Method for obtaining a welding electrode

A method for manufacturing, comprising the steps of:

• continuous casting a welding electrode being comprised of copper, chromium, zirconium alloy, and at least one of phosphorus and magnesium,
• starting from an alloy comprising chromium in a proportion between 0.4 and 0.8% by weight, zirconium in a proportion between 0.02 and 0.09% by weight, said alloy comprising at least one of phosphorus and magnesium in a total proportion higher than 0.005% by weight, the proportion of magnesium being less than 0.1% by weight and the proportion of phosphorus being less than 0.03% by weight, the rest of the composition being copper,
• wherein said method comprises at least the following steps:
• a) melting of the various components of the alloy, namely copper, chromium, zirconium and phosphorus and/or magnesium at a temperature above 1200° C.;
• b) continuous casting through a cylindrical die having a diameter d, which permits to obtain a bar having a diameter close to the diameter d of the die with maintaining the metal liquid in the casting furnace at a temperature between 1100 and 1300° C.;
• c) solidification of said bar and cooling to a temperature below 100° C., the cooling rate being at least equal to 10° C./s until reaching a bar temperature of 1060° C., then at least equal to 15° C./s between 1060 and 1040° C., then at least equal to 20° C./s between 1040 and 1030° C., then at least equal to 25° C./s between 1030 and 1000° C., then at least equal to 30° C./s between 1000 and 900° C., then at least equal to 20° C./s for temperatures below 900° C., until the bar is cooled down to a temperature of not more than 100° C.;
• d) cold deformation in order to obtain a bar with a diameter smaller than 20 mm;
• e) aging or tempering treatment;
• and
• f) shearing of said bar in order to obtain pieces, then punching or machining by removal of material in order to provide said electrode with its final shape,
• wherein the metallurgical structure of the active face of said electrode includes incoherent chromium precipitates, more than 90% of which have a projected surface area of less than 1 μm2, said incoherent chromium precipitates having dimensions between at least 10 and 50 nm, said electrode having in addition a fibrous structure, visible in a cross-sectional view of the active face of said electrode after surfacing and chemical etching, said structure being comprised, on the one hand, of a plurality of radial fibers, said fibers having a thickness of less than 1 mm, and a substantially fiberless central zone having a diameter of less than 3 mm, and the electrical conductivity of said electrode being higher than 85% IACS (for International Annealed Copper Standard).

A welding method, comprising the steps of:

• spot resistive welding at least two steel sheets coated with a coating of zinc or an alloy comprising zinc with a welding electrode being comprised of copper, chromium, zirconium alloy, and at least one of phosphorus and magnesium,
• wherein the proportion of chromium is between 0.4 and 0.8% by weight,
• wherein the proportion of zirconium is between 0.02 and 0.09% by weight, the total proportion of phosphorus and/or magnesium being higher than 0.005% by weight, with a proportion of magnesium lower than 0.1% by weight and a proportion of phosphorus lower than 0.03% by weight, the rest of the composition being copper,
• wherein the structure of the electrode comprises incoherent chromium precipitates, more than 90% of which have a projected surface area of less than 1 μm2, said incoherent chromium precipitates having dimensions at least between 10 and 50 nm, said electrode having in addition a fibrous structure, visible in a cross-sectional view of the active face of said electrode after surfacing and chemical etching,
• wherein said structure comprises a plurality of radial fibers, said fibers having a thickness of less than 1 mm, and a substantially fiberless central zone having a diameter of less than 5 mm, and the electrical conductivity of said electrode being higher than 85% IACS (International Annealed Copper Standard), and
• wherein said coating has a thickness of less than 10 μm and said sheets having a thickness of less than or equal to 1.5 mm.