Spot welding structure and method for manufacturing spot welding structure

Abstract

This spot welding structure comprises: a first lap joint; a second lap joint; a lap part in which one or more ends of a metal plate of the first lap joint and one or more ends of a metal plate of the second lap joint are overlapped; a main nugget joining the first lap joint and the second lap joint in the lap part; a metal plate-form member that is superposed on the first lap joint, the second lap joint, and the lap part; and a plurality of sub-nuggets joining the plate-form member and the first lap joint, and joining the plate-form member and the second lap joint. The number of metal plates to which the main nugget is joined is three. An indentation of the main nugget is provided only to the metal plate included in the first lap joint or the second lap joint. The plate-form member is located outside the main nugget. The sub-nuggets are located outside the lap part.

Description

Spot welded structure and method for manufacturing spot welded structure

[0001] This application claims priority to Japanese Patent Application No. 2023-150978, filed on September 19, 2023, the contents of which are incorporated herein by reference.

[0002] Spot welding is a resistance welding technique in which overlapping base metals are clamped between the tips of electrodes, and a current is passed through them while applying pressure, causing the metals to melt and join with the Joule heat generated. In lap resistance welding such as spot welding, the molten and solidified part that forms at the weld is called a nugget.

[0003] Because spot welding is highly efficient, it is used to manufacture various mechanical structural parts. For example, in the manufacture of automobiles, thousands of spot welds are formed per vehicle.

[0004] Patent Document 1 discloses a joint structure for a vehicle body frame in which a side sill is formed by joining ends of divided panel pieces obtained by dividing the inner panel longitudinally with a reinforcement interposed between them, and in which the flange portions of both panels and the reinforcement are overlapped with each other, spot welds that can be overlapped with each other are formed at one end of each divided panel piece and joined by spot welding, and the flange portion of the outer panel and the flange portion of the reinforcement are overlapped with one of the flange portions of both divided panel pieces in a three-layer configuration and then joined by spot welding.

[0005] The vehicle center pillar structure disclosed in Patent Document 2 has an upper reinforcement disposed in the upper portion of the interior space formed by the outer center pillar and the inner center pillar, and a lower reinforcement disposed in the lower portion of the interior space. The lower end of the upper reinforcement and the upper end of the lower reinforcement are disposed so as to overlap each other. The two reinforcements are combined with each other, with one different side of each side edge being cut out. The upper main reinforcement portion extends toward the lower main reinforcement portion, and the two reinforcements overlap each other, forming a partition wall that divides the interior space into upper and lower sections.

[0006] The impact energy absorbing structure for the upper body of an automobile disclosed in Patent Document 3 is a structure for absorbing impact energy for the upper body of an automobile formed into a closed structure by an outer panel and an inner panel and equipped with a roof side rail extending in the fore-and-aft direction, and includes two energy absorbing panels made of steel plate with a thickness smaller than that of the outer panel and the inner panel. The two energy absorbing panels have the same cross-sectional shape and the same plate thickness, are arranged in the fore-and-aft direction of the roof side rail so that adjacent terminal portions overlap, and are joined to the roof side rail at their flanges without joining their web portions.

[0007] In the resistance spot welding method disclosed in Patent Document 4, after the applied pressure reaches a set value, a preliminary current is started at a low current; when the gap is eliminated and all of the overlapping plates are in contact, the electrode position is fixed, and then main current is started; by fixing the electrode position, thermal expansion of the plate assembly after the start of main current is suppressed, thereby increasing the applied pressure, and the initial set applied pressure P and the maximum applied pressure Pm are set to Pm≧1.5×P.

[0008] Patent Document 5 discloses a resistance spot welding method for high-strength thin steel sheets, which comprises: sandwiching overlapping high-strength thin steel sheets between a pair of electrodes and applying a pressure while performing resistance spot welding; after welding a first point, moving the position of the electrodes; and, after the first welded point is cooled to a temperature below the Mf point, performing a second weld so that a part of the second welded point overlaps the first welded point.

[0009] Japanese Patent Application Laid-Open No. 2012-30746 Japanese Utility Model Application Laid-Open No. 5-16559 Japanese Patent Application Laid-Open No. 9-39834 Japanese Patent Application Laid-Open No. 2008-290098 Japanese Patent Application Laid-Open No. 2010-172945

[0010] In recent years, the use of inner frame structures in automobile bodies has been increasing. In an inner frame structure, the automobile body is divided into a framework (frame) and outer panel members (outer panels), and the entire frame is assembled first, after which the outer panels are welded. This allows the framework members to be joined even more firmly.

[0011] However, spot welding has the problem of prone to welding defects when welding four or more overlapping plates. In the manufacture of automobile bodies, welding defects are likely to occur at the spot welding points where two frame members and an outer plate member are spot welded. In particular, when the outer plate member is made of thin mild steel, the nugget may not grow all the way to the thin plate. It is difficult to stabilize the quality of spot welding at such joints.

[0012] One way to avoid poor welding of outer plate members is to increase the welding current to promote nugget growth, but spot welding, which uses a large current, is prone to expulsion.

[0013] Furthermore, during the manufacturing of automotive parts, gaps are likely to form between steel sheets due to a phenomenon known as springback. Spot welding of sheet assemblies with gaps between the steel sheets can result in explosions and large blowholes in the nugget. Blowholes reduce the joint strength of the weld.

[0014] Another method for avoiding poor joints of outer plate members is to reduce the number of metal plates included in the joint and thereby reduce the total thickness of the joint. For example, as shown in FIG. 2 , a conventional method involves overlapping three steel plates at the joint between two skeletal members. In the conventional technique, only one of two or more steel plates in each skeletal member (first lap joint 11 and second lap joint 12 in FIG. 2 ) is the target of joining. Then, the steel plates of the two skeletal members (the metal plate 111 on the upper side of the drawing in FIG. 2 and the metal plate 121 on the lower side of the drawing in FIG. 2 ) and the outer plate member (the plate-like member 13 in FIG. 2 ) are overlapped and spot-welded to form a nugget (the main nugget 15 in FIG. 2 ).

[0015] However, in the spot welding structure illustrated in Fig. 2, the joint is made of three layers including an outer plate member, which reduces the load transmission efficiency between the frame members, which can result in a decrease in the rigidity of the car body and a decrease in the joint strength.

[0016] None of the techniques disclosed in Patent Documents 1 to 5 assume an inner frame structure, and do not disclose means for solving the above-mentioned problems.

[0017] In view of the above circumstances, an object of the present disclosure is to provide a spot welded structure that suppresses poor joining of outer plate members and has high load transmission efficiency between frame members, and a method for manufacturing the spot welded structure.

[0018] The gist of the present disclosure is as follows.

[0019] (1) A spot welding structure according to one aspect of the present disclosure includes a first lap joint having two or more metal plates, a second lap joint having two or more metal plates, a lap portion where end portions of one or more of the metal plates of the first lap joint and end portions of one or more of the metal plates of the second lap joint are overlapped, a main nugget that joins the first lap joint and the second lap joint at the lap portion, a metal plate-shaped member overlapped on the first lap joint, the second lap joint, and the lap portion, and a plurality of secondary nuggets that join the plate-shaped member and the first lap joint, and the plate-shaped member and the second lap joint, wherein the number of metal plates joined by the main nugget is three, an indentation of the main nugget is provided only on the metal plates included in the first lap joint or the second lap joint, the plate-shaped member is outside the main nugget, and the secondary nugget is outside the lap portion. (2) Preferably, in the spot welding structure described in (1) above, the distance between the center of the secondary nugget joining the plate-like member and the first lap joint and the center of the secondary nugget joining the plate-like member and the second lap joint is 200 mm or less. (3) Preferably, in the spot welding structure described in (1) or (2) above, one or more of the metal plates of the first lap joint and the metal plates of the second lap joint have a seam-processed portion in the lap portion, and two of the three metal plates joined by the primary nugget are arranged on approximately the same plane outside the lap portion. (4) Preferably, in the spot welding structure described in any one of (1) to (3) above, the number of joining interfaces between the first lap joint and the second lap joint is two or more. (5) Preferably, in the spot welding structure described in any one of (1) to (4) above, the spot welding structure has the main nugget joining the two metal plates of the first lap joint and the one metal plate of the second lap joint, and the main nugget joining the one metal plate of the first lap joint and the two metal plates of the second lap joint. (6) Preferably, in the spot welding structure described in any one of (1) to (5) above, the spot welding structure further has an adhesive between the plate-shaped member and one or both of the first lap joint and the second lap joint.(7) Preferably, in the spot welded structure described in any one of (1) to (6) above, the spot welded structure further includes an additional nugget in the lap portion, the additional nugget joining two or more of the metal plates and the plate-like members included in the lap portion, and an indentation of the additional nugget is provided in the plate-like member. (8) Preferably, in the spot welded structure described in any one of (1) to (7) above, the plate-like member is separated from the first lap joint and the second lap joint over the entire lap portion. (9) Preferably, in the spot welded structure described in any one of (1) to (8) above, the first lap joint includes a nugget outside the lap portion that joins only the metal plates of the first lap joint, and the second lap joint includes a nugget outside the lap portion that joins only the metal plates of the second lap joint. (10) Preferably, in the spot welding structure described in any one of (1) to (9) above, the first lap joint and the second lap joint are skeletal components of an automobile, and the plate-like member is an outer panel component of the automobile.

[0020] (11) A manufacturing method of a spot welded structure according to another aspect of the present invention includes the steps of preparing a first lap joint having two or more metal plates and a second lap joint having two or more metal plates; overlapping end portions of one or more of the metal plates of the first lap joint and end portions of one or more of the metal plates of the second lap joint to form a lap portion; primary spot welding the first lap joint and the second lap joint at the lap portion to form a main nugget; overlapping the first lap joint, the second lap joint, and the lap portion on which the main nugget is formed with a metal plate-shaped member; and secondary spot welding the plate-shaped member and the first lap joint, and the plate-shaped member and the second lap joint to form a plurality of secondary nuggets, wherein the number of metal plates to be primary spot welded is three, and the welding points for the secondary spot welding are outside the lap portion. (12) A manufacturing method of a spot welded structure according to another aspect of the present invention includes the steps of preparing a plate assembly in which two of the metal plates that are components of a first lap joint are overlapped at their ends, overlapping an end of one of the metal plates that are components of a second lap joint with the end of the plate assembly to form a lap portion, primarily spot welding two of the metal plates that are components of the first lap joint and one of the metal plates that are components of the second lap joint at the lap portion to form a main nugget and the first lap joint, and welding the second lap joint in which the main nugget is provided. The method includes the steps of: overlapping one of the metal plates and the remaining metal plate of the second lap joint outside the lap portion and spot welding them to form the second lap joint; overlapping a metal plate-shaped member on the lap portion where the first lap joint, the second lap joint, and the main nugget are formed; and secondary spot welding the plate-shaped member and the first lap joint, and the plate-shaped member and the second lap joint to form a plurality of secondary nuggets, wherein the number of metal plates to be primarily spot welded is three, and the welding points for the secondary spot welding are outside the lap portion.(13) Preferably, in the manufacturing method of the spot welded structure described in (11) or (12) above, the distance between the center of the weld point in the secondary spot welding of the plate-shaped member and the first lap joint and the center of the weld point in the secondary spot welding of the plate-shaped member and the second lap joint is 200 mm or less. (14) Preferably, in the manufacturing method of the spot welded structure described in any one of (11) to (13) above, a seam processing portion is provided on the end portion of one or more of the metal plates of the first lap joint and the metal plates of the second lap joint, and when forming the lap portion, two of the three metal plates joined by the main nugget are arranged in approximately the same plane outside the lap portion. (15) Preferably, in the manufacturing method of the spot welded structure described in any one of (11), (13), and (14), when forming the lap portion, the lap portion is formed by overlapping two of the metal sheets of the first lap joint with one of the metal sheets of the second lap joint, and the lap portion is formed by overlapping one of the metal sheets of the first lap joint with two of the metal sheets of the second lap joint, and the primary spot welding is performed on each of the plurality of lap portions. (16) Preferably, in the manufacturing method of the spot welded structure described in any one of (12) to (14), after forming the main nugget, the method further includes a step of overlapping the remaining metal sheet of the second lap joint on the one of the metal sheets of the second lap joint on which the main nugget is provided, so as to form an additional lap portion by overlapping one of the metal sheets of the first lap joint with two of the metal sheets that are components of the second lap joint, and a step of further forming the main nugget by performing primary spot welding on the additional lap portion. (17) Preferably, the manufacturing method of the spot welded structure described in any one of (11) to (16) above further includes a step of applying an adhesive between the plate-shaped member and one or both of the first lap joint and the second lap joint after the primary spot welding and before overlapping the plate-shaped member onto the first lap joint, the second lap joint, and the lap portion on which the main nugget is formed.(18) Preferably, the manufacturing method of the spot welded structure described in any one of (11) to (17) above further includes a step of performing additional spot welding on the lap portion and the plate-like member after the secondary spot welding. (19) Preferably, in the manufacturing method of the spot welded structure described in any one of (11) to (18) above, all spot welding on the plate-like member is performed outside the lap portion. (20) Preferably, in the manufacturing method of the spot welded structure described in any one of (11) to (19) above, the first lap joint and the second lap joint are automotive frame components, and the plate-like member is an exterior panel component of the automotive vehicle.

[0021] According to the present disclosure, it is possible to provide a spot welded structure that suppresses poor joining of outer plate members and has high load transmission efficiency between frame members, and a method for manufacturing the spot welded structure.

[0022] FIG. 1 is a cross-sectional view of a spot welding structure according to an embodiment of the present disclosure. FIG. 2 is a cross-sectional view of a conventional spot welding structure. FIG. 3 is a schematic diagram of a seam structure. FIG. 4 is a cross-sectional view of an example of a spot welding structure having a seam structure. FIG. 5 is a cross-sectional view of an example of a spot welding structure having a seam structure. FIG. 6 is a cross-sectional view of an example of a spot welding structure having a seam structure. FIG. 7 is a cross-sectional view of an example of a spot welding structure having a seam structure. FIG. 8 is a cross-sectional view of an example of a spot welding structure having a seam structure. FIG. 9 is a cross-sectional view of an example of a spot welding structure having a seam structure. FIG. 10 is a cross-sectional view of an example of a spot welding structure having a seam structure. FIG. 11 is a cross-sectional view of an example of a spot welding structure having a lap portion divided into two. FIG. 12 is a cross-sectional view of an example of a spot welding structure having a lap portion divided into two. FIG. 13 is a cross-sectional view of an example of a spot welding structure having an adhesive. FIG. 14 is a cross-sectional view of an example of a spot welding structure having an additional nugget. FIG. 15 is a schematic diagram of a step of preparing a lap joint in a first manufacturing method for a spot welding structure according to an embodiment of the present disclosure. FIG. 16 is a schematic diagram of a step of overlapping ends of a lap joint to form a lap portion in a first manufacturing method for a spot welding structure according to an embodiment of the present disclosure.

[0023] FIG. 1 is a schematic diagram of a step of forming a main nugget by primary spot welding a lap portion in a first manufacturing method of a spot welded structure according to an embodiment of the present disclosure.

[0024] FIG. 1 is a schematic diagram of a step of overlapping a metal plate-shaped member on another member in a first manufacturing method of a spot welded structure according to an embodiment of the present disclosure.

[0025] FIG. 2 is a schematic diagram of a step of secondary spot welding the plate-shaped member and another member to form a sub-nugget in a first manufacturing method of a spot welded structure according to an embodiment of the present disclosure.

[0026] FIG. 3 is a schematic diagram of a step of preparing a plate assembly corresponding to a first lap joint in a second manufacturing method of a spot welded structure according to an embodiment of the present disclosure.

[0027] FIG. 4 is a schematic diagram of a step of overlapping ends of metal plates to form a lap portion in a second manufacturing method of a spot welded structure according to an embodiment of the present disclosure.

[0028] FIG. 5 is a schematic diagram of a step of primary spot welding a lap portion to form a main nugget and a first lap joint in a second manufacturing method of a spot welded structure according to an embodiment of the present disclosure.

[0029] FIG. 1 is a schematic diagram of a step of overlapping a metal plate-shaped member on another member in a second manufacturing method for a spot welded structure according to an embodiment of the present disclosure. FIG. 2 is a schematic diagram of a step of forming a secondary nugget by secondary spot welding a plate-shaped member and another member in a second manufacturing method for a spot welded structure according to an embodiment of the present disclosure. FIG. 3 is a schematic diagram of an automobile body. FIG. 4 is a cross-sectional example of a spot welded structure according to an embodiment of the present disclosure. FIG. 5 is a cross-sectional example of a spot welded structure according to an embodiment of the present disclosure. FIG. 6 is a cross-sectional example of a spot welded structure according to an embodiment of the present disclosure. FIG. 7 is a cross-sectional example of a spot welded structure according to an embodiment of the present disclosure. FIG. 8 is a cross-sectional example of a spot welded structure according to an embodiment of the present disclosure. FIG. 9 is a perspective view of a first simulation model (Comparative Example No. 1). FIG. 10 is a side view of the first simulation model (Comparative Example No. 1). FIG. 11 is a perspective view of a second simulation model (Invention Example No. 2). FIG. 12 is a perspective view of a second simulation model (Invention Example No. 2). 1 is a perspective view of a third simulation model (invention example No. 3); FIG. 2 is a perspective view of a third simulation model (invention example No. 3); FIG. 3 is a side view of a third simulation model (invention example No. 3); FIG. 4 is a schematic diagram for explaining three-point bending conditions; and FIG. 5 is a graph showing simulation results.

[0023] (1. Spot welding structure 1) A spot welding structure 1 according to one aspect of the present disclosure is applied to, for example, a flange portion of a frame member of an automobile body, and as shown in FIG. 1 etc., includes a first lap joint 11 having two or more metal plates, a second lap joint 12 having two or more metal plates, a lap portion 14 where end portions of one or more of the metal plates 111 of the first lap joint and end portions of one or more of the metal plates 121 of the second lap joint are overlapped, a main nugget 15 that joins the first lap joint 11 and the second lap joint 12 at the lap portion 14, and The joint comprises a metal plate-like member 13 overlapped on the lap joint 11, the second lap joint 12, and the lap portion 14, and a plurality of secondary nuggets 16 joining the plate-like member 13 and the first lap joint 11, and the plate-like member 13 and the second lap joint 12, the number of metal plates joined by the main nugget 15 is three, the indentation 151 of the main nugget 15 is only provided on the metal plates included in the first lap joint 11 or the second lap joint 12, the plate-like member 13 is outside the main nugget 15, and the secondary nuggets 16 are outside the lap portion 14.

[0024] (First lap joint 11 and second lap joint 12) The spot welded structure 1 is a welded structure obtained by further spot welding the ends of two lap joints. Hereinafter, for convenience of explanation, the two lap joints will be referred to as the first lap joint 11 and the second lap joint 12. Either side of the two lap joints can be considered the first lap joint 11. For example, if one lap joint is a B-pillar and the other lap joint is a side sill, the B-pillar may be considered the first lap joint 11, or the side sill may be considered the first lap joint 11. In the spot welded structure 1 illustrated in FIG. 1 and other figures, for convenience, the lap joint on the left side of the page is considered the first lap joint 11, and the lap joint on the right side of the page is considered the second lap joint 12. Hereinafter, the term "lap joint" refers to both the first lap joint 11 and the second lap joint 12 unless otherwise specified.

[0025] A lap joint is a joint in which parts are placed approximately parallel and overlap each other. In this embodiment, the parts of the lap joint are metal plates. The lap joint is a joint obtained by overlap welding multiple metal plates. An example of a lap joint is a frame member of an automobile body to which an inner frame structure is applied.

[0026] The metal plate can have any shape suitable for spot welding. In other words, the term "metal plate" is a concept that encompasses not only flat metal plates but also flat plate-like regions of metal members having a three-dimensional shape. For example, the flange portion of a hat-shaped member is also considered to be a metal plate. The plate-like member described below is also a concept that encompasses flat plate-like regions of metal members having a three-dimensional shape.

[0027] In the spot welding structure 1 illustrated in Fig. 1, the number of metal plates in the lap joint is two. However, the number of metal plates in the lap joint may be any value equal to or greater than three.

[0028] (Lap portion 14 and main nugget 15) End portions of one or more of the metal plates of the first lap joint 11 and end portions of one or more of the metal plates of the second lap joint 12 are overlapped at the lap portion 14. The first lap joint 11 and the second lap joint 12 are spot welded at the lap portion 14. The lap portion 14 is provided with a main nugget 15 that is formed by spot welding and joins the first lap joint 11 and the second lap joint 12 together.

[0029] The main nugget 15 joins three metal plates. In Fig. 1, the main nugget 15 joins one metal plate 111 of the first lap joint 11 and two metal plates 121 of the second lap joint 12. Alternatively, the main nugget 15 may join two metal plates of the first lap joint 11 and one metal plate of the second lap joint 12.

[0030] During spot welding, an indentation is formed so as to overlap the nugget. The indentation is a depression on the surface of the base material caused by the electrode tip as a result of spot welding. The indentation 151 of the main nugget 15 is formed only on the metal plates included in the first lap joint 11 or the second lap joint 12. The indentation 151 of the main nugget is not formed on the plate-like member 13 described below.

[0031] (Plate-like member 13) The spot welding structure 1 according to this embodiment includes a metal plate-like member 13 in addition to the two lap joints. The plate-like member 13 is made of a metal plate. The plate-like member 13 is overlapped with the first lap joint 11, the second lap joint 12, and the lap portion 14. The plate-like member 13 is, for example, an outer plate member of an automobile body to which an inner frame structure is applied.

[0032] The plate-like member 13 is located outside the above-described main nugget 15. The above-described main nugget 15 does not have the function of joining the plate-like member 13 to the first lap joint 11 or the second lap joint 12. The main nugget 15 is formed by spot welding (primary spot welding, described later) that only targets the metal plate 111 of the first lap joint 11 and the metal plate 121 of the second lap joint 12. Therefore, the indentation 151 of the main nugget 15 is not formed on the plate-like member 13.

[0033] (Secondary nugget 16) The plate-like member 13 and the first lap joint 11 are joined by a secondary nugget 16. The plate-like member 13 and the second lap joint 12 are also joined by a secondary nugget 16. This firmly joins the first lap joint 11, the second lap joint 12, and the plate-like member 13 together. An impression 161 of the secondary nugget 16 is provided on the plate-like member 13.

[0034] The secondary nugget 16 is provided outside the lap portion 14. Therefore, the secondary nugget 16 does not join the first lap joint 11 and the second lap joint 12. The secondary nugget impression 161 is also necessarily provided outside the lap portion 14.

[0035] 2 , the main nugget 15 joining the first lap joint 11 and the second lap joint 12 joins two metal plates and plate-like members. Only two frame members (metal plates) are directly involved in joining the first lap joint 11 and the second lap joint 12. In this case, it is difficult to sufficiently increase the load transmission efficiency of the spot welding structure 1.

[0036] In the spot welding structure 1 according to this embodiment as illustrated in Fig. 1 , the main nugget 15 that joins the first lap joint 11 and the second lap joint 12 joins three skeletal members (metal plates). In the spot welding structure 1 according to this embodiment, the plate-shaped member 13 is located outside the main nugget 15. The main nugget 15 is separated from the plate-shaped member 13. Therefore, the number of skeletal members (metal plates) that are directly involved in joining the first lap joint 11 and the second lap joint 12 is three. As a result, the load transmission efficiency is dramatically improved in the spot welding structure 1 according to this embodiment.

[0037] Furthermore, when manufacturing the spot welded structure 1 according to this embodiment, it is not necessary to grow the main nugget 15 up to the plate-like member 13. Therefore, it is not necessary to use an excessively large welding current in the spot welding (primary spot welding) for forming the main nugget 15. This makes it possible to suppress the occurrence of expulsion and blowholes, and further increase the joint strength of the main nugget 15.

[0038] Furthermore, in the spot welding structure 1 according to this embodiment, the secondary nugget 16 that joins the plate-shaped member 13 to another member is provided outside the lap portion 14. This makes it possible to suppress poor joining of the plate-shaped member 13.

[0039] The reason for this can be explained by the sheet thickness ratio. The sheet thickness ratio is a value calculated by dividing the total thickness of the sheet assembly to be spot welded by the thickness of the thinner of the metal sheets arranged on the surface of the sheet assembly. When the thicknesses of the metal sheets arranged on the surface of the sheet assembly are the same, the sheet thickness ratio is the value obtained by dividing the total thickness of the sheet assembly by that thickness. The sheet thickness ratio is used as an indicator of the ease of spot welding. The larger the sheet thickness ratio, the more difficult it is to grow a nugget to the metal sheet arranged on the surface of the sheet assembly. If a nugget does not grow to the metal sheet arranged on the surface of the sheet assembly, the metal sheet will not be joined to the adjacent metal sheet. Therefore, the larger the sheet thickness ratio, the more likely it is that poor joining will occur.

[0040] The total thickness of the metal plate outside the lap portion 14 is smaller than the total thickness of the metal plate at the lap portion 14. Therefore, the plate thickness ratio outside the lap portion 14 is smaller than the plate thickness ratio at the lap portion 14. By providing the secondary nugget 16 outside the lap portion 14, poor joining of the plate-like member 13 can be suppressed extremely effectively.

[0041] When the spot welding structure 1 according to this embodiment is applied to an inner frame structure of an automobile body, the plate-like member 13 becomes an outer plate member of the vehicle body, and the first lap joint 11 and the second lap joint 12 become frame members of the vehicle body. The spot welding structure 1 according to this embodiment can suppress poor joining of the outer plate members and increase the load transmission efficiency between the frame members.

[0042] The most basic aspect of the spot welding structure 1 according to this embodiment has been described above. A more preferred aspect will now be described.

[0043] (Spacing of secondary nuggets) It is preferable that the distance d between the center of the secondary nugget 16 joining the plate-like member 13 and the first lap joint 11 and the center of the secondary nugget 16 joining the plate-like member 13 and the second lap joint 12 is 200 mm or less. This further improves the load transmission efficiency of the spot welded structure 1. The distance d may be 190 mm or less, 180 mm or less, 160 mm or less, or 140 mm or less. The center-to-center distance of the secondary nuggets 16 can be determined by measuring the center-to-center distance of the indentations 161 of the secondary nuggets 16.

[0044] A plurality of secondary nuggets 16 may be provided in the first lap joint 11 and the second lap joint 12. In this case, the distance d between the center of the secondary nugget 16 joining the plate-like member 13 and the first lap joint 11 and the center of the secondary nugget 16 joining the plate-like member 13 and the second lap joint 12 means the minimum value of the center-to-center distance of the secondary nuggets 16.

[0045] (Side Joint Structure) The lap portion 14 is preferably a side joint. Generally, a side joint is understood to be a type of joint in which one member and / or part of a lap joint is stepped so that the base material surfaces are substantially flush (JIS Z 3001-1:2018). In the spot welding structure according to this embodiment, at least one of the metal plates 111 of the first lap joint 11 and the metal plates 121 of the second lap joint 12 preferably has a side joint in the lap portion 14, and two of the three metal plates joined by the main nugget 15 are preferably arranged on substantially the same plane outside the lap portion 14.

[0046] The seam structure will be described in detail below with reference to Figure 3. In the seam structure of Figure 3, a metal plate 2 having a seam processing portion 21 is stacked on a metal plate 3 without a seam processing portion. The seam processing portion 21 is formed, for example, by bending the metal plate 2 in a stepped shape. The seam processing portion 21 has a tip portion 211 and a connection portion 212. In the following description, the portion of the metal plate 2 having the seam processing portion 21, excluding the seam processing portion 21, is referred to as a main body portion 22.

[0047] The tip 211 of the seam processing portion 21 is approximately parallel to the main body portion 22 of the metal plate 2, but is not on the same plane as the main body portion 22 of the metal plate 2. The plane that includes the tip 211 is separated by a predetermined distance x from the plane that includes the main body portion 22. It is preferable that the predetermined distance x is approximately the same as the thickness y of the metal plate 3 that is overlapped with the seam processing portion 21. The tip 211 constitutes the lap portion 14. The tip 211 has a width that is at least large enough to allow spot welding.

[0048] The connection portion 212 of the edge processing portion 21 connects the tip portion 211 of the edge processing portion 21 and the main body portion 22 of the metal plate 2. The connection portion 212 forms a predetermined angle with respect to the tip portion 211 of the edge processing portion 21 and the main body portion 22 of the metal plate 2. As a result, the connection portion 212 separates the plane containing the tip portion 211 of the edge processing portion 21 by a predetermined distance x from the plane containing the main body portion 22 of the metal plate 2. The connection portion 212 may be flat or curved.

[0049] 3, the metal plate 3 is disposed in the space between a plane including the tip 211 of the edge processing portion 21 and a plane including the main body 22 of the metal plate 2. One surface of the metal plate 2 having the edge processing portion 21 is disposed on approximately the same plane as one surface of the metal plate 3 that is overlapped with the edge processing portion 21.

[0050] Next, a specific example of the welded joint structure in the spot welding structure according to this embodiment will be described with reference to FIG. 1 and FIGS. 4A to 4E.

[0051] 1, a seam 1211 is provided at the end of each of the two metal plates 121 of the second lap joint 12. The two metal plates 111 of the first lap joint 11 do not have a seam.

[0052] 1, the metal plate 111 on the upper side of the first lap joint 11 is disposed between the seam-processed portion 1211 of the metal plate 121 on the upper side of the second lap joint 12 and the plate-like member 13. The seam-processed portion 1211 of the metal plate 121 on the upper side of the second lap joint 12 is disposed between the seam-processed portion 1211 of the metal plate 121 on the lower side of the second lap joint 12 and the metal plate 111 on the upper side of the first lap joint 11. As a result, the metal plate 111 of the first lap joint 11 and the metal plate 121 of the second lap joint 12 are disposed on approximately the same plane.

[0053] 1 , (1) the metal plate 111 on the upper side of the first lap joint 11, (2) the edged portion 1211 of the metal plate 121 on the upper side of the second lap joint 12, and (3) the edged portion 1211 of the metal plate 121 on the lower side of the second lap joint 12 are overlapped in this order. A main nugget 15 joins the three metal plates of the lap portion 14. The metal plate 111 on the lower side of the first lap joint 11 is excluded from the lap portion 14.

[0054] 4A, a seam 1211 is provided only on the end of the metal plate 121 on the lower side of the paper of the second lap joint 12. The two metal plates 111 of the first lap joint 11 and the metal plate 121 on the upper side of the paper of the second lap joint 12 do not have a seam.

[0055] 4A, the two metal plates 111 of the first lap joint 11 are arranged between the seam processing portion 1211 of the metal plate 121 on the lower side of the paper surface of the second lap joint 12 and the plate-like member 13. As a result, the metal plates 111 of the first lap joint 11 and the metal plates 121 of the second lap joint 12 are arranged on approximately the same plane.

[0056] 4A , (1) the metal plate 111 on the upper side of the first lap joint 11, (2) the metal plate 111 on the lower side of the first lap joint 11, and (3) the seam 1211 of the metal plate 121 on the lower side of the second lap joint 12 are overlapped in this order. The main nugget 15 joins the three metal plates of the lap portion 14. The metal plate 121 on the upper side of the second lap joint 12 is excluded from the lap portion 14.

[0057] 4B , a seam 1111 is provided at the end of the metal plate 111 on the lower side of the first lap joint 11, and a seam 1211 is provided at the end of the metal plate 121 on the upper side of the second lap joint 12. The metal plate 111 on the upper side of the first lap joint 11 and the metal plate 121 on the lower side of the second lap joint 12 do not have a seam.

[0058] 4B , the metal plate 111 on the upper side of the first lap joint 11 is disposed between the seam-processed portion 1211 of the metal plate 121 on the upper side of the second lap joint 12 and the plate-like member 13. The seam-processed portion 1211 of the metal plate 121 on the upper side of the second lap joint 12 is disposed between the metal plate 111 on the upper side of the first lap joint 11 and the seam-processed portion 1111 of the metal plate 111 on the lower side of the first lap joint 11. As a result, the metal plate 111 of the first lap joint 11 and the metal plate 121 of the second lap joint 12 are disposed on approximately the same plane.

[0059] 4B , (1) the metal plate 111 on the upper side of the first lap joint 11, (2) the edged portion 1211 of the metal plate 121 on the upper side of the second lap joint 12, and (3) the edged portion 1111 of the metal plate 111 on the lower side of the first lap joint 11 are overlapped in this order. A main nugget 15 joins the three metal plates of the lap portion 14. The metal plate 121 on the lower side of the second lap joint 12 is excluded from the lap portion 14.

[0060] 4C, a seam 1211 is provided only on the end of the metal plate 121 on the upper side of the paper of the second lap joint 12. The two metal plates 111 of the first lap joint 11 and the metal plate 121 on the lower side of the paper of the second lap joint 12 do not have a seam.

[0061] 4C , the two metal plates 111 of the first lap joint 11 are arranged between the seam processing portion 1211 of the metal plate 121 on the upper side of the paper of the second lap joint 12 and the plate-like member 13. As a result, the metal plate 111 of the first lap joint 11 and the metal plate 121 of the second lap joint 12 are arranged on approximately the same plane.

[0062] 4C , (1) the metal plate 111 on the upper side of the first lap joint 11, (2) the metal plate 111 on the lower side of the first lap joint 11, and (3) the seam 1211 of the metal plate 121 on the upper side of the second lap joint 12 are overlapped in this order. The main nugget 15 joins the three metal plates of the lap portion 14. The metal plate 121 on the lower side of the second lap joint 12 is excluded from the lap portion 14.

[0063] The plate-shaped member 13 may have a seam-processed portion. In the spot-welded structure of Fig. 4D, a seam-processed portion 1211 is provided on the two metal plates 121 of the second lap joint 12. Furthermore, a seam-processed portion 131 is also provided on the plate-shaped member 13. The two metal plates 111 of the first lap joint 11 do not have a seam-processed portion.

[0064] 4D , the seam processing portion 1211 of the metal plate 121 on the lower side of the second lap joint 12 is disposed between the seam processing portion 1211 of the metal plate 121 on the upper side of the second lap joint 12 and the metal plate 111 on the lower side of the first lap joint 11. As a result, the metal plate 111 of the first lap joint 11 and the metal plate 121 of the second lap joint 12 are disposed on approximately the same plane.

[0065] In the lap portion 14 of the spot welding structure 1 in FIG. 4D , the seam 1211 of the metal plate 121 on the upper side of the second lap joint 12 forms a convex portion facing upward. However, this convex portion is accommodated in the seam 131 of the plate-shaped member 13. Therefore, no large gap is formed between the plate-shaped member 13 and the main body 1112 of the metal plate 111 of the first lap joint 11 and / or the main body 1212 of the metal plate 121 of the second lap joint 12. In the lap portion 14 of the spot welding structure 1, a gap may be formed between the plate-shaped member 13 and the first lap joint 11 and / or the second lap joint 12. By providing a gap between the plate-shaped member 13 and the first lap joint 11 and / or the second lap joint 12, the rigidity of the lap portion 14 of the spot welding structure 1 is improved.

[0066] 4D , (1) the edged portion 1211 of the metal plate 121 on the upper side of the second lap joint 12, (2) the edged portion 1211 of the metal plate 121 on the lower side of the second lap joint 12, and (3) the metal plate 111 on the lower side of the first lap joint 11 are overlapped in this order. A main nugget 15 joins the three metal plates of the lap portion 14. The metal plate 111 on the upper side of the first lap joint 11 is excluded from the lap portion 14.

[0067] A plurality of main nuggets 15 may be provided in the lap portion 14. In the spot welded structure 1 of Fig. 4E, a shoulder 1211 is provided on the two metal sheets 121 of the second lap joint 12. The two metal sheets 111 of the first lap joint 11 do not have a shoulder.

[0068] 4E, the metal plate 111 of the first lap joint 11 on the upper side of the drawing sheet is disposed between the seam processing portion 1211 of the two metal plates 121 of the second lap joint 12 and the plate-like member 13. As a result, the metal plate 111 of the first lap joint 11 and the metal plate 121 of the second lap joint 12 are disposed on approximately the same plane.

[0069] 4E , (1) the metal plate 111 on the upper side of the first lap joint 11, (2) the edged portion 1211 of the metal plate 121 on the upper side of the second lap joint 12, and (3) the edged portion 1211 of the metal plate 121 on the lower side of the second lap joint 12 are overlapped in this order. There are two main nuggets 15, and the two main nuggets 15 join the three metal plates of the lap portion 14. The metal plate 111 on the lower side of the first lap joint 11 is excluded from the lap portion 14.

[0070] In either configuration, the metal plate 111 of the first lap joint 11 and the metal plate 121 of the second lap joint 12 are arranged on approximately the same plane, thereby further improving the load transmission efficiency.

[0071] 1, 4A, 4C, 4D, and 4E, there is only one joint interface between the first lap joint 11 and the second lap joint 12. However, the number of joint interfaces between the first lap joint 11 and the second lap joint 12 may be two or more. By making the number of joint interfaces between the first lap joint 11 and the second lap joint 12 two or more, the rigidity of the spot welded structure 1 is further improved.

[0072] An example of a spot welding structure 1 having two or more joining interfaces is the spot welding structure 1 shown in Fig. 4B . In the spot welding structure 1 of Fig. 4B , one metal plate 121 of the second lap joint 12 is sandwiched between two metal plates 111 of the first lap joint 11 at the location where the main nugget 15 is formed.

[0073] Another example of a spot welded structure 1 having two or more bonding interfaces is a spot welded structure 1 in which three different metal plates are joined by multiple main nuggets 15, as shown in Figures 5A to 5D. Specifically, in the spot welded structure 1 shown in Figures 5A to 5D, it is preferable that some of the main nuggets 15 join two metal plates 111 of the first lap joint 11 to one metal plate 121 of the second lap joint 12, and the other main nuggets 15 join one metal plate 111 of the first lap joint 11 to two metal plates 121 of the second lap joint 12. This further improves the load transfer efficiency of the spot welded structure 1. Furthermore, the number of bonding interfaces between the first lap joint 11 and the second lap joint 12 is two or more, further improving the rigidity of the spot welded structure 1.

[0074] 5A , the lap portion 14 is divided into two. In the lap portion 14 on the first lap joint 11 side, the metal plate 111 on the upper side of the first lap joint 11 in the drawing, the metal plate 111 on the lower side of the first lap joint 11 in the drawing, and the edged portion 1211 of the metal plate 121 on the lower side of the second lap joint 12 in the drawing are overlapped in this order. In the lap portion 14 on the second lap joint 12 side, the metal plate 111 on the upper side of the first lap joint 11 in the drawing, the edged portion 1211 of the metal plate 121 on the upper side of the second lap joint 12 in the drawing, and the edged portion 1211 of the metal plate 121 on the lower side of the second lap joint 12 in the drawing are overlapped in this order. In each of the two lap portions 14, a main nugget 15 is formed that joins the three metal plates.

[0075] 5B , the lap portion 14 is also divided into two. In the lap portion 14 on the first lap joint 11 side, the metal plate 111 on the upper side of the first lap joint 11 in the drawing, the edged portion 1211 of the metal plate 121 on the upper side of the second lap joint 12 in the drawing, and the edged portion 1111 of the metal plate 111 on the lower side of the first lap joint 11 in the drawing are overlapped in this order. In the lap portion 14 on the second lap joint 12 side, the main body 1212 of the metal plate 121 on the upper side of the second lap joint 12 in the drawing, the metal plate 121 on the lower side of the second lap joint 12 in the drawing, and the edged portion 1111 of the metal plate 111 on the lower side of the first lap joint 11 in the drawing are overlapped in this order. In each of the two lap portions 14, a main nugget 15 is formed that joins the three metal plates.

[0076] 5C , the lap portion 14 is also divided into two. In the lap portion 14 on the first lap joint 11 side, the edged portion 1211 of the metal plate 121 on the upper side of the paper of the second lap joint 12, the metal plate 111 on the upper side of the paper of the first lap joint 11, and the metal plate 111 on the lower side of the paper of the first lap joint 11 are overlapped in this order. In the lap portion 14 on the second lap joint 12 side, the edged portion 1211 of the metal plate 121 on the upper side of the paper of the second lap joint 12, the edged portion 1211 of the metal plate 121 on the lower side of the paper of the second lap joint 12, and the metal plate 111 on the lower side of the paper of the first lap joint 11 are overlapped in this order. In each of the two lap portions 14, a main nugget 15 is formed that joins the three metal plates. Furthermore, the seam processing portion 1211 of the metal plate 121 on the upper side of the drawing of the second lap joint 12 forms a convex portion in the lap portion 14. The convex portion is housed inside the seam processing portion 131 provided in the plate-like member 13.

[0077] 5D , the lap portion 14 is also divided into two. In the lap portion 14 on the first lap joint 11 side, the metal plate 111 on the upper side of the first lap joint 11 in the drawing, the edged portion 1211 of the metal plate 121 on the upper side of the second lap joint 12 in the drawing, and the edged portion 1111 of the metal plate 111 on the lower side of the first lap joint 11 in the drawing are overlapped in this order. In the lap portion 14 on the second lap joint 12 side, the metal plate 111 on the upper side of the first lap joint 11 in the drawing, the edged portion 1211 of the metal plate 121 on the upper side of the second lap joint 12 in the drawing, and the edged portion 1211 of the metal plate 121 on the lower side of the second lap joint 12 in the drawing are overlapped in this order. In each of the two lap portions 14, a main nugget 15 is formed that joins the three metal plates.

[0078] In either configuration, the lap portion 14 is divided into two, and a main nugget 15 that joins three metal plates is formed in each of the two lap portions 14, thereby forming two joint structures. This further improves the load transmission efficiency. Also, two or more main nuggets 15 may be formed in each lap portion 14. This further improves the load transmission efficiency.

[0079] (Adhesive 17) It is preferable that the spot welded structure 1 further includes an adhesive 17. For example, as shown in FIG. 6 , the adhesive 17 is disposed between the plate-shaped member 13 and one or both of the first lap joint 11 and the second lap joint 12. This further increases the joint strength between the plate-shaped member 13 and other members. The adhesive 17 may also be disposed between the metal plate 111 of the first lap joint 11 and the metal plate 121 of the second lap joint 12. This further increases the joint strength of the two lap joints and improves load transmission efficiency. The adhesive 17 may be disposed between the multiple metal plates 111 included in the first lap joint 11, or between the multiple metal plates 121 included in the second lap joint 12. This further increases the joint strength of the first lap joint 11 and / or the second lap joint 12. The type of adhesive 17 is not particularly limited. Examples of the adhesive 17 include epoxy-based, rubber-based, silicone-based, and mixtures thereof. The adhesive may be of the thermosetting or reactive curing type, but when applied to automobile bodies, the thermosetting type that cures during the heat treatment process (140°C to 190°C) of electrodeposition coating is preferred.

[0080] (Additional nugget 18) As shown in Fig. 7 , the spot welded structure 1 may further include an additional nugget 18 in the lap portion 14. The additional nugget 18 joins two or more of the metal plates and the plate-like members 13 included in the lap portion 14. The additional nugget 18 is similar to the main nugget 15 in that it is provided in the lap portion 14. However, unlike the main nugget 15, one of the two indentations of the additional nugget 18 is provided in the plate-like member 13. The additional nugget 18 is obtained by spot welding to the lap portion 14 with another plate-like member 13 superimposed on the lap portion 14.

[0081] When an additional plate-shaped member 13 is placed on the lap portion 14, the lap portion 14 includes four or more metal plates. Spot welding of such a plate assembly typically tends to cause expulsion. However, the gaps in the lap portion 14 are removed when the main nugget 15 and the sub-nugget 16 are formed. Therefore, expulsion caused by the gaps is suppressed when the additional nugget 18 is formed.

[0082] The additional nugget 18 only needs to join at least two of the metal plates included in the lap portion 14 and the plate-like member 13. The additional nugget 18 does not have to grow to the plate-like member 13. That is, the additional nugget 18 does not have to join the plate-like member 13 to another member. Alternatively, the additional nugget 18 may press the plate-like member 13 to another member. Even if the additional nugget 18 is located outside the plate-like member 13, the additional nugget 18 can still improve the joining strength and load transmission efficiency of the two lap joints. The additional nugget 18 may be provided spaced apart from the main nugget 15. On the other hand, the additional nugget 18 and the main nugget 15 may be partially fused together. Figure 15 shows a cross-sectional example of a case where the additional nugget 18 and the main nugget 15 are partially fused together.

[0083] On the other hand, the spot welded structure 1 does not have to have the additional nugget 18 in the lap portion 14. That is, the plate-like member may be separated from the first lap joint and the second lap joint over the entire lap portion. By omitting the additional nugget 18, the manufacturing cost of the spot welded structure 1 can be reduced, and the time required to manufacture the spot welded structure 1 can be shortened.

[0084] The first lap joint may have a joint outside the lap portion that joins only the metal plates of the first lap joint. The second lap joint may have a joint outside the lap portion that joins only the metal plates of the second lap joint. The joints are, for example, the nugget 112 of the first lap joint 11 and the nugget 122 of the second lap joint 12 shown in FIG. 1 . The nugget 112 of the first lap joint 11 joins the metal plates 111 of the first lap joint 11, but does not join the first lap joint 11 to another member. The nugget 122 of the second lap joint 12 joins the metal plates 121 of the second lap joint 12, but does not join the second lap joint 12 to another member. The metal plates included in the lap joint may be joined by arc weld metal, seam weld metal, laser weld metal, or a combination thereof.

[0085] (2A. First Manufacturing Method of Spot Welded Structure 1) As shown in Figures 8A to 8E, a first manufacturing method of a spot welded structure 1 according to another aspect of the present invention includes: (S1) preparing a first lap joint 11 having two or more metal plates and a second lap joint 12 having two or more metal plates; (S2) overlapping end portions of one or more of the metal plates 111 of the first lap joint and end portions of one or more of the metal plates 121 of the second lap joint to form a lap portion 14; (S3) primarily spot welding the first lap joint 11 and the second lap joint 12 at the lap portion 14 to form a main nugget 15; and (S4) overlapping a metal plate-like member 13 on the lap portion 14 on which the first lap joint 11, the second lap joint 12, and the main nugget 15 have been formed. (S5) A step of secondary spot welding the plate-shaped member 13 and the first lap joint 11, and the plate-shaped member 13 and the second lap joint 12 to form a plurality of secondary nuggets 16, wherein the number of metal plates to be primarily spot welded is three, and the welding points in the secondary spot welding are outside the lap portion 14.

[0086] A first manufacturing method for a spot welded structure 1 according to another aspect of the present invention will be described in detail below. Note that the preferred aspects of the spot welded structure 1 described above can also be applied to the first manufacturing method for the spot welded structure 1 and the second manufacturing method described below. For example, the configuration of the lap portion 14 described with reference to Figures 4A to 4E can also be appropriately adopted in the first and second manufacturing methods for the spot welded structure 1.

[0087] (S1) First, a first lap joint 11 having two or more metal plates and a second lap joint 12 having two or more metal plates are prepared. The process of preparing the lap joints may be a process of manufacturing the first lap joint 11 having two or more metal plates and the second lap joint 12 having two or more metal plates. The process of manufacturing the lap joints may be, for example, a spot welding process shown in FIG. 8A. The process of manufacturing the lap joints may be, for example, a laser welding process, a seam welding process, a lap arc welding process, or the like. On the other hand, lap joints manufactured by others may be used as the first lap joint 11 and the second lap joint 12. Examples of the first lap joint and the second lap joint are automotive frame components, and examples of the plate-like member 13 are automotive exterior panel components.

[0088] When preparing the first lap joint 11 and the second lap joint 12, it is preferable to shape the ends of these. For example, it is preferable to make only one of the two or more metal plates in the lap joint protrude. By machining the end of the lap joint or positioning the end of the metal plate before manufacturing the lap joint, it is possible to make only one of the two or more metal plates in the lap joint protrude. It is also preferable to provide a seam processing portion in at least one of the two or more metal plates. The seam processing portion can be formed by machining the end of the metal plate before or after manufacturing the lap joint.

[0089] In the first lap joint 11 illustrated in Fig. 8A, only one of the two or more metal plates 111 protrudes. In the second lap joint 12 illustrated in Fig. 8A, only one of the two or more metal plates 121 protrudes, and a seam is provided on both of the two metal plates 121. In addition, various embodiments such as those illustrated in Fig. 1, 4A to 4E, and 5A to 5D can be applied to the ends of the first lap joint 11 and the second lap joint 12.

[0090] (S2) Next, the end portions of one or more of the metal plates 111 of the first lap joint and the end portions of one or more of the metal plates 121 of the second lap joint are overlapped to form the lap portion 14. As shown in Fig. 8B, an area where three metal plates are overlapped is formed in the lap portion 14. The number of metal plates in the lap portion 14 can be adjusted by appropriately adjusting the shape of the end portions of the lap joint.

[0091] 4B , the two lap joints are aligned so that the ends of the metal plates are inserted into the spaces formed by the seams. This alignment is also considered to be the overlapping of the ends of one or more of the metal plates 111 of the first lap joint and the ends of one or more of the metal plates 121 of the second lap joint. In other words, all of the actions for forming the lap portion 14 are considered to be the actions of overlapping the ends of the metal plates.

[0092] (S3) After the ends of the first lap joint 11 and the second lap joint 12 are overlapped, the first lap joint 11 and the second lap joint 12 are primarily spot welded together at the lap portion 14 to form a main nugget 15. At this time, a main nugget impression 151 is formed in the metal plate disposed on the outermost surface of the lap portion 14 by the primary spot welding.

[0093] 8C , when forming main nugget 15, the number of metal sheets to be primarily spot-welded is three. Therefore, it is preferable to use the region of lap portion 14 where three metal sheets are overlapped as the welding point for the primary spot welding. The number of times primary spot welding may be one, two or more.

[0094] (S4) Following the primary spot welding, a metal plate-like member 13 is placed on the lap portion 14 where the first lap joint 11, the second lap joint 12, and the main nugget 15 are formed. One of the pair of indentations formed by the primary spot welding is covered by the plate-like member 13.

[0095] (S5) After the plate-shaped member 13 and the other members are overlapped, the plate-shaped member 13 and the first lap joint 11, and the plate-shaped member 13 and the second lap joint 12 are secondarily spot welded to form multiple minor nuggets 16. The welding points of the secondary spot welds are outside the lap portion 14. The secondary spot welding is performed at least once on the plate-shaped member 13 and the first lap joint 11, and at least once on the plate-shaped member 13 and the second lap joint 12. Therefore, the secondary spot welding is performed at least two times. The total number of secondary spot weldings may be three or more.

[0096] (Operation and Effect) In the first manufacturing method of the spot welded structure 1 according to this embodiment, three metal plates are overlapped at the joint of the two lap joints. Therefore, the first manufacturing method according to this embodiment can firmly join the two lap joints. Furthermore, the spot welded structure 1 obtained by the first manufacturing method according to this embodiment has excellent load transmission efficiency.

[0097] Furthermore, by overlapping three metal sheets at the joints of the two lap joints, the occurrence of expulsion is suppressed. Generally, in spot welding of sheet pairs with a large total thickness, a high welding current is required to grow the nugget. However, a high welding current is likely to cause expulsion. In particular, in cases where there is a gap between the metal sheets, expulsion caused by a high current is likely to form large blowholes in the nugget. In the first manufacturing method according to this embodiment, by limiting the number of metal sheets to be primarily spot-welded to three, the total thickness of the sheet pair to be primarily spot-welded can be reduced, thereby suppressing the occurrence of expulsion.

[0098] Additionally, in the first manufacturing method according to this embodiment, the welding point in the secondary spot welding for joining the plate-shaped member 13 to the other member is located outside the lap portion 14. Therefore, in the first manufacturing method according to this embodiment, the total thickness of the plate assembly in the secondary spot welding is also reduced. This also suppresses the occurrence of expulsion in the secondary spot welding. In addition, poor joining between the plate-shaped member 13 and the other member is avoided.

[0099] (2B. Second Manufacturing Method of Spot Welded Structure 1) As shown in Figures 9A to 9F, a second manufacturing method of a spot welded structure 1 according to another aspect of the present invention includes the following steps: (s1) preparing a plate assembly by overlapping two of the metal plates that are components of a first lap joint at their ends; (s2) overlapping an end of one of the metal plates that are components of a second lap joint on the end of the plate assembly to form a lap portion; (s3) primarily spot welding two of the metal plates that are components of the first lap joint and one of the metal plates that are components of the second lap joint at the lap portion to form a main nugget and a first lap joint; (s4) overlapping and spot welding one metal plate of the second lap joint on which the main nugget is provided and the remaining metal plate of the second lap joint outside the lap portion to form a second lap joint; and (s5) overlapping a metal plate-like member on the lap portion on which the first lap joint, the second lap joint, and the main nugget are formed. (s6) forming a plurality of sub-nuggets by secondarily spot welding the plate-shaped member and the first lap joint, and the plate-shaped member and the second lap joint, wherein the number of metal plates to be primarily spot-welded is three, and the welding points in the secondary spot welding are outside the lap portion.

[0100] A second manufacturing method for a spot welded structure 1 according to another embodiment of the present invention will be described in detail below. The second manufacturing method differs from the first manufacturing method in that the first lap joint and the second lap joint are not formed before the primary spot welding. However, in other respects, the second manufacturing method is very similar to the first manufacturing method. In particular, the first manufacturing method and the second manufacturing method are identical in terms of the process of stacking the plate-like members and subsequent processes. Therefore, in the following description of the second manufacturing method, details that overlap with those of the first manufacturing method may be omitted. Suitable aspects of the first manufacturing method can be applied to the second manufacturing method.

[0101] (s1) First, as shown in Fig. 9A, a plate set is prepared by overlapping two metal plates at their ends. These two metal plates are components of a first lap joint. However, at this point, the metal plates of the first lap joint are not joined.

[0102] (s2) Next, as shown in Figure 9B, the end of another metal plate is overlapped on the end of the plate set to form a lap portion. The other metal plate is a part of a second lap joint. However, at this point, the metal plates of the second lap joint are also not joined.

[0103] (s3) Then, as shown in Figure 9C, two of the metal plates that are components of the first lap joint and one of the metal plates that are components of the second lap joint are primarily spot welded at the lap portions. This brings the number of metal plates that are primarily spot welded to three. A main nugget is formed by the primary spot welding. Furthermore, a first lap joint is formed by the primary spot welding, which has two metal plates and a main nugget that joins the two metal plates.

[0104] (s4) Furthermore, as shown in Fig. 9D, one metal plate of the second lap joint on which the main nugget is provided is overlapped with the remaining metal plates of the second lap joint outside the lap portion, and the overlapping portion is spot welded to form the second lap joint.

[0105] 9D, the second lap joint has only two metal plates. However, a second lap joint having three or more metal plates can be manufactured by spot welding two or more metal plates to the one metal plate of the second lap joint in which the main nugget is provided.

[0106] In addition, in order to increase the joining strength of the first lap joint, additional spot welding may be performed on the first lap joint as shown in Fig. 9D. When additional spot welding is performed, a metal plate may be added, so that the number of metal plates in the first lap joint can be three or more.

[0107] (s5, s6) Then, as shown in Fig. 9E, a metal plate-shaped member is placed on the lap portion where the first lap joint, the second lap joint, and the main nugget have been formed. Furthermore, as shown in Fig. 9F, the plate-shaped member and the first lap joint, and the plate-shaped member and the second lap joint are secondarily spot welded to form multiple sub-nuggets. The welding points for the secondary spot welding are outside the lap portion. In the second manufacturing method, the process of welding the plate-shaped member to another member by placing it on top of another member is substantially the same as in the first manufacturing method.

[0108] (Operation and Effect) As with the first manufacturing method, the second manufacturing method of the spot welded structure 1 according to this embodiment also uses three overlapping metal plates at the joint between the two lap joints. Also, as with the first manufacturing method, the second manufacturing method according to this embodiment also uses a welding point outside the lap portion 14 for the secondary spot welding to join the plate-shaped member 13 to another member. Therefore, the second manufacturing method according to this embodiment provides a spot welded structure with excellent load transmission efficiency. Furthermore, the second manufacturing method can suppress the occurrence of expulsion. Additionally, the second manufacturing method avoids poor joining between the plate-shaped member 13 and another member.

[0109] The most basic aspects of the first and second manufacturing methods for the spot welded structure 1 according to this embodiment have been described above. More preferred aspects will now be described. Unless otherwise specified, the preferred aspects described below are applicable to both the first and second manufacturing methods.

[0110] (Spacing of Welding Points) It is preferable that the distance between the center of the welding point in the secondary spot welding between the plate-like member 13 and the first lap joint 11 and the center of the welding point in the secondary spot welding between the plate-like member 13 and the second lap joint 12 be 200 mm or less. This allows the distance d between the center of the secondary nugget 16 joining the plate-like member 13 and the first lap joint 11 and the center of the secondary nugget 16 joining the plate-like member 13 and the second lap joint 12 to be 200 mm or less. This further improves the load transmission efficiency of the spot welding structure 1. The distance between the centers of the welding points may be 190 mm or less, 180 mm or less, 160 mm or less, or 140 mm or less.

[0111] It should be noted that there are cases where secondary spot welding is performed two or more times on the first lap joint 11 and the second lap joint 12. In this case, the distance between the center of the welding point in the secondary spot welding of the plate-like member 13 and the first lap joint 11 and the center of the welding point in the secondary spot welding of the plate-like member 13 and the second lap joint 12 means the minimum value of the center-to-center distance of the welding points.

[0112] (Seaming Structure) Seaming may be provided on the ends of one or more of the metal plates 111 of the first lap joint 11 and the metal plates 121 of the second lap joint 12, and when forming the lap portion 14, two of the three metal plates joined by the main nugget 15 may be arranged on approximately the same plane outside the lap portion 14. By arranging the metal plates 111 of the first lap joint 11 and the metal plates 121 of the second lap joint 12 on approximately the same plane, the load transmission efficiency is further improved. Specific examples of the seaming structure are the same as the preferred aspects of the spot welding structure 1 according to this embodiment, as illustrated in Figures 1 and 4A to 4E.

[0113] (Multiple Primary Spot Welding) In the first manufacturing method, primary spot welding may be performed on the lap portion 14 multiple times. In this case, as shown in FIG. 4E , the sheet combinations may be the same in the multiple primary spot weldings. On the other hand, as shown in FIG. 5A and the like, the sheet combinations may be different in the multiple primary spot weldings. Specifically, when forming the lap portion 14, a lap portion 14 formed by overlapping two metal sheets 111 of the first lap joint 11 with one metal sheet 121 of the second lap joint 12, and a lap portion 14 formed by overlapping one metal sheet 111 of the first lap joint 11 with two metal sheets 121 of the second lap joint 12 may be formed, and primary spot welding may be performed on each of these multiple lap portions 14.

[0114] In the second manufacturing method, primary spot welding may also be performed multiple times on the lap portion 14. In this case, as shown in Fig. 4E, the sheet combinations may be the same in the multiple primary spot weldings. On the other hand, as shown in Fig. 5A etc., the sheet combinations may be different in the multiple primary spot weldings.

[0115] In the second manufacturing method, in order to perform multiple primary spot welding operations on different sheet pairs, the primary spot welding must be performed in two stages. This is because only one of the metal sheets of the second lap joint is included in the lap portion during the first primary spot welding. Therefore, during the first primary spot welding, a main nugget is formed that joins the two metal sheets 111 of the first lap joint 11 and the one metal sheet 121 of the second lap joint 12. Next, one metal sheet 121 of the second lap joint 12 is overlapped on the other metal sheet 121 of the second lap joint 12 to form an additional lap portion in which the one metal sheet 111 of the first lap joint 11 and the two metal sheets 121 of the second lap joint 12 are overlapped. Then, primary spot welding is performed on the additional lap portion to form a main nugget that joins the one metal sheet 111 of the first lap joint 11 and the two metal sheets 121 of the second lap joint 12.

[0116] Either manufacturing method can be used to manufacture a spot welded structure 1 having a main nugget 15 that joins the two metal plates 111 of the first lap joint 11 to the one metal plate 121 of the second lap joint 12, and a main nugget 15 that joins the one metal plate 111 of the first lap joint 11 to the two metal plates 121 of the second lap joint 12. This further improves the load transmission efficiency of the spot welded structure 1. The lap portion 14 when performing multiple primary spot welding operations can have the configurations shown in Figures 5A to 5D, for example.

[0117] (Application of Adhesive 17) The secondary spot welding may be a weld bond, i.e., a method in which adhesive 17 is applied to the joint interface before spot welding. Specifically, the manufacturing method of the spot welded structure 1 may further include a step of applying adhesive 17 between the plate-shaped member 13 and one or both of the first lap joint 11 and the second lap joint 12 after the primary spot welding and before overlapping the plate-shaped member 13 onto the lap portion 14 in which the first lap joint 11, the second lap joint 12, and the main nugget 15 are formed. This makes it possible to manufacture a spot welded structure 1 in which adhesive 17 is applied between the plate-shaped member 13 and one or both of the first lap joint 11 and the second lap joint 12, as illustrated in FIG. 6 . This further increases the joint strength between the plate-shaped member 13 and another member.

[0118] (Additional Spot Welding) The manufacturing method of the spot welded structure 1 may further include a step of performing additional spot welding on the lap portion 14 and the plate-shaped member 13 after the secondary spot welding. This makes it possible to manufacture a spot welded structure 1 that further has an additional nugget 18 in the lap portion 14, as shown in FIG.

[0119] When an additional plate-shaped member 13 is placed on the lap portion 14, the lap portion 14 includes four or more metal plates. The metal plates referred to here include the metal plates that make up the plate-shaped member 13. Spot welding of such a plate assembly typically tends to result in expulsion. However, the gaps in the lap portion 14 are removed when the main nugget 15 and the sub-nugget 16 are formed. Therefore, expulsion caused by the gaps is suppressed during additional spot welding.

[0120] The additional spot welding only needs to join at least two of the metal plates included in the lap portion 14 and the plate-like member 13. The additional spot welding does not need to grow the additional nugget 18 to the plate-like member 13. That is, the additional nugget 18 does not need to join the plate-like member 13 to another member. Alternatively, the additional nugget 18 may press the plate-like member 13 to another member. Even if the additional nugget 18 is located outside the plate-like member 13, the additional nugget 18 can still improve the joining strength and load transmission efficiency of the two lap joints. The welding point of the additional spot welding may be separated from or overlap with the welding point of the primary spot welding.

[0121] On the other hand, the manufacturing method of the spot-welded structure 1 does not have to include the step of performing additional spot welding. That is, all spot welding of the plate-like member may be performed outside the lap portion. By omitting the additional spot welding, the manufacturing cost of the spot-welded structure 1 can be reduced and the time required to manufacture the spot-welded structure 1 can be shortened.

[0122] Although the embodiment of the present invention has been described above, the present invention is not limited thereto and can be modified as appropriate without departing from the technical concept of the invention. A more preferred example of the spot welding structure 1 according to this embodiment and its manufacturing method will be described below. The preferred example described below is applicable to both the spot welding structure 1 and its manufacturing method.

[0123] (Thickness Ratio) The thickness ratio of the lap portion 14 is the total thickness of the three metal plates and the plate-shaped member 13 at the location where the main nugget 15 is provided, divided by the thickness of the plate-shaped member 13. The thickness ratio of the lap portion 14 is preferably within a range of 6.0 to 11.0. The larger the thickness ratio of the lap portion 14, the thinner the plate-shaped member 13 can be, thereby reducing the weight of the spot-welded structure 1. Generally, the larger the thickness ratio, the more difficult it is to join the plate-shaped member 13. However, in the spot-welded structure 1 and its manufacturing method according to this embodiment, the plate-shaped member 13 is firmly joined outside the lap portion 14. Therefore, it is not essential that the plate-shaped member 13 be welded at the lap portion 14. Even if the thickness ratio of the lap portion 14 is large, poor joining of the plate-shaped member 13 does not pose a problem.

[0124] The thickness ratio of the secondary nugget forming portion is the total thickness of the plurality of metal plates and the plate-like member 13 at the location where the secondary nugget 16 is provided, divided by the thickness of the plate-like member 13. The thickness ratio of the secondary nugget forming portion is preferably within the range of 3.0 to 6.0, and more preferably has an upper limit of 5.5. The smaller the thickness ratio of the secondary nugget forming portion, the more likely it is that poor joining of the plate-like member 13 will be avoided.

[0125] The plate thickness ratio is determined by measuring the thickness of the metal plate and the plate-shaped member 13 in a cross section that passes through the center of the indentation and is perpendicular to the surface of the plate-shaped member 13. If the thickness of the metal plate and the plate-shaped member 13 is reduced by the indentation, the thickness measured at a location that is not affected by the indentation is used to calculate the plate thickness ratio.

[0126] (Types of metal plates and plate-shaped member 13) The types of metal plates of the lap joint are not particularly limited. The types of metal plates constituting the plate-shaped member 13 are also not particularly limited. In the spot welded structure 1 and its manufacturing method according to this embodiment, any material that can be spot welded can be used as the metal plates of the lap joint and the metal plates constituting the plate-shaped member 13. A suitable material can be adopted depending on the application of the spot welded structure 1. Examples of metal plates include steel plates, aluminum plates, and stainless steel plates.

[0127] A suitable example of the metal plate is a steel plate. In particular, it is preferable that one or more of the metal plates be a high-strength steel plate. For example, when the spot welded structure 1 is a car body, by using a high-strength steel plate for one or more of the metal plates, the thickness of the metal plate can be reduced while ensuring the strength of the spot welded structure 1. By reducing the thickness of the metal plate, the weight of the spot welded structure 1 can be reduced.

[0128] When the metal plate is a steel plate, it is preferable that the plate-shaped member 13 be mild steel and that one or more of the metal plates included in the lap joint be a high-strength steel plate. For example, it is preferable that the Vickers hardness of the plate-shaped member 13 be 150 HV or less, 140 HV or less, 130 HV or less, or 120 HV or less. It is preferable that the tensile strength of the plate-shaped member 13 be 440 MPa or less, 400 MPa or less, or 370 MPa or less. It is preferable that the hardness of one or more of the metal plates included in the lap joint be 300 HV or more, 380 HV or more, or 440 HV or more. It is preferable that the tensile strength of one or more of the metal plates included in the lap joint be 980 MPa or more, 1200 MPa or more, or 1350 MPa or more. The plate-shaped member 13 can be arranged on the outermost surface of the spot-welded structure 1, and is therefore suitable, for example, for use as an exterior component of an automobile. By using thin mild steel for the plate-shaped member 13, the formability of the plate-shaped member 13 can be improved, and the aesthetic appearance of the automobile can be improved. The metal plates included in the lap joint are suitable for, for example, automobile frame parts. By using high-strength steel for one or more of the metal plates included in the lap joint, the collision safety of the automobile can be improved.

[0129] (Surface Treatment of Metal Sheet and Plate-Like Member 13) One or more of the plate-like member 13 and the metal sheet may have a surface treatment layer. Examples of the surface treatment layer include a chemical conversion coating layer and a plating layer. The chemical conversion coating layer and the plating layer may have a composition suitable for the composition of the underlying metal sheet. For example, when the metal sheet or plate-like member 13 of the lap joint is a steel sheet, the plating layer is preferably a zinc-based plating layer and an aluminum-based plating layer. A zinc-based plating layer is a plating layer having a Zn content of 30% by mass or more. Examples of zinc-based plating layers include a hot-dip galvanized layer, a galvannealed hot-dip galvanized layer, an electrogalvanized layer, a zinc-aluminum-plated layer, a zinc-aluminum-magnesium plating layer, and a hot-stamped hot-dip galvanized layer. An aluminum-based plating layer is a plating layer having an Al content of 20% by mass or more. An example of a steel sheet having an aluminum-based plating layer is a hot-stamped aluminum-silicon-plated steel sheet. A layer of Al oxide, Ti oxide or zinc oxide may be provided on the surface of the aluminum plated after hot stamping.

[0130] (Thickness of Metal Plates) There are no particular limitations on the thickness of the metal plates of the lap joint and the thickness of the plate-shaped member 13. For example, the thickness of the plate-shaped member 13 is preferably 0.5 to 1.1 mm. The thickness of the metal plates of the lap joint is preferably 0.9 to 2.6 mm.

[0131] (Nugget Diameter) The diameter of the main nugget 15 is not particularly limited, but is preferably, for example, 3√t or more, 4√t or more, or 5√t or more. This increases the joining strength of the main nugget 15. On the other hand, from the viewpoint of suppressing the heat input and further preventing the generation of spatter, it is preferable that the diameter of the main nugget 15 be 9√t or less, 8√t or less, or 7√t or less. The numerical range of the diameter of the main nugget 15 can also be applied to the sub nugget 16, the additional nugget 18, the nugget 112 of the first lap joint, and the nugget 122 of the second lap joint.

[0132] The nugget diameter refers to the diameter of the nugget measured along the joint interface in a cross-sectional test of the weld. t refers to the plate thickness of the thinner of the two steel plates constituting the joint interface at which the nugget diameter was measured. When the number of steel plates joined by the nugget is three or more and there are two or more joint interfaces, it is preferable that the nugget diameter be within the above-mentioned range at each joint interface, and it is even more preferable that the nugget diameter be within the above-mentioned range at all joint interfaces.

[0133] (Applications) The application of the spot welding structure 1 according to this embodiment is not particularly limited. A suitable application is a joint in an automobile body to which an inner frame structure is applied. FIG. 10 shows a joint in an automobile body 4. For example, by applying the spot welding structure 1 according to this embodiment to flanges at joints in the framework of the automobile body, such as a joint 41 between an A-pillar lower and a side sill, a joint 42 between a B-pillar and a side sill, a joint 43 between a B-pillar and a roof rail, a joint 44 between an A-pillar lower and an A-pillar upper, a joint 45 between an A-pillar upper and a roof rail, a joint between a C-pillar and a roof rail, and a joint 46 between a C-pillar and a side sill, an automobile body with high load transmission efficiency can be realized.

[0134] Cross-sectional examples of spot welding according to the present disclosure are shown in Figures 11 to 15. In the spot-welded joint 1 shown in Figure 11, the first and second plates from the bottom at the left edge of the page are two metal plates that make up the first lap joint. The first and second plates from the bottom at the right edge of the page are two metal plates that make up the second lap joint. The location where the second nugget from the right is provided is a lap portion where the end of one of the metal plates in the first lap joint and the end of two or more of the metal plates in the second lap joint overlap. The second nugget from the right is a main nugget that joins the first lap joint and the second lap joint at the lap portion. The top plate on the page is a plate-like member that overlaps the first lap joint, the second lap joint, and the lap portion. The first nugget from the right on the page is a secondary nugget that joins the plate-like member and the first lap joint, and the first nugget from the left is a secondary nugget that joins the plate-like member and the second lap joint. The number of metal plates joined by the main nugget is three. The main nugget indentation is only provided on the metal plates included in the first lap joint or the second lap joint. The plate-like member is located outside the main nugget. The secondary nugget is located outside the lap portion. Note that the second nugget from the left on the page in Figure 11 does not correspond to either the main nugget or the secondary nugget of the spot welding structure according to this embodiment. The location where the second nugget from the left on the page is provided corresponds to the lap portion where the end of one of the metal plates of the first lap joint and the end of one of the metal plates of the second lap joint overlap. Since this nugget is provided in the lap portion, it does not correspond to a secondary nugget. On the other hand, since the plate-like member is joined to this nugget, this nugget does not correspond to a main nugget. However, adding such an additional nugget to the spot welded structure according to this embodiment is naturally permitted. Furthermore, as described above, the spot welded joint of Figure 11 has all the characteristic features of the spot welded joint according to this embodiment. Therefore, even if the spot welded joint of Figure 11 has an additional nugget, it is still considered to be a spot welded joint according to this embodiment. If the second nugget from the left on the page is ignored, the spot welded joint 1 shown in Figure 11 has a configuration similar to that of Figure 4A flipped left and right.The spot welded joint 1 shown in Fig. 12 has the same configuration as that shown in Fig. 1. The spot welded joint 1 shown in Fig. 13 has the same configuration as that shown in Fig. 5A flipped left and right. The spot welded joint 1 shown in Fig. 14 has the same configuration as that shown in Fig. 5B. The spot welded joint 1 shown in Fig. 15 has the same configuration as that shown in Fig. 7 flipped left and right.

[0135] (Example) The effects of one embodiment of the present disclosure will be explained more specifically through experiments using simulations. However, the following conditions are merely one example of conditions adopted to confirm the effects of the present disclosure. The present disclosure is not limited to this one example of conditions. Various conditions may be adopted in the present disclosure as long as they do not deviate from the gist of the disclosure and achieve the purpose.

[0136] The following three simulation models, No. 1 to No. 3, were created, and a simulation assuming three-point bending was performed on each of the created models. Models No. 1 to No. 3 are models assuming structural components in which hat-shaped components (metal plates) made of steel plates with thicknesses of 1.2 mm and 1.6 mm and tensile strengths of 1500 MPa are combined and the flange portions are spot-welded. The outer plate member (plate-shaped member 13) is made of steel plate with a thickness of 0.65 mm and tensile strength of 270 MPa.

[0137] The spot welding structure of Comparative Example No. 1 is shown in Figures 16A to 16C. The spot welding structure of Comparative Example No. 1 simulates a conventional spot welding structure. Note that the outer plate member (plate-like member 13) is omitted from Figure 16A. As shown in Figure 16C, the spot welding structure of Comparative Example No. 1 targets only one of the two steel plates in each of the first lap joint 11 and the second lap joint 12. The spot welding structure of Comparative Example No. 1 is a model in which three steel plates (metal plate 111 and metal plate 121 on the upper side of the page in Figure 16C) of two frame members are spot-welded to the outer plate member (plate-like member 13) to form a nugget (main nugget 15). Note that the holes in Figures 16A and 16B represent spot welds in the simulation model.

[0138] The spot welding structure 1 of Example No. 2 is shown in Figures 17A to 17C. The spot welding structure 1 of Example No. 2 simulates the spot welding structure of the present disclosure. Note that the outer plate member (plate-like member 13) is not shown in Figure 17A. As shown in Figure 17C, in the spot welding structure 1 of Example No. 2, the main nugget 15 joining the first lap joint 11 and the second lap joint 12 joins three metal plates. In the spot welding structure 1 of Example No. 2, the metal plate 111 on the upper side of the page in Figure 17C, the metal plate 112 on the upper side of the page, and the metal plate 112 on the lower side of the page are stacked in this order. In the spot welding structure 1 of Example No. 2, the plate-like member 13 is located outside the main nugget 15. The main nugget 15 is separated from the plate-like member 13. Therefore, there are three metal plates directly involved in joining the first lap joint 11 and the second lap joint 12. The spot welding structure of Example No. 2 has the same configuration as that shown in Fig. 1. Note that the holes in Figs. 17A and 17B represent spot welds on the simulation model.

[0139] The spot welding structure 1 of Example No. 3 is shown in Figures 18A to 18C. The spot welding structure 1 of Example No. 3 simulates the spot welding structure of the present disclosure. Note that the outer plate member (plate-like member 13) is not shown in Figure 18A. In the spot welding structure 1 of Example No. 3, the lap portion 14 is divided into two. In the lap portion 14 on the side of the first lap joint 11, the metal plate 111 on the upper side of the first lap joint 11 in the drawing, the metal plate 111 on the lower side of the first lap joint 11 in the drawing, and the metal plate 121 on the lower side of the second lap joint 12 in the drawing are overlapped in this order. In the lap portion 14 on the side of the second lap joint 12, the metal plate 111 on the upper side of the first lap joint 11 in the drawing, the edged portion 1211 of the metal plate 121 on the upper side of the second lap joint 12 in the drawing, and the edged portion 1211 of the metal plate 121 on the lower side of the second lap joint 12 in the drawing are overlapped in this order. A main nugget 15 joining the three metal plates is formed in each of the two lap portions 14. In the spot welding structure 1 of Example No. 3, the plate-shaped member 13 is located outside the main nugget 15. The main nugget 15 is separated from the plate-shaped member 13. Therefore, in each of the two lap portions 14, three metal plates are directly involved in joining the first lap joint 11 and the second lap joint 12. The spot welding structure of Example No. 3 has a configuration similar to that shown in FIG. 5A . In addition, the parts that look like holes in Figures 18A and 18B indicate spot welds on the simulation model.

[0140] The simulation model is a long structural member as shown in Figure 19. The basic conditions for the simulation assuming three-point bending are as follows: In Nos. 1 and 2, the impactor was pressed against the position in the longitudinal direction of the structural member where the main nugget 15 was formed, and in No. 3, the impactor was pressed against the position in the longitudinal direction of the structural member midway between the two main nuggets 15. Total length L of structural member: 800 mm Spot welding: 50 mm pitch Support points: R30 Distance between support points: 700 mm Impactor: R50

[0141] The results of the simulation are shown in Figure 20. Compared to the maximum load of Comparative Example No. 1, the maximum load of Inventive Example No. 2 was improved by 22%, and the maximum load of Inventive Example No. 3 was improved by 50%.

[0142] DESCRIPTION OF SYMBOLS 1 Spot welding structure 11 First lap joint 111 Metal plate of first lap joint 1111 Seam processing portion of first lap joint 112 Nugget of first lap joint 12 Second lap joint 121 Metal plate of second lap joint 1211 Seam processing portion of second lap joint 122 Nugget of second lap joint 13 Plate-shaped member 131 Seam processing portion of plate-shaped member 14 Lap portion 15 Main nugget 151 Indentation of main nugget 16 Sub-nugget 161 Indentation of sub-nugget 17 Adhesive 18 Additional nugget 181 Indentation of additional nugget 2 Metal plate with seam processing portion 21 Seam processing portion 211 Tip portion 212 Connection portion 22 Main body portion 3 Metal plate without seam processing portion 4 Vehicle body 41 Joint between A-pillar lower and side sill 42 Joint between B-pillar and side sill 43 Joint between B-pillar and roof rail 44 Joint between A-pillar lower and A-pillar upper 45 Joint between A-pillar upper and roof rail 46 Joint between C-pillar and side sill

Claims

1. A spot welding structure comprising: a first lap joint having two or more metal plates; a second lap joint having two or more metal plates; a lap portion where end portions of one or more of the metal plates of the first lap joint and end portions of one or more of the metal plates of the second lap joint are overlapped; a main nugget joining the first lap joint and the second lap joint at the lap portion; a metal plate-like member overlapped on the first lap joint, the second lap joint, and the lap portion; and a plurality of auxiliary nuggets joining the plate-like member and the first lap joint, as well as the plate-like member and the second lap joint, wherein the number of metal plates joined by the main nugget is three, an impression of the main nugget is provided only on the metal plates included in the first lap joint or the second lap joint, the plate-like member is located outside the main nugget, and the auxiliary nugget is located outside the lap portion.

2. A spot welding structure as described in claim 1, characterized in that the distance between the center of the secondary nugget joining the plate-like member and the first lap joint and the center of the secondary nugget joining the plate-like member and the second lap joint is 200 mm or less.

3. The spot welding structure according to claim 1 or 2, characterized in that one or more of the metal plates of the first lap joint and the metal plates of the second lap joint have a seam processing portion in the lap portion, and two of the three metal plates joined by the main nugget are arranged in approximately the same plane outside the lap portion.

4. A spot welded structure as described in claim 1 or 2, characterized in that the number of joint interfaces between the first lap joint and the second lap joint is two or more.

5. The spot welding structure according to claim 1 or 2, characterized in that the spot welding structure has: a main nugget joining the two metal plates of the first lap joint and one metal plate of the second lap joint; and a main nugget joining the one metal plate of the first lap joint and two metal plates of the second lap joint.

6. The spot welded structure according to claim 1 or 2, further comprising an adhesive between the plate-like member and one or both of the first lap joint and the second lap joint.

7. The spot welding structure as described in claim 1 or 2, characterized in that the spot welding structure further has an additional nugget in the lap portion, the additional nugget joins two or more of the metal plates and the plate-like members included in the lap portion, and an impression of the additional nugget is provided on the plate-like member.

8. The spot welded structure according to claim 1 or 2, characterized in that the plate-like member is separated from the first lap joint and the second lap joint over the entire lap portion.

9. A spot welding structure as described in claim 1 or 2, characterized in that the first lap joint has a nugget that joins only the metal plates of the first lap joint outside the lap portion, and the second lap joint has a nugget that joins only the metal plates of the second lap joint outside the lap portion.

10. The spot welded structure according to claim 1 or 2, characterized in that the first lap joint and the second lap joint are structural components of an automobile, and the plate-like member is an exterior panel component of the automobile.

11. A manufacturing method for a spot welding structure, comprising the steps of: preparing a first lap joint having two or more metal plates, and a second lap joint having two or more metal plates; overlapping end portions of one or more of the metal plates of the first lap joint and end portions of one or more of the metal plates of the second lap joint to form a lap portion; primarily spot welding the first lap joint and the second lap joint at the lap portion to form a main nugget; overlapping a metal plate-shaped member on the lap portion where the first lap joint, the second lap joint, and the main nugget are formed; and secondary spot welding the plate-shaped member and the first lap joint, and the plate-shaped member and the second lap joint to form a plurality of sub-nuggets, wherein the number of metal plates to be primary spot welded is three, and the welding points in the secondary spot welding are outside the lap portion.

12. A process of preparing a plate assembly by overlapping two of the metal plates that are components of a first lap joint at their ends; a process of overlapping an end of one of the metal plates that are components of a second lap joint on the end of the plate assembly to form a lap portion; a process of primarily spot welding two of the metal plates that are components of the first lap joint and one of the metal plates that are components of the second lap joint at the lap portion to form a main nugget and the first lap joint; a process of overlapping and spot welding one of the metal plates of the second lap joint on which the main nugget is provided and the remaining metal plate of the second lap joint outside the lap portion to form the second lap joint; a process of overlapping a metal plate-like member on the lap portion on which the first lap joint, the second lap joint, and the main nugget are formed; a step of secondary spot welding the plate-like member and the first lap joint, and the plate-like member and the second lap joint to form a plurality of sub-nuggets, wherein the number of the metal plates to be primary spot welded is three, and the welding points in the secondary spot welding are outside the lap portion.

13. A method for manufacturing a spot welded structure as described in claim 11 or 12, characterized in that the distance between the center of the welding point in the secondary spot welding of the plate-like member and the first lap joint and the center of the welding point in the secondary spot welding of the plate-like member and the second lap joint is 200 mm or less.

14. A method for manufacturing a spot welded structure as described in claim 11 or 12, characterized in that a seam processing portion is provided on the end portions of one or more of the metal plates of the first lap joint and the metal plates of the second lap joint, and when forming the lap portion, two of the three metal plates joined by the main nugget are arranged in approximately the same plane outside the lap portion.

15. A method for manufacturing a spot welded structure as described in claim 11, characterized in that when forming the lap portion, the lap portion is formed by overlapping two of the metal plates of the first lap joint with one of the metal plates of the second lap joint, and the lap portion is formed by overlapping one of the metal plates of the first lap joint with two of the metal plates of the second lap joint, and the primary spot welding is performed on each of the multiple lap portions.

16. A method for manufacturing a spot welded structure as described in claim 12, further comprising the steps of: after forming the main nugget, overlapping the remaining metal plate of the second lap joint onto the one metal plate of the second lap joint on which the main nugget is provided, so as to form an additional lap portion in which the one metal plate of the first lap joint and the two metal plates that are parts of the second lap joint are overlapped; and further performing primary spot welding on the additional lap portion to further form the main nugget.

17. A method for manufacturing a spot welded structure as described in claim 11 or 12, characterized in that after the primary spot welding and before overlapping the plate-shaped member onto the first lap joint, the second lap joint, and the lap portion on which the main nugget is formed, a step of applying an adhesive between the plate-shaped member and one or both of the first lap joint and the second lap joint is further included.

18. A method for manufacturing a spot welded structure as claimed in claim 11 or 12, further comprising a step of performing additional spot welding on the lap portion and the plate-like member after the secondary spot welding.

19. A method for manufacturing a spot welded structure as claimed in claim 11 or 12, characterized in that all spot welding to the plate-like member is performed outside the lap portion.

20. A method for manufacturing a spot welded structure as described in claim 11 or 12, characterized in that the first lap joint and the second lap joint are structural components of an automobile, and the plate-like member is an exterior panel component of the automobile.