Optimization of 18-karat yellow gold alloys for the additive manufacturing of jewelry and watch parts

Gold and silver alloys are very difficult to process by laser melting because of their high reflectivity and thermal conductivity. The processing of 18-karat gold alloys was optimized in several steps by suitable alloying additions and a variation of the process parameters. The first step of the optimization was to improve the density and the surface quality of selective laser-melted (SLM) parts made of standard 3N yellow gold. The residual porosity was about 3-4%. In a second step, the gold alloy powder was surface treated to reduce the reflectivity and thereby increase the interaction with the laser beam. This resulted in a significant reduction of porosity to below 1%. Finally, the thermal conductivity of the alloys was reduced by microalloying. The type and amount of alloying elements were determined by means of thermodynamic simulations. Additions of iron and germanium were found to be suitable additions and resulted in a reduction of porosity to 0.3%. At the same time, the surface roughness was reduced significantly.