2 . A number of sheet sets formed of two lapped steel sheets of the same grade having the same thickness, for use as test specimens, were prepared from hot stamped steel sheets (non-plated) having a tensile strength of 1500 MPa Grade with a thickness of 1.6 mm DR type electrode tips were used as the electrode tip of the first electrode body and the electrode tip of the second electrode body. The DR type electrode tips were made from a copper-chromium alloy (1 mass % Cr—Cu), having an outside diameter of 12 mm with an end diameter of 6 mm and having a radius of curvature R of the end surface of 40 mm. The first rigid body and the second rigid body were made from a copper-chromium alloy (1 mass % Cr—Cu), having an inside diameter of 13 mm 
     The welding conditions are shown in Table 1 below. The welding current was varied for each run of spot welding, and the behavior of the nugget growth and the current value at which expulsion occurs were investigated. In Table 1, 1 cycle indicates 1/60 seconds. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Sheet 
                 Applied 
                 Welding 
                 Welding 
                 Holding 
               
               
                 thickness 
                 force 
                 time 
                 current 
                 time 
               
               
                   
               
             
            
               
                 1.6 mm 
                 400 kgf 
                 20 cyc. 
                 4.0-10.5 kA 
                 10 cyc. 
               
               
                   
                 (3.92 kN) 
               
               
                   
               
               
                 Remarks) 1 cyc. indicates 1/60 seconds 
               
            
           
         
       
     
     For comparison, a test was conducted in which spot welding was performed using a typical conventional method simply with a pair of electrode tips alone clamping the sheet set. The test specimens and the electrode tips were prepared in the same manner as in the above inventive example and the welding conditions were the same as in the above inventive example. 
     A torsion test was conducted for each sheet set that had undergone the spot welding. The nugget diameter was measured from the appearance of the nugget, which was made visible by the torsion test. Specifically, diameters of the nugget were measured in two orthogonal directions, and the average of the obtained results was designated as the nugget diameter. 
       FIG. 6  is a graph showing relationships between the welding current values and nugget diameters obtained in the tests of the examples. The test specimens were prepared from hot stamped steel sheets (non-plated) of 1500 MPa Grade with a thickness t of 1.6 mm 
     As shown in  FIG. 6 , in the inventive examples, the suitable current range and the maximum nugget diameter were significantly increased than in the comparative examples. In the comparative examples, the maximum nugget diameter was approximately 5 √t, whereas, in the inventive examples, the maximum nugget diameter was greater than 6 √t. Furthermore, in the comparative examples, the suitable current range was approximately 2.6 kA, whereas, in the inventive examples, the suitable current range was expanded to approximately 4.0 kA. These results demonstrate that the present invention is capable of expanding the suitable current range and enlarging the nugget diameter in spot welding of a super high tensile steel, and therefore capable of increasing the weld joint strength. 
     The present invention is not limited to the embodiments described above, but may be modified in various ways without departing from the spirit and scope of the present invention. For example, the shape of the rigid bodies is not limited to cylindrical, but may be modified depending on the shape of the sheet set to be welded. That is, the shape of the rigid bodies may be such that the inner periphery of the end surface is circular and the outer periphery of the end surface is oval, elliptical, or substantially rectangular. 
     INDUSTRIAL APPLICABILITY 
     The present invention is capable of being effectively utilized in production of structural parts from a super high tensile steel. 
     REFERENCE SIGNS LIST 
       1 : sheet set,  2 A: metal sheet,  2 B: metal sheet,  3 : nugget, 
       10 : first composite electrode,  11 : first electrode body, 
       11   la:  electrode tip,  11   aa:  end surface of electrode tip, 
       11   b:  shank,  11   ba:  flange portion of shank, 
       12 : first rigid body,  12   a:  end surface of first rigid body, 
       12   b:  through hole of first rigid body,  12   c:  stopper surface of first rigid body, 
       13 : resilient member,  13 A: compression coil spring, 
       13 B: molded polymeric component,  14 : holder,  15 : retainer plate, 
       16 : movable plate,  17 : guide bolt, 
       20 : second composite electrode,  21 : second electrode body, 
       21   a:  electrode tip,  21   aa:  end surface of electrode tip, 
       21   b:  shank,  21   ba:  flange portion of shank, 
       22 : second rigid body,  22   a:  end surface of second rigid body, 
       22   b:  through hole of second rigid body,  22   c:  stopper surface of second rigid body, 
       23 : resilient member,  23 A: compression coil spring, 
       23 B: molded polymeric component,  24 : holder,  25 : retainer plate, 
       26 : movable plate,  27 : guide bolt.