Method for spot welding of dissimilar metals and welded rivet

Abstract

A dissimilar metal joint welding method is provided for welding a first metal with a melting point higher than 1200 DEG C and a second metal with a melting point lower than 800 DEG C, and provides a welding rivet with a melting point higher than 1200 DEG C, which includes a shaft portion, a cap and a top wall, wherein the length of the shaft portion is shorter than the edge of the cap, and the welding method includes a preheating stage and a welding stage, in which the cap is at least partially pressed into the surface of the second metal to form a mechanical lock, and then the welding stage is performed to directly weld the shaft portion with the first metal plate. The method can improve the strength of the dissimilar metal spot welding joint, and improve the surface quality and the sealing of the welding spot. The invention also provides a welding rivet.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of welding, and particularly relates to a dissimilar metal joint spot welding method and a welding rivet. BACKGROUND

[0002] When spot welding is performed between dissimilar metals, especially between light metals and steel, a large amount of brittle intermetallic compounds is easily formed in the welding area, thereby causing the mechanical performance of the joint to deteriorate. Improving the performance of dissimilar joints by setting various auxiliary welding workpieces has always been a popular topic in the industry. Some welding processes use steel rivets as welding auxiliary tools. During welding, the rivets are stacked on the steel plate and the aluminum plate to be welded, and the aluminum is discharged to directly weld the rivet and the steel plate together to improve the strength of the joint.

[0003] However, this welding method still cannot avoid the formation of some intermetallic compounds around the welding spot. When the welded joint bears a load, all the stresses are concentrated on the welding spot, and these brittle intermetallic compounds are prone to crack, thereby causing the strength and durability of the joint to decrease. Therefore, it is of high practical value to propose a welding process that can improve the stress concentration state of the welding spot. SUMMARY

[0004] The present application aims to provide a dissimilar metal joint spot welding method to improve the mechanical performance of the spot welded joint. The present application also provides a welding rivet.

[0005] According to one aspect of an embodiment of the present application, a dissimilar metal joint spot welding method is provided, which provides a welding rivet to perform the overlay welding of a first metal plate and a second metal plate, the melting point of the welding rivet is higher than 1200℃, the melting point of the second metal plate is lower than 800℃, the welding rivet comprises a top wall, a shaft part arranged at the center of the top wall, and a cap arranged around the top wall, the first metal plate, the second metal plate, and the welding rivet are arranged in a stacked manner, and the second metal plate is arranged between the first metal plate and the welding rivet, wherein the shaft part of the welding rivet is arranged inside the semi-open space formed by the cap, and the resistance welding is performed at the center of the welding spot where the shaft part is arranged. The welding process comprises a preheating process and a welding process. Specifically, in the preheating process, a preheating pressure and a preheating current are applied to the welding spot to soften part of the second metal plate, and the edges of the cap are at least partially embedded in the second metal plate to generate mechanical locking. In the welding process, a welding pressure and a welding current are applied to the welding spot, and the shaft part discharges the melted second metal plate and directly welds the second metal plate and the first metal plate together.

[0006] In the spot welding process, the welding rivet and the first metal plate are deformed due to the extrusion of the welding electrode, and the rivet is usually locally warped due to uneven force in the central and edge regions during the deformation, which may cause the risk of not providing sufficient mechanical locking effect for the welded joint. The method adopted by the present application first locally softens the second metal plate in the preheating step by using a preheating current, and then makes the welding rivet partially embedded in the second metal plate when the deformation does not occur or the deformation amount is small. When the rivet is deformed by increasing the welding current and pressure in the welding step, the shaft part flows away the main welding current, so that the area where the cap is in contact with the second metal plate maintains a low temperature and a high hardness, and the second metal plate provides constraint to the embedded cap part to avoid its warping deformation. After the welding is completed, the complete embedded state of the metal cap can provide additional mechanical locking effect for the welding spot. When bearing load, especially positive tensile load, the cap compresses the second metal plate to disperse the load of the welding spot by its own structural strength, which helps to improve the mechanical properties. At the same time, since the cap itself has a certain elasticity, it can absorb the deformation of the welding spot when bearing stress within a small load range, to a certain extent, inhibit the initiation of intermetallic compound internal cracks, and prolong the fatigue life of the welded joint. At the same time, the cap also plays a role in improving the sealing performance of the welding spot.

[0007] Further, the duration T and the current intensity I of the preheating current satisfy: 30ms≤T≤350ms; when t≤1mm, 2kA≤I≤7kA; when 1mm

[0008] Further, the welding process includes a spatter stage and a fusion stage. In the spatter stage, a spatter current is applied to the welding spot to melt the second metal plate and make it in the form of spatter to separate from the area between the shaft part and the first metal plate and be contained in the cap. In the fusion stage, a fusion current is applied to the welding spot, and the contact area between the shaft part and the first metal plate is melted to form a welding nugget. The second metal plate between the shaft part and the first metal plate is discharged by the spatter process, which can reduce the contact time of the dissimilar metals at high temperature and inhibit the generation of intermetallic compounds. At the same time, the cap can contain and shield the spattered second metal, avoiding the adverse effects of the solidified structure of the spattered second metal on the surface quality of the welded workpiece. At the same time, the second metal is filled in the cap, which further improves the sealing performance of the welding spot.

[0009] Further, the ratio of the distance between the top wall and the end of the shaft to the distance between the top wall and the edge of the cap is 0.5-0.75, and the cap is partially embedded into the second metal sheet until the end of the shaft abuts against the surface of the second metal sheet in the preheating step. The cap is partially embedded into the surface of the second metal sheet until the end of the shaft abuts against the surface of the second metal sheet in the preheating step. Since the shaft has contacted the surface of the second metal sheet at this time, the welding current is mainly conducted through the shaft when the welding step is performed, so that the welding is quickly and effectively achieved.

[0010] Further, the ratio of the length of the shaft to the thickness of the second metal sheet is 0.5-1.5. In order to maintain the mechanical locking function of the rivet after welding deformation, the length of the shaft should match the thickness of the second metal sheet. When the length of the shaft is too short, the rivet is severely deformed during welding, and the edge region of the cap is prone to obvious warping, affecting the mechanical locking effect. When the length of the shaft is too long, the shaft is prone to twisting deformation, affecting the welding effect, and also causing the rivet to protrude severely and increasing the weight of the welding spot.

[0011] Further, the process of discharging the melted second metal sheet is not more than 0.5s. The contact time of the melted second metal sheet with the first metal sheet is shortened to inhibit the generation of intermetallic compounds.

[0012] According to another aspect of the embodiments of the present application, a welding rivet is provided for the overlay welding of a first metal sheet and a second metal sheet, the melting point of the welding rivet is higher than 1200°C, the melting point of the second metal sheet is lower than 800°C, the welding rivet has the same base material as the first metal sheet, and the second metal sheet is arranged between the first metal sheet and the welding rivet during welding, wherein the rivet comprises a top wall, a shaft arranged at the center of the top wall, and a cap arranged around the top wall; the height of the shaft protruding from the top wall is less than the height of the edge of the cap protruding from the top wall; and a plurality of pinning portions are arranged on the edge of the cap protruding from the top wall, for pinning into the surface of the metal sheet to be welded during welding to provide mechanical locking. During welding, the cap of the rivet is first embedded into the surface of the second metal sheet to be welded to form mechanical locking, and then the shaft contacts the second metal sheet and conducts the welding current mainly through the shaft to melt, discharge and form a welding nugget between the shaft and the first metal sheet. After welding, the cap of the welding rivet is connected to the second metal sheet to be welded through mechanical locking, and can disperse the load, especially the tensile load, borne by the welding spot, to achieve the effect of improving the strength of the joint. By arranging the pinning portions on the edge of the cap of the rivet, the mechanical locking effect between the cap and the metal sheet to be welded is improved, and the bonding force between the cap of the rivet and the metal sheet is enhanced.

[0013] Further, the ratio of the total height of the pinning part to the thickness of the second metal plate is 1 / 10 to 4 / 5, preferably 3 / 5 to 4 / 5, which ensures that the pinning part and the second metal plate can generate sufficient mechanical interlocking strength, and avoids the pinning part piercing the second metal plate and weakening the second metal plate.

[0014] Preferably, the cap has a higher electrical resistivity than the shaft, so that most of the welding current flows through the shaft. The cap can have a higher electrical resistivity by applying a coating with a higher electrical resistivity on the surface or by applying more mechanical deformation to the cap during processing.

[0015] Further, the cap gradually expands from the top wall to the cap edge in a trumpet shape. The expanded cap structure helps the cap to embed into the surface of the second metal plate during welding and maintain the stability of the structure to form a stable mechanical interlocking.

[0016] Further, the ratio of the volume of the space enclosed by the inner surface of the cap and the outer surface of the shaft to the volume of the shaft is 1.0-1.5, preferably 1.0-1.2. There should be sufficient space in the cap to accommodate the liquid metal generated during welding to improve the surface quality of the welded part.

[0017] Further, the ratio of the distance from the top wall to the end of the shaft to the distance from the top wall to the cap edge is 0.5-0.75. The drop between the cap and the shaft should be adapted to the application scenario of the rivet, too short shaft will cause the rivet to deform severely during welding, and too long shaft will easily cause the cap to embed insufficiently and have lower mechanical interlocking strength.

[0018] Optionally, the top wall is centrally provided with a concave structure facing the direction of the end of the shaft, and the depth of the concave structure is 10%-50% of the length of the shaft. The concave structure helps to position the welding electrode and further shortens the conduction path of the welding current through the shaft.

[0019] Optionally, the top wall is centrally provided with a convex structure away from the direction of the end of the shaft, and the height of the convex structure is 10%-50% of the length of the shaft. The convex structure on the top wall can provide material supplement for the formation of the weld nugget, avoiding the occurrence of voids in the weld or welding through the thin welding plate.

[0020] Further, the end of the shaft is configured as a circular arc surface or a conical surface. The shaft configured as a circular arc surface or a conical surface helps to discharge the second metal plate.

[0021] Further, the connection between the top wall and the cap is provided with a step structure in the circumferential direction. The step structure in the circumferential direction at the connection between the top wall and the cap helps to concentrate the deformation of the rivet during the welding process in this area, so as to avoid the warping of the edge of the cap and affect the mechanical locking.

[0022] According to yet another aspect of the present application, there is provided a method for spot welding a dissimilar metal joint, using any of the rivets as described above, to perform any of the methods for welding a dissimilar metal joint as described above. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1a A schematic view of the structure of the upper surface of the rivet in an embodiment;

[0024] Figure 1b A schematic view of the structure of the lower surface of the rivet in an embodiment;

[0025] Figure 1c A schematic view of the structure of the cross section of the rivet in an embodiment;

[0026] Figures 2a-2d A schematic view of the welding process in an embodiment;

[0027] Figure 3 A schematic view of the current and pressure of the welding process in an embodiment;

[0028] Figure 4 A schematic view of the structure of the cross section of the rivet in another embodiment;

[0029] Figure 5 A schematic view of the structure of the cap of the rivet in yet another embodiment;

[0030] Figures 6a-6c A schematic view of the structure of the cap of the rivet in still another embodiment;

[0031] Figures 7a-7c A schematic view of the structure of the rivet in yet another embodiment;

[0032] Figures 8a-8d A schematic view of the structure of the shaft of the rivet in yet another embodiment;

[0033] Figure 9a A schematic view of the cross tensile failure of the welded joint in a comparative example;

[0034] Figure 9b A schematic view of the tensile shear failure of the welded joint in a comparative example;

[0035] Figure 10 A photograph of the rivet in yet another comparative example;

[0036] Figure 11This is a photograph of the interface after the welding rivets have been preheated and peeled off in another embodiment.

[0037] Figure 12 Here is a partial photograph of the welded joint in another embodiment;

[0038] Figure 13 Here is a metallographic photograph of the welded joint cross-section in another embodiment;

[0039] Figure 14a This is a schematic diagram of the cross-shaped tensile failure of the welded joint in another embodiment;

[0040] Figure 14b for Figure 14a Enlarged view of a portion of the image;

[0041] Figure 14c This is a schematic diagram of the tensile shear failure of a welded joint in another embodiment;

[0042] Figure 14d for Figure 14c Enlarged view of a portion of the image;

[0043] Figure 15 The cross tensile test curves of another embodiment and the comparative example welded joint are shown below.

[0044] Figure 16 The figure shows a comparison curve of the tensile shear test of the welded joint in another embodiment and a comparative example.

[0045] The purpose of the above-described drawings is to provide a detailed description of the present invention so that those skilled in the art can understand the technical concept of the invention, and is not intended to limit the invention. For the sake of brevity, the above-described drawings only schematically illustrate the structures related to the technical solution of the present invention, and do not depict the complete structure and all details strictly according to actual proportions. Detailed Implementation

[0046] The present invention will now be described in further detail with reference to specific embodiments and accompanying drawings.

[0047] The term "embodiment" as used herein means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment herein. The phrase appearing in various places in the specification does not necessarily refer to the same embodiment, nor is it limited to mutually exclusive, independent, or alternative embodiments. Those skilled in the art will understand that the embodiments herein can be combined with other embodiments without causing structural conflicts.

[0048] In the description of the present application, unless otherwise explicitly specified and limited, the technical terms "mounting", "connection", "connection" and the like should be understood in a broad sense, which can be active connection, or fixed connection or integrated. For those skilled in the art, the specific meaning of the above terms in the embodiments of the present application can be understood according to the specific circumstances.

[0049] In the description of the present application, the terms indicating the orientation or position relationship such as "upper", "lower", "left", "right", "transverse", "longitudinal", "height", "length", "width" and the like are intended to accurately describe the embodiments and simplify the description, and are not intended to limit the parts or structures involved to have a specific orientation, to be installed or operated in a specific orientation, and cannot be understood as a limitation on the embodiments in the present application.

[0050] In the description of the present application, the terms "first", "second" and the like are only used to distinguish different objects, and cannot be understood as indicating relative importance or limiting the number, specific order or primary and secondary relationship of the described technical features. In the description of the present application, the meaning of "multiple" is at least two.

[0051] Because direct fusion welding between aluminum and steel can cause a large number of brittle intermetallic compounds between Al and Fe elements, thereby seriously weakening the mechanical properties of the joint, some schemes propose to use steel-aluminum-steel lap welding to perform spot welding, which melts and extrudes the inner metal aluminum, and directly forms a welded connection between the outer steel plates, thereby avoiding the destructive effect of intermetallic compounds on the joint strength. However, the inventors realize that this welding method still has certain deficiencies: although the aluminum in the center of the welding spot has been extruded, liquid aluminum still exists around the welding spot and forms an intermetallic compound ring around the welding spot with the steel, and cracks are easily induced in this area to cause joint failure. In one comparative example, the typical failure mode in the cross tensile test is as shown in Figure 9a As shown in the figure, by stacking the steel welding piece 400 with a sheet structure on the steel plate (not shown) and the aluminum plate 201, a relatively firm heterogeneous metal spot welding connection can be achieved, but the welding piece 400 will be deformed and warped 401 during the spot welding process due to the extrusion and heat input of the welding electrode, which on the one hand causes the load to directly concentrate on the welding spot area during the tensile process, causing the joint to fail earlier, and on the other hand, the melted aluminum overflows from the gap between the welding piece 400 and the aluminum plate 201 to form spatters 402, affecting the surface quality of the welding. The tensile shear typical failure mode of the comparative example is as shown in Figure 9b As shown in the figure, the welding piece 400 is obviously warped 401, the second metal plate 201 forms a semicircular fracture around the welding nugget, and the welding piece 400 does not provide additional mechanical locking effect.

[0052] To address the aforementioned problems, one embodiment of the present invention provides a welding method. This method utilizes welding rivets 100, such as... Figures 1a-1c As shown, it includes a shaft portion 101 located at the center of the rivet, a top wall 103, and a cap 102 surrounding the shaft portion 101. The shaft portion 101 and the cap 102 are connected together through the top wall 103. Taking the highest point of the top wall 103 on the upper surface of the rivet 100 as the measurement starting point, the length h1 of the shaft portion 101 is less than the length h2 of the cap 102, so that the shaft portion 101 is located inside the semi-open space 105 formed by the cap 102. During welding, resistance spot welding is performed with the position of the shaft portion as the center of the weld point.

[0053] Combination Figures 2a-2d The specific welding process is as follows: First metal plate 202, second metal plate 201, and welding rivets 100 are stacked sequentially. The first metal plate 202 and welding rivets 100 are made of metals with a melting point higher than 1200℃, such as steel (including but not limited to quenched steel, dual-phase steel, transformation-induced plastic deformation steel, multiphase steel, twin-induced plastic deformation steel, hot-formed steel, cold-rolled steel, or hot-rolled steel, etc., with a surface coating such as Zn-based coating, Zn-Fe-based coating, Zn-Ni-based coating, Zn-Al-based coating, Zn-Mg-based coating, or aluminum-based coating, etc.), with a thickness typically between 0.4mm and 4mm. In the preferred embodiment, [the following configuration is described]. The thickness is 0.5mm-3mm; the second metal plate is made of metals with a melting point below 800℃, such as aluminum or magnesium (including but not limited to aluminum-magnesium alloys, aluminum-copper alloys, aluminum-magnesium-silicon alloys, aluminum-silicon alloys, aluminum-magnesium-aluminum alloys, magnesium-manganese alloys, magnesium-zinc-zirconium alloys, etc.), and the thickness is usually 0.5mm-3mm, and in the preferred embodiment it is configured to be 0.8mm-2.5mm; with the shaft portion 101 of the welding rivet 100 as the center of the welding point, the welding electrodes 300 are brought together to clamp the stacked structure to be welded, so that the edge of the cap 102 abuts against the surface of the second metal plate 201. At this time, because the shaft portion 101 is relatively short, it does not temporarily come into contact with the second metal plate. Figure 3 Resistance spot welding includes two stages: a preheating process and a welding process. In a preferred embodiment, the welding process is further divided into a spattering stage and a fusion stage. During the preheating process, the welding electrode 300 applies preheating pressure and a small preheating current, causing the second metal plate contacted by the cap 102 to soften or partially melt. Under the pressure of the welding electrode 300, the edge of the cap 102 is at least partially embedded in the second metal plate 201, creating a mechanical lock. Next, in the welding stage, a larger current is applied to fully melt the second metal plate in the welding area. The shaft passes through the second metal plate 201, discharging the molten second metal plate 201 and directly welding it to the first metal plate 202. In a preferred embodiment, the process of the shaft 101 discharging the second metal plate 201 lasts no more than 0.5 seconds to minimize the formation of intermetallic compounds.

[0054] In particular, in the preferred embodiment, the welding process first is a spatter stage, in which the welding electrode 300 applies several spatter current pulses at high current intensity to melt the second metal sheet 201 between the shaft 101 and the first metal sheet 202 and discharge the molten metal in the form of spatters 203, which are subsequently accommodated inside the cap 102; and a fusion stage, in which the welding electrode applies a welding current to form a fusion core between the shaft 101 of the welding rivet 100 and the first metal sheet 202 and connect them together.

[0055] In the preferred embodiment, the preheating current has a duration T and an intensity I, which satisfy the following conditions: 30ms≤T≤350ms; 2kA≤I≤7kA when t≤lmm; 3kA≤I≤lOkA when lmm

[0056] In the preferred embodiment, the ratio h1 :h2 is between 0.5 and 0.75, and the cap 102 is partially embedded into the surface of the second metal sheet 201 until the shaft 101 abuts against the surface of the second metal sheet 201 during the preheating stage. If the shaft 101 is too long, the shaft will contact the metal sheet to be welded at a small embedding depth of the cap 102, which will cause a current shunt; and if the shaft is too short, the shaft will not contact the metal sheet to be welded before the welding starts, which will affect the joint bonding. In the preferred embodiment, the ratio of the shaft length h1 to the thickness of the second metal sheet is between 0.5 and 1.5, so as to avoid too severe deformation of the welding rivet 100 during the welding process and a long time to achieve the melting and spattering of the second metal workpiece in the welding zone.

[0057] The welding method makes the welding rivet 100 and the second metal sheet 201 have a strong mechanical locking, because the cap 102 is embedded into the surface of the second metal sheet 201 during the preheating process, the second metal sheet 201 around the cap 102 can constrain the welding rivet 100 even if it is deformed due to the electrode pressure and heat input during the welding process, so as to avoid the cap edge from being warped; and after the welding is completed, when the welding joint bears a tensile load such as cross tensile load, the cap can provide a mechanical locking to the second metal sheet and elastically absorb part of the deformation, so as to avoid stress and strain from concentrating on the welding spot and the intermetallic compound region around the welding spot, thereby achieving the effect of improving the mechanical properties of the joint. On the other hand, because the cap 102 is embedded into the surface of the second metal sheet 201 during the welding stage, the spatters 203 generated during the welding process are all accommodated inside the cap 102, which also has the effect of improving the surface quality of the welded part. Meanwhile, the solidification structure of the cap 102 and the second metal sheet 201 accommodated therein can also improve the sealing of the welding spot.

[0058] According to another aspect of the embodiment of the present application, a welding rivet 100 is provided, which comprises a shaft 101 and a cap 102, whereinFigure 4 and Figure 10 As shown, it can be used for resistance spot welding between the first metal plate and the second metal plate in the aforementioned embodiments. The welding rivet 100 is made of metal exceeding 1200°C (such as steel or various iron-based alloys, such as DP600 steel), and includes a shaft portion 101, a cap 102, a top wall 103 connecting the shaft portion 101 and the cap 102, and a stapling portion 104 protruding from the edge of the cap 102, wherein the length h1 of the shaft portion is less than the length h2 of the cap. During welding, in the preheating stage, the stapling portion 104 first contacts the surface of the second metal plate 201. Under the action of the preheating current, its contact position softens first, making it easier for the welding rivet 100 to be driven into the surface of the second metal plate 201 and providing additional mechanical locking. Simultaneously, when welding conditions limit the cap 102 from being easily embedded into the second metal plate 201 in the circumferential direction, the stapling portion 104 can also provide sufficient mechanical locking.

[0059] In one embodiment, such as Figure 11 As shown, the second metal plate 201 is configured as a 1.6mm thick AA6061 aluminum plate. Welding rivets are set on its surface, and a welding pressure of 5000N and a preheating current of 150ms and 6kA are applied. Preheating stops after the rivet shaft contacts the aluminum plate. After peeling off the welding rivet, indentations 111 caused by the rivet insertion into the aluminum plate surface and indentations 112 left at the center of the weld point are visible. The 1.6mm thick AA6061 aluminum plate and a 1.2mm thick QP1180 steel plate are then overlapped and welded using the same preheating parameters. During the welding process, three 17kA spatter currents are applied for 65ms each during the spatter phase, and a 14kA fusion current is applied for 650ms during the fusion phase. The resulting weld joint is shown below. Figure 12 As shown, the metallographic photograph of the joint structure is as follows: Figure 13 As shown, the stud portion 104 and part of the cap 102 of the welding rivet 100 are embedded in the surface of the second metal plate 201, creating a mechanical locking effect. The welding spatter 203 is completely contained inside the cap 102, the surface of the second metal plate 201 is clean and free of spatter, and a weld nugget 204 is formed at the center of the weld point, indicating that the welding rivet 100 and the first metal plate 202 are fully fused together. The welding electrode, plate material, and... (The sentence is incomplete and requires further context to translate accurately.) Figure 9a , Figure 9b The comparisons shown are the same.

[0060] A cross tensile test was performed on the spot welded joint provided in this embodiment, and its failure mode is as follows: Figure 14a , 14b As shown, with Figure 9a The comparative examples shown demonstrate that the welding rivet 100 remains firmly attached to the second metal plate 201 without warping. This is consistent with... Figure 9a The stretching curves shown in the comparative example are as follows:Figure 15 As shown in the figure, the peak load of this embodiment reaches 4700.8N, while the peak load of the comparative example is only 3278.8N. It can be seen that the pinning part and part of the cap of the welding rivet 100 embedded in the second metal plate 201 disperses the load borne by the welding spot, absorbs part of the deformation in the cross tensile with its own elasticity, reduces the strain borne by the brittle area around the welding spot, and effectively improves the connection strength of the welding spot.

[0061] The tensile shear test was performed on the spot-welded joint provided by this embodiment, and the failure mode was as shown in Figure 14c , 14d As compared with the comparative example shown in Figure 9b , it can be seen that the welding rivet 100 always keeps in close contact with the second metal plate 201 and does not warp, and the mechanical locking action between the welding rivet 100 and the second metal plate 201 on the surface of the second metal plate 201 can be seen as the failure trace 113 left when the tensile occurs. As compared with the tensile curve of the comparative example shown in Figure 9b , as shown in Figure 16 , the peak load of the present application is 8170.4N, while the peak load of the comparative example is 6249.4N. It can be seen that the pinning part and part of the cap of the welding rivet 100 embedded in the second metal plate disperses the load borne by the welding spot, effectively improving the connection strength of the welding spot.

[0062] In the preferred embodiment, the resistivity of the cap 102 is greater than the resistivity of the shaft part 101, which makes the current mainly flow through the shaft part 101 after the shaft part 101 is in contact with the second metal plate 201, reducing the shunt effect of the cap 102. The resistivity can be realized by setting a high-resistance coating (such as stainless steel surface, zirconium oxide, silicon carbide, aluminum oxide, etc.) on the surface of the cap 102, or by applying higher cold deformation to the cap 102 during processing.

[0063] The welding rivet 100 can be selected according to the size of the welded part, and the overall height thereof can be set to 2mm-10mm. h1 can be specifically set, and is usually 1mm-4mm, and in the preferred embodiment, it is configured to be 1.5mm-3mm. In the preferred embodiment, the value range of h1:h2 is configured to be 0.5-0.75. In the preferred embodiment, the space surrounded by the inner surface of the cap 102 and the outer surface of the shaft part 101 is 1-1.5 times the volume of the shaft part itself, and is further preferably 1-1.2 times, so that the cap 102 can fully accommodate the melted second metal plate 201 discharged during welding.

[0064] In the preferred embodiment, as shown in Figure 5As shown, the cap 102 is gradually enlarged from the top wall 103 to the edge in a flared shape, and the inclination angle a is not more than 45°. In this way, when welding is performed, the load applied by the welding electrode 300 can be transmitted radially outward along the cap 102, which is counteracted by the second metal plate 201 around the cap 102, thereby reducing the risk of deformation and warping of the cap 102.

[0065] In a preferred embodiment, as shown in Figures 6a-6c The inner and / or outer side of the wall of the cap 102 is provided with a stepped structure 106 and 107 at the connection area with the top wall 103, which is used to induce deformation to occur at the upper part of the cap 102 when the welded rivet 100 is compressed by the welding electrode 300, avoiding deformation and warping of the cap edge.

[0066] In an optional embodiment, as shown in Figure 7a , Figure 7b The central part of the top wall 103 is provided with a concave structure 108a towards the end of the shaft 101, and the depth h3 of the concave structure 108a is 10%-50% of the length of the shaft 101, which is configured as 0.1mm-2mm in a preferred embodiment. The concave structure 108a facilitates positioning of the welding electrode, preventing the position of the welding spot from deviating from the center of the welded rivet, affecting the welding effect.

[0067] In other optional embodiments, as shown in Figure 7c The surface of the top wall 103 is provided with a protruding structure 108b away from the end of the shaft 101, and the height h4 of the protruding structure 108b is configured as 10%-50% of the length h1 of the shaft 101. Such a rivet can be used for thinner welding joints to avoid the situation of welding through; or for welding joints with poor flowability of the to-be-welded plate material, to supplement the material in the welding nugget with the material of the welded rivet 100 itself, so as to avoid holes.

[0068] As shown in Figure 8a The end of the shaft 101 can be configured as a flat surface. In some preferred embodiments, as shown in Figures 8b-8d The end of the shaft 101 is configured as a circular arc surface or a conical surface. In a preferred embodiment, the diameter d1 of the shaft 101 is configured as 4mm-8mm, further preferably 4.5mm-7mm, and the end is configured as a spherical surface with a radius of curvature not less than 10mm; the side wall surface of the shaft 101 has an inclination angle γ, which can be selected as 5°-30°, further preferably 8°-20°. In a preferred embodiment, the end of the shaft 101 is configured as a conical surface with a conical angle β>90°, further preferably β>120°. The circular arc surface or the conical surface is helpful for the molten second metal plate to be quickly discharged during welding. In another preferred embodiment, the end edge of the shaft 101 is configured with a rounded corner 109, and the side wall surface is provided with an annular concave surface 110, which is also used to promote the molten second metal plate to be quickly discharged and guide the discharge of spatter during the welding stage.

[0069] The above embodiments are intended to further illustrate the technical concept of the present application in conjunction with the drawings, so that those skilled in the art can understand the technical concept of the present application. Within the scope of the claims of the present application, the materials or method steps involved are optimized or equivalently replaced, and the embodiments in different embodiments are combined without structural and principle conflicts, which fall within the protection scope of the present application.

Claims

1. A method for spot welding dissimilar metal joints, providing welding rivets for overlapping welding of a first metal plate and a second metal plate, wherein the melting point of the welding rivet and the first metal plate is higher than 1200°C, and the melting point of the second metal plate is lower than 800°C, the welding rivet comprising a top wall, a shaft portion disposed at the center of the top wall, and a cap disposed around the top wall, wherein the first metal plate, the second metal plate, and the welding rivet are arranged in a stacked manner, with the second metal plate placed between the first metal plate and the welding rivet, characterized in that: The shaft portion of the weld rivet is located within the semi-open space formed by the surrounding cap. Resistance welding is performed with the shaft portion as the center of the weld point. The welding process includes a preheating process and a welding process. During the preheating process, preheating pressure and preheating current are applied to the solder joint to soften part of the second metal plate, and the edge of the cap is at least partially embedded in the second metal plate to create a mechanical lock. During the welding process, welding pressure and welding current are applied to the weld joint, and the shaft passes through the second metal plate to discharge the molten second metal plate and weld it directly to the first metal plate.

2. The method for spot welding dissimilar metal joints according to claim 1, characterized in that, The duration T and current intensity I of the preheating current satisfy: 30ms≤T≤350ms; When t≤1mm, 2kA≤I≤7kA; When 1mm<t≤2.5mm, 3kA≤I≤10kA; When t > 2.5 mm, 5 kA ≤ I ≤ 12 kA; Where t is the thickness of the second metal plate.

3. The method for spot welding dissimilar metal joints according to claim 1 or 2, characterized in that, The welding process includes a spatter stage and a fusion stage. In the spatter stage, a spatter current is applied to the weld joint, causing the second metal plate to melt and detach from the area between the shaft and the first metal plate in the form of spatter, and be contained within the cap. During the welding stage, a welding current is applied to the weld joint, and the contact area between the shaft and the first metal plate melts to form a weld nugget.

4. The method for spot welding dissimilar metal joints according to claim 1 or 2, characterized in that, The ratio of the distance from the top wall of the welding rivet to the end of the shaft portion to the distance from the top wall to the edge of the cap is 0.5-0.

75. During the preheating process, the cap portion is embedded in the second metal plate until the end of the shaft portion abuts against the surface of the second metal plate.

5. The method for spot welding dissimilar metal joints according to claim 4, characterized in that, The ratio of the shaft length to the thickness of the second metal plate is 0.5 to 1.

5.

6. The method for spot welding dissimilar metal joints according to claim 1 or 2, characterized in that, The process of expelling the molten second metal plate shall not exceed 0.5 seconds.

7. A welding rivet, the welding rivet having a melting point higher than 1200°C, comprising a top wall, a shaft portion disposed at the center of the top wall, and a cap disposed around the top wall, the shaft portion and the cap being connected through the top wall; the height by which the shaft portion protrudes from the top wall is less than the height by which the edge of the cap protrudes from the top wall; characterized in that, The cap has multiple protruding studs on its edge, which are used to stud the surface of the metal plate to be welded during welding to provide mechanical locking.

8. The welding rivet according to claim 7, characterized in that, The resistivity of the cap is greater than that of the shaft, so that most of the welding current flows through the shaft.

9. The welding rivet according to claim 7, characterized in that, The ratio of the overall height of the stapling portion to the thickness of the second metal plate is 1 / 10 to 4 / 5.

10. The welding rivet according to claim 7 or 8, characterized in that, The cap gradually widens into a trumpet shape from the top wall to the edge of the cap.

11. The welding rivet according to claim 7 or 8, characterized in that, The ratio of the space enclosed by the inner surface of the cap and the outer surface of the shaft to the volume of the shaft is 1.0-1.

5.

12. The welding rivet according to claim 11, characterized in that, The ratio of the space enclosed by the inner surface of the cap and the outer surface of the shaft to the volume of the shaft is 1.0-1.

2.

13. The welding rivet according to claim 7 or 8, characterized in that, The ratio of the distance from the top wall of the welded rivet to the end of the shaft portion to the distance from the top wall to the edge of the cap is 0.5-0.

75.

14. The welding rivet according to claim 7 or 8, characterized in that, The top wall has a concave structure in the center facing the end of the shaft, and the depth of the concave structure is 10%-50% of the length of the shaft.

15. The welding rivet according to claim 7 or 8, characterized in that, The top wall has a raised structure in the center, away from the end of the shaft, and the height of the raised structure is 10%-50% of the length of the shaft.

16. The welding rivet according to claim 7 or 8, characterized in that, The end of the shaft is configured as an arc surface or a conical surface.

17. The welding rivet according to claim 7 or 8, characterized in that, The connection between the top wall and the cap is provided with a stepped structure along the circumference.

18. A method for spot welding dissimilar metal joints, characterized in that, The dissimilar metal joint spot welding method according to any one of claims 1 to 6 is carried out using the welding rivets as described in any one of claims 7 to 17.

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