Electrode cap for resistance spot welding of aluminum workpieces and spot welding method

Abstract

The application discloses an electrode cap for resistance spot welding of aluminum workpieces and a spot welding method. The electrode cap comprises a first surface located in the center of the electrode cap and recessed towards the same side of the electrode cap around a central axis, a second surface around the outside of the first surface, and 2-5 annular ridges with convex or concave around the central axis on the second surface. The central axis of the cross section of each annular ridge is perpendicular to the second surface. The highest points of the annular ridges are distributed on a spherical surface, and the center of the spherical surface and the center of the second surface are one center. The electrode cap can effectively weld aluminum alloy workpieces, make the current and pressure directional flow during the welding process, make the pressure borne by the annular ridges more uniform, control the nugget formation process, avoid the generation of defects such as cracks and shrinkage holes, improve the mechanical properties of the welding spot, and make the welding structure be able to bear a higher stress load.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of resistance spot welding, more particularly to an electrode cap for resistance spot welding of aluminum workpieces and a spot welding method. BACKGROUND

[0002] Aluminum alloy resistance spot welding is an important manufacturing process in the current industry. Due to the high specific strength, aluminum alloy is a material that is more commonly used in the automobile, aerospace and other industries, but due to its large solidification shrinkage and the presence of alloying elements such as Mg, Si, Cu and Zn in the alloy, low-melting eutectic is easily produced inside the nugget during spot welding, and the solidification shrinkage is serious during the cooling process of spot welding, the tensile stress is large, and cracks and shrinkage holes are easily formed inside the nugget. The presence of defects greatly reduces the mechanical properties of the weld, causing the welded structure to permanently fail under a relatively low stress load, thereby hindering the application of aluminum alloy resistance spot welding. At the same time, aluminum alloy resistance spot welding is prone to spatter and cannot form a large enough nugget, resulting in reduced joint performance. Moreover, the copper electrode cap is prone to electrode sticking to the surface of the aluminum workpiece during aluminum alloy resistance spot welding, resulting in unstable heat generation and unstable weld performance during continuous welding, reduced electrode service life, high frequency of grinding and dressing, and increased manufacturing cost and reduced efficiency.

[0003] Therefore, there is still a lack of an electrode cap for effectively welding aluminum alloy workpieces and a spot welding method for the same to avoid cracks, shrinkage holes and other defects, improve the mechanical properties of the weld, enable the welded structure to withstand a relatively high stress load, and at the same time improve the electrode service life, manufacturing efficiency and reduce cost. SUMMARY

[0004] The present application relates to the field of resistance spot welding, more particularly to an electrode cap for resistance spot welding of aluminum workpieces and a spot welding method.

[0005] In a first aspect of the present application, an electrode cap for resistance spot welding of aluminum workpieces is provided, the electrode cap comprising: a first surface located at the center of the electrode cap and recessed towards the inside of the electrode cap around a central axis; a second surface around the outside of the first surface; and 2-5 annular ridges with a convex shape around the central axis on the second surface; wherein the central axis of the cross section of each annular ridge is perpendicular to the second surface, so that the electrode pressure during resistance spot welding can make the pressure on the annular ridge more uniform in the direction of the central axis of the annular ridge; the highest points of the annular ridges are distributed on a spherical surface, and the center of the spherical surface and the center of the second surface are a center.

[0006] In another preferred embodiment, the number of the annular ridges is 3-4.

[0007] In another preferred embodiment, the depth h of the first surface is 0.01-0.5mm, and h is not more than 0.2T, wherein T is the thickness of the aluminum workpiece.

[0008] In another preferred embodiment, in the case where the thickness of the aluminum workpiece is not uniform, the T is the thickness of the thinner one of the aluminum workpieces.

[0009] In another preferred embodiment, the depth h of the first surface is 0.02-0.3mm.

[0010] In another preferred embodiment, the first surface is a smooth curved surface, and the cross-sectional shape of the first surface on a vertical plane is a cambered surface.

[0011] In another preferred embodiment, the first surface and the second surface are connected by a circular arc transition, and the radius r of the circular arc is not less than 0.1mm.

[0012] In another preferred embodiment, the radius r of the circular arc is 0.6-3mm.

[0013] In another preferred embodiment, the outermost peripheral diameter d0 of the first surface is 2-6mm.

[0014] In another preferred embodiment, the outermost peripheral diameter of the first surface is wherein T is the thickness of the aluminum workpiece.

[0015] In another preferred embodiment, the electrode cap further comprises a third surface, which is located between the first surface and the second surface, is an annular surface, and the third surface is not provided with the annular ridges.

[0016] In another preferred embodiment, the third surface is a plane, and the width L of the third surface is not less than 0.2mm, so as to ensure that the initial contact surface is large enough when the electrode is energized for spot welding pre-pressing, so as to avoid too small electrode / material contact area and too large current density, which results in high heat production and electrode sticking.

[0017] In another preferred embodiment, the width L of the third surface is 0.2≤L≤0.5mm.

[0018] In another preferred embodiment, the second surface is a curved surface with a radius of curvature R between 20-100mm, and satisfies 15T≤R≤50T, wherein T is the thickness of the aluminum workpiece.

[0019] In another preferred embodiment, with the center of the electrode cap as the center, the diameter is A point p on the intersection of the cylindrical side surface of the electrode cap and the second surface (a circle centered on the center of the electrode cap) is at a height h2 from the third surface, satisfying h2≤0.15T, where T is the thickness of the aluminum workpiece.

[0020] In another preferred embodiment, the outer diameter d1 of the surface circumference where the second surface is located is 8-15 mm; and the height difference h1 of the outer diameter of the surface circumference where the second surface is located from the third surface is no more than 0.2T, where T is the thickness of the aluminum workpiece.

[0021] In another preferred embodiment, the height difference h1 of the outer diameter of the surface circumference where the second surface is located from the third surface is 10%T≤h1≤30%T, where T is the thickness of the aluminum workpiece.

[0022] In another preferred embodiment, the fourth surface located outside the second surface and adjacent to the second surface is a conical surface, and the included angle between the conical surface and a plane perpendicular to the central axis of the electrode cap (also referred to as the "base surface") is α, where 20°≤α≤70°.

[0023] In another preferred embodiment, 30°≤α≤60°.

[0024] In another preferred embodiment, the fourth surface located outside the second surface and adjacent to the second surface is an arc surface, and the radius of curvature of the arc surface is r2≥10 mm.

[0025] In another preferred embodiment, the height H of the annular ridge is 20-300 μm, and the width B of the bottom of the annular ridge is 0.1-0.5 mm; the width-height ratio B / H satisfies 0.5≤B / H≤10.

[0026] In another preferred embodiment, the cross-sectional shape of the annular ridge can be a triangle, a trapezoid, a semicircle, a parabolic shape, or any shape composed of straight lines and curves.

[0027] In another preferred embodiment, the distance between the central axes of adjacent annular ridges is 200-1500 μm.

[0028] In another preferred embodiment, the height of the innermost annular ridge is 50-200 μm, preferably 100-200 μm.

[0029] In another preferred embodiment, when the electrode cap performs the welding operation, the annular ridge on the second surface first contacts the aluminum workpiece to be welded, the electrode pressure and current pass through the annular ridge and initially form a melting core, as the electrode pressure and welding current flow, the third surface, the first surface of the recess and the annular ridge on the outside gradually contact the aluminum workpiece, the melting core expands inward and outward at the same time, but due to the outside being pressed by the electrode pressure and the inside having the electrode recess, the workpiece is compressed and deformed inward, so that the central electrode in the recess contacts the aluminum workpiece first, the current passes through, melts the central base material of the workpiece, and the melting core expands inward, forming a melting core from the outside to the inside, while avoiding too much concentration of energy at the contact position to cause heat concentration and serious electrode sticking.

[0030] In a second aspect of the application, a resistance spot welding method is provided, the method comprising:

[0031] (a) providing a pair of the above-mentioned electrode caps;

[0032] (b) pre-pressing stage: placing a pair of the electrode caps on both sides of the stacked aluminum workpieces respectively, performing pre-pressing, applying an electrode pressure of 2000-8000 N for a duration of 200-1500 ms, so that the innermost annular ridge of the electrode cap contacts the aluminum workpiece;

[0033] (c) welding stage: conducting one or more welding currents, the effective value of the welding current being 22 KA-65 KA, as time progresses, the central contact position of the aluminum workpiece corresponding to the inner annular ridge melts first, forming a melting center, the outer aluminum workpiece gradually deforms and expands inward and outward, and the third surface, the first surface, the outer annular ridge and the outer second surface of the electrode cap successively contact the electrode cap, and the melting center simultaneously expands inward and outward to form a complete aluminum melting core;

[0034] (d) condensation stage: stopping the welding current, maintaining the state of the electrode cap and the aluminum workpiece for 30-300 ms, and the molten metal solidifies to form a complete welding spot.

[0035] In another preferred embodiment, the welding current is in the form of multiple pulses, and the welding stage includes at least 3 or more welding current pulses, each welding current pulse being separated by a cooling interval;

[0036] wherein the action time t n of each welding current pulse is not more than 20 ms, the cooling interval time t n-n+1 is not more than 5 ms, and

[0037] In another preferred example, the action time t of the welding current pulse is not more than 15 ms n not more than 15 ms, the time t of the cooling interval n-n+1 is 2-5 ms.

[0038] In another preferred example, the number of the welding current pulses is not less than 4.

[0039] In another preferred example, the amplitude of each of the welding current pulses can be different, for example, in a gradually increasing trend.

[0040] The electrode cap of the present application can be applied to high-quality resistance spot welding of different aluminum workpieces by setting specific topography structure and size. The main mechanism is to use the raised annular ridge and the central recessed surface on the surface of the electrode cap to deliver the electrode pressure and the welding current according to the predetermined path, to control the welding nugget melting formation process and the cooling and solidification process; to enable directional nucleation expansion during melting to reduce the generation of spatter and surface adhesion, and to facilitate the reduction of internal shrinkage and crack formation during solidification by distributing the electrode pressure according to the electrode surface topography. And using multiple short-time main welding pulses during welding can reduce the temperature between the electrode and the aluminum plate surface without reducing the internal temperature, thereby reducing the generation of surface adhesion; using preheating pulses to establish stable electrical contact, using tempering current to reduce the solidification shrinkage stress of the welding spot and reduce the formation of solidification cracks, and improve the welding quality.

[0041] It should be understood that, within the scope of the present application, each of the above technical features of the present application and the technical features specifically described in the following (such as the examples) can be combined with each other to form a new or preferred technical solution. Due to the limited space, they will not be listed one by one here. BRIEF DESCRIPTION OF DRAWINGS

[0042] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or prior art description will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without creative labor.

[0043] Figure 1 is a sectional view of an electrode cap for resistance spot welding of aluminum workpieces in an example of the present application;

[0044] Figure 2 is Figure 1 a partial view of the first surface of the electrode cap in

[0045] Figure 3 is a schematic view of the circumferential diameter size of a plurality of annular ridges in an example of the present application;

[0046] Figure 4 is a cross-sectional view of the annular ridge involved in the present invention;

[0047] Figure 5 is a schematic diagram of the current versus time during a resistance spot welding process in one example of the present invention;

[0048] Figure 6A is a schematic diagram of the current versus time during a resistance spot welding process in another example of the present invention;

[0049] Figure 6B is a schematic diagram of the current versus time during a resistance spot welding process in yet another example of the present invention;

[0050] Figure 7 is a schematic diagram of the current versus time during a resistance spot welding process in still another example of the present invention;

[0051] Figure 8 is a schematic diagram of the actual current output during a resistance spot welding process in one example of the present invention;

[0052] Figure 9 is a schematic diagram of the process of welding aluminum alloy workpieces with the electrode cap in one example of the present invention;

[0053] Figure 10 is a cross-sectional view of a weld spot resulting from welding 0.8 mm 6-series aluminum alloy and 1.0 mm 6-series aluminum alloy with the electrode cap shown in Figure 1 ;

[0054] Figure 11 is a cross-sectional view of a weld spot resulting from welding 3.0 mm cast aluminum alloy with the electrode cap shown in Figure 1 ;

[0055] In each of the drawings, the following are indicated:

[0056] 1 - electrode cap;

[0057] 12 - first surface;

[0058] 11 - second surface;

[0059] 111 - annular ridge;

[0060] 10 - third surface;

[0061] 13 - circular arc transition between the first surface and the second surface;

[0062] 14 - outer surface;

[0063] 21 - first welding electrode cap;

[0064] 22 - second welding electrode cap;

[0065] 3 - first aluminum alloy workpiece;

[0066] 4 - second aluminum alloy workpiece;

[0067] 5 - welding current;

[0068] 6 - initial nugget;

[0069] 7 - final nugget;

[0070] d1 - diameter of the surface of the outermost circumference of the second surface; d0 - diameter of the surface of the outermost circumference of the first surface; R - radius of curvature of the circular arc of the second surface; h1 - height difference between the outermost circumference of the second surface and the third surface; h2 - height of the cylindrical side surface with a circumferential diameter from the intersection line with the second surface to the third surface; L - width of the third surface; and a - included angle between the fourth surface and the base surface. DETAILED DESCRIPTION

[0071] The inventors have developed, for the first time, an electrode cap and a spot welding method for resistance spot welding of aluminum workpieces, which can effectively avoid defects such as cracks and shrinkage holes, improve the mechanical properties of the welding points, and enable the welded structure to withstand a higher load, on the basis of which the present application is completed.

[0072] TERMS

[0073] As used herein, the terms "second surface" and "second base surface" and the like are used interchangeably.

[0074] As used herein, the terms "aluminum workpiece", "workpiece", and "aluminum alloy workpiece" and the like are used interchangeably.

[0075] As used herein, the terms "electrode cap" and "welding electrode" and the like are used interchangeably.

[0076] The present application provides an electrode cap for resistance spot welding of aluminum workpieces, which is an electrode cap with a specific structure.

[0077] Typically, the electrode cap 1 of the present application includes a first surface 12 recessed at the center of the electrode cap and facing the same side of the electrode cap around the central axis, and a second base surface 11 surrounding the outside of the first surface, and having 2-5 annular ridges 111 around the central axis and convex or recessed on the base surface; the central axis of the cross section of the single annular ridge is perpendicular to the base surface, so that the electrode pressure can make the annular ridge bear more uniform pressure in the direction of the central axis of the annular ridge when performing resistance spot welding, and the highest points of the annular ridges are distributed on a spherical surface, the center of which is the same as the center of the base surface.

[0078] Reference will now be made to Figure 2 ​The first surface 12 has a concave recess with a depth h of 0.01-0.5 mm (preferably 0.02-0.3 mm) and a cross-sectional shape of an arc surface, and h is not more than 0.2T, which is transitioned to the base surface with a circular arc r, and the transition round angle r is not less than 0.1 mm, preferably 0.5-5 mm.

[0079] The first surface 12 has an outer diameter d0 of 2-6 mm; preferably

[0080] The third surface 10 is a plane, and the width L is not less than 0.2 mm; to ensure that the initial contact surface is large enough during spot welding pre-pressing, so as not to make the electrode / material contact area too small and the current density too large, resulting in high heat production and electrode sticking; preferably 0.2≤L≤0.5 mm, preferably 0.25≤L≤0.4 mm.

[0081] The second base surface 11 is a curved surface with a radius of curvature R of 20-100, and satisfies 20T≤R≤50T.

[0082] The electrode is revolved around the center, and the circumferential surface with a diameter The intersection point of the circumferential surface and the second base surface is located at p, and the height of the third surface (also referred to as "top surface") 10 is h2, which satisfies h2≤0.15T.

[0083] The outer diameter d1 of the circumferential surface of the second base surface is 8-15 mm; and the height difference h1 is not more than 0.3T; preferably 10%T≤h≤30%T.

[0084] The outer surface 14 of the second base surface is a conical surface, and the included angle with the electrode end plane is α, which is generally 20°≤α≤70°, preferably 30°≤α≤60°.

[0085] The height H of each of the annular ridges is 20-300 μm, and the width B of the bottom of the annular ridges is 0.1-0.5 mm; the width-height ratio B / H satisfies 0.5≤B / H≤20; the number of annular ridges is 2-5; preferably 3-4.

[0086] The cross-sectional shape of the annular ridges can be triangular, trapezoidal, semicircular, parabolic, etc. composed of any straight line and curve; which can be the same or different.

[0087] The height of the innermost annular ridge is 50-200 μm.

[0088] The spacing between adjacent annular ridges is 100-500 μm.

[0089] The second base surface 10 and the annular ridge are first in contact with the workpiece when welding is performed, and a molten core is first formed. As the electrode pressure and welding current flow, the recessed first surface 12, the outer annular ridge and the second base surface 11 are all in contact with the workpiece, and the molten core expands both inward and outward. However, because the outer side is pressed by the electrode and the inner side has the electrode recess, the workpiece is compressed and deformed inward, the inner electrode first contacts the workpiece, the current flows, the center base material melts, and the molten core expands inward, forming a molten core from the outside to the inside, and avoiding too much energy concentration at the contact position to cause serious electrode sticking.

[0090] The present application also provides a resistance spot welding method. Typically, the resistance spot welding method utilizes a pair of the above-mentioned electrode caps, and one resistance spot welding process performed by the electrode caps includes the following steps:

[0091] The welding electrodes are respectively placed on both sides of the stacked aluminum alloy workpieces, pre-pressing is performed, the electrode pressure is 3000-6000 N, and the time is 200-1000 ms, so that the innermost annular ridge and the second base surface 11 of the electrode cap are in contact with the aluminum workpiece;

[0092] One or more welding currents are passed, and as time progresses, the annular ridge corresponds to the inner aluminum workpiece melting the outer aluminum workpiece gradually deforming and expanding inward to contact the third surface 10 and the outer annular ridge and the second base surface 11 of the welding electrode, and the welding core expands in this direction to form a complete aluminum molten core;

[0093] The welding current is stopped for a period of time, and the molten metal solidifies to form a complete welding spot.

[0094] The welding current pulse includes multiple welding current pulses, and the process includes at least 3 or more welding current pulses; each welding current pulse has a cooling time interval; and generally, the welding current effective value is 25 KA-50 KA.

[0095] wherein the current action time t n of each welding pulse is not more than 20 ms, the cooling interval time t n-n+1 is not more than 5 ms, and and satisfies

[0096] When the welding current pulse time t n is not more than 15 ms, the number of welding pulses is not less than 4; the cooling time interval t n-n+1 is 2-5 ms;

[0097] The current amplitude of each welding pulse can be different and gradually increasing.

[0098] The main advantages of the present application include:

[0099] (a) effectively weld aluminum alloy workpieces;

[0100] (b) avoid the generation of defects such as cracks, shrinkage, etc;

[0101] (c) improve the mechanical properties of the weld, so that the welded structure can withstand higher stress loads;

[0102] (d) the surface of the weld is free of defects such as adhesion and cracks, and has good surface quality;

[0103] (e) use shorter welding heat input to achieve the final weld nugget, saving welding energy.

[0104] The present application will be further described in conjunction with specific examples. It should be understood that these examples are only used to illustrate the present application and not to limit the scope of the present application. In addition, the drawings are schematic drawings, so the dimensions and proportions of the devices and equipment of the present application are not limited by the schematic drawings.

[0105] It should be noted that in the claims and specification of the present patent, relationship terms such as first and second are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between the entities or operations. Moreover, the terms "include", "contain" or any other variant thereof are intended to cover non-exclusive inclusion, so that the process, method, article or equipment including a series of elements not only includes those elements, but also includes other elements not explicitly listed or inherent to such process, method, article or equipment. Without further limitation, the element defined by the statement "including one" does not exclude the presence of another identical element in the process, method, article or equipment including the element.

[0106] Examples

[0107] The electrode cap for resistance spot welding of aluminum workpieces of the present example is as follows Figure 1The electrode cap 1 comprises a first surface 12 in the center of the electrode cap and recessed towards the same side of the electrode cap around the central axis, and a second surface 11 around the first surface, and 2-5 annular ridges 111 are convex or concave around the central axis on the second surface, the central axis of the cross section of each annular ridge is perpendicular to the base surface, so that the electrode pressure can be more uniform in the direction of the central axis of the annular ridge during resistance spot welding, the highest points of the annular ridges are distributed on a spherical surface, and the center of the spherical surface is the same as the center of the base surface; the innermost annular ridge is in contact with the aluminum workpiece first during the pre-pressing stage of resistance spot welding, and then the welding current is turned on to make the 1-2 annular ridges outside the first annular ridge contact the workpiece surface, and the aluminum workpiece is deformed outward to make the first surface 12 gradually contact the aluminum workpiece, the current passes through the first surface 12, and a complete nugget is formed inside the aluminum workpiece; at least the third and fourth annular ridges outside the first annular ridge also gradually contact the aluminum workpiece and the current passes through during the current passing through the first surface 12, so that the current passing sequence is in the order of outside-in-out, the current density is more uniformly distributed and controllable during welding, the adhesion between the electrode and the aluminum workpiece is reduced, and a large enough nugget is formed.

[0108] As Figure 2 The cross-sectional view of the first surface of the center of the welding electrode is shown in the figure. The first surface 12 has a maximum depth h of 0.01-0.3mm, the cross-sectional shape of the recess is arc surface, and the transition between the arc surface and the base surface is circular arc, which is called the circular arc transition surface 13 between the first surface and the second surface, and the radius r is not less than 0.1mm, preferably 0.6-3mm.

[0109] The outermost diameter d0 of the first surface is 2-6mm, and satisfies Where T is the thickness of the aluminum workpiece. The electrode cap further comprises a third surface 10 between the first surface and the second surface, which is an annular surface, and the third surface is not provided with the annular ridge. In another preferred example, the third surface is a plane, and the width B of the third surface is not less than 0.2mm, so as to ensure that the initial contact surface is large enough during spot welding pre-pressing, so as to avoid too small electrode / material contact area and too large current density, resulting in high heat production and electrode adhesion. In another preferred example, the width L of the third surface is 0.2≤L≤0.5mm.

[0110] The second surface is a curved surface with a radius of curvature R of 20-100mm, and satisfies 15T≤R≤50T, where T is the thickness of the aluminum workpiece. In another preferred example, the center of the electrode cap is taken as the center, and the diameter is The distance between a point p on the intersection of the cylindrical side surface of the electrode cap and the second surface (a circle with the center of the electrode cap as the center) and the third surface is h2, which satisfies h2≤0.15T, where T is the thickness of the aluminum workpiece. The outer diameter d1 of the surface circumference where the second surface is located is 8-15 mm; and the height difference h1 between the outer diameter of the surface circumference where the second surface is located and the third surface is not more than 0.3T, where T is the thickness of the aluminum workpiece. In another preferred embodiment, the height difference h1 between the outer diameter of the surface circumference where the second surface is located and the third surface is 10%T≤h1≤30%T, where T is the thickness of the aluminum workpiece.

[0111] In another preferred embodiment, the fourth surface located outside the second surface and adjacent to the second surface is a conical surface, as shown in Figure 1 , the included angle between the conical surface and the plane perpendicular to the central axis of the electrode cap is α, where 20°≤α≤70°. In another preferred embodiment, 30°≤α≤60°. In another preferred embodiment, the fourth surface located outside the second surface and adjacent to the second surface is an arc surface, as shown in Figure 3 , the radius of curvature of the arc surface is r2≥10 mm.

[0112] Referring to Figure 3 and Figure 4 , the second surface 11 is a plane or a curved surface with a radius of curvature R not less than 20 mm, generally R≤100 mm. There are a plurality of annular ridges 111 on the second surface 11, the annular ridge bottom width is 0.1-0.5 mm, preferably 0.2-0.4 mm, and the width-height ratio B / H satisfies: 0.5≤B / H≤10. The height of each annular ridge is 20-300 μm, preferably 50-250 μm, and the shape size of each annular ridge can be the same or different; wherein the height of the first annular ridge on the innermost side is 50-200 μm, preferably 100-200 μm, which helps to ensure that it is the first to contact the workpiece and gradually transitions to the subsequent annular ridges in a fixed manner during the execution of resistance spot welding, so that the electrode pressure and welding current distribution are more controllable, the heat generation efficiency is improved, and the electrode sticking is reduced. As shown in Figure 4 , the cross-sectional shape structure of the annular ridge in the present application; the cross-sectional shape of the annular ridge can be any shape composed of triangles, trapezoids, semicircles, and other straight lines and curves.

[0113] The distance between the central axes of each annular ridge is 200-1500 μm, preferably 400-1000 μm. The diameter of the surface where the center of the first annular ridge bottom is located is d2, the diameter of the surface where the center of the second annular ridge bottom is located is d3, the diameter of the surface where the center of the third annular ridge bottom is located is d4, and the diameter of the surface where the center of the fourth annular ridge bottom is located is d5, wherein:

[0114] 3≤d2≤5mm, 4≤d3≤6mm, 6≤d4≤8mm, 7≤d5≤9mm.

[0115] In another preferred embodiment, the annular ridges further comprise a fifth annular ridge, wherein the surface on which the center of the bottom of the fifth annular ridge is located has a diameter d6, and d6 satisfies 8≤d6≤10mm.

[0116] The power-on sequence of the electrode cap in the welding process is in the order of the first annular ridge, the second annular ridge, the first surface (i.e., the central concave surface) 12, the third annular ridge, the fourth annular ridge, and optionally the nth annular ridge. This welding sequence ensures that the nucleation mode is in the outer-inner-outer mode, so that the nugget size is controllable; and different target sizes of the nugget can be formed according to different plate thicknesses and the formation of spatter is reduced.

[0117] The present application also provides a resistance spot welding method, comprising the following steps:

[0118] (a) providing any of the electrode caps described above;

[0119] (b) pre-pressing stage: placing a pair of the electrode caps described above on both sides of the stacked aluminum workpieces, performing pre-pressing, applying an electrode pressure of 2000-8000N, preferably 3000-7500N, for a duration of 200-1500ms, preferably 300-1000ms, so that the innermost annular ridges of the electrode caps are in contact with the aluminum workpieces;

[0120] (c) power-on welding stage: conducting one or more welding currents, the effective value of the welding current being 22KA-65KA, and as time progresses, the center contact position of the aluminum workpiece corresponding to the inner annular ridges melts first to form a melting center, and the outer aluminum workpiece gradually deforms and expands inward, and the third surface, the first surface, the outer annular ridges, and the outer second surface of the electrode cap successively come into contact with the electrode cap, and the melting center simultaneously expands inward and outward to form a complete aluminum nugget;

[0121] (d) condensation stage: stopping the welding current, maintaining the state of the electrode cap and the aluminum workpiece for 300-300ms, and the molten metal solidifies to form a complete welding spot;

[0122] wherein the welding current is in the form of multiple pulses, the power-on welding stage comprises at least 3 or more main welding current pulses, and each main welding current pulse has a cooling interval; the action time t n of each welding current pulse is 0.5-2ms, n is the total number of welding pulses, and satisfies preferably T is in mm, and tn is in ms; each main welding current pulse In is greater than 25KA;

[0123] wherein the duration of each main welding current pulse t n is not more than 20 ms, the duration of the cooling interval t n-n+1 is 3 ms≤t n ≤5 ms. When the duration of the welding current pulse t n-n+1 is not more than 15 ms, the duration of the cooling interval t n is 2-3 ms. When the duration of the welding current pulse t n-n+1 is not more than 10 ms, the duration of the cooling interval t n+1 is 1-2 ms. The number of the welding current pulses n is not less than 4. The amplitude of each of the welding current pulses can be the same or different. For example, as shown in Figure 5 , they are the same; for example, as shown in Figure 6A and 6B , they are gradually increasing, and I n ≥2 KA, I n+1 is the amplitude of the (n+1)th current pulse.

[0124] It is worth noting that the main welding current can further include a preheating current pulse before it, the amplitude of the preheating current pulse I0 is 10-20 KA, and the duration is 20-60 ms; the cooling interval between the preheating pulse and the main welding current pulse is t 0-1 is 0-5 ms, as shown in Figure 6A ; preferably 0 ms, as shown in Figure 6B .

[0125] In addition, at least one tempering current pulse is included after the main welding current pulse, as shown in Figure 7 ; the amplitude of the tempering current pulse I h is 25-35 KA, and the duration is 40-100 ms; in particular, it is not more than the average value of all the amplitudes of the preceding main welding pulses I w , more narrowly, it is not more than 0.8 I w . The cooling interval between the tempering current pulse and the last main welding current pulse is 5-50 ms, the application time point of the tempering current is after the inside of the weld nugget begins to solidify and before the complete solidification ends, generally, when the plate thickness T is not more than 1.0 mm, the interval is 5-10 ms; when the plate thickness is more than 1.0 mm, it is 10-30 ms; in this way, the shrinkage stress during the solidification of the weld nugget is reduced, and the generation of internal heat cracks and other defects is reduced.

[0126] It is also worth noting that the amplitudes of all the current pulses described above are the effective values of the current in a single phase. Since the actual current rising process has a ramping and fluctuation phase, and considering the time for current ramping and dropping, the actual output cannot guarantee to be completely zero. For example, as shown in Figure 8 All the figures are schematic diagrams with constant average values, and the meanings are understood by those skilled in the art.

[0127] The present embodiment discloses a process for welding aluminum alloy workpieces using the electrode cap of the present application, as shown in Figure 9 The first and second aluminum alloy workpieces 3 and 4 are made of aluminum alloys such as aluminum-magnesium alloy, aluminum-silicon alloy, aluminum-magnesium-silicon alloy, or aluminum-copper alloy, and the thickness of the aluminum alloy workpieces is 0.5-4.0 mm, preferably 0.8-3.5 mm. The aluminum workpieces can be various aluminum workpieces formed by stamping, extrusion, or casting. The aluminum alloy workpieces can be two (for example, only 3 and 4) or more than two in combination, and the thickness of each aluminum alloy workpiece can be the same or different. It should be noted that the term “workpiece” used herein refers to a wide range of metal sheet layers, protruding parts, castings, and other aluminum alloy parts that can be resistance spot welded.

[0128] At the beginning of the resistance spot welding, the welding current 5 passes through the edge first contact annular ridge part, the second base surface 10 and the annular ridge 111 first contact the workpiece, and the initial nugget 6 is formed at the beginning. With the electrode pressure and the flow of the welding current 5, the concave first surface 12, the outer annular ridge, and the second base surface 11 are in contact with the workpiece at the same time, and the initial nugget 6 expands inward and outward at the same time. However, due to the outer side being pressed by the electrode and the inner side having the electrode concave, the workpiece is compressed and deformed inward, the inner electrode first contacts the workpiece, the current passes through, the center base material is melted, the nugget expands inward, and an outward-to-inward nugget is formed, which avoids too much concentration of energy at the contact position and prevents serious electrode sticking caused by heat concentration. Finally, a complete final nugget 7 is formed.

[0129] The time for the first annular ridge to contact the workpiece and fully penetrate the workpiece is no more than 30 ms (calculated from the start of power-on), and the time for the top of the first surface 12 to contact the workpiece is no more than 300 ms (calculated from the start of power-on). In particular, when the thickness of the workpiece is less than 1.0 mm, the time for the first surface 12 to contact the workpiece is no more than 150 ms (calculated from the start of power-on).

[0130] Figure 10The figure shows the cross section of the welding spot for 0.8mm 6 series aluminum alloy and 1.0mm unequal thickness 6 series aluminum alloy welding, the electrode center has a recess (first surface 12) with a depth of 0.1mm, the outer diameter of the second base surface of the electrode cap is 6mm, and the curvature radius of the second base surface is 40mm; the second base surface has two rings of annular ridges with trapezoidal cross section, the annular ridge height is 0.15mm, and the width is 0.2mm. The welding process used is: electrode pressure 3500N, preheating 10KA, 50ms, the main welding current pulse is composed of 6 segments, each segment is 32KA, the current-on time is 10ms, the cooling interval is 2ms, the tempering pulse current is 25KA, and the tempering time is 50ms; according to the cross section shape, the welding nugget diameter reaches 7.18mm without spatter, and the electrode of the application helps to expand the welding nugget diameter and improve the joint strength.

[0131] Figure 11 The figure shows the cross section of the welding spot of the resistance spot welding of 3.0mm cast aluminum alloy; the welding electrode used has a recess (first surface 12) with a depth of 0.15mm at the center, the outer diameter of the second base surface of the electrode cap is 6mm, and the curvature radius of the second base surface is 40mm; the second base surface has two rings of annular ridges with triangular cross section, the annular ridge height is 0.2mm, and the width is 0.3mm; the welding process is: electrode pressure 4500N, preheating 15KA, 50ms, each main welding current pulse is 34KA, the welding time is 20-30ms, the pulse number is 8-10, the cooling interval is 2-5ms, the tempering current is 25KA, and the time is 80ms; the pressure holding time is 100ms. According to the cross section shape, the welding nugget diameter reaches 9.81mm without spatter, the welding nugget diameter is expanded, which is conducive to improving the joint strength.

[0132] All the documents mentioned in the present application are incorporated by reference in the present application as if each document was individually incorporated by reference. In addition, it should be understood that various changes and modifications can be made to the present application by those skilled in the art after reading the above description of the present application, and these equivalent forms also fall within the scope defined by the appended claims of the present application.

Claims

1. An electrode cap for resistance spot welding of aluminum workpieces, characterized in that, The electrode cap includes: A first surface located at the center of the electrode cap and facing the recessed area inside the electrode cap around the central axis; The second surface surrounding the outer side of the first surface; and The second surface has 2-5 annular ridges that radiate around the central axis. Wherein, the central axis of the cross section of each of the annular ridges is perpendicular to the second surface, so that the electrode pressure during resistance spot welding can be more uniform along the central axis of the annular ridge. The highest point of the annular ridge is distributed on a spherical surface, the center of the spherical surface and the center of the second surface are the same center, and the height of the innermost annular ridge is 50-200μm; The electrode cap further includes a third surface, which is located between the first surface and the second surface. The third surface is an annular surface and does not have the annular ridge. The third surface is a plane and the width L of the third surface is not less than 0.2 mm.

2. The electrode cap as described in claim 1, characterized in that, The outermost circumferential diameter d0 of the first surface is 2-6 mm.

3. The electrode cap as described in claim 1, characterized in that, The first surface is a smooth curved surface, and the cross-sectional shape of the first surface in the vertical plane is an arc surface.

4. The electrode cap as described in claim 1, characterized in that, The second surface is a curved surface with a radius of curvature R between 20-100 mm, and satisfies 15T≤R≤50T, where T is the thickness of the aluminum workpiece.

5. The electrode cap as described in claim 1, characterized in that, The outer diameter d1 of the circumference of the second surface is 8-15mm; and the height difference h1 between the outer diameter of the second surface and the third surface does not exceed 0.3T, and satisfies 0.1T≤h1≤0.3T, where T is the thickness of the aluminum workpiece.

6. The electrode cap as described in claim 5, characterized in that, When the thickness of the aluminum workpieces is inconsistent, T refers to the thickness of the thinner aluminum workpiece.

7. A resistance spot welding method, characterized in that, The method includes: (a) Provide a pair of electrode caps as described in any one of claims 1-6; (b) Pre-compression stage: Place a pair of electrode caps on both sides of the stacked aluminum workpieces and perform pre-compression. The applied electrode pressure is 2000-8000N and the duration is 200-1500ms, so that the innermost annular ridge of the electrode cap contacts the aluminum workpiece. (c) Welding stage: One or more welding currents are turned on, the effective value of which is 22KA-65KA. As time goes on, the aluminum workpiece center contact position corresponding to the innermost annular ridge melts first to form a melting center. The workpieces corresponding to the third surface, the first surface, the outer annular ridge and the outer second surface of the electrode cap come into contact with the electrode cap in turn. The melting center expands inward and outward at the same time to form a complete aluminum melt nugget. (d) Condensation stage: Stop the welding current and maintain the state of the electrode cap and the aluminum workpiece for 30-300ms. The molten metal solidifies to form a complete weld point.

Images