The electrodes are one of the most important factors in the resistance welding process but often the most abused. It is important to consider the electrode material, shape, size, tip profile and cooling.
Electrode materials are covered by ISO 5182. These are mainly copper alloys with a small percentage of alloying element to improve hardness, while maintaining good conductivity.
The most common materials are Class 2 (e.g. copper/chromium or copper/chromium/zirconium) and may be used for low carbon and high strength steels in general. Higher conductivity alloys, such as copper/zirconium and dispersion strengthened copper, show some benefits when welding coated steels as they provide less surface heating because of their low contact resistance.
When welding harder sheet materials, such as stainless steels, much higher electrode forces are required but lower welding current. These materials are better welded with the harder Class 3 electrodes such as copper/nickel/silicon. This is replacing the superior copper/cobalt/beryllium alloy because of the potential beryllium hazard (mainly as a dust from machining or dressing operations).
Refractory electrode materials, such as tungsten/copper, tungsten, or molybdenum are used for applications such as projection welding inserts, where the electrode contact area is at least three times the weld size. These materials have higher hardness but lower conductivity than the Class 2 electrodes. They are unsuitable for spot welding as they suffer localised heating at the tip contact, which can lead to cracking of the electrode. The exception is for joining high conductivity metals such as copper wire or foil, where heat is generated mainly within the refractory electrode tip and conducted into the materials to be joined.
ISO 5182 Materials for resistance welding electrodes and ancillary equipment, Second Edition, 1991
See also FAQ: How can I minimise electrode wear in resistance spot welding? and Resistance Welding of Sheet Metals Best Practice Guide