Chromium Zirconium Copper
Copper for resistance welding applications

Manufacture

Pure copper is a soft metal and its use in electrical applications is usually based on its high conductivity rather than mechanical properties. The strength of copper can be improved to a certain extent by cold working, but copper tends to soften already at comparatively low temperatures.

The production of a high conductivity material which possesses considerable strength at both room and higher temperatures and which also retains its strength if subjected to high temperatures, is of great practical importance.

A typical application where this combination of properties is required are the electrodes for resistance welding.

A copper alloy containing 1% chromium and 0.1% zirconium such as Thatcher Alloys Cu.Cr.Zr. is often the best choice for applications where a combination of high electrical and thermal conductivity and high strength at higher temperatures is required. Studies have shown that of all commercially available electrode materials for spot welding mid stee, the lowest electrode costs are obtained with chromium-zirconium copper.

The properties of Cu-Cr-Zr are obtained by alloying and through heat treatments combined with cold working. The metallurgical behaviour of this alloy is based on transformations at the atomic level.

Applications

The main application for Cu.Cr.Zr. is for electrodes which must have a high electrical conductivity to conduct the heat away from the weld and be strong enough to withstand the mechanical pressure required for welding.

Other applications are contact tips for MIG-welding guns and components for electrical switches and certain stressed parts in turbine generators.

Production

The first heat treatment stage is solution annealing at approximately 1000 °C. At this temperature the chromium and zirconium atoms are randomly distributed in the copper matrix. The material is then quenched in water. Between 400 and 700 °C the chromium and zirconium atoms have a tendency to form precipitates in the copper matrix, but the cooling is too rapid for precipitation to take place. The resuly is a supersaturated solid solution. The strength of this structure is only slightly higher than that of pure copper and foreign atoms in the copper matrix considerably reduce the electrical conductivity.

The supersaturated solid solution serves as a starting point for further heat treatments.

The properties of Cu.Cr.Zr. are acheived by ageing the material at a temperature below 500 °C.

The ageing conditions are very critical and are chosen so that the precipitations are coherent with the copper matrix, i.e. the atomic layers of copper continue through the precipitations. Although the atomic layers continue through the boundary of precipitates the lattices on both sides of the booundary do not match each other completely. This mismatch causes strain which is responsible for the high mechanical strength.

Another consequence of ageing is that, as the number of foreign atoms in the matrix decreases the electrical conductivity becomes considerably higher.

The mechanical properties of Cu.Cr.Zr. are further improved by introducing a cold working process between solution annealing and ageing.

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