Electrode cap for adhesive spot welding of aluminum workpieces and welding method thereof

Abstract

The application discloses an electrode cap for adhesive spot welding of aluminum workpieces and a welding method thereof, comprising: a first surface located in the center of the electrode cap and protruding outward from the center axis of the electrode cap; a second surface surrounding the outside of the first surface; and a plurality of annular ridges provided on the second surface and protruding or recessed around the center axis, the number of the annular ridges being 2-5; wherein the center axis of the cross section of a single annular ridge is perpendicular to the second surface; the height of the first surface is not less than 0.02 mm higher than the height of the annular ridge; a point p on the intersection of a cylindrical side curve with a diameter of T centered on the center of the electrode cap and the second surface is a distance h0 from the highest point of the first surface, and h0 satisfies h0<=0.15T, wherein T is the thickness of the aluminum workpiece. The application can effectively prevent welding defects caused by residual adhesive, and due to the presence of the surface annular ridges, the temperature between the electrode and the workpiece can be reduced, heat dissipation can be accelerated, electrode adhesion can be reduced, and the service life of the electrode can be improved.

Description

TECHNICAL FIELD

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

[0002] Aluminum alloy resistance spot welding is an important manufacturing process method in current industry. In actual production, in order to improve the strength of the welded joint and improve the sealing, a layer of structural adhesive is often needed to be added; when resistance spot welding is performed, a certain pressure needs to be applied in advance to remove the adhesive layer in the intermediate interface, and then the welding current is connected to form a resistance spot welding spot; and in actual production, due to the incomplete removal of the center adhesive layer, the high-resistivity adhesive layer remains, which will cause welding through after the welding current is connected, and even the gun explosion phenomenon, which seriously affects the welding quality and production efficiency; however, if a spherical electrode with a small radius of curvature is used, a large amount of heat will be concentrated in the center of the welding spot during the welding process, which will cause serious electrode sticking and surface cracks, resulting in unstable heat generation and unstable welding spot performance during continuous welding, reduced electrode service life, high frequency of grinding, and increased manufacturing cost and reduced efficiency.

[0003] Therefore, there is an urgent need in the art for a method and a welding electrode cap that can ensure high welding quality during adhesive bonding spot welding. SUMMARY

[0004] The purpose of the present application is to provide an electrode cap for adhesive bonding spot welding of aluminum workpieces, which can effectively prevent welding defects caused by adhesive layer residues, and at the same time, due to the presence of the surface annular ridge, the temperature between the electrode and the workpiece can be reduced, heat dissipation can be accelerated, electrode sticking can be reduced, and electrode life can be improved, and at the same time, due to the presence of the outer annular ridge, the temperature of the heat affected zone around the nugget is reduced, which helps to reduce the generation of liquefied cracks.

[0005] In a first aspect of the present application, an electrode cap for adhesive bonding spot welding of aluminum workpieces is provided, which comprises: a first surface in the center of the electrode cap and protruding outward from the center axis of the electrode cap in a whole dome shape; a second surface surrounding the outside of the first surface; and a plurality of annular ridges provided on the second surface and protruding or recessed around the center axis, the number of annular ridges being 2-5; wherein the center axis of the cross section of a single annular ridge is perpendicular to the second surface; the height of the first surface is not less than 0.02mm higher than the height of the annular ridge; and the distance from a point p on the intersection of a cylindrical side surface with a diameter of T centered on the center of the electrode cap and the second surface (which is a circle centered on the center of the electrode cap) to the highest point of the first surface is h0, which satisfies h0≤0.15T, wherein T is the thickness of the aluminum workpiece. In a first aspect of the present application, an electrode cap for adhesive bonding spot welding of aluminum workpieces is provided, which comprises: a first surface in the center of the electrode cap and protruding outward from the center axis of the electrode cap in a whole dome shape; a second surface surrounding the outside of the first surface; and a plurality of annular ridges provided on the second surface and protruding or recessed around the center axis, the number of annular ridges being 2-5; wherein the center axis of the cross section of a single annular ridge is perpendicular to the second surface; the height of the first surface is not less than 0.02mm higher than the height of the annular ridge; and the distance from a point p on the intersection of a cylindrical side surface with a diameter of T centered on the center of the electrode cap and the second surface (which is a circle centered on the center of the electrode cap) to the highest point of the first surface is h0, which satisfies h0≤0.15T, wherein T is the thickness of the aluminum workpiece.

[0006] In another preferred embodiment, the outer diameter d1 of the first surface is 3-8 mm, and Where T is the thickness of the aluminum workpiece.

[0007] In another preferred embodiment, the height h1 of the protrusion on the first surface is 0.02-0.6 mm; preferably, 0.02-0.5 mm; more preferably, 0.05-0.4 mm.

[0008] In another preferred embodiment, the cross-sectional shape of the protrusion is an arc surface.

[0009] In another preferred embodiment, the first surface is an arc surface with a radius of curvature R of 15-100 mm; preferably, it is 20-80 mm; and Where T is the thickness of the aluminum workpiece.

[0010] In another preferred embodiment, the second surface is a plane, or a curved surface with a radius of curvature r between 20 and 100 mm;

[0011] In another preferred embodiment, the outer diameter d2 of the second surface is 8-15 mm, preferably 10-12 mm.

[0012] In another preferred embodiment, the height of each of the annular ridges is 20-300 μm; preferably, 50-250 μm; more preferably, 100-200 μm; and the bottom width of the annular ridges is 0.1-0.5 mm.

[0013] In another preferred embodiment, the height of the innermost annular ridge is 50-200 μm.

[0014] In another preferred embodiment, the height of the outer diameter of the second surface from the highest point of the first surface is h2, wherein h2 does not exceed 0.3 times the thickness of the plate, and preferably does not exceed 0.2 times the thickness of the plate.

[0015] In another preferred embodiment, the vertices of the plurality of annular ridges lie on a sphere, the center of which is the same as the center of the sphere containing the second surface.

[0016] In another preferred embodiment, the annular ridge protrudes downwards along the axial direction of the electrode cap.

[0017] A second aspect of the present invention provides a resistance spot welding method, the method comprising:

[0018] (a) Provide a pair of the above-described electrode caps;

[0019] (b) pre-pressing stage: a pair of said electrode caps are placed on both sides of the stacked aluminum workpieces, pre-pressing is performed, the electrode pressure applied is 2000-8000N, the duration is 200-1500ms, the most central dome surface of the electrode cap is first in contact with the aluminum workpieces; a layer of glue for gluing is contained between the aluminum workpieces;

[0020] (c) current welding stage: one or more welding currents are conducted through the electrode cap, the effective value of the welding current is 22KA-65KA, as time goes on, the aluminum workpieces gradually expand outward in contact with the first surface contact area and gradually contact the annular ridge on the second surface from inside to outside; the current first passes through the electrode center and gradually expands outward to the annular ridge position, as the welding time goes on, the aluminum workpiece center contact position melts to form a welding pool;

[0021] (d) condensation stage: stop the welding current, maintain the state of the electrode cap and the aluminum workpiece for 30-300ms, and the molten metal solidifies to form a complete welding spot.

[0022] In another preferred embodiment, the welding current is in the form of multiple pulses, and the current welding stage includes at least 3 or more welding current pulses, and each welding current pulse has a cooling interval therebetween;

[0023] wherein the action time t n of each welding current pulse is not more than 20ms, the cooling interval time t n-n+1 is not more than 5ms, and

[0024] In another preferred embodiment, the action time t n of each welding current pulse is not more than 15ms, and the cooling interval time t n-n+1 is 2-5ms.

[0025] In another preferred embodiment, the number of welding current pulses is not less than 4.

[0026] In another preferred embodiment, the amplitude of each welding current pulse can be different, for example, gradually increasing.

[0027] The main advantages of the present application include:

[0028] The present application can ensure the smooth outward discharge of the glue layer in the initial stage by arranging the highest contact surface in the center position, and can establish more stable contact with the aluminum workpiece, reduce the interface contact resistance and increase the contact surface, improve the heat dissipation capacity, and ultimately reduce the alloying reaction between the electrode and the aluminum workpiece, and improve the electrode life. Compared with the traditional spherical electrode, the electrode of the present application first contacts the center in the initial stage, and the annular ridge gradually contacts the outside with time, which expands the heat dissipation area and provides more sufficient pressure, which is beneficial to reduce the temperature of the nugget affected zone and reduce the shrinkage stress during cooling, thereby reducing the generation of cracks in the heat affected zone.

[0029] It should be understood that, within the scope of the present application, each of the above technical features of the present application and each of the technical features specifically described below (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

[0030] 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 the prior art description will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and those skilled in the art can also obtain other drawings according to these drawings without creative labor.

[0031] Figure 1 is a cross-sectional view of an electrode cap for adhesive spot welding of aluminum workpieces in an example of the present application;

[0032] Figure 2 is Figure 1 is an enlarged view of the first surface of the electrode cap;

[0033] Figure 3 is a cross-sectional view of another example of the first surface of the electrode cap for adhesive spot welding of aluminum workpieces of the present application;

[0034] Figure 4 is a cross-sectional view of still another example of the first surface of the electrode cap for adhesive spot welding of aluminum workpieces of the present application;

[0035] Figure 5 is a cross-sectional view of yet another example of the first surface of the electrode cap for adhesive spot welding of aluminum workpieces of the present application;

[0036] Figure 6 is a cross-sectional view of the annular ridge of the electrode cap for adhesive spot welding of aluminum workpieces in an example of the present application;

[0037] Figure 7This is a schematic diagram showing the circumferential diameter of multiple annular ridge positions of an electrode cap for adhesive spot welding of aluminum workpieces in one embodiment of the present invention.

[0038] Figure 8 This is a schematic diagram of multiple annular ridges with different cross-sectional shapes in an electrode cap for adhesive spot welding of aluminum workpieces, according to an example of the present invention.

[0039] Figure 9 This is a schematic diagram of the cross-sectional shape of multiple annular ridges of the electrode cap used for adhesive spot welding of aluminum workpieces in another embodiment of the present invention.

[0040] Figure 10 In one embodiment of the present invention, the diameter is centered on the center of the electrode cap. A schematic diagram showing the location of point p at the intersection of the cylindrical side surface and the second surface;

[0041] Figure 11 This is a schematic diagram of the electrode cap welding process for an aluminum alloy workpiece in one embodiment of the present invention;

[0042] Figure 12 This is a schematic diagram illustrating the change of current over time during resistance spot welding in an embodiment of the present invention.

[0043] Figure 13A This is a schematic diagram illustrating the change of current over time during resistance spot welding in another embodiment of the present invention;

[0044] Figure 13B This is a schematic diagram illustrating the change of current over time during resistance spot welding in yet another embodiment of the present invention;

[0045] Figure 14 This is a schematic diagram illustrating the change of current over time during resistance spot welding in another embodiment of the present invention;

[0046] Figure 15 This is a schematic diagram of the actual current output during the resistance spot welding process in one embodiment of the present invention;

[0047] Figure 16 This is a schematic diagram showing the residual surface caused by the inability of the adhesive layer at the central interface to be effectively removed when using traditional electrodes for adhesive spot welding.

[0048] Figure 17 The cross-section of the weld joint after adhesive bonding using conventional electrodes;

[0049] Figure 18 for Figure 17 A magnified view of position 'a' in the middle;

[0050] Figure 19 The image shown is a cross-sectional view of the weld joint obtained using the welding electrode and method of the present invention.

[0051] In the various drawings, the various signs are as follows:

[0052] 1, 2 - electrode cap;

[0053] 12 - first surface;

[0054] 11 - second surface;

[0055] 111 - annular ridge;

[0056] 13 - third surface;

[0057] 14 - transition fillet;

[0058] 3, 4 - aluminum workpiece;

[0059] 5 - adhesive layer;

[0060] 6 - current. DETAILED DESCRIPTION

[0061] The inventor, through extensive and in-depth research, through a large number of screening, first developed an electrode cap for adhesive spot welding of aluminum workpieces. Compared with the prior art, the present application can ensure that the adhesive layer is smoothly discharged outward at the initial stage by providing the highest contact surface at the center. Since the outer ring of the electrode has multiple annular protruding ridges that can pierce the oxide film and establish a more stable contact with the aluminum workpiece, the interface contact resistance is reduced and the contact surface is increased, the heat dissipation capacity is improved, and finally the alloying reaction between the electrode and the aluminum workpiece is reduced, the electrode life is improved. At the same time, compared with the traditional spherical electrode, the center of the electrode of the present application first contacts at the initial stage, and the outer side contacts the annular ridge gradually with time, which expands the heat dissipation area and provides more sufficient pressure, which is beneficial to reduce the temperature of the nugget affected zone and reduce the shrinkage stress when it cools, thereby reducing the generation of cracks in the heat affected zone. On this basis, the present application is completed.

[0062] Terminology

[0063] As used herein, the terms "aluminum workpiece", "workpiece", and "aluminum alloy workpiece" and the like are used interchangeably.

[0064] As used herein, the terms "electrode cap" and "welding electrode" and the like are used interchangeably.

[0065] The present application will be further described below 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, therefore the dimensions or proportions of the devices and equipment of the present application are not limited by the schematic drawings.

[0066] 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.

[0067] Example

[0068] Figure 1 The figure shown is a cross-sectional schematic diagram of the electrode cap used for adhesive spot welding of aluminum workpieces in this embodiment. The electrode cap 1 has a central rotation structure along the axis of symmetry OO. Its center has a first surface 12 that is dome-shaped away from the electrode cap body and a second surface 11 surrounding the outside of the first surface. A third surface (i.e., side surface) 13 is also included outside the second surface. The second surface 11 has a plurality of raised annular ridges 111.

[0069] Now for reference Figure 2 , Figure 2 for Figure 1 An enlarged view of the first surface of the middle electrode cap. Preferably, the first surface 12 and the second surface 11 are connected by a transition fillet 14, wherein the radius of curvature of the transition fillet 14 is 0.5-3mm. Taking the highest point A of the first surface as a reference point, its height from the plane containing the outer circumference of the first surface is h1, and the diameter of the plane containing the outer circumference of the first surface is d1; generally, h1 is not less than 0.02mm, generally 0.02≤h1≤0.6mm, preferably 0.05≤h1≤0.4mm; and d1 is generally 3-8mm, and satisfies Where T represents the thickness of the welded aluminum workpiece.

[0070] The first surface 12 is generally dome-shaped away from the electrode cap body, and it can be composed of a single circular arc surface. Figure 2 The example shown is where the first surface 12 is a single, integral arc surface. In this case, the radius of curvature R of the first surface 12 is 15-100 mm, preferably 20-80 mm, and satisfies... Where T represents the thickness of the welded aluminum workpiece.

[0071] The first surface 12 can also be a dome-shaped structure composed of multiple surfaces. For example... Figure 3The first surface is composed of two surfaces 1231 and 1232, wherein the surface 1231 is a central curved surface, and the surface 1232 is a conical surface between the surface 1231 and the second surface 11. The diameter of the outer circumferential of the surface 1231 is d131, and the diameter of the outer circumferential of the surface 1232 is d132. In this case, 30%≤d131 / d132≤85%, preferably 40%≤d131 / d132≤75%, d132 is 3-8 mm, and The surface 1231 is a spherical surface as a whole, and the radius of curvature thereof is not less than 15 mm. The surface 1232 can be a conical surface or an arc surface, and when it is a conical surface, the included angle a thereof with the horizontal plane is not less than 120°. The height h1 of the outer diameter of the surface 1232 from the vertex of the surface 1231 is not more than 0.8 mm, generally 0.02≤h1≤0.8 mm, and preferably 0.04≤h1≤0.3 mm.

[0072] The first surface 12 can also be composed of more than two surfaces. For example, Figure 4 The cross-sectional profile of the first surface 12 is composed of a plurality of straight lines and curves, and from the inside to the outside, they are surfaces 1241, 1242, 1243, 1244 and 1245, i.e. the innermost surface is 1241, and the outermost surface is 1245. The diameters of the outer circumferences of the surfaces 1241, 1242, 1243, 1244 and 1245 are d141, d142, d143, d144 and d145 respectively. The diameter of the outer circumferential of the outermost surface 1245 is d145, and the height thereof from the central vertex is h1, d145 is 3-8 mm, and wherein T is the thickness of the welded aluminum workpiece, and generally 0.02≤h1≤0.8 mm, and preferably 0.04≤h1≤0.3 mm. The innermost surface 1241 is an arc surface, and the radius of curvature thereof is not less than 15 mm, and preferably more than 20 mm. The ratio of the diameter of the outer circumferential of each surface to the diameter of the outer circumferential of the adjacent surface is generally 0.3-0.85, and preferably 0.4-0.75; for example, 30%≤d143 / d144≤85%, 30%≤d142 / d143≤85%. The plurality of surfaces 1241, 1242, 1243, 1244 and 1245 which constitute the first surface 12 as a whole can be a plane, a conical surface or a curved surface, etc. For example, Figure 4 As shown in the middle, the surface 1241 is a curved surface, the surface 1242 is a plane, the surface 1243 is a conical surface, the surface 1244 is a plane, and the surface 1245 is a conical surface. The number and type of the surfaces can be selected according to the actual use (for example, the thickness and type of the intermediate adhesive layer). For example, Figure 5The first surface is shown, which has only three surfaces: a curved surface, a plane, and a cone surface, respectively, from the center outwards.

[0073] The second surface 11 can have various structures, with its outermost circumference having a diameter d2, typically 8-15 mm, preferably 10-12 mm. The second surface can be a flat end face or an arc surface with a certain radius of curvature r, where the radius of curvature r is not less than 20 mm, typically 20 ≤ r ≤ 100 mm. The second surface 11 has multiple raised annular ridges 111, typically 2-5 in number. The bottom width B of each annular ridge is 0.15-0.4 mm, preferably 0.2-0.35 mm; the height H of each annular ridge is 15-350 μm, preferably 50-250 μm; and the aspect ratio B / H satisfies: 0.4 ≤ B / H ≤ 12. The shape and dimensions of each annular ridge can be the same or different. The height of the innermost first annular ridge is 30-220 μm, preferably 100-200 μm. This helps ensure that it first contacts the workpiece during resistance spot welding and transitions gradually to subsequent annular ridges in a fixed manner, thereby making the electrode pressure and welding current distribution more directional and controllable, improving heat generation efficiency and reducing electrode adhesion. Figure 6 The diagram shows the cross-sectional shape of the annular ridge in this invention. The cross-sectional shape of the annular ridge can be any shape that includes triangles, trapezoids, semicircles, and other straight lines and curves.

[0074] The central axis spacing of each annular ridge is 150-1200 μm, preferably 350-800 μm. For example... Figure 7 As shown, the diameter of the surface where the center of the bottom of the first annular ridge is located is d21, the diameter of the surface where the center of the bottom of the second annular ridge is located is d31, the diameter of the surface where the center of the bottom of the third annular ridge is located is d41, and the diameter of the surface where the center of the bottom of the fourth annular ridge is located is d51, wherein the following conditions are met: d21>d1; 3.5≤d21≤6mm, 5≤d31≤6.5mm, 6.5≤d41≤8.5mm, and 7.5≤d51≤9.5mm.

[0075] In another preferred embodiment, the annular ridge further includes a fifth annular ridge, wherein the diameter of the surface where the bottom center of the fifth annular ridge is located is d61, and d61 satisfies 8.5≤d61≤10mm.

[0076] Multiple ring-shaped ridges can have the same or different shapes. Figure 8 The diagram shows that the innermost annular ridge has a triangular cross-section, the second annular ridge has an arc-shaped cross-section, the third annular ridge has a triangular cross-section, and the fourth annular ridge has a trapezoidal cross-section. Furthermore, the cross-sectional shapes of all annular ridges can be combinations of other shapes. For example... Figure 9The cross-sectional shape of the annular ridges is shown as trapezoidal, and the size of each can also be different.

[0077] In a preferred embodiment, the annular ridge apex is on a spherical surface, and the second surface of the welding electrode is located at the same center.

[0078] The electrode cap gradually expands from inside to outside during the nugget formation process during welding. This welding sequence ensures that the nucleation mode is in an inside-out manner, and the size of the nugget is controllable; and the annular ridges on the outer surface can reduce the heat on the surface, reduce electrode sticking, and easily form different target sizes of nuggets according to different plate thicknesses to ensure the strength of the welded joint.

[0079] The intersection of the cylindrical side surface with a diameter of T of the aluminum workpiece; and the height difference h2 between the outer diameter of the surface circle where the second surface is located and the lowermost end of the first surface is not more than 0.3T, where T is the thickness of the aluminum workpiece, as shown in Figure 10 .

[0080] The third surface 13 located outside the second surface and adjacent to the second surface can be a conical surface or an arc surface. When it is a conical surface, the included angle between it and the plane perpendicular to the central axis of the electrode cap is β, where 20°≤β≤70°; in another preferred embodiment, 30°≤β≤60°. In another preferred embodiment, the third 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 not less than 10 mm.

[0081] The present application also provides a resistance spot welding method, which comprises the following steps:

[0082] Now referring to Figure 11 , (a) first providing a pair of the aforementioned electrode caps 1 and 2;

[0083] (b) pre-pressing stage: placing a pair of electrode caps on both sides of the stacked aluminum workpieces 3, 4, respectively, 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 most central domed surfaces 12, 22 of the electrode caps first come into contact with the outer surfaces 31, 41 of the aluminum workpieces; a layer of adhesive 5 for gluing is contained between the aluminum workpieces; the adhesive is a heat-curable epoxy resin, and the thickness of the adhesive layer is generally 0.1-1.0mm. With the continuous action of the electrode pressure, the adhesive layer 5 between the aluminum workpieces is gradually extruded outward and melted, evaporated, and vaporized, so that the inner surfaces 32, 42 of the aluminum workpieces come into contact with each other;

[0084] (c) the welding current stage: one or more welding currents are conducted, the effective value of the welding current is 22 KA-65 KA, as time goes on, the aluminum workpiece corresponding to the center dome welding surface 12 is first melted to form a melting center 34, and as the current 6 continues to pass, the aluminum melting core gradually expands outward, and the first surface of the electrode cap and the annular ridge above gradually contact the aluminum workpiece outward;

[0085] (d) the condensation stage: stop the welding current, maintain the state 300-300 ms of the electrode cap and the aluminum workpiece, and the molten metal solidifies to form a complete welding point 34.

[0086] Wherein, the welding current is in the form of multiple pulses, the welding current stage includes at least 3 or more main welding current pulses, and each main welding current pulse has a cooling interval; the action time of each welding current pulse is tn, n is the total number of welding pulses, and satisfies T is mm, and tn is ms; each main welding current pulse In is greater than 25 KA.

[0087] Wherein, the action time of each main welding current pulse t n is not more than 20 ms, and the cooling interval time is 3 ms≤t n-n+1 ≤5 ms. When the action time of the welding current pulse t n is not more than 15 ms, the cooling interval time t n-n+1 is 2-3 ms. When the action time of the welding current pulse t n is not more than 10 ms, the cooling interval time t n-n+1 is 1-2 ms. The number of welding current pulses n is not less than 4. The amplitude of each welding pulse current can be the same or different. For example, as shown in Figure 12 , they are the same; for example, as shown in Figure 13A and 13B , they gradually increase, and I n+1 -I n ≥2 KA, I n+1 is the amplitude of the n+1 current.

[0088] It is worth noting that a preheating current pulse can be included before the main welding current, the amplitude of the preheating current pulse I0 is 10-20 KA, and the duration is 20-60 ms; the cooling interval time between the preheating pulse and the main welding current pulse is t 0-1 is 0-5 ms, as shown in Figure 13A ; preferably 0 ms, as shown in Figure 13B .

[0089] In addition, at least one tempering current pulse is included after the main welding current pulse, as shown in Figure 14 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 the amplitudes of all the preceding main welding pulses I w , more narrowly, it is not more than 0.8 I w . The cooling interval between the tempering pulse current and the last main welding current pulse is 5-50 ms, and the application time point of the tempering current is after the inside of the welding nugget has just started to solidify and before the complete solidification is finished; in general, 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 welding nugget is reduced, and the generation of internal heat cracks and other defects is reduced.

[0090] The time from the contact of the electrode center first surface 12 with the workpiece to the complete exclusion of the intermediate adhesive layer in the aluminum workpiece is not more than 1500 ms, and the time from the appearance of the melting center at the center of the current to the formation of the complete welding nugget is not more than 300 ms, in particular, when the workpiece thickness is less than 1 mm, the time is not more than 100 ms, and when the workpiece thickness is 1.0-2.0 mm, the time is not more than 150 ms.

[0091] In addition, it is worth noting that the amplitudes of all the current pulses described above are the effective values of the current in a single stage. Due to the existence of the ramping-up and fluctuation stages in the actual current rising process, the actual output cannot guarantee that it will be completely reduced to 0, as shown in Figure 15 All the figures are schematic diagrams with constant average values, and their meanings are understood by those skilled in the art.

[0092] It is worth noting that the electrode cap in the present application can be made of any electrically conductive and thermally conductive material, for example, it can be made of a copper alloy, including a copper-chromium (CuCr) alloy, a copper-chromium-zirconium (CuCrZr) alloy, a copper alloy with added aluminum oxide particles, or other various copper alloys that can be used as electrode materials; the above-mentioned aluminum alloys can include wrought aluminum alloys or cast aluminum alloys, including aluminum alloy substrates with or without coatings on the surface, such as aluminum-magnesium alloys, aluminum-silicon alloys, aluminum-magnesium-silicon alloys, aluminum-zinc alloys, aluminum-copper alloys, and the like. Moreover, the material state can include various tempering, including annealing, strain hardening, solid solution strengthening, and the like. The thickness of the aluminum substrate is generally between 0.3 mm and 6.0 mm, preferably between 0.5 mm and 3.0 mm.

[0093] It is worth noting that 3, 4 in the present application are used for resistance spot welding to connect the first aluminum alloy workpiece and the second aluminum workpiece; 1 and 2 are respectively the first welding electrode cap and the second welding electrode cap, at least one of which is the welding electrode cap described above. The aluminum workpiece can be various aluminum workpieces formed by stamping, extrusion or casting. The aluminum alloy workpieces can be two (such as only 3 and 4) or more than two combinations during welding, 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.

[0094] The present application can ensure that the glue layer is smoothly discharged outward at the initial stage by arranging the highest dome contact surface at the center position, and can establish more stable contact with the aluminum workpiece, reduce the interface contact resistance and increase the contact surface, improve the heat dissipation capacity, and ultimately reduce the alloying reaction between the electrode and the aluminum workpiece, thereby improving the electrode life. At the same time, compared with the traditional spherical electrode, the electrode of the present application first contacts at the center at the initial stage, and the annular ridge gradually contacts at the outer side with the passage of time, thereby expanding the heat dissipation area and providing more sufficient pressure, which is beneficial to reduce the temperature of the heat affected zone of the nugget and reduce the shrinkage stress during cooling, thereby reducing the generation of crack defects in the heat affected zone.

[0095] As shown in Figure 16 the residual surface caused by the fact that the glue layer at the center interface cannot be effectively discharged when using the traditional electrode for glue spot welding, Figure 17 the cross section of the welding spot after resistance spot welding, a large number of cracks caused by glue layer residues exist in the nugget; Figure 18 is a partial enlarged view thereof; Figure 19 the cross section of the welding spot obtained by using the welding electrode and method of the present application, as can be seen from the figure, the method of the present application can reduce the glue layer residues to a minimum, and greatly reduce the length of internal defects; at the same time, the presence of the surface annular ridge can reduce the temperature between the electrode and the workpiece, accelerate heat dissipation, reduce electrode adhesion, and improve electrode life; at the same time, the presence of the outer annular ridge reduces the temperature of the heat affected zone around the nugget, which helps to reduce the generation of liquefied cracks, and the maximum length of the cracks is only 0.14 mm, which helps to greatly improve the joint strength.

[0096] All the documents mentioned in the present application are incorporated by reference in the present application, as if each document is individually incorporated by reference. In addition, it should be understood that those skilled in the art can make various modifications or modifications to the present application after reading the above description of the present application, and these equivalent forms also fall within the scope defined by the claims attached to the present application.

Claims

1. An electrode cap for use in the friction stir spot welding of aluminium workpieces, characterised in that, The electrode cap comprises: a first surface in the center of the electrode cap and protruding outward from the center axis of the electrode cap in the shape of a whole dome; a second surface surrounding the first surface; a plurality of annular ridges provided on the second surface and protruding or recessing around the center axis, the number of annular ridges being 2-5; wherein the center axis of the cross section of a single annular ridge is perpendicular to the second surface; the height of the first surface is not less than 0.02mm higher than the height of the annular ridge; Wherein, with the center of the electrode cap as the center, a diameter of A point p on the intersection of the cylindrical side surface and the second surface is a distance h0 from the highest point of the first surface, and h0 satisfies h0≤0.15T, wherein T is the thickness of the aluminum workpiece. the first surface and the second surface are connected by a transition round corner.

2. The electrode cap of claim 1, wherein The outer diameter d1 of the first surface is 3-8 mm, and where T is the thickness of the aluminum workpiece.

3. The electrode cap of claim 1, wherein, The height h1 of the protrusion on the first surface is 0.02-0.6mm.

4. The electrode cap of claim 1, wherein, The first surface is a curved surface with a radius of curvature R of 15-100 mm; and where T is the thickness of the aluminum workpiece.

5. The electrode cap of claim 1, wherein, The second surface is a flat surface, or a curved surface with a radius of curvature r of 20-100mm.

6. The electrode cap of claim 1, wherein The outer diameter d2 of the second surface is 8-15mm.

7. The electrode cap of claim 1, wherein The height of each annular ridge is 20-300μm; the bottom width of the annular ridge is 0.1-0.5mm.

8. The electrode cap of claim 7, wherein, The height of the innermost annular ridge is 50-200μm.

9. The electrode cap of claim 1, wherein, The height from the highest point of the first surface to the outer diameter of the second surface is h2, wherein h2 is not more than 0.3 times the thickness of the aluminum workpiece.

10. The electrode cap of claim 1, wherein, The vertices of the plurality of annular ridges are on a spherical surface, and the spherical center of the spherical surface is the same as the spherical center of the spherical surface on which the second surface is located.

11. A resistance spot welding method characterized by, The method comprises: (a) providing a pair of electrode caps as claimed in any one of claims 1-10; (b) pre-pressing stage: placing a pair of electrode caps on both sides of the stacked aluminum workpieces respectively, performing pre-pressing, applying an electrode pressure of 2000-8000N for a duration of 200-1500ms, so that the most central dome surface of the electrode cap first contacts the aluminum workpiece; containing a layer of adhesive layer for gluing between the aluminum workpieces; (c) current welding stage: conducting one or more welding currents through the electrode cap, the effective value of the welding current being 22KA-65KA, as time progresses, the aluminum workpiece contact area with the first surface gradually expands outward and the annular ridges on the second surface gradually contact from the inside out; the current first passes through the center of the electrode cap and gradually expands outward to the annular ridge position, as the welding time progresses, the aluminum workpiece center contact site melts to form a welding pool; (d) condensation stage: stop the welding current and maintain the state of the electrode cap and the aluminum workpiece for 30-300ms, and the molten metal solidifies to form a complete weld.

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