Resistance spot welding electrodes: Understand the variables

It is well known that high conductivity electrode materials (grade 1 and 2 under ISO 5182 system) are ideal for welding low conductivity workpiece. Conversely, high conductivity metals require electrodes with lower conductivity, such as refractory metal electrodes referred to as Class 3 electrodes under ISO 5182.

For example, the widely used copper/chromium and copper/chromium/zirconium electrodes are ideal for mild steel and high strength steels. In order to spot weld this family of ferrous metals, various strategies were employed to strengthen the copper to achieve the necessary material hardness. (It is worth noting that copper alloys are still recommended for high carbon stainless steels; However, the resistance welding process is modified to provide the higher force and lower current required. Alternatively, when welding copper, low conductivity metals (such as the refractory metal electrode series, including pure tungsten, molybdenum, and tungsten/copper electrodes) and some other variants work best.

When resistance spot welds low conductivity metals, the workpiece material (rather than the welding electrode) is heated. Copper is ideal because it allows current and heat to flow to the workpiece. On the other hand, when you weld a highly conductive metal, the workpiece will allow heat to dissipate, acting as a radiator. In this case, you need an electrode that can hold heat, especially in the tip, and is rigid enough to hold position at high temperatures to maximize contact between the electrode and the workpiece.

Despite these principles, no electrode material performs well in every application. For example, refractory metal electrodes are often erroneously thought to crack or delaminate at the tip due to thermal cycling, but have certain advantages. While this is true if the choice of spot welding is truly unsuitable for high resistivity workpiece metals, there are strategies to eliminate tip delamination. In successful applications, the advantage of refractory materials to survive high current, high repeat cycles makes them indispensable.

Problems with high conductivity electrodes can be found in precipitate hardened alloys such as chromium-copper (CrCu). During use, it has been found that repeated thermal cycles lead to further diffusion of sediment into the copper matrix, resulting in an increase in electrode hardness and, ultimately, a decrease in electrical conductivity. However, this metallurgical transition can be controlled during use, and the advantages of class 1 and class 2 remain attractive for properly welded workpiece metals.

To learn more about the variables involved in choosing the right resistance welding electrode for your resistance spot welding application.

Please contact us at zhang@pride-cnc.com