METHOD FOR JOINING ADJUSTING OVERLAPING COPPER WORKPIECES BY RESISTANCE SPOT WELDING

Abstract

Method for joining adjacent overlapping copper workpieces (10, 12) by resistance spot welding, the method comprising: Providing a stack of workpieces (14) comprising a first copper workpiece (10) and a second copper workpiece (12) arranged adjacent to the first copper workpiece (10), wherein the first copper workpiece (10) has a first mating surface (18) and the second copper workpiece (12) has a second mating surface (20) opposite the first mating surface (18) to form a mating surface (22), wherein the first mating surface (18) includes a boss (24) projecting beyond a surrounding base surface (26) of the first mating surface (18) and contacting the second mating surface (20), and wherein the second mating surface (20) also includes a boss (124) projecting beyond a surrounding base surface (126) of the second mating surface (20); Applying a compressive force (86) to the first and second copper workpieces (10, 12) which presses the bump (24) of the first mating surface (18) onto the second mating surface (20); Briefly passing an electric current through the first and second copper workpieces (10, 12), wherein the electric current initially flows through the bump (24) present on the first mating surface (18) to generate and concentrate heat within the bump (24) before the bump (24) collapses to bring the first mating surface (18) and the second mating surface (20) into greater interfacial contact along the mating surface (22) of the first and second copper workpieces (10, 12), and wherein, upon cooling of the first and second copper workpieces (10, 12), a metallurgical connection (90) is formed between the first and second copper workpieces (10, 12) via their mating surface (22) at a point where the bump (24) has collapsed.

Description

[0001] Resistance spot welding relies on the resistance to the flow of an electric current through the bulk material and across the interface between overlapping metal workpieces to generate and concentrate heat. The generated heat typically melts the overlapping metal workpieces into a molten pool, which is drawn from each workpiece. This molten pool extends into each of the adjacent metal workpieces, consuming the interface. After the electric current ceases, the molten pool solidifies into a weld nugget, which welds the adjacent, overlapping metal workpieces together through fusion welding.In practice, the instantaneous flow of electric current through the overlapping metallic workpieces is guided by a set of spot welding electrodes clamped against opposing exposed surfaces of the metal workpieces, ensuring that the welding surfaces of the electrodes are aligned at the weld point. Resistance spot welding has long been used for fusion welding stacks of steel workpieces and, more recently, for stacks of aluminum alloys. Resistance spot welding is a popular joining method for these metals because it can be carried out relatively quickly and cost-effectively compared to other joining methods.

[0002] Resistance spot welding was traditionally considered an impractical method for joining copper workpieces. The high electrical conductivity of copper makes it difficult to generate resistance heat in the copper workpieces along the current flow between the opposing spot welding electrodes. To be certain, the electrical conductivity of pure annealed copper at 20 °C is 5.80 × 10⁻⁶. 7S / m, and this value is used as the standard (100% IACS) to express the electrical conductivity of other materials as a percentage. In comparison, aluminum alloys are at least 35% less electrically conductive than copper, and steels are even less electrically conductive than aluminum alloys. Even the copper alloys commonly used to manufacture spot welding electrodes are more electrically resistant than copper. And as long as sufficient current can be passed through overlapping copper workpieces to generate some resistance heat without thermally damaging the electrodes, this heat is rapidly dissipated due to copper's correspondingly high thermal conductivity. Passing a large current through overlapping copper workpieces therefore generally results in a relatively uniform heating of the copper workpieces over an extended area, as opposed to heat concentration between the electrodes.In reality, it is more likely that the spot welding electrodes made of a copper alloy will be welded to their respective copper workpieces by the supply of a large electric current than that the copper workpieces will be welded together via their interface.

[0003] When joining copper workpieces, the choice is usually between soldering, arc welding (such as MIG welding (GMAW), TIG welding (GTAW), and plasma welding (PAW), or electron beam welding. While each of these currently available methods can work under certain circumstances, each has a general disadvantage compared to resistance spot welding, namely the need to heat the joint from an external source (e.g., a flame, an arc, a beam of high-speed electrons, etc.). The heat supplied by the external source is intense enough to melt the copper workpieces or filler at the joint. However, the applied heat is difficult to control precisely and often results in excessive heat input into the joint, making it difficult to avoid thermal damage to any heat-sensitive materials that may be present nearby.The ability to control resistance heat generation in overlapping copper workpieces in a targeted manner, so that resistance spot welding can be used as a practical joining method, would give a high degree of flexibility to manufacturing processes that require a copper-copper connection and would be particularly valuable where it is a matter of protecting nearby heat-sensitive materials.

[0004] JP 2002-346 757 A discloses a projection welding process in which a copper connection to be welded, pressed into a resin molded part, and a copper welding terminal are placed one on top of the other and projection welded. A weld nugget is formed between the connection to be welded and the welding terminal when the size of the connection to be welded protruding from the resin molded part, the size of the welding terminal, and the size of the projection are set to a predetermined size and projection welded.

[0005] It can be seen as a task to increase the heat concentration within a joint in order to bring a first and second mating surface into a wider interfacial contact along the common mating surface.

[0006] A method for joining adjacent, overlapping copper workpieces by resistance spot welding can involve several steps. In one step, a stack of workpieces is provided, comprising a first copper workpiece and a second copper workpiece adjacent to the first. The first copper workpiece has a first mating surface, and the second copper workpiece has a second mating surface opposite the first mating surface to form a mating surface. Furthermore, the first mating surface includes a projection that extends beyond a surrounding base surface of the first mating surface and contacts the second mating surface. In a further step, a compressive force is applied to the first and second copper workpieces, pressing the projection of the first mating surface onto the second mating surface. In a further step, an electric current is passed through the first and second copper workpieces.The electric current initially flows through the bump on the first mating surface to generate and concentrate heat before the bump collapses, bringing the first and second mating surfaces into wider interfacial contact along the mating surface of the first and second copper workpieces. After the first and second copper workpieces have cooled, a metallurgical bond is formed between them via their mating interface at the point where the bump collapsed.

[0007] The second mating surface of the second copper workpiece also includes a boss that projects beyond a surrounding base surface of the second mating surface. Furthermore, the boss of the first mating surface and the boss of the second mating surface can come into contact and be pressed against each other when a compressive force is applied to the first and second copper workpieces. In this scenario, conducting an electric current through the first and second copper workpieces can initially cause the current to flow through the bosses to generate and concentrate heat before the boss collapses, bringing the first mating surface and the second mating surface into wider interfacial contact along the mating surface of the first and second copper workpieces. As another example, an intermediate reactive material can be placed between the boss of the first mating surface and the second mating surface.The intermediate reaction material can be a copper alloy containing between 2.0 wt.% and 10 wt.% phosphorus.

[0008] Furthermore, the method of the aforementioned embodiment can include the step of supplying an electric current to a capacitor bank to store energy in the capacitor bank and the step of discharging the energy stored in the capacitor bank to provide the electric current currently being passed through the first and second copper workpieces. In addition, each of the first and second copper workpieces can comprise 99.9 wt.% copper or more. In a further implementation, the method of the aforementioned embodiment can include the step of forming the bump on the first mating surface before the first copper workpiece and the second copper workpiece are joined to form the workpiece stack. The formation of the bumps on the first mating surface can involve mechanical deformation of the first copper workpiece to displace material from the first copper workpiece into the bumps.Or, in another technique, the formation of the protrusions on the first mating surface can involve applying a copper material to the first copper workpiece to form the protrusions without mechanically deforming the first copper workpiece.

[0009] Another method for joining adjacent, overlapping copper workpieces by resistance spot welding can involve several steps. In one step, at least one projection can be formed on a first mating surface of a first copper workpiece. This projection can extend beyond a surrounding base surface of the first mating surface. In another step, a stack of workpieces can be assembled, comprising the first copper workpiece and a second copper workpiece adjacent to the first. The projection on the first mating surface of the first copper workpiece can contact a second mating surface of the second copper workpiece. In yet another step, a compressive force can be applied to the first and second copper workpieces, pressing the projection on the first mating surface onto the second mating surface.In a further step, the energy stored in a capacitor bank can be discharged to provide an electric current that rises to a peak current between 30 kA and 80 kA within a period of 2.0 ms to 10 ms and then falls to below 1 kA over a period of 5.0 ms to 20 ms. In a further step, a metallurgical connection between the first and second copper workpieces can be established by passing the electric current through them. The electric current initially flows through the at least one bump located on the first mating surface to generate and concentrate heat within it before the bump collapses, bringing the first and second mating surfaces into wider interfacial contact.After the first and second copper workpieces have cooled, the metallurgical connection between the first and second copper workpieces is made via their interface at a point where at least one bump has broken in.

[0010] The method of the aforementioned embodiment can include various steps or be further defined. For example, each of the first and second copper workpieces can comprise 99.9 wt.% copper or more. The aforementioned method also includes the step of forming at least one projection on the second mating surface of the second copper workpiece. The at least one projection of the second mating surface extends beyond a surrounding base surface of the second mating surface. In this case, the step of assembling the workpiece stack can consist of bringing the at least one projection of the first mating surface and the at least one projection of the second mating surface into contact with each other, such that the application of the compressive force to the first and second copper workpieces presses the projections against each other.The step of passing electric current through the first and second copper workpieces also initially involves the flow of electric current through the protrusions to generate and concentrate heat within the protrusions before the protrusions collapse.

[0011] Furthermore, the aforementioned method can include the step of introducing an intermediate reaction material between the first and second mating surfaces, such that the intermediate reaction material is positioned between the bump of the first and second mating surfaces when the first and second copper workpieces are assembled into the workpiece stack. The intermediate reaction material is a copper alloy containing between 2.0 wt.% and 10 wt.% phosphorus. Alternatively, the intermediate reaction material can be a Cu-P-Ag alloy containing copper and between 3 wt.% and 8 wt.% phosphorus and between 0.1 wt.% and 20 wt.% silver.

[0012] Furthermore, the step of forming at least one boss on the first copper workpiece can involve punching the first copper workpiece, whereby a die is driven into the first copper workpiece from the opposite back side to displace the material of the first copper workpiece into the boss. Alternatively, the step of forming at least one boss on the first copper workpiece can involve applying a copper material to the first copper workpiece to form the boss without mechanically deforming the first copper workpiece.

[0013] Another method for joining adjacent, overlapping copper workpieces by resistance spot welding can involve several steps. In one step, at least one projection can be formed on a first mating surface of a first copper workpiece. This projection can extend beyond a surrounding base surface of the first mating surface. In a further step, a stack of workpieces is assembled, comprising the first copper workpiece and a second copper workpiece adjacent to the first. The projection on the first mating surface of the first copper workpiece can come into contact with a second mating surface of the second copper workpiece through an intervening reaction material. This intervening reaction material can be a copper alloy containing between 2.0 wt.% and 10 wt.% copper.The copper contains -% phosphorus and has an electrical conductivity lower than that of the first and second copper workpieces. In a further step, a compressive force is applied to the first and second copper workpieces, pressing at least one bump of the first mating surface onto the second mating surface. In yet another step, the energy stored in a capacitor bank is discharged to provide an electric current that rises to a peak current between 30 kA and 80 kA within a period of 2.0 ms to 10 ms and then falls to below 1 kA over a period of 5.0 ms to 20 ms. In a further step, the electric current is passed through the first and second copper workpieces.The electric current can initially flow through the at least one bump located on the first mating surface to generate and concentrate heat within it. This heat is then released, bringing the first and second mating surfaces into wider contact. The heat generated within the bump can cause the intervening material layer to liquefy and be displaced laterally between the first and second mating surfaces. After the first and second copper workpieces have cooled, a metallurgical bond can be formed between them at their interface, specifically at the point where the bump has collapsed.

[0014] The method of the aforementioned embodiment can include various steps or be further defined. For example, the method can include forming at least one bump on the second mating surface of the second copper workpiece. The at least one bump of the second mating surface can project beyond a surrounding base surface of the second mating surface. In this case, the step of assembling the workpiece stack can consist of bringing the at least one bump of the first mating surface and the at least one bump of the second mating surface into indirect contact with each other through the intervening layer of reaction material, such that applying the compressive force to the first and second copper workpieces presses the projections against each other.Furthermore, the step of passing the electric current through the first and second copper workpieces can initially allow the electric current to flow through the at least one bump of the first mating surface and the at least one bump of the second mating surface to generate and concentrate heat within the projections before the two projections collapse to bring the first mating surface and the second mating surface into a wider interfacial contact.

[0015] In another example, the method of the aforementioned embodiment can include various steps. In one of these additional steps, the first copper workpiece can be brought into contact with a first spot welding electrode made of a copper alloy. Likewise, in a further step, the second copper workpiece can be brought into contact with a second spot welding electrode made of a copper alloy, which is aligned with the first spot welding electrode made of a copper alloy. Furthermore, the compressive force exerted on the first and second copper workpieces is applied by pressing the first and second spot welding electrodes made of a copper alloy against the first and second copper workpieces, respectively.Furthermore, the electric current can be passed through the first and second copper workpieces by exchanging the electric current between the first and second copper alloy welding electrodes located on opposite sides of the workpiece stack. Fig. 1 is a schematic representation of a stack of workpieces comprising a first copper workpiece and an adjacent overlapping second copper workpiece, and an embodiment of a resistance spot welding device capable of welding the first and second copper workpieces together, wherein at least one first mating surface of the first copper workpiece includes at least one bump according to an embodiment of the present disclosure; Fig. 2 is an enlarged view of the in Fig. 1 illustrated stack of workpieces according to an embodiment of the present disclosure and isolated from the resistance spot welding device; Fig. 3 is a generalized representation of a stamping process which, according to an embodiment of the present disclosure, can be used to form the at least one bump on the first mating surface (or the first and second mating surfaces) by mechanically deforming and displacing the material of the first copper workpiece (or the first and second copper workpiece); Fig. Figure 4 is a generalized representation that depicts a further stamping operation which, according to an embodiment of the present disclosure, can be used to form the at least one bump on the first mating surface (or the first and second mating surfaces) by mechanically deforming and displacing the material of the first copper workpiece (or the first and second copper workpiece); Fig. Figure 5 is a generalized representation which represents the deposition of a copper material according to an embodiment of the present disclosure in order to form the at least one bump on the first mating surface (or the first and second mating surfaces) without mechanically deforming the first copper workpiece (or the first and second copper workpiece); Fig. Figure 6 is a top view of the first mating surface of the first copper workpiece, which includes a plurality of projections according to an embodiment of the present disclosure; Fig. Figure 7 shows the first and second spot welding electrodes of the welding gun, which are pressed against the opposite sides of the stack to exert a pressure force on the workpieces during joining according to an embodiment of the present disclosure; Fig. Figure 8 shows a possible welding plan, which is combined with the one in Fig. 1 resistance spot welding device shown according to an embodiment of the present disclosure can be used, wherein the y-axis is the current level in kiloamperes (kA) and the x-axis is the time in milliseconds (ms); Fig. Figure 9 shows the first and second spot welding electrodes of the welding gun, which are pressed against the opposite sides of the stack to apply a pressure force to the workpieces at the joining point, after the at least one bump on the first mating surface (or the first and second mating surfaces) has collapsed and the first and second mating surfaces have been brought into wider interfacial contact according to a representation of the present disclosure; Fig. Figure 10 shows the first and second copper workpieces and a metallurgical connection that was made between the first and second copper workpieces via their fitting interface at the joining point, where the at least one bump was initially present and later collapsed according to an embodiment of the present disclosure; Fig. Figure 11 is a schematic representation of a stack of workpieces comprising a first copper workpiece and an adjacent overlapping second copper workpiece, and a further embodiment of a resistance spot welding device capable of welding the first and second copper workpieces together, wherein at least one first mating surface of the first copper workpiece includes at least one bump according to an embodiment of the present disclosure; Fig. Figure 12 illustrates a possible welding plan that is combined with the one in Fig. 11 can be used in the resistance spot welding device shown in an embodiment of the present disclosure, wherein the y-axis is the current level in kiloamperes (kA) and the x-axis is the time in milliseconds (ms); Fig. Figure 13 shows the first and second copper workpieces from the same perspective as Fig. 2, wherein the second mating surface of the second copper workpiece also includes at least one bump together with the first mating surface according to an embodiment of the present disclosure; Fig. Figure 14 shows the first and second copper workpieces from the same perspective as Fig. 2 and an intermediate reaction material arranged between the first mating surface and the second mating surface to facilitate the production of the metallurgical compound according to an embodiment of this disclosure; and Fig. Figure 15 shows the first and second copper workpieces from the same perspective as Fig. 2 and an intermediate reaction material arranged between the first mating surface and the second mating surface to facilitate the production of the metallurgical compound according to a further embodiment of the present disclosure.

[0016] A method for resistance spot welding of copper workpieces is disclosed, despite the high electrical and thermal conductivities of copper. The term "resistance spot welding" is used in connection with the currently known method to essentially denote processes in which an electric current is passed through overlapping copper workpieces to resistively generate the heat required to join the copper workpieces via their mating interface. The resulting heat can melt the copper workpieces, thus creating a fused joint, or it can merely soften the copper workpieces to form a solid-state joint. As part of the disclosed method, a projection is provided on a mating surface of at least one pair of adjacent, overlapping copper workpieces.The boss extends beyond the surrounding base section of the mating surface and serves to initially concentrate the current flow through the copper workpieces, resulting in a relatively rapid and targeted increase in temperature. This rapid temperature increase is sufficient to form a metallurgical bond between the copper workpieces—either the solid or the molten form—while maintaining an overall low energy input. Furthermore, in certain embodiments, an intermediate reactive material can be placed between the copper workpieces to facilitate the formation of the metallurgical bond, and / or the electric current can be conducted through the copper workpieces via capacitive discharge.

[0017] Now, with reference to the Fig. Figure 1-10 illustrates a method for resistance spot welding a first copper workpiece 10 and a second copper workpiece 12. A workpiece stack 14 is provided, containing the first copper workpiece 10 and the second copper workpiece 12 for resistance spot welding with a resistance spot welding device 16. In the assembled state within the workpiece stack 14, the first copper workpiece 10 and the second copper workpiece 12 are adjacent to each other and overlap at least partially. The first copper workpiece 10 includes a first mating surface 18, and the second copper workpiece 12 includes a second mating surface 20, which is opposite the first mating surface 18. The overlapping and opposite mating surfaces 18, 20 form a mating interface 22 between the first and second copper workpieces 10, 12.The interface 22 thus refers largely to the opposing first and second mating surfaces 18, 20 of the first and second copper workpieces 10, 12, although, as will be explained in more detail below, parts of these surfaces 18, 20 can initially be separated from each other by a gap before an electric current flows through the copper workpieces 10, 12. Each of the first and second copper workpieces 10, 12 can have a thickness of 101, 121, which is in the range of 1.0 mm to 4.0 mm.

[0018] Each of the first and second copper workpieces 10, 12 can consist of unalloyed copper containing at least 99.3 wt.% copper and preferably at least 99.9 wt.% copper, the remainder being acceptable impurities. Some examples of suitable unalloyed copper compositions are, for instance, certain forged and cast coppers. Suitable forged coppers are those designated C10100-C15760, which include, in particular, oxygen-free copper, oxygen-containing copper, phosphorus-deoxidized copper, and free-working copper. Suitable cast coppers are those designated C80100-C81200. In addition, each of the first and second copper workpieces 10, 12 can consist of a high-copper alloy containing between 94 wt.% and 99.2 wt.% copper, the remainder being one or more alloying elements.The one or more alloying elements may be selected from Cr, Zr, Mg, Fe, P, Be, Co, S, Te, and Pb. Suitable forged high-grade copper alloys are usually designated C16200-C19750, and suitable cast high-grade copper alloys as C81300-C82800. The term "copper workpiece," as used herein, thus includes a workpiece made of unalloyed copper or a high-grade copper alloy. The first and second copper workpieces 10, 12 may both be made of unalloyed copper, both of a high-grade copper alloy, or one of the copper workpieces 10, 12 may be made of unalloyed copper while the other copper workpiece 10, 12 is made of a high-grade copper alloy.

[0019] The first copper workpiece 10 can include at least one bump 24 on the mating surface 18, as shown in Fig. Figure 2 best illustrates this. The boss 24 projects beyond a surrounding base surface 26 of the first mating surface 18 and contacts the second mating surface 20 directly or indirectly, creating a gap 28 between the first and second mating surfaces 18, 20 on the outside of the boss 24. Direct contact between the boss 24 and the second mating surface 20 occurs when there is physical contact between the boss 24 and the second mating surface 20. Indirect contact between the boss 24 and the second mating surface 20 occurs when there is an intervening material between the boss 24 and the second mating surface 20, but there is contact continuity (i.e., no gap) between the boss 24 and the intervening material and the second mating surface 20. The boss 24 can assume a variety of shapes and contours, as shown here in Fig. As shown in Figure 2, the boss 24 can, for example, be hemispherical. In further embodiments, and as described in more detail below, the boss 24 can, however, be spherical, pyramidal, conical, frustoconical, ringed, or any other conceivable shape that extends beyond the surrounding base surface 26 and is capable of creating the gap 28 between the first and second mating surfaces 18, 20 outside the boss 24. In certain applications, the boss 24 can project beyond a surrounding base surface 26 of the first mating surface 18 to a height 241 of at least 0.5 mm. More narrowly defined, the boss 24 can have a convex dome with a height 241 between 0.5 mm and 2.5 mm and a diameter along the surrounding base surface 26 of 0.5 mm to 2.0 mm.

[0020] The boss 24 can be formed in the first mating surface 18 in numerous ways. More precisely, and depending on several factors, including the desired shape and contour, the boss 24 can be formed by mechanically deforming the first copper workpiece 10, by applying copper material to the copper workpiece 10, or by other means. Regarding the first option, the first copper workpiece 10 can be mechanically deformed to displace the material of the first copper workpiece 10 into the boss 24. As shown in Fig. As shown in Figure 3, the boss 24 can, for example, be formed by a stamping process. During stamping, the first copper workpiece 10 is supported on a carrier substrate 30, such as an anvil, which has a recess 32 with a shape complementary to the boss to be formed. A punch 34 is driven into the first copper workpiece 10 from a rear surface 36. The punch 34 presses into the first copper workpiece 10 and plastically deforms it, while the material of the first copper workpiece 10 is forced into the recess 32. In this process, the material of the copper workpiece is displaced over the base surface 26 of the first mating surface 18 on the opposite side of the workpieces 10 in the shape of the punch 34 to produce the boss 24.The die 34 is then withdrawn, leaving an impression 38 on the reverse 36 behind the boss 24, while the first copper workpiece 10 is removed from the support substrate 30.

[0021] The boss 24 can also be formed in other ways, depending on the mechanical deformation of the first copper workpiece 10 and the displacement of the material into the boss 24. In an alternative approach, as in Fig. As shown in Figure 4, the same punching process as described above can be carried out, except that there is no recess within the support substrate 30'. In this way, the back surface 36' of the first copper workpiece 10 is supported on the support substrate 30', and the punch 34 is driven into the first mating surface 18. The punch 34 presses into the first copper workpiece 10 as before and plastically deforms it; however, here, material from the copper workpiece 10 is displaced upwards and around the punch 34 in the opposite direction to the force applied by the punch 34, since the back surface 36' of the workpiece 10 is completely supported against the support substrate 30'. As a result, the material of the copper workpiece is displaced beyond the base surface 26 of the first mating surface 18 in the form of a protruding ring surrounding a penetrating recess 40.Other methods can also be used, although they are not shown here, such as mechanical upsetting.

[0022] As already mentioned, the boss 24 can also be formed by applying copper material to the copper workpiece 10. This option can be used if the projections 24 are simply to be applied to the first mating surface 18 without having to mechanically deform the first copper workpiece 10. For example, in Fig. As shown in Figure 5, a copper material 42 (either unalloyed copper or a high-copper alloy) can be applied to the first copper workpiece 10 such that it projects beyond the surrounding base surface 26 of the first mating surface 18 and forms the boss 24. The copper material 42 can assume a wide variety of shapes and contours and can be attached to the first copper workpiece 10 via an interface connection 44, such as a soldered joint or a solid-state connection. Several methods are available for applying the copper material 42 to the first copper workpiece 10 as a boss 24, including the cold metal transfer process by oscillating wire arc welding disclosed in US application 2016 / 0008911A1, the entire contents of which are incorporated herein by reference. As another example, the copper material 42 can be deposited by vacuum tack welding.Furthermore, there are other ways to form the bump 24 that do not rely on mechanical deformation or the deposition of the copper material 42. These other ways include laser scribing or chemical etching of the first copper workpiece 10 to form the bump 24 on the first mating surface 18.

[0023] As in Fig. As illustrated in Figure 6, more than one projection 24 can be formed on the first mating surface 18 of the first copper workpiece 10. In fact, a plurality of projections 24 can be present, each of which may be identical or different from the others in the plurality of projections 24. As shown here, for example, four projections 24 with similar shapes and contours can be formed on the first mating surface 18 in a diamond arrangement, although more or fewer projections 24 may well be present in a plurality of predetermined or random arrangements. In certain preferred applications, from two to ten, or somewhat narrower, from three to six, projections 24 can be formed on the first mating surface 18. All of the plurality of projections 24 can be formed independently on the first mating surface 18 by one of the methods described above.Furthermore, if all of the plurality of projections 24 are to be constructed identically, the first copper workpiece 10 can be mechanically deformed by rolling a cylindrical tool with a knurled surface over the first copper workpiece 10 to form the plurality of projections 24 on the first mating surface 18 in a single step, as opposed to individually forming the plurality of projections 24.

[0024] The disclosed method utilizes the at least one projection 24 formed on the first mating surface 18 of the first copper workpiece 10 to make the first and second copper workpieces 10, 12 resistance spot weldable. For this purpose, the workpiece stack 14 is prepared in the assembled state. After the workpiece 14 is stacked, a compressive force is applied to the first and second copper workpieces 10, 12, pressing the projection 24 of the first mating surface 18 against the second mating surface 20, either directly or through an intervening material, and an electric current is passed through the first and second copper workpieces 10, 12. This electric current initially flows through the projection 24 to generate and concentrate heat within the projection 24 relative to the first two copper workpieces 10, 12. The heat generated within the projection 24 ultimately causes the projection 24 to collapse under the applied compressive force.This brings the first mating surface 18 and the second mating surface 20 into wider interfacial contact along their mating interface 22, with the mating surfaces 18, 20 interacting either by sintering or by mixing molten copper. Finally, the conduction of the electric current through the first and second copper workpieces 10, 12 is stopped, allowing the workpieces 10, 12 to cool. After cooling, a metallurgical bond is established between the first and second copper workpieces 10, 12 via their mating interface 22 at the point where the boss 24 has collapsed.

[0025] Referring back to Fig. In Figure 1, the resistance spot welding device 16 shown is a special device suitable for carrying out the disclosed method. The resistance spot welding device 16 includes a welding gun 50 (partially shown), a power supply 52, a transformer 54, a rectifier 56, and a welding controller 58. The welding gun 50 can be mounted on a robot or on a stationary base. The welding gun 50 includes a first gun arm 60 and a second gun arm 62, which are mechanically movable towards and away from each other. These gun arms 60, 62 can be arranged in a C-configuration, an X-configuration, or another configuration. The first gun arm 60 holds a first spot welding electrode 64 with a first welding surface 66, and the second gun arm 60 holds a second spot welding electrode 68 with a second welding surface 70.Each of the first and second spot welding electrodes 64, 68 can be made of a copper alloy, such as a zirconium-copper alloy (ZrCu) containing approximately 0.10 wt.% to approximately 0.20 wt.% zirconium and copper as the balance material. Copper alloys meeting this composition and designated as C15000 are preferred. Other copper alloy compositions possessing suitable mechanical and electrical conductivity properties may also be used. A mechanism for cooling the first and second welding electrodes 64, 68 with water is also typically integrated into the clamping arms 60, 62 to manage the temperatures of the spot welding electrodes 64, 68 during operation.

[0026] The power supply 52, the transformer 54, and the rectifier 56 are electrically connected to the first and second spot welding electrodes 62 and 66, respectively, and supply the electrical current exchanged between the electrodes 64 and 68 during welding. The power supply 52 receives a three-phase AC mains current and supplies a high-voltage AC input current to power the transformer 54. For example, the AC mains current can first be rectified and then inverted within the power supply 52 to produce a single-phase AC input current, typically a higher-voltage square wave. The AC input current is fed to a primary winding 72 of the transformer 54, typically at 1000 Hz, generating a magnetic flux that induces a lower-voltage, higher-power AC current in a secondary winding 74 of the transformer 54.The alternating current in the secondary winding 74 is then supplied to the rectifier 56, where an array of semiconductor diodes converts the supplied alternating current into a low-frequency direct current, which can be supplied as electric current through the first and second spot welding electrodes 64, 68. The transformer 54 and the rectifier 56 can be combined into a single housing, such as a medium-frequency direct current welding machine (MFDC), which is available on the market from various sources such as ARO Welding Technologies (US headquarters in Chesterfield Township, Michigan) and Bosch Rexroth (US headquarters in Charlotte, North Carolina).

[0027] The welding controller 58 controls how the electric current is supplied between the first and second spot welding electrodes 64, 68. The welding controller 58 can be connected to the transformer 54 and allows the input of a welding schedule that defines and manages the waveform of the electric current exchanged between the spot welding electrodes 64, 68 during a welding operation. The welding controller 58 can be set to manage the electric current at a constant current level, or it can be set to manage the electric current as a series of current pulses that may exhibit constant or increasing peak currents over time.Depending on the composition of the first and second copper workpieces 10, 12, the thickness of the first and second copper workpieces 10, 12, the size and shape of the bump 24, the number of projections 24 and whether or not there is an intermediate layer of reaction material (see below) between the first and second copper workpieces 10, 12, the welding control 58 can execute any welding plans.

[0028] Still referring to Fig. 1. The disclosed method is carried out using the resistance spot welding device 16, by first providing the workpiece stack 14. The workpiece stack 14 can be provided by joining the stack 14, wherein the first and second copper workpieces 10, 12 can be joined together such that the workpieces 10, 12 and their respective mating surfaces 18, 20 are positioned opposite each other such that the at least one projection 24 on the first mating surface 18 (either directly or indirectly) comes into contact with the second mating surface 20 at a joint 76, at which a metallurgical connection is finally formed ( Fig. 10). The first and second copper workpieces 10, 12 can be assembled and held together as a workpiece stack 14 using suitable holding devices or other process hardware. After the workpiece stack 14 has been provided in its assembled state, it is positioned between the first and second spot welding electrodes 64, 68 relative to the welding gun 50 in preparation for welding. Depending on the design of the overall process, this may involve a robot movement of the welding gun 50 in spatial relation to the workpiece stack 14 or the transport of the workpiece stack 14 to the welding gun 50.

[0029] Once in position, the first and second spot welding electrodes 64, 68 are used to conduct the electric current through the workpiece stack 14 and across the interface 22 of the adjacent overlapping copper workpieces 10, 12 at the joining point 76. In this context, the welding gun 50 is operated to bring the first and second spot welding electrodes 64, 68 against opposite first and second sides 78, 80 of the workpiece stack 14, as shown in Fig. 7 best illustrated, to converge, with the first side 78 of the stack 14 being exposed by an exposed back face 82 of the first copper workpiece 10 (which is the same face as the back 36, 36' according to the Fig. 3-4) and the second side of the stack 14 is provided by an exposed back surface 84 of the second copper workpiece 12. In particular, the first welding surface 66 of the first spot welding electrode 64 is pressed against the exposed back surface 82 of the first copper workpiece 10 and the second welding surface 70 of the second welding electrode 68 is pressed against the exposed back surface 84 of the second copper workpiece 12 in alignment with each other. The first welding surface 66 can, as shown, at least partially cover the impression 38 remaining behind the projection 24 in the exposed back surface 82 of the first copper workpiece 10 by the process for forming the projection (e.g., punching).

[0030] The convergence of the first and second spot welding electrodes 64, 68 against the first and second copper workpieces 10, 12, respectively, generates a compressive force 86 against the workpieces 10, 12 at the joint 76. This compressive force 86 presses the projection 24 on the first mating surface 18 against the second mating surface 20, either directly or indirectly through an intervening material. The compressive force 86 applied by the first and second spot welding electrodes 64, 68 is preferably in the range of 250 lbf (pound force) to 1000 lbf, or more precisely, 350 lbf to 500 lbf. Once the compressive force 86 is reached, an electric current is passed between the first and second spot welding electrodes 64, 68 and through the first and second copper workpieces 10, 12. This electric current is preferably a direct current supplied by the rectifier 56 associated with the welding gun 50.The electric current can be constant or pulsed over time, according to a welding plan that is controllable by the welding control 58. In a particular embodiment, however, the electric current is conducted at a constant current level in the range of 25 kA to 35 kA, as in the one in . Fig. The welding plan shown in Figure 8 illustrates this. The electric current may take 5 ms to 20 ms to reach its constant current level and can then be nominally held at this current level for a period of 30 ms to 100 ms before dropping to 0 kA.

[0031] The electric current passed through the first and second copper workpieces 10, 12 initially flows through the bump 24 located on the first mating surface 18, as this is the only direct electrical path between the copper workpieces 10, 12 within the joint 76 at the time the current flow begins. The initial current flow through the bump 24 increases the current density of the electric current flowing through the mating interface 22 of the first and second copper workpieces 10, 12 by a factor of ten or more compared to the current density of the electric current at the interfaces of the weld surfaces 66, 70 and their respective back faces 82, 84 of the first and second copper workpieces 10, 12. The increased current density achieved in the bump 24 generates and concentrates heat within the bump 24, as long as the bump 24 remains structurally intact.This concentrated heat surge softens at least the hump 24 as well as the immediately adjacent areas of the first and second mating surfaces 18, 20 and can even melt them.

[0032] When the electric current passes through the first and second copper workpieces 10, 12, and as a result of the locally concentrated heat generated in the boss 24, the boss 24 collapses and the first and second mating surfaces 18, 20 are pressed along the interface 22 of the copper workpieces 10, 12, as shown in Fig. Figure 9 shows that the first and second mating surfaces 18, 20 (generally represented by arrows 88) are brought into broader interfacial contact, thereby eliminating the sharp increase in current density that prevailed before the collapse of the bump 24. At the time of the collapse of the bump 24, and while current is still flowing, the first and second mating surfaces 18, 20 interact in a manner that leads to the formation of a metallurgical bond. Such interaction between the first and second mating surfaces 18, 20 can occur either through the softening and diffusion of solid particles without melting the first and second copper workpieces 10, 12, or through the melting of the first and second copper workpieces 10, 12 at their contacting mating surfaces 18, 22, such that the molten portions of the mating surfaces 18, 22 absorb the mating surfaces 22 and flow into a common basin that extends into each of the copper workpieces 10, 12.

[0033] The conduction of electric current through the first and second copper workpieces 10, 12 eventually ceases. Upon the cessation of current flow, the first and second copper workpieces 10, 12 cool relatively quickly due to their high thermal conductivity. After cooling, a metallurgical connection 90, in the form of a solid-state connection or a fusion connection, is formed between the first and second copper workpieces 10, 12 via their interface 22 at the joining point 76, whereby the boss 24 was initially present and later collapsed, as shown in Fig. Figure 10 illustrates this. The metallurgical connection 90 is formed by the interaction 88 between the first and second mating surfaces 18, 20 as a result of the heat concentration within the bump 24 and the immediately surrounding parts of the mating surfaces 18, 20. After the metallurgical connection 90 has been formed, the welding gun 50 inserts the first and second spot welding electrodes 64, 68, whereby the joined workpiece stack 14 is then removed from the spot welding device 16 or realigned at another location where welding is to be carried out in the same manner.

[0034] The embodiment of the method described above is carried out with at least one boss 24 on the first mating surface 18 and a standard design for the welding gun 52. Although this is possible, other embodiments are also conceivable, including those described below in conjunction with the Fig. 11-15. In the embodiments described below, similar numbers are used to identify corresponding features, and consequently, the above description of the corresponding features applies unless otherwise specified. In a particular embodiment, as in Fig. As shown in Figure 11, the resistance spot welding device designated here by the number 116 can include a capacitor bank 200, so that the electric current between the first and second copper workpieces 10, 12 can be conducted more directly than in the embodiment described above. The capacitor bank 200 comprises a plurality of capacitors connected in series or parallel, which can store energy in the form of an electric field. This energy can later be released to provide the electric current flowing through the first and second copper workpieces 10, 12, albeit more quickly than in the embodiment described above. Fig. 1. Standard welding gun configuration shown with the same welding gun 50.

[0035] In the embodiment of Fig. The resistance spot welding device 116, for example, includes a power supply 202, a capacitor bank 200, a pulse transformer 204, and a welding controller 206. The power supply 202 receives a three-phase AC mains current and provides a high-voltage AC input current to supply the capacitor bank 200. For example, the AC mains current can be passed through a high-voltage transformer and converted into DC within the power supply 202 to generate the high-voltage DC input current. The DC input current is then supplied to the capacitor bank 200 for energy storage and accumulation. When an electric current is required for the passage through the first and second copper workpieces 10, 12, a switch 208 (e.g., an IGBT, an SCR, etc.) is used.The capacitor bank 200 is closed, and the energy stored in it is rapidly discharged into a primary winding 210 of the pulse transformer 204. This generates a magnetic flux that induces a lower-voltage, higher-power DC pulse in a secondary winding 212 of the transformer 204. This pulse can be supplied as an electric current through the first and second spot welding electrodes 64 and 68. The capacitor bank 200 and the pulse transformer 204 can be combined into a single housing known as a capacitive discharge welding unit, which is commercially available from various sources, such as TJ Snow Company (Chattanooga, Tennessee). The welding controller 206 can interact with the pulse transformer 204 to control the current supply between the first and second spot welding electrodes 64 and 68.

[0036] When using capacitive discharge to generate the electric current for the first and second spot welding electrodes 64, 68, which is passed through the first and second copper workpieces 10, 12, the electric current rises to a higher peak current level over a shorter period of time compared to the current profile usually associated with the resistance spot welding device 16. Fig. 1 is produced as shown in the welding plan in Fig. Figure 8 shows that, for example, in a representative embodiment, the electric current passed through the discharge capacitor bank 190 can rise to a peak current of 30 kA to 80 kA over a period of 2.0 ms to 10 ms, as shown in the welding plan in Figure 8. Fig. 12. The electric current can then decrease or decay over a period of 5.0 ms to 20 ms before falling below 1.0 kA. The shorter duration of the electric current passed through the first and second copper workpieces 10, 12 by means of capacitive discharge is noteworthy because it can lead to a reduced energy input into the workpieces 10, 12, even though the electric current briefly rises to a higher current level. The resistance spot welding device 116 and its use of capacitive discharge to provide the electric current passed through the first and second copper workpieces 10, 12 can therefore have the advantage of reducing the metallurgical joint 90 ( Fig. 10) to create between the workpieces 10, 12 while minimizing the risk of thermal damage to nearby heat-sensitive materials.

[0037] In yet another embodiment and with reference to Fig. The second mating surface of the second copper workpiece, identified by reference numerals 120 and 112 respectively, can also include at least one projection 124. Like the at least one projection 24 on the first mating surface 18, the at least one projection 124 on the second mating surface 120 extends beyond a surrounding base surface 126 of the second mating surface 120 and contacts the first mating surface 18 directly or indirectly. The projection 124 on the second mating surface 120 can come into contact with the first mating surface 18 at the projection 24 of the first mating surface 18, or the projections 24, 124 can be offset from one another. By using the bump 124 on the second mating surface 120 in conjunction with the bump 24 on the first mating surface 18, the heat concentration within the joint 76 can be increased until the projections 24, 124 collapse, in order to bring the first and second mating surfaces 18, 20 into a wider interface contact along the mating surface 22.The bump 124 contained on the second mating surface 120 can be replaced by one of the above in conjunction with the . Fig. The methods described in 3-5 can be used. Just as with the bump 24 on the first mating surface 18, the bump 124 on the second mating surface can include a plurality of projections 124.

[0038] In the here in Fig. In the embodiment shown in Figure 13, the bump 24 of the first mating surface 18 and the bump 124 of the second mating surface 120 contact each other (directly or indirectly through an intervening material) and are pressed together when the compressive force 86 is applied ( Fig. 7) pressed against each other on the first and second copper workpieces 10, 112. Consequently, when the electric current is passed between the first and second copper workpieces 10, 112, the current initially flows through each of the projections 24, 124, since this is the only direct electrical path between the copper workpieces 10, 112 within the joint 76 at the time the current flow begins. The initial current flow through the projections 24, 124 generates and concentrates heat within the projections 24, 124. This concentrated heat surge softens at least each of the projections 24, 124, as well as the immediately adjacent areas of the first and second mating surfaces 18, 20, and may even melt them. The concentrated heat eventually causes each of the projections 24, 124 to collapse.At the time of the collapse of the projections 24, 124 and during the current flow, the first and second mating surfaces 18, 20 interact as previously described, and during the cooling of the first and second copper workpieces 10, 112, a metallurgical bond similar to that in . Fig. 10 shown connection 90.

[0039] In another embodiment and as in Fig. As shown in Figure 14, an intermediate reaction material 214 can be introduced between the first mating surface 18 and the second mating surface 20, 120 to facilitate the metallurgical connection 90. The intermediate reaction material layer 214 is a copper alloy that has a lower electrical conductivity (i.e., is more electrically resistant) than either of the first and second copper workpieces 10, 12, 112 and preferably contains between 2.0 wt.% and 10 wt.% phosphorus. In a particular embodiment, the intermediate reaction material 214 can be a Cu-P-Ag copper alloy containing copper and between 3 wt.% and 8 wt.% phosphorus and between 0.1 wt.% and 20 wt.% silver. The intermediate reaction material 214 can be arranged between the first and second mating surfaces 18, 20, 120 in the form of a deposited material layer, a foil, a strip, or any other applicable constitution.The arrangement of the intermediate reaction material 214 between the first and second mating surfaces 18, 20, 120 - together with the at least one bump 24, 124 on the first mating surface 18 or the first and second mating surfaces 18, 120 - enables the formation of the metallurgical compound 90 when the electric current is passed through at a moderate current level for a relatively short period of time, which in turn can reduce the energy input into the first and second copper workpieces 10, 12.

[0040] The intermediate reaction material 214 is arranged between the first mating surface 18 and the second mating surface 20, 120 such that the material 214 is positioned between the bump 24 on the first mating surface 18 and the second mating surface 20, 120. In this context, if the second mating surface 120 includes the at least one bump 124, the intermediate reaction material 214 can be positioned between the at least one bump 24 of the first mating surface 18 and the at least one bump 124 of the second mating surface 120, as shown in Fig. Figure 15 illustrates this. Thus, when the interposed reaction material 214 is present, the bump 24 on the first mating surface 18 indirectly comes into contact with the second mating surface 20, 120 through the reaction material 214, regardless of whether the second mating surface 20, 120 contains the bump 124 or not. The arrangement of the interposed reaction material 214 between the bump 24 on the first mating surface 18 and the second mating surface 20, 120 helps to focus the initial concentrated heat generation within the joint 76.

[0041] The intermediate reaction material 214 aids in the formation of the metallurgical compound 90 by cleaning the first and second mating surfaces 18, 20, 120 along their interface 22 within the joint 76, thus facilitating the interaction of the mating surfaces 18, 20, 120 to form the metallurgical compound 90. In particular, when an electric current is passed through the first and second copper workpieces 10, 12, 112, the intermediate reaction material 214, due to its lower electrical conductivity, heats up more rapidly and liquefies between the first and second mating surfaces 18, 20, 120.The liquefied reaction material is ejected laterally outwards along the interface 22 when the boss 24 or the projections 24, 124 collapse and the first and second mating surfaces 18, 20, 120 are brought closer together and into broader interfacial contact under the pressure of the applied pressure force 86. In this process, the liquefied reaction material breaks up surface oxide layers and cleans the resulting oxide layer residues and other materials laterally outwards, thereby bringing cleaned portions of the first and second mating surfaces 18, 20, 120 into direct contact under the pressure of the applied pressure force 86 and in the locally heated environment facilitated by the boss 24 on the first mating surface 18 or the bosses 24, 124 on the first and second mating surfaces 18, 120.The cleaning effect achieved by the lateral displacement of the liquefied reaction material is at least partially due to the presence of phosphorus in the copper alloy, which acts as a flux when the reaction material 214 melts. The intermediate reaction material 214 is self-flowing in this respect.

[0042] The various embodiments of the method described above can be combined with one another as required. For example, the intermediate material 214 can be used or not, regardless of whether the electric current is passed through the first and second copper workpieces 10, 12, 112 using the resistance spot welding device 16. Fig. 1 or the resistance spot welding device 116 of Fig. 11 is conducted, which includes a capacitive discharge to provide the electric current. In fact, the use of the resistance spot welding device 116 of Fig.11 in combination with the intermediate reaction material 214 is suitable for producing the metallurgical connection 90 between the first and second copper workpieces 10, 12, 122 with the lowest total energy input into the workpieces 10, 12, 112, which can be advantageous for avoiding thermal damage to nearby heat-sensitive materials that are in contact with the first and / or second copper workpieces 10, 12, 112. Likewise, the formation of the at least one projection 24, 124 on each of the first and second mating surfaces 18, 20, 120 can be used with or without the intermediate reaction material 214, and each of the resistance spot welding devices 16, 116 described above can be used to conduct the electric current through the copper workpieces 10, 12, 112 and their respective projections 24, 124.

Claims

[1] Method for joining adjacent overlapping copper workpieces (10, 12) by resistance spot welding, the method comprising: Providing a stack of workpieces (14) comprising a first copper workpiece (10) and a second copper workpiece (12) arranged adjacent to the first copper workpiece (10), wherein the first copper workpiece (10) has a first mating surface (18) and the second copper workpiece (12) has a second mating surface (20) opposite the first mating surface (18) for forming a mating surface (22), wherein the first mating surface (18) includes a boss (24) projecting beyond a surrounding base surface (26) of the first mating surface (18) and contacting the second mating surface (20), and wherein the second mating surface (20) also includes a boss (124) projecting beyond a surrounding base surface (126) of the second mating surface (20); Applying a compressive force (86) to the first and second copper workpieces (10, 12) which presses the bump (24) of the first mating surface (18) onto the second mating surface (20); Briefly passing an electric current through the first and second copper workpieces (10, 12), wherein the electric current initially flows through the bump (24) present on the first mating surface (18) to generate and concentrate heat within the bump (24) before the bump (24) collapses to bring the first mating surface (18) and the second mating surface (20) into greater interfacial contact along the mating surface (22) of the first and second copper workpieces (10, 12), and wherein, upon cooling of the first and second copper workpieces (10, 12), a metallurgical connection (90) is formed between the first and second copper workpieces (10, 12) via their mating surface (22) at a point where the bump (24) has collapsed. [2] Method according to claim 1, wherein the boss (24) of the first mating surface (18) and the boss (124) of the second mating surface (20) are in contact with each other and are pressed against each other when the pressure force (86) is applied to the first and second copper workpieces (10, 12), and wherein the conduction of the electric current through the first and second copper workpieces (10, 12) initially involves the flow of the electric current through the projections (24, 124) to generate and concentrate heat within the projections (24, 124) before the projections (24, 124) collapse to bring the first mating surface (18) and the second mating surface (20) into wider interfacial contact along the mating interface (22) of the first and second copper workpieces (10, 12). [3] Method according to claim 1, wherein an intermediate reaction material (214) is arranged between the bump (24) of the first mating surface (18) and the second mating surface (20), wherein the intermediate reaction material (214) is a copper alloy containing between 2.0 wt.% and 10 wt.% phosphorus. [4] The method of claim 1, further comprising: Supplying an electric current to a capacitor bank (200) in order to store energy in the capacitor bank (200); and Discharging the energy stored in the capacitor bank (200) to provide the electric current that is briefly passed through the first and second copper workpieces (10, 12). [5] Method according to claim 1, wherein each of the first and second copper workpieces (10, 12) comprises 99.9 wt.% copper or more. [6] The method of claim 1, further comprising: Forming the bump (24) on the first mating surface (18) before the first copper workpiece (10) and the second copper workpiece (12) are inserted into the workpiece stack (14). [7] Method for joining adjacent overlapping copper workpieces (10, 12) by resistance spot welding, the method comprising: Forming at least one boss (24) on a first mating surface (18) of a first copper workpiece (10), wherein the at least one boss (24) of the first mating surface (18) projects beyond a surrounding base surface (26) of the first mating surface (18); Forming at least one further boss (124) on the second mating surface (20) of the second copper workpiece (12), wherein the at least one boss (124) of the second mating surface (20) projects beyond a surrounding base surface (126) of the second mating surface (20); Assembling a stack of workpieces (14) comprising the first copper workpiece (10) and a second copper workpiece (12) adjacent to the first copper workpiece (10), wherein the at least one bump (24) of the first mating surface (18) of the first copper workpiece (10) comes into contact with a second mating surface (20) of the second copper workpiece (12); Applying a compressive force (86) to the first and second copper workpieces (10, 12) which presses the at least one bump (24) of the first mating surface (18) against the second mating surface (20); Discharging the energy stored in a capacitor bank (200) to provide an electric current which rises to a peak current level between 30 kA and 80 kA within a period of 2.0 ms to 10 ms and then falls over a period of 5.0 ms to 20 ms before dropping below 1 kA; Forming a metallurgical connection (90) between the first and second copper workpieces (10, 12) by passing an electric current through the first and second copper workpieces (10, 12), wherein the electric current initially flows through the at least one bump (24) present on the first mating surface (18) to generate and concentrate heat in the at least one bump (24) before the at least one bump (24) collapses to bring the first mating surface (18) and the second mating surface (20) into wider interfacial contact, wherein, upon cooling of the first and second copper workpieces (10, 12), the metallurgical connection (90) between the first and second copper workpieces (10, 12) is formed via their mating interface (22) at a location where the at least one bump (24) has collapsed. [8] The method of claim 7, further comprising: Arranging an intermediate reaction material (214) between the first mating surface (18) and the second mating surface (20), such that the intermediate reaction material (214) is arranged between the bump (24) of the first mating surface (18) and the second mating surface (20) when the first copper workpiece (10) and the second copper workpiece (12) are assembled into the workpiece stack (14), wherein the intermediate reaction material (214) is a copper alloy containing between 2 wt.% and 10 wt.% phosphorus.

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