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Plasma arc welding of precious metals and their alloys

A project was set up to investigate the weldability of precious metals used in jewellery manufacturing and silversmithing. The weldability of gold alloys and silver alloys and the appearance of the welds were excellent. Shielding with inert gases of the weld pool has to be good to avoid oxidation of the copper normally present in the alloys. Most alloys showed a lot of internal porosity after welding, probably caused by a too high content of impurities. Silver alloys have a very high heat conductivity, which leads to high heat inputs and large welding speeds and distortion. Silver alloy's tendency for hot-cracking can be avoided by careful setting of the welding parameters. A new type of silver alloy did give welds with a very small grain size. This has a strong beneficial effect on the formability and ductility of the welds.

The seam welding of gold alloys, platinum alloys and silver alloys is readily feasible, but in end-products are often not acceptable from an aesthetic viewpoint. Examples of successful welding are the longitudinal seams in gold bracelets and silverware, where after-treatment (hammering, drawing or coating) removes all visible traces of the former welding. Most parts in the manufacturing of jewellery are small and tack welding is all that is needed for joining. Relatively high currents (20 to 80 A) and very short welding times (60 ms to 1 s) are needed to get the desired results. In gold products tack welds are mostly sufficient for joining but in silver products the tack welding is often used for assembling parts and easing the consecutive brazing operation.

Special welding equipment was specified and developed. The microplasma equipment can also be used for tungsten inert gas (TIG) welding. Special features of the equipment are the stable welding arc for low current and very short welding times (from 10ms).

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Vitobello VAN DER SCHOOT
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