Adding Si Co. 18k Yellow Gold Alloy

Gold alloy is one of the most important jewelry material. Higher quality was inquired to the gold alloy with the developing of jewelry industry. The aim of this research is at high performance gold alloy. For this reason, a typical alloy containing Silver, Copper and Zinc usually used for jewelry manufacturing, has been used to prepare for master alloys with variable concentrations of Silicon and Cobalt; 18-ct gold alloys were also prepared.The effect of Silicon additions on gold alloys was studied first, and determined the best percentage composition of Silicon element. Then we studied the effect of combinations of minor Silicon and Cobalt additions on microstructure and hardness of alloys. We also discussed the behavior of age-hardening of Cobalt additions. All these were investigated by metallographic examination, X-ray diffraction (XRD), scanning electron microscope (SEM), Energy Dispersive Spectrometer(EDS), microhardness test. The results obtained were summarized as follows:It has been demonstrated that Small amounts of silicon can increase melt fluidity improving capability and mould filling, silicon can also act as deoxidizer conferring a clean aspect to the surface of final products.On the other hand, high silicon concentrations favor a grain coarsening that reduces the resistance to cycling heating. The increasing of Silicon concentration induces embrittlement in gold alloys. EDS and XRD analysis on fractured samples revealed the low-melting compound Au₂Si were exist at the grain boundaries of samples. The optimum Silicon concentration is 0.05% at 18-ct gold alloys.Cobalt is an effective grain refiner in the solidification of casting. With micro-addition of Co element, the dendritic crystals are cut into pieces in the master alloys while the metamorphism is inapparent in gold alloys. Grain size of master alloys and gold alloys decrease first and increase later with increasing Co addition. If the solubility of cobalt at solidification is exceeded(0.3%), detrimental side effects will occur, coarse segregations can be formed. These nests of particles detrimentally affect the polishing behavior.The alloys of Cobalt additions have considerable hardening behavior both in casting and age-treating. With the annealing temperature improving, the hardness of the alloys increases; as temperature reaches to 250℃, the hardness of the alloys reaches the maximum(242Hv); Then with the temperature increasing the hardness decreases. The optimum Cobalt concentration is 0.2%.The strengthening mechanisms of The alloy of Cobalt additions are that the casting is considered as fine grain strengthening and solution strengthening and the age-treating considered as dispersion strengthening and precipitation strengthening.The alloy of combinations of Silicon and Cobalt additions to form the compounds CoSi in gold alloys. The precipitation of these phases causes a stabilization of Si within the alloy, which decreases the Au₂Si and SiO₂ forming at grain boundaries thus preventing embrittlement.With the addition of Si elements, the color of gold alloys have a bit change in a axis of CIE lab Chroma space. The chrominance become pale. Micro-addition of RE has little influence in gold alloys. While With the addition of Co elements, the color of gold alloys change little.