LAP WELDED JOINT, METHOD OF PRODUCTION THEREOF AND CLOSED CROSS SECTION MEMBER.

Abstract

A method for manufacturing a lap welded joint is provided, which allows for an effective, easy, and low-cost improvement of the joint's fatigue strength. On one side of a first edge (12), a first metal plate (10) is provided with a convex portion (13) having an internal space S of a predetermined size, and a second metal plate (20) has a protruding portion (22) facing the convex portion and insertable into the convex portion (13). In the state where the protruding portion (22) is inserted into the convex portion (13), the first metal plate (10) and a second edge (21) of the second metal plate (20) are welded together, forming a first weld bead (30).

Description

LAP WELDED JOINT, METHOD OF PRODUCTION THEREOF AND CLOSED CROSS SECTION MEMBER TECHNICAL FIELD The present invention relates to a lap welded joint, a method for producing the same, and a member with a closed cross-section. BACKGROUND OF THE INVENTION A structural member of a car must possess not only static strength and stiffness but also high fatigue resistance. Furthermore, steel or aluminum alloy sheets are often joined by welding due to the superior efficiency and cost-effectiveness when using similar materials. On the other hand, there is always a need to reduce the weight of a vehicle body to improve fuel economy, and as a countermeasure, recent advances have been made in thinning steel or aluminum alloy materials by increasing their strength. However, it is well known that the strength of a welded joint does not increase linearly with the material's strength and is significantly improved. The fatigue resistance problem of a joint is one of the issues that is difficult to solve using high-strength materials and thinning techniques. It is said that there are primarily three reasons why the fatigue of a welded joint is less than that of a sheet metal material. Hereafter in this document, as shown in Fig. 12, a lap welded joint 1 will be described as an example, in which a first metal sheet 10 and a second metal sheet 20 overlap, and a portion of the edge 21 of the second metal sheet 20 and a surface 11 in the vicinity of a portion of the edge 12 of the first metal sheet 10 are welded together by means of a weld bead 30. The first cause is the geometric concentration of stresses. As shown in Fig. 13, a boundary between the weld bead 30, which is a portion Lcannn / pznz / e / Yi / u welded, and the first metal plate 10 and the second metal plate 20, which are metal members, is a dashed line, so that the stress is concentrated at the toe of the weld bead. The magnitude of the stress concentration is inversely proportional to the smoothness of the toe of the weld bead. The second cause is material deterioration. The first metal sheet 10 and the second metal sheet 20 are subjected to rapid heating and cooling to form a heat-affected portion 31 whose crystalline structure changes from that of the original material. Consequently, material characteristics such as hardness, toughness, and elongation are locally modified in the heat-affected portion 31. The third cause is tensile residual stress. In addition to a series of thermal hysteresis effects due to temperature rise and fall, tensile residual stress generally remains in the vicinity of the welded portion after cooling to room temperature, due to a restraint around the weld. Tensile residual stress is considered a factor that reduces fatigue strength. Geometric stress concentration, local material deterioration, and residual tensile stress overlap to deteriorate the fatigue characteristics of the joint. Given this background, various efforts and studies have been undertaken to improve the fatigue strength of welded joints with respect to the related technique, considering one of its mechanisms. For example, in Patent Bibliography 1, the thickness of a portion of a material is locally reduced to separate a stress concentration area from a weld bead. In Patent Bibliography 2 and 3, several shot blasting treatments are performed after arc welding, and a compressive residual stress is applied to a stress concentration area. In Patent Bibliography 4, a welding material with a composition is adopted. A special Lcannn / pznz / e / Yi / u compressive stress is applied using martensite transformation. In Patent Bibliographies 5 and 6, after welding is complete, the periphery of a toe is subjected to further heat treatment using a heat source such as a plasma arc or laser. In Patent Bibliographies 7 and 8, another weld metal called a reinforcing bead is provided. In Patent Bibliography 9, a convex pressure bead is provided adjacent to a weld bead. Additionally, a method has been proposed in which the foot is smoothed by a grinding medium, or the product itself is placed in a furnace and annealed to reduce residual tensile stress. LIST OF CITED DOCUMENTS PATENT BIBLIOGRAPHY Patent Bibliography 1: JP2018-30169A Patent Literature 2: JP5880260B Patent Bibliography 3: JP3899007B Patent Bibliography 4: JP5450293B Patent Bibliography 5: JP2014-4609A Patent Bibliography 6: JP6515299B Patent Bibliography 7: JP5522317B Patent Bibliography 8: JP5843015B Patent Bibliography 9: JP6008072B SUMMARY OF THE INVENTION TECHNICAL PROBLEM At the same time, in the case of thin sheet metal, there is another deterioration factor besides the previously described factors of decreased fatigue strength. This is due to poor clamping of the metal element, distortion and deflection of the metal member itself, deterioration of the sheet metal alignment accuracy caused by thermal deformation generated during welding, and similar factors; that is, enlargement of the gap G Lcannn / pznz / e / YiAi of the root, which is the so-called gap between plates. When there is no root gap G, the portion of maximum stress concentration in joint 1 is the toe of the weld bead. However, as shown in Figs. 14 and 15, when root gap G is generated, the toe does not necessarily become the portion of maximum stress concentration due to stiffness deterioration, and the portion of the root gap G becomes the portion of maximum stress concentration, making fracture more likely to occur at an earlier stage. The various means of improving the fatigue strength of joints, described in Patent Bibliographies 1 to 9, that have been devised so far do not have the effect of suppressing an increase in root gap G, and therefore there is room for further improvements. The present invention has been made in view of the problems described above, and one object thereof is to provide a lap welded joint and a method of producing the same capable of effectively improving fatigue resistance with a simple structure. SOLUTION TO THE PROBLEM Accordingly, the above object of the present invention is achieved by the following configuration (1) according to a lap welded joint. (1) A lap welded joint produced by overlapping and welding a first sheet metal and a second sheet metal, the first sheet metal having at least one convex portion on an edge portion of a first side which is the side welded to the second sheet metal, the convex portion extending along a lengthwise direction from the edge portion of the first side towards an edge portion of the other side of the first sheet metal, and convex in a direction oriented towards the second sheet metal with respect to a flat sheet portion, the second sheet metal having at least one projecting portion on an edge portion of a first side which is the side welded to the first sheet metal, the projecting portion projecting in a direction from a Lcannn / pznz / e / Yi / u edge portion of the other side of the second metal sheet towards the edge portion of the first side, and being able to be inserted into the convex portion, the lap welded joint comprises a first weld bead formed by welding the first metal sheet and the edge portion of the second metal sheet, on the first side of the second metal sheet, in a state in which the protruding portion is inserted into the convex portion, and the convex portion is formed so that the height of the first metal sheet, in the direction of the sheet thickness, is equal to or greater than the sum of the sheet thickness of the first metal sheet and the sheet thickness of the second metal sheet, and the length of the first metal sheet, in the direction of the length, exceeds the sum of the leg length of the first weld bead, between the first metal sheet and the second metal sheet,and the length of a heat-affected weld zone extending from the first weld bead to the other side of the first metal sheet. According to this configuration, with a simple structure, the stress concentration in the welded portion is relieved, and the welded portion is formed in which the root separation is suppressed, so the fatigue resistance of the joints can be effectively improved. Furthermore, a preferred embodiment of the present invention relating to the lap welded joint relates to points (2) to (8) below. (2) In the lap welded joint according to point (1) above, in the second metal sheet, the protruding portion is formed between a pair of cutouts that are formed along the length direction from the edge portion of the first side of the second metal sheet. According to the configuration, the protruding portion can be easily formed by providing the cutouts. (3) In the lap welded joint according to points (1) or (2) above, the bulged portion is formed by pressure molding. According to this configuration, the bulging portion can be formed Lcannn / pznz / e / Yi / u with precision and ease. (4) The lap welded joint according to any of the above (1) to (3), further comprising a second weld bead formed by welding the second metal sheet and the edge portion of the first metal sheet, on the first side of the first metal sheet, in a position corresponding to the convex portion. According to this configuration, the fatigue strength of the lap welded joint can be further improved. (5) In the lap welded joint according to point (4) above, the first weld bead and the second weld bead are formed continuously. According to this configuration, the welding operation of the first weld bead and the second weld bead is facilitated. (6) In the lap welded joint according to any of the above points (1) to (5), the first metal sheet has at least a cut-out portion in the edge portion of the first side of the first metal sheet, and the bulged portion is formed in a position corresponding to the cut-out portion. According to this configuration, the overlapping area between the first metal sheet and the second metal sheet can be increased, and fatigue resistance is further improved. (7) In the lap welded joint according to any of the above points (1) to (6), the height of the first metal sheet, in the direction of the sheet thickness, in an internal space formed by the bulged portion is substantially equal to the sum of the sheet thickness of the first metal sheet and the sheet thickness of the second metal sheet. According to this configuration, the effect of suppressing root separation is further improved. (8) In the lap welded joint according to any of the preceding points (1) to (7), the convex portion is formed so that the height of the Lcannn / pznz / e / Yi / u first metal sheet, in the direction of the thickness of the sheet, gradually decrease from the entry side of the protruding portion towards the edge portion of the first metal sheet, on the other side of the first metal sheet. According to this configuration, the first metal sheet and the second metal sheet can be held together without loosening while the protruding portion is easily inserted into the bulged portion. The previous object of the present invention is achieved by the following configuration (9) related to a member with a closed cross-section. (9) A member with a closed cross section to which the lap welded joint is applied in accordance with any one of the above points (1) to (8). According to this configuration, the closed cross-section member can be produced whose joint has better fatigue resistance. The previous object of the present invention is achieved by the following configuration (10) related to a method of producing a lap welded joint. (10) A method for producing a lap welded joint by overlapping and welding a first metal sheet and a second metal sheet, comprising: a step of forming at least one bulged portion on an edge portion of the first metal sheet, on a first side of the first metal sheet, which is the side to be welded to the second metal sheet, the bulged portion extending along the length direction from the edge portion of the first side towards an edge portion of the other side of the first metal sheet, and bulging in a direction oriented towards the second metal sheet with respect to a flat sheet portion; a step of forming at least one projecting portion on an edge portion of the second metal sheet, on a first side of the second metal sheet, which is the side to be welded to the first metal sheet, the projecting portion projecting in a direction from an edge portion on the other side of Lcannn / pznz / e / Yi / u the second metal plate towards the edge portion of the first side, and being able to be inserted into the bulged portion; a step of overlapping the first metal sheet with the second metal sheet while inserting the protruding portion into the convex portion; and a step of forming a first weld bead by welding the first metal sheet and the edge portion of the second metal sheet, on the first side of the second metal sheet, and the convex portion is formed so that the height of the first metal sheet, in the direction of the sheet thickness, is equal to or greater than the sum of the sheet thickness of the first metal sheet and the sheet thickness of the second metal sheet, and the length of the first metal sheet, in the direction of the length, exceeds the sum of the leg length of the first weld bead, between the first metal sheet and the second metal sheet, and the length of a heat-affected weld zone extending from the first weld bead to the other side of the first metal sheet. According to this configuration, with a simple structure, the stress concentration in the welded portion is relieved, and the welded portion is formed in which the root separation is suppressed, so the fatigue resistance of the joints can be effectively improved. Furthermore, a preferred embodiment of the present invention, related to the method of producing a lap welded joint, relates to the following points (11) to (16). (11) In the method of producing a lap welded joint according to the previous point (10), in the second metal sheet, the protruding portion is formed between a pair of cutouts that are formed along the length direction from the edge portion of the first side of the second metal sheet. According to the configuration, the protruding portion can be easily formed by providing the cutouts. (12) In the method of producing a lap welded joint according to Lcannn / pznz / e / Yi / u with the previous points (10) or (11), the bulbous portion is formed by pressure molding. According to this configuration, the domed portion can be formed accurately and easily. (13) In the method of producing a lap welded joint according to any one of the above points (10) to (12), the step of forming the first weld bead is carried out by any of the methods of arc welding, laser welding and hybrid laser arc welding. According to this configuration, the fatigue strength of the joint can be effectively improved by any one of the welding methods. (14) The method of producing a lap welded joint according to any one of the above points (10) to (13) further includes: a step of forming a second weld bead by welding the second metal sheet and the edge portion of the first metal sheet, on the first side of the first metal sheet, in a position corresponding to the convex portion. According to this configuration, the fatigue strength of the lap welded joint can be further improved. (15) In the method of producing a lap welded joint according to the above point (14), the first weld bead and the second weld bead are formed by continuous welding. According to this configuration, the welding operation of the first weld bead and the second weld bead is facilitated. (16) The method of producing a lap welded joint according to any one of the above points (10) to (15) further includes: a step of forming at least a cut-out portion in the edge portion of the first metal sheet, on the first side of the first metal sheet, before forming the domed portion in the first metal sheet, and the domed portion is formed in a position corresponding to the cut-out portion. According to this configuration, the overlapping area can be increased. Lcannn / pznz / e / Yi / u between the first metal sheet and the second metal sheet, and fatigue resistance is further improved. ADVANTAGEOUS EFFECTS OF THE INVENTION According to the lap welded joint and the method of producing a lap welded joint of the present invention, with a simple structure, the stress concentration in the welded portion is relieved, and the welded portion is formed in which the root separation is suppressed, in order to effectively improve the fatigue strength of the joint. BRIEF DESCRIPTION OF THE DRAWINGS [Fig. 1] Fig. 1 is a perspective view showing a lap welded joint according to a first embodiment of the present invention. [Fig. 2] Fig. 2 is a perspective view showing the production stages of the lap weld joint shown in Fig. 1. [Fig. 3] Fig. 3 is a cross-sectional view taken along line CC of the lap weld joint shown in Fig. 2. [Fig. 4] Fig. 4 is a cross-sectional view taken along line DD of a first metal sheet shown in Fig. 1. [Fig. 5] Fig. 5 is a perspective view of a lap welded joint according to a first modification of the first embodiment. [Fig. 6] Fig. 6 is a perspective view of a lap welded joint according to a second modification of the first embodiment. [Fig. 7] Fig. 7 is a perspective view of a lap welded joint according to a third modification of the first embodiment. [Fig. 8] Fig. 8 is a perspective view showing the production stages of a lap welded joint according to a second embodiment of the present invention. [Fig. 9] Fig. 9 is a perspective view showing the production stages of a lap welded joint according to a first modification of the second embodiment. Lcannn / pznz / e / Yi / u [Fig. 10] Fig. 10 is a perspective view of a suspension arm that is an example of the lap welded joint of the present invention. [Fig. 11] Fig. 11 is a perspective view schematically showing a cross-section taken along line EE of Fig. 10. [Fig. 12] Fig. 12 is a perspective view of a lap welded joint of the related technique produced by overlapping and welding two metal sheets. [Fig. 13] Fig. 13 is a cross-sectional view taken along line AA of the lap weld joint shown in Fig. 12. [Fig. 14] Fig. 14 is a perspective view of a lap welded joint of the related technique having a gap. [Fig. 15] Fig. 15 is a cross-sectional view taken along line BB of the lap welded joint shown in Fig. 14. DESCRIPTION OF EMBODIMENTS Hereafter in this document, embodiments of a lap welded joint according to the present invention will be described, with reference to the drawings. (First realization) Fig. 1 is a perspective view showing a lap welded joint according to a first embodiment of the present invention, Fig. 2 is a perspective view showing the production stages of the lap welded joint of Fig. 1, and Fig. 3 is a cross-sectional view taken along a line CC of the lap welded joint of Fig. 2. As shown in Figs. 2 and 3, a lap welded joint 1 according to the first embodiment of the present invention has a configuration in which a first metal plate 10 and a second metal plate 20 overlap each other, and a second edge portion 21 of the second metal plate 20 is welded to the first metal plate 10. Lcannn / pznz / e / Yi / u In the following description, the side of the first metal sheet 10 welded to the second metal sheet 20 is called the first side, the opposite side is called the other side, and the edge portion of the first side of the first metal sheet 10 is called the first edge portion 12. Furthermore, the side of the second metal sheet 20 welded to the first metal sheet 10 is called the first side, the opposite side is called the other side, and the edge portion of the first side of the second metal sheet 20 is called the second edge portion 21. That is, the first edge portion 12 and the second edge portion 21 are facing each other, and the first side and the other side of the first metal sheet 10 and the second metal sheet 20 are opposite each other. Furthermore, in the present embodiment, the first and second metal plates 10, 20 are thin, flat plates, each of which has a substantially rectangular shape. The direction in which the first and second metal plates 10, 20 overlap each other is also called the height direction, the direction along the weld line (each of the edge portions 12, 21) is also called the width direction, and the direction that departs from the weld line is also called the length direction. In the first metal sheet 10, a plurality of convex portions 13 are formed in the first edge portion 12. These portions are convex relative to a flat sheet portion in a direction (surface side) oriented towards the second metal sheet 20 and form internal spaces S on one side of the rear surface, which is the side welded to the second metal sheet 20, such that they are spaced apart from each other along the edge portion 12. For example, in the embodiment shown in Fig. 1, two convex portions 13 are provided. The height Xt of the convex portion 13 in the sheet thickness direction of the first metal sheet 10 is set to be equal to or greater than ta + tb, which is the sum of the sheet thickness ta of the first metal sheet 10 and the sheet thickness tb of the second metal sheet 20. The length Xl in the length direction of the first metal sheet 10 is adjusted to a length Lcannn / pznz / e / Yi / u greater than L1 + L2, which is the sum of the length L1 of the leg of a first weld bead 30, between the first metal sheet 10 and the second metal sheet 20, and the length L2 of a heat-affected weld zone 31 extending from the first weld bead 30 to the other side of the first metal sheet 10 (see Fig. 13). On the other hand, the second metal sheet 20 includes a plurality of protruding portions 22 formed by cutouts 23 provided in the second edge portion 21, which is the side welded to the first metal sheet 10, and which protrude from the second edge portion 21 into the first metal sheet 10. For example, in the embodiment shown in Fig. 1, two protruding portions 22 are provided. The two protruding portions 22 are shaped to correspond to the intervals of the two convex portions 13, and can be inserted into the internal spaces S of the convex portions 13. The domed portion 13 of the first metal sheet 10 can be easily and accurately formed by cold or hot pressing of the first metal sheet 10. Then, the protruding portion 22 of the second metal sheet 20 is inserted into the internal space S of the convex portion 13 of the first metal sheet 10, and the first metal sheet 10 and the second metal sheet 20 overlap each other. Consequently, a surface of the protruding portion 22 of the second metal sheet 20 comes into contact with a rear surface of the convex portion 13, and a portion of the rear surface of the second metal sheet 20, on one side of the second edge portion 21 that excludes the protruding portion 22, comes into contact with a surface 11 of the first metal sheet 10, so that the second metal sheet 20 is supported by the first metal sheet 10. The height Xt in the direction of the thickness of the first metal sheet 10 in the internal space S is preferably substantially equal to the sum of the sheet thickness ta of the first metal sheet 10 and the sheet thickness tb of the second Lcannn / pznz / e / Yi / u sheet metal 20. However, since there is a possibility that the protruding portion 22 will not fit into the internal space S of the convex portion 13 if the height Xt is exactly equal to the sum of the sheet thickness ta and the sheet thickness tb, a small margin is actually provided. However, in this case, it is also desirable that at least a portion of the surface of the protruding portion 22 of the second sheet metal 20 be physically in contact with the back surface of the convex portion 13, and therefore the root gap G between the first sheet metal 10 and the second sheet metal 20 is suppressed to a minimum gap. Furthermore, in this embodiment, the surface of the protruding portion 22 of the second metal sheet 20 and the back surface of the convex portion 13 are substantially parallel to each other, but the convex portion 13 may be slightly inclined so that its height gradually decreases from the inlet side of the protruding portion 22 towards the side of the root portion 40 (an edge portion on the other side of the first metal sheet 10) which will be described later.Specifically, in the bulged portion 13, the height Xt of the side inlet opening of the protruding portion 22 is set to be equal to or greater than ta + tb, which is the sum of the sheet thickness ta of the first metal sheet 10 and the sheet thickness tb of the second metal sheet 20, and the height Xt of the bulged portion 13 is set to be less than ta + tb, which is the sum of the sheet thickness ta of the first metal sheet 10 and the sheet thickness tb of the second metal sheet 20, in a region where the bulged portion 13 and the protruding portion 22 overlap each other in the direction of the length of the first metal sheet 10.Such a convex portion 13 facilitates the insertion of the protruding portion 22 into the convex portion 13 and, at the same time, the protruding portion 22 is pushed into the convex portion 13 to abut the convex portion 13, so that the first metal plate 10 and the second metal plate 20 can be held without loosening. Next, in a state where the protruding portion 22 is Lcannn / pznz / e / Yi / u inserted into the domed portion 13, a part of the second edge portion 21 of the second metal sheet 20 and a part of the surface 11 of the first metal sheet 10 are welded by arc welding or hybrid laser arc welding to form the first weld bead 30. In this case, the welded part is the second edge portion 21 of the second metal sheet 20 exposed on the surface 11 of the first metal sheet 10, and the second edge portion 21 of the protruding portion 22 inserted into the domed portion 13 is not welded. The first weld bead 30 can be formed in contact with an outer side surface 41 of the convex portion 13 extending from the first edge portion 12 of the convex portion 13 to the other side. In the welded joint 1 of the present embodiment, when a load in the direction of the sheet thickness acts on the welded joint 1, a root portion 40 on the separated side of the first edge portion 12 of the convex portion 13 becomes part of a stress concentration portion, thus relieving the stress concentration in the first weld bead 30, which is a weld portion. Therefore, the stress concentration does not act on the heat-affected weld zone 31 (see Fig. 13) where the strength decreases due to the influence of the welding heat, and the strength of the base metal of the first sheet metal 10 and the second sheet metal 20 is maintained. Consequently, since the root portion 40 serving as a stress concentration portion is separated from the first weld bead 30, the influence of the heat-affected weld zone 31, where the strength decreases due to welding heat, on the fatigue strength of the lap welded joint 1 is small, and the strength of the first metal sheet 10 and the second metal sheet 20, which are the base metal, is maintained. As in the present embodiment, when the convex portion 13 of the first metal sheet 10 and the edge portion 21 of the second metal sheet 20 overlap each other by the cutouts 23 formed in the second metal sheet Lcannn / pznz / e / Yi / u rectangular 20, to make the first metal sheet 10 and the second metal sheet 20 overlap each other, it is preferable that the length Xl of the convex portion 13, in the direction of the length of the first metal sheet 10, also takes into account the length L3 of the overlapping portion of the first metal sheet 10 and the second metal sheet 20. That is, in the present embodiment, in the convex portion 13, the length Xl in the direction of the length of the first metal sheet 10 is set at a length that exceeds the sum L1 + L2 + L3, that is, the length L1 of the leg of the first weld bead 30, between the first metal sheet 10 and the second metal sheet 20, the length L2 of the heat-affected weld zone 31 extending from the first weld bead 30 to the other side of the first metal sheet 10, and the length L3 of the overlapping portion of the first metal sheet 10 and the second metal sheet 20. Furthermore, the gap G between the first metal plate 10 and the second metal plate 20 is suppressed to a minimum gap without using a clamping jig or similar commonly used to reduce the gap G between plates, and the fatigue strength of the welded joint 1 is improved. It should be noted that the greater the number of clamping jigs, the greater the effect of suppressing the gap G, but when the number of clamping jigs increases, the number of fitting stages and the cost of the jig also increase, which is not preferable. Furthermore, by appropriately adjusting the length Xl of the single domed portion 13, the width Xw of the single domed portion 13, the ratio (ZXb2 / ZB) between the total length (ZXw) of the widths Xw of the domed portions 13 and the total length (ΣB; ΣB = Bi + B2 + B3 in the example shown in Fig. 1) of the first weld bead 30, and the radius Xr of the root portion 40 of the domed portion 13, as shown in Fig. 4, the strength can be optimized. Since the domed portion 13 is formed by press working (press molding), the radius Xr of the root portion 40 of the Lcannn / pznz / e / Yi / u the bulging portion 13 can be given a smooth and stable shape. As described above, according to the lap weld joint 1 of the present embodiment, since the gap G between the first metal plate 10 and the second metal plate 20 can be eliminated, the weld performance is stabilized. Furthermore, the shape of the convex portion 13 formed by press working is not damaged by the welding, and fatigue resistance is improved. In the present embodiment, a pointed end portion of the protruding portion 22 of the second metal sheet 20 protrudes from the second edge portion 21 from the point of view of the rigidity of the joint, but the present invention is not limited to this, and the pointed end portion of the protruding portion 22 can be located in the same position as the second edge portion 21 in the direction of the length. Fig. 5 is a perspective view of a lap welded joint of a first modification of the first embodiment, wherein the second edge portion 21 of the second metal sheet 20 and the surface 11 of the first metal sheet 10 are laser welded together to form the first weld bead 30. At the time of laser welding, a separate filler wire can be used. Next, with reference to Figs. 6 and 7, the lap welded joints of a second modification and a third modification of the first embodiment will be described. In welded joint 1 of the second modification, as shown in Fig. 6, the protruding portions 22 of the second metal sheet 20 are inserted and fitted into the convex portions 13 of the first metal sheet 10, and the second edge portion 21 of the second metal sheet 20 and the surface 11 of the first metal sheet 10 are welded by arc welding or hybrid laser arc welding to form the first weld bead 30. In addition, the first edge portions 14 of the convex portions 13 and a The surface 24 of the second metal sheet 20 is welded by arc welding or hybrid laser arc welding to form a second weld bead 30A. The first weld bead 30 and the second weld bead 30A can be welded continuously. Consequently, the number of welding stages is reduced compared to a case where the first weld bead 30 and the second weld bead 30A are welded in separate stages. In the lap weld joint 1 of the third modification, as shown in Fig. 7, the protruding portions 22 of the second metal sheet 20 are inserted and fitted into the convex portions 13 of the first metal sheet 10. The second edge portion 21 of the second metal sheet 20 and the surface 11 of the first metal sheet 10 are laser welded together to form the first weld bead 30, and the first edge portions 14 of the convex portions 13 and the surface 24 of the second metal sheet 20 are laser welded together to form the second weld beads 30A. The first weld bead 30 and the second weld bead 30A can be continuously welded together by laser welding. (Second embodiment) Figure 8 is a perspective view showing the production stages of a lap welded joint according to a second embodiment of the present invention. As shown in Figure 8, in the first edge portion 12 of the first metal sheet 10 of the second embodiment, there are a plurality of cut-out portions 15 formed along the first edge portion 12. For example, in the embodiment shown in Figure 8, two cut-out portions 15 are provided. The domed portions 13 are formed in the cut-out portions 15, respectively. Similar to the first metal sheet 10 of the first embodiment, the convex portion 13 has the internal space S in which the height Xt in the sheet thickness direction of the first metal sheet 10 is equal to or greater than ta + tb, Lcannn / pznz / e / YiAi, which is the sum of the sheet thickness ta of the first metal sheet 10 and the sheet thickness tb of the second metal sheet 20, and the length in the direction from the first edge portion 12 to the other side is the length Xl (see Fig. 2). The length Xl exceeds L1 + L2, which is the sum of the length L1 of the leg of the first weld bead 30, between the first metal sheet 10 and the second metal sheet 20, and the length L2 of the heat-affected weld zone 31 extending from the first weld bead 30 to the other side of the first metal sheet 10 (see Fig. 13). On the other hand, the second metal sheet 20 has a plurality of protruding portions 22 that project in the direction of the first metal sheet 10 from the second edge portion 21, which is the side welded to the first metal sheet 10. The protruding portion 22 is formed so as to correspond to the intervals of the convex portions 13, and can be inserted into the internal space S of the convex portion 13. The protruding portion 22 of the second sheet metal 20 is then inserted and fitted into the convex portion 13 of the first sheet metal 10, and the first sheet metal 10 and the second sheet metal 20 overlap each other. Consequently, a surface of the protruding portion 22 of the second sheet metal 20 contacts a rear surface of the convex portion 13, and a portion of the rear surface of the second sheet metal 20, on the side of the second edge portion 21 that excludes the protruding portion 22, contacts surface 11 of the first sheet metal 10, so that the second sheet metal 20 is supported by the first sheet metal 10. Consequently, the gap G between the first sheet metal 10 and the second sheet metal 20 is reduced to a minimum. Next, a portion of the edge portion 21 of the second metal sheet 20 and a portion of the surface 11 of the first metal sheet 10 are welded together, either by arc welding, laser welding, or hybrid laser-arc welding, to form the first weld bead 30. Furthermore, the second Lcannn / pznz / e / Yi / u 30A weld bead can be formed by welding the first edge portion 14 of the domed portion 13 and the surface 24 of the second metal sheet 20 either by arc welding, laser welding and hybrid laser arc welding. Figure 9 is a perspective view showing the production stages of a lap welded joint according to a first modification of the second embodiment. In the present modification, the cut portion 15, formed along the first edge portion 12 of the first metal plate 10, has corner portions each shaped like an R, and each of the domed portions 13 is formed on the cut portion 15. The convex portion 13 has the internal space S of the same size as that of the first metal sheet 10 of the second embodiment. That is, the convex portion 13 has the internal space S in which the height Xt in the direction of the sheet thickness of the first metal sheet 10 is equal to or greater than ta + tb, which is the sum of the sheet thickness ta of the first metal sheet 10 and the sheet thickness tb of the second metal sheet 20, and the length in the direction from the first edge portion 12 to the other side is the length Xl. On the other hand, in the second metal sheet 20, the plurality of protruding portions 22 that protrude in the direction of the first metal sheet 10 are formed in the second edge portion 21 with which the first metal sheet 10 overlaps, corresponding to the intervals of the bulged portions 13. The shape of the protruding portion 22 is an R shape. Then, the protruding portion 22 of the second metal sheet 20 is inserted and fitted into the convex portion 13 of the first metal sheet 10, the first metal sheet 10 and the second metal sheet 20 are overlapped, and the second edge portion 21 of the second metal sheet 20 and the surface 11 of the first metal sheet 10 are welded together either by arc welding, laser welding, or hybrid laser arc welding, to form the first weld bead 30. Lcannn / pznz / e / Yi / u By shaping the cut portion 15 of the first metal sheet 10 and the protruding portion 22 of the second metal sheet 20 into an R shape, the overlap area between the first metal sheet 10 and the second metal sheet 20 can be increased, and the strength of the joint is improved. Fig. 10 is a perspective view of a suspension arm 100 that is an example of the lap welded joint 1 described above, and Fig. 11 is a perspective view schematically showing a cross-section taken along line EE of Fig. 10. In the suspension arm 100, a first member 110 and a second member 120, both substantially bowl-shaped, butt against each other, and their joining surfaces are welded either by arc welding, laser welding, or hybrid laser arc welding. As shown in Fig. 11, both the first member 110 and the second member 120 are substantially shaped like a channel whose cross-section is curved inwards, and there are upright portions 111 on both end portions in the width direction of the first member 110, and there are upright portions 121 on both end portions in the width direction of the second member 120 that correspond to the upright portions 111. On each of the upright portions 111 of the first member 110, the plurality of bulbous portions 13 described in the first embodiment (see Fig. 1) are shaped to be spaced from each other in the length direction. Furthermore, the plurality of protruding portions 22 described in the first embodiment (see Fig. 1) are formed in each upright portion 121 of the second member 120 so as to correspond to the intervals of the bulging portions 13. Then, after inserting the protruding portion 22 of the second member 120 into the bulbous portion 13 of the first member 110 and overlapping the end portions of the upright portion 111 and the upright portion 121 with each other, the edge portion 114 of the second member 120 and the portion Lcannn / pznz / e / Yi / u upright 111 of the first member 110 either by arc welding, laser welding or hybrid laser arc welding, to form the first weld bead 30. In addition, although not shown, the second weld bead 30A can be formed by welding the first edge portion 14 of the bulbous portion 13 and the second member 120 either by arc welding, laser welding and hybrid laser arc welding. Consequently, the lap weld joint 1 also applies to a hollow member with a closed cross-section that has any shape, light weight and high fatigue strength. It should be noted that the present invention is not limited to the embodiments and modifications described above, and that modifications, improvements, or similar actions may be made as appropriate. For example, in the embodiment described above, arc welding, laser welding, and hybrid laser arc welding are described as joining methods, but the joining method is not limited to these, and brazing or similar methods can also be used. In this case, different materials can also be joined together. Furthermore, in the embodiment described above, the second metal sheet is provided with the protruding portion providing the trim of the edge portion on the first side thereof, but for example, the edge portion of the first side may have a wavy shape in which a concave portion and a convex portion are continuous, and the convex portion may be formed as the protruding portion. Furthermore, according to the present invention, the root separation can be eliminated, provided that the second metal sheet is supported by the first metal sheet, by placing the rear surface of the second metal sheet, on a second side of the edge portion, in contact with the surface of the first metal sheet and placing at least one rear surface of the plurality of bulbous portions in contact with at least one surface of the Lcannn / pznz / e / Yi / u plurality of protruding portions of the second metal sheet. Although various embodiments have been described above with reference to the drawings, it is clear that the present invention is not limited to such examples. It is evident to those skilled in the art that various changes and modifications can be conceived within the scope of the claims, and it is also understood that such changes and modifications fall within the technical scope of the present invention. Furthermore, the components of the embodiments described above can be freely combined within a range that does not depart from the spirit of the invention. This application is based on the Japanese Patent Application filed on July 15, 2020 (Japanese Patent Application No. 2020121667), and its contents are incorporated herein by reference. LIST OF REFERENCE SIGNS lap welded joint first metal sheet first edge portion (edge ​​portion on a first side of the first metal sheet) bulged portion trimmed portion second metal sheet second edge portion (edge ​​portion on a first side of the second metal sheet, end surface) protruding portion trimmed portion first weld bead 30A second weld bead heat-affected weld zone 100 suspension arm (member with closed cross-section) Lcannn / pznz / e / Yi / u L1 length of the weld bead leg L2 length of the heat-affected weld zone S interior space sheet thickness ta of the first metal sheet sheet thickness tb of the second metal sheet Xl length of the bulged portion (length in the direction from the edge portion of the first side to the edge portion of the other side of the first metal sheet) Xt height of the bulged portion in the direction of the sheet thickness 10 (height in the direction of the sheet thickness of the first metal sheet) Xw width of the bulged portion

Claims

1. A lap welded joint produced by overlapping and welding a first metal sheet and a second metal sheet, wherein the first metal sheet has at least one convex portion on an edge portion of a first side, which is the side welded to the second metal sheet, the convex portion extending along a longitudinal direction from the edge portion of the first side to an edge portion of the other side of the first metal sheet, and convex in a direction oriented towards the second metal sheet with respect to a flat portion of the sheet, wherein the second metal sheet has at least one projecting portion on an edge portion of a first side, which is the side welded to the first metal sheet, the projecting portion projecting in a direction from an edge portion of the other side of the second metal sheet towards the edge portion of the first side, and being insertable into the convex portion,wherein the lap welded joint comprises a first weld bead formed by welding the first metal sheet and the edge portion of the second metal sheet to the first side of the second metal sheet, in a state in which the protruding portion is inserted into the convex portion, and wherein the convex portion is formed such that the height of the first metal sheet, in the sheet thickness direction, is equal to or greater than the sum of the sheet thickness of the first metal sheet and the sheet thickness of the second metal sheet, and the length of the first metal sheet, in the length direction, exceeds the sum of the leg length of the first weld bead between the first metal sheet and the second metal sheet, and the length of a heat-affected weld zone extending from the first weld bead to the other side of the first metal sheet. Lcannn / pznz / e / Yi / u, 2. The lap welded joint according to claim 1, wherein, in the second metal sheet, the protruding portion is formed between a pair of cutouts that are formed along the length direction from the edge portion of the first side of the second metal sheet.

3. The lap welded joint according to claim 1, wherein the convex portion is formed by pressure molding.

4. The lap welded joint according to claim 2, wherein the convex portion is formed by pressure molding.

5. The lap welded joint according to any one of claims 1 to 4, further comprising: a second weld bead formed by welding the second metal sheet and the edge portion of the first metal sheet, on the first side of the first metal sheet, in a position corresponding to the convex portion.

6. The lap welded joint according to claim 5, wherein the first weld bead and the second weld bead are formed continuously.

7. The lap welded joint according to any one of claims 1 to 4, wherein the first metal sheet has at least one portion cut out in the edge portion of the first side of the first metal sheet, and wherein the convex portion is formed in a position corresponding to the cut-out portion. Lcannn / pznz / e / Yi / u 8. The lap welded joint according to claim 5, wherein the first metal sheet has at least a cut-out portion in the edge portion of the first side of the first metal sheet, and wherein the bulged portion is formed in a position corresponding to the cut-out portion.

9. The lap welded joint according to claim 6, wherein the first metal sheet has at least a cut-out portion in the edge portion of the first side of the first metal sheet, and wherein the bulged portion is formed in a position corresponding to the cut-out portion.

10. The lap welded joint according to any one of claims 1 to 4, wherein the height of the first metal sheet in the direction of the sheet thickness, in an internal space formed by the bulged portion, is substantially equal to the sum of the thickness of the first metal sheet and the thickness of the second metal sheet.

11. The lap welded joint according to any one of claims 1 to 4, wherein the domed portion is formed so that the height of the first metal sheet in the direction of the sheet thickness gradually decreases from the entry side of the protruding portion towards the edge portion of the first metal sheet on the other side of the first metal sheet.

12. A closed cross-section member to which the lap welded joint is applied according to any one of claims 1 to 4. Lcannn / pznz / e / Yi / u 13. A method of producing a lap welded joint produced by overlapping and welding a first metal sheet and a second metal sheet, comprising: a step of forming at least one bulged portion in an edge portion of the first metal sheet, on a first side of the first metal sheet, which is the side to be welded to the second metal sheet, the bulged portion extending along the length direction from the edge portion of the first side towards an edge portion of the other side of the first metal sheet, and bulging in a direction oriented towards the second metal sheet with respect to a flat sheet portion;a step of forming at least one projecting portion on an edge portion of the second metal sheet, on a first side of the second metal sheet, which is the side to be welded to the first metal sheet, the projecting portion projecting in a direction from an edge portion of the other side of the second metal sheet towards the edge portion of the first side, and being able to be inserted into the convex portion; a step of overlapping the first metal sheet with the second metal sheet while inserting the projecting portion into the convex portion;and a step of forming a first weld bead by welding the first metal sheet and the edge portion of the second metal sheet to the first side of the second metal sheet, wherein the convex portion is formed such that the height of the first metal sheet, in the direction of the sheet thickness, is equal to or greater than the sum of the sheet thickness of the first metal sheet and the sheet thickness of the second metal sheet, and the length of the first metal sheet, in the direction of length, exceeds the sum of the leg length of the first weld bead, between the first metal sheet and the second metal sheet, and the length of a heat-affected weld zone extending from the first weld bead to the other side of the first metal sheet. Lcannn / pznz / e / Yi / u; 14. The method of producing a lap welded joint according to claim 13, wherein, in the second metal sheet, the protruding portion is formed between a pair of cutouts that are formed along the length direction from the edge portion of the first side of the second metal sheet.

15. The method of producing a lap welded joint according to claim 13, wherein the convex portion is formed by pressure molding.

16. The method of producing a lap welded joint according to claim 13, wherein the step of forming the first weld bead is performed by any one of the methods of arc welding, laser welding, and hybrid laser arc welding.

17. The method of producing a lap welded joint according to any one of claims 13 to 16, further comprising: a step of forming a second weld bead by welding the second metal sheet and the edge portion of the first metal sheet, on the first side of the first metal sheet, in a position corresponding to the convex portion.

18. The method of producing a lap welded joint according to claim 17, wherein the first weld bead and the second weld bead are formed by continuous welding. Lcannn / pznz / e / Yi / u 19. The method of producing a lap welded joint according to any one of claims 13 to 16, further comprising: a step of forming at least a cut-out portion in the edge portion of the first metal sheet, on the first side of the first metal sheet, before forming the domed portion in the first metal sheet, wherein the domed portion is formed in a position corresponding to the cut-out portion.

20. The method of producing a lap welded joint according to claim 17, further comprising: a step of forming at least a cut-out portion in the edge portion of the first metal sheet, on the first side of the first metal sheet, before forming the domed portion in the first metal sheet, wherein the domed portion is formed in a position corresponding to the cut-out portion.

21. The method of producing a lap welded joint according to claim 18, further comprising: a step of forming at least a cut-out portion in the edge portion of the first metal sheet, on the first side of the first metal sheet, before forming the domed portion in the first metal sheet, wherein the domed portion is formed in a position corresponding to the cut-out portion.

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