Fastener for welding dissimilar metals, welding electrode, and welding method

Abstract

The present invention provides a fastener for welding dissimilar metals. The fastener comprises a shaft portion, a flange portion, and a cap surrounding the periphery of the shaft portion. The outer contour of the cross-section of the shaft portion is configured as a first arc which is outwardly convex, thereby improving the strength of the shaft portion and its capability of penetrating a metal workpiece, and preventing excessive flattening deformation of the shaft portion during welding. The outer side of the shaft portion is connected to the lower surface of the flange portion by means of a second arc, so that the size of the flange portion is reduced, thereby increasing the volume of an accommodating cavity formed by the cap while ensuring a compact size of the fastener, and also improving the compatibility of the flange portion and the fastener in welding metal workpieces of different thicknesses. In addition, in order to improve weld spot formation and achieve stable welding of dissimilar metals, the present invention further provides a welding electrode for welding the fastener to dissimilar metals and a welding method.

Description

Fastener, welding electrode and welding method for welding dissimilar metals TECHNICAL FIELD

[0001] The present invention belongs to the field of welding dissimilar metals, and particularly relates to a fastener, welding electrode and welding method for welding dissimilar metals. BACKGROUND

[0002] In the automobile industry, the manufacture of the vehicle body is usually made of a mixture of various metal materials, especially the vehicle body of new energy vehicles. Aluminum metal and steel are the main materials used in the vehicle body. In particular, integrated die-cast aluminum metal is popularly used in the rear floor and engine compartment of the vehicle body, while high-strength steel or ultra-high-strength steel is used in A / B columns, door rings, floor channels, and side impact steel beams. Therefore, in the automobile industry, the connection of dissimilar metals represented by aluminum / steel has a wide range of application requirements. However, due to the large difference in thermophysical properties of dissimilar metals, cracks and brittle intermetallic compounds are easily formed in the weld during welding, making it difficult to achieve the mechanical properties of the joint required for industrial applications. In particular, under the fast-paced manufacturing environment of the vehicle body, higher requirements are placed on the welding of dissimilar metals. Therefore, the welding technology of dissimilar metals is difficult to be applied in the manufacture of the vehicle body.

[0003] In order to achieve reliable welding of dissimilar metals, a promising technical solution is to use a welding and riveting (weld-riveting) combined welding technology for connection. For example, when connecting aluminum alloy and steel, the weld-riveting combined technology can use a steel fastener (or rivet and welding element, etc.) combined with external force or a pre-process hole to be pre-placed on the aluminum alloy, and then the fastener is welded with the steel, so that the fastener can firmly rivet the aluminum alloy workpiece. Since this process solution excludes all aluminum alloy in the weld, it can avoid the problems caused by direct welding of dissimilar metals, and the joint strength obtained is relatively reliable. However, pre-placing a process hole on an aluminum plate or stamping a pre-placed fastener on an aluminum alloy increases the complexity of the connection and reduces the welding efficiency.

[0004] In order to achieve efficient and reliable welding of dissimilar metals, patent CN114211104A discloses a method for high-speed welding of dissimilar metals, which induces low-melting-point metal spatter to achieve high-speed discharge of low-melting-point metal in the weld, while driving the fastener to pierce the low-melting-point metal layer and weld with the high-melting-point metal workpiece. For this welding method, a special fastener is needed. Under the premise of ensuring the compact size of the fastener, it can simultaneously achieve sufficient accommodation of induced discharged spatter metal and axial penetration of the metal layer without excessive flattening deformation. Designing the structure of the above fastener is particularly important in the field.

[0005] Since the welding technology disclosed in patent CN114211104A actively induces the ejection of low-melting-point metal, the material in the welding point is missing, and the metal workpiece is softened by heat, so the fastener shaft part can easily pierce the low-melting-point metal layer under the action of a small pressure. Therefore, the height of the fastener shaft part is relatively small. The shaft part disclosed in patent CN104540628B has a high aspect ratio, which is not suitable for the welding technology disclosed in patent CN114211104A. Patent CN114932302B proposes another fastener structure, in which the shaft part is a cylindrical structure, and the end face of the shaft part is relatively flat. When the shaft part of this structure is welded by the welding method disclosed in patent CN114211104A, the shaft part is easily deformed plastically due to excessive flattening when the diameter of the shaft part is too small, resulting in poor piercing effect of the shaft part. In order to improve the strength of the shaft part, the diameter of the shaft part needs to be increased. However, as the diameter of the shaft part increases, it is difficult to completely eject the low-melting-point metal in the welding point and increase the difficulty of piercing the metal workpiece.

[0006] Patent CN115780980A discloses a welding element (hereinafter referred to as a fastener) for welding dissimilar metals. The shaft part is similar to an inverted circular table structure, and the shaft part side forms an inclination angle α with the axis, which is beneficial to the piercing of the workpiece and the ejection of the liquid metal. However, when the inclination angle is small, for example, α is less than 30°, the shaft part is easily collapsed after being heated. When the angle α is increased, in order to ensure that the cavity formed by the cap can completely accommodate the splashed metal, the diameter of the fastener as a whole must be increased. In particular, when welding thick dissimilar metal workpieces, the height of the fastener shaft part and the diameter of the fastener are further increased, resulting in an excessively large size of the fastener and an excessively bulky welding point, which is not conducive to the design of the welding point in structure.

[0007] Therefore, there is an urgent need in the art for an innovative fastener structure that avoids excessive collapse of the shaft part during welding, improves the piercing ability of the shaft part, and ensures that the size of the fastener is compact and the cap of the fastener can completely accommodate the liquid metal ejected from the welding point.

[0008] SUMMARY

[0009] The present application proposes a fastener for welding dissimilar metals, a welding electrode, and a welding method to overcome the above problems, achieve compact design of the fastener, improve the piercing effect of the shaft part on low-melting-point workpieces, and improve the welding quality and stability of dissimilar metals.

[0010] The application provides a fastener for welding dissimilar metals, which is used for welding a laminated structure of a first metal workpiece and a second metal workpiece, and the melting point of the first metal workpiece is less than 750 DEG C, and the melting point of the second metal workpiece is greater than 1300 DEG C, the fastener is an axis rotationally symmetrical structure, comprising a shaft part, a flange part and a cap, and the material type is the same as that of the second metal workpiece, the shaft part is used for penetrating the first metal workpiece and being welded with the second metal workpiece, the lower end surface of the shaft part is a plane, and the lower end size of the shaft part is smaller than the upper end, and the side surface of the shaft part is a curved surface formed by the rotation of a circular arc around the axis; the flange part is gradually extended outward and upward from the upper end of the shaft part, a thin-walled cap is further extended and bent outward from the edge of the flange part, and the end part of the cap extends to the horizontal plane where the lower end surface of the shaft part is located, so that the cap is distributed around the periphery of the shaft part and forms a semi-open annular cavity with the outer side of the shaft part, and the volume V1 of the semi-open annular cavity and the volume V2 of the fastener entity satisfy the relationship: V2 < V1 < 2 * V2.

[0011] In the cross section passing through the axis of the fastener, the profile of the fastener comprises:

[0012] a first circular arc, which is located on the side surface of the shaft part, and the center of the first circular arc is located in the shaft part, and one end of the first circular arc intersects with the lower end surface of the shaft part, and the other end extends upward and does not exceed the horizontal plane where the center of the circular arc is located; a second circular arc, which is located on the upper side of the first circular arc and constitutes at least part of the lower surface of the flange part, one end of the second circular arc is connected with the first circular arc, so that the side surface of the shaft part is smoothly connected to the lower surface of the flange part, and the center of the second circular arc is located on the outer side of the shaft part; and a third circular arc, which is located in the inner surface of the bent part of the cap, one end of the third circular arc is connected with the outer edge of the lower surface of the flange part, and the other end is connected with the first straight line extending from the end part of the cap.

[0013] Further, the distance H1 from the connecting point of the first circular arc and the second circular arc to the plane where the lower end surface of the shaft part is located satisfies the relationship: 0.5 * T < H1 < T, wherein T is the total thickness of the first metal workpiece.

[0014] Further, the distance H2 between the horizontal plane where the connecting point of the first circular arc and the second circular arc is located and the horizontal plane where the connecting point of the third circular arc and the outer edge of the flange part is located satisfies the relationship: H2 ≤ H1.

[0015] Further, the first circular arc and the second circular arc are tangent at the connecting point, and the centers of the first circular arc and the second circular arc are located on the same horizontal line.

[0016] Further, one end of the third circular arc passes through the second straight line and is connected with the second circular arc, and the second straight line is tangent to the second circular arc at the connecting point.

[0017] Optionally, there is a circular chamfer at the connecting position of the third circular arc and the second straight line.

[0018] Further, the ratio of the diameter D2 of the shaft portion at the intersection of the first and second circular arcs to the diameter D1 is in the range of 1.4 to 2.2.

[0019] Preferably, the diameter D1 of the lower end surface of the shaft portion is in the range of 4 to 6 mm, when the total thickness T of the first metal workpiece to be welded is less than 2.5 mm, D1 is in the range of 4 to 5 mm; when 2.5 mm≤T<5 mm, D1 is in the range of 4.8 to 5.5 mm; and when T≥5 mm, D1 is in the range of 5 to 6 mm.

[0020] Further, the edge wall thickness t1 of the flange portion is not more than 0.55 mm, and the maximum wall thickness of the area where the third circular arc region is located on the cap is not more than 0.5 mm; preferably, the wall thickness of the area where the third circular arc region is located on the cap is in the range of 0.35 to 0.42 mm.

[0021] Further, the radius (R1) of the first circular arc is greater than the radius (R2) of the second circular arc.

[0022] In order to improve the effect of welding dissimilar metals by the fastener of the present application, a welding electrode is provided for welding the fastener of the present application into the laminated structure of the first metal workpiece and the second metal workpiece, and the fastener is placed on the welding point of the first metal workpiece before welding. The welding electrode is divided into a first electrode and a second electrode; wherein the first electrode is placed on one side of the fastener, and the diameter D4 of the working end surface thereof has a relationship with the diameter D1 of the lower end surface of the shaft portion of the fastener: D1<D4<1.4×D1, and the spherical radius R4 of the working end surface is not more than 60 mm; the second welding electrode is placed on one side of the second metal workpiece, the ratio of the diameter D5 of the working end surface thereof to the diameter D4 is greater than 1.5, and the spherical radius R5 is greater than 95 mm; preferably, the spherical radius R4 of the first welding electrode is in the range of 50 to 60 mm, and the spherical radius R5 of the second welding electrode is in the range of 100 to 150 mm.

[0023] Further, the working end surface of the second welding electrode is divided into a first working end surface and a second working end surface; wherein the first working end surface is located inside and is used to concentrate most of the welding current to promote the formation of the welding point, and the end surface diameter D6 thereof has a relationship with the working end surface diameter D4 of the first electrode: D6≤D4; the second working end surface is located on the periphery of the first working end surface and is used to limit the first working end surface from producing serious electrode indentation on the welding point, the second working end surface is composed of a plurality of concentric annular ridges, and the tips of the annular ridges are located on the same spherical surface as the first working end surface.

[0024] Preferably, the tip width of the concentric annular ridges is not more than 1 mm, and the annular grooves existing between the concentric annular ridges have a depth in the range of 0.5 to 1 mm.

[0025] In order to improve the stability of the fastener welding, a welding method is provided, which adopts the fastener and welding electrode of the present application to weld the laminated structure of the first metal workpiece and the second metal workpiece, and the welding electrode is used to input the welding current and apply the electrode pressure to the welding spot during welding, the welding current includes the preheating current (I1), the piercing current (I2) and the nucleation current (I3), the piercing current (I2) is used to discharge the first metal in the welding spot and realize the piercing of the fastener shaft to the first metal workpiece, the piercing current (I2) is divided into two parts, the first part is the induced spatter current (I2-1), which is mainly used to induce the spatter phenomenon of the first metal in the welding spot, and the first metal in the welding spot is discharged at high speed through the liquid metal spatter process, so that the shaft of the fastener is efficiently pierced to the first metal workpiece under the action of the electrode pressure and is in contact with the second metal workpiece, and the induced spatter current includes a plurality of pulse currents.

[0026] Further, the current intensity, the maintenance time and the cooling time between the pulse currents of the induced spatter current (I2-1) show a downward trend, the maximum maintenance time of a single pulse is not more than 75 ms, and the maximum cooling time is not more than 20 ms; the cooling time between the induced spatter current and the interface stabilization current ranges from 25 ms to 35 ms; the maximum maintenance time of a single pulse of the interface stabilization current (I2-2) is not more than 60 ms, and the current intensity is not more than that of the induced spatter current (I2-1).

[0027] The beneficial effects of the present application are as follows:

[0028] (1) The fastener structure of the present application is compact, which avoids the excessive bulkiness of the joint structure after welding, and is beneficial to the production and manufacturing of the fastener, and the manufacturing cost is low.

[0029] (2) The shaft side is provided with a first circular arc, and the first circular arc is transitioned to the lower surface of the flange part, on the one hand, the diameter of the shaft side is rapidly increased from the lower side to the upper side, the strength of the lower side of the shaft is improved, and excessive plastic deformation of the shaft is avoided, and at the same time, the size of the lower end of the shaft is smaller than that of the upper end, which ensures that the spatter metal can be smoothly discharged when the shaft pierces the first metal workpiece, and the mechanical locking strength of the shaft after piercing the first metal workpiece is ensured; on the other hand, the first circular arc gradually approaches the vertical direction when extending upward, so that the flange part is arranged closer to the axis, and the compactness of the fastener size is improved while ensuring that the semi-open annular cavity formed by the cap has sufficient volume.

[0030] (3) The second arc is connected to the shaft side and the lower surface of the flange part, so that the lower surface of the flange gradually increases, and then can sink to the surface of the first metal workpiece with different thickness, which is beneficial to the compatible welding of the fastener with the same size to the first metal workpiece with larger thickness range.

[0031] (4) The straight line connection is arranged between the second arc and the third arc, the size of the flange part is further extended outward, the volume of the accommodating cavity formed by the cap is expanded, and the strength of the flange part locking the first metal workpiece is increased.

[0032] (5) The working end face of the second welding electrode is set to a larger spherical radius and end face diameter, the electrode is limited to sink to the surface of the second metal workpiece, the risk of excessive deformation of the material around the welding spot caused by the indentation of the welding spot on the second metal workpiece is avoided, and the welding spot forming quality is improved.

[0033] (6) The piercing current is divided into two parts, and an interface stabilizing current is arranged after the induction of the spatter current. In mass continuous welding, the interface stabilizing current can further remove the first metal remaining in the welding spot, and ensure the stability of the welding quality. BRIEF DESCRIPTION OF DRAWINGS

[0034] Fig. 1 is a three-dimensional schematic view of the fastener of the present application;

[0035] Fig. 2 is a cross-sectional schematic view of a typical fastener of the present application;

[0036] Fig. 3 is a schematic view of the setting of the first arc on the shaft side of the fastener of the present application;

[0037] Fig. 4 is a schematic view of the position and connection relationship between the first arc and the second arc of the fastener of the present application;

[0038] Fig. 5 is a schematic view of the size change of the second arc of the fastener of the present application;

[0039] Fig. 6 is a schematic view of the connection relationship between the second arc, the second straight line and the third arc of another typical fastener of the present application;

[0040] Fig. 7 is a schematic view of the position and thickness change of the flange part and the cap bending area of the fastener of the present application;

[0041] Fig. 8 is a schematic view of the comparison between the arc-shaped profile and the straight line profile of the shaft part cross section of the fastener of the present application;

[0042] Fig. 9 is a schematic view of the size comparison between the fastener of the present application and the fastener with straight line profile of the shaft part cross section;

[0043] Fig. 10 is a schematic view of the appearance of the welding electrode for welding dissimilar metals of the fastener of the present application;

[0044] Fig. 11 is a schematic view of the welding process of the welding method of the present application;

[0045] Figure 12 is a schematic diagram of the welding process of the welding method of the present application;

[0046] Figure 13 is a schematic diagram of the effect of different currents on the piercing stage of the welding method of the present application;

[0047] Figure 14 is a schematic diagram of the cross-sectional morphology of the joint after welding of the welding electrode of the present application and a typical symmetrical welding electrode;

[0048] Figure 15 is a schematic diagram of the morphology of a typical welding electrode of the present application;

[0049] Figure 16 is a schematic diagram of the contact state of the welding electrode of the present application during the welding process;

[0050] Figure 17 is a schematic diagram of the cross-section of a fastener used in Example 1 of the present application;

[0051] Figure 18 is a metallographic cross-sectional micrograph of the dissimilar metal welded joint of Example 1 of the present application. DETAILED DESCRIPTION

[0052] The embodiments of the present application will be further described in detail below with reference to the accompanying drawings.

[0053] According to an aspect of the embodiments of the present application, a fastener 100 for welding dissimilar metals is provided, which is used for welding a laminated structure of a first metal workpiece and a second metal workpiece, and the melting point of the first metal workpiece is less than 750°C, and the melting point of the second metal workpiece is greater than 1300°C. The dissimilar metal combination includes aluminum alloy and steel, magnesium alloy and titanium alloy, etc. The material for manufacturing the fastener 100 is the same as the second metal workpiece, such as steel and titanium alloy, etc., and the fastener is usually processed by stamping, cold heading, powder metallurgy and 3D printing, etc. As shown in FIGS. 1 and 2, the fastener 100 is an axis A1 rotationally symmetrical structure, which has a shaft portion 101, a flange portion 108 and a cap 109. Among them, the shaft portion 101 is used for piercing the first metal workpiece and welding with the second metal workpiece, the lower end surface 102 of the shaft portion is a plane, and the side surface of the shaft portion is a curved surface formed by rotating a circular arc around the axis A1. The flange portion 108 gradually extends outward and upward from the upper end of the shaft portion. The cap 109 is a thin-walled structure which is bent and extended outward from the flange portion 108, and the end portion 110 of the cap extends to the horizontal plane where the lower end surface 102 of the shaft portion is located, so that the cap is distributed around the periphery of the shaft portion and forms a semi-open annular cavity with the outer side of the shaft portion. In order to make the semi-open annular cavity completely accommodate the liquid metal discharged from the welding point during the welding process, the volume V1 of the semi-open annular cavity and the volume V2 of the fastener satisfy the relationship: V2 < V1 < 2 × V2, under the premise of maintaining the compactness of the size of the fastener. For the upper surface of the fastener, it is set according to the specific circumstances of the welding electrode topography adopted. For example, the upper end 103 of the shaft portion and the upper surface of the flange portion 108 on the periphery can be set as a concave topography which is conformal to the welding electrode.

[0054] As shown in FIGS. 2 and 3, in the cross section passing through the axis, the side contour of the shaft portion 101 is composed of a first circular arc 104, and the center of the circular arc is located in the shaft portion. According to the size requirements of the shaft portion, the first circular arcs 104 and 104' on both sides of the shaft portion can be on the same circle, or on two circles with the same radius and a distance d apart on the same horizontal line. Among them, one end of the first circular arc 104 intersects the lower end surface of the shaft portion at the P1 point, and the other end extends upward without exceeding the horizontal plane where the center of the circular arc is located.

[0055] As shown in Fig. 4, the second circular arc 105 is arranged on the upper side of the first circular arc 104, and the center of the second circular arc is located on the outside of the shaft portion of the fastener, constituting at least part of the lower surface of the flange portion 108. One end of the second circular arc 105 is connected with the first circular arc 104 at the P2 point, so that the shaft portion side surface can be smoothly transitioned to the lower surface of the flange portion. The first circular arc radius R1 is greater than the second circular arc radius R2. In the preferred embodiment, the first circular arc 104 and the second circular arc 105 are tangent at the connection point P2, and the centers of the first circular arc and the second circular arc are located on the same horizontal line, as shown in Fig. 5. For different thicknesses of the workpiece to be welded, the radius R2 of the second circular arc 105 can be increased to R2' to adapt the fastener to weld thicker metal workpieces, while maintaining the tangency of the first circular arc and the second circular arc at P2.

[0056] As shown in Figs. 2, 3 and 4, in order to better fit the lower end surface 102 of the shaft portion before welding to the workpiece and to distribute the welding current more evenly on the shaft portion lower end surface, the shaft portion lower end surface is arranged as a flat surface, and the diameter D1 thereof is in the range of 4 to 6 mm. The side wall of the shaft portion 101 is formed by rotating the first circular arc 104 around the axis A1, so that the side wall gradually converges when the shaft portion extends upward, avoiding excessive diameter on the upper side of the shaft portion. The ratio of the shaft portion diameter D2 at the intersection P2 of the first circular arc and the second circular arc to D1 is in the range of 1.4 to 2.2. The maximum diameter D3 of the fastener is not more than 20 mm. Preferably, when the total thickness T of the first metal workpiece to be welded is less than 2.5 mm, D1 is in the range of 4 to 5 mm, and D3 is not more than 15 mm; when 2.5 mm≤T<5 mm, D1 is in the range of 4.8 to 5.5 mm, and D3 is in the range of 13 to 17 mm; when T≥5 mm, D1 is in the range of 5 to 6 mm, and D3 is in the range of 15 to 20 mm.

[0057] The inner surface of the bending part of the cap is formed as a third circular arc 106, one end of which is connected to the outer edge of the lower surface of the flange part, and can form an intersecting or tangent connection with the second circular arc at the connection point, and the other end is connected to the first straight line 111 extending from the end of the cap. As shown in Fig. 6, in another embodiment, in order to properly expand the width of the flange part, a second straight line 107 is further provided between the third circular arc 106 and the second circular arc 105. The second straight line 107 serves as another part of the lower surface of the flange part, one end of which is connected to the second circular arc 105 at point P3 through a tangent connection, and the other end serves as the edge of the lower surface of the flange part, and is connected to the third circular arc at point P4. Preferably, a round chamfer is further provided at point P4 to make the transition at the connection between the third circular arc and the second straight line more gradual. In addition, under the premise that the fastener can be effectively welded, the compactness of the fastener in the height dimension is improved, and the distance H1 from the connection point P2 between the first circular arc and the second circular arc to the horizontal plane of the lower end surface 102 of the shaft part is set to satisfy the relationship: 0.5 x T < H1 < T, where T is the total thickness of the first metal workpiece. The distance H2 between the horizontal plane of the connection point P2 and the horizontal plane of the connection point P4 between the third circular arc and the outer edge of the lower surface of the flange part is set to satisfy the relationship: H2 < H1.

[0058] In a preferred example, the center of the third circular arc 106 is located on the perpendicular of the edge of the flange part 108 (as shown in Fig. 7), and the wall thickness t1 of the edge of the flange part is not more than 0.55 mm, and the maximum wall thickness of the area on the cap where the third circular arc is located is not more than 0.5 mm. Preferably, the wall thickness t2 of the area on the cap where the third circular arc is located is in the range of 0.36 mm to 0.42 mm, and the maximum wall thickness of the area extending from the third circular arc to the end 110 of the cap is not more than 0.45 mm.

[0059] By setting the profiles of the shaft part and the flange part, plastic deformation similar to "bumping" of the shaft part under the action of heat and force during welding can be effectively avoided, which causes the shaft part to fail to fully penetrate the first metal workpiece. As shown in FIG. 8, if the range of the cross-sectional shaft part side profile is a straight line profile between L1 and L2, and the angle α is about 0° to 35°, plastic deformation similar to "bumping" is prone to occur near the side of the lower end surface 102 of the shaft part during welding, especially when welding a thicker first metal workpiece, the effect of the shaft part penetrating the first metal workpiece is poor. After the shaft part side wall is set to the first circular arc 104, for example, compared with the shaft part of the fastener with a L2 straight side, the shaft part of the present application has an additional area S1, which can further improve the support strength of the shaft part and reduce the excessive collapse deformation of the shaft part. As shown in FIG. 9, although the shaft side straight profile of the fastener is extended to L3 and L4, the rigidity of the shaft part can be further improved, but as the angle α increases, on the one hand, the penetration ability of the shaft part decreases, on the other hand, in order to maintain the same height as the fastener of the present application and the volume of the accommodating cavity formed by the cap, the cap 109 needs to be expanded outward to 109' and 109", which leads to further increase of the diameter of the fastener, such as D3' and D3", and thus significantly reduces the compactness of the fastener.

[0060] According to another aspect of the embodiments of the present application, a welding electrode is provided for welding the fastener of the present application into a weld joint of dissimilar metals, and the cross-sectional profile of the welding electrode is shown in FIG. 10. The diameter D4 of the working end surface 401 of the first welding electrode on the side of the fastener has a relationship with the diameter D1 of the lower end surface of the shaft part of the fastener: D1 < D4 < 1.4 x D1, and the spherical radius R4 is not more than 60 mm, preferably the range of R4 is 50 mm to 60 mm. The ratio of the diameter D5 of the working end surface of the second welding electrode on the side of the second metal workpiece to D4 is greater than 1.5, and the spherical radius R5 is greater than 95 mm. Preferably, the range of R5 is 100 mm to 150 mm.

[0061] According to another aspect of the embodiments of the present application, a welding method is provided, the welding process is shown in Fig. 11 and the welding procedure is shown in Fig. 12, the welding method comprises a preheating stage, a piercing stage and a nucleation stage. In the preheating stage, a small preheating current (I1) is inputted to the welding spot, for example, I1=4kA and maintained for 160ms, to improve the contact state between the electrode, the fastener and the workpiece, and since the inputted current I1 in the preheating stage is small, the generated resistance heat is also small, only causing a small range of melting zone 201 (A1 in Fig. 12) in the first metal workpiece 200. A cooling stage 1 is arranged between the preheating stage and the piercing stage, and the cooling time of the cooling stage 1 is not more than 15ms. In the piercing stage, a piercing current (I2) is inputted, the piercing current comprises a plurality of pulse currents, for generating an instantaneously increased resistance heat, combined with the extrusion effect of the electrode pressure, to make the first metal quickly melt and expand to generate a fast spatter ejection welding spot. Since the cap is wrapped around the periphery of the shaft portion 101 and forms a closed cavity with the surface of the first metal workpiece during the welding process, the expelled metal 202 is finally completely accommodated in the cap of the fastener. The shaft portion 101 is smoothly pierced into the first metal workpiece under the driving of the electrode pressure and forms a contact interface 301 with the second metal workpiece, as shown in B1 in Fig. 12. In the nucleation stage, a nucleation current (I3) is inputted, for example, I3=12kA and maintained for 350ms, to make the contact interface 301 melt and form a firm fusion nucleus 304, to realize the firm connection between the fastener and the second metal workpiece 300. In addition, under the action of the nucleation current, the melting range of the first metal on the shaft portion side is further expanded, and part of the liquid metal is again extruded into the cap of the fastener, so that the expelled metal 202 in the cap is further compacted. In addition, the melting zone 201 of the first metal around the shaft portion and part of the expelled metal 202 in the cap are also melted into one, further enhancing the metallurgical and mechanical connection strength between the first metal workpiece and the fastener, as shown in C1 in Fig. 12.

[0062] In order to ensure the stability and reliability of the fastener piercing the first metal workpiece in the fast-paced and large-batch continuous welding process of the present welding method, the piercing current (I2) is divided into two parts, the first part is the induced spatter current (I2-1), which includes a number of pulse currents, mainly used to induce spatter and achieve the fastener piercing the first metal workpiece. As shown in Figure 11, in the preferred embodiment, the current intensity, holding time and cooling time between pulses of each current pulse of the induced spatter current (I2-1) show a downward trend, the maximum holding time of a single pulse does not exceed 75 ms, and the cooling time between pulses does not exceed 20 ms (cooling stage 2). For example, the first pulse current intensity is 17 kA, the holding time is 55 ms; the second pulse current intensity is 16 kA, the holding time is 52 ms; the third pulse current intensity is 15 kA, the holding time is 50 ms, and the cooling time between the first and second pulses is 20 ms, and the cooling time between the second and third pulses is 10 ms. By regulating the current intensity, holding time and cooling time of the induced spatter current (I2-1), on the one hand, it ensures that the first metal in the weld is fully discharged into the fastener cap through the spatter process, and on the other hand, it avoids the transition growth of the internal melting area 302 of the shaft and the second metal to the surface, effectively preventing the mixing of liquid first metal and second metal to produce brittle phases.

[0063] The second part of the piercing current (I2) is an interface stabilizing current (I2-2), which is not more than three current pulses, and the current intensity of a single pulse is not more than the splashing current (I2-1), the single pulse maintaining time is not more than 60 ms, and the cooling stage 4 between the pulses is not more than 20 ms. A cooling stage 3 is further arranged between the splashing current and the interface stabilizing current, and the time range is 25 ms to 35 ms. Arranging the interface stabilizing current (I2-2) can further ensure that the first metal is fully discharged in the weld, so that a stable contact state is achieved between the shaft part of the fastener and the second metal in the weld, and a basic condition is provided for the subsequent nucleation stage. In particular, as shown in A2 in FIG. 13, in a large number of continuous welds, the splashing current (I2-1) can fully discharge the first metal in most of the welds, and there is no a large amount of residual first metal on the contact interface 301 between the shaft part of the fastener and the second metal. Since the interface reaches a relatively ideal stable contact state at this time, the resistance heat generated by the interface stabilizing current (I2-2) can promote the formation of a small fusion nucleus 303 on the contact interface 301. Due to some unstable factors such as the working end surface state of the welding electrode, the workpiece assembly gap and the workpiece surface state, there are differences in the initial state of each weld, as shown in B2 in FIG. 13, after the action of the splashing current I2-1, the first metal cannot be fully discharged in some welds, resulting in residual first metal between the shaft part of the fastener and the second metal. At this time, with the input of the interface stabilizing current (I2-1), the residual first metal in the weld is splashed or melted and squeezed out of the weld again, so that a stable contact interface 301 is formed between the shaft part of the fastener and the second metal.

[0064] As shown in A3 in FIG. 14, it is a joint obtained by using the welding electrode of the present application. As shown in B3 in FIG. 14, it is a joint obtained by using a conventional symmetrical welding electrode. Since the working end size of the second welding electrode 500 of the present application is larger, the electrode indentation depth t3 formed on the weld surface of the second metal workpiece is much smaller than the electrode indentation depth t5 formed by the symmetrical electrode, which on the one hand avoids the risk of cracks in the first metal in the weld area due to excessive deformation, and on the other hand, the protrusion height t4 towards the fastener direction is also reduced, which is beneficial to reduce the excessive flattening deformation of the shaft part, prevent the locking effect of the first metal workpiece by the fastener from being reduced, and avoid more liquid metal being squeezed into the cap and not being fully accommodated.

[0065] In another embodiment, as shown in Fig. 15, the working end face of the second welding electrode 500 is divided into an inner first working end face 501 and an outer second working end face. The diameter D6 of the first working end face 501 satisfies the relationship: D6≤ D4, where D4 is the diameter of the working end face of the first welding electrode 400. The second working end face is composed of a plurality of concentric annular ridges 503, and the tip width of the concentric annular ridges 503 is not more than 1 mm. The tips of the concentric annular ridges 503 are located on the spherical surface of the first working end face 501, and annular grooves 502 are arranged between the concentric annular ridges, and the depth of the annular grooves ranges from 0.5 to 1 mm. As shown in Fig. 16, during welding, the annular grooves 502 do not contact the surface of the second metal workpiece, reducing the contact area between the second working end face and the workpiece, on the one hand, reducing the heat dissipation rate of the electrode on this side, and on the other hand, more conducive to the first working end face 501 concentrating most of the welding current to promote the formation of the nugget 304, while also reducing the tendency of the nugget 304 to deviate upward on the shaft portion of the fastener. The outer concentric annular ridges 503 can effectively limit the formation of severe electrode indentations on the welding spot by the first working end face 501 during welding, improving the forming effect of the welding spot.

[0066] Embodiment 1

[0067] In this embodiment, the cross-sectional morphology of the fastener 100 used is shown in Fig. 17. The diameter D1 of the lower end face of the shaft portion is 5 mm, the shaft side expands upward and outward through a first circular arc 104, expanding the shaft diameter to D2 = 8.6 mm, where the radius R1 of the first circular arc 104 = 4.3 mm, and the distance H1 from the center of the first circular arc 104 to the plane of the lower end 102 of the shaft portion = 3.5 mm. The radius R2 of the second circular arc 105 = 1.7 mm, and the center of the second circular arc 105 is on the same horizontal line as the center of the first circular arc 104, and the first circular arc 104 and the second circular arc 105 are tangent at the connection. The upper end of the second circular arc extends outward through a second straight line 107 and is connected to a third circular arc 106, the distance H2 from the connection point to the horizontal line passing through the center of the first circular arc = 2.1 mm, and the radius R3 of the third circular arc = 1.75 mm. In addition, the rim has a wall thickness t1 = 0.45 mm, the third circular arc region on the cap has a wall thickness t2 = 0.4 mm, and the diameter D3 = 17 mm.

[0068] The fastener of Fig. 17 was used to weld dissimilar metals, wherein the first metal workpiece 200 was a 4mm thick cast aluminum alloy, and the second metal workpiece 300 was a 1.6mm thick hot-formed steel, and the welding was performed using the welding process shown in Fig. 11. The cross-section metallograph of the joint obtained after welding is shown in Fig. 18, the shaft portion of the fastener completely penetrates the first metal workpiece with large thickness, without serious flattening deformation, and the shaft portion is firmly connected with the second metal workpiece through the nugget. The first metal is locked by the shaft portion and the flange portion, realizing the connection of the first metal and the second metal. The end of the fastener cap 109 is always in contact with the surface of the first metal workpiece during the welding process, forming a closed cavity to fully accommodate the expelled metal 202.

[0069] It should be understood that the purpose of the above embodiments 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 structure or method steps involved in the above embodiments, especially the combination of different embodiments without structural or principle conflict, falls within the protection scope of the present application.

Claims

1. A fastener for welding dissimilar metals, the fastener being used for welding a stack structure of a first metal workpiece and a second metal workpiece, and the melting point of the first metal workpiece being less than 750℃, the melting point of the second metal workpiece being greater than 1300℃, the fastener being an axis rotationally symmetrical structure, comprising a shaft portion, a flange portion and a cap, and the material type being the same as that of the second metal workpiece, characterized in that: the shaft portion is used for piercing the first metal workpiece and welding with the second metal workpiece, the lower end surface of the shaft portion being a plane, and the lower end size of the shaft portion being smaller than the upper end, the side surface of the shaft portion being a curved surface formed by the rotation of a circular arc around the axis; the flange portion is gradually extended outward and upward from the upper end of the shaft portion, a thin-walled cap is bent out from the edge of the flange portion, and the end of the cap extends to the horizontal plane where the lower end surface of the shaft portion is located, so that the cap is distributed around the periphery of the shaft portion and forms a semi-open annular cavity with the outer side of the shaft portion, and the volume V1 of the semi-open annular cavity and the volume V2 of the fastener satisfy the relationship: V2 < V1 < 2 × V2; in the cross section passing through the axis of the fastener, the profile of the fastener comprises: a first circular arc, which is located on the side surface of the shaft portion, and the center of the first circular arc is located inside the shaft portion, and one end of the first circular arc intersects with the lower end surface of the shaft portion, and the other end extends upward without exceeding the horizontal plane where the center of the circular arc is located; a second circular arc, which is located on the upper side of the first circular arc and constitutes at least part of the lower surface of the flange portion, one end of the second circular arc is connected with the first circular arc, so that the side surface of the shaft portion is smoothly connected to the lower surface of the flange portion, and the center of the second circular arc is located outside the shaft portion; and a third circular arc, which is located in the inner surface of the bending part of the cap, one end of the third circular arc is connected with the outer edge of the lower surface of the flange portion, and the other end is connected with the first straight line extending from the end of the cap. The radius (R1) of the first circular arc is greater than the radius (R2) of the second circular arc, the distance H1 from the connection point of the first circular arc and the second circular arc to the plane where the lower end surface of the shaft portion is located satisfies the relationship: 0.5 × T < H1 < T, where T is the total thickness of the first metal workpiece. The distance H2 between the horizontal plane where the connection point of the first circular arc and the second circular arc is located and the horizontal plane where the connection point of the third circular arc and the outer edge of the flange portion is located satisfies the relationship: H2 ≤ H1. The first circular arc and the second circular arc are tangent at the connection point, and the centers of the first circular arc and the second circular arc are located on the same horizontal line. One end of the third circular arc passes through the second straight line and is connected with the second circular arc, and the second straight line is tangent to the second circular arc at the connection point. The ratio of the diameter D2 of the shaft portion at the intersection point of the first circular arc and the second circular arc to the diameter D1 of the lower end surface of the shaft portion is in the range of 1.4 to 2.

2. The edge thickness t1 of the flange portion is not more than 0.55mm, and the maximum wall thickness of the region where the third circular arc of the cap is located is not more than 0.5mm.

2. The fastener of claim 1, wherein, The welding electrode is divided into a first electrode and a second electrode; 3. The fastener of claim 1 or 2, wherein, The first electrode is arranged on one side of the fastener, and the diameter D4 of the working end surface of the first electrode satisfies the relationship: D1 < D4 < 1.4 × D1, and the spherical radius R4 of the working end surface is not more than 60mm.

4. The fastener of claim 1, wherein ​ 5. The fastener of claim 1, wherein, ​ 6. The fastener of claim 1, wherein ​ 7. The fastener of claim 1, wherein ​ 8. A welding electrode for welding the fastener of any one of claims 1 to 7 into a stacked structure of a first metal workpiece and a second metal workpiece, the fastener being placed on a weld point of the first metal workpiece before welding, characterized in that: ​ ​ The second welding electrode is arranged on the side of the second metal workpiece, and the ratio of the diameter D5 of the working end face of the second welding electrode to the diameter D4 is greater than 1.5, and the spherical radius R5 is greater than 95 mm.

9. The welding electrode of claim 8, wherein, The working end face of the second welding electrode is divided into a first working end face and a second working end face. The first working end face is located in the interior and is used for concentrating most of the welding current to promote the formation of the welding spot, and the end face diameter D6 and the working end face diameter D4 of the first electrode satisfy the relationship D6≤D4. The second working end face is located on the periphery of the first working end face and is used for limiting the first working end face from producing a serious electrode indentation on the welding spot, the second working end face is composed of a plurality of concentric annular ridges, and the tips of the annular ridges are located on the same spherical surface as the first working end face.

10. A welding method, which uses any one of the fasteners of claims 1 to 7 and any one of the welding electrodes of claims 8 to 9 to weld the laminated structure of the first metal workpiece and the second metal workpiece, and when welding, the welding electrode is used to input the welding current to the welding spot and apply the electrode pressure, the welding current includes the preheating current (I1), the piercing current (I2) and the nucleation current (I3), and the method is characterized in that: The piercing current (I2) is used to discharge the first metal in the welding spot and realize the piercing of the shaft of the fastener into the first metal workpiece, the piercing current (I2) is divided into two parts, the first part is the induced spatter current (I2-1), which is mainly used to induce the first metal in the welding spot to produce the spatter phenomenon, and through the high-speed discharge of the first metal in the welding spot by the liquid metal spatter process, the shaft of the fastener is efficiently pierced into the first metal workpiece under the action of the electrode pressure and contacts the second metal workpiece, and the induced spatter current (I2-1) includes a plurality of pulse currents. The second part is the interface stabilizing current (I2-2), which is not more than three current pulses, and is used to ensure that the first metal in the welding spot is completely discharged from the welding spot, so that the interface between the lower end face of the shaft of the fastener and the second metal workpiece reaches a stable contact state.

11. The welding method of claim 10, wherein, The current intensity, the maintenance time and the cooling time between the pulses of each current pulse of the induced spatter current (I2-1) present a downward trend, the maximum maintenance time of a single pulse is not more than 75 ms, and the maximum cooling time is not more than 20 ms; the cooling time between the induced spatter current (I2-1) and the interface stabilizing current (I2-2) ranges from 25 ms to 35 ms; the maximum maintenance time of a single pulse of the interface stabilizing current (I2-2) is not more than 60 ms, and the current intensity is not more than that of the induced spatter current (I2-1).

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