Fastener for resistance spot welding of dissimilar metals

Abstract

The present invention provides a fastener for resistance spot welding of dissimilar metals. The fastener comprises a shaft part, a top cover and a cap; the cap extends from the periphery of the top cover to the plane where the end of the shaft part is located, to sequentially form a transition area, an arc-shaped area and a vertical wall area; the vertical wall area vertically supports the cap during welding, forcing the deformation of the cap to concentrate in the arc-shaped area, and preventing collapse of the cap after welding, which would reduce the volume of an accommodating cavity; the vertical wall area is almost vertically pressed to the surface of a first workpiece after being welded, to provide extra mechanical locking force for a welding joint; and the wall thickness of the transition area is gradually reduced from the thickness of the peripheral side of the top cover to the wall thickness of the arc-shaped area, preventing the transition area from cracking due to stress concentration during manufacturing and welding, and ensuring that plastic deformation of the cap is concentrated in the arc-shaped area. By means of the fastener provided by the present invention, the high-strength welding effect of dissimilar metals can be achieved.

Description

Resistance spot welding of fasteners of dissimilar metals TECHNICAL FIELD

[0001] The present invention belongs to the field of welding of dissimilar metals, and particularly relates to a resistance spot welding of fasteners of dissimilar metals. BACKGROUND

[0002] In the automotive industry, the application of various metal materials to manufacture the vehicle body is more conducive to achieving the goals of safety performance and lightweight of the vehicle body. Among them, steel and aluminum alloy are widely used in the manufacture of vehicle body, especially in the current popular new energy vehicle manufacturing, a large number of large-scale integrated die-cast aluminum alloy is widely used to reduce the number and weight of vehicle body parts. In addition, in the vehicle body framework and key parts, multi-material advanced high-strength steel or even super-high-strength steel is used to ensure the safety performance of the vehicle body, for example, 1500MPa level or even higher strength level of hot forming steel is used in A / B column, longitudinal beam and other parts. Therefore, the reliable welding of dissimilar metals (such as aluminum / steel, magnesium / steel, etc.) is conducive to promoting the development of automotive lightweight, and the dissimilar metal welding technology has a wide application demand in the automotive field. However, when connecting dissimilar metals by traditional welding process, it is easy to be affected by brittle intermetallic compounds and cracks, etc., resulting in poor mechanical properties of the weld, therefore, the current traditional welding process cannot weld dissimilar metals with high quality.

[0003] In order to realize the efficient and reliable welding of dissimilar metals, patent document CN114211104B discloses a dissimilar metal joint and a resistance spot welding method thereof, which realizes high-quality connection effect of dissimilar metals by inducing low-melting-point metal to be high-speed discharged from the weld spot in the form of spatter, and then welding the two steel plates. However, the spatter metal high-speed discharged from the weld spot will cause the surface and surrounding components of the weld spot to be contaminated; in addition, when welding thicker low-melting-point metal, it is difficult to achieve a better weld spot forming effect. Patent document CN115570251A discloses a dissimilar metal welding rivet, which realizes welding of dissimilar metals and effectively accommodates the spatter metal discharged from the weld spot. However, the shape structure and size of the shaft and the cap of the rivet are not carefully designed, and the cap cannot adaptively deform during the welding process due to the change in the position of the shaft, that is, after the edge (end) of the cap contacts the metal workpiece to be welded, the cap cannot deform plastically, which makes it difficult for the central shaft to sink relative to the cap and pierce the low-melting-point metal to be welded at high speed, which is not conducive to welding of low-melting-point metal with large thickness (such as low-melting-point metal with a thickness greater than 2.5mm). Since the shaft cannot effectively pierce the low-melting-point metal, the high-melting-point metal needs to produce more significant concave deformation, which significantly increases the depth of the electrode indentation of the weld spot, and increases the risk of cracks in the weld spot. In addition, the structure of the cap is bulky, and the wall thickness of the cap cross section is large, so a larger size rivet needs to be designed to form the same accommodation cavity volume, which increases the weight of the joint and reduces the flexibility of the joint structure design.

[0004] Patent document CN116810111A discloses a welding element for dissimilar metal resistance spot welding, which is provided with a cap structure outside the shaft part, achieving the effect of accommodating splashed metal. At the same time, one or more recessed auxiliary deformation zones are provided in the connection area between the cap structure and the top cover, achieving the effect of preferential deformation of the cap structure in the auxiliary deformation zone during welding, and improving the plastic deformation capacity of the cap structure. However, due to the thin-walled structure of the cap, the auxiliary deformation zone is further thinned, which on the one hand increases the difficulty of manufacturing and increases the manufacturing cost. On the other hand, in a complex welding environment, the auxiliary deformation zone has the risk of excessive plastic deformation, which leads to easy cracking of the auxiliary deformation zone, especially when the welding electrode and the shaft center line of the welding element cannot be accurately centered, the probability of this problem will further increase. Once the auxiliary deformation zone cracks, it will cause the splashed metal to be discharged from the crack, on the one hand affecting the welding spot quality stability and the welding spot surface quality, on the other hand the splashed metal will pollute the electrode surface, reducing the service life of the welding electrode. In addition, the auxiliary deformation zone is easily stretched and deformed, and other areas are less deformed, resulting in a more prominent cap protruding from the workpiece surface after welding. Therefore, there is an urgent need in the art for a structure design that is more reasonable, can efficiently accommodate splashed metal (discharged metal) and achieve high-strength welding of dissimilar metals, while also ensuring that the post-weld fastener protrudes from the workpiece surface with a smaller height.

[0005] SUMMARY

[0006] The purpose of the present application is to provide a resistance spot welding fastener for welding dissimilar metals, to improve the welding quality of dissimilar metal resistance spot welding, to improve the deformation capacity of the cap structure of the fastener during the welding process, to ensure that the cap structure of the fastener maintains the cavity structure during the welding process, and to solve the problems of improving the spot forming and achieving low-cost and efficient manufacturing of the fastener.

[0007] A resistance spot welding fastener for welding dissimilar metals, the fastener is used for resistance spot welding of a first metal and a second metal stack, the melting point of the first metal is lower than the melting point of the second metal, the melting point of the fastener metal is the same as or similar to that of the second metal, the fastener comprises a cap, a shaft part extending from the center of the cap, and a cap cover arranged around the outer periphery of the cap, the cap cover and the horizontal plane where the side of the shaft part and the end of the shaft part are located form a containing cavity, and the containing cavity is used to collect the first metal squeezed out or sprayed from the welding spot, characterized in that:

[0008] The side wall of the shaft part gradually expands when extending from the end of the shaft part to the outer periphery of the cap, which is beneficial to the high-speed penetration of the shaft part into the first metal and the discharge of molten first metal from the welding spot during welding, and provides the fastener with a main mechanical locking force to the first metal;

[0009] The cap is extended from the outer periphery of the top cover to the horizontal plane where the end of the shaft part is located, sequentially forming a transition zone, an arc-shaped zone and a straight wall zone; wherein,

[0010] The arc-shaped zone has good plastic deformation capacity, and can adapt to adaptive deformation at the position of the shaft part during welding, avoiding the cap hindering the shaft part from piercing the first metal;

[0011] The straight wall zone vertically supports the cap during welding, forcing the cap to deform and concentrate in the arc-shaped zone, while avoiding the cap from collapsing after welding to reduce the volume of the accommodating cavity;

[0012] The straight wall zone is almost vertically extruded to the surface of the first workpiece after welding, providing additional mechanical locking force for the welded joint;

[0013] The wall thickness of the transition zone gradually decreases from the thickness of the outer periphery of the top cover to the wall thickness of the arc-shaped zone, avoiding stress concentration and cracking of the transition zone during manufacturing and welding, and ensuring that plastic deformation of the cap concentrates in the arc-shaped zone.

[0014] Further, the wall thickness of the arc-shaped zone gradually decreases when extending from the top cover side to the straight wall zone, so that plastic deformation of the arc-shaped zone occurs near the straight wall zone side during welding, and the height of the cap protruding from the surface of the first metal after welding is controlled.

[0015] Further, the wall thickness of the straight wall zone gradually increases from one end near the arc-shaped zone to the other end in contact with the first metal, and the increase is not more than 65%.

[0016] Further, the wall thickness of the arc-shaped zone gradually decreases when extending from the top cover side to the straight wall zone, and the decrease is not more than -50%.

[0017] Further, the minimum height H2 of the shaft part protruding from the top cover satisfies H2=K1×T, wherein T is the thickness of the first metal, and K1 is a coefficient considering the thickness of the first metal, and the value range is K1=0.6-1.2,

[0018] The thickness H of the center of the fastener satisfies H=1.2×H2-2×H2,

[0019] The minimum horizontal width from the outer periphery of the top cover to the side wall of the shaft part is L, and the minimum horizontal distance from the outer periphery of the top cover to the straight wall zone is L1, wherein L1 is greater than 1.5×L,

[0020] The horizontal width L2 of the transition zone is less than 0.5×L1,

[0021] Through a line connecting the connecting point of the arc-shaped zone and the straight wall zone and tangent to the arc-shaped zone, the line forms an angle δ with the straight wall zone, and the angle δ ranges from 0° to 40°,

[0022] The straight wall region surrounds the fastener shaft portion and forms an angle β with the central axis of the fastener, and β = 0°-5°, the height H1 of the straight wall region < H,

[0023] The included angle Ω between the inner wall of the top cover and the horizontal plane satisfies Ω = 10°-50°.

[0024] Further, the included angle γ between the side wall of the shaft portion and the horizontal plane ranges from 45° to 80°, and γ > Ω.

[0025] Further, there is an included angle α between the line connecting the end of the cap cover close to the outer periphery of the top cover and the end of the arc-shaped region close to the straight wall region and the horizontal plane, and α = 15°-65°.

[0026] Further, the height H1 of the straight wall region is less than the height H2.

[0027] Further, the connection area between the arc-shaped region and the straight wall region is tangential.

[0028] Further, the arc-shaped region is a circular arc with a radius ranging from 0.6 mm to 4 mm.

[0029] Preferably, the horizontal width of the transition region is wherein L1 is the minimum horizontal distance from the outer periphery of the top cover to the straight wall region.

[0030] Further, the wall thickness of the arc-shaped region and the straight wall region is 0.25 mm to 0.6 mm; the horizontal width L of the outer periphery of the top cover to the side wall of the shaft portion is 0.3 mm to 1.6 mm; the minimum horizontal distance L1 from the outer periphery of the top cover to the straight wall region ranges from 1.2 mm to 3.2 mm; and the diameter D of the end of the shaft portion ranges from 3 mm to 6 mm.

[0031] Preferably, when the thickness of the first metal is less than 1.5 mm, the value of the coefficient K1 ranges from 0.6 to 0.8; when the thickness of the first metal is greater than or equal to 1.5 mm and less than 2.5 mm, the value of the coefficient K1 ranges from 0.7 to 1.0; and when the thickness of the first metal is greater than or equal to 2.5 mm, the value of the coefficient K1 ranges from 0.8 to 1.2.

[0032] Preferably, the angle β ranges from 2° to 4°.

[0033] Preferably, the angle Ω ranges from 25° to 40°.

[0034] Further, the intersection of the outer periphery of the top cover and the inner wall of the top cover is connected by a circular arc R1, and the radius of the circular arc R1 is greater than 0.3 mm. Preferably, the inner surface of the arc-shaped region and the top cover are connected by a circular arc R2, wherein the circular arc R2 is tangent to the inner surface of the arc-shaped region and the circular arc R1 respectively, and the radius of the circular arc R2 is greater than 0.5 mm.

[0035] Further, the melting point temperature of the first metal is lower than 750°C, and the melting point temperature of the second metal is higher than 1300°C.

[0036] Advantages of the present application:

[0037] (1) The side wall of the shaft portion gradually expands when extending from the end of the shaft portion to the outer periphery of the top cover, which is conducive to the high-speed penetration of the shaft portion into the first metal and the discharge of molten first metal from the welding point during welding, and provides the main mechanical locking force of the fastener to the first metal; the arc-shaped area has good plastic deformation capability, which can adapt to the deformation of the shaft portion position during the welding process, avoiding the cap cover hindering the penetration of the shaft portion into the first metal; the wall thickness of the transition area gradually decreases from the thickness of the outer periphery of the top cover to the wall thickness of the arc-shaped area, avoiding stress concentration and cracking of the transition area during manufacturing and welding, and ensuring that the plastic deformation of the cap cover occurs in the arc-shaped area; the straight wall area vertically supports the structure of the cap cover during the welding process, forcing the deformation of the cap cover to concentrate in the arc-shaped area, avoiding collapse deformation in this area, and ensuring that the cap cover structure maintains the cavity structure after welding, while the straight wall area is almost vertically extruded to the surface of the first workpiece after welding, providing additional mechanical locking force effect.

[0038] (2) According to a preferred embodiment of the present application, the straight wall area vertically supports the structure of the cap cover during the welding process, avoiding collapse deformation in this area, ensuring that the cap cover structure maintains the cavity structure after welding, while the straight wall area is almost vertically extruded to the surface of the first workpiece after welding, providing additional mechanical locking force effect.

[0039] (3) According to a preferred embodiment of the present application, the thickness of the straight wall area gradually increases when extending from the arc-shaped area to the end, forming a trend of decreasing and then increasing wall thickness, which on the one hand facilitates the control of the plastic flow of the cap cover material and the bending forming of the cap cover during manufacturing, reducing the difficulty of fastener stamping manufacturing; on the other hand, the gradual increase of the wall thickness of the straight wall area towards the end improves the vertical support effect of the straight wall area during the welding process, avoiding its collapse.

[0040] (4) According to a preferred embodiment of the present application, the included angle Ω between the inner wall of the top cover of the fastener and the horizontal plane satisfies Ω = 10°-50°, and the inner wall 1061 of the top cover extends upwards from the side of the shaft portion to the outer periphery of the top cover, which improves the compatibility of the fastener in welding first metals of different thicknesses.

[0041] (5) According to a preferred embodiment of the present application, the fastener of the present application has simple structure and more compact size, ensuring efficient collection of splashed metal (discharged metal) and implementation of high-strength welding of dissimilar metals, while also ensuring smaller height of the fastener protruding from the surface of the workpiece and smaller diameter of the welding point after welding. BRIEF DESCRIPTION OF DRAWINGS

[0042] FIG. 1 is a cross-sectional schematic view of the fastener of the present disclosure;

[0043] Figure 2 is a cross-sectional view of a fastener transition zone of the present disclosure;

[0044] Figure 3 is a cross-sectional view of a fastener cap of the present disclosure;

[0045] Figure 4 is a cross-sectional view of a fastener arc zone of the present disclosure;

[0046] Figure 5 is a cross-sectional view of a fastener cap of an embodiment of the present disclosure;

[0047] Figure 6 is a cross-sectional view of a fastener cap of another embodiment of the present disclosure during welding;

[0048] Figure 7 is a cross-sectional view of a fastener cap of another embodiment of the present disclosure during welding;

[0049] Figure 8 is a cross-sectional view of a fastener cap of another embodiment of the present disclosure during welding;

[0050] Figure 9 is a cross-sectional view of a fastener cap of another embodiment of the present disclosure during welding;

[0051] Figure 10 is a cross-sectional view of a fastener of another embodiment of the present disclosure during welding of a first metal of different thickness;

[0052] Figure 11 is a cross-sectional view of a fastener arc zone of another embodiment of the present disclosure;

[0053] Figure 12 is a graph of welding current, electrode pressure, and time during resistance spot welding of dissimilar metals of the present disclosure;

[0054] Figure 13 is a graph of the pre-pressing stage of a welding electrode during welding of dissimilar metals of the present disclosure;

[0055] Figure 14 is a graph of the pre-heating stage during welding of dissimilar metals of the present disclosure;

[0056] Figure 15 is a graph of the piercing stage of a fastener during welding of dissimilar metals of the present disclosure;

[0057] Figure 16 is a graph of the welding stage of a fastener during welding of dissimilar metals of the present disclosure;

[0058] Figure 17 is a cross-sectional view of a fastener of Example 1 of the present disclosure;

[0059] Figure 18 is a cross-sectional metallograph of a welded joint of Example 1 of the present disclosure; and

[0060] Figure 19 is a size histogram of the weld joints in Example 1 of the present disclosure and the weld joints of the comparative examples;

[0061] Figure 20 is a cross-sectional schematic view of the fastener in Example 2 of the present disclosure;

[0062] Figure 21 is a cross-sectional metallographic view of the weld joint in Example 2 of the present disclosure;

[0063] Figure 22 is a surface and cross-sectional view of the fastener in Example 3 of the present disclosure;

[0064] Figure 23 is a view of the distribution of the expelled metal in the cap of the fastener after the piercing stage of the fastener during welding in Example 3 of the present disclosure;

[0065] Figure 24 is a cross-sectional metallographic view of the weld joint in Example 3 of the present disclosure;

[0066] Figure 25 is a surface topography and cross-sectional metallographic view of the weld joint in Example 4 of the present disclosure;

[0067] Figure 26 is a cross-sectional metallographic view of the fastener and a surface topography view of the joint after welding in Comparative Example 1 of the present disclosure;

[0068] Figure 27 is a force-displacement curve obtained after cross-tension testing of the weld joints in Example 4 and Comparative Examples 1 and 2 of the present disclosure;

[0069] Figure 28 is a fracture topography view after cross-tension testing of the weld joints in Example 4 and Comparative Examples 1 and 2 of the present disclosure;

[0070] Figure 29 is a schematic view of the force on the joint of the fastener of the present disclosure during cross-tension testing; DETAILED DESCRIPTION

[0071] The embodiments of the present application will be described in further detail by the specific embodiments in conjunction with the accompanying drawings.

[0072] The horizontal plane in the present application refers to the plane in which the end portion is located.

[0073] According to one aspect of the embodiments of the present application, a fastener 100 for resistance spot welding of dissimilar metals is provided as shown in Figure 1. The fastener 100 is rotationally symmetrical about a central axis and comprises a cap 106, a shaft portion 102 extending from the center of the cap 106, and a cap 104 arranged around the outer periphery 1062 of the cap. When the cap 104 extends from the outer periphery 1062 of the cap to the level of the end portion 103 of the shaft portion, it forms a transition region 1043, an arc-shaped region 1041, and a straight wall region 1042. The cross-sectional thickness of the transition region 1043 gradually decreases from the thickness on the side of the outer periphery 1062 of the cap to the wall thickness of the arc-shaped region, and the arc-shaped region is curvedly connected to the straight wall region.

[0074] The minimum height H2 of the shaft portion protruding from the top cover 106 of the fastener 100 satisfies the relationship H2 = K1 x T, where T is the first metal thickness and K1 is a coefficient considering the first metal thickness, and K1 is in the range of 0.6 to 1.2. The thickness H of the center of the fastener 100 satisfies H = 1.2 x H2 ~ 2 x H2. The minimum horizontal width L of the outer periphery 1062 of the top cover to the shaft portion side wall and the minimum horizontal distance L1 of the outer periphery of the top cover to the straight wall region satisfy the relationship L1 > 1.5 x L. The horizontal width L2 of the transition region 1043 satisfies the relationship L2 < 0.5 x L1. In this way, the fastener can effectively penetrate the first metal to form a high-quality welding spot while making the fastener size compact and effectively accommodating the splashed metal during welding.

[0075] As shown in FIG. 2, the transition region 1043 is a cross-sectional view of the transition region 1043. The arc-shaped region 1041 extends from the top cover in the following forms: (1) the arc-shaped region 1041 intersects with the outer periphery 1062 of the top cover or the upper surface 101 of the top cover, and the transition region 1043 is designed to be filled from the upper and lower sides, as shown in A1 of FIG. 2; (2) the arc-shaped region 1041 is tangent to the upper surface of the top cover, and the transition region 1043 is mainly filled from the lower side, as shown in B1 of FIG. 2; (3) the arc-shaped region 1041 is tangent to the outer periphery 1062 of the top cover, and the transition region 1043 is mainly filled from the upper side, as shown in C1 of FIG. 2. Since the arc-shaped region 1041 is a thin-walled structure, the transition region 1043 gradually thins the wall thickness from the top cover 106 to the arc-shaped region, avoiding excessive plastic deformation of the fastener at the transition region position during welding and causing cracks. In addition, the horizontal width L2 of the transition region 1043 is controlled in the range of L2 < 0.5 x L1, avoiding the transition region from being too large to affect the plastic deformation of the arc-shaped region during the welding process.

[0076] As shown in FIG. 3, the connection state of the transition region and the top cover in a preferred embodiment is shown. The arc-shaped region 1041 is a circular arc and intersects with the outer periphery of the top cover 106. On the upper surface, the arc-shaped region 1041 is connected to the upper surface 101 of the top cover through a circular arc R3, and the circular arc R3 is tangent to the arc-shaped region 1041 and the upper surface 101 of the top cover. At the intersection of the outer periphery of the top cover and the inner wall 1061 of the top cover, a circular arc R1 is connected, and the circular arc R1 is tangent to the outer periphery of the top cover and the inner wall 1061 of the top cover. Between the inner surface of the arc-shaped region 1041 and the top cover, a circular arc R2 is connected, and the circular arc R2 is tangent to the inner surface of the arc-shaped region 1041 and the circular arc R1, respectively. The radius of the circular arc R1 is greater than 0.3 mm, and the radius of the circular arc R2 is greater than 0.5 mm.

[0077] As shown in FIG. 4, the cross-sectional view of the connection between the arc-shaped area and the straight wall area of the fastener, through the line tangent to the arc-shaped area 1041 and the connection point between the arc-shaped area 1041 and the straight wall area 1042, the line forms an angle δ with the straight wall area 1042, and the angle δ ranges from 0° to 40°. Preferably, the angle δ is set to 0°, i.e., the arc-shaped area 1041 is tangent to the straight wall area 1042, as shown in A2 in FIG. 4. According to different requirements of application scenarios, the connection between the arc-shaped area 1041 and the straight wall area 1042 can also be set to an intersecting relationship, but the angle δ formed is not more than 40°, as shown in B2 in FIG. 4. In this way, while maintaining the compact size of the fastener, the volume of the cavity formed by the cap is ensured, and the manufacturability of the cap is improved.

[0078] More specifically, the diameter D of the end 103 of the shaft of the fastener ranges from 3 mm to 6 mm, the horizontal width L of the outer periphery 1062 of the cap to the side wall of the shaft 102 ranges from 0.3 mm to 1.6 mm, preferably from 0.5 mm to 1 mm, the minimum horizontal distance L1 of the outer periphery of the cap to the straight wall area 1042 ranges from 1.2 mm to 3.2 mm, preferably from 1.8 mm to 2.5 mm, and in addition, the angle Ω between the inner wall 1061 of the cap and the horizontal plane ranges from 10° to 50°. Preferably, when the thickness of the first metal ranges from less than 1.5 mm, the diameter D ranges from 3 mm to 4.5 mm, L ranges from 0.3 mm to 0.6 mm, L1 ranges from 1.2 mm to 2.0 mm, and Ω ranges from 10° to 25°; when the thickness of the first metal ranges from 1.5 mm to 3 mm, the diameter D ranges from 4.2 mm to 5.2 mm, L ranges from 0.5 mm to 1 mm, L1 ranges from 1.8 mm to 2.5 mm, and Ω ranges from 20° to 40°; and when the thickness of the first metal is greater than 3 mm, the diameter D ranges from 4.8 mm to 6 mm, L ranges from 0.9 mm to 1.6 mm, L1 ranges from 2.3 mm to 3.2 mm, and Ω ranges from 30° to 50°. This ensures that the shaft can efficiently pierce the first metal and form a reliable welding point while the cap structure can accommodate the molten metal discharged from the welding point, while maintaining the compact size of the fastener.

[0079] Figure 5 shows a cross-sectional view of the cap of the fastener in one embodiment. When the arc region 1041 extends from the transition region 1043 to the straight wall region 1042, there is a thickness relationship t1≥t4>t3>t2, such that the wall thickness of the cap presents a trend of first decreasing and then increasing, where t1 is the wall thickness of the arc region near the end of the transition region, t2 is the wall thickness of the arc region near the end of the straight wall region, t3 is the wall thickness of the straight wall region near the end of the arc region, and t4 is the wall thickness of the straight wall region away from the end of the arc region. In addition, the amplitude of the gradual thinning of the wall thickness of the arc region 1041 extending from the top cap side to the straight wall region does not exceed -50%, and the amplitude of the gradual thickening of the wall thickness of the straight wall region 1042 extending from the end of the arc region to the end 105 does not exceed 65%. In this way, the cap can be better formed during manufacturing, and the deformation of the arc region 1041 during welding is controlled to be closer to the side of the straight wall region 1042, which is beneficial for reducing the height of the cap protruding above the first metal surface after welding. In addition, the straight wall region 1042 gradually increases from top to bottom, which is more conducive to maintaining the vertical support effect.

[0080] As shown in Figure 6, in yet another embodiment, a schematic diagram of the deformation process of the cap of the fastener during welding. In the electrode pre-pressing stage (A3 stage in Figure 6), the electrode exerts an electrode pressure F e on the fastener, so that the shaft portion 102 of the fastener and the end of the straight wall region 1042 of the cap are pressed onto the surface of the first metal 200, and the first metal 200 exerts a reaction force F s on the straight wall region 1042 of the cap. In the welding stage (B3 stage in Figure 6), as the shaft portion 102 quickly penetrates the first metal 200 and sinks downward, the arc region 1041 on the cap is also deformed downward under the linkage of the top cap. Due to the upward support of the straight wall region 1042, the arc region 1041 is deformed around the upper end of the straight wall region, thereby maintaining the cavity structure of the cap. The rapid deformation of the arc region 1041 during welding facilitates the penetration of the shaft portion of the fastener into the first metal 200, and on the other hand, the vertical support of the straight wall region effectively ensures that the cap maintains a sufficient cavity volume for accommodating the expelled metal during welding.

[0081] The straight wall area 1042 is distributed around the fastener shaft portion 102, as shown in FIG. 1, and the height H1 of the straight wall area is less than H. In addition, the straight wall area 1042 is designed to extend downward at an angle β with the central axis, and the angle β is in the range of 0° to 5°. This aspect ensures that the cap is easy to manufacture and compact in size, and at the same time, the straight wall area can provide better vertical support to the arc-shaped area during welding, thereby reducing the risk of the straight wall area collapsing, and at the same time, promoting the deformation of the cap during welding to concentrate on the arc-shaped area. As shown in FIG. 7, the β angle of the straight wall area 1042 is in the range of 0° to 5°, which can more effectively maintain the vertical support effect after welding, thereby ensuring that the cap maintains a larger cavity volume V1 of the accommodating cavity (as shown in B4 of FIG. 7) after welding, and at the same time, it is beneficial to provide additional mechanical locking force to the workpiece by the cap after welding. When the straight wall area 1042 is designed to extend downward and at the same time to tilt inward, especially as the angle β' with the central axis increases, as shown in C4 of FIG. 7, this will increase the risk of the straight wall area collapsing during welding. Once the straight wall area collapses at a larger angle, it will further compress the volume V1' of the accommodating cavity of the cap after welding, thereby reducing the effect of receiving the discharged metal. In addition, the excessively inclined straight wall area provides limited additional mechanical locking force to the workpiece.

[0082] When the fastener is used to weld a thicker first metal, for example, the thickness of the first metal is 4mm to 7mm, and the shaft portion penetrates the thickness of the first metal, the volume of the first metal discharged from the welding point is significantly increased, which requires the cap to form an accommodating cavity with a large enough volume. In the case of ensuring compact transverse size, if the cross-sectional length of the arc-shaped area is too short, excessive folding deformation occurs between the straight wall area and the arc-shaped area during welding (as shown in A5 and B5 of FIG. 8), which reduces the volume (V2) of the accommodating cavity formed by the cap, and is not conducive to the cap fully receiving the discharged metal. In another preferred embodiment, the cross-sectional shape of the arc-shaped area is designed as shown in C5 of FIG. 8, the height H1 of the straight wall area 10421 is less than the minimum height H2 of the shaft portion protruding from the cap, and the highest point of the arc-shaped area is arranged near the cap 106 side, so that the cap forms a significant protruding shape near the cap side, the height of the arc-shaped area protruding from the cap is H3, and H3 is not more than H1, and the cross-sectional length La of the arc-shaped area and the cross-sectional length Lb of the straight wall area satisfy the relationship: La≥2×Lb. In this way, the cross-sectional length (La) of the arc-shaped area 1041 can be increased, and at the same time, the height H1 can be appropriately reduced. In addition, due to the large amplitude reduction of the arc-shaped area near the cap side after welding, the horizontal distance L3 from the outer periphery 1062 of the cap to the highest point of the arc-shaped area is designed as: L3≤0.4×L1, which is conducive to the arc-shaped area to produce a larger curvature deformation during welding, as shown in D5 of FIG. 8. At the same time, the cap maintains a larger transverse width at the upper end (as shown in D5 of FIG. 8) after welding, thereby forming a sufficient volume (V2') to accommodate the discharged metal.

[0083] Fig. 9 shows a cross-sectional view of a fastener and a schematic view of a deformation of its cap in yet another embodiment. When welding a first metal with a relatively small thickness, for example, the thickness of the first metal is 1 mm to 2 mm, the height H1 of the straight wall region 1042 of the fastener is set to be smaller than the minimum height H2 of the protruding top cap of the shaft portion, the cross-sectional thickness of the transition region gradually decreases from the thickness of the outer peripheral side of the top cap to the wall thickness of the arc region, and the arc region 1041 is connected to the straight wall region by a rapid downward bending of the transition region, so that an angle a exists between the line connecting the two ends of the arc region and the horizontal line, and the value of a is in the range of 15° to 65°. In addition, the wall thickness t1 of most of the arc region 1041 is almost equal to the wall thickness t2 of the straight wall region 1042. The schematic views of the shapes of the cap before and after welding are shown as A6 and B6 in Fig. 9, respectively. Since the thickness of the first metal to be welded is small, the amount of metal expelled from the welding point is relatively small, the height H1 of the straight wall region can be reduced, the arc region 1041 is deformed with a larger curvature, and the height of the cap after welding is reduced while maintaining a sufficient volume of the accommodation cavity for accommodating the expelled metal.

[0084] The included angle Ω between the inner wall 1061 of the top cap and the horizontal plane is set to be in the range of 10° to 50°, and the inner wall 1061 of the top cap extends upward from the side of the shaft portion to the outer periphery of the top cap, so that the inner wall 1061 of the top cap forms an obtuse angle with the side wall 1021 of the shaft portion (as shown in Fig. 1). This design can embed the top cap at least partially into the first metal during welding, expand the contact range of the fastener with the first metal, and form a firm locking effect. On the other hand, it is also beneficial to expel the liquid first metal from the welding point. Fig. 10 shows a schematic view of the deformation of the fastener when the fastener of the same size is welded with a first metal 200 of different thicknesses. When the first metal 200 with a thickness of T0 is welded (as shown in A7 in Fig. 10), the shaft portion 102 can almost penetrate the first metal 200 and contact the second metal 300 due to the small thickness of the first metal, so that the top cap 106 cannot continue to sink, and only a small part of the inner wall 1061 of the top cap contacts the surface of the first metal. When the first metal 200 with a thickness of T1 (T1 > T0) is welded (as shown in B7 in Fig. 10), the shaft portion cannot produce a large penetration distance in the first metal 200 due to the large thickness of the first metal 200 and the short size of the shaft portion. Therefore, under the combined action of the electrode pressure and the resistance heat, the top cap 106 produces a more significant bending deformation in the direction of the central axis, further promotes the sinking of the top cap 106 into the first metal 200, and causes the inner wall 1061 of the top cap to be in complete contact with the first metal. Therefore, the same fastener can be used to weld first metals with a large thickness range. In addition, when welding a first metal with the same thickness, by adjusting the welding process, for example, by increasing the electrode pressure and the welding current, the shaft portion 102 is plastically deformed and shortened, and the inner wall 1061 of the top cap is embedded into the surface of the first metal.

[0085] In order to further improve the plastic deformation ability of the arc-shaped area 1041 during welding, the arc-shaped area is provided with radially distributed knurling indentations 1044, which can weaken the strength of the arc-shaped area 1041 to some extent, and is conducive to improving the bending deformation of the arc-shaped area and the compression deformation of the circumferential material when the shaft portion penetrates the first metal, thereby improving the ability of the fastener shaft portion to penetrate the first metal.

[0086] In one welding embodiment of the fastener of the present disclosure for resistance spot welding of dissimilar metals, the welding process is shown in FIG. 12. In the welding process, a current (I1) in the preheating stage, a current (I2) in the fastener penetration stage, and a current (I3) in the welding stage are provided. The electrode pressures in the preheating stage, the fastener penetration stage, and the welding stage are F1, F2, and F3, respectively, and the electrode pressure relationship is F1≥F2≥F3. The currents in each stage are designed for different purposes in combination with the thickness and strength of the first metal and the second metal to be welded. The process of welding dissimilar metals by the fastener 100 of the present disclosure is further described in combination with the welding process schematic as shown in FIGS. 13-16. First, the upper electrode 400 and the lower electrode 500 apply pressure F1 to the fastener 100, the first metal 200, and the second metal 300, so that the fastener and the workpiece are in close contact, and the shaft portion and the end of the cap straight wall area of the fastener are completely extruded onto the surface of the first metal workpiece, as shown in FIG. 13.

[0087] In the preheating stage, as shown in FIG. 14, in order to make the shaft portion and the end of the cap straight wall area of the fastener in closer contact with the surface of the first metal 200 to be welded. The preheating stage uses a small current I1 and maintains a relatively long time, for example, the current is set to 4kA and maintained for 150ms. Further, in the case of welding thicker first metals, for example, welding first metals with a thickness greater than 4.0mm of aluminum alloy, the preheating stage can further extrude the shaft portion 102 of the fastener into the first metal 200, reducing the remaining first metal thickness T2 between the end surface 103 of the shaft portion and the second metal 300, creating more favorable conditions for the shaft portion 102 to quickly penetrate the first metal 200 in the subsequent fastener penetration stage. Therefore, the electrode pressure is preferably set to satisfy the relationship: F1>1.5×F2, and F2=F3. At this time, the preheating current is appropriately increased. For example, I1 is increased to 6kA and the time is increased to 250mm, so that part of the first metal melting area 201 is extruded into the cap to form the discharged metal (spatter metal) 202.

[0088] In the fastener piercing stage as shown in FIG. 15, the electrode exerts a pressure F2 on the weld, and several relatively large and short current pulses I2 are applied in the weld. For example, 4 current pulses are applied, and each current pulse has a current I2 of 18 kA and is maintained for 50 ms, so that the first metal 200 in the weld is quickly melted, and the melted liquid metal is extruded by the fastener shaft and the expansion force of the liquid metal itself, forming the expelled metal (spatter metal) 202 and quickly entering the receiving cavity formed by the fastener cap. The shaft of the fastener is quickly pierced into the first metal 200 under the pressure of the electrode and achieves contact with the second metal 300. During the piercing of the shaft of the fastener into the first metal, the straight wall area 1042 of the cap supports upward, and the arc-shaped area 1041 is deformed downward as the shaft of the fastener sinks, thereby causing a large bending deformation of the straight wall area 1042 and the arc-shaped area 1041, which ensures that the cap can maintain the structure of the receiving cavity. The shaft of the fastener is quickly pierced into the first metal, and the piercing process ends after the shaft contacts the surface of the second metal 300. In addition, the weld area of the second metal 300 is depressed under the resistance heat and the extrusion of the lower electrode 500, resulting in a raised area with a height T3 on the surface of the second metal in the weld, so that the raised area directly contacts the end 103 of the shaft of the fastener. Since the current pulse in the fastener piercing stage has a short maintenance time, it is beneficial to control the heat input, so that in this stage, isolated local melting areas 107 and 301 are formed in the shaft of the fastener and the weld of the second metal, respectively, and the periphery of these local melting areas 107 and 301 is still in a solid state, avoiding contact with the first metal melting area 201 and preventing the formation of a large amount of brittle intermetallic compounds in the weld. However, in some embodiments, due to the excessive current pulses in the fastener piercing stage or due to the excessive maintenance time of a single current pulse, after the aluminum between the shaft of the fastener and the second metal is fully expelled to form a contact surface, excessive resistance heat promotes the formation of a small nugget 600 on the contact surface by local melting, so that the fastener 100 can form a stronger connection with the second metal 300 in this stage.

[0089] The welding stage process is shown in FIG. 16, in which the electrode exerts a pressure F3 on the weld, and a relatively large and long welding current I3 is applied, for example, a current of 12 kA is maintained for 400 ms. Under the action of the continuous input resistance heat, the contact surface between the shaft of the fastener and the second metal melts and forms a firm nugget 600. The first metal around the shaft of the fastener is affected by the resistance heat, and the first metal melting area 201 further expands, and a metallurgical connection is formed on the interface between the first metal melting area 201 and the part of the fastener and the second metal in contact. In addition, part of the melted first metal is again extruded into the receiving cavity formed by the cap 104 of the fastener, so that the receiving cavity is filled with more expelled metal 202.

[0090] Example 1:

[0091] In this embodiment, the cross-sectional structure of the fastener used is shown in Figure 17, the side wall of the shaft portion gradually expands when extending from the end of the shaft portion to the outer periphery of the cap, and the wall thickness of the transition zone gradually decreases from the thickness of the outer periphery of the cap to the wall thickness of the arc-shaped zone. The fastener is designed with the following dimensions: L = 0.7 mm, L1 = 1.71 mm, L2 = 0.45 mm, D = 5.0 mm, Dmax = 12.0 mm, H = 2.2 mm, H1 = 1.45 mm, H2 = 1.2 mm, γ = 60°, Ω = 30°. The arc-shaped zone 1041 is a circular arc, and the straight wall zone is designed to be perpendicular to the horizontal plane. The first metal and the second metal are welded using the welding process shown in Figure 12, wherein the fastener is made of stainless steel, the first metal 200 is 1.6 mm thick 6061 aluminum alloy, and the second metal 300 is 1.2 mm thick QP1180 steel (the first metal and the second metal are the same as the materials used in one embodiment of the published patent CN115570251A). After welding, the cross-sectional metallographic structure of the joint is shown in Figure 18, the shaft portion of the fastener effectively penetrates the first metal 200 and forms a firm connection with the second metal 300 through the fusion core 300. The straight wall zone 1042 of the cap vertically extrudes to the surface of the first metal 200 and cannot move downward, vertically supporting one end of the arc-shaped zone 1041, while the other end of the arc-shaped zone moves downward under the traction of the transition zone, causing the arc-shaped zone to bend and deform near the straight wall zone, so that the cap maintains a cavity structure that completely accommodates the expelled metal 202. Compared with the joint in the comparative document CN115570251A, as shown in Figure 19. The maximum diameter Dmax of the fastener after welding and the height H5 of the cap protruding from the surface of the first metal in the present invention are 12.18 mm and 2.06 mm, respectively, which are much smaller than those of the joint in the comparative document. In addition, the electrode indentation depth H6 on the surface of the second metal of the joint of the fastener in the present invention is 0.38 mm, which is much smaller than that of the joint in the comparative document. This fully demonstrates that the fastener size in the present invention is more compact, effectively reduces the indentation depth and the height of the cap protruding from the surface of the first metal under the premise of ensuring sufficient accommodation of the expelled metal, and at the same time can achieve firm connection.

[0092] Example 2:

[0093] In this embodiment, the cross section of the fastener used is shown in Figure 20, the side wall of the shaft portion gradually expands when extending from the end of the shaft portion to the outer periphery of the cap, the wall thickness of the transition zone gradually decreases from the thickness of the outer periphery of the cap to the wall thickness of the arc-shaped zone, and the design size of the fastener is: L = 0.68 mm, L1 = 2.45 mm, L2 = 1.0 mm, D = 4.8 mm, Dmax = 15.0 mm, H1 = 1.6 mm. The arc-shaped zone 1041 is a circular arc, and the outer wall circular arc radius is 2.5 mm, and the wall thickness of the arc-shaped zone is 0.4 mm, which is connected through a circular arc R1 at the intersection of the outer periphery 1062 of the cap and the inner wall 1061 of the cap, and the circular arc R1 is tangent to the outer periphery of the cap and the inner wall 1061 of the cap. The circular arc R2 is connected between the inner surface of the arc-shaped zone 1041 and the cap, wherein the circular arc R2 is tangent to the inner surface of the arc-shaped zone 1041 and the circular arc R1, respectively, forming a transition zone. The radius of the circular arc R1 and the circular arc R2 is 0.8 mm. The wall thickness of the transition zone gradually decreases from the thickness of the outer periphery of the cap (greater than 1 mm) to the wall thickness of the arc-shaped zone (0.4 mm), so that the plastic deformation of the cap mainly occurs in the arc-shaped zone. In addition, the straight wall zone 1042 is perpendicular to the horizontal plane. The fastener is welded to the first metal and the second metal, and the welding process shown in Figure 12 is used during welding, and the electrode pressure is 5.5 kN during welding. The fastener is made of stainless steel, the first metal 200 is 2.5 mm thick 6000 series aluminum alloy, and the second metal 300 is 1.5 mm thick and has a strength of 1500 MPa grade aluminum-silicon plated hot formed steel. The metallographic section of the joint after welding is shown in Figure 21, the fastener successfully penetrates the first metal 200, and forms a common fusion zone 600 with the second metal 300 to achieve firm connection. The arc-shaped zone 1041 of the fastener is bent, and the straight wall zone 1042 supports upward, so that the cap maintains the cavity structure to accommodate and discharge the metal 202. The height of the cap of the fastener protruding from the surface of the first metal after welding is only 2.66 mm, the maximum diameter Dmax is 15.28 mm, and the electrode indentation depth on the second metal side is 0.61 mm. As can be seen, in the case of welding thicker first metal, the fastener of the present application not only achieves firm connection, but also has a lower cap height protruding from the surface of the first metal than the joint in the comparative document CN115570251 A; in addition, the electrode indentation depth on the second metal side is also much smaller than that in the comparative document (as shown in Figure 19), which indicates that the shaft portion of the fastener of the present application has better ability to penetrate the first metal.

[0094] Embodiment 3:

[0095] In this embodiment, the surface topography and cross-sectional view of the fastener 100 is shown in FIG. 22. The sidewall of the shaft portion gradually expands when extending from the end of the shaft portion to the outer periphery of the cap. The wall thickness of the transition region gradually decreases from the thickness at the outer periphery of the cap to the wall thickness of the arc region. The fastener is made of low carbon steel. The arc region 1041 of the fastener is a circular arc and is connected to the straight wall region 1042 by tangency. The wall thickness of the arc region gradually decreases when extending from the cap side to the straight wall region, so that the plastic deformation of the arc region occurs on the side close to the straight wall region during welding, thereby achieving the control of the post-weld protrusion of the cap above the first metal surface. The dimensions of the fastener are as follows: H = 3.2 mm, H1 = 2.6 mm, H2 = 2.0 mm, L = 0.85 mm, L1 = 2.22 mm, L2 = 0.6 mm, γ = 55°, Ω = 32°, and D = 5.1 mm. The wall thickness of the cap first decreases and then increases, where t1 = 0.42 mm, t2 = 0.31 mm, t3 = 0.34 mm, and t4 = 0.38 mm. The angle β of the outward inclination of the straight wall region 1042 is designed to be 2°. The first metal 200 is selected to be 2.5 mm thick 6000 series aluminum alloy, and the second metal 300 is selected to be 2.0 mm thick and 980 MPa grade advanced high-strength steel (QP 980). The welding process shown in FIG. 12 is used during welding, and the fixed electrode pressure is 5.5 kN during welding. First, the preheating current I1 and the fastener piercing current I2 are applied to the weld point. FIG. 23 is a topography diagram of the fastener 100 after the fastener pierces the first metal, and then the fastener 100 is peeled off from the weld point. It can be seen that a larger hole is formed on the first metal 200, which indicates that the shaft portion successfully pierces the first metal 200. In addition, the expelled metal 202 from the weld point is completely accommodated in the accommodation cavity formed by the cap of the fastener, and the volume of the accommodation cavity is much larger than the volume of the expelled metal 202.

[0096] Figure 24 shows the cross-section metallographic image of the joint after welding, it can be seen that the shaft of the fastener penetrates the first metal 200 and forms a common nugget 600 with the second metal 300, and the first metal is firmly locked in the joint. Due to the thin thickness of the first metal 200, only a partial area of the inner wall 1061 of the cap is extruded on the surface of the first metal. In addition, the straight wall area 1042 of the cap is extruded to the surface of the first metal 200 and vertically supports upward, causing the area near the arc-shaped area 1041 and the straight wall area 1042 to be deformed by a large bending, avoiding the collapse of the cap structure, so that the cap maintains the cavity shape to completely accommodate the expelled metal 202. It can be seen from the present embodiment that due to the vertical support of the straight wall area 1042, the arc-shaped area 1041 is deformed by bending near the upper end of the straight wall area. The height of the cap of the fastener protruding from the surface of the first metal after welding is 3.23 mm, the depth of the electrode indentation on the second metal side is 0.62 mm, and the maximum diameter Dmax is 15.25 mm. Compared with the fastener in Example 2, since the height H1 of the straight wall area 1042 in the present embodiment is larger, the height of the cap protruding from the surface of the first metal after welding is also higher, but the volume of the accommodation cavity formed by the cap after welding is also increased. As can be seen from Figure 24, the left cap is not filled with expelled metal 202, which also indicates that the fastener of the present application can control the volume of the accommodation cavity formed by the cap after welding and the height of the cap protruding from the surface of the first metal by controlling the height of the straight wall area 1042.

[0097] Example 4:

[0098] In this embodiment, the same fastener 100 as in Example 3 was used, the first metal 200 was welded as a 3 mm thick cast aluminum alloy, and the second metal was still a 1.5 mm thick aluminum-silicon plated hot-formed steel with a strength of 1500 MPa level, and the welding process was still the process shown in Figure 12, and the fixed electrode pressure during welding was 5.5 kN. The joint surface and cross-sectional morphology after welding is shown in Figure 25, it can be seen that the shaft part of the fastener 100 can also pierce the thicker first metal 200, and form a common nugget 600 with the second metal 300, so that the fastener and the second metal form a firm connection. Because the shaft side wall 1021 and the inner wall 1061 of the cap gradually expand when extending upward from the second metal side, the first metal is firmly locked in the joint, so that the first metal and the second metal form a high-strength connection. Compared with Example 3, since the thickness of the first metal 200 is increased by 0.5 mm, the piercing depth of the fastener shaft 102 is further increased under the condition that the size of the fastener does not change, which makes the cap 106 further sink and embed into the first metal, resulting in a larger area on the inner wall 1061 of the cap being in contact with the first metal. The discharged metal 202 can be completely accommodated in the cap of the fastener, and there is no first metal overflow from the cap. The height of the fastener cap protruding from the first metal surface after welding is 3.04 mm, the electrode indentation depth on the second metal side is 0.62 mm, and the maximum diameter Dmax is 15.28 mm, which is close to the measurement results of Example 3, indicating that the same fastener of the present application has stable joint structure size when welding different thicknesses of the first metal. In addition, the results of this embodiment show that the fastener structure can firmly weld the thicker first metal, and the same fastener can be compatible with the welding of a wide range of thicknesses of the first metal.

[0099] Comparative Example 1:

[0100] In this comparative example, the fastener 100 used was the same as in Example 3, and the cap of the fastener was cut as shown in the schematic diagram A8 in Figure 26 to obtain a fastener without a cap for welding with dissimilar metals. The welding parameters, the first metal and the second metal welded were exactly the same as in Example 4. The surface morphology of the weld after welding is shown in Figure 26 B8. Since the fastener has no cap structure to accommodate the discharged metal 202, the discharged metal is distributed around the weld, which not only affects the surface quality of the weld, but also the high-speed ejected splash metal can also contaminate the surrounding components. This comparative example 1 fully shows the importance, necessity and effectiveness of the cap of the fastener of the present application in accommodating the discharged metal.

[0101] Comparative Example 2:

[0102] The first metal of the same kind was welded by using the traditional resistance spot welding process, wherein the welded first metal was the cast aluminum alloy with a thickness of 3.0 mm in Example 4, and the welding parameters were the optimized resistance spot welding parameters. In this embodiment, the welding of the cast aluminum alloy of the same kind was realized.

[0103] The cross tensile mechanical property test was performed on the welded joints of Example 4, Comparative Example 1 and Comparative Example 2 (the cross tensile sample size was 150 mm x 50 mm), and the force-displacement curve obtained after the test is shown in FIG. 27, wherein curve ① is the joint obtained in Example 4, curve ② is the joint obtained in Comparative Example 1, and curve ③ is the joint obtained in Comparative Example 2. The fracture morphology of the joint after the tensile test is shown in FIG. 28, wherein A9 is the joint of Example 4 (curve ①), B9 is the joint of Comparative Example 1 (curve ②), and C9 is the joint of Comparative Example 2 (curve ③).

[0104] The load of the resistance spot welded joint of the cast aluminum alloy of the same kind was 3443.4 N, and the joint was a button pull-out fracture, as shown in C9 in FIG. 28, indicating that the resistance spot welded joint of the cast aluminum alloy of the same kind has high connection performance. However, the cross tensile load of the heterogeneous metal welded joint of the fastener 100 of the present application was as high as 5424.6 N, and the displacement was larger than curve ③ (as shown in curve ① and curve ③ in FIG. 27), which indicated that the fastener of the present application could realize high-strength reliable connection of heterogeneous metals.

[0105] The only difference between Example 4 and Comparative Example 1 is that the fastener used in this example does not have a cap structure. However, there is a great difference between the cross tensile performance and failure mode of the joint in Example 4 and Comparative Example 1. In terms of cross tensile performance, the maximum load of the joint is almost similar, which indicates that the structural design of the shaft and cap of the fastener of the present application can provide high strength connection performance for the joint. However, the curve ① of the joint in Example 4 has a greater displacement than the curve ② of the joint in Comparative Example 1. In addition, after the load reaches the maximum value, the curve ② suddenly decreases, while the curve ① continues to produce displacement S, and then the load suddenly decreases to a certain extent and then slowly decreases (as shown in FIG. 27). In terms of joint failure, the cast aluminum alloy around the joint in Example 4 produces a significant crack, and the arc-shaped area 1041 of the cap of the fastener breaks, resulting in the straight wall area 1042 remaining on the aluminum side weld (A9 in FIG. 28). The cast aluminum alloy around the joint in Comparative Example 1 does not produce a crack (B9 in FIG. 28), and a circular hole is left on the first metal weld after the mechanical locking of the shaft and cap of the fastener is broken. The reason for the difference in cross tensile load and failure mode of the joint between Example 4 and Comparative Example 1 is that the straight wall area 1042 provided by the fastener 100 of the present application can be vertically extruded onto the surface of the first metal 200 after welding, as shown in FIG. 29. During the cross tensile process, the straight wall area generates an additional extrusion force Fc on the outer periphery of the joint, which provides additional mechanical locking force for the joint, thereby improving the mechanical performance of the joint.

[0106] It should be understood that the purpose of the above examples is only to illustrate the technical concept of the present application to facilitate the understanding of those skilled in the art, and is not intended to limit the protection scope of the present application. Any improvement and equivalent replacement of the parts, structures or method steps involved in the above examples, especially the combination of different examples without structural or principle conflict, fall within the protection scope of the present application.

Claims

1. A fastener for resistance spot welding dissimilar metals, the fastener being used for resistance spot welding a stack of a first metal and a second metal, the first metal having a lower melting point than the second metal, the fastener metal having a melting point same as or close to that of the second metal, the fastener comprising a top cover, a shaft portion extending from the center of the top cover, and a cap cover arranged around the outer periphery of the top cover, the cap cover and the side wall of the shaft portion and the end of the shaft portion forming a receiving cavity, the receiving cavity being used to collect the first metal squeezed out or sprayed out of the weld point, characterized in that: the side wall of the shaft portion gradually expands when extending from the end of the shaft portion to the outer periphery of the top cover, which is beneficial to the high-speed penetration of the shaft portion into the first metal and the discharge of the molten first metal from the weld point during welding, and provides the fastener with a main mechanical locking force to the first metal; the cap cover extends from the outer periphery of the top cover to the level of the end of the shaft portion, sequentially forming a transition zone, an arc-shaped zone, and a straight wall zone; wherein the arc-shaped zone has good plastic deformation capability, and can adaptively deform during the welding process to avoid the cap cover hindering the penetration of the shaft portion into the first metal; the straight wall zone vertically supports the cap cover during the welding process, forcing the cap cover to deform mainly in the arc-shaped zone, while avoiding the cap cover from collapsing after welding to reduce the volume of the receiving cavity; the straight wall zone is almost vertically extruded to the surface of the first workpiece after welding, providing the welded joint with additional mechanical locking force; the wall thickness of the transition zone gradually decreases from the thickness of the outer periphery of the top cover to the wall thickness of the arc-shaped zone, avoiding stress concentration and cracking of the transition zone during manufacturing and welding, and ensuring that plastic deformation of the cap cover mainly occurs in the arc-shaped zone. the wall thickness of the arc-shaped zone gradually decreases when extending from the side of the top cover to the straight wall zone, causing the plastic deformation of the arc-shaped zone to occur on the side close to the straight wall zone during the welding process, and controlling the height of the cap cover protruding out of the surface of the first metal after welding. the wall thickness of the straight wall zone gradually increases when extending from the end close to the arc-shaped zone to the other end in contact with the first metal, and the increase is not more than 65%. the wall thickness of the arc-shaped zone gradually decreases when extending from the side of the top cover to the straight wall zone, and the decrease is not more than -50%. the minimum height H2 of the shaft portion protruding out of the top cover satisfies H2 = K1 x T, where T is the thickness of the first metal, and K1 is a coefficient considering the thickness of the first metal, and the value range of K1 is K1 = 0.6-1.2, the thickness H of the center of the fastener satisfies H = 1.2 x H2-2 x H2, the minimum horizontal width from the outer periphery of the top cover to the side wall of the shaft portion is L, and the minimum horizontal distance from the outer periphery of the top cover to the straight wall zone is L1, where L1 is greater than 1.5 x L, 2. The fastener of claim 1, wherein, the horizontal width L2 of the transition zone is less than 0.5 x L1, 3. The fastener of claim 1, wherein, a line passing through the connection point of the arc-shaped zone and the straight wall zone and tangent to the arc-shaped zone forms an angle δ with the straight wall zone, and the range of angle δ is 0°-40°, 4. The fastener of claim 1 or 2, wherein the straight wall zone is arranged around the shaft portion of the fastener, and forms an angle β with the central axis of the fastener, and β = 0°-5°, and the height H1 of the straight wall zone is less than H, 5. The fastener of claim 1, wherein, the angle Ω between the inner wall of the top cover and the horizontal plane satisfies Ω = 10°-50°. the angle γ between the side wall of the shaft portion and the horizontal plane ranges from 45° to 80°, and γ > Ω. ​ ​ ​ ​ ​ 6. The fastener of claim 1 or 5, wherein, ​ 7. The fastener of claim 1 or 5, wherein An angle α between a line connecting an end of the cap close to the outer periphery of the cap and an end of the arc-shaped area close to the straight wall area and a horizontal plane exists, and α = 15°-65°.

8. The fastener of claim 1 or 5, wherein, A height H1 of the straight wall area is less than a minimum height H2 of the convex cap of the shaft portion.

9. The fastener of claim 1, wherein, A connecting area of the arc-shaped area and the straight wall area is in a tangential relationship.

10. The fastener of claim 1, wherein, The arc-shaped area is a circular arc, and a radius of the circular arc ranges from 0.6 mm to 4 mm.

11. The fastener of claim 1, wherein, A wall thickness of the arc-shaped area and the straight wall area ranges from 0.25 mm to 0.6 mm; a horizontal width L of the outer periphery of the cap to the side wall of the shaft portion ranges from 0.3 mm to 1.6 mm; a minimum horizontal distance L1 of the outer periphery of the cap to the straight wall area ranges from 1.2 mm to 3.2 mm; and a diameter D of the end of the shaft portion ranges from 3 mm to 6 mm.

12. The fastener of claim 1, wherein, A junction of the outer periphery of the cap and an inner wall of the cap is connected by a circular arc R1, and a radius of the circular arc R1 is greater than 0.3 mm.

13. The fastener of any one of claims 1-12, wherein, A melting point temperature of the first metal is lower than 750 °C, and a melting point temperature of the second metal is higher than 1300 °C.

Images