Welding electrode arrangement for ejecting organic material during welding.

Abstract

Welding electrode arrangement with: a first welding electrode (16), comprising: a body (20); and a weld surface (22) for pressing against a first side (84) of a workpiece stack arrangement (80) supported by an end of the body (20), wherein the weld surface (22) comprises a central upright plateau (40) with a plateau surface (52) and a convex dome section (42) surrounding the central upright plateau (40); and a second welding electrode (18), comprising: a welding surface (26) for pressing against a second side (86) of the workpiece stack arrangement (80), wherein the welding surface (26) of the second welding electrode (18) is aligned with the welding surface (22) of the first welding electrode (16) when the welding surfaces (22, 26) of the first and second welding electrodes (16, 18) are pressed against the respective first and second sides (84, 86) of the workpiece stack arrangement (80); wherein the welding surface (26) of the second welding electrode (18) comprises a convex basic welding surface (58) and a plurality of upright circular ribs (60) projecting outwards from the convex basic welding surface (58), such that the upright circular ribs (60) are separated by intervening circular sections (62) of the convex basic welding surface (58); characterized by the fact that the convex dome section (42) of the welding surface (22) of the body (20) of the first welding electrode (16) further comprises a base surface (44) and a plurality of trapezoidal welding surface sections (46) spaced circumferentially on the base surface (44) around the central upright plateau (40), each of the trapezoidal welding surface sections (44) comprising a plurality of transversely directed, upright, curved ribs (48) spaced radially apart along the base surface (44) of the convex dome section (42); wherein a plurality of radial slots (56) communicate with an annular channel surrounding the central upright plateau (40) and extend outwards in the direction of an outer circumference of the base surface (44) of the convex dome section (42), each of the plurality of radial slots (56) extending between two adjacent trapezoidal weld surface sections (44); wherein the diameter of the innermost upright circular rib (60') of the welding surface (26) of the second welding electrode (18) is smaller than the diameter of the plateau surface (52) of the central upright plateau (40) of the welding surface (22) of the first welding electrode (16).

Description

TECHNICAL AREA

[0001] This invention relates to a welding electrode arrangement according to the preamble of claim 1, as is known essentially from US 2013 / 0306604A1.

[0002] Regarding the further state of the art, reference is made here to the publications DE 20 2012 009 321 U1 and US 8 502 105 B2.

[0003] The welding electrode assembly can be used to perform resistance spot welding on a workpiece stack assembly comprising a steel workpiece and an overlapping and adjacent aluminum workpiece. The radially slotted welding electrode is particularly advantageous when an intervening layer of organic material is present between the aluminum and steel workpieces in the workpiece stack assembly. INTRODUCTION

[0004] Joining a steel workpiece to a lighter aluminum workpiece (unalloyed aluminum or an aluminum alloy with 85 wt.% or more aluminum) provides weight reduction while maintaining adequate strength in many manufactured items. Practical applications offer numerous weight-saving opportunities, such as in the manufacture of structural frame elements (e.g., body sides and crossmembers) and vehicle closure elements (e.g., doors, hoods, trunk lids, and tailgates). Many such vehicle component elements feature inner and outer sheets that are complementarily shaped to provide a stronger, stiffer structure, as well as enclosing windows, closures, insulation, electrical wiring, and the like.The steel and aluminum workpieces are typically formed with similarly shaped circumferential edges or flanges, allowing them to be joined by a series of suitably spaced resistance spot welds strategically placed along their circumferences. An interlayer organic material may also be applied between the facing surfaces to be joined when closing the sheet metal structure, in order to provide additional bond strength, sound-dampening properties, and / or other characteristics.

[0005] In resistance spot welding, the steel and aluminum workpieces are stacked and clamped in a surface contact with interlocking butt surfaces at each point where a spot weld is to be formed. Two welding electrodes, positioned in collinear plane opposition to their respective outer surfaces within the stacked workpieces, then deliver an electric current through overlapping and adjacent steel and aluminum workpieces. The flowing electric current briefly forms a weld pool within the aluminum workpiece, which in turn wets the adjacent butt surface of the steel workpiece. The steel workpiece does not melt and does not contribute to the weld pool.After a brief period of electric current flow, typically no longer than one or two seconds, the welding current is terminated, with the opposing electrodes continuing to be pressed against their respective workpiece stack outer surfaces. The steel and aluminum workpieces cool in the surrounding environment—although differences in the thermal conductivities of the two materials cause heat to spread differently from the weld point through the workpieces—and the weld pool in the aluminum workpiece solidifies to form a weld bonded to the steel workpiece. The weld includes a weld nugget and typically a brittle intermetallic layer between the weld nugget and the steel workpiece. The intermetallic layer may comprise various Fe-Al intermetallic compounds.

[0006] In practice, spot welding a steel workpiece to an aluminum workpiece presents a challenge, as several properties of these two metals can negatively affect the strength—especially peel strength and tensile strength—of the weld. Regarding the properties of these dissimilar metals, aluminum has a relatively low melting point (~600 °C) and relatively low electrical and thermal resistance, while steel has a relatively high melting point (~1500 °C) and relatively high electrical and thermal resistance. As a result of these physical differences, most of the heat generated during current flow is dissipated within the steel workpiece, creating a thermal imbalance between the steel and aluminum workpieces.The combination of the heat imbalance generated during the current flow and the high thermal conductivity of the aluminum workpiece means that, immediately after the electric current stops, a situation arises in which heat is not distributed symmetrically from the weld. Instead, heat is conducted from the hotter steel workpiece through the aluminum workpiece towards the spot welding electrode on the other side of the aluminum workpiece, creating a steep thermal gradient in that direction.

[0007] It is assumed that the development of a steep thermal gradient between the steel workpiece and the spot welding electrode on the other side of the aluminum workpiece weakens the resulting weld in several ways. First, because the steel workpiece retains heat longer than the aluminum workpiece after the current has ceased, the aluminum weld pool generated during the current flow solidifies in a directed manner, starting from the area closest to the colder spot welding electrode (often water-cooled), near the aluminum workpiece, and spreading towards the joint interface. A solidification front of this kind tends to carry or drive defects—such as gas porosity, shrinkage voids, and microcracking—towards and along the joint interface within the weld, where the residual oxide film and / or composite residue film are already present.Secondly, the persistently elevated temperature in the steel workpiece promotes the growth of a hard and brittle intermetallic Fe-Al layer at and along the butt joint interface. The presence of a distribution of weld defects, together with excessive growth of the intermetallic Fe-Al layer along the butt joint interface, tends to reduce the peel and tensile strength of the weld.

[0008] The challenges that tend to complicate resistance spot welding of steel and aluminum workpieces extend beyond their materially divergent properties. Each steel and aluminum workpiece may, in some cases, contain applied or natural surface coatings that differ in composition from their underlying substrates. The aluminum workpiece, for example, may contain a refractory surface oxide layer. This oxide layer is typically composed of aluminum oxide compounds, although other oxide compounds, such as magnesium oxide compounds, may be present if the aluminum workpiece contains a magnesium-containing aluminum alloy. The oxide layer present on the aluminum workpiece is electrically insulating and mechanically tough.As a result, a residual oxide film, which includes remnants of the surface oxide layer, tends to remain intact on and along the interface of the steel workpiece, where it can impede the ability of the aluminium weld pool to wet the steel workpiece.

[0009] The complications attributed to the refractory surface oxide layer of the aluminum workpiece can be exacerbated if an intervening layer of organic material, such as a layer of uncured, thermosetting adhesive, is present between the mating surfaces of the aluminum and steel workpieces at the weld point. An uncured, but thermosetting adhesive layer may be placed between the mating surfaces of the stacked workpieces to provide additional bonding. When the workpieces are clamped together by the strong pressure applied by the welding electrodes, and prior to the exchange of current, some of the adhesive is squeezed laterally out of the weld zone. Subsequently, the remaining adhesive at the weld point is decomposed during the current flow. After completion of the spot welding step(s), the adhesive-containing areas of the welded workpieces are, for example,The material is heated in an ELPO heating oven (ELPO refers to an electrophoretic preparation step). The applied heat cures the adhesive layer to achieve a strong supporting bond between the opposing butt surfaces of the metal workpieces around the point(s) where spot welding was performed.

[0010] The intervening layer of organic material tends to interact with the refractory surface oxide layer to form a stickier material at spot welding temperatures. It is thought that the thermal residues obtained from the decomposition of the intervening organic layer—e.g., carbon ash, filler particles (e.g., silicon dioxide, rubber, etc.), and other derived materials—combine with the residual oxide film to form a composite residue film that is more resistant to mechanical disruption and dispersal during current flow compared to the residual oxide film alone.The formation of a more viscous composite residue film results in fragments of this film remaining grouped and aggregated on and along the interface of the steel workpiece in a far more destructive manner compared to cases where no layer of organic material is present between the steel and aluminum workpieces. In particular, the composite residue film is thought to block the diffusion of iron into the aluminum weld pool, leading to excessive thickening of the hard and brittle intermetallic Fe-Al layer and thus potentially weakening the weld. Furthermore, any gases generated during the decomposition of the organic material can become trapped in the metal weld pool and may eventually lead to voids or porosity within the solidified weld.Furthermore, the composite residue film can provide a finished crack path along the bonding interface of the weld joint and the steel workpiece, which can in turn weaken the weld joint.

[0011] In light of the challenges mentioned above, previous attempts to spot weld a steel workpiece to an aluminum workpiece have employed a welding plan that specifies higher currents, longer welding times, or both (compared to spot welding steel to steel) in an attempt to achieve and maintain a reasonable weld fusion area. These efforts have been largely unsuccessful in a manufacturing environment and tend to damage the welding electrodes. Given the lack of success of previous spot welding efforts, mechanical fasteners, including self-piercing rivets and flow-drilling screws, have been predominantly used. However, mechanical fasteners take longer to position and involve high consumable costs compared to spot welding.They also add weight to the vehicle – weight that is avoided when joining is accomplished by spot welding – thus negating some of the weight savings achieved primarily by using an aluminum workpiece. Advances in spot welding that would make it easier to join steel and aluminum workpieces, despite the many challenges involved, would therefore be a welcome addition to the state of the art. SUMMARY

[0012] According to the invention, a welding electrode arrangement is presented which is characterized by the features of claim 1.

[0013] A method for resistance spot welding a stack of workpieces comprising an aluminum workpiece and an overlapping adjacent steel workpiece can include several steps. Initially, a stack of workpieces is provided, having a first side and an opposite second side. The stack comprises an aluminum workpiece and an adjacent steel workpiece that overlaps the aluminum workpiece, and further includes an intervening layer of organic material positioned between the opposing abutting surfaces of the overlapping aluminum and steel workpieces. Additionally, within the stack, the aluminum workpiece is located near the first side of the stack, and the steel workpiece is located near the second side of the stack.

[0014] Next, after the workpiece stack arrangement has been provided, a welding surface of a radially slotted welding electrode is pressed against the first side of the workpiece stack arrangement, and a welding surface of a second welding electrode is pressed against the second side of the workpiece stack arrangement. The welding surface of the radially slotted welding electrode comprises a central upright plateau and a convex dome section surrounding the central upright plateau, which includes a plurality of trapezoidal welding surface segments spaced circumferentially on the base around the central upright plateau. Each of the trapezoidal welding surface segments includes a plurality of transversely oriented, upright, curved ribs spaced radially along the base of the convex dome section.Furthermore, an annular channel surrounds the central upright plateau, and a multitude of radial slots communicate with the annular channel and extend outwards towards an outer circumference of the base between the trapezoidal welding surface segments. The welding surface of the second welding electrode is aligned with the welding surface of the radially slotted welding electrode when the welding surfaces are pressed against their respective first and second sides of the workpiece stack arrangement.

[0015] Once the welding electrodes are in position, an electric current is passed between the welding surface of the radially slotted welding electrode and the welding surface of the second welding electrode to create a weld pool contained within the aluminum workpiece and wetting the mating surface of the steel workpiece. Finally, the flow of the electric current between the welding surface of the radially slotted welding electrode and the welding surface of the second welding electrode is stopped to allow the weld pool to solidify into a weld joint that joins the aluminum and steel workpieces together.

[0016] The resistance spot welding procedure for the workpiece stack described above can be further defined. For example, the first side of the workpiece stack can be an outer surface of the aluminum workpiece, and the second side of the workpiece stack can be an outer surface of the steel workpiece. In another example, the workpiece stack can include an additional aluminum workpiece and / or an additional steel workpiece, in addition to the aluminum and steel workpieces, which overlap and are adjacent to each other.

[0017] Furthermore, during the pressing of the welding surface of the radially slotted welding electrode against the first side of the workpiece stack arrangement, a plateau surface of the central upright plateau can contact the first side of the stack arrangement before any of the transverse, upright, curved ribs encompassed by the plurality of trapezoidal welding surface segments contact the first side. This allows a clamping load applied by the welding surfaces of the radially slotted and second welding electrodes to initially be transmitted through the central upright plateau. In another implementation of the aforementioned method, only the plateau surface of the central upright plateau contacts the first side of the workpiece stack arrangement before the passage of the electric current.Then, during a pass of the electric current, the trapezoidal weld surface sections of the convex dome section surrounding the central upright plateau progressively come into contact with the first side of the workpiece stack arrangement, starting with the innermost upright curved ribs of the trapezoidal weld surface sections and continuing radially outwards towards the outermost upright curved ribs of the trapezoidal weld surface sections.

[0018] The welding surface of the second welding electrode, which is pressed against the second side of the workpiece stack, can assume various configurations. For example, the welding surface of the second welding electrode can comprise a convex base welding surface and a multitude of upright circular ribs projecting outwards from the convex base welding surface, such that the upright circular ribs are separated by intervening circular sections of the convex base welding surface. Furthermore, in a specific implementation, the diameter of the innermost upright circular rib of the welding surface of the second welding electrode can be smaller than the diameter of the plateau surface of the central upright plateau of the welding surface of the radially slotted welding electrode.

[0019] The radially slotted welding electrode according to the embodiment described above can have certain structural design features. For example, each of the trapezoidal weld surface sections can include two to ten transversely oriented, upright, curved ribs, each of which has a rib height ranging from 20 µm to 400 µm and is separated from each of its adjacent ribs by a radial spacing of 50 µm to 1800 µm. Furthermore, as another example, the plateau surface of the central upright plateau can be positively displaced above the surrounding base weld surface surface of the convex dome section, such that the plateau surface is raised above an innermost transversely oriented, upright, curved rib of each of the plurality of trapezoidal weld surface sections.Of course, other structural design features may be implemented. BRIEF DESCRIPTION OF THE DRAWINGS Fig. Figure 1 is an enlarged schematic side view in cross-section of a workpiece stack arrangement comprising an aluminium workpiece on a steel workpiece with an intermediate layer of organic material, wherein a radially slotted welding electrode, welding according to an embodiment of the invention, is shown in engagement with the aluminium workpiece, and another welding electrode is shown in engagement with the steel workpieces immediately before the application of an electric current between the opposing welding electrodes; Fig. Figure 2 is a fragmentary cross-sectional view of the weld surface of the in Fig. 1 radially slotted welding electrode shown, which makes contact with the aluminium workpiece; Fig. Figure 3 is a top view of the welding surface of the radially slotted welding electrode, which is located in the Fig. 1-2 is shown; Fig. Figure 4 is a fragmentary cross-sectional view of the weld surface of the in Fig. 1 welding electrode shown, which makes contact with the steel workpiece; Fig. Figure 5 is a perspective view of a ball-nose welding electrode, which is presented as an alternative to the one in Fig. The welding electrode shown in 1 can be used, which establishes contact with the steel workpiece; Fig. Figure 6 is a perspective view of a ball-nose welding electrode with a central flat area, which is presented as an alternative to the one in Fig. The welding electrode shown in 1 can be used, which establishes contact with the steel workpiece; Fig. Figure 7 is a perspective view of a truncated cone welding electrode, which is presented as an alternative to the one in Fig. The welding electrode shown in section 1 can be used, which establishes contact with the steel workpiece; and Fig. Figure 8 is an enlarged schematic side view in cross-section of the workpiece stack arrangement and opposing welding electrodes, as shown in Fig., wherein the welding electrodes are in full engagement with their respective workpieces after the passage of electric current has ceased. DETAILED DESCRIPTION

[0020] The present invention relates to the design of a welding electrode that can be used in conjunction with another, opposing welding electrode to perform resistance spot welding between a steel workpiece and an aluminum workpiece arranged adjacent to each other in a workpiece stack arrangement, particularly when an intervening organic material is arranged between the steel and aluminum workpieces. The disclosed welding electrode is radially slotted and is intended to engage with and be pressed against the workpiece stack arrangement near the aluminum workpiece, in contrast to a planar orientation with another welding electrode that engages with and is pressed against the stack arrangement near the steel workpiece.The radially slotted welding electrode comprises a central plateau surrounded by a section of a convex dome. This dome contains a plurality of circumferentially spaced trapezoidal weld surface sections, each featuring transversely oriented, upright curved ribs spaced radially apart. The central plateau and the trapezoidal weld surface sections define an annular channel surrounding the central plateau and a plurality of radial slots extending outward from the annular channel along one circumference of the weld surface. The design of the radially slotted welding electrode is effective in expelling organic material laterally away from the weld zone between the steel and aluminum workpieces before the weld joint is formed.

[0021] Now referring to the Fig. Figures 1-8 describe a radially slotted welding electrode 16 together with an opposing accompanying welding electrode 18, together with a method for using these spot welding electrodes 16, 18. In the enlarged schematic view of Fig. Figure 1 illustrates a workpiece stack arrangement 80 comprising a section of a steel workpiece sheet 10 and an adjacent overlapping section of an aluminum workpiece 12. The composite steel and aluminum workpieces 10, 12 can, for example, each be sections of an outer aluminum body side panel and an inner steel body side panel for a motor vehicle. Each sheet may have been previously formed. The steel and aluminum workpieces 10, 12 establish indirect interfacial contact at an butt joint 82 extending through a spot weld where joining is desired. The indirect contact experienced between the workpieces 10, 12 is a result of an intervening layer of organic material 14, which was generously applied through the weld zone between the workpieces 10, 12 prior to spot welding. The illustrated workpiece sections can, for example,at the perimeter of the stack arrangement 80, where a series of resistance spot welds are to be formed in a spaced orientation along the circumferential edges.

[0022] The steel workpiece 10 comprises a steel substrate of any number of strengths and grades, which may be coated or uncoated. The steel substrate may be hot-rolled or cold-rolled and may be composed of steel such as mild steel, interstitial-free (IF) steel, bake-hardening steel, high-strength low-alloy (HSLA) steel, dual-phase (DP) steel, complex-phase (CP) steel, martensitic (MART) steel, transformation-induced plasticity (TRIP) steel, twinning-induced plasticity (TWIP) steel, and boron steel, e.g., if the steel workpiece 10 comprises press-hardened steel (PHS).If coated, the steel substrate preferably comprises a surface layer of zinc (galvanized), a zinc-iron alloy (galvanneal), a zinc-nickel alloy, nickel, aluminum, an aluminum-magnesium alloy, an aluminum-zinc alloy, or an aluminum-silicon alloy, each of which may have a thickness of up to 50 µm and may be present on either side of the steel substrate. Taking into account the thickness of the steel substrate and any optional surface layer that may be present, the steel workpiece 10 may have a thickness ranging from 0.3 mm to 6.0 mm, or more narrowly, from 0.6 mm to 2.5 mm, at least at the spot weld.

[0023] The aluminum workpiece 12 comprises an aluminum substrate that is either coated or uncoated. The aluminum substrate can be composed of unalloyed aluminum or an aluminum alloy containing at least 85 wt.% aluminum. Some notable aluminum alloys that can form the coated or uncoated aluminum substrate are an aluminum-magnesium alloy, an aluminum-silicon alloy, an aluminum-magnesium-silicon alloy, and an aluminum-zinc alloy. If coated, the aluminum substrate may include a surface layer of a refractory oxide material composed of aluminum oxide compounds and possibly other oxide compounds, such as magnesium oxide compounds if the aluminum substrate is an aluminum-magnesium alloy.The aluminum substrate may also be coated with a layer of zinc, tin, or a metal oxide conversion coating consisting of oxides of titanium, zirconium, chromium, or silicon, as described in U.S. Patent Publication No. 2014 / 036 O 986. The surface layer may have a thickness ranging from 1 nm to 10 µm and may be present on either side of the aluminum substrate. Taking into account the thickness of the aluminum substrate and any optional surface layer that may be present, the aluminum workpiece 12 may have a thickness ranging from 0.3 mm to approximately 6.0 mm, or more narrowly, from 0.5 mm to 3.0 mm, at least at the spot weld location.

[0024] The aluminum substrate of the aluminum workpiece 12 can be in machined or cast form. The aluminum substrate can be composed, for example, of a machined aluminum alloy sheet layer of the 4xxx, 5xxx, 6xxx, or 7xxx series, an extruded, forged, or other workpiece. Alternatively, the aluminum substrate can be composed of an aluminum alloy casting of the 4xx.x, 5xx.x, 6xx.x, or 7xx.x series. Some more specific types of aluminum alloys that can form the aluminum substrate include, but are not limited to, the aluminum-magnesium alloys AA5754 and AA5182, the aluminum-magnesium-silicon alloys AA6111 and AA6022, the aluminum-zinc alloys AA7003 and AA7055, and the Al-10Si-Mg aluminum die-casting alloy. The aluminium substrate can also be used in many degrees of hardness, including annealed (O), cold-hardened (H) and solution-annealed (T), if desired.

[0025] The workpiece stack arrangement 80 comprises a first outer surface 84 near the aluminum workpiece 12 and a second outer surface 86 near the steel workpiece 10. The term "near," as used in this context, means a surface of the actual aluminum or steel workpiece 10, 12, or the surface of a nearby workpiece on the same side of the stack arrangement 80 as the aluminum or steel workpiece 10, 12. For example, if only the steel and aluminum workpieces 10, 12 are present in the stack arrangement 80 (e.g., a "2T" stack), as shown here in Fig. As shown in Figure 1, an outer surface 12' of the aluminum workpiece 12 provides the first outer surface 84, and an outer surface 10' of the steel workpiece 10 provides the second outer surface 86. In other embodiments, however, an additional workpiece can be arranged on one or both sides of the adjacent steel and aluminum workpieces 10, 12, provided that the additional workpieces are of the same base metal type as the immediately adjacent steel or aluminum workpiece 10, 12. The workpiece stack arrangement 80 can, for example, include an additional aluminum workpiece that is arranged adjacent to and covers the outer surface 12' of the aluminum workpiece 12 that is adjacent to the steel workpiece 10, and / or it can include an additional steel workpiece that is arranged adjacent to and covers the outer surface 10' of the steel workpiece 10 that is adjacent to the aluminum workpiece 12.If an additional workpiece is available, the first and / or the second outer surface 84, 86 of the stacking arrangement 80 can be provided by an additional workpiece.

[0026] The fact that the workpiece stack arrangement 80 may include at least one additional workpiece besides the adjacent steel and aluminum workpieces 10, 12, which overlap to form the butt joint 82, does not change the basic function of the welding electrodes 16, 18 or their effect on a spot weld formed between the adjacent steel and aluminum workpieces 10, 12. In any case, as further explained below, a spot weld comprising a weld joint contained in the aluminum workpiece 12 is formed by an electric current supplied between the spot welding electrodes 16, 18, and the resulting weld joint bonds to the adjacent butt joint of the steel workpiece 10 upon solidification. Consequently, the Fig. For the sake of simplicity, the generalized figures of the workpiece stacking arrangement 80 shown in Figures 1-3 are illustrated only with the adjacent steel and aluminum workpieces 10, 12, since the bonding effect that occurs between these two workpieces 10, 12 is practically the same regardless of whether an additional workpiece is arranged next to one or both workpieces 10, 12. However, the structure of the welding electrodes 16, 18 and their use in conjunction with the workpiece stacking arrangement 80 is also fully applicable to arrangements comprising only the adjacent steel and aluminum workpieces 10, 12, as well as to others comprising an additional workpiece or workpieces.

[0027] The intermediate layer 14 of organic material applied between the steel and aluminum workpieces 10, 12 can be a weld-through adhesive, a sealant, a sound-dampening material, or any other intermediate material that can be spot-welded under the clamping force of the welding electrodes and the magnitude and duration of the electric current flow between the electrodes. The intermediate layer 14 of organic material can, for example, be an uncured but thermosetting adhesive. Such an adhesive can be generously applied between a butt surface 10" of the steel workpiece 10 and a butt surface 12" of the aluminum workpiece 12, so that after spot welding, the workpiece stack assembly 80 can be heated in an (ELPO) primer-coat drying oven or other device to cure the adhesive and provide additional bonding between the workpieces 10, 12.Suitable adhesives, which may contain one or more chemical units, can be any of a wide variety of thermosetting polymer systems, such as thermosetting epoxies and polyurethanes. In fact, a specific example of a suitable thermosetting adhesive is a thermosetting epoxy resin, which may include filler particles such as silicon dioxide particles to modify the viscosity profile or other properties of the adhesive for manufacturing steps. A variety of thermosetting epoxies, including DOW Betamate 1486, Henkel Terokal 5089, and Uniseal 2343, are commercially available. The intermediate layer 14 of organic material is typically applied to a thickness of 0.1 mm to 2.0 mm between the workpiece butt surfaces 10", 12" before the clamping force of the welding electrodes 16, 18 is applied to the workpiece stack 80.

[0028] With particular reference to Fig. Figure 1 shows the radially slotted welding electrode 16 and the opposite accompanying second welding electrode 18 engaged with the workpiece stack 80 in a planar orientation. More specifically, the radially slotted welding electrode 16 engages with its first outer surface 84 near the aluminum workpiece 12, and the opposite second welding electrode engages with its second outer surface 86 near the steel workpiece 10. The welding electrodes 14, 16 can be made of any suitable electrically and thermally conductive material. For example, the welding electrodes 14, 16 can be made of a material with an electrical conductivity of at least 45% IACS and a thermal conductivity of at least 180 W / mK. Some material classes that meet this criterion include a copper alloy and a refractory-based material containing at least 35 wt.% and preferably at least 50 wt.% of the total material content.-% of a refractory metal. Specific examples of a suitable copper alloy include a C15000 copper-zirconium (CuZr) alloy, a C18200 copper-chromium (CuCr) alloy, and a C18150 copper-chromium-zirconium (CuCrZr) alloy, while a suitable refractory-based material comprises a molybdenum or tungsten particle phase, such as a tungsten-copper metal composite, containing between 50 wt.% and 90 wt.% of a tungsten particle phase dispersed in a copper matrix, forming the remainder (between 50 wt.% and 10 wt.%) of the composite. Of course, other materials not expressly listed here may also be used, provided they meet the applicable electrical and thermal conductivity standards.

[0029] The radially slotted welding electrode 16 comprises a body 20 and a first welding surface 22, and likewise the second welding electrode 18 comprises a second welding surface 26 of the body 24. The first and second welding surfaces 22, 26 of the first and second welding electrodes 16, 18 have aligned central axes 28, 30 which extend through the center of the intended welding zone, but not necessarily through the centers of the bodies 20, 24 of the electrodes 14, 16, since one or both of the welding surfaces 22, 26 may be inclined for better accessibility relative to their respective bodies 20, 24, or the electrodes 16, 18 may have a single or double curved design.The body 20, 24 of each welding electrode 14, 16, which may have a diameter between 12 mm and 22 mm, or more preferably between 15 mm and 20 mm, is machined or formed to create a hollowed-out interior 32, 34 for attaching the welding electrode 16, 18 to a separate, movable, opposing welding arm (not illustrated), which is configured to supply a programmed electric current between the electrodes 16, 18. A coolant may also be supplied through each welding arm to maintain the attached welding electrode 16, 18 at a suitable temperature. The respective movements of the welding arms are programmed to position the two welding electrodes 16, 18 on opposite sides of the workpiece stack arrangement 80 such that the welding surfaces 22, 26 are coaxially aligned with respect to their axes 28, 30 to form a resistance spot weld.

[0030] The shapes of the welding electrodes 16, 18 differ fundamentally in the design of their welding surfaces 22, 26. For each of the first and second spot welding electrodes 14, 16, the end of the body 20, 24 transitions, opposite the entrance to the hollow interior 32, 34, into a section 36, 38 that is profiled inwards from the outer diameter of the body 20, 24. The inwardly profiled section 36, 38 preferably has the shape of a truncated cone or a truncated sphere and possesses an annular surface that accommodates the central welding surface section of the electrode 16, 18. As is best demonstrated in the Fig. As shown in Figures 2-3, the radially slotted welding electrode 16 comprises a central, upright plateau 40 surrounded by a section of a convex dome 42. The convex dome section 42 has a base 44, which in a particular embodiment may be part of a sphere having a radius of curvature Rq ranging from 15 mm to 300 mm or, more narrowly, from 20 mm to 50 mm, and may further have a diameter (i.e., a planar extension) extending over an outer circumference 440 of the base 44 at the transition between the inwardly profiled section 34 and the base 44. This diameter preferably ranges from 3 mm to 20 mm or, more narrowly, from 4 mm to 12 mm.

[0031] The convex dome section 42 comprises a plurality of circumferentially spaced trapezoidal welded surface segments 46, which include transversely oriented, upright, curved ribs 48. The upright, curved ribs 48 within each trapezoidal welded surface segment 46 project outwards from the base 44 of the convex dome section 42 and are radially spaced from one another – and thus separated by intervening, circumferentially extending sections 50 of the base 44 – while having arc lengths that extend circumferentially around the central, upright plateau 40. In fact, the arc lengths of the curved ribs 48 within each trapezoidal weld surface section 46 increase when moving from an innermost rib 48' closest to the central round, upright plateau 40 to an outermost rib 48'' furthest from the plateau 40, as best illustrated in Fig. Figure 3 shows that in a particular implementation of the radially slotted welding electrode 16, the convex dome section 42 of the welding surface 22 can comprise three to eight trapezoidal welding surface segments 46, which preferably include two to ten upright, curved ribs 48. Furthermore, each of these two to ten transversely oriented, upright curved ribs 48 can have a rib height ranging from 20 µm to 400 µm, or more narrowly from 50 µm to 300 µm, an arc length ranging from 1 mm to 15 mm, and a radial spacing between each of its adjacent ribs 48 ranging from 50 µm to 1800 µm, or more narrowly from 80 µm to 1500 µm. The transversely directed, upright, curved ribs 48 of the several trapezoidal weld surface sections 48 can be aligned in the circumferential direction, although such a relationship is not mandatory.

[0032] The central, upright plateau 40 has a plateau surface 52 which is positioned at the center of the weld area 26 around the axis 28 of the weld area 22. In plan view, the central, upright plateau 40 is preferably cylindrical and the plateau surface 52 is preferably circular, as shown in Fig. Figure 3 shows that it can have a diameter ranging from 2 mm to 7 mm, or more narrowly, from 3 mm to 5 mm. Furthermore, as is best described in Figure 3, Fig. As shown in Figure 2, the plateau surface 52 is positively displaced over the surrounding base 44 of the convex dome section 42 of the welded surface 22 by a distance “d”, which can range from 100 µm to 500 µm or, more narrowly, from 200 µm to 300 µm. In most cases, this distance “d” is greater than the rib heights of the innermost ribs 48’, so that the plateau surface 52 extends axially beyond all of the upright, curved ribs 48 present on the convex dome section 42. Regarding its surface contour, the plateau surface 52 can be planar or slightly convex in profile. If the plateau surface 52 has a convex shape, it can be a section of a sphere with a radius of curvature Rp greater than 50 mm, such as… B. from 50 mm to 400 mm. If the radius of curvature Rp is greater than 400 mm, the plateau surface 52 is considered planar.However, the plateau surface 52 of the central, upright plateau 52 is preferably flatter than the base surface 44 of the convex dome section 42, regardless of its profile.

[0033] The central, upright plateau 40 and the trapezoidal welded surface sections 46 of the surrounding convex dome section 42 interact to define an annular channel 54 surrounding the central plateau 40 and a plurality of radial slots 56 communicating with the annular channel 54 and extending outward from the annular channel 54 toward the outer circumference 440 of the base 44. More specifically, the annular channel 54 is defined by the central, upright plateau 40 and the innermost upright, curved ribs 48', and the radial slots 54 are defined by the ends of the transversely oriented, upright, curved ribs 48 of adjacent trapezoidal welded surface sections 46.The radial slots 56 have individual widths between adjacent trapezoidal weld surface sections 46, which preferably range from 1 / 20 to 1 / 10 of the length of the outer circumference 440 of the base surface 44, which in most spot welding processes forms a minimum width of 1 mm to 4 mm.As further explained below, the combination of the central plateau 40, the trapezoidal weld surface sections 46 with their transverse, upright, curved ribs 48 and the annular channel 54, which communicates with the radial slots 56, serves to apply pressure to the intermediate layer 14 of organic material, while at the same time providing radially slotted low-pressure flow paths between the workpiece butt surfaces 10", 12", where, in addition to providing an outlet for any gaseous products generated at the butt interface 84 during the current flow, organic material can be more easily ejected from the weld zone.

[0034] Now returning to the Fig. 1 and Fig. 4. The welding surface 26 of the accompanying second welding electrode 18 can assume a variety of designs that can interact with the welding surface 22 of the radially slotted welding electrode 16 to facilitate the lateral ejection of organic intermediate material from the weld zone. In the particular embodiment shown and described here, the welding surface 26 of the second welding electrode 18 comprises a convex base welding surface 58 and a plurality of upright, circular ribs 60 (in plan view) projecting outwards from the base welding surface 58 and thus separated by interposed circular sections 62 of the base welding surface 58. The base welding surface 58 can, for example, be a section of a sphere with a radius of curvature Rg ranging from 15 mm to 300 mm or, more narrowly, from 20 mm to 50 mm, and can furthermore have a diameter (i.e.,(a planar extent) which extends over an outer circumference 580 of the basic weld surface 58 at the transition between the inwardly profiled section 36 and the basic weld surface 58. This diameter preferably ranges from 3 mm to 20 mm or, more narrowly, from 4 mm to 12 mm.

[0035] The upright, circular ribs 60 are centered concentrically around the axis 30 of the welding surface 26 and are radially spaced apart such that the planar diameters of the ribs increase from an innermost rib 60', which directly surrounds the welding surface axis 30, to an outermost rib 60'' furthest from the welding surface axis 30. Accordingly, in this particular embodiment, and in contrast to the welding surface 22 of the radially slotted welding electrode 16, the upright, circular ribs 60 on the welding surface 26 of the second welding electrode 18 are closed in the circumferential direction, meaning that each rib 60 has a continuously curved perimeter that completely surrounds the welding surface axis 60 without interruption. The welding surface 26 of the second welding electrode 18 preferably comprises anywhere from two to ten upright, curved ribs 60, with three to five being most preferred.Each of these upright, circular ribs 60 has a rib height that can range from 20 µm to 400 µm or, more narrowly, from 50 µm to 300 µm, and a radial spacing between each of its radially adjacent (radially inward and radially outward) ribs 60 can range from 50 µm to 1800 µm or, more narrowly, from 80 µm to 1500 µm.

[0036] The accompanying second welding electrode 18 is not necessarily connected to the one in the Fig. 1 and Fig. The design shown and described above is limited to the fourth. Other electrode designs are certainly possible, e.g., those described in the... Fig. 5-7 illustrated, are used, highlighting only the main differences compared to the one in the Fig. 1 and Fig. The welding electrode 18 shown in section 4 can be described with the understanding that the remaining sections of the welding electrode correspond to the above teaching. Now referring to Fig. 5 The second welding electrode, designated by reference numeral 118, can be a conventional spherical nose welding electrode having a convex welding surface 126 with a basic welding surface 158 that extends directly from the end of the body 24 opposite the entrance to the hollow interior 34, and consequently does not include an inwardly profiled section between the body 24 and the welding surface 126. The convex welding surface 126 used here can be part of a spherical surface or a “sphere” with a diameter extending beyond its outer diameter, which ranges from 12 mm to 22 mm, and a radius of curvature ranging from 12 mm to 400 mm.

[0037] Alternatively, and now referring to Fig. 6 comprises the second welding electrode, designated by reference numeral 218, a welding surface 226, and an inwardly profiled section 238 in the shape of a truncated sphere. The welding surface 226 comprises a basic welding surface 258 having a diameter ranging from 3 mm to 16 mm, or more narrowly, from 4 mm to 8 mm, and being either planar or convex. If the basic welding surface 258 is convex, it may be a section of a sphere with a radius of curvature ranging from 25 mm to 400 mm. Furthermore, and now referring to Fig. 7. The second welding electrode, designated by reference numeral 318, can produce a welding surface 326 similar to the welding surface 226 of the one described in Fig. The second welding electrode 218 shown in Figure 6 is included, although in this embodiment the inwardly profiled section 338 is more of a truncated cone than a truncated sphere. The truncated cone can be inclined by 15° to 40° relative to the axis 332 of the welding surface 326. Of course, the details shown in the Fig. The welding electrodes 118, 218, 318 shown in Figures 5-7 are not the only alternative electrode designs suitable for use with the radially slotted welding electrode 116; rather, other welding electrodes may also be used which are not explicitly shown here.

[0038] Now referring to the Fig. Section 1-2 describes a method for using the radially slotted welding electrode 16 and the accompanying second welding electrode 18 in the context of resistance spot welding a stack arrangement comprising only the overlapping steel and aluminum workpieces 10, 12, although, as described above, this same method can be practiced on stack arrangements comprising additional steel and / or aluminum workpieces. Generally, the workpiece stack arrangement 80 is first prepared by applying the organic coating material to the butt surface 10" of the steel workpiece 10, the butt surface 12" of the aluminum workpiece 12, or both butt surfaces 10", 12", and subsequently assembling the steel and aluminum workpieces 10, 12 in an overlapping manner to form the stack arrangement 80 by jointly aligning and fitting the workpieces 10, 12 together with a suitable clamping device.Then the welding surface 22 of the radially slotted welding electrode 16 and the welding surface 26 of the accompanying second welding electrode, with their respective first and second sides 84, 86 (in this embodiment also the workpiece outer surfaces 10', 12') of the workpiece stack arrangement 80, are brought into contact with each other in a planar orientation, so that their respective welding surface central axes 28, 30 are collinearly aligned. This general setup prior to the electrical current exchange between the welding electrodes 16, 18 is generally described in . Fig. 1 shown. 1.

[0039] The term “planar alignment,” as used herein, does not necessarily mean that the weld surface center axes 28, 30 of the welding electrodes 16, 18 must be perfectly collinear, as will be obvious to professionals with practical experience in spot welding processes. While some lateral and angular misalignment may occur between the respective weld surfaces 22, 26 of the welding electrodes 16, 18 due to misalignment of the welding equipment or electrode wear, this does not impair the ability of the welding electrodes 16, 18 to function as intended. The degree of misalignment that can be tolerated between the opposing weld surfaces 22, 26 in any given resistance spot welding event depends on many factors, including the shapes and diameters of the weld surfaces 22, 26.Under many circumstances, and as a general but not mandatory rule, a slight angular misalignment of 3° or less and / or a slight lateral misalignment of 2.5 mm or less can be tolerated between the welding surface 22, 26 of the welding electrodes 16, 18 during the resistance spot welding of the workpiece stack arrangement 80 according to the practical embodiments of the present invention.

[0040] Once in position, the welding surfaces 22, 26 of the welding electrodes 16, 18 are pressed directly against their respective first and second sides 84, 86 of the workpiece stack arrangement 80 to achieve a predetermined clamping load between the welding surfaces 22, 26. The clamping load applied by the welding electrodes 16, 18 typically ranges from 400 lb to 2000 lb or, more narrowly, from 600 lb to 1300 lb and can be increased or decreased within this range based on factors such as the thickness of the workpieces 10, 12, their composition, and the surface area of ​​the plateau 52.Because of its position and displacement from the surrounding convex dome section 42, the plateau surface 52 of the central upright plateau 40 engages in the first side 84 of the workpiece stack arrangement 80 before any section of the surrounding convex dome section 42 engages in the same side 84 of the stack arrangement 80 with the transversely directed, upright, curved ribs 48 of the trapezoidal weld surface sections 44.The plateau surface 52 initially supports the entire clamping load and exerts a concentrated pressure through the aluminum workpiece 12 on the intervening layer 14 of organic material within the weld zone between the facing weld surfaces 22, 26. This causes the layer 14 of organic material to be compressed and initially ejected laterally along the impact interface 84, bringing the opposing workpiece impact surfaces 10", 12" into more direct and closer contact. It is assumed that this initial squeezing and lateral ejection of organic material significantly reduces the volume of organic matter that is ultimately exposed to the elevated temperatures reached during the current flow.

[0041] Shortly after the welding surfaces 22, 26 of the welding electrodes 16, 18 are pressed against the workpiece stack 80, an electric current is exchanged between the welding electrodes 16, 18 and, more specifically, between the sections of the welding surfaces 22, 26 that are in pressed contact with the first and second sides 84 and 86, respectively, of the stack 80. The magnitude of the electric current can range from 5 kA to 50 kA, and the duration of the current flow can range from 40 ms to 2500 ms, or more narrowly, from 200 ms to 1000 ms. The electric current rapidly heats the electrically and thermally more resistant steel workpiece 10 to a temperature above the melting point or range of the adjacent section of the aluminum workpiece 12.Heat from the steel workpiece 10 is transferred to the adjacent section of the aluminum workpiece 2 to create a weld pool contained within the aluminum workpiece 12, which wets the adjacent butt surface 10" of the steel workpiece 10. Such wetting of the steel workpiece butt surface 10" with molten aluminum tends to cause molten aluminum to react with or dissolve iron from the steel workpiece 10, while simultaneously growing a brittle intermetallic layer along the butt surface 10" of the steel workpiece 10, which may include FeAl3 compounds, Fe2Al5 compounds, and possibly other intermetallic Fe-Al compounds.

[0042] During current flow, the welding surface 22 of the radially slotted welding electrode 16 displaces the intervening layer 14 of organic material further laterally away from the welding zone. Specifically, when the welding surface 22 of this welding electrode 16 initially engages with and is pressed against the first side 84 of the workpiece stack arrangement 80, only the plateau surface 52 is in contact with the stack arrangement 80, as just described. As the electric current heats the steel workpieces 10 and the steel workpiece 10 heats the aluminum workpiece 12, the welding surface 22 presses further into the first side 84 of the workpiece stack arrangement 80, bringing the convex dome section 42 of the welding surface 22 into pressed engagement with the first side of the stack arrangement 80 and, in particular, with the trapezoidal welding surface sections 44 that encompass the transversely oriented, curved ribs 48.These ribs 48 help to establish good mechanical and electrical contact with the first side 84 of the workpiece stack arrangement 80. This is particularly true if the first side 84 of the stack arrangement 80 comprises a surface layer of a refractory oxide material such as those commonly found on the surfaces of aluminum workpieces, since the upright, curved ribs 48 can help to penetrate such insulating and mechanically hard surface layers.

[0043] As the convex dome section 42 progressively presses further into the first side 84 of the workpiece stack arrangement 80 and the upright, curved ribs 48 of the trapezoidal weld surface sections 46 are successively brought into contact with the first side 84, the annular channel 54 surrounding the central plateau 40 and a plurality of radial slots 56 communicating with the circular channel 54 influence the pressure distribution on the intermediate layer 14 of organic material at the impact interface 84.This means that the annular channel 54 and a plurality of radial slots 56 do not transmit the applied clamping load to the first side 84 of the workpiece stack arrangement 80, and consequently the corresponding sections of the butt surface 12" of the aluminium workpiece 12, which mimic the channel and slot geometry of the welding surface 22, will experience less pressure from the welding electrode 16 compared with parts corresponding to the central, upright plateau 40 and the trapezoidal welding surface sections 46.These low-pressure regions of the impact surface 12" result in the development of radially slotted low-pressure flow paths between the workpiece impact surfaces 10", 12", through which the intervening layer 14 of organic material can be displaced further laterally away from the weld zone as the spot welding process progresses and the welding surface of the radially slotted welding electrode 22 continues to press into the workpiece stack arrangement 80. In other words, the radially slotted welding electrode 16 is designed such that it does not trap organic material within the weld zone at the impact interface 84, but rather facilitates its ejection while maintaining spot welding functionality.

[0044] The radially slotted welding electrode 16 can also offer other advantages. In particular, if an electric current is first initiated between the welding electrodes 16, 18, at least a large portion of the electric current flows through the central, upright plateau 40, provided that the plateau 40 is initially in sole contact with the first side 84 of the workpiece stack arrangement 80. In this respect, heating is initiated in and around the central upright plateau 40, and any residual organic material located at the interface 84 in a region mirroring the annular channel 54 of the weld surface 22 is exposed to the heat first. This residual organic material can release gaseous products during thermal decomposition.The radially slotted low-pressure flow paths developed between the workpiece butt surfaces 10", 12" allow these gaseous decomposition products to escape and, if necessary, pressurize organic residue material and expel it further from the weld zone as the escaping gases flow through the low-pressure flow paths. As such, the design of the weld surface 22 of the radially slotted welding electrode 16 and its tendency to drive organic material and gaseous thermal decomposition products away from the weld zone reduce or eliminate the overall amount of thermal residue that can be exposed to the weld pool and ultimately find its way into the weld joint as imbalances and defects that weaken the weld.

[0045] After the electric current flow between the welding surfaces 22, 26 of the welding electrodes 16, 18 has ceased, and while the welding electrodes 16, 18 are still engaged with their respective sides 84, 86 of the workpiece stack arrangement 80, the weld pool generated within the aluminum workpiece 12 solidifies to form a weld 64, as shown in Fig. Figure 8 shows that the weld 64 has a bonding interface 66 with the abutment surface 10" of the steel workpiece 12 and generally comprises a lens 68 of resolidified aluminum workpiece material, typically having a nominal diameter in the range of 6 mm to 8 mm, and an intermetallic layer located along a bonding interface 66 of the weld 64 and the steel workpiece 10, which may include FeAl3 compounds, Fe2Al5 compounds, and possibly other Fe-Al intermetallic compounds. As shown, the weld 66 can penetrate the entire aluminum workpiece 12 and typically has a truncated cone cross-sectional shape.

[0046] The bonding interface 66 between the weld joint 64 and the butt surface 10" of the steel workpiece 12 can be bent inwards into the weld joint 66, at least partially, due to the structure of the weld surface 22 of the radially slotted welding electrode 16. Since the electric current is initially conducted mainly through the central, upright plateau 40 of the radially slotted welding electrode 16, the progressive heating and the resulting softening of the steel and aluminum workpieces 10, 12 effectively allow both welding electrodes 16, 18 to deform the workpieces 10, 12, as larger sections of the weld surfaces 22, 26 are involved in the conduction of the electric current. For example, in Fig. As shown in Figure 8, the welding surface 22 of the radially slotted welding electrode 16 ends fully recessed into the first side 84 of the stack arrangement 80, while, in contrast, the welding surface 26 of the second welding electrode 18 experiences a smaller indentation, but the workpiece 10 is locally deformed in such a way that the second side 86 of the stack arrangement 80 deforms and wraps around the welding surface 36, so that the welding surface 36 is fully involved in the exchange of electric current. The physical deformation of the steel workpiece 10 ultimately results in the curved entry of its contact surface 10" into the weld pool and ultimately its arrival at the curved bonding interface 66.

[0047] The development of low-pressure regions on the contact surface 12" of the aluminum workpiece 12, which manifest themselves as radially slotted low-pressure flow paths between the workpiece contact surfaces 10", 12", can generally be achieved despite the special design of the welding surface 26 of the second welding electrode 18. In some cases, however, the design of the welding surface 26 of the second welding electrode 18 can increase the effectiveness of the radially slotted low-pressure flow paths. For example, if the second welding electrode 18 is designed as in Fig. As shown in Figure 4, it may be desirable to provide the innermost rib 60', which directly surrounds the weld surface axis 30, with a smaller diameter than that of the plateau surface 52 of the central, upright plateau 40. In this scenario, the innermost circular, upright rib 60' on the weld surface 26 of the second welding electrode 18 will first make contact with the second side 86 of the workpiece stack arrangement 80, resulting in the clamping pressure being transferred between the plateau surface 52 of the central, upright plateau 40 on one side and the innermost circular rib 60' on the other. In this way, the intermediate layer 14 of organic material experiences more concentrated pressure from both the steel workpiece 10 and the aluminum workpiece 12.Such a concentrated application of pressure through both workpiece contact surfaces 10", 12" will, as the clamping force increases, displace and squeeze out a very significant volume of the intervening layer 14 of organic material laterally from the weld zone. Furthermore, with continued indentation of the weld surfaces 22, 26, an additional sequential outward radial displacement of organic material from the weld zone will occur.

[0048] While the innermost circular rib 60' on the welding surface 26 of the second welding electrode 18 preferably has a smaller diameter than the plateau surface 52 of the central upright plateau 40 on the welding surface 22 of the radially slotted welding electrode 16, such a relationship is not the only practical option. If, for example, the innermost circular rib 60' has a larger diameter than the plateau surface 82 and is thus located radially outside the central, circular plateau 40 when the two welding surfaces 22, 26 are aligned, the low-pressure regions on the butt surface 12" of the aluminum workpiece 12, which manifest themselves as radially slotted low-pressure flow paths between the workpiece butt surfaces 10", 12", are further developed to facilitate the lateral displacement of the intervening layer 14 of organic material and the outgassing of gaseous thermal decomposition products.This relationship between the weld surfaces 22, 26 is further supported by the fact that, due to their stiffnesses, the aluminium workpiece 12 and the steel workpiece 10 are physically deformed as opposed to bending, which contributes to the squeezing out or lateral ejection of the intermediate layer 14 of organic material.

[0049] For similar reasons to those just discussed, the second welding electrodes 218, 318, which are in the Fig.Figures 6-7 illustrate how the operation and functionality of the radially slotted welding electrode 16 are supported when its basic welding surface surfaces 258, 358 are planar. However, the basic welding surface surfaces 268, 358 of these electrode welding surfaces 226, 326 are preferably convexly profiled. When the basic welding surface surfaces 258, 358 are profiled in this way, each of these surfaces has a central location (similar to the basic welding surface surface 126 of the ball-nose welding electrode 118) that establishes initial contact with the second side 86 of the workpiece stack arrangement 80. This results in the clamping pressure being transferred between the plateau surface 52 of the central, upright plateau 40 on one side and the central location of the basic welding surface surface 258, 358 of the second welding electrode 218, 318 on the other side.In this way, the intervening layer 14 of organic material experiences more concentrated pressure from both the steel workpiece 10 and the aluminum workpiece 12. Such concentrated pressure application by both workpiece contact surfaces 10", 12" will, as the clamping force increases, displace and squeeze out a very significant volume of the intervening layer 14 of organic material laterally from the weld zone. Furthermore, with continued indentation of the weld surfaces 22, 226, 326, an additional sequential outward radial displacement of organic material from the weld zone will occur.

Claims

[1] Welding electrode arrangement with: a first welding electrode (16), comprising: a body (20); and a weld surface (22) for pressing against a first side (84) of a workpiece stack arrangement (80) supported by an end of the body (20), wherein the weld surface (22) comprises a central upright plateau (40) with a plateau surface (52) and a convex dome section (42) surrounding the central upright plateau (40); and a second welding electrode (18), comprising: a welding surface (26) for pressing against a second side (86) of the workpiece stack arrangement (80), wherein the welding surface (26) of the second welding electrode (18) is aligned with the welding surface (22) of the first welding electrode (16) when the welding surfaces (22, 26) of the first and second welding electrodes (16, 18) are pressed against the respective first and second sides (84, 86) of the workpiece stack arrangement (80); wherein the welding surface (26) of the second welding electrode (18) comprises a convex basic welding surface (58) and a plurality of upright circular ribs (60) projecting outwards from the convex basic welding surface (58), such that the upright circular ribs (60) are separated by intervening circular sections (62) of the convex basic welding surface (58); characterized by , that the convex dome section (42) of the welding surface (22) of the body (20) of the first welding electrode (16) further comprises a base surface (44) and a plurality of trapezoidal welding surface sections (46) spaced circumferentially on the base surface (44) around the central upright plateau (40), each of the trapezoidal welding surface sections (44) comprising a plurality of transversely directed, upright, curved ribs (48) spaced radially apart along the base surface (44) of the convex dome section (42); wherein a plurality of radial slots (56) communicate with an annular channel surrounding the central upright plateau (40) and extend outwards in the direction of an outer circumference of the base surface (44) of the convex dome section (42), each of the plurality of radial slots (56) extending between two adjacent trapezoidal weld surface sections (44); wherein the diameter of the innermost upright circular rib (60') of the welding surface (26) of the second welding electrode (18) is smaller than the diameter of the plateau surface (52) of the central upright plateau (40) of the welding surface (22) of the first welding electrode (16). [2] Welding electrode arrangement according to claim 1, wherein each of the trapezoidal welding surface sections (44) comprises two to ten transversely directed, upright, curved ribs (48), and wherein each of the transversely directed, upright, curved ribs (48) in each of the trapezoidal welding surface sections (44) has a rib height in a range of 20 µm to 400 µm and is separated from each of its adjacent ribs (48) by a radial spacing of 50 µm to 1800 µm. [3] Welding electrode arrangement according to claim 1, wherein the plateau surface (52) of the central upright plateau (40) is positively displaced over the surrounding base welding surface surface (58) of the convex dome section (42), such that the plateau surface (52) is raised over an innermost, transversely directed, upright, curved rib of each of the plurality of trapezoidal welding surface sections (44).

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