It’s commonly understood that high conductivity electrode materials (Classes 1 and 2 under the ISO 5182 system) are ideal for welding low conductivity workpieces. And conversely, high conductivity metals require electrodes with lower conductivity, such as refractory metal electrodes referred to as the Class 3 electrodes under ISO 5182.
For example, widely available copper/chromium and copper/chromium/zirconium electrodes work great with low carbon steels and high strength steels. To spot weld this ferrous family of metals, various strategies to strengthen the copper are employed to achieve the necessary material hardness. (Notably, for high carbon stainless steels, alloys of copper are still recommended; however, the resistance welding process is adjusted to provide the higher force and lower current that is required.) Alternatively when welding copper, low conductivity metals such as the family of refractory metal electrodes including pure tungsten, molybdenum and tungsten/copper electrodes, along with a few other variations, work best.
When resistance spot welding low conductivity metals, the workpiece material (and not the welding electrode) becomes heated. Copper is ideal in that it allows the current and heat to flow to the workpiece. On the other hand, when you are welding a highly conductive metal, the workpiece will allow the heat to dissipate, acting in the manner of a heat sink. In this case, you need an electrode that can retain the heat, especially in the tip, and be stiff enough at high temperatures to maintain position to maximize the contact between the electrode and the workpiece.
Despite these principles, no single electrode material excels at every application. For example, refractory metal electrodes are perceived, often mistakenly but with some merit, to crack or delaminate at the tip due to thermal cycling. While true if selected to spot weld truly unsuitable high resistivity workpiece metals, there are strategies to eliminate tip delamination. In applications where it is successful, the refractory’s advantage of surviving high current, high repetition cycling makes them indispensable.
Problems with high conductivity electrodes can be found in precipitation hardened alloys, such as chromium copper (CrCu). During use, the repeated heat cycling has been found to cause further diffusion of the precipitates into the copper matrix, which leads to an increase in electrode hardness and ultimately to a reduction in electrical conductivity. This metallurgical transformation during use can be managed, however, and the advantages of the Class 1 and Class 2 remain compelling for welding the right workpiece metals.
To learn more about the variables involved in choosing the right resistance welding electrodes for your resistance spot welding application, download our free white paper on Resistance Welding Electrode Materials: Selecting the Right One for Your Application.