Spot welded joint and its manufacturing method

Abstract

This invention discloses a spot welded joint and its manufacturing method. The spot welded joint includes a first workpiece, a second workpiece, and a weld nugget for fixing the first and second workpieces together. At least one of the outer surfaces of the first and second workpieces includes a base surface and an indentation. The indentation includes a first surface protruding outward from the center and a second surface recessed inward from the outer periphery of the first surface. The maximum distance h1 between the first surface and the base surface is less than the maximum distance h2 between the second surface and the base surface. The spot welded joint of this invention features stronger edge welding and a thinner center, which helps reduce surface cracks and internal defects, thereby improving weld strength.

Description

Technical Field

[0001] This invention relates to the field of resistance spot welding, and more specifically to spot welded joints formed by resistance spot welding of two or more layers of metal workpieces (especially aluminum alloy workpieces) and their manufacturing methods. Background Technology

[0002] With the increasing severity of global warming and energy depletion, automobile emissions and energy consumption are becoming more and more serious. Experiments have shown that reducing the weight of a car by half can reduce fuel consumption by nearly half. Due to the need for environmental protection and energy conservation, lightweighting of automobiles has become a global trend in automotive development. Aluminum alloys, with their advantages of high strength, light weight, excellent corrosion resistance, and suitability for various forming methods, are widely used in automobile bodies because replacing steel plates with aluminum alloys in welding can reduce structural weight by more than 50%.

[0003] Currently, the main method for joining aluminum alloy bodies in automobile manufacturing is riveting. Riveting is a costly, complex process with poor surface quality, and it increases the weight of the car body. A typical all-aluminum or hybrid body requires more than 1500 nails. Resistance spot welding utilizes the resistance between the workpieces themselves and each other to generate heat and melt the material to achieve a connection. Because it does not require filler material during the connection process, it has high production efficiency and is easily automated. Therefore, this method is widely used in automobile body manufacturing, such as for engine hoods and doors. With the increasing use of aluminum alloys in automobiles, automakers hope to continue using resistance spot welding for joining aluminum alloys.

[0004] However, due to the inherent physical properties of aluminum alloys, there are numerous problems when using ordinary spot welding processes. The high electrical and thermal conductivity of aluminum alloys necessitates exceptionally high current and pressure during spot welding, leading to increased manufacturing costs. Furthermore, the narrow molding temperature range of aluminum alloys results in severe spatter and internal defects during welding, along with numerous surface cracks. The presence of a high-resistivity oxide film on the surface also causes rapid electrode wear and short electrode life during spot welding, ultimately leading to reduced weld strength, poor surface quality, and low aesthetic appeal.

[0005] Patent CN104043898A discloses a method for using multiple raised annular ridges on a spherical electrode to puncture the surface oxide film, thereby improving electrode life and surface quality. However, because the welding heat is still mainly concentrated in the center of the weld point, this method is very easy to form cracks on the surface of the weld point, which in turn leads to a decrease in the strength of the weld point.

[0006] US6646221B2 discloses a method for secondary welding of aluminum spot welds. For welds with small weld nuggets and substandard quality, a ring-shaped concave electrode is used to re-weld on the outside to enlarge the weld nugget size. Although this method can make the final weld nugget quality meet the standard, it is a method for existing welds with small weld nuggets, which requires multiple applications. Furthermore, it cannot suppress or eliminate the generation of defects inside the weld nugget and surface cracks, and the electrode life is shorter.

[0007] Therefore, there is a need in the field for a resistance spot welding joint for aluminum alloys that can achieve higher welding strength, longer electrode life, lower cost, better surface quality, higher aesthetics, and is easier to promote, as well as a method for manufacturing the same. Summary of the Invention

[0008] The purpose of this invention is to provide a resistance spot welding head and its manufacturing method, so as to obtain a spot welding head with high weld strength and fewer welding defects, and solve the problems of welding spatter, severe defects and low welding strength, unstable welding quality and short electrode life in the prior art of aluminum alloy resistance spot welding.

[0009] In a first aspect of the invention, a spot welding head is provided, the spot welding head including a first workpiece, a second workpiece, and a weld nugget for fixing the first workpiece and the second workpiece together; at least one of the outer surfaces of the first workpiece and the second workpiece includes a base surface and an indentation; wherein the indentation includes a first surface that protrudes outward from the center and a second surface that is recessed inward from the outer periphery of the first surface, and the maximum distance h1 between the first surface and the base surface is less than the maximum distance h2 between the second surface and the base surface.

[0010] In another preferred embodiment, the first surface and / or the second surface have a plurality of discontinuously distributed protruding or recessed annular ridge structures; the height of the annular ridges is 15-300 μm.

[0011] In another preferred embodiment, the weld nugget is a disc-shaped material that is thin in the middle and thick at the periphery.

[0012] In another preferred embodiment, the longest distance s1 of the thinner central region of the weld nugget from the first surface 111 of the workpiece and the shortest distance s2 of the thicker peripheral region from the second surface satisfy the following: s2≤0.9w; s1≥s2, preferably s1≥1.2*s2, where w is the workpiece thickness.

[0013] In another preferred embodiment, the first surface and / or the second surface is composed of a combination of multiple continuous planes and arc surfaces.

[0014] In another preferred embodiment, the spot weld joint has a centrally symmetrical structure.

[0015] In a second aspect of the invention, a pair of spot welding electrode caps are provided for welding to obtain the above-mentioned spot welded joint, wherein the welding surfaces of the spot welding electrode caps have shapes adapted to the outer surfaces of the first workpiece and the second workpiece, respectively.

[0016] A third aspect of the present invention provides a method for welding the aforementioned welded joint, the method comprising:

[0017] (1) A pair of welding electrode caps are provided, wherein the welding surfaces of the spot welding electrode caps have shapes adapted to the outer surfaces of the first workpiece and the second workpiece, respectively;

[0018] (2) Pre-pressing stage, in which the welding electrode cap applies electrode pressure to the first workpiece and the second workpiece;

[0019] (3) Welding stage, wherein one or more stages of welding current are applied to the first workpiece and the second workpiece to form a weld nugget; and

[0020] (4) Condensation stage: In the condensation stage, the weld nugget is condensed to form the final spot weld joint.

[0021] In another preferred embodiment, the welding stage includes a preheating stage, a main welding stage, and a post-heating stage; the effective value of the welding current I1 in the preheating stage is 10-30KA, and the energizing time t1 is 30-60ms; the effective value of the welding current I2 in the main welding stage is 20-60KA, and the total energizing time t2 is 50-300ms; the welding current I3 in the post-heating stage is 15-40KA, and the duration t3 is 20-100ms.

[0022] In another preferred embodiment, the main welding stage forms a weld nugget through multiple identical or different current pulses, 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 3, and the pulse interval cooling time being 1-10 ms.

[0023] It should be understood that, within the scope of this invention, the above-described technical features of this invention and the technical features specifically described below (such as in the embodiments) can be combined with each other to form new or preferred technical solutions. Due to space limitations, they will not be described in detail here. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 The figure shown is a cross-sectional schematic diagram of a typical spot welded joint involved in this invention;

[0026] Figure 2 The image shown is a partial schematic diagram of the surface indentation of the spot welded joint in this invention;

[0027] Figure 3 The image shown is a partial schematic diagram of the surface indentation of another spot weld joint in this invention;

[0028] Figure 4 The image shown is a partial schematic diagram of the surface indentation of another spot weld joint in this invention;

[0029] Figure 5 The image shown is a partial schematic diagram of the surface indentation of another spot weld joint in this invention;

[0030] Figure 6 The image shown is a partial schematic diagram of the surface indentation of another spot weld joint in this invention;

[0031] Figure 7 The image shown is a partial schematic diagram of the surface indentation of another spot weld joint in this invention;

[0032] Figure 8 The image shown is a partial schematic diagram of the second surface in the indentation on the surface of the spot welded joint in this invention;

[0033] Figure 9 The image shown is a partial schematic diagram of the second surface in the surface indentation of another spot welded joint in this invention;

[0034] Figure 10 The diagram shows a schematic of the cross-sectional shape of the annular ridge on the surface of the spot weld joint.

[0035] Figure 11 The diagram shows another cross-sectional shape of the annular ridge on the surface of the spot welded joint;

[0036] Figure 12 The diagram shows another cross-sectional shape of the annular ridge on the surface of the spot welded joint.

[0037] Figure 13 The image shown is a partial schematic diagram of the surface indentation of another spot weld joint in this invention;

[0038] Figure 14 The image shown is a partial schematic diagram of the surface indentation of another spot weld joint in this invention;

[0039] Figure 15 The figure shown is a cross-sectional schematic diagram of another spot welded joint involved in this invention;

[0040] Figure 16 The figure shown is a cross-sectional schematic diagram of another spot welded joint involved in this invention;

[0041] Figure 17 The figure shown is a cross-sectional schematic diagram of another spot welded joint involved in this invention;

[0042] Figure 18 The figure shown is a cross-sectional schematic diagram of another spot welded joint involved in this invention;

[0043] Figure 19 The figure shown is a cross-sectional schematic diagram of another spot welded joint involved in this invention;

[0044] Figure 20 The figure shown is a cross-sectional schematic diagram of another spot welded joint involved in this invention;

[0045] Figure 21 The figure shown is a cross-sectional schematic diagram of another spot welded joint involved in this invention;

[0046] Figure 22 The figure shown is a partial cross-sectional schematic diagram of a welding electrode involved in this invention;

[0047] Figure 23 The figure shown is a timing diagram of spot welding current and pressure involved in this invention;

[0048] Figure 24 The figure shown is another timing diagram of spot welding current and pressure involved in this invention.

[0049] Figure 25 The image shown is a welding image of the surface of a conventional spot weld joint in the prior art;

[0050] Figure 26 The image shown is a welding image of the surface of the spot welded joint in this invention;

[0051] Figure 27 The figure shown is a cross-sectional view of a common spot weld joint in the prior art;

[0052] Figure 28 The figure shown is a cross-sectional view of the spot welded joint in this invention;

[0053] Figure 29 The figure shown is a cross-sectional view of another spot welded joint in this invention;

[0054] Figure 30 The figure shown is a cross-sectional view of another spot welded joint in this invention;

[0055] Figure 31 The figure shown is a cross-sectional view of another spot welded joint in this invention;

[0056] Figure 32 The figure shown is a cross-sectional view of another spot welded joint in this invention;

[0057] Figure 33 The figure shown is a cross-sectional view of another spot welded joint in this invention.

[0058] The labels in each of the attached figures are as follows:

[0059] 1-First workpiece;

[0060] 2-Second workpiece;

[0061] 10 - First workpiece surface;

[0062] 11 - First indentation surface;

[0063] 12-Combined interface;

[0064] 111 - First surface;

[0065] 112 - Second surface;

[0066] 3-Melting zone (weld nugget);

[0067] 31-Melting nucleus boundary;

[0068] 20 - Second workpiece surface;

[0069] 1111-Ring Ridge;

[0070] 1122-Base surface;

[0071] 1123 - Inner surface;

[0072] 1121 - Outer surface;

[0073] 21 - Second indentation surface;

[0074] 4-Welding electrode;

[0075] 41 - Center surface of the welding electrode;

[0076] 42 - Outer surface of the welding electrode. Detailed Implementation

[0077] Through extensive and in-depth research and screening, the inventors have developed a spot welded joint and its manufacturing method for the first time. The spot welded joint of the present invention forms a disc-shaped weld nugget that is thin in the middle and thick at the periphery by an indentation comprising a first surface that protrudes outward from the center and a second surface that is recessed inward from the outer periphery of the first surface. This results in better surface quality and welding performance, and higher joint strength. In addition, the spot welded joint of the present invention can form a weld nugget by multi-segment pulse segmented welding and inter-segment cooling, which makes the formed weld nugget more stable and requires less welding energy. The present invention was completed on this basis.

[0078] This invention provides a spot welding joint, wherein the joint has a centrally symmetrical structure. When viewed along the cross-section of the joint, the joint includes a molten zone and an unmolten zone. The weld joint has a bonding interface and opposing upper and lower surfaces. The outer surface of the workpiece in the weld area has an indentation that is generally far away from the molten zone of the workpiece.

[0079] At least one of the indentations comprises at least a central first surface and a second surface that gradually extends outwards; and the maximum distance h1 from the central first surface of the indentation to the outer surface of the workpiece is lower than the maximum distance h2 from the outer second surface of the central indentation to the workpiece surface.

[0080] In a preferred embodiment, the joint is obtained by a spot welding method, which sequentially includes: providing a pair of welding electrode caps, wherein at least one electrode cap surface has a shape similar to the joint indentation; and then providing a resistance spot welding process comprising the following steps:

[0081] Pre-compression stage: Electrode pressure is applied to the workpiece during this stage;

[0082] Welding stage: In this stage, one or more stages of welding current are applied to the workpiece to form a molten core;

[0083] Condensation stage: This stage allows the molten core to condense and form the final spot weld joint.

[0084] In a preferred embodiment, the welding electrode cap has a central recess, wherein the maximum depth of the recess is 1%-20% of its outer circumferential diameter, preferably 2%-10%.

[0085] In a preferred embodiment, the circumference diameter of the first surface edge of the indentation center does not exceed 8 mm, and preferably does not exceed 7 mm;

[0086] In a preferred embodiment, the diameter of the circumference of the outer side of the indentation does not exceed 14 mm, and is preferably 8-13 mm;

[0087] In a preferred embodiment, the first surface of the indentation center is generally arc-shaped, and the distance h1 between the top of the arc surface and the workpiece surface (here referring to the base surface in the workpiece surface) is within -0.3 to +0.3 mm, preferably -0.15 to +0.15 mm, wherein the negative sign "-" indicates that the top is inside (below) the base surface, and "+" indicates that the top is outside (above) the base surface;

[0088] In a preferred embodiment, the first surface of the indentation center has a plurality of discontinuously distributed annular ridge structures with protrusions or depressions; the height of the annular ridges is 15-300 μm; preferably 30-250 μm;

[0089] In a preferred embodiment, the second surface on the outer side of the indentation has a plurality of raised or recessed annular ridge features, the height of which is 15-300 μm; preferably 30-250 μm.

[0090] In a preferred embodiment, the second surface on the outer side of the indentation is composed of a combination of multiple continuous planes and arc surfaces;

[0091] A spot weld joint is also provided, the joint comprising, when viewed along its cross-section:

[0092] The central melting zone and the non-melting zone; the boundary of the melting zone is the boundary of the molten core;

[0093] The projection length of the melting zone on the bonding interface is d, the maximum projection height of the melt nugget boundary in the workpiece thickness direction within 40% of the center of the melting zone is h5, and the maximum projection height of the melt nugget boundary in the workpiece thickness direction within 60% of the outer side of the melting zone is h6, where h6≥h5.

[0094] In a preferred embodiment, the thickness of the thinnest workpiece in the welded joint is w, where 0.5 ≤ w ≤ 4.0 mm; wherein the following condition is satisfied:

[0095] In a preferred embodiment, h6 ≥ 0.3w and h5 ≥ 0.2w in the welded joint;

[0096] A spot welding joint is provided, wherein the joint has a centrally symmetrical structure. When viewed along the cross-section of the joint, the joint includes a molten zone and an unmolten zone; the weld point has a bonding interface and upper and lower surfaces of the workpiece to be welded; the workpiece surface in the weld point area has an indentation that is generally far away from the workpiece surface; the boundary between the molten zone and the unmolten zone is a weld nugget boundary.

[0097] The indentation on at least one side is formed by at least a central first surface and a second surface that gradually extends outwards; and the maximum distance between the central first surface and the workpiece surface is lower than the maximum distance between the edge of the indentation and the second surface and the workpiece surface.

[0098] The projection length of the melting zone on the bonding interface is d, the maximum projection height of the melt nugget boundary in the workpiece thickness direction within 40% of the center of the melting zone is h5, and the maximum projection height of the melt nugget boundary in the workpiece thickness direction within 60% of the outer side of the melting zone is h6, where h6≥h5.

[0099] In a preferred embodiment, the joint is obtained by a spot welding method, which sequentially includes: providing a pair of welding electrode caps, wherein at least one electrode end face has a shape similar to the joint indentation; and then providing a resistance spot welding process comprising the following steps:

[0100] Pre-compression stage: Electrode pressure is applied to the workpiece during this stage;

[0101] Welding stage: In this stage, one or more stages of welding current are applied to the workpiece to form a molten core;

[0102] Condensation stage: This stage allows the molten core to condense and form the final spot weld joint.

[0103] In a preferred embodiment, the circumference diameter of the first surface edge of the indentation center does not exceed 8 mm;

[0104] In a preferred embodiment, the thickness of the thinnest workpiece in the welded joint is w, where 0.5 ≤ w ≤ 4.0 mm; wherein the following condition is satisfied:

[0105] In a preferred embodiment, h6 ≥ 0.3w and h5 ≥ 0.2w in the welded joint.

[0106] The main advantages of this invention include:

[0107] (a) The provided spot welds have a shallower central indentation than the outer indentation in the weld surface indentation, which helps to reduce crack-level weld penetration on the central surface of the weld, improve weld strength and surface quality, and enhance aesthetics.

[0108] (b) The thickness of the weld nugget at the center does not exceed the thickness at the edge, which helps to improve the load-bearing capacity of the weld edge, reduce the generation of internal cracks in the center, and thus improve the weld strength;

[0109] (c) The multi-stage pulsed current welding method can effectively avoid overheating of the weld surface, accelerate heat dissipation, reduce weld surface adhesion, improve surface quality and electrode life, and at the same time avoid defects such as spatter caused by excessive internal heating speed, reduce energy dissipation and save energy.

[0110] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, the accompanying drawings are schematic diagrams, and therefore the apparatus and device of the present invention are not limited to the size or scale of the schematic diagrams.

[0111] It should be noted that in the claims and specification of this patent, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one" does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0112] Example

[0113] Now for reference Figure 1 , Figure 1 The diagram shown is a cross-sectional schematic of a spot welded joint according to the present invention; the joint is obtained by resistance spot welding of a first workpiece 1 and a second workpiece 2; the joint includes a weld nugget 3 formed by a molten zone, and the boundary of the weld nugget is 31; the joint surface has a formed spot weld indentation; the joint has a first workpiece surface 10, a second workpiece surface 20, and a bonding interface 12; the indentation surface is composed of a central first surface 111 and a second surface 112 away from the center; and the highest point of the first surface 111 of the indentation is at a distance from the base surface of the first workpiece surface 10 (the base surface is the base surface of the first workpiece surface 10). The base surface (referring to the surface on the first workpiece surface 10 excluding the indentation) has a height of h1, and the height of the lowest point of the second surface 112 from the base surface of surface 10 is h2, where h1 ≤ h2. That is, facing the melting center, the first surface 111 located at the center of the indentation is shallower and more outward than the second surface 112 located outside the indentation. Here, a value far from the melting center is defined as positive, and a value close to the melting center is negative. Therefore, h1 is generally between -0.3 and 0.3 mm, preferably between -0.2 and 0.2 mm. The diameter of the outer circumference of the first surface 111 is d1, which is generally 2-6 mm, preferably 3-5 mm; the diameter of the outer circumference of the second surface 112 is 6-16 mm, preferably 8-14 mm.

[0114] like Figure 2 The image shown is a partial enlarged view of the indentation surface 11; the first surface 111 located at the center of the indentation can have various shape features; Figure 2The first surface 111 shown is a surface formed by rotating a circular arc; Figure 3 The first surface 111 shown is composed of an arc surface with a certain radius of curvature. Generally, the radius of curvature R of this arc surface is not less than 5 mm, preferably 5 ≤ R ≤ 100 mm; and the positions of the first surface 111 and the second surface 112 in the indentation relative to the workpiece surface 10 can be of various types; for example... Figure 2 As shown, along the melting center direction, the first indentation surface 111 is higher than the workpiece surface 10, while Figure 3 As shown, the first surface 111 is lower than the workpiece surface 10, that is, h1 is located on the outer or inner side of the workpiece surface, which is possible in this invention.

[0115] It is worth noting that the first surface 111 of the indentation center can have various shape characteristics; for example, its cross-sectional shape can be composed of any combination of multiple straight lines or curves. Figure 4 As shown, the first surface 111 is a surface of revolution composed of multiple smoothly transitioning curved sections; the first surface is a flat end face, as... Figure 5 As shown, the first surface 111 is generally a surface with a continuous and smooth transition to ensure that the material undergoes smooth and stable deformation during welding without causing defects such as cracks. In particular, the first surface at the center of the indentation may also have multiple protruding or recessed annular ridge structures, such as... Figure 6 The diagram shows a structure with two protruding annular ridges 1111 on the first surface, and the height h3 of the annular ridges 1111 is generally 15-300μm.

[0116] The second surface 112 of the indentation surface can also include a variety of structural shapes. Figures 1 to 5 The cross-sectional shapes of the second surface shown are all composed of a combination of circular arcs and straight lines. In fact, the second surface 112 can also contain multiple annular ridge structures with protrusions or depressions. For example... Figure 7 The diagram shows a second surface 112 with two concave annular ridges (or simply "annular recesses"). The number of annular ridges is 1-5, preferably 2-4. The distance between the concave annular ridge and its base surface is h4. The base surface containing the annular ridge is either planar or curved. Figure 8The image shown is a partially enlarged view of the 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 relative to the central weld nugget 3; the one closer to the weld nugget 3 is called the inner surface, and the one farther from the weld nugget 3 is called the outer surface). The inner surface 1123 is composed of a combination of protrusions and depressions formed by multiple straight lines and curves. The base surface 1122 on which the inner surface 1123 is located can be a plane and / or a curved surface. When the base surface 1122 is a plane, the height of the maximum depression of the annular ridge relative to the base surface 1122 is h4, which is generally 15-300 μm; preferably 30-280 μm.

[0117] The center-to-center distance B between the annular ridges is generally 300μm≤B≤2000μm, and preferably 400μm≤B≤1500μm.

[0118] The diameter of the outer circumference of the base surface 1122 is d3, generally 7-13 mm, preferably 8-12 mm. The outer surface 1121 can be a conical surface or an arc surface. When the outer surface 1121 is a conical surface, the angle formed between it and the workpiece surface 10 is generally α, and α does not exceed 40°, preferably 5°≤α≤30°. Figure 8 As shown. The outer surface 1121 can also be an entire curved surface. When it is curved, its radius r1 is not less than 20 mm, preferably 25 ≤ r1 ≤ 100 mm. The base surface 1122 can also be a curved surface, with its radius r2 not less than 25 mm, preferably 30 ≤ r2 ≤ 100 mm. In a specific embodiment, the outer surface 1121 and the base surface 1122 are the same surface, i.e., both are curved surfaces, such as... Figure 9 As shown.

[0119] The annular ridges on both the first surface 111 and the second surface 112 can have various structural types. An annular ridge is a structure that can be formed by rotating any combination of straight lines and curves. For example... Figure 10 As shown, the annular ridge has a trapezoidal structure; its bottom surface width is B1, and its top surface width is B2. Generally, 200μm≤B1≤1000μm, 0≤B2≤500μm, preferably 300μm≤B1≤600μm, 0≤B2≤300μm. Specifically, when B2 is zero, it is triangular, as shown... Figure 11 As shown. It is worth noting that the cross-sectional shape of the annular ridge can be composed of any straight lines and curves, for example, as... Figure 12 As shown, the whole is composed of curves and forms a shape similar to a triangle. Various transition rounded corners can be included between any straight lines and curves, and any other shape is also possible, which will not be elaborated here.

[0120] The shape of the indentation surface of the joint can be formed by any combination of the first and second surfaces mentioned above. For example... Figure 13 As shown, the first surface 111 is an arc surface, and the second surface 112 is configured with the outer side of the arc surface combined with the planar base surface, and has two integrally inverted triangular annular recesses on the base surface. Figure 14 The diagram shows a raised annular ridge on the second surface.

[0121] Now for reference Figure 15 The joint described in this invention, viewed along its cross-section, comprises a central molten zone 3 (i.e., the "weld nugget") and an outer non-molten zone; the junction of these two zones is referred to as the weld nugget boundary 31. The projected length of the molten zone 3 on the interface between workpiece 1 and workpiece 2 is d. The maximum projected height of the weld nugget boundary in the workpiece thickness direction within 60% of the central area of ​​the molten zone is h5, and the maximum projected height of the weld nugget boundary in the workpiece thickness direction within 40% of the outer area of ​​the molten zone is h6, where h6 ≥ h5. The thickness of workpiece 1 in the welded joint is w, 0.5 ≤ w ≤ 4.0 mm; where... Preferably Simultaneously, h6 ≥ 0.3w and h5 ≥ 0.2w. Melting zone 3 has a peanut-shell-like shape, and its maximum melt depth is not located at the very center, but rather within a 60%d region near the edge. This weld nugget morphology is beneficial for achieving better melt depth at the weld nugget edge, resulting in better load-bearing capacity and fracture energy absorption.

[0122] The morphology of the melt nucleus within the 60%d and 40%d range of the melting zone can include various types; such as Figure 16 As shown, the shape of the melting zone 3 is basically close to an elliptical shape, but in fact, the height h5 within the range of 0.6d is still less than or equal to h6.

[0123] The shape of the molten zone and the indentation on the joint surface can be combined in various ways. For example... Figure 17 As shown, the first surface 111 located at the center of the indentation is an arc surface, the second surface 112 is a surface with an annular depression having a trapezoidal cross-section, and the melting zone 3 is a combination of weld nuggets with a peanut shell shape. Figure 18 As shown, the first surface 111 at the center of the indentation is an arc surface, the second surface 112 is a surface with a triangular cross-section and a concave ring ridge, and the melting zone 3 is still a weld nugget assembly with a peanut shell shape. The peanut shell shape described in this article refers to the cross-sectional shape of the weld nugget as observed from the center section of the welded workpiece, and its overall shape should be a disc shape that is thin in the middle and thick at the periphery.

[0124] The shortest distance s1 (in mm) from the thicker area around the weld nugget to the second surface 112 and the longest distance s2 (in mm) from the thinner area in the middle to the first surface 111 of the workpiece satisfy the following: s2≤0.9w; s1≥s2, preferably s1≥1.2*s2, where w is the workpiece thickness.

[0125] The shape of the melting zone 3 is not necessarily symmetrical. Asymmetrical structures are often formed when welding workpieces of unequal thickness or dissimilar materials. For example... Figure 19 The diagram shows a scenario where the two workpieces have different thicknesses. The first workpiece has a thickness of w1, and the second workpiece has a thickness of w2, generally w1 < w2. In this case, the melting height within the inner melting zone of the first workpiece side is at least h5 within a range of 0.4d, and the melting height within the range of 0.6d is h6, satisfying the condition that h6 > h5.

[0126] Similarly, the surfaces of the joint indentation are not necessarily identical on both the upper and lower surfaces. For example... Figure 20 The first and second workpiece surfaces are not entirely identical. The first workpiece surface has a centrally protruding arc-shaped first surface 111 and an outer surface with a ring-shaped ridge distribution of recessed structures; while the second workpiece surface is a single arc surface with a radius of curvature r3. Generally, the outer circumferential diameter d5 of the second indentation surface is 6-14 mm, preferably 8-12 mm, while r3 is not less than 20 mm, preferably 25-200 mm. The first indentation surface 11 and the second indentation surface 21 can also have other different combinations. Figure 21 The first indentation surface 11 has a raised arc surface at its center and a second surface with two annular recesses on its outer side. The second indentation surface 21 has a first surface with an integral arc structure at its center and a second surface with three annular recesses on its outer side. Therefore, the first surface of the first indentation and the second surface of the second indentation can be composed of various shapes and sizes, including different recess / ridge structures, dimensions, planes, or curved surfaces.

[0127] In this invention, the first workpiece 1 and the second workpiece 2 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-zirconium alloys. Their form can be deformed or cast; and their material state can include various tempering processes, including annealing, strain strengthening, and other heat treatment states. Additionally, an inorganic or organic oil film (e.g., a lubricating oil film) can be formed on the surface of the coating. The joint can be formed by resistance spot welding of two or more workpieces, and the thickness of each workpiece can be the same or different. It should be noted that the term "workpiece" as used herein broadly includes metal sheets, protrusions, castings, and other workpieces that can be resistance spot welded.

[0128] An uncured but heat-curable adhesive may also be included between workpieces 1 and 2. The thickness of this adhesive is typically 0.1-5 mm, preferably 0.2-2 mm. The thickness of this adhesive layer in the weld area may vary under the electrode pressure F during the welding process. Generally, the adhesive is applied to the contact surfaces of the first metal workpiece 1 and the second metal workpiece 2. After resistance spot welding, the stacked assembly is cured in an oven or other heating device to achieve a strong bond. Thermocurable adhesives are generally thermocurable epoxy resins, which is readily understood in this field.

[0129] The joint is obtained by a spot welding method, which sequentially includes: providing a pair of welding electrode caps, at least one of which has a shape similar to the joint indentation; followed by a resistance spot welding process:

[0130] Pre-compression stage: Electrode pressure is applied to the workpiece during this stage;

[0131] Welding stage: In this stage, one or more stages of welding current are applied to the workpiece to form a molten core;

[0132] Condensation stage: This stage allows the molten core to condense and form the final spot weld joint.

[0133] The shape of the welding electrode cap is similar to the shape of the indentation on the joint surface; for example, when the indentation on the joint surface is a symmetrical, centrally convex arc surface with concave ring ridges at the edges, then the electrode cap 4 has a structure 42 with a concave surface 41 at the center and a convex ring ridge at the edges; wherein the height of the central concavity is h7 (relative to the height of the surface corresponding to the base surface of the joint), and the diameter of the outer circumference of the concavity is d7. Generally, h7 is 0.05-1.0 mm, preferably 0.1-0.5 mm; d7 is 2-8 mm, preferably 3-7 mm; and h7 / d7 is 1-20%, preferably 2-15%. Figure 22 The cross-sectional shape of electrode cap 4 is shown. Similarly, when the indentation surface of the joint is of other shapes or the indentations on both sides of the joint have asymmetrical surface structures, the corresponding welding electrode cap also has a shape similar to its shape. Its welding surface can be a spherical surface, an end plane, or other special shaped surfaces, such as electrode caps with protruding or recessed end faces, which will not be elaborated here. In general, the welding electrode can be made of any conductive and thermally conductive material, such as copper alloys, including copper-chromium (CuCr) alloys, copper-chromium-zirconium (CuCrZr) alloys, copper alloys with added alumina particles, or other various copper alloys that can be used as electrode materials.

[0134] When performing resistance spot welding, such as Figure 23The diagram shows a timing sequence of a spot welding process involved in this invention, including a preheating stage, a main welding stage, and a post-heating stage. Generally, the welding process can involve one or more welding currents, especially in the main welding stage. In the preheating stage, the effective value of the welding current I1 is generally 8-30 kA, preferably 10-20 kA, and more preferably 12-15 kA. The energizing time t1 in this stage is 30-60 ms. The preheating stage allows the workpiece to form sufficient contact and reduces the contact resistance between the welding electrode and the workpiece, thereby improving electrode life and weld stability. In the main welding stage, a stable weld nugget is formed. The effective value of the current I2 is generally 20-60 kA, and the total energizing time t2 is 50-300 ms. Preferably, the effective value of the current I2 is 25-50 kA, and the total energizing time t2 is 60-200 ms. When the main welding stage involves forming a sufficiently large weld nugget through multiple identical or different current pulses, such as... Figure 24 As shown, in this process, the welding current I21, I22...I2n is generally 25-50KA, and the duration t2n of a single welding pulse is generally 5-30ms, preferably 6-20ms, where n is an integer greater than or equal to 1, preferably n≥3, and the pulse interval tc is 1-10ms. The number of pulses n satisfies... w1 is in mm; the pulse interval time tc satisfies tc≥w1; preferably tc≥1.5*w1, and the number of pulses n≥3. This process continuously heats and cools the electrode while maintaining a sufficient temperature for the internal weld nugget to form a larger weld nugget, reducing adhesion on the weld surface. The pulse interval cooling time is 1-10ms; the total main welding time is 50-150ms, preferably 60-130ms. The current amplitude of each pulse can be the same or different to ensure sufficient weld nugget size while reducing heat generation between the electrode and the workpiece, thus extending electrode life.

[0135] Resistance heat is generated between the workpieces, forming a weld nugget 3, which, under the action of electrode pressure, forms an indentation on the weld joint surface. The relatively convex first surface at the center prevents internal heat from concentrating in the center and causing welding defects such as hot cracks. The thicker unmelted zone in the center of the weld nugget also prevents internal burn-through and other defects such as hot cracks, avoiding defects such as electrode adhesion caused by heat concentration in the center of the weld joint. Generally, in existing technologies, the central area generates more heat and dissipates it slowly, which easily causes the weld nugget to not grow fully, resulting in spatter, internal cracks, and adhesion. However, the weld joint of this invention has a more uniform internal and external temperature and stress distribution, making it easier to form a uniform weld nugget. The edge of the internal molten zone has a thicker melting depth, which helps to improve the joint strength, make the stress distribution within the entire weld nugget more uniform, reduce residual stress, and improve weld performance. When there is an annular ridge on the outside of the indentation, and the aluminum oxide film at the annular ridge is peeled off, the contact resistance is reduced during welding contact, and the contact area is increased, enhancing heat dissipation. This reduces the heat between the electrode welding surface and the workpiece contact surface, thereby improving the service life of the electrode and the surface quality of the weld joint.

[0136] 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 not less than 15 ms, generally 15-40 ms; this ensures that the weld nugget solidifies 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 less shrinkage stress during the cooling stage, thus reducing the generation of internal shrinkage cavities and cracks. Meanwhile, throughout the welding stage, the electrode pressure F is generally 2000-10000 N, preferably 3000-8000 N, which can vary during the welding stage and is not necessarily constant. The pressurization method can be pneumatic or servo-driven, which is readily understood in the art.

[0137] Now for reference Figure 25 , Figure 25 The image shows the surface morphology of a traditional spot-welded joint obtained using 5-series aluminum alloy. It can be seen that the surface indentation is generally spherical, and there is a large amount of copper-aluminum alloying structure adhering to the surface. This severely affects electrode life, increases manufacturing costs, and results in a poor weld appearance. In contrast, Figure 26 The image shown is a picture of the surface indentation of the solder joint as shown in this invention. The obtained surface indentation of the solder joint is uniform and beautiful and there are no alloying reaction layer deposits. It can greatly improve manufacturing efficiency, reduce costs, and improve surface quality.

[0138] Figure 27The image shows a cross-sectional view of the weld nugget in traditional spot welding of aluminum workpieces. Cracks and internal defects are easily generated in the center of the molten zone, which can easily lead to electrode adhesion due to weld penetration in the center of the molten zone. This results in unstable welding quality, reduced electrode life, and high manufacturing costs. Figure 28 The diagram shows a typical joint cross-sectional shape formed by spot welding of 6-series aluminum alloys according to the present invention. The indentation surface is composed of a central first surface 111 and an outer second surface 112, forming a centrally rotating structure. The highest point of the first surface 111 is at a height h1 above the workpiece surface 10, and the lowest point of the second surface 112 is at a height h2 above the workpiece surface 10. The second surface 112 is generally lower than the workpiece surface 10, while the first surface 111 is higher than the workpiece surface 10. The weld nugget 3 in the molten zone has a diameter d, a molten height of h5 in the innermost region, and a molten height of h6 at the outer edge, where h6 > h5. This weld nugget shape can avoid surface defects such as cracks caused by overheating at the center and improve the joint strength.

[0139] In another embodiment, for 1.2mm thick 6-series aluminum alloys, the weld nugget size can reach 6.55mm without surface burn-through, cracks, or other defects. Figure 29 As shown, the first surface 111 and the second surface 112 of the indentation on the surface of the weld joint are both lower than the workpiece surface 10. The second surface 112 of the indentation has two concave triangular ring-shaped recessed structures; however, its h2 is still greater than h1.

[0140] For example Figure 30 The cross-sectional morphology of a 2mm 7-series aluminum alloy spot weld joint is shown; the first surface 111 of the indentation center is generally higher than the second surface 112 of the indentation, wherein the second surface 112 has two concave trapezoidal ring structures, which helps to reduce the formation of cracks on the first surface 111. Figure 31 The figure shows the cross-sectional morphology of the weld joint obtained by resistance spot welding of two layers of AZ31 magnesium alloy. The indentation is composed of the first surface 111 in the center and the second surface 112 on the outside. The height of the center part of the weld nugget 3 formed by the molten zone is h5, while the maximum height of the outer edge is h6, where h6 > h5.

[0141] For workpieces with unequal thickness, such as Figure 32The diagram shows a welded joint obtained by resistance spot welding of dissimilar materials of unequal thickness, 4.0mm cast aluminum alloy and 2.3mm aluminum alloy, according to the present invention. The indentations on both sides of the joint can be different. The indentation on the cast aluminum side has a central first surface 111 and an outer edge second surface 112, wherein the second surface 112 has a three-ring annular recessed structure. On the 5-series aluminum alloy side, the indentation 21 is a spherical recess. This combination helps to form a sufficiently large weld nugget size while also creating a sufficiently deep molten zone on the thinner plate side without causing defects such as weld burn-through or cracks on the thicker plate side. This is highly adaptable to applications where special requirements exist for surface protrusions or recesses.

[0142] Of course, the indentations on both sides of the solder joint can also be symmetrical, such as... Figure 33 The image shows the morphology of the joint cross-section obtained by spot welding cast aluminum and 5-series aluminum alloy with unequal thicknesses. Both sides of the weld point include a first surface 111 with a central protrusion and a second surface 112 with three annular recesses on the outer side. The maximum height of the molten zone center is h6 located at the edge, and the minimum height is h5 located at the center.

[0143] All documents mentioned in this invention are incorporated herein by reference as if each document were individually incorporated by reference. Furthermore, it should be understood that after reading the foregoing teachings of this invention, those skilled in the art can make various alterations or modifications to this invention, and these equivalent forms also fall within the scope defined by the appended claims.

Claims

1. A spot welding joint, characterized in that, The spot weld joint includes a first workpiece, a second workpiece, and a weld nugget for fixing the first workpiece and the second workpiece together; at least one of the outer surfaces of the first workpiece and the second workpiece includes a base surface and an indentation. The indentation includes a first surface that protrudes outward from the center and a second surface that is recessed inward from the outer periphery of the first surface. The maximum distance h1 between the first surface and the base surface is less than the maximum distance h2 between the second surface and the base surface. The longest distance s1 from the thinner central region of the weld nugget to the first surface of the workpiece and the shortest distance s2 from the thicker peripheral region to the second surface satisfy the following: s2≤0.9w; s1≥1.2*s2, where w is the workpiece thickness.

2. The spot welded joint as described in claim 1, characterized in that, The first surface and / or the second surface have a plurality of discontinuously distributed protruding or recessed annular ridge structures; the height of the annular ridges is 15-300 μm.

3. The spot welded joint as described in claim 1, characterized in that, The weld nugget is a disc-shaped material that is thin in the middle and thick at the edges.

4. The spot welded joint as described in claim 1, characterized in that, The first surface and / or the second surface are composed of a combination of multiple continuous planes and arc surfaces.

5. The spot welded joint as described in claim 1, characterized in that, The spot weld joint has a centrally symmetrical structure.

6. A pair of spot welding electrode caps, characterized in that, The spot welding electrode cap is used to weld a spot welded joint as described in any one of claims 1-5, wherein the welding surface of the spot welding electrode cap has a shape adapted to the outer surface of the first workpiece and the outer surface of the second workpiece, respectively.

7. A method for welding a spot weld joint as described in any one of claims 1-5, characterized in that, The method includes: (1) A pair of spot welding electrode caps are provided, wherein the welding surfaces of the spot welding electrode caps have shapes adapted to the outer surfaces of the first workpiece and the second workpiece, respectively; (2) Pre-pressing stage, in which the spot welding electrode cap applies electrode pressure to the first workpiece and the second workpiece; (3) Welding stage, wherein one or more stages of welding current are applied to the first workpiece and the second workpiece to form a weld nugget; and (4) Condensation stage: In the condensation stage, the weld nugget is condensed to form the final spot weld joint.

8. The method as described in claim 7, characterized in that, The welding stage includes a preheating stage, a main welding stage, and a post-heating stage; The effective value of the welding current I1 during the preheating stage is 10-30KA, and the energizing time t1 is 30-60ms. The effective value of the welding current I2 in the main welding stage is 20-60KA, and the total energizing time t2 is 50-300ms. The welding current I3 in the post-heating stage is 15-40KA, and the duration t3 is 20-100ms.

9. The method as described in claim 8, characterized in that, The main welding stage forms a weld nugget through multiple identical or different current pulses. Each welding current is 25-50 kA, the duration of a single welding pulse is 5-30 ms, the pulse interval is 1-10 ms, the number of pulses is at least 3, and the pulse interval cooling time is 1-10 ms.

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