Spot welded joint and its manufacturing method

Abstract

A spot welded joint and a manufacturing method thereof, the spot welded joint comprising a first workpiece (1), a second workpiece (2), and a weld nugget for fixedly joining the first workpiece (1) and the second workpiece (2), wherein at least one of the outer surfaces of the first workpiece (1) and the second workpiece (2) comprises a base surface and a recess, wherein the recess comprises a first surface (111) located at the center and protruding outward, and a second surface (112) located around the first surface (111) and recessed inward, wherein a maximum distance h1 between the first surface (111) and the base surface is smaller than a maximum distance h2 between the second surface (112) and the base surface, and the spot welded joint has stronger joint edge welds and is thinner in the center, which reduces the production of surface cracks and the formation of internal defects and helps to improve the strength of the weld spot.

Description

[Technical Field]

[0001] The present invention relates to the field of resistance spot welding, and more particularly to a spot weld joint formed by resistance spot welding two or more layers of metal workpieces (especially aluminum alloy workpieces) and a method for manufacturing the same. [Background technology]

[0002] As the problems of global warming and energy depletion become increasingly serious, automobile exhaust gases and energy consumption become increasingly serious. Experiments have shown that if the weight of an automobile is halved, fuel consumption will also be reduced by almost half. Due to the need for environmental protection and energy conservation, automobile weight reduction has become a global trend in automobile development. Aluminum alloy materials have advantages such as high strength, light weight, excellent corrosion resistance, and suitability for various forming methods, so if aluminum alloys are used instead of welding steel plate materials, the structural weight can be reduced by more than 50%, and they are widely used in automobile bodies.

[0003] Currently, mechanical joining using rivets is the mainstream method for joining aluminum alloy car bodies in automobile manufacturing. Riveting is expensive, requires a complex process, results in poor surface quality, and increases vehicle weight; a single all-aluminum or hybrid car body typically requires more than 1,500 nails. Resistance spot welding utilizes heat generated by the resistance between the workpieces themselves and between them to melt and join the materials. Because no filler material is required for the joining process, it offers high production efficiency and is easily automatable. Therefore, this method is widely used in the manufacture of automobile body parts, such as engine hoods and car doors. As aluminum alloys become more widely used in automobiles, automakers are expected to continue using resistance spot welding to join aluminum alloys.

[0004] However, due to the physical properties of aluminum alloys themselves, welding them using conventional spot welding techniques poses numerous challenges. Because aluminum alloys have high electrical and thermal conductivity, particularly large currents and pressures are required during spot welding. The use of high currents and high electrode pressures increases production costs during welding. Furthermore, the plastic temperature range of aluminum alloys is relatively narrow, resulting in severe spatter and internal defects during welding, as well as frequent surface cracks. The presence of a highly resistive oxide film on the surface accelerates wear of the welding electrode during the spot welding process, shortening the electrode's lifespan and resulting in reduced strength, surface quality, and aesthetics of the welded spot.

[0005] Patent CN104043898A discloses a method of using a plurality of convex annular protrusions arranged on a spherical electrode to break through the surface oxide film and improve the electrode's lifespan and surface quality. However, in this method, the welding heat is still mainly concentrated in the center of the welding spot, which makes it easy for cracks to form on the surface of the welding spot, thereby reducing the strength of the welding spot.

[0006] Patent US6646221B2 discloses a secondary repair welding method for aluminum spot welding, in which, for weld spots with small weld nuggets and substandard quality, the size of the weld nugget is enlarged by rewelding the outside using an annular cavity electrode. This method can bring the final weld nugget quality up to standard, but this method is only applicable to existing weld spots with small weld nuggets, and must be performed multiple times. This method cannot suppress or remove defects and surface cracks inside the weld spot, and further shortens the life of the electrode. Therefore, there is a need in the art for resistance spot welded aluminum alloy joints and methods for manufacturing the same that provide higher weld strength, longer electrode life, lower cost, excellent surface quality, enhanced aesthetics, and easier promotion. Summary of the Invention [Problem to be solved by the invention]

[0007] The object of the present invention is to provide a resistance spot welded joint and a manufacturing method thereof, which can achieve a spot welded joint with higher weld spot strength and fewer weld defects, thereby solving the problems of prior art aluminum alloy resistance spot welding, such as weld spatter, serious defects, reduced weld strength, unstable weld quality, and short electrode life. [Means for solving the problem]

[0008] A first aspect of the present invention provides a spot welded joint, the spot welded joint including a first workpiece, a second workpiece, and a weld nugget for fixedly joining the first workpiece and the second workpiece, at least one of the outer surfaces of the first workpiece and the second workpiece includes a base surface and a depression, wherein the depression includes a first surface located at a center and protruding outward and a second surface located around the periphery of the first surface and recessed inward, and a maximum distance h1 between the first surface and the base surface is smaller than a maximum distance h2 between the second surface and the base surface.

[0009] In another preferred example, the first surface and / or the second surface has a plurality of discontinuously distributed protruding or recessed annular protrusion structures, and the height of the annular protrusions is 15 to 300 μm. In another preferred example, the weld nugget has a cake-like shape that is thin in the center and thick around the periphery. In another preferred example, the maximum distance s1 between the central thin zone of the weld nugget and the first surface 111 of the workpiece and the minimum distance s2 between the peripheral thick zone and the second surface are:

number

[0010] In another preferred example, the first surface and / or the second surface is composed of a plurality of continuous flat surfaces and arcuate surfaces. In another preferred example, the spot welded joint has a centrally symmetric structure.

[0011] A second aspect of the present invention provides a pair of spot welding electrode caps, the spot welding electrode caps being used for welding to obtain the above-mentioned spot welded joint, and the welding surfaces of the spot welding electrode caps having shapes that match the outer surfaces of the first workpiece and the second workpiece, respectively.

[0012] A third aspect of the present invention provides a method for welding the above-described weld joint, said method comprising: (1) providing a pair of welding electrode caps, the welding surfaces of the spot welding electrode caps having shapes corresponding to the outer surfaces of the first workpiece and the second workpiece, respectively; (2) a pre-pressurizing step in which the welding electrode cap applies electrode pressure to the first workpiece and the second workpiece; (3) applying one or more stages of welding current to the first workpiece and the second workpiece to form a weld nugget; and (4) a condensation step of condensing the weld nugget to form the final spot weld joint.

[0013] In another preferred example, the welding step includes a preheating step, a main welding step, and a post-heating step, and the effective value of the welding current I1 in the preheating step is 10 to 30 KA and the current application time t1 is 30 to 60 ms, the effective value of the welding current I2 in the main welding step is 20 to 60 KA and the total current application time t2 is 50 to 300 ms, and the welding current I3 in the post-heating step is 15 to 40 KA and the duration t3 is 20 to 100 ms.

[0014] In another preferred example, the main welding stage uses multiple identical or different current pulses to form a weld nugget, each welding current being 25-50 KA, the duration of a single welding pulse being 5-30 ms, the pulse interval being 1-10 ms, the number of pulses being at least three, and the pulse interval cooling time being 1-10 ms. [Effects of the Invention]

[0015] It should be understood that within the scope of the present invention, the above-mentioned technical features of the present invention and the technical features specifically described below (e.g., in the Examples) can be combined with each other to form new or preferred technical solutions, which will not be repeated here due to space limitations.

[0016] In order to more clearly describe the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings that need to be used in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention, and those skilled in the art can also obtain other drawings according to these drawings without creative efforts. [Brief explanation of the drawings]

[0017] [Figure 1] 1 shows a cross-sectional schematic view of a typical spot weld joint referred to in the present invention. [Figure 2] 1 shows a schematic topographical view of the surface depressions of the spot welded joint of the present invention. [Figure 3] 1 shows a schematic topographical view of the surface depressions of another spot welded joint of the present invention. [Figure 4] 1 shows a schematic topographical view of the surface depressions of another spot welded joint of the present invention. [Figure 5] 1 shows a schematic topographical view of the surface depressions of another spot welded joint of the present invention. [Figure 6] 1 shows a schematic topographical view of the surface depressions of another spot welded joint of the present invention. [Figure 7]1 shows a schematic topographical view of the surface depressions of another spot welded joint of the present invention. [Figure 8] 1 shows a schematic topographical view of a second surface in a surface depression of a spot welded joint of the present invention. [Figure 9] 1 shows a schematic topographical view of a second surface in a surface depression of another spot welded joint of the present invention. [Figure 10] 1 shows a schematic diagram of the cross-sectional shape of an annular ridge on the surface of a spot welded joint. [Figure 11] 10A-10C show schematic diagrams of alternative cross-sectional shapes of annular ridges on the surface of a spot weld joint; [Figure 12] 10 shows a schematic diagram of another cross-sectional shape of an annular ridge on the surface of a spot weld joint. [Figure 13] 1 shows a schematic topographical view of the surface depressions of another spot welded joint of the present invention. [Figure 14] 1 shows a schematic topographical view of the surface depressions of another spot welded joint of the present invention. [Figure 15] 1 shows a cross-sectional schematic view of another spot welded joint referred to in the present invention. [Figure 16] 1 shows a cross-sectional schematic view of another spot welded joint referred to in the present invention. [Figure 17] 1 shows a cross-sectional schematic view of another spot welded joint referred to in the present invention. [Figure 18] 1 shows a cross-sectional schematic view of another spot welded joint referred to in the present invention. [Figure 19] 1 shows a cross-sectional schematic view of another spot welded joint referred to in the present invention. [Figure 20] 1 shows a cross-sectional schematic view of another spot welded joint referred to in the present invention. [Figure 21] 1 shows a cross-sectional schematic view of another spot welded joint referred to in the present invention. [Figure 22] 1 shows a schematic partial cross-sectional view of a welding electrode according to the present invention. [Figure 23] 1 shows a timing diagram of spot welding current and pressure referred to in the present invention. [Figure 24]4 shows another spot welding current and pressure timing diagram referred to in the present invention. [Figure 25] 1 shows a welding image of the surface of a typical spot welded joint in the prior art. [Figure 26] 1 shows a weld image of the surface of a spot welded joint of the present invention. [Figure 27] 1 shows a cross-sectional view of a typical spot welded joint in the prior art. [Figure 28] 1 shows a cross-sectional view of a spot welded joint of the present invention. [Figure 29] 1 shows a cross-sectional view of another spot welded joint of the present invention. [Figure 30] 1 shows a cross-sectional view of another spot welded joint of the present invention. [Figure 31] 1 shows a cross-sectional view of another spot welded joint of the present invention. [Figure 32] 1 shows a cross-sectional view of another spot welded joint of the present invention. [Figure 33] 1 shows a cross-sectional view of another spot welded joint of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0018] The inventors, through extensive and thorough research and extensive screening, have developed for the first time a spot welded joint and a method for manufacturing the same. The spot welded joint of the present invention forms a cake-like weld nugget that is thin in the center and thick on the periphery via a depression including a first surface located in the center and protruding outward, and a second surface located around the first surface and recessed inward, thereby giving the spot welded joint superior surface quality and welding performance and high joint strength. Furthermore, the spot welded joint of the present invention can be section-welded using multi-segment pulses, and the weld nugget is formed with cooling between segments, making the formed weld nugget more stable and requiring less welding energy. This is the basis for the present invention.

[0019] The present invention provides a spot welded joint, the joint having a generally centrally symmetrical structure, the joint including a fused zone and a non-fused zone when viewed along a cross section of the joint, the weld spot joint having a binding interface and opposing upper and lower surfaces, the outer surface of a workpiece at the weld spot zone having a depression generally spaced from the workpiece fused zone; wherein at least one side of the recess includes at least a central first surface and a second surface gradually extending outward, and the maximum distance h1 between the central first surface of the recess and the outer surface of the workpiece is smaller than the maximum distance h2 between the outer second surface of the central recess and the surface of the workpiece; In a preferred embodiment, the joint is obtained by a spot welding method, which comprises providing a pair of welding electrode caps, wherein at least one electrode cap surface has a shape similar to the recess of the joint; and then a pre-pressurizing step of applying electrode pressure to the workpiece; a welding step of applying a welding current to the workpiece in one or more stages to form a molten core; and a condensation step in which the molten core condenses to form a final spot weld joint. In a preferred embodiment, the welding electrode cap has a central concave feature, wherein the ratio of the maximum depth of the concave to its peripheral diameter is 1% to 20%, preferably 2% to 10%. In a preferred embodiment, the diameter of the circumference on which the first surface edge of the recess is located does not exceed 8 mm, preferably does not exceed 7 mm; In a preferred example, the diameter of the circumference where the outside of the depression is located does not exceed 14 mm, preferably does not exceed 8 to 13 mm, In a preferred example, the first surface at the center of the depression has an overall arcuate surface structure, and a distance h1 between the apex of the arcuate surface and the workpiece surface (here, referring to the basic surface of the workpiece surface) is within −0.3 to +0.3 mm, preferably −0.15 to +0.15 mm, where the minus sign “−” indicates that the apex is inside (below) the basic surface, and the “+” indicates that the apex is outside (upper) the basic surface; In a preferred example, the first surface at the center of the depression has a plurality of discontinuously distributed protruding or recessed annular protrusion structures, and the height of the annular protrusion is 15 to 300 μm, preferably 30 to 250 μm; In a preferred example, the second surface outside the depression has a plurality of convex or concave annular raised features, and the height of the annular raised features is 15 to 300 μm, preferably 30 to 250 μm; In a preferred example, the second surface outside the depression is composed of a plurality of continuous flat and arcuate surfaces, There is further provided a spot welded joint, the joint having, when viewed along a cross section thereof, a central molten zone and a non-molten zone, the boundary of the molten zone being the boundary of the molten nugget; The projected length of the fusion zone at the binding interface is d, the maximum projected height of the fusion nugget boundary within 40% of the center of the fusion zone in the thickness direction of the workpiece is h5, and the maximum projected height of the fusion nugget boundary within 60% of the outer fusion zone in the thickness direction of the workpiece is h6, where h6≧h5; In a preferred example, the thickness of the thinnest workpiece in the weld joint is w, and 0.5≦w≦4.0 mm, where:

number

number

[0020] The main advantages of the present invention are: (a) In the case of the proposed spot welding, the depression on the surface of the welding spot is shallower in the center than in the outer depression, thereby reducing the crack and burn-off situation on the surface of the center of the welding spot, improving the strength and surface quality of the welding spot, and improving the aesthetics; (b) The thickness of the weld nugget in the center of the fusion nugget does not exceed the thickness of the edge, which improves the supporting force of the weld spot edge, reduces the production of cracks inside the center, and improves the strength of the weld spot; (c) The use of multi-stage pulsed current welding method fully avoids overheating of the welding spot surface, promotes heat dissipation, reduces adhesion on the welding spot surface, improves surface quality and electrode life, and at the same time avoids defects such as spatter caused by excessive internal heating rate, reduces energy dissipation, and saves energy.

[0021] Hereinafter, the present invention will be further described in conjunction with specific examples. It should be understood that these examples are used only to explain the present invention and do not limit the scope of the present invention. Furthermore, since the drawings are schematic diagrams, the apparatus and device of the present invention are not limited to the size or proportion of said schematic diagrams.

[0022] It should be noted that in the claims and the specification, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another and do not necessarily require or imply an actual relationship or order between those entities or operations. Furthermore, terms such as "comprise," "comprises," or any other variation thereof are intended to cover a non-exclusive inclusion, such that a process, method, article, or device comprising a set of elements may further include those elements, as well as other elements not expressly listed, or elements inherent in such process, method, article, or device. Absent further limitations, an element defined by the phrase "comprises one of" does not exclude the presence of other identical elements within the process, method, article, or device of that element.

[0023] Example

[0023] Now, referring to Figure 1, Figure 1 shows a cross-sectional schematic diagram of a spot welded joint referred to in the present invention, the joint is obtained by resistance spot welding a first workpiece 1 and a second workpiece 2, the joint includes a weld nugget 3 formed by a fusion zone, the boundary of the fusion nugget is 31, the joint surface has a spot weld depression formed thereon, the joint has a first workpiece surface 10, a second workpiece surface 20 and a binding interface 12, the depression surface is composed of a central first surface 111 and a second surface 112 remote from the center, the most of the depression first surface 111 The height between the highest point and the base surface of first workpiece surface 10 (the base surface refers to the surface on first workpiece surface 10 other than the depression) is h1, and the height between the lowest point of second surface 112 and the base surface of surface 10 is h2, where h1≦h2. That is, facing the melt center, first surface 111 located at the center of the depression is shallower and located further outward than second surface 112 located outside the depression. Here, h1 is defined as a positive value for distance from the melt center and a negative value for distance from the melt center. Therefore, h1 is generally −0.3 to 0.3 mm, preferably −0.2 to 0.2 mm. The diameter of the outer circumference of first surface 111 is d1, which is generally 2 to 6 mm, preferably 3 to 5 mm. The diameter of the circumference of second surface 112 located outside is 6 to 16 mm, preferably 8 to 14 mm.

[0024] Figure 2 is a local enlarged view of the depression surface 11. The first surface 111 located in the center of the depression can have various shape features. The first surface 111 shown in Figure 2 is a surface formed by arc rotation. The first surface 111 shown in Figure 3 is composed of an arc surface with a certain radius of curvature. The radius of curvature R of the arc surface is generally 5 mm or more, preferably 5≦R≦100 mm. The positions of the first surface 111 and the second surface 112 in the depression relative to the workpiece surface 10 can have various types. As shown in Figure 2, the first surface 111 of the depression is higher than the workpiece surface 10 along the melt center direction. Figure 3 shows that the first surface 111 is lower than the workpiece surface 10, that is, h1 is located outside or inside the workpiece surface, respectively. All of these are possible in the present invention.

[0025] It should be noted that the first surface 111 at the center of the depression can have various shape features. For example, its cross-sectional shape can be composed of any combination of multiple straight or curved lines. As shown in FIG. 4, the first surface 111 is a surface of revolution composed of multi-segment curves smoothly joined with transitions, and the first surface is a flat end face. As shown in FIG. 5, the first surface 111 is usually a continuous, smooth transition surface, ensuring that the material can be flat and smoothly deformed without producing defects such as cracks during the welding process. In particular, the first surface at the center of the depression can further have multiple protruding or recessed annular ridge structures. FIG. 6 shows a schematic diagram of two circular protruding annular ridge structures 1111 on the first surface. The height h3 of the annular ridges 1111 is generally 15 to 300 μm.

[0026] The second surface 112, which is the surface of the recess, can further include various structural shapes. The cross-sectional shapes of the second surface shown in FIGS. 1 to 5 are all composed of arcs and straight lines. In practice, the second surface 112 can also include multiple convex or concave annular ridge structures. FIG. 7 shows a schematic diagram of two circular concave annular ridges (or simply referred to as "annular concaves") on the second surface 112. The number of annular ridges is 1 to 5, preferably 2 to 4. The distance between the concave annular ridges and their base surface is h4. The base surface on which the annular ridges are located is flat or curved. FIG. 8 shows a locally enlarged view of an annular ridge portion on the second surface 112. The second surface 112 is composed of an outer surface 1121 and an inner surface 1123 (the outer and inner surfaces are based on the central weld nugget 3; the side closer to the weld nugget 3 is called the inner surface, and the side farther from the weld nugget 3 is called the outer surface). The inner surface 1123 is composed of a combination of convex and concave shapes formed by a plurality of straight and curved lines. Here, the base surface 1122 on which the inner surface 1123 is located may be flat and / or curved. When the base surface 1122 is flat, the maximum concave height of the annular ridge relative to the base surface 1122 is h4, which is generally 15 to 300 μm, preferably 30 to 280 μm.

[0027] The central spacing distance B between the annular ridges is generally 300 μm≦B≦2000 μm, preferably 400 μm≦B≦1500 μm.

[0028] The diameter d3 of the outer circumference of the base surface 1122 is generally 7-13 mm, preferably 8-12 mm. The outer surface 1121 may be a conical or arcuate surface. If the outer surface 1121 is a conical surface, the included angle α formed with the workpiece surface 10 is generally not greater than 40°, preferably 5°≦α≦30°, as shown in FIG. 8 . The outer surface 1121 may also be an arcuate surface as a whole, and if it is an arcuate surface, the radius r1 of the arcuate surface is 20 mm or more, preferably 25≦r1≦100 mm. The base surface 1122 may also be an arcuate surface, and the radius r2 of the arcuate surface is 25 mm or more, preferably 30≦r2≦100 mm. In a particular embodiment, the outer surface 1121 and the base surface 1122 are the same surface, i.e., both are arcuate surfaces, as shown in FIG.

[0029] The annular ridge on either the first surface 111 or the second surface 112 can have various structural types. The annular ridge is a structure that can be formed by combining any straight and curved lines and then rotating them. As shown in FIG. 10, the annular ridge has an overall trapezoidal structure, with the width of its bottom surface being B1 and the width of its top surface being B2, generally 200 μm≦B1≦1000 μm, 0≦B2≦500 μm, and preferably 300 μm≦B1≦600 μm, 0≦B2≦300 μm. In particular, when B2 is zero, it becomes triangular, as shown in FIG. 11. Note that the cross-sectional shape of the annular ridge can be composed of any straight or curved lines. For example, as shown in FIG. 12, the entire annular ridge is composed of curved lines and forms a shape similar to a triangular structure, and various transition fillets can be included between any straight and curved lines. Any other shape is also possible, and will not be repeated here.

[0030] The shape of the joint recess surface is composed of any of the first and second surface features described above. As shown in Figure 13, the first surface 111 is a circular arc surface, and the second surface 112 is a combination of the outer circular arc surface and a flat base surface, with two inverted triangular ring-shaped recesses on the base surface. Figure 14 shows a schematic diagram of the convex ring-shaped protrusions on the second surface.

[0031] 15, the joint of the present invention, when viewed along its cross section, has a central fused zone 3 (i.e., "weld nugget") and a peripheral unfused zone, the intersection of both of which is referred to as the fused nugget boundary 31. The projected length of the fused zone 3 at the binding interface of workpieces 1 and 2 is d, the maximum projected height of the fused nugget boundary in the thickness direction of the workpieces within the central 60% of the fused zone is h5, and the maximum projected height of the fused nugget boundary in the thickness direction of the workpieces within the outer 40% of the fused zone is h6, where h6≧h5. The thickness of workpiece 1 at the weld joint is w, and 0.5≦w≦4.0 mm, where

number

[0032] The molten nugget morphology within the 60% d range and the 40% d range of the fusion zone can include various types, and as shown in FIG. 16, the shape of the fusion zone 3 is basically close to an ellipse, but in reality, the height h5 within the 0.6d range is still equal to or less than h6.

[0033] The shape of the melting zone and the joint surface depression can have various combinations. As shown in FIG. 17, the first surface 111 located at the center of the depression is an arcuate surface, the second surface 112 is a surface having an annular concave shape with a trapezoidal cross section, and the melting zone 3 is a combination of welding nuggets having a peanut shell shape. As shown in FIG. 18, the first surface 111 located at the center of the depression is an arcuate surface, the second surface 112 is a surface having an annular raised concave shape with a triangular cross section throughout, and the melting zone 3 is still a combination of welding nuggets having a peanut shell shape. The peanut shell shape described in this specification is the cross-sectional shape of the welding nugget observed from the central cross section of the welded workpiece, and its overall shape is in the form of a cake with a thin center and thick periphery.

[0034] The shortest distance s1 (unit: mm) between the thick zone around the welding nugget and the second surface 112, and the longest distance s2 (unit: mm) between the thin zone in the center and the first surface 111 of the workpiece are

Number

[0035] The shape of the melting zone 3 is not necessarily an axially symmetric structure. When welding some workpieces with different thicknesses or different materials, etc., an asymmetric structure is often formed. As shown in FIG. 19, when the thicknesses of the two workpieces are different, the thickness of the first workpiece is w1, the thickness of the second workpiece is w2, and generally, w1 < w2. In this case, the melting height within the range of 0.4d of the inner melting zone on at least the first workpiece side is h5, the melting height within the range of 0.6d is h6, satisfying h6 > h5. In this case,

Number

[0036] Similarly, the upper and lower surfaces of the joint recessed surface do not necessarily have to be completely identical. As shown in FIG. 20, the first workpiece surface and the second workpiece surface are not completely identical. The first workpiece surface has a convex arc-shaped first surface 111 in the center, a concave structure with annular protrusions distributed on the outside, and the second workpiece surface is an arc-shaped surface with a radius of curvature r3 as a whole. Generally, the outer circumferential diameter d5 of the second recessed surface is 6 to 14 mm, preferably 8 to 12 mm, and r3 is 20 mm or more, preferably 25 to 200 mm. The first recessed surface 11 and the second recessed surface 21 can have other different combination types. 21, the central surface of the first recessed surface 11 is a convex arc surface, and the outer surface is a second surface with two annular concave structures, and the central portion of the second recessed surface 21 is a first surface with an arc surface structure as a whole, and the outer surface is a second surface with three annular concave structures. Therefore, the first surface of the first recess and the second surface of the second recess can be formed in any shape or size, including different concave / protruding structures, sizes, flat or curved surfaces.

[0037] The first workpiece 1 and the second workpiece 2 in the present invention are aluminum workpieces, specifically aluminum alloys such as aluminum-magnesium alloys, aluminum-silicon alloys, aluminum-magnesium-silicon alloys, or aluminum-copper alloys, or magnesium alloys such as magnesium-aluminum alloys, magnesium-manganese alloys, or magnesium-zinc-zirconium alloys. Their forms may be wrought or cast, and their material states may include various quenching processes, including heat treatments such as annealing and strain hardening. Furthermore, an inorganic or organic oil film (e.g., a lubricating oil film) may be formed on the surface of the plating layer. The joint may be formed by resistance spot welding two or more workpieces, and the thicknesses of the workpieces may be the same or different. It should be noted that the term "workpiece" used herein broadly includes metal sheet layers, protrusions, castings, and other workpieces that can be resistance spot welded.

[0038] A heat-curable adhesive may be included between workpiece casting 1 and workpiece casting 2, with the adhesive typically having a thickness of 0.1-5 mm, preferably 0.2-2 mm, and the thickness of the bonding layer in the weld spot zone may vary depending on the electrode pressure F during the welding process. Typically, the adhesive is applied to the contact surfaces of first metal workpiece 1 and second metal workpiece 2, and after resistance spot welding, the stack assembly is cured in an oven or other heating device to achieve a strong adhesive bond. The heat-cured adhesive is typically a heat-curable epoxy resin, which is well known in the art.

[0039] The joint is obtained by a spot welding method, which comprises providing a pair of welding electrode caps, at least one of which has a shape similar to the recess of the joint, and then a pre-pressurizing step of applying electrode pressure to the workpiece; a welding step of applying a welding current to the workpiece in one or more stages to form a molten core; and performing a resistance spot welding process, followed by a condensation step in which the molten core condenses to form a final spot weld joint.

[0040] The shape of the welding electrode cap is similar to the shape of the joint surface depression. For example, if the joint surface depression has symmetrical central convex arc surfaces and edge convex annular protrusions, the shape of the electrode cap 4 has a central concave surface 41 and edge convex annular protrusions 42. Here, the height of the central concave surface is h7 (based on the height of the surface corresponding to the joint base surface), and the diameter of the outer circumference of the concave surface is d7. Generally, h7 is 0.05 to 1.0 mm, preferably 0.1 to 0.5 mm, and d7 is 2 to 8 mm, preferably 3 to 7 mm, with the ratio h7 / d7 being 1 to 20%, preferably 2 to 15%. Figure 22 shows the cross-sectional shape of the electrode cap 4 in this case. Similarly, if the surface of the recess of the joint has another shape, or if the recesses on both sides of the joint have an asymmetric surface structure, the corresponding welding electrode cap also has a similar shape, and its welding surface can be spherical, flat, or other specially shaped surface, for example, the surface has an electrode cap with an end surface having a protruding structure or a concave structure, which will not be repeated here. Generally, the welding electrode can be made of any electrically and thermally conductive material, for example, can be made of a copper alloy, including a copper chromium (CuCr) alloy, a copper chromium zirconium (CuCrZr) alloy, a copper alloy with added alumina particles, or other copper alloys that can be used as various electrode materials.

[0041] FIG. 23 shows a timing diagram of a resistance spot welding process, including a preheating stage, a main welding stage, and a post-heating stage, as referred to in the present invention. Generally, the welding process involves passing a welding current through one or more segments, particularly during the main welding stage. The effective value of the welding current I1 during the preheating stage is typically 8-30 kA, preferably 10-20 kA, and more preferably 12-15 kA. The welding time t1 during this stage is 30-60 ms. The preheating stage ensures sufficient contact with the workpiece and reduces the contact resistance between the welding electrode and the workpiece, thereby extending the electrode's life and improving the stability of the weld spot. During the main welding stage, which is a proper and stable stage, the effective value of the current I2 is typically 20-60 kA, and the total welding time t2 is 50-300 ms. Preferably, the effective value of the current I2 is 25-50 kA, and the total welding time t2 is 60-200 ms. When the main welding stage is to form a sufficiently large weld nugget through multiple identical or different current pulses, as shown in Figure 24, each welding current I21, I22...I2n in this process is generally 25-50 KA, and in this case, the duration t2n of a single welding pulse is generally 5-30 ms, preferably 6-20 ms, where n is an integer greater than or equal to 1, preferably n≧3, and the pulse interval time tc is 1-10 ms. Here, the number of pulses n is:

number

[0042] Resistance heat is generated between the workpieces, forming a fused nugget 3, and a depression forms on the surface of the weld spot as electrode pressure is applied. The first surface, with a relatively convex center, prevents internal heat from concentrating in the center and causing welding defects such as thermal cracking. The thicker central unfused zone within the weld nugget prevents defects such as internal burnout and thermal cracking, and avoids defects such as electrode adhesion caused by heat concentration in the center of the weld spot. Generally, in prior art, the central zone generates more heat and dissipates more slowly, preventing the weld nugget from fully growing, potentially resulting in spatter, internal cracking, and adhesion. However, the weld joint of the present invention has more uniform temperature and stress distribution inside and outside, making it easier to form a uniform weld nugget. The edges of the internal fused zone have a thicker fusion depth, which improves joint strength, more uniform stress distribution throughout the weld nugget, reduces residual stress, and improves the performance of the weld spot. When there is an annular protrusion outside the depression and the aluminum oxide film on the annular protrusion is peeled off, the contact resistance during welding contact is reduced, the contact area is increased, and heat dissipation is enhanced, thereby reducing the heat between the electrode welding surface and the workpiece contact surface, thereby improving the service life of the electrode and the quality of the welding spot surface.

[0043] The current pulse I3 in the post-heating stage is generally 15-40 KA, preferably 25-35 KA, the duration t3 is 20-100 ms, preferably 30-80 ms, more preferably 30-60 ms, and I3 does not exceed 0.8 × I2max (where I2max is the maximum current peak value in the post-heating stage). The cooling interval between the post-heating stage and the main welding stage is 15 ms or more, generally 15-40 ms, so that when the weld nugget condenses, it is completed at a lower cooling rate, thereby avoiding the formation of internal shrinkage cavities and cracks. The presence of the post-heating stage allows the molten workpiece to have smaller shrinkage stress during the cooling stage, thereby reducing the production of internal shrinkage cavities and cracks. At the same time, during the entire welding stage, the electrode pressure F is generally 2000-10000 N, preferably 3000-8000 N, and can be changed during the welding stage, but is not necessarily constant. The pressure application method is air pressure application or servo pressure application, which is easily understood in the art.

[0044]

[0003] Referring now to Figure 25, which shows the surface morphology of a conventional spot-welded joint obtained using a 5-based aluminum alloy, it can be seen that the entire surface depressions are spherical and that a large amount of copper-aluminum alloying structure is attached to the surface, which can seriously affect the electrode life, increase manufacturing costs, and cause the appearance of the weld spot to deteriorate. In contrast, Figure 26 shows a photograph of the depressions on the surface of the weld spot shown in the present invention, which shows that the depressions on the surface of the weld spot obtained are uniform and beautiful, and are free of alloying reaction layer deposits, greatly improving manufacturing efficiency, reducing costs, and improving surface quality.

[0045] FIG. 27 shows a cross-sectional view of a weld nugget from a conventional spot weld of an aluminum workpiece. In the center of the fusion zone, there is a high possibility of cracks and internal defects occurring, and burnout and electrode adhesion in the center of the fusion zone are likely to occur, resulting in unstable weld quality, shortened electrode life, and increased manufacturing costs. 28 shows a typical cross-sectional shape of a joint formed by spot welding of the 6-based aluminum alloy of the present invention, in which the recessed surface is composed of a central first surface 111 and an outer second surface 112, and the entire structure has a central rotation structure. The height between the highest point of the first surface 111 and the workpiece surface 10 is h1, and the height between the lowest point of the second surface 112 and the workpiece surface 10 is h2, where the entire second surface 112 is lower than the workpiece surface 10, and the first surface 111 is higher than the workpiece surface 10. The fusion nugget diameter of the weld nugget 3 in the fusion zone is d, the fusion height near the center is h5, and the fusion height at the outer edge is h6, where h6 > h5. This type of weld nugget shape can prevent overheating at the center and the formation of surface defects such as cracks, thereby improving the strength of the weld spot joint.

[0046] In another example, in the case of a 1.2 mm 6-series aluminum alloy, even if the weld nugget size reaches 6.55 mm, defects such as surface burn-off and cracks still do not appear. As shown in Figure 29, the first surface 111 and the second surface 112 of the depression on the surface of the welded spot joint are both lower than the workpiece surface 10, and the second surface 112 of the depression has two concave triangular annular concave structures, but its h2 is still larger than h1.

[0047] Fig. 30 shows the cross-sectional shape of a 2 mm 7 series aluminum alloy spot welded joint, in which the central first surface 111 of the indentation is usually higher than the second surface 112 of the indentation, where the second surface 112 has two concave trapezoidal ring structures, which are advantageous for reducing the formation of cracks on the first surface 111. Fig. 31 shows the cross-sectional shape of a welded spot joint obtained by resistance spot welding two layers of AZ31 magnesium alloy, in which the indentation is composed of the central first surface 111 and the outer second surface 112, and the height of the central part of the weld nugget 3 formed in the fusion zone is h5, but the maximum height of the outer edge is h6, where h6 > h5.

[0048] For workpieces with uneven thicknesses, Figure 32 shows a weld joint obtained by resistance spot welding dissimilar materials of uneven thickness, namely, a 4.0 mm cast aluminum alloy and a 2.3 mm aluminum alloy. The recesses on both sides of the joint can be different. The recess on the cast aluminum side has a central first surface 111 and an outer edge second surface 112, where the second surface 112 has three annular recesses, while the recess 21 on the 5-type aluminum alloy side has an overall spherical recess shape. This type of combination allows for a sufficiently large weld nugget size while simultaneously forming a sufficiently deep fusion zone on the thin plate, while avoiding defects such as burn-through and cracks on the thick plate. This provides excellent adaptability when specific application environments have special requirements for surface protrusions or recesses.

[0049] Of course, the depressions on both sides of the weld spot can also be symmetrical. Figure 33 shows the cross-sectional shape of a joint obtained by spot welding unevenly thick cast aluminum and a 5-series aluminum alloy. Both sides of the weld spot include a first surface 111 with a central convex shape and a second surface 112 with three annular depressions on the outside. The maximum height h6 of the center of the fusion zone is located at the edge, and the minimum height h5 is located in the center.

[0050] All documents mentioned in this application are incorporated by reference in this application as if each document were incorporated by reference individually. Furthermore, after reading the above teachings of the present invention, those skilled in the art will be able to make various changes or modifications to the present invention, and these equivalents will also fall within the scope defined by the appended claims of this application. [Explanation of symbols]

[0051] 1-First workpiece 2-Second workpiece 10—First workpiece surface 11-First recessed surface 12-Binding Interface 111-First Surface 112-Second Surface 3-Fusion zone (weld nugget) 31-Melting nugget boundary 20-Second workpiece surface 1111-Annular ridge 1122-Base surface 1123-Inner surface 1121-Outer surface 21-Second recessed surface 4- Welding electrodes 41-Welding electrode center surface 42-Welding electrode outer surface

Claims

1. A spot welded joint, the spot weld joint includes a first workpiece, a second workpiece, and a weld nugget for fixedly joining the first workpiece and the second workpiece, and at least one of an outer surface of the first workpiece and an outer surface of the second workpiece includes a base surface and a recess; Here, the depression includes a first surface located at the center and protruding outward, and a second surface located around the periphery of the first surface and recessed inward, and the maximum distance h1 between the first surface and the base surface is smaller than the maximum distance h2 between the second surface and the base surface.

2. The first surface and / or the second surface have a plurality of discontinuously distributed protruding or recessed annular protrusion structures, and the height of the annular protrusions is 15 to 300 μm. The spot welded joint of claim 1.

3. The weld nugget is characterized in that it has a cake-like shape with a thin center and a thick periphery. The spot welded joint of claim 1.

4. The longest distance s1 between the central thin zone of the weld nugget and the first surface 111 of the workpiece, and the shortest distance s2 between the peripheral thick zone and the second surface are: [Equation 8] s2≦0.9w, s1≧s2, where w is the thickness of the workpiece. The spot welded joint of claim 1.

5. The first surface and / or the second surface is characterized in that it is composed of a plurality of continuous flat surfaces and arc surfaces. The spot welded joint of claim 1.

6. The spot welded joint is characterized in that it has a centrally symmetrical structure. The spot welded joint of claim 1.

7. A spot welding electrode cap, The spot welding electrode cap is used for welding to obtain the spot welded joint according to any one of claims 1 to 6, and characterized in that welding surfaces of the spot welding electrode cap have shapes that match the outer surfaces of the first workpiece and the second workpiece, respectively.

8. A method for welding the welded joint according to any one of claims 1 to 6, The method comprises: (1) providing a pair of spot welding electrode caps, the welding surfaces of the spot welding electrode caps having shapes corresponding to the outer surfaces of the first workpiece and the second workpiece, respectively; (2) a pre-pressurizing step in which the welding electrode cap applies electrode pressure to the first workpiece and the second workpiece; (3) applying one or more stages of welding current to the first workpiece and the second workpiece to form a weld nugget; and (4) a condensation step of condensing the weld nugget to form the final spot weld joint.

9. The welding step includes a preheating step, a main welding step, and a post-heating step, The effective value of the welding current I1 in the preheating stage is 10 to 30 KA, and the current application time t1 is 30 to 60 ms. The effective value of the welding current I2 in the main welding stage is 20 to 60 KA, and the total welding time t2 is 50 to 300 ms. The spot welded joint according to claim 8, wherein the welding current I3 in the post-heating stage is 15 to 40 KA, and the duration t3 is 20 to 100 ms.

10. The main welding step is characterized in that a plurality of the same or different current pulses are used to form a weld nugget, each welding current is 25-50 KA, the duration of a single welding pulse is 5-30 ms, the interval between pulses is 1-10 ms, the number of pulses is at least three, and the interval between pulses is 1-10 ms. The spot welded joint of claim 9.

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