WELDING ELECTRODE AND USE OF THE WELDING ELECTRODE

Abstract

The invention relates to a resistance welding electrode, comprising a welding tool made of metal. This welding tool has a contact surface (1) that comes into contact with the workpiece (9) to be welded. To prevent adhesion between the contact surface (1) and a workpiece, particularly one made of aluminum, the invention suggests that the contact surface (1) be made of boron-doped diamond.

Description

WELDING ELECTRODE AND USE OF THE WELDING ELECTRODE FIELD OF INVENTION The invention relates to a welding electrode for resistance welding according to the preamble of claim 1. It further relates to a use of said welding electrode. BACKGROUND OF THE INVENTION Welding electrodes for resistance welding, particularly resistance spot welding, are known, for example, from JF Key, TH Courtney: Refractory Metal Composite Tips for Resistance Spot Welding of Galvanized Steel, Welding Research Supplement, 261-266, 1974. Resistance spot welding electrodes typically have a cap as the welding tool, which can be connected to an electrode holder of a resistance spot welding machine. For seam welding using rollers, a disc is used as the welding tool. To produce a welded joint between steel sheets, these welding electrodes are made, for example, of sintered CuAl₂O₃ or CuCr or CuCrZr alloys. 52 / 1756 / 21 MA / Recently, there has been a demand, particularly in the automotive industry, for the production of welded joints between aluminum sheets. Especially in the production of spot welds, conventional welding electrodes adhere poorly to the aluminum sheets being welded. SUMMARY OF THE INVENTION The object of the invention is to eliminate the disadvantages of the prior art. In particular, a universal welding electrode is provided with which it is possible to obtain a large number of joints by resistance welding or a long seam length between metal sheets. A further object of the invention will specify a use for the welding electrode. This task is solved by the features of claims 1 and 14. Practical embodiments of the invention are shown in the separate patent claims. According to the invention, a welding electrode for resistance welding is proposed in which the contact surface is formed of diamond doped with boron and / or phosphorus. With the electrode 52 / 1756 / 21 MA / t / ZUZZ / UU4OyU proposed welding, it is surprisingly possible to produce more than 1,400 welded joints, particularly spot welds, between metal sheets, particularly aluminum sheets, without adhesion. In particular, in the case of producing a spot weld between two aluminum sheets, it appears, according to the present state of knowledge, that by means of the diamond layer according to the invention, a passivation layer of A12O3 formed on the surface of the aluminum sheets is mechanically broken, at least in sections, so that the diamond layer comes into direct contact with the metallic aluminum. As a result, the contact resistance between the welding electrode and the aluminum sheet can be considerably reduced. This, in turn, prevents the aluminum sheet from melting in an area toward the contact surface of the welding electrode and thus from adhering to the welding electrode. According to one advantageous modality, the diamond is doped with 500 to 20,000 ppm of boron, preferably 2,000 to 10,000 ppm. The diamond may be additionally or alternatively doped with 500 to 20,000 ppm of phosphorus. This allows a resistance welding process to be carried out with a current density 52 / 1756 / 21 of 30 kA / cm² or more. This corresponds to approximately 30 times the current density compared to the conventional resistance welding process for welding steel sheets. In that case, a current density of 1 kA / cm² is usually used. The possibility of using a particularly high current density makes it possible to perform a resistance welding joint quickly. In particular, it avoids undesirable heating of large areas of the workpieces to be welded. According to an additional advantageous method, the diamond is produced as a diamond coating by means of a chemical vapor deposition (CVD) process. In the CVD process, the diamond coating is deposited from the gas phase in situ onto the welding electrode. A diamond coating produced in this way has been shown to have surprisingly good durability even under the extreme conditions of resistance welding. Advantageously, the diamond layer thickness ranges from 0.5 to 50 µm, preferably from 1 to 10 µm. The diamond layer advantageously has a surface roughness with an average roughness depth Rz > 1 µm. A diamond coating with the above parameters is characterized by an improved service life of MA / t / ZUZZ / UU4O»U 52 / 1756 / 21 welding electrode. According to a further advantageous embodiment, more than 50% of the contact surface is formed by facets that form the (111 or 001) planes of diamond crystals, preferably of individual diamond crystals grown together. A growth zone of the diamond layer opposite the contact surface is conveniently in contact with an intermediate layer on the cap side. In particular, the individual diamond crystals extend predominantly in one direction [111 or 110] from the intermediate layer to the contact surface. That is, the individual diamond crystals extend from the intermediate layer to the contact surface such that their grain boundaries are predominantly approximately perpendicular to the contact surface. A diamond layer with the proposed formation is characterized by excellent electrical and thermal conductivity. According to an additional advantageous embodiment, the intermediate layer is formed from a carbide and / or nitride and / or boride compound of the first metal or of a second metal different from the first metal. In particular, the first and / or second metals form a carbide and / or nitride and / or boride compound that is stable up to MA / t / ZUZZ / UU4OyU 52 / 1756 / 21 at a temperature of 800°C. The first and / or second metals may be formed in particular by one or more of Cr, Ti, Nb, Mo, W, Ta. The intermediate layer may be formed in situ directly during the CVD process or may be formed separately at a temperature of 600°C to 1,050°C. For example, the first metal could be W containing Cu as an alloying component. In this case, the intermediate layer could be formed directly in the CVD process by which the diamond layer is deposited. In this case, WC is formed as the intermediate layer. Alternatively, the first metal could be W containing Fe as an alloying component. In this case, a TiN layer is deposited on top of the first metal as an intermediate layer in a first CVD process. This layer could be doped with B. Then, the diamond layer is deposited on top of the intermediate layer in a second CVD process. The first metal may preferably include Cu, Fe or even Ag as an alloying component. In addition to the first metal, the welding tool can also be formed in sections from a third metal. The third metal may consist primarily of copper. That is, the welding tool can be made from, for example, a W alloy or MA / t / ZUZZ / UU4OyU 52 / 1756 / 21 Mo, at least in a section that forms the contact surface. Incidentally, the welding tool can also be made of another metal, for example, a copper alloy. Such a welding tool can be manufactured relatively inexpensively. The welding tool can be a cap that fits over the electrode holder of a resistance spot welding machine. However, the welding tool can also be a disc for a roller seam welding machine. According to a further stipulation of the invention, the welding electrode according to the invention is proposed to be used to make a welded joint between workpieces made of a fourth metal having a passivating layer of metal oxide. The term "metal" shall be understood in general within the sense of the present invention. That is, it may also refer to an alloy. The fourth metal is understood to be a metal that spontaneously forms an oxide layer on its surface when in contact with air. The fourth metal is preferably selected from the following group: Al, Mg, Ni, Ti, Zn, Cr, Fe, Nb, Ta, Cu. In particular, aluminum spontaneously forms a passivation layer of 52 / 1756 / 21 MA / Al₂O₃ is applied to the surface of the welding electrode. Al₂O₃ is electrically insulating and has a high hardness (Vickers hardness of approximately 2,000). The diamond coating provided on the welding electrode according to the invention has a higher hardness, i.e., a Vickers hardness of 7,000 to 10,000. Consequently, the welding electrode according to the invention successfully penetrates the passivating layer that forms, for example, on aluminum sheets, establishing direct electrical contact between the diamond coating and the metallically conductive section beneath the passivating layer. As a result, the welding electrode according to the invention successfully produces a welded joint without the welding electrode adhering to the sheet being welded. The described effect also applies to four other metals that form a passivating metal oxide layer, for example, Al, Mg, Ni, Ti, Zn, Cr, Fe, Nb, Ta, and Cu. It is preferable that the welded joint be produced by resistance spot welding. However, with a corresponding design of the welding electrode according to the invention, it is also conceivable, for example, to produce linear welded joints. 52 / 1756 / 21 BRIEF DESCRIPTION OF THE DRAWINGS OR FIGURES One embodiment of the invention will now be explained in more detail with reference to the drawings. These show that: Figure 1 is a top view of a weld-on cap, Figure 2 is a cross-sectional view through the weld-on cap according to section line AA' of Figure 1, Figure 3 is a bottom view according to Figure 1, and Figure 4 is a schematic cross-sectional view through the surface of a weld-on cap and a sheet to be welded. DETAILED DESCRIPTION OF THE INVENTION Figures 1 to 3 show a welding electrode in the form of a cap or welding cap. The welding electrode has a contact surface (1) that forms the free surface of a diamond layer (2). The reference number (3) denotes a portion formed, for example, of W or Mo or an alloy containing Mo or W as the main component. The reference symbol (4) denotes an intermediate layer, which in the specific example is formed substantially of WC or MA / t / ZUZZ / UU4OyU 52 / 1756 / 21 MA / MoC. The intermediate layer (4) can be formed in situ during the manufacture of the diamond layer (2) by means of a CVD process. The reference number (5) indicates a base portion of the solder cap. The base portion (5) may be made of a third metal different from the first metal that forms portion (3). A third metal may be chosen for the base portion (5) that is less expensive than the first metal used to make portion (3). For example, the base portion (5) may be made of pure copper or a copper alloy, particularly CuAl₂O₃, CuCr, or CuCrZr alloys. It is also possible, of course, to omit the base portion (5) and for the cap to be made of the first metal that forms portion (3). According to an additional embodiment not shown in the figures, section (3) may also be omitted. In this case, the solder cap is manufactured, for example, from a conventional copper alloy. In this case, the intermediate layer (4) must be applied separately. The intermediate layer may be formed from carbide-forming metals. For example, the intermediate layer may comprise Ti. The diamond layer (2) may then be deposited onto this intermediate layer (4) by means of a CVD process. 52 / 1756 / 21 MA / E / ZUZZ / UU4ÜUU Figure 4 schematically shows section (3), which is made of a W or Mo alloy. The alloy may have a grain boundary phase (6) at the grain boundaries; only one is shown here as an example, which is composed of, for instance, Fe, Ni, Co, or Cu. In the case of an in-situ coating, it is advantageous to remove the surface grain boundary phase (6) by chemical etching and / or particle blasting. This increases the bond strength between the diamond layer (2) and the intermediate layer (4). The diamond crystals (7) extending from the intermediate layer (4) are more than 50% single diamond crystals. The facets of the diamond crystals (7), denoted by the reference symbol (8), are formed by the (111) plane or the (001) plane. The reference symbol (P) denotes arrows representing the direction of current flow through the diamond layer (2). The current flow is parallel to the

[111] direction as well as the

[110] direction of the diamond crystals (7). The contact surface (1) of the diamond layer (2) is formed by all the facets (8). Opposite the contact surface (1) is a workpiece (9) to be welded, which is 52 / 1756 / 21 ML / made, for example, of an aluminum alloy. The workpiece (9) has a layer of metal oxide (10) on its surface. Although not shown in the figures, the welding tool may consist of a disc instead of a cover. Such a disc is used in roller seam welding devices. In this case, the contact surface (1) is formed on the peripheral edge of the disc. The section (3) and, where applicable, the base section (5) are arranged radially inward on the disc in a sequence analogous to that of the cover shown in Figures 1 to 3. To produce a welded joint between the workpiece (9) and another workpiece (not shown here), the diamond layer (2) is pressed against the metal oxide layer (10). A current density in the range of 5 to 60 kA / cm², preferably in the range of 10 to 20 kA / cm², is generated. In this process, the workpiece (9) is welded to an additional workpiece (not shown here) arranged in the opposite direction, which is pressed against the workpiece (9) with an additional welding electrode (not shown here) according to the invention. With the proposed welding electrode, more than 1000 spot welds can be formed, especially 52 / 1756 / 21 MA / t / ZUZZ / UU4OyU on aluminum sheets, without adhesion occurring between the welding electrode and the aluminum sheets. List of reference symbols: Contact surface Diamond layer Section Intermediate layer Base section Grain boundary phase Diamond crystal Facet Workpiece Metal oxide layer P Arrow 52 / 1756 / 21

Claims

1. A welding electrode for resistance welding, comprising a welding tool which is manufactured at least in sections from a first metal and has a contact surface (1) that is brought into contact with the workpiece (9) to be welded, characterized in that the contact surface (1) is formed of diamond doped with boron and / or phosphorus.

2. The welding electrode according to claim 1, wherein the diamond is doped with 500 to 20,000 ppm of boron.

3. The welding electrode according to any of the preceding claims, wherein the diamond is produced as a diamond layer (2) by the chemical vapor deposition (CVD) process.

4. The welding electrode according to any of the preceding claims, wherein the thickness of the diamond layer (2) is from 0.5 to 50 pm, preferably from 1 to 10 pm.

5. The welding electrode according to any of the preceding claims, wherein the diamond layer (2) has a surface roughness with an average roughness depth of Rz > 1 pm.

6. The welding electrode according to any of the preceding claims, wherein the contact surface (1) is formed by more than 50% facets forming the planes (111 or 001) of diamond crystals, preferably individual diamond crystals.

7. The welding electrode according to any of the preceding claims, wherein a diamond layer growth zone (2) opposite the contact surface (1) is in contact with an intermediate layer (4).

8. The welding electrode according to any of the preceding claims, wherein the diamond crystals (7) extend in a direction [111 or 110] from the intermediate layer (4) to the contact surface (1).

9. The welding electrode according to any of the preceding claims, wherein the intermediate layer (4) is formed of a carbide and / or nitride and / or boride compound of the first metal or of a second metal different from the first metal.

10. The welding electrode according to any of the preceding claims, wherein the first and / or second metals form a carbide and / or nitride and / or boride compound stable up to a temperature of 800°C. 52 / 1756 / 21 MA / t / ZUZZ / UU4OyU 11. The welding electrode according to any of the preceding claims, wherein the first and / or second metals are formed from one or more of the following elements: Cr, Ti, Nb, Mo, W, Ta.

12. The welding electrode according to any of the preceding claims, wherein the welding tool is formed in sections of a third metal.

13. The welding electrode according to any of the preceding claims, wherein the third metal contains Cu as a main component.

14. The use of the welding electrode according to any of the preceding claims to produce a resistance welded joint between workpieces (9) made of a fourth metal with a passivating metal oxide layer (10).

15. The use according to claim 14, wherein the resistance welded joint is made by resistance spot welding, resistance projection welding, or resistance seam welding.

16. The use according to claim 14 or 15, wherein the fourth metal is selected from the following group: Al, Mg, Ni, Ti, Zn, Cr, Fe, Nb, Ta, Cu.