Fastener, welding electrode and welding method for welding dissimilar metals

Abstract

The present invention discloses a fastener, a welding electrode and a welding method for welding dissimilar metals. The fastener comprises a shaft portion, a flange portion and a cap surrounding the outer periphery of the shaft portion. The outer contour of the cross section of the shaft portion is set to a first arc protruding outward, so as to improve the strength of the shaft portion and the ability to pierce metal workpieces, and avoid excessive flattening and deformation of the shaft portion during welding. The outer side of the shaft portion is connected to the lower surface of the flange portion through a second arc to narrow the size of the flange portion. On the premise of ensuring the compact size of the fastener, the volume of the accommodating cavity formed by the cap is increased, and at the same time, the compatibility of the flange portion and the fastener for welding metal workpieces of different thicknesses is improved. In addition, in order to improve the formation of weld spots and achieve stable welding of dissimilar metals, the present invention also provides a welding electrode and a welding method for welding fasteners to dissimilar metals.

Description

Technical Field

[0001] The invention belongs to the field of welding of dissimilar metals, and in particular relates to a fastener, a welding electrode and a welding method for welding dissimilar metals. Background Art

[0002] In the automotive industry, the manufacture of car bodies usually uses a mixture of multiple metal materials, especially for the bodies of new energy vehicles, where aluminum and steel are the main materials used on the body. In particular, integrated die-cast aluminum metal is popular in parts such as the rear floor and engine compartment of the car body, while high-strength steel or ultra-high-strength steel is used for A / B pillars, door rings, floor channels, side anti-collision steel beams, etc. Therefore, in the automotive industry, dissimilar metal connections represented by aluminum / steel have a wide range of application needs. However, due to the large differences in the thermophysical properties of dissimilar metals, welding can easily lead to problems such as cracks and brittle intermetallic compounds in the weld, making it difficult for the mechanical properties of the joint to meet the requirements of industrial applications. Especially under the fast-paced manufacturing environment of the car body, higher requirements are placed on the welding of dissimilar metals. Therefore, the welding technology of dissimilar metals is difficult to be applied in car body manufacturing.

[0003] In order to achieve reliable welding of dissimilar metals, a promising technical solution is to use a welding technology that combines welding and riveting (welding-riveting) for connection. For example, when connecting aluminum alloy and steel, the welding-riveting technology can use steel fasteners (also known as rivets and welding elements, etc.) combined with external force or prefabricated process holes to pre-set them on the aluminum alloy, and then weld the fasteners to the steel so that the fasteners can firmly rivet the aluminum alloy workpiece. Since this process solution excludes all aluminum alloys in the weld, it can avoid the problems caused by direct welding of dissimilar metals, and the obtained joint strength is more reliable. However, pre-setting process holes on aluminum plates or stamping pre-setting fasteners on aluminum alloys will increase the complexity of the connection and reduce 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 uses the technology of actively inducing low-melting-point metal to produce splashes 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 required, which can fully accommodate the induced discharge of splashing metal and the shaft firmly pierces the metal layer without excessive flattening and deformation while ensuring the compact size of the fastener. The structure of the fastener designed for the above purpose is particularly important in this field.

[0005] Since the welding technology disclosed in patent CN114211104A uses active induction of spatter to discharge low-melting-point metal, which causes the loss of material in the weld and the softening of the metal workpiece by heat, the fastener shaft can easily pierce the low-melting-point metal layer under a relatively small pressure, so the fastener shaft height dimension suitable for welding is relatively small. Similar to the welding auxiliary joint parts (referred to as fasteners in this article) with a large height / width ratio as disclosed in patent CN104540628B, they are not suitable for the welding technology of patent CN114211104A. Patent CN114932302B proposes another fastener structure, in which the shaft of the fastener structure is a cylindrical structure, and the end face of the shaft is relatively flat. When the shaft of this structure is welded using the welding method of patent CN114211104A, if the diameter of the fastener shaft is too small, it is easy to cause excessive flattening plastic deformation, resulting in poor shaft puncture effect. In order to improve the strength of the shaft, the diameter of the shaft needs to be increased. However, as the diameter of the shaft increases, it is difficult to completely discharge the low-melting-point metal in the weld and it is more difficult for the shaft to pierce the metal workpiece.

[0006] Patent CN115780980A discloses a welding element for welding dissimilar metals (referred to herein as a fastener), the shaft portion of which is similar to an inverted truncated cone structure, and the side surface of the shaft portion forms an inclination angle α with the axis, which is conducive to the shaft portion piercing the workpiece and discharging liquid metal. However, when the inclination angle is small, for example, α is less than 30°, the shaft portion is easily collapsed after being heated. After increasing the angle α, in order to ensure that the volume of the cavity formed by the cap can completely accommodate the splashing metal, the overall diameter of the fastener must be increased. In particular, when welding thick dissimilar metal workpieces, the height of the fastener shaft and the diameter of the fastener will further increase, resulting in the fastener being too large and the weld being too bloated, which is not conducive to the structural weld design.

[0007] To this end, the art urgently needs an innovative fastener structure to prevent the shaft from excessively collapsing and deforming during welding, improve the shaft's ability to pierce the workpiece, and at the same time ensure that the fastener is compact in size and the fastener cap can fully accommodate the liquid metal discharged from the weld. Summary of the invention

[0008] In order to overcome the above problems, the present invention proposes a fastener, a welding electrode and a welding method for welding dissimilar metals, which can realize a compact design of the fastener, improve the effect of the shaft piercing a low-melting-point workpiece, and enhance the welding quality and welding stability of the dissimilar metals.

[0009] Provided is a fastener for welding dissimilar metals, the fastener is used for welding a laminated structure of a first metal workpiece and a second metal workpiece, the melting point of the first metal workpiece is less than 750°C, the melting point of the second metal workpiece is greater than 1300°C, the fastener is an axis rotationally symmetrical structure, comprises a shaft portion, a flange portion and a cap, and the material type is the same as that of the second metal workpiece,

[0010] The shaft portion is used to pierce the first metal workpiece and weld with the second metal workpiece. The lower end surface of the shaft portion is a plane, and the size of the lower end of the shaft portion is smaller than the upper end. The side surface of the shaft portion is a curved surface formed by the rotation of a circular arc around the axis. The flange portion gradually extends outward and upward from the upper end of the shaft portion, and a thin-walled cap is bent outward 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 outer periphery of the shaft portion and forms a semi-open annular cavity with the outer side of the shaft portion. 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] The fastener profile in a cross section through the fastener axis consists of:

[0012] A first arc is located on the side of the shaft portion, with its center located inside the shaft portion, and one end of the first arc intersects with the lower end surface of the shaft portion, and the other end does not exceed the horizontal plane where the center of the arc is when extending upward; a second arc is located on the upper side of the first arc, constituting at least part of the lower surface of the flange portion, one end of the arc is connected to the first arc, so that the side of the shaft portion smoothly transitions to the lower surface of the flange portion, and the center of the second arc is located outside the shaft portion; a third arc is located on the inner surface of the bend of the cap, one end of the arc is connected to the outer edge of the lower surface of the flange portion, and the other end is connected to the first straight line extending from the end of the cap.

[0013] Furthermore, a distance H1 from the connection point of the first arc and the second arc to the plane where the lower end surface of the shaft portion is located satisfies: 0.5×T

[0014] Furthermore, a distance H2 between a horizontal plane where the connection point between the first arc and the second arc is located and a horizontal plane where the connection point between the third arc and the outer edge of the lower surface of the flange portion is located satisfies the relationship: H2≤H1.

[0015] Furthermore, the first arc and the second arc are in a tangent relationship at a connection point, and the centers of the first arc and the second arc are located on the same horizontal line.

[0016] Furthermore, one end of the third arc is connected to the second arc through a second straight line, and the second straight line and the second arc are tangent to each other at a connection point.

[0017] Optionally, a round chamfer is provided at a connection point between the third arc and the second straight line.

[0018] Furthermore, a ratio of the shaft diameter D2 to D1 at the intersection of the first arc and the second arc is in a range of 1.4 to 2.2.

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

[0020] Furthermore, the edge wall thickness t1 of the flange portion does not exceed 0.55 mm, and the maximum wall thickness of the area where the third arc area on the cap is located does not exceed 0.5 mm; preferably, the wall thickness of the area where the third arc area on the cap is located ranges from 0.35 mm to 0.42 mm.

[0021] Furthermore, the first arc radius R1 is greater than the second arc radius R2.

[0022] In order to improve the effect of the fastener of the present invention in welding dissimilar metals, a welding electrode is provided, which is used to weld a fastener of the present invention to a laminated structure of a first metal workpiece and a second metal workpiece. Before welding, the fastener is placed on the welding point of the first metal workpiece, and the welding electrode is divided into a first welding electrode and a second welding electrode; wherein the first welding electrode is placed on one side of the fastener, and the diameter D4 of its working end face is in a relationship with the diameter D1 of the lower end face of the fastener shaft: D1<D4<1.4×D1, and the spherical radius R4 of the working end face does not exceed 60 mm; the second welding electrode is placed on one side of the second metal workpiece, and the ratio of the diameter D5 of its working end face 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 mm to 60 mm, and the spherical radius R5 of the second welding electrode is in the range of 100 mm to 150 mm.

[0023] Furthermore, the working end face of the second welding electrode is divided into a first working end face and a second working end face; wherein, the first working end face is located inside, and is used to concentrate most of the welding current to promote the formation of the weld, and its end face diameter D6 is in a relationship with the working end face diameter D4 of the first welding electrode: D6≤D4; the second working end face is located outside the first working end face, and is used to limit the first working end face from producing severe electrode indentations on the weld, and the second working end face is composed of a number of concentric annular ridges, and the tip of the annular ridge is located on the same spherical surface as the first working end face.

[0024] Preferably, the tip width of the concentric annular ridges does not exceed 1 mm, and the depth of the annular grooves between the concentric annular ridges ranges from 0.5 to 1 mm.

[0025] In order to improve the stability of fastener welding, a welding method is provided, wherein the fastener and the welding electrode of the present invention are used to weld the laminated structure of the first metal workpiece and the second metal workpiece, during welding, the welding electrode inputs a welding current to the weld point and applies electrode pressure, the welding current includes a preheating current I1, a piercing current I2 and a nucleation current I3, the piercing current I2 is used to discharge the first metal in the weld point and realize that the shaft of the fastener pierces the first metal workpiece, the piercing current I2 is divided into two parts, the first part is the spatter induction current I2-1, which is mainly used to induce the first metal in the weld point to produce spatter phenomenon, and the first metal in the weld point is discharged at a high speed through the liquid metal spatter process, so that the shaft of the fastener efficiently pierces the first metal workpiece and contacts the second metal workpiece under the action of the electrode pressure, which includes a number of pulse currents. The second part is the interface stabilization current I2-2, which does not exceed three current pulses, and is used to ensure that the first metal in the weld point is completely discharged from the weld point, 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.

[0026] Furthermore, 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 maximum cooling time does not exceed 20 ms; the cooling time between the induced spatter current and the interface stabilization current ranges from 25 ms to 35 ms; the maximum holding time of a single pulse of the interface stabilization current I2-2 does not exceed 60 ms, and the current intensity does not exceed the current intensity of the induced spatter current I2-1.

[0027] Beneficial effects of the present invention:

[0028] (1) The fastener of the present invention has a compact structure, which avoids the joint structure being too bulky after welding. It is also convenient for the production of the fastener and has a low manufacturing cost.

[0029] (2) The side surface of the shaft is set as a first arc, and transitions to the lower surface of the flange portion through the first arc. On the one hand, the diameter of the shaft side increases rapidly from bottom to top, thereby improving the strength of the lower side of the shaft and avoiding excessive flattening and plastic deformation. At the same time, the size of the lower end of the shaft is smaller than the upper end, thereby ensuring that the splashing metal can be smoothly discharged when the shaft pierces the first metal workpiece and that reliable mechanical locking strength is provided to the first metal after the shaft pierces the first metal workpiece. On the other hand, when extending upward from the first arc, it gradually approaches the vertical direction, so that the flange portion is set closer to the axis, thereby improving the compactness of the fastener size while ensuring that the volume of the semi-open annular cavity formed by the cap is sufficient.

[0030] (3) A second arc is connected to the shaft side and the lower surface of the flange portion, so that the lower surface of the flange is gradually enlarged, and can then be sunk under the surface of the first metal workpiece with different thicknesses, which is conducive to the compatibility of the same size fastener with the first metal workpiece with a larger thickness range for welding.

[0031] (4) A straight line connection is provided between the second arc and the third arc to further extend the flange portion outward, thereby increasing the volume of the accommodating cavity formed by the cap and the strength of the flange portion locking the first metal workpiece.

[0032] (5) The working end face of the second welding electrode is set to a larger spherical radius and end face diameter to limit the electrode from sinking into the surface of the second metal workpiece, thereby avoiding the risk of excessive indentation of the weld on the second metal workpiece and excessive deformation of the material around the weld causing cracking, thereby improving the quality of the weld formation.

[0033] (6) The puncture current is divided into two parts. After inducing the spatter current, an interface stabilization current is set. In large-scale continuous welding, the interface stabilization current can further remove the first metal remaining in the weld spot and ensure the stability of the welding quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 is a three-dimensional schematic diagram of the fastener of the present invention;

[0035] Figure 2 is a schematic cross-sectional view of a typical fastener of the present invention;

[0036] Figure 3 It is a schematic diagram of the arrangement of the first arc on the axial side of the fastener of the present invention;

[0037] Figure 4 It is a schematic diagram of the position and connection relationship of the first arc and the second arc of the fastener of the present invention;

[0038] Figure 5 A schematic diagram of the change in the second arc size of the fastener of the present invention;

[0039] Figure 6 A schematic diagram of the connection relationship between the second arc, the second straight line and the third arc of another typical fastener of the present invention;

[0040] Figure 7 It is a schematic diagram of the position and thickness variation of the flange portion and the cap bending area of ​​the fastener of the present invention;

[0041] Figure 8 A schematic diagram comparing the arc-shaped profile and the straight-line profile of the cross-section of the shaft portion of the fastener of the present invention;

[0042] Fig. 9 It is a schematic diagram showing the comparison of the dimensions of the fastener of the present invention and the fastener whose shaft section is a straight-line profile;

[0043] Fig.10 A schematic diagram of the morphology of the welding electrode used for welding dissimilar metals to fasteners of the present invention;

[0044] Fig.11 A schematic diagram of the welding process of the welding method of the present invention;

[0045] Fig.12 A schematic diagram of the welding process of the welding method of the present invention;

[0046] Fig.13 It is a schematic diagram of the effects of different currents in the puncture stage of continuous welding in the welding method of the present invention;

[0047] Fig.14 It is a schematic diagram of the cross-sectional morphology of a joint after welding between the welding electrode of the present invention and a typical symmetrical welding electrode;

[0048] Fig.15 A schematic diagram of the morphology of a typical welding electrode of the present invention;

[0049] Fig.16 A schematic diagram of the contact state of the welding electrodes of the present invention during the welding process;

[0050] Fig.17 is a schematic cross-sectional view of a fastener used in Example 1 of the present invention;

[0051] Fig.18 This is a cross-sectional metallographic image of the dissimilar metal welded joint according to Example 1 of the present invention. DETAILED DESCRIPTION

[0052] The implementation of the present invention is further described in detail below through specific examples in conjunction with the accompanying drawings.

[0053] According to one aspect of an embodiment of the present invention, a fastener 100 for welding dissimilar metals is provided, and the fastener 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. Dissimilar metal combinations include aluminum alloy and steel, magnesium alloy and titanium alloy, etc. The type of material used to manufacture the fastener 100 is the same as that of the second metal workpiece, such as steel and titanium alloy, etc. The fastener is usually processed and formed by stamping, cold heading, powder metallurgy and 3D printing. Figure 1 and 2As shown, the fastener 100 is a rotationally symmetrical structure about the axis A1, and has a shaft 101, a flange 108 and a cap 109. The shaft 101 is used to pierce the first metal workpiece and weld with the second metal workpiece, the lower end face 102 of the shaft is a plane, and the side face of the shaft is a curved surface formed by the arc rotating around the axis A1. The flange 108 gradually extends outward and upward from the upper end of the shaft. The thin-walled cap 109 is further extended outward from the flange 108, and the cap end 110 extends to the horizontal plane where the lower end face 102 of the shaft is located, so that the cap is distributed around the outer periphery of the shaft and forms a semi-open annular cavity with the outer side of the shaft. Under the premise of maintaining the compactness of the fastener size, in order to enable the semi-open annular cavity to 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 entity satisfy the relationship: V2<V1<2×V2. The upper surface of the fastener is set according to the specific conditions of the welding electrode shape used. For example, the upper surface of the shaft upper end 103 and the outer flange 108 can be set to a concave shape that conforms to the welding electrode.

[0054] like Figure 2 and Figure 3 As shown, in the cross section passing through the axis, the side contour of the shaft portion 101 is composed of a first arc 104, the center of which is located inside the shaft portion. According to the shaft portion size requirements, the first arcs 104 and 104' on both sides of the shaft portion can be on the same circle, or on two circles of the same radius offset by a distance d on the same horizontal line. Among them, one end of the first arc 104 intersects with the lower end surface of the shaft portion at point P1, and the other end does not exceed the horizontal plane where the center of the arc is when extending upward.

[0055] like Figure 4 As shown, the second arc 105 is disposed on the upper side of the first arc 104, and the center of the second arc is located outside the fastener shaft, forming at least part of the lower surface of the flange portion 108. One end of the second arc 105 is connected to the first arc 104 at point P2, so that the side surface of the shaft portion can smoothly transition to the lower surface of the flange portion. The first arc radius R1 is greater than the second arc radius R2. In a preferred embodiment, the first arc 104 and the second arc 105 form a tangent relationship at the connection point P2, and the centers of the first arc and the second arc are located on the same horizontal line, as shown in FIG. Figure 5 For workpieces to be welded of different thicknesses, while maintaining the tangent relationship between the first arc and the second arc at position P2, the radius R2 of the second arc 105 can be increased to R2' to enable the fastener to adapt to welding thicker metal workpieces.

[0056] like Figure 2 , Figure 3 and Figure 4As shown, in order to make the lower end face 102 of the shaft part fit better with the workpiece before welding and to make the welding current more evenly distributed on the lower end face of the shaft part, the lower end face of the shaft part is set to a plane, and its diameter D1 ranges from 4 to 6 mm. The side circumference of the shaft part 101 is formed by rotating the first arc 104 around the axis A1, so that the side circumference gradually closes when the shaft part extends upward to avoid the upper diameter of the shaft part being too large. The ratio of the shaft diameter D2 to D1 at the intersection P2 of the first arc and the second arc ranges from 1.4 to 2.2. The maximum diameter D3 of the fastener does not exceed 20 mm. Preferably, when the total thickness T of the first metal workpiece to be welded is less than 2.5 mm, D1 ranges from 4 mm to 5 mm, and D3 does not exceed 15 mm; when 2.5 mm≤T<5 mm, D1 ranges from 4.8 mm to 5.5 mm, and D3 ranges from 13 mm to 17 mm; when T≥5 mm, D1 ranges from 5 mm to 6 mm, and D3 ranges from 15 mm to 20 mm.

[0057] The inner surface of the bend of the cap is formed into a third arc 106, one end of the third arc is connected to the outer edge of the lower surface of the flange, and can form an intersecting or tangent connection relationship with the second arc at the connection point, and the other end of the third arc is connected to the first straight line 111 extending the end of the cap. Figure 6 As shown, in another embodiment, in order to appropriately expand the width of the flange portion, a second straight line 107 is also provided between the third arc 106 and the second arc 105. The second straight line 107 is another part of the lower surface of the flange portion, and one end of it is connected to the second arc 105 at point P3 through a tangent relationship, and the other end is the edge of the lower surface of the flange portion, intersecting and connecting with the third arc at position P4. Preferably, a round chamfer is also provided at position P4, so that the transition at the connection between the third arc and the second straight line is smoother. In addition, under the premise of ensuring that the fastener can be effectively welded, the compactness of the fastener in height dimension is improved, and the distance H1 from the connection point P2 of the first arc and the second arc to the horizontal plane where the lower end surface 102 of the shaft portion is located is set to satisfy the relationship: 0.5×T

[0058] In a preferred example, the center of the third arc 106 is located on a perpendicular line to the edge of the flange portion 108 (eg Figure 7 ), and the edge wall thickness t1 of the flange portion does not exceed 0.55 mm, and the maximum wall thickness of the area where the third arc area on the cap is located does not exceed 0.5 mm. Preferably, the wall thickness t2 of the area where the third arc area on the cap is located ranges from 0.36 mm to 0.42 mm, and the maximum wall thickness of the area extending from the third arc to the end 110 of the cap does not exceed 0.45 mm. ​

[0059] The configuration of the shaft and flange profiles of the present invention can effectively prevent the shaft from undergoing plastic deformation similar to "roughening" under the action of heat and force during welding, which may result in the shaft being unable to completely penetrate the first metal workpiece. Figure 8 As shown, if the side profile of the cross-sectional shaft portion is a straight line profile between L1 and L2, and the angle α is in the range of about 0° to 35°, the side close to the lower end face 102 of the shaft portion is prone to plastic deformation similar to "roughening" during the welding process, especially when welding a thicker first metal workpiece, the effect of the shaft portion piercing the first metal workpiece is poor. After the side perimeter of the shaft portion is set to the first arc 104, for example, compared with the fastener shaft portion with the straight line L2 as the side, the side perimeter of the shaft portion of the present invention has an extra area of ​​S1, which can further enhance the support strength of the shaft portion and reduce the occurrence of excessive collapse and deformation of the shaft portion. Fig. 9 As shown, although the stiffness of the shaft portion can be further improved when the axial straight line profile of the fastener is extended to L3 and L4, as the angle α increases, on the one hand, the piercing ability of the shaft portion decreases, and on the other hand, while maintaining the same height as the fastener of the present invention and the volume of the accommodating cavity formed by the cap, the cap 109 needs to be expanded outward to 109' and 109'', which results in a further increase in the diameter of the fastener, such as D3' and D3'', thereby significantly reducing the compactness of the fastener.

[0060] According to another aspect of the embodiment of the present invention, a welding electrode is provided for welding the fastener of the present invention to a welding point of dissimilar metals, and its cross-sectional profile is as follows: Fig.10 As shown. The diameter D4 of the working end face 401 of the first welding electrode on the fastener side is related to the diameter D1 of the lower end face of the fastener shaft: D1<D4<1.4×D1, and the spherical radius R4 does not exceed 60 mm, preferably R4 ranges from 50 mm to 60 mm. The ratio of the working end face diameter D5 of the second welding electrode on the second metal workpiece side to D4 is greater than 1.5, and the spherical radius R5 is greater than 95 mm. Preferably, R5 ranges from 100 mm to 150 mm.

[0061] According to another aspect of the embodiment of the present invention, a welding method is provided, wherein the welding process is as follows: Fig.11 As shown, the welding process is as follows Fig.12 As shown, the welding method specifically includes a preheating stage, a puncture stage and a nucleation stage. In the preheating stage, a small preheating current I1 is input to the welding point, for example, I1=4 kA and maintained for 160 ms, to improve the contact state between the electrode, the fastener and the workpiece. Since the input current I1 in the preheating stage is relatively small, the resistance heat generated is also small, so that only a small range of melting zone 201 (such as Fig.12A1 in the figure). A cooling stage 1 is set between the preheating stage and the puncture stage, and the cooling time of the cooling stage 1 does not exceed 15 ms. In the puncture stage, a puncture current I2 is input, and the puncture current includes a number of pulse currents, which are used to generate instantaneous resistance heat, combined with the squeezing effect of the electrode pressure, so that the first metal melts and expands rapidly to generate splashes that quickly eject from the weld. Since the cap surrounds the periphery of the shaft 101 and always forms a closed cavity with the surface of the first metal workpiece during the welding process, the discharged metal 202 is finally completely received by the cap of the fastener. Driven by the electrode pressure, the shaft 101 smoothly punctures the first metal workpiece and forms a contact interface 301 with the second metal workpiece, as shown in FIG. Fig.12 As shown in B1. In the nucleation stage, a nucleation current I3 is input, for example, I3 = 12 kA and maintained for 350 ms, so that the contact interface 301 is melted and a strong molten core 304 is formed, thereby realizing a strong 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 side of the fastener is further expanded, and part of the liquid metal is squeezed into the fastener cap again, so that the discharged metal 202 in the cap is further compacted. In addition, the first metal melting zone 201 around the shaft side and part of the discharged metal 202 in the cap will also be melted into one, further enhancing the metallurgical and mechanical connection strength between the first metal workpiece and the fastener, as shown in FIG. Fig.12 As shown in C1.

[0062] In order to ensure the stability and reliability of the fastener piercing the first metal workpiece during the continuous welding process of large quantities at a fast beat, the piercing current I2 is divided into two parts. The first part is the spatter inducing current I2-1, which includes a number of pulse currents and is mainly used to induce spatter and realize the fastener piercing the first metal workpiece. Fig.11 As shown, in the preferred embodiment, the current intensity, holding time and cooling time between pulses of each current pulse that induces the 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 current intensity of the first pulse is 17 kA, and the holding time is 55 ms; the current intensity of the second pulse is 16 kA, and the holding time is 52 ms; the current intensity of the third pulse is 15 kA, and 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 spatter current I2-1 that induces spatter, on the one hand, it is ensured that the first metal in the weld is fully discharged into the fastener cap through the spatter process, and on the other hand, it is prevented that the internal melting area 302 of the shaft and the second metal grows to the surface, effectively preventing the liquid first metal and the second metal from mixing to produce a brittle phase.

[0063] The second part of the puncture current I2 is the interface stabilization current I2-2, which does not exceed three current pulses, and the current intensity of a single pulse does not exceed the spatter-inducing current I2-1, the single pulse maintenance time does not exceed 60 ms, and the cooling stage 4 between pulses does not exceed 20 ms. A cooling stage 3 is also set between the spatter-inducing current and the interface stabilization current, and its time range is 25 ms to 35 ms. Setting the interface stabilization current I2-2 can further ensure that the first metal in the weld is fully discharged, so that a stable contact state is achieved between the fastener shaft and the second metal in the weld, providing basic conditions for the subsequent nucleation stage. In particular, in large-scale continuous welding, such as Fig.13 As shown in A2, in most welds, the spatter-induced current I2-1 can completely expel the first metal in the weld, and there is no large amount of first metal remaining on the contact interface 301 between the fastener shaft and the second metal. Since the interface has reached a relatively ideal stable contact state at this time, the resistance heat generated by the interface stabilization current I2-2 may promote the formation of a smaller molten core 303 on the contact interface 301. Due to some unstable factors such as the change in the working end face state of the welding electrode, the workpiece assembly gap and the workpiece surface state, the initial state of each weld is different, such as Fig.13 As shown in B2, after the spatter current I2-1 is induced, the first metal cannot be completely expelled from the weld, resulting in the first metal remaining between the fastener shaft and the second metal. At this time, with the input of the interface stabilization current I2-2, the first metal remaining in the weld is spattered again or melted and squeezed out of the weld, forming a stable contact interface 301 between the fastener shaft and the second metal.

[0064] like Fig.14 A3 shows a joint obtained by using the welding electrode of the present invention. Fig.14 B3 in the figure shows a joint obtained by using a conventional symmetrical welding electrode. Since the working end of the second welding electrode 500 of the present invention is larger in size, the electrode indentation depth t3 formed on the surface of the second metal workpiece weld spot is much smaller than the electrode indentation depth t5 formed by the symmetrical electrode. This avoids the risk of cracks in the weld spot area of ​​the first metal due to excessive deformation, and on the other hand, the bulge height t4 in the direction of the fastener is also reduced, which is beneficial to reduce excessive flattening deformation of the shaft, prevent the locking effect of the fastener on the first metal workpiece from decreasing, and avoid more liquid metal being squeezed into the cap and unable to be fully accommodated.

[0065] In yet another embodiment, Fig.15As shown, the working end face of the second welding electrode 500 of the present invention is divided into an inner first working end face 501 and an outer second working end face. The end face diameter D6 of the first working end face 501 satisfies the relationship: D6≤D4, D4 is the working end face diameter of the first welding electrode 400. The second working end face is composed of a number of concentric annular ridges 503, and the tip width of the concentric annular ridges 503 does not exceed 1 mm. The tip of the concentric annular ridge 503 is located on the spherical surface where the first working end face 501 is located, 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. Fig.16 As shown, during the welding process, the annular groove 502 does 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 being more conducive to the first working end face 501 concentrating most of the welding current to promote the formation of the molten core 304, while also reducing the tendency of the molten core 304 to deviate to the upper side of the shaft of the fastener. The concentric annular ridge 503 on the periphery can effectively limit the first working end face 501 from producing severe electrode indentations on the weld point during the welding process, thereby improving the weld point forming effect.

[0066] Example 1

[0067] In this embodiment, the cross-sectional morphology of the fastener 100 is as follows: Fig.17 As shown. The diameter D1 of the lower end face of the shaft is 5 mm, and the shaft side expands upward and outward through the first arc 104, so that the diameter of the shaft body is expanded to D2=8.6 mm, wherein the radius R1 of the first arc 104 is 4.3 mm, and the distance H1=3.5 mm between the plane where its center is located and the lower end face 102 of the shaft. The radius R2=1.7 mm of the second arc 105, and its center is on the same horizontal line as the center of the first arc 104, and the first arc 104 and the second arc 105 are tangent at the connection. The upper end of the second arc extends outward through the second straight line 107 and is connected to the third arc 106. The distance H2=2.1 mm from the connection point to the horizontal line where the center of the first arc is located, and the radius R3=1.75 mm of the third arc. In addition, the edge wall thickness of the flange portion is t1=0.45 mm, the wall thickness of the area where the third arc is located on the cap is t2=0.4 mm, and the diameter D3=17 mm.

[0068] use Fig.17 The first metal workpiece 200 is a 4 mm thick cast aluminum alloy, and the second metal workpiece 300 is a 1.6 mm thick hot-formed steel. Fig.11 The cross-sectional metallographic structure of the joint obtained after welding is as follows: Fig.18As shown, the shaft of the fastener completely penetrates the first metal workpiece with a large thickness without serious flattening deformation, and the shaft and the second metal workpiece are firmly connected through the molten core. The first metal is locked by the shaft and the flange to achieve the connection between the first metal and the second metal. During the welding process, the end of the fastener cap 109 is always in contact with the surface of the first metal workpiece, forming a closed cavity to fully receive the discharged metal 202.

[0069] It should be understood that the purpose of the above embodiments is only to illustrate the technical concept of the present invention for the convenience of understanding of those skilled in the art, and is not intended to limit the scope of protection of the present invention. Within the scope of the claims of the present invention, any improvement or equivalent replacement of the structure or method steps involved in the above embodiments, especially the combination of different embodiments without structural or principle conflicts, falls within the scope of protection of the present invention.

Claims

1. A fastener for welding dissimilar metals, the fastener is used to weld a laminated structure of a first metal workpiece and a second metal workpiece, 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 fastener is an axis rotationally symmetrical structure, including a shaft portion, a flange portion and a cap, and the material type is the same as that of the second metal workpiece, characterized in that: The shaft portion is used to pierce the first metal workpiece and weld with the second metal workpiece. The lower end surface of the shaft portion is a plane. The lower end of the shaft portion is smaller than the upper end. The side surface of the shaft portion is a curved surface formed by the arc rotating around the axis. The flange portion gradually extends outward and upward from the upper end of the shaft portion, and a thin-walled cap is bent outward 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 outer periphery of the shaft portion and forms a semi-open annular cavity with the outer side of the shaft portion. The volume V1 of the semi-open annular cavity and the volume V2 of the fastener entity satisfy the relationship: V2<V1<2×V2; The fastener profile in a cross section through the fastener axis consists of: A first arc, which is located on the side of the shaft portion, with its center located inside the shaft portion, and one end of the first arc intersects with the lower end surface of the shaft portion, and the other end does not exceed the horizontal plane where the center of the arc is when extending upward; a second arc, which is located above the first arc and constitutes at least a portion of the lower surface of the flange portion, one end of which is connected to the first arc, so that the side surface of the shaft portion smoothly transitions to the lower surface of the flange portion, and the center of the second arc is located outside the shaft portion; The third arc is located on the inner surface of the bend of the cap, one end of the arc is connected to the outer edge of the lower surface of the flange, and the other end is connected to the first straight line extending from the end of the cap.

2. The fastener according to claim 1, characterized in that The first arc radius (R1) is greater than the second arc radius (R2), and a distance H1 from a connection point between the first arc and the second arc to a plane where a lower end surface of the shaft portion is located satisfies: 0.5×T<H1<T, where T is a total thickness of the first metal workpiece.

3. The fastener according to claim 1 or 2, characterized in that: A distance H2 between a horizontal plane where the connection point between the first arc and the second arc is located and a horizontal plane where the connection point between the third arc and the outer edge of the lower surface of the flange portion is located satisfies the relationship: H2≤H1.

4. The fastener according to claim 1, characterized in that The first arc and the second arc are in a tangent relationship at a connection point, and the centers of the first arc and the second arc are located on the same horizontal line.

5. The fastener according to claim 1, characterized in that One end of the third arc is connected to the second arc through a second straight line, and the second straight line and the second arc are tangent to each other at a connection point.

6. The fastener according to claim 1, characterized in that The ratio of the shaft diameter D2 at the intersection of the first arc and the second arc to the shaft lower end surface diameter D1 ranges from 1.4 to 2.

2.

7. The fastener according to claim 1, characterized in that The edge wall thickness t1 of the flange portion does not exceed 0.55 mm, and the maximum wall thickness of the area where the third arc area on the cap is located does not exceed 0.5 mm.

8. A welding electrode for welding the fastener according to any one of claims 1 to 7 to a laminated structure of a first metal workpiece and a second metal workpiece, wherein the fastener is placed on a welding point of the first metal workpiece before welding, characterized in that: The welding electrode is divided into a first welding electrode and a second welding electrode; The first welding electrode is placed on one side of the fastener, and the diameter D4 of the working end face thereof is related to the diameter D1 of the lower end face of the fastener shaft: D1<D4<1.4×D1, and the spherical radius R4 of the working end face does not exceed 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.

9. The welding electrode according to claim 8, characterized in that The working end surface of the second welding electrode is divided into a first working end surface and a second working end surface; The first working end face is located inside and is used to concentrate most of the welding current to promote the formation of the welding spot. The end face diameter D6 of the first welding electrode is related to the working end face diameter D4 of the first welding electrode: D6≤D4. The second working end face is located at the periphery of the first working end face, and is used to limit the first working end face from producing severe electrode indentations on the welding point. The second working end face is composed of a number 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, using the fastener according to any one of claims 1 to 7 and the welding electrode according to any one of claims 8 to 9 to weld a laminated structure of a first metal workpiece and a second metal workpiece, wherein during welding, the welding electrode inputs a welding current to a welding point and applies an electrode pressure, wherein the welding current includes a preheating current (I1), a puncture current (I2) and a nucleation current (I3), and wherein: The piercing current (I2) is used to discharge the first metal in the weld and to enable the shaft of the fastener to pierce the first metal workpiece. The piercing current (I2) is divided into two parts. The first part is a spatter induction current (I2-1), which is mainly used to induce the first metal in the weld to generate spatter, and discharge the first metal in the weld at a high speed through the liquid metal spattering process, so that the shaft of the fastener efficiently pierces the first metal workpiece and contacts the second metal workpiece under the action of electrode pressure, and includes a number of pulse currents; the second part is an interface stabilization current (I2-2), which does not exceed three current pulses, and is used to ensure that the first metal in the weld is completely discharged from the weld, 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 according to claim 10, characterized in that: 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 maximum cooling time does not exceed 20 ms; the cooling time between the induced spatter current and the interface stabilization current ranges from 25 ms to 35 ms; the maximum holding time of a single pulse of the interface stabilization current (I2-2) does not exceed 60 ms, and the current intensity does not exceed the current intensity of the induced spatter current (I2-1).

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