Arc spot welding method for joining dissimilar materials

Abstract

To provide an arc spot welding method for dissimilar material joining which can obtain a dissimilar material weld joint that is excellent in both tensile shear strength and cross tensile strength.SOLUTION: An arc spot welding method for dissimilar material joining joins a first plate 10 made of an Al-based material or a Mg-based material, and a second plate 20 made of superhigh tensile steel having tensile strength of 1,180 MPa or more includes steps of: boring a hole in the first plate 10; overlapping the first plate 10 and the second plate 20; inserting a steel-made joint auxiliary member 30 having a hollow part into the hole of the first plate 10; and joining the first plate 10 and the second plate 20 through the joint auxiliary member 30 using a weld material containing 13 mass% or more of Ni, wherein the step of joining the first plate 10 and the second plate 20 is a step of melting the second plate 20 and the joint auxiliary member 30 while melting the weld material, and filling the hollow part of the joint auxiliary member 30 with weld metal 40.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to arc spot welding for joining dissimilar materials. In the law

Background Art

[0002] For transportation equipment represented by automobiles, improvement of fuel efficiency during driving is constantly required for the purpose of suppressing various factors such as (a) consumption of petroleum fuel which is a finite resource, (b) CO2 which is a greenhouse gas generated by combustion, and (c) driving costs. As means for this, in addition to improvement of power system technologies such as utilization of electric drive, weight reduction of the vehicle body is also one of the improvement measures. For weight reduction, there is a means of replacing steel, which is the current main material, with lightweight materials such as aluminum alloy, magnesium alloy, and carbon fiber. However, replacing everything with these lightweight materials has problems such as increased costs and insufficient strength, and as a solution, a design method called so-called multi-material, in which steel and lightweight materials are combined in appropriate places, has attracted attention.

[0003] In order to combine steel and the above lightweight materials, there are inevitably places where these need to be joined. It is known that welding is easy between steels, between aluminum alloys, and between magnesium alloys, but extremely difficult between dissimilar materials. The reason for this is that an intermetallic compound (IMC), which has extremely brittle properties, is generated in the molten mixing part of steel and aluminum or magnesium, and the molten mixing part is easily broken by external stresses such as tension and impact. For this reason, welding methods such as resistance spot welding method and arc welding method cannot be adopted for joining dissimilar materials, and it is common to use other joining methods. Welding cannot be used for joining steel and carbon fiber either because the latter is not a metal.

[0004] ​As an example of conventional dissimilar material joining technology, Patent Document 1 discloses an arc spot welding method in which an upper plate made of aluminum alloy or magnesium alloy and a lower plate made of steel are overlapped and welded together via a steel joining auxiliary member. The arc spot welding method described in Patent Document 1 involves inserting a joining auxiliary member having a hollow portion into a hole provided in the upper plate, filling this hollow portion with weld metal, and welding the lower plate and the joining auxiliary member.

[0005] Furthermore, Patent Document 2 proposes an arc spot welding method for joining dissimilar materials, which is an improved version of the joining auxiliary member described in Patent Document 1. The joining auxiliary member described in Patent Document 2 has a stepped outer shape with a shaft portion and a flange portion, the maximum outer diameter of the shaft portion and the width of the flange portion are larger than the diameter of the hole in the upper plate, and the shaft portion has a constricted portion on the flange portion side.

[0006] According to the welding methods described in Patent Documents 1 and 2, high joint strength and reliability can be obtained because steel is welded to steel. Furthermore, according to Patent Document 2, since the maximum outer diameter of the shaft is larger than the diameter of the hole in the upper plate, a force is obtained that restrains the upper and lower plates in the horizontal direction, thereby improving the strength against horizontal shear stress. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2018-034164 [Patent Document 2] Japanese Patent Publication No. 2018-103240 [Overview of the project] [Problems that the invention aims to solve]

[0008] Incidentally, in welded joints obtained by joining two overlapping plates, tensile shear strength (TSS) and cross tensile strength (CTS) are used as indicators to determine the joint strength. Therefore, even in dissimilar material welded joints that combine steel and lightweight materials as described above, it is required that both TSS and CTS be excellent. For example, if a steel plate of 1180 MPa class or lower is used as the base plate, and dissimilar materials are joined by the welding method described in Patent Documents 1 and 2, a welded joint with good TSS and CTS can be obtained.

[0009] On the other hand, in the fields of transportation equipment and the like, the demand for increased strength in steel components is growing, and it is becoming necessary to consider ways to further enhance the strength of multi-materials using ultra-high-strength steel with a carbon (C) content of 0.1% by mass or more and a tensile strength of 1180 MPa or more (approximately 1.2 GPa class or higher). However, when welding aluminum or aluminum alloy material to ultra-high-strength steel with a tensile strength of 1180 MPa or higher using the above welding method, the desired CTS may not be obtained.

[0010] The present invention has been made in view of the above problems, and aims to provide an arc spot welding method for joining dissimilar materials, which allows joining of dissimilar materials, such as a material made of pure aluminum or an aluminum alloy (hereinafter also referred to as "Al-based material") or pure magnesium or a magnesium alloy (hereinafter also referred to as "Mg-based material") and a steel material, using inexpensive arc welding equipment that is already widely available, and to provide a dissimilar material welded joint that has excellent tensile shear strength and cross tensile strength, and a dissimilar material welded joint that has excellent tensile shear strength and cross tensile strength. [Means for solving the problem]

[0011] In order to solve the above problems, the inventors of this invention conducted diligent research and found that when the steel material to be welded is an ultra-high-strength steel with a tensile strength of 1180 MPa or more, the strength of the dissimilar material welded joint can be improved by using a welding material containing 13% by mass or more of Ni.

[0012] Therefore, the above objective of the present invention is achieved by the configuration of (1) below.

[0013] (1) An arc spot welding method for joining dissimilar materials, which joins a first plate made of an Al-based material or an Mg-based material and a second plate made of ultra-high-strength steel having a tensile strength of 1180 MPa or more, The process of making a hole in the first plate, The process of stacking the first plate and the second plate, A step of inserting a steel joining auxiliary member, which has a hollow portion formed that penetrates the thickness direction of the first plate and the second plate, into a hole provided in the first plate, The process includes a step of joining the first plate and the second plate via the joining auxiliary member using a welding material containing 13% by mass or more of Ni, An arc spot welding method for joining dissimilar materials, wherein the step of joining the first plate and the second plate is to melt the second plate and the joining auxiliary member, melt the welding material, and fill the hollow portion of the joining auxiliary member with weld metal.

[0014] Furthermore, preferred embodiments of the present invention relating to an arc spot welding method for joining dissimilar materials are as follows (2) to (4).

[0015] (2) The arc spot welding method for joining dissimilar materials according to (1), wherein in the step of joining the first plate and the second plate, the weld metal is melted into the second plate to the extent that a back bead is produced. (3) The arc spot welding method for joining dissimilar materials according to (1) or (2), wherein the joining auxiliary member has a stepped outer shape having an insertion portion and a non-insertion portion, and the hollow portion is formed to penetrate the insertion portion and the non-insertion portion. (4) The step of joining the first plate and the second plate uses any one of the welding methods (a) to (e) below, and is the arc spot welding method for joining dissimilar materials according to any one of (1) to (3). (a) Gas shielded arc welding method using the welding material as an electrode type wire. (b) Non-gas arc welding method using the welding material as an electrode type wire. (c) Gas tungsten arc welding method using the welding material as a non-electrode type filler. (d) Plasma arc welding method using the welding material as a non-electrode type filler. (e) Covered arc welding method using the welding material as an electrode type welding rod.

[0016] Further, the above object of the present invention is achieved by the configuration of the following (5) related to a dissimilar material welded joint.

[0017] (5) A dissimilar material welded joint including a first plate made of an Al-based material or a Mg-based material, a second plate made of a high-tensile steel having a tensile strength of 1180 MPa or more, and a joint portion joining the first plate and the second plate, The first plate has holes facing the overlapping surface with the second plate, The joint portion A steel joint auxiliary member inserted into the holes provided in the first plate and having a hollow portion penetrating in a direction perpendicular to the overlapping surface, It includes a part of the joint auxiliary member and a part of the second plate, and has welding metal filled in the hollow portion of the joint auxiliary member. A dissimilar material welded joint joined by the arc spot welding method for joining dissimilar materials according to any one of (1) to (4).

[0018] Further, preferred embodiments of the present invention related to a dissimilar material welded joint relate to the following (6) and (7).

[0019] (6) The second plate has a heat affected zone at a position adjacent to the joint portion. The maximum hardness of the heat affected zone is 130% or more with respect to the average hardness of the region excluding the heat affected zone in the second plate. The maximum hardness of the weld metal is 50% or less with respect to the average hardness, the dissimilar material joining joint according to (5).

[0020] (7) The joining auxiliary member has a stepped outer shape having an insertion portion and a non-insertion portion, and the insertion portion is inserted into a hole provided in the first plate, the dissimilar material welding joint according to (5) or (6).

Advantages of the Invention

[0021] According to the present invention, it is possible to join dissimilar materials of an Al-based material or a Mg-based material and a steel material using inexpensive arc welding equipment that has already become widespread, and to obtain a dissimilar material welding joint excellent in both tensile shear strength and cross tensile strength, and an arc spot welding method for dissimilar material joining can be provided. Further, according to the present invention, it is possible to provide a dissimilar material welding joint excellent in both tensile shear strength and cross tensile strength.

Brief Description of the Drawings

[0022] [Figure 1A] FIG. 1A is a perspective view showing the arc spot welding method for dissimilar material joining according to an embodiment of the present invention in the order of steps, and is a view showing step S1. [Figure 1B] FIG. 1B is a perspective view showing the arc spot welding method for dissimilar material joining according to an embodiment of the present invention in the order of steps, and is a view showing step S2. [Figure 1C] FIG. 1C is a perspective view showing the arc spot welding method for dissimilar material joining according to an embodiment of the present invention in the order of steps, and is a view showing step S3. [Figure 1D] FIG. 1D is a perspective view showing the arc spot welding method for dissimilar material joining according to an embodiment of the present invention in the order of steps, and is a view showing step S4. [Figure 2] FIG. 2 is a cross-sectional view showing a dissimilar material welding joint obtained by the arc spot welding method for dissimilar material joining according to an embodiment of the present invention. [Figure 3]Figure 3 is a graph showing the relationship between the type of steel plate and the strength of the joint when using wire that does not contain nickel. [Figure 4] Figure 4 is a graph showing the cross-sectional hardness of a joint when using a wire that does not contain Ni, with the vertical axis representing Vickers hardness and the horizontal axis representing the distance from the center line L of the weld. [Figure 5] Figure 5 is a graph showing the cross-sectional hardness of a joint when steel plate C is used, with the vertical axis representing Vickers hardness and the horizontal axis representing the distance from the center line L of the weld. [Figure 6] Figure 6 is a schematic diagram illustrating the specific method of the cross tensile test. [Figure 7] Figure 7 is a schematic cross-sectional view showing a dissimilar material joint after welding. [Figure 8A] Figure 8A is a schematic cross-sectional view showing the condition after a cross-tensile test has been performed on a dissimilar material welded joint, which is made by joining steel plates A and B, each with a tensile strength of 1.0 GPa or less, using wire I that does not contain Ni. [Figure 8B] Figure 8B is a schematic cross-sectional view showing the condition after a cross-tensile test has been performed on a dissimilar material welded joint, which is made by joining steel plate C with a tensile strength of 1.5 GPa using wire I that does not contain Ni. [Figure 8C] Figure 8C is a schematic cross-sectional view showing the condition after a cross-tensile test has been performed on a dissimilar material welded joint, which is made by joining steel plate C, which has a tensile strength of 1.5 GPa, with wire III, which has a Ni content of 96.3 mass%. [Figure 9] Figure 9 shows a photograph in lieu of a drawing illustrating the cross-section of a joint obtained by welding using stainless steel welding wire, and a diagram showing the relationship between the cross-sectional position of the joint and its cross-sectional hardness. [Figure 10] Figure 10 shows a photograph in lieu of a drawing, illustrating the cross-section of a joint obtained by welding using a Ni-free welding wire for high-tensile steel, and the relationship between the cross-sectional position of the joint and its cross-sectional hardness. [Figure 11] Figure 11 is a cross-sectional view showing another example of a dissimilar material welded joint obtained by the arc spot welding method for joining dissimilar materials according to an embodiment of the present invention. [Figure 12] Figure 12 is a side view showing the size of the joining auxiliary member used in this embodiment. [Figure 13] Figure 13 is a top view showing the size of the test specimen for tensile shear testing. [Figure 14] Figure 14 is a top view showing the size of the test specimen for the cross tensile test. [Figure 15] Figure 15 is a graph showing the relationship between wire type and joint strength when steel plate C is used. [Figure 16] Figure 16 is a graph showing the relationship between the strength of the joint and the type of wire with various Ni content, using steel plate C. [Figure 17] Figure 17 is a graph showing the relationship between the type of wire and the strength of the joint when steel plate A is used. [Figure 18] Figure 18 is a graph showing the relationship between wire type and joint strength when steel plate B is used. [Modes for carrying out the invention]

[0023] Hereinafter, an arc spot welding method and a dissimilar material welding joint according to an embodiment of the present invention will be described in detail with reference to the drawings.

[0024] [Arc spot welding method for joining dissimilar materials] Figures 1A to 1D are perspective views showing the arc spot welding method for joining dissimilar materials according to an embodiment of the present invention, in order of the process steps. Figure 2 is a cross-sectional view showing a dissimilar material welded joint obtained by the arc spot welding method for joining dissimilar materials according to an embodiment of the present invention. As shown in Figures 1A to 1D and Figure 2, the arc spot welding method for joining dissimilar materials according to this embodiment is a welding method that joins an upper plate (first plate) 10 made of Al-based material or Mg-based material and a lower plate (second plate) 20 made of steel, which are superimposed on each other, by arc spot welding via a joining auxiliary member 30.

[0025] A specific arc spot welding method for joining dissimilar materials will be explained with reference to Figures 1A to 1D. First, as shown in Figure 1A, a hole 11 is made in the upper plate 10, penetrating in the thickness direction and facing the overlapping surface of the lower plate 20 (Step S1). Specific methods for the hole-making process include (A) cutting using rotary tools such as electric drills or drill presses, (B) punching using a punch, or (C) press die-cutting using a die.

[0026] Next, as shown in Figure 1B, the top plate 10 and the bottom plate 20 are placed on top of each other. (Step S2). In this embodiment, the lower plate 20 is made of ultra-high-strength steel with a tensile strength of 1180 MPa or more, for example, 1.5 GPa. Furthermore, as shown in Figure 1C, the insertion portion 31 of the joining auxiliary member 30 is inserted into the hole 11 of the upper plate 10 from the upper surface of the upper plate 10 (step S3). The joining auxiliary member 30 has a stepped outer shape, for example, an insertion portion 31 that is inserted into the hole 11 of the upper plate 10, and a flange-shaped non-insertion portion 32 that is positioned on the upper surface of the upper plate 10. The joining auxiliary member 30 also has a hollow portion 33 that penetrates the insertion portion 31 and the non-insertion portion 32. That is, the joining auxiliary member 30 is inserted into the hole of the upper plate 10 so that the direction in which the hollow portion 33 penetrates is in the thickness direction of the upper plate 10 and the lower plate 20. Note that the outer shape of the non-insertion portion 32 is not limited to a circle as shown in Figure 1C, but can be any shape. Similarly, the shape of the hollow portion 33 is not limited to a circle, but can be any shape.

[0027] Next, as shown in Figure 1D, using a welding wire (welding material) 50 containing 13% by mass or more of Ni, the lower plate 20 and the joining auxiliary member 30 are melted by arc welding, and the welding wire 50 is also melted to fill the hollow portion 33 of the joining auxiliary member 30 with weld metal 40, thereby joining the upper plate 10 and the lower plate 20 (step S4). In this way, the dissimilar material welded joint 1 shown in Figure 2 can be obtained. Note that Figure 1D shows an example of arc welding using the consumable electrode gas shielded arc welding method.

[0028] Here, we will explain the mechanism by which the joint strength changes depending on the type of steel and welding wire when an Al-based material (upper plate 10) and a steel material (lower plate 20) are joined by the method shown in steps S1 to S4 above, based on the various studies conducted by the inventors of this invention.

[0029] First, the inventors of the present invention joined the upper plate 10 and the lower plate 20 using steel plates with different tensile strengths as the lower plate 20 and an aluminum alloy plate as the upper plate 10, using a wire I that does not contain Ni, and employing the welding conditions described in steps S1 to S4 above for the other welding conditions. Then, they measured the tensile shear strength (TSS) and cross tensile strength (CTS) of the resulting joint.

[0030] Figure 3 is a graph showing the relationship between the type of steel plate and the strength of the joint when using wire that does not contain nickel. As shown in Figure 3, when steel plate A, with a tensile strength of 0.6 GPa, and steel plate B, with a tensile strength of 1.0 GPa, are joined to an aluminum alloy plate using wire I that does not contain Ni, no significant difference is observed in the TSS and CTS values. On the other hand, as shown in Figure 3, it can be seen that the CTS decreases significantly when steel plate C with a tensile strength of 1.5 GPa is used.

[0031] Therefore, the inventors of the present invention considered that the hardness at each position of the joint might be affecting the tensile strength of the joint. They joined the upper plate 10 and the lower plate 20 under the same conditions as those used to measure the strength of the joint, and measured the cross-sectional hardness of the resulting joint.

[0032] Figure 4 is a graph showing the cross-sectional hardness of a joint when using a wire that does not contain Ni, with the vertical axis representing Vickers hardness and the horizontal axis representing the distance from the center line L of the weld. In Figure 4, the graphs indicated by ○ represent the hardness of a welded joint using steel plate A with a tensile strength of 0.6 GPa as the lower plate 20, the graphs indicated by □ represent the hardness of a welded joint using steel plate B with a tensile strength of 1.0 GPa as the lower plate 20, and the graphs indicated by △ represent the hardness of a welded joint using steel plate C with a tensile strength of 1.5 GPa as the lower plate 20. Furthermore, the area from 0 mm to approximately 2 mm from the center line L of the weld represents the weld metal (the part where the weld metal is formed), the area from approximately 2 mm to approximately 4 mm represents the heat-affected zone (HAZ), and the area beyond approximately 4 mm represents the steel plate (lower plate 20).

[0033] As shown in Figure 4, the welded joint using steel plate C, which has a tensile strength of 1.5 GPa, had a HAZ Vickers hardness of approximately 650 HV 0.5, which was more than 1.5 times higher than the HAZ Vickers hardness of approximately 370 HV 0.5 for welded joints using other steel plates A and B. This is likely because steel plate C has a higher carbon content than steel plates A and B.

[0034] Next, the inventors of the present invention joined the upper plate 10 and the lower plate 20 using a steel plate C with a tensile strength of 1.5 GPa as the lower plate 20, an aluminum alloy plate as the upper plate 10, wires with different Ni content, and the welding conditions other than those described in steps S1 to S4 above, and measured the cross-sectional hardness of the resulting joint.

[0035] Figure 5 is a graph showing the cross-sectional hardness of a joint when steel plate C is used, with the vertical axis representing Vickers hardness and the horizontal axis representing the distance from the center line L of the weld. In Figure 5, the graphs indicated by △ represent the hardness of a welded joint using wire I, which does not contain Ni; the graphs indicated by ◇ represent the hardness of a welded joint using wire II, which has a Ni content of 66.0 mass% of the total wire mass; and the graphs indicated by × represent the hardness of a welded joint using wire III, which has a Ni content of 96.3 mass% of the total wire mass. Also, similar to Figure 4, the region from 0 mm to approximately 2 mm from the center line L of the weld represents the weld metal, the region from approximately 2 mm to approximately 4 mm represents the heat-affected zone (HAZ), and the region beyond approximately 4 mm represents the steel plate (bottom plate 20).

[0036] As shown in Figure 5, when using steel plate C with a tensile strength of 1.5 GPa, using wires II and III containing Ni resulted in a Vickers hardness of approximately 170 HV 0.5 for the weld metal, which showed a tendency towards softening compared to the Vickers hardness of approximately 370 HV 0.5 for the weld metal when using wire I, which does not contain Ni. Furthermore, when using steel plate C with a tensile strength of 1.5 GPa and wires II and III containing Ni, the same CTS (Critical Tension Score) was obtained as when using steel plates A and B with a tensile strength of 1.0 GPa or less and wire I without Ni.

[0037] Based on the above results, the inventors of this application inferred the fracture mechanism in the cross tensile test (CTS). Figure 6 is a schematic diagram illustrating the specific method of the cross tensile test. As shown in Figure 6, an upper plate 10 and a lower plate 20 were prepared, with one side being longer than the other in a plan view. The upper plate 10 and the lower plate 20 were then placed on top of each other so that they formed a cross in a plan view. In the study test, welding was performed using joining auxiliary members in the manner shown in Figures 1(C) and 1(D), using lower plates 20 with different tensile strengths and wires with different Ni content. After that, both longitudinal ends of the upper plate 10 were pulled in the direction indicated by arrow A10, and both longitudinal ends of the lower plate 20 were pulled in the direction indicated by arrow A20, and the maximum tensile load until the test specimen fractured was measured.

[0038] Figure 7 is a schematic cross-sectional view showing a dissimilar metal welded joint after welding. Figure 8A is a schematic cross-sectional view showing the result after a cross-tensile test on a dissimilar metal welded joint made using steel plates A and B with a tensile strength of 1.0 GPa or less, joined with wire I that does not contain Ni. Furthermore, Figure 8B is a schematic cross-sectional view showing the result after a cross-tensile test on a dissimilar metal welded joint made using steel plate C with a tensile strength of 1.5 GPa, joined with wire I that does not contain Ni. Figure 8C is a schematic cross-sectional view showing the result after a cross-tensile test on a dissimilar metal welded joint made using steel plate C with a tensile strength of 1.5 GPa, joined with wire III that has a Ni content of 96.3 mass%. Figures 8A to 8C show only the portion enclosed by the dashed line in the cross-sectional view of the dissimilar material welded joint 1 shown in Figure 7.

[0039] As shown in Figure 7, the upper plate 10 and the lower plate 20 are joined by a joint 46, which comprises a joining auxiliary member 30 and weld metal 40. Regardless of the type of wire and steel material, in the resulting dissimilar material welded joint 1, a HAZ 45 is formed in the region adjacent to the weld metal 40 on the lower plate (steel plate) 20. The weld metal 40 also has an interface (bond) 41 between it and the HAZ 45. As shown in Figure 8A, the welded joint using steel plates A and B with a tensile strength of 1.0 GPa or less and wire I that does not contain Ni, showed no significant hardening of HAZ45, and the cross tensile test revealed that the lower plate 20 itself deformed in the direction indicated by the arrow. This indicates a high CTS. On the other hand, in a welded joint using steel plate C with a tensile strength of 1.5 GPa and wire I that does not contain Ni, the HAZ hardens significantly due to the high carbon content in the lower plate (steel plate) 20. Therefore, as shown in Figure 8B, stress concentrates at the interface (bond) 41 between the weld metal 40 and the hardened HAZ 45. As a result, it is thought that brittle fracture occurred at the interface 41 in the cross tensile test, leading to a decrease in CTS.

[0040] Furthermore, in a welded joint using steel plate C with a tensile strength of 1.5 GPa and wire III with a Ni content of 96.3 mass%, the HAZ hardens significantly, similar to the case shown in Figure 8B. However, because wire III with a Ni content of 96.3 mass% is used, Ni is incorporated into the weld metal 40, and the structure of the weld metal 40 becomes an austenite crystal structure with high elongation, resulting in softening. Therefore, when the upper plate 10 and the lower plate 20 are joined by the arc spot welding method for dissimilar material joining according to this embodiment, as shown in Figure 8C, the weld metal 40 deforms in the direction indicated by the arrow in the cross tensile test, which is thought to suppress brittle fracture and improve the CTS.

[0041] Furthermore, the inventors of this invention conducted further investigations into welding materials that can produce a weld metal 40 with a soft yet highly ductile austenite crystal structure and are also lower in cost. Specifically, they used a relatively inexpensive stainless steel welding wire (JIS Z 3321 YS310) as the welding material that produces an austenite crystal structure in the weld metal, performed arc spot welding, and measured the cross-sectional hardness of the joint. For comparison, they also performed arc spot welding using a high-tensile steel welding wire (JIS Z 3317 G52A-1CM3) that does not contain Ni, and similarly measured the cross-sectional hardness of the joint. The Ni content of the stainless steel welding wires used was 20.0 to 22.5% by mass.

[0042] Figure 9 shows a photograph (in lieu of a drawing) of a cross-section of a joint obtained by welding with stainless steel welding wire, and a diagram illustrating the relationship between the cross-sectional position and the cross-sectional hardness of the joint. The nickel content of the stainless steel welding wire (JIS Z3321 YS310) used to weld the joint shown in Figure 9 is 21.21% by mass. Figure 10 shows a photograph (in lieu of a drawing) of a cross-section of a joint obtained by welding with nickel-free high-tensile steel welding wire, and a diagram illustrating the relationship between the cross-sectional position and the cross-sectional hardness of the joint. In Figures 9 and 10, the horizontal axis of the graph represents the distance (mm) from the center line L of the weld and corresponds to the position in the photograph (in lieu of a drawing) showing the cross-section of the joint.

[0043] In Figures 9 and 10, the circled areas in the graphs represent the weld metal region. As shown in Figures 9 and 10, the maximum hardness of the weld metal obtained using stainless steel welding wire (approximately 210 Hv) was significantly lower than that of the weld metal obtained using high-tensile steel welding wire (approximately 380 Hv). From this result, it can be concluded that even when stainless steel welding wire is used, the weld metal has a soft, highly ductile austenite crystal structure. Therefore, the bending deformation capacity of the weld metal is increased, and the cross-tensile strength can be improved.

[0044] Next, the upper plate 10, lower plate 20, welding material, and joining auxiliary member 30 used in the welding method according to this embodiment, as well as a specific welding method, will be described in detail below.

[0045] <Lower plate (steel plate)> As described above, the lower plate 20 to be welded in this embodiment is made of steel with a tensile strength of 1180 MPa or more. Preferably, such steel has a carbon content of 0.2% by mass or more and 0.5% by mass or less relative to the total mass of the steel. As shown in Figure 8A, steel materials with a tensile strength of less than 1180 MPa can achieve high strength regardless of the type of wire used, and therefore do not require a specific welding material. For this reason, they are not used for welding in this embodiment.

[0046] <Top plate (a plate made of Al-based or Mg-based material)> In this embodiment, since the purpose is to join dissimilar materials with the lower plate 20, the upper plate 10 is made of a different material from the lower plate 20, specifically an Al-based material or a Mg-based material. In this embodiment, the composition of the upper plate 10 is not particularly limited. As mentioned above, Al-based material means pure aluminum or an aluminum alloy, and Mg-based material means pure magnesium or a magnesium alloy.

[0047] <Welding materials> In this embodiment, any welding wire commonly used can be used as a welding material, as long as it contains 13% by mass or more Ni. Specifically, the Ni content relative to the total mass of the welding material should be 13% by mass or more, preferably 21% by mass or more, and more preferably 96% by mass or more when considering only the strength of the joint. Furthermore, there is no particular upper limit for the Ni content, but it is preferably 98% by mass or less, and more preferably 22.5% by mass or less when considering the cost of the welding material, etc. Specifically, the following can be used: stainless steel filler materials YS310 and YS309 as described in JIS Z 3321; nickel and nickel alloy coated arc welding electrodes as described in JIS Z 3224:2010; flux-cored wires for nickel and nickel alloy arc welding as described in JIS Z 3335:2014; and filler rods and solid wires for nickel and nickel alloy welding as described in JIS Z 3334:2011.

[0048] In addition to Ni, other components in the welding material include C, Si, Mn, Cr, Ti, Al, Fe, Mo, Ca, etc. The composition of the welding material preferably has a total content of Cr, Ni, and Fe of 85% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more.

[0049] Thus, the arc spot welding method for joining dissimilar materials according to this embodiment is used for joining high-tensile steel of 1180 MPa or higher to plates made of Al-based or Mg-based materials, and since it uses a wire containing 13% by mass or more of Ni, it is possible to obtain a welded joint with excellent TSS and CTS.

[0050] <Joining support member> Furthermore, the steel joining auxiliary member 30 used in the above-described arc spot welding method acts as a protective barrier to prevent melting of, for example, Al-based or Mg-based materials. In the upper plate 10 made of Al-based or Mg-based material, the areas most likely to melt during welding are the inner surface of the hole 11 and the surface surrounding the inner surface. By covering these surfaces with the joining auxiliary member 30, it is possible to prevent the heat of the arc welding from being directly transferred to the upper plate 10 and to prevent it from mixing with the components of the welding material to form intermetallic compounds (IMC). In other words, if the penetration range of the arc welding is limited to the joining auxiliary member 30 and the lower plate 20, the dilution of the components of the upper plate 10 (Al-based or Mg-based material) into the weld metal 40 becomes zero, and the formation of IMC is completely prevented, thus achieving high joint strength.

[0051] However, in this embodiment, it is not necessary for IMC to occur to be zero, and some IMC formation is acceptable. Even if IMC is formed on the inner surface of the hole 11, if the weld metal 40 has ductility and appropriate strength, the weld metal 40 acts as a resistance to external stress in the plate width direction (two-dimensional direction), so the influence of the IMC layer formed around the weld metal 40 is small. Furthermore, although IMC is brittle, even if tensile stress acts on the structure, compressive and tensile stresses act simultaneously at the joint, and since IMC maintains sufficient strength against compressive force, the formation of the IMC layer does not lead to fracture propagation. Therefore, the insertion portion 31 of the joining auxiliary member 30 does not necessarily have to be the same thickness as the upper plate 10.

[0052] Furthermore, in this embodiment, as shown in Figure 2, it is preferable that the joining auxiliary member 30 has a stepped outer shape with an insertion portion 31 and a non-insertion portion 32. When the joining auxiliary member 30 has the above shape, it can maintain its strength even when an external force is applied in the thickness direction of the plate, such as in a cross tensile test. Furthermore, as the steel material constituting the joining auxiliary member 30, for example, mild steel, carbon steel, and stainless steel can be used.

[0053] <Specific welding methods> The step of joining the upper plate 10 and the lower plate 20 by arc welding in step S4 is necessary, as described above, to melt the lower plate 20 and the joining auxiliary member 30, and to fill the hollow portion 33 provided in the joining auxiliary member 30 with weld metal 40. Furthermore, in order to obtain good CTS, it is necessary to form a weld metal containing Ni. Therefore, in arc welding, it is essential to insert a welding wire (welding material) 50 made of a material containing 13 mass% or more of Ni as a welding material that serves as a filler. Therefore, in this embodiment, for example, the welding methods (a) to (e) below can be used.

[0054] "(a) A gas shielded arc welding method using a welding material containing 13% by mass or more of Ni as a consumable electrode wire." Gas-shielded arc welding, commonly known as MAG or MIG, is a welding method that uses solid wire or flux-cored wire as both filler and arc-generating electrode, and forms a sound weld by shielding the weld area from the atmosphere with a shielding gas such as CO2, Ar, or He.

[0055] "(b) A non-gas arc welding method using a welding material containing 13% by mass or more of Ni as a consumable electrode wire." Non-gas arc welding, also known as self-shielded arc welding, is a welding method that uses a special flux-cored wire as both filler and arc-generating electrode, while eliminating the need for shielding gas, and forming a sound weld.

[0056] "(c) Gas tungsten arc welding method using a welding material containing 13% by mass or more of Ni as a non-consumable electrode filler." Gas tungsten arc welding is a type of gas shielded arc welding, but it is a non-consumable electrode type and is commonly called TIG (Tinted Ion Grazing). An inert gas of Ar or He is used as the shielding gas. An arc is generated between the tungsten electrode and the base metal, and the filler wire is fed into the arc from the side. Generally, filler wires are not energized, but there are also hot-wire TIG methods that energize them to increase the melting rate. In this case, no arc is generated in the filler wire.

[0057] "(d) Plasma arc welding method using a welding material containing 13% by mass or more of Ni as a non-consumable electrode filler." Plasma arc welding is based on the same principle as TIG welding, but it is a welding method that tightens the arc and increases the arc force by using a dual gas system and high speed.

[0058] "(e) Covered arc welding method using a welding material containing 13% by mass or more of Ni as a consumable electrode welding rod." Shielded metal arc welding is an arc welding method that uses a shielded metal arc welding rod, which has flux coated on a metal core wire, as a filler, and does not require shielding gas.

[0059] In this embodiment, as described above, a welding material containing 13% by mass or more of Ni is used to fill the hollow portion 33 of the joining auxiliary member 30 with weld metal. Generally, however, it is not necessary to move the target position of the wire or welding rod, and the welding can be completed by cutting the arc after an appropriate feeding time. However, if the area of ​​the hollow portion 33 is large, the target position of the wire or welding rod may be moved in a circular motion within the hollow portion 33.

[0060] The weld metal 40 fills the hollow portion 33 of the joining auxiliary member 30, and it is also desirable to form an excess weld on the surface of the joining auxiliary member 30 (in Figure 2, the portion of the weld metal 40 that protrudes above the joining auxiliary member 30). By forming this excess weld, high strength can be obtained, especially against external stress in the thickness direction (three-dimensional direction).

[0061] Furthermore, regarding the penetration on the opposite side from the excess weld, as shown in Figure 2, it is preferable to penetrate the weld metal 40 to the point where a back bead is produced that exceeds the thickness of the lower plate 20. By penetrating the weld metal 40 to the point where a back bead is produced in the lower plate 20, the upper plate 10 and the lower plate 20 can be joined with high strength. Furthermore, since the strength of the joint interface can be estimated by checking for the occurrence of a back bead during welding, it is preferable to allow the weld to penetrate to the point where a back bead is produced. However, in this embodiment, as shown in Figure 2, it is not necessarily required to melt the material to the point where a back bead appears; as shown in Figure 11, it is sufficient if the lower plate 20 is moderately melted.

[0062] While the thickness of the upper plate 10 and lower plate 20 does not necessarily need to be limited, considering the construction efficiency and the shape as an overlap weld, it is desirable that the thickness of the upper plate 10 be 4.0 mm or less. On the other hand, considering the heat input of arc welding, if the plate thickness is too thin, it will melt through during welding, making welding difficult. Therefore, it is desirable that both the upper plate 10 and the lower plate 20 be 0.5 mm or thicker.

[0063] [Welded joints of dissimilar materials] The dissimilar material welded joint according to this embodiment is manufactured by the above-described arc spot welding method for joining dissimilar materials. That is, the dissimilar material welded joint according to this embodiment comprises a first plate made of an Al-based material or an Mg-based material, a second plate made of ultra-high-strength steel having a tensile strength of 1180 MPa or more, and a joint that joins the first plate and the second plate. Here, "joint" refers to the part involved in joining the first plate and the second plate, and may also be called the "welded part." As shown in Figure 2, the upper plate (first plate) 10 has a hole 11 facing the overlapping surface with the lower plate (second plate) 20. The joint also includes a joining auxiliary member 30 and weld metal 40. The joining support member 30 has a hollow portion that penetrates in a direction perpendicular to the overlapping surface and is inserted into a hole 11 provided in the upper plate 10. Furthermore, the welding metal 40 includes a part of the joining support member 30 and a part of the lower plate 20 and fills the hollow portion of the joining support member 30. Furthermore, the weld metal 40 contains Ni, as well as the components of the steel constituting the joining auxiliary member 30 and the components of the ultra-high-strength steel constituting the lower plate 20.

[0064] Furthermore, in this embodiment, as shown in Figure 7, a heat-affected zone 45 is formed in the lower plate 20 at a position adjacent to the joint 46. Since the lower plate 20 targeted by this invention is made of ultra-high-strength steel with a tensile strength of 1180 MPa or more, the hardness of the heat-affected zone 45 is higher than the hardness of the area of ​​the lower plate 20 excluding the heat-affected zone 45. That is, if the maximum hardness of the heat-affected zone 45 is 130% or more of the average hardness of the area of ​​the lower plate 20 excluding the heat-affected zone 45 (the base material portion of the lower plate 20), it can be determined that the lower plate 20 is made of ultra-high-strength steel with a tensile strength of 1180 MPa or more. In other words, in this embodiment, a significant effect can be obtained when the maximum hardness of the heat-affected zone 45 is 130% or more of the average hardness of the base material portion of the lower plate 20.

[0065] Furthermore, in this embodiment, welding is performed using a welding material containing 13% by mass or more of Ni, so the weld metal 40 softens, and its maximum hardness becomes lower than the average hardness of the area of ​​the lower plate 20 excluding the heat-affected zone 45. When the maximum hardness of the weld metal 40 is 50% or less of the average hardness, the weld metal 40 softens sufficiently, resulting in a homogeneous soft structure without hardness unevenness, which further improves the CTS. Therefore, it is preferable that the maximum hardness of the weld metal 40 is 50% or less of the average hardness.

[0066] The maximum hardness of the weld metal 40 is the value obtained by measuring the Vickers hardness at a distance of 0.3 mm in accordance with JIS Z 2244:2009, using a point 0.7 mm below the upper surface of the lower plate 20 (the surface in contact with the upper plate 10) in the thickness direction as the reference point, and reading the maximum hardness of the weld metal 40. The maximum hardness of the heat-affected zone 45 is the value obtained by measuring the Vickers hardness at a distance of 0.3 mm in the same manner as the measurement method for the maximum hardness of the weld metal 40, and reading the maximum hardness of the heat-affected zone 45. Furthermore, the average hardness of the area of ​​the lower plate 20 excluding the heat-affected zone 45 is the value obtained by measuring the Vickers hardness at a distance of 0.3 mm in the same manner as the measurement method for the maximum hardness of the weld metal 40, starting from a point 6 mm from the center line L of the weld metal 40 in the thickness direction, and measuring up to a point 8.1 mm, and averaging the total measured values ​​of 8 points. Furthermore, the location where the maximum hardness of the weld metal, the maximum hardness of the heat-affected zone, and the average hardness of the area of ​​the lower plate excluding the heat-affected zone are measured can be any location where the maximum hardness or average hardness of the required location can be correctly measured. For example, the measurement may be taken along a direction perpendicular to the plate thickness, with the center of the plate thickness of the lower plate 20 as the reference point.

[0067] Furthermore, the maximum hardness (%) of the heat-affected zone relative to the average hardness of the area of ​​the lower plate excluding the heat-affected zone (average hardness of the lower plate) can be calculated using the following formula. ((Maximum hardness of the heat-affected zone) / (Average hardness of the base plate)) × 100

[0068] Furthermore, the maximum hardness (%) of the weld metal relative to the average hardness of the lower plate (average hardness of the lower plate) can be calculated using the following formula. ((Maximum hardness of weld metal) / (Average hardness of the base plate)) × 100

[0069] In a dissimilar metal welded joint configured in this way, since the weld metal 40 contains Ni, which is the main component of the welding material used in the welding method according to this embodiment, the weld metal 40 becomes softer, brittle fracture is suppressed, and excellent TSS and CTS can be obtained.

[0070] Furthermore, the present invention is not limited to the embodiments described above, and can be modified, improved, and so on as appropriate. [Examples]

[0071] <1. First Example> The following describes in detail an example of the arc spot welding method for joining dissimilar materials according to this embodiment, in comparison with a comparative example. In the first embodiment, arc spot welding was performed using wire I, which does not contain Ni, wire II, which has a Ni content of 66.0 mass%, and wire III, which has a Ni content of 96.3 mass%, and the strength was measured by tensile shear tests and cross tensile tests.

[0072] First, in step S1, a steel joining auxiliary member 30 was fabricated, and an aluminum alloy plate (A6022-T4) with a thickness of 2.0 mm was prepared as the upper plate (first plate) 10, and a 1.5 GPa class ultra-high tensile strength steel plate (steel plate C) with a thickness of 1.4 mm and a carbon (C) content of 0.40 mass% was prepared as the lower plate (second plate) 20, to prepare test specimens for tensile shear testing and test specimens for cross tensile testing.

[0073] Figure 12 is a side view showing the size of the joining auxiliary member used in this embodiment. Figure 13 is a top view showing the size of the test material for tensile shear testing. Figure 14 is a top view showing the size of the test material for cross tensile testing.

[0074] As shown in Figure 12, the joining auxiliary member 30 is made of mild steel, with the insertion portion 31 having a diameter of 6.9 mm and a height of 1.9 mm, the non-insertion portion 32 having a diameter of 11 mm and a height of 1.6 mm, and the hollow portion 33 having a diameter of 4.9 mm. Furthermore, as shown in Figure 13, the test specimen for tensile shear testing had a length of 125 mm in the longitudinal direction and a width of 40 mm, and the hole 11 was formed so that the center of the hole was located 20 mm from one end face in the longitudinal direction and from the end face in the width direction. Furthermore, as shown in Figure 14, the test specimen for the cross tensile test had a longitudinal length of 150 mm and a width of 50 mm. In addition, holes 11 were formed so that their centers were located 75 mm from the longitudinal end face and 25 mm from the width end face, and bolt holes 15 were formed in two locations so that their centers were located 25 mm from both longitudinal and width end faces. The lower plates 20 of each test specimen were the same size as the upper plates 10, but no holes 11 were formed in them.

[0075] Next, in step S2, as shown in Figure 1B, the upper plate 10 and the lower plate 20 were stacked on top of each other, and then in step S3, as shown in Figure 1C, the joining auxiliary member 30 was inserted into the hole 11 from the top surface of the upper plate 10. Subsequently, in step S4, as shown in Figures 1D and 2, arc welding was performed at a fixed point for a certain period of time using MAG welding (Metal Active Gas Welding). This melted the lower plate 20 and the joining auxiliary member 30, as well as the welding wire 50, filling the hollow portion 33 of the joining auxiliary member 30 with weld metal 40, and obtaining a dissimilar material welded joint 1 in which the upper plate 10 and the lower plate 20 were joined. Detailed welding conditions are shown in Table 1 below, and the chemical composition of the welding wire 50 used is shown in Table 2 below.

[0076] Subsequently, a tensile test was performed on dissimilar material welded joint 1 in accordance with JIS Z3136 "Specimen dimensions and test method for shear test of resistance spot and projection welded joints" and JIS Z3137 "Cross tensile test of resistance spot and projection welded joints". The measurement results of the tensile test are shown in Table 3 and Figure 15 below. In Table 3 and Figure 15 below, TSS represents the tensile strength measured by the tensile shear test, and CTS represents the tensile strength measured by the cross tensile test.

[0077] [Table 1]

[0078] [Table 2]

[0079] [Table 3]

[0080] As shown in Table 3 and Figure 12 above, Invention Examples No. 1 and 2 were joined using the dissimilar material joining arc spot welding method according to the present invention, using wire containing 13% by mass or more of Ni (especially wire with a Ni content of more than 50% by mass). Therefore, inexpensive arc welding equipment that is already widely available could be used, and dissimilar material welded joints with excellent TSS and CTS were obtained.

[0081] On the other hand, Comparative Example No. 1, which used wire I that does not contain Ni and was joined using the same welding method as the inventive example, showed a TSS value equivalent to that of the inventive example, but a significant decrease in CTS.

[0082] <2. Second Example> Next, as a second embodiment, arc spot welding was performed using various wires (W1 to W6) with different Ni content, and the strength was measured by tensile shear tests and cross tensile tests. The hardness of the weld metal, heat-affected zone, and lower plate was also compared. The specific welding method and test method are shown below.

[0083] The method for preparing the test specimens for tensile shear testing and cross-tensile testing, as well as the size of the test specimens, were the same as in the first embodiment described above, and three test specimens were prepared for each test. Detailed welding conditions are shown in Table 4 below, and the chemical composition of the welding wire used is shown in Table 5 below. However, in Table 5 below, "-" indicates that it was below the detection limit, and "0.00" indicates that it was less than 0.005.

[0084] Subsequently, as in the first embodiment, tensile tests were performed on each of the obtained dissimilar-material welded joint specimens in accordance with JIS Z3136 "Specimen dimensions and test method for shear tests of resistance spot and projection welded joints" and JIS Z3137 "Cross tensile test of resistance spot and projection welded joints". The measurement results of the tensile tests are shown in Table 6 and Figure 16 below. In Table 6 and Figure 16, TSS represents the tensile strength measured by the tensile shear test, and CTS represents the tensile strength measured by the cross tensile test. The TSS and CTS values ​​shown in Table 6 and Figure 16 are the average values ​​of the three specimens, respectively.

[0085] Furthermore, the maximum hardness of the weld metal and the maximum hardness of the heat-affected zone were measured, and the hardness ratio to the average hardness of the base plate was calculated. First, in the cross-section of the obtained joint (test specimen), Vickers hardness was measured at 0.3 mm intervals in accordance with JIS Z 2244:2009, using a point 0.7 mm below the upper surface of the lower plate (the surface in contact with the upper plate) in the plate thickness direction as the reference point, along a direction perpendicular to the plate thickness. The maximum hardness in the weld metal was then read and defined as the maximum hardness of the weld metal. In addition, Vickers hardness was measured at 0.3 mm intervals using the same method as for measuring the maximum hardness of the weld metal, and the maximum hardness in the heat-affected zone was read and defined as the maximum hardness of the heat-affected zone. Furthermore, Vickers hardness was measured at 0.3 mm intervals from a point 6 mm from the centerline of the weld metal along a direction perpendicular to the plate thickness to a point 8.1 mm from there, and the average of the total 8 measured values ​​was defined as the average hardness of the lower plate (average hardness of the area of ​​the lower plate excluding the heat-affected zone).

[0086] The maximum hardness (%) of the weld metal relative to the average hardness of the lower plate in Table 6 was calculated using the following formula. ((Maximum hardness of weld metal) / (Average hardness of the base plate)) × 100

[0087] Furthermore, the maximum hardness (%) of the heat-affected zone relative to the average hardness of the bottom plate in Table 6 was calculated using the following formula. ((Maximum hardness of the heat-affected zone) / (Average hardness of the base plate)) × 100

[0088] [Table 4]

[0089] [Table 5]

[0090] [Table 6]

[0091] As shown in Table 6 and Figure 16 above, Invention Examples No. 3 to 6 were joined using the arc spot welding method for dissimilar material joining according to the present invention, with wire (welding material) W3 to W6 containing 13% by mass or more of Ni. Therefore, inexpensive arc welding equipment could be used, and dissimilar material welded joints with excellent TSS and CTS were obtained. In particular, Invention Examples No. 3 and 4 used SUS wire with a lower Ni content compared to Invention Examples No. 5 and 6, thus reducing the cost related to the wire.

[0092] Furthermore, since Invention Examples No. 3 to 6 and Comparative Examples No. 1 and 2 all used 1500 MPa class medium-high carbon high-tensile steel plates as the base plates, the maximum hardness of the heat-affected zone was 650 HV or higher, and the maximum hardness of the heat-affected zone relative to the average hardness of the base plate was 130% or higher. In addition, since Invention Examples No. 3 to 6 used wire (welding material) containing 13 mass% or more of Ni, the maximum hardness of the weld metal was 50% or less of the average hardness of the base plate, which was lower compared to Comparative Examples No. 2 and 3. Therefore, it is considered that the TSS and CTS of Invention Examples No. 3 to 6 showed superior results compared to the comparative examples.

[0093] On the other hand, Comparative Example No. 2, which was joined using wire W1 with a Ni content of 0.01 mass% and the same welding method as the inventive example, showed a TSS of the same value as the inventive example, but a significant decrease in CTS. Furthermore, Comparative Example No. 3, which was joined using wire W2 and the same welding method as the inventive example, had a higher Ni content compared to the case using wire W1, but the microstructure of the weld metal became non-uniform, and it is thought that both TSS and CTS decreased compared to Comparative Example No. 2.

[0094] <3.Reference example> Next, as a reference example, steel plate A, having a tensile strength of approximately 0.6 GPa and a carbon (C) content of 0.06 mass%, and steel plate B, having a tensile strength of approximately 1.0 GPa and a carbon (C) content of 0.09 mass%, were used as the lower plate 20. Using wires I and III from the first embodiment, the upper plate 10 and the lower plate 20 were joined in the same manner as in the first embodiment, and the TSS and CTS were measured. In this reference example, GA (Galvannealed Steel) 590DP (Dual Phase) was used as steel plate A, and GA980DP was used as steel plate B.

[0095] The chemical compositions of wire I and wire III used are shown in Table 2 above. Figure 17 shows a graph illustrating the relationship between wire type and joint strength when steel plate A is used, and Figure 18 shows a graph illustrating the relationship between wire type and joint strength when steel plate B is used.

[0096] As shown in Figure 17, when using steel plate A of 0.6 GPa class, there was no significant difference between wire I, which does not contain Ni, and wire III, which contains 96.3 mass% Ni, and both were able to obtain good TSS and CTS. Furthermore, as shown in Figure 18, even when using 1.0 GPa class steel plate B, there was no significant difference between wire I and wire III, and good TSS and CTS were obtained in both cases.

[0097] These findings demonstrate that when using steel with a tensile strength of less than 1180 MPa as the base plate, good tensile strength can be obtained regardless of the Ni content of the welding material. In other words, when using a steel plate with a tensile strength of less than 1180 MPa as the base plate, it is not necessary to use a wire containing a predetermined amount of Ni as the welding material. However, when a steel plate with a tensile strength of 1180 MPa or more is used as the base plate, welding with a wire that does not contain Ni results in a significant decrease in CTS. Therefore, the arc spot welding method for joining dissimilar materials according to the present invention, which can prevent a decrease in CTS, has been shown to be extremely effective. [Explanation of Symbols]

[0098] 1. Welded joints of dissimilar materials 10. Top board (first board) 11 holes 20. Bottom board (second board) 30 Joining support member 31 Insertion part 32 Non-insertion portion 33 Hollow part 40 Weld metal 41 Interface (bond) 45 HAZ 50 Welding wire (welding material)

Claims

1. An arc spot welding method for joining dissimilar materials, comprising joining a first plate made of an Al-based material or an Mg-based material and a second plate made of ultra-high-strength steel having a tensile strength of 1180 MPa or more, The process of making a hole in the first plate, The process of stacking the first plate and the second plate, A step of inserting a steel joining auxiliary member, which has a hollow portion formed that penetrates the thickness direction of the first plate and the second plate, into a hole provided in the first plate, The process includes a step of joining the first plate and the second plate via the joining auxiliary member using a welding material containing 21% by mass or more of Ni, An arc spot welding method for joining dissimilar materials, wherein the step of joining the first plate and the second plate is to melt the second plate and the joining auxiliary member, melt the welding material, and fill the hollow portion of the joining auxiliary member with weld metal.

2. The arc spot welding method for joining dissimilar materials according to claim 1, wherein in the step of joining the first plate and the second plate, the weld metal is melted into the second plate to the extent that a back bead is produced.

3. The arc spot welding method for joining dissimilar materials according to claim 1 or 2, wherein the joining auxiliary member has a stepped outer shape having an insertion portion and a non-insertion portion, and the hollow portion is formed to penetrate the insertion portion and the non-insertion portion.

4. The arc spot welding method for joining dissimilar materials according to any one of claims 1 to 3, wherein the step of joining the first plate and the second plate is performed using any of the welding methods (a) to (e) below. (a) A gas shielded arc welding method using the welding material as a consumable electrode wire. (b) A non-gas arc welding method using the welding material as a consumable electrode wire. (c) A gas tungsten arc welding method using the welding material as a non-consumable electrode filler. (d) A plasma arc welding method using the welding material as a non-consumable electrode filler. (e) A covered arc welding method using the welding material as a consumable electrode welding rod.

5. The arc spot welding method for joining dissimilar materials according to any one of claims 1 to 4, wherein the second plate is made of ultra-high-strength steel having a tensile strength of 1.5 GPa or more.

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