Influence of casting techniques on hardness, tarnish behavior and microstructure of Ag-Cu-Zn-Si sterling silver jewelry alloys
The effects of Zn and Si on the hardness, tarnish resistance and microstructures of experimental Ag-Cu-Zn-Si alloy were investigated by means of vickers hardness, tarnish testing, optical and scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The various compositions of 935 sterling silver alloys produced by different casting techniques, investment (lost wax) process and metal mould casting, were compared. An induction casting machine with a vacuum system was used for investment casting at 1,025°C with the mould temperature at 600°C whereas a LPG/oxygen single flame tip torch was used for the metal mould process. Vickers hardness of the samples produced via metal mould casting has higher hardness than that of the samples obtained by investment casting. The hardness of standard 935Ag sterling samples produced by metal mould casting was around 66 HV while it was around 56 HV for the investment cast samples. The increase in hardness is related to differences in the cooling rate of solidification rather than to the addition of third elements. The tarnish test was performed by immersing the samples in 0.1% Na2S solutions for 15-180 min. To specify the color change by tarnishing, the surface color differences (DE*) were measured according to the Commission International d’ Eclairage (CIELAB) standard. It was found that the tarnish resistance was improved with high additions of elements especially Si. Furthermore, Si addition plays an important role in a complete microstructural modification, which coincides with the increase in tarnish resistance of sterling silver alloys, in terms of transforming a lamellar eutectic structure to a predominantly Cu-Si continuous structure.
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