Electrode properties in resistance spot welding: Why are they important

Unlike pure copper resistance welding electrodes, which conduct heat due to their properties and design, pure refractory electrodes were chosen precisely because they retain heat. Tungsten (W) is notable for having the highest melting point of any metal (3,387 °C) and being extremely hard at both room temperature and high temperature. It usually resists oxidation and its resistance is very high.

Molybdenum (MO), tungsten's "sister" among refractory metals, also has a high melting point (2610°C) and high mechanical strength at high temperatures, although it is relatively lower than tungsten. The resistance of Mo is also very high. For a more detailed review of the resistivity of various elements, including tungsten and molybdenum, the resistivity of the selected elements includes charts showing the resistance over the full temperature range, which are more useful than individual values.

When selecting electrode materials for your resistance welding application, it is important to enter the resistance spot welding process cycle by design so that the electrodes are as cool as possible, thereby controlling the inevitable oxidation consumption. Tungsten is more easily stratified by thermal shock than molybdenum, so this is an additional selection consideration. The trade-off, however, is that molybdenum oxidizes faster than tungsten.

In parallel with the development of copper alloy materials designed to improve the properties and behavior of pure copper, there are tungsten and molybdenum alloys that improve the pure elements themselves. (Please note that since tungsten and molybdenum are powder metallurgical products, the term "alloy" is applied in a different way than non-powder metals. In order to disperse completely into refractory metals, the alloy material must survive or adapt to the high-temperature processing requirements of tungsten and molybdenum.

Copper tungsten (CuW) has various composition ratios and is widely used because of the balance of advantages of copper and tungsten. Silver tungsten (AgW) is also produced in different proportions. The advantage is that silver (Ag) is difficult to alloyed with iron (Fe) and nickel (Ni), so AgW can be a good choice for welding stainless steel and other nickel-based alloys.

These alloy electrode materials are the result of sintering hardening of permeable structure. Basically, precisely measured sheets of pure copper or silver are placed in molds containing precisely measured amounts of tungsten or molybdenum powder and baked in an inert atmosphere oven. The result is an electrode with high temperature strength and high conductivity.

Sometimes resistance welding applications require unique performance structures. In these cases, a variety of special electrodes are available. Silver carbide (AgWC) is an unusual refractory electrode material because tungsten carbide (WC) has excellent oxidation resistance while retaining shape throughout the welding thermal cycle.

Finally, heavy metals -- also known as heavy tungsten alloys, which consist of W, Fe, Ni and Cu, depending on each manufacturer's respective grade -- are another unusual refractory metal for spot welding electrodes. Its advantages include excellent oxidation resistance and ease of processing compared to pure tungsten and tungsten carbide (WC).

If you are choosing resistance spot welding electrodes for your application, understanding material properties may be critical. For more information on material properties and their many interactions,

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