Machinable porous gold structures for decorative applications made via supersolidus liquid phase sintering

Abstract

Typical 18 ct. gold alloys like white, yellow, and red gold lend themselves for supersolidus liquid phase sintering due to a sufficient difference between solidus and liquidus temperature. In search of non-conventional design opportunities for decorative applications, this fact has been successfully exploited in this feasibility study for the manufacturing of sintered 18 ct. gold fiber structures from all three gold varieties as well as a foam-type material from yellow gold. Obtained porosities ranged from approximately 50 to 70 % for the fiber structures and reached up to 94.5 % for the foams. In order to make precisely shaped parts from these porous gold materials, a high-speed milling route was developed. Determination of favourable sintering conditions A literature survey on sintering of gold alloys was conducted, the most relevant findings of which are discussed below. [1] reports on the powder metallurgical production of wedding rings made from 9 ct. gold alloy (37.5 wt% Au, rest Ag, Cu, Zn and others). In order to prevent Cu and Zn from oxidising, a controlled oxygen-free atmosphere is recommended. Sintering was carried out 20 degrees below the solidus temperature at 780 °C for 24 hours in an atmosphere comprised of 95 % N 2 and 5 % H 2 with good results. The production of metal injection moulded parts from 18 ct. gold alloys is reported in [2]. In comparison with precision-cast parts, the obtained microstructure is much finer and thus provides better properties. Debindering and sintering takes about 65 hours. From the diagrams provided in the paper, it can be deduced that the sintering time amounts to roughly 24 hours. According to the authors, sintering was carried out at a temperature of 80 to 90 % of the melting temperature. The quasi-binary phase diagram (Fig. 1) reproduced from [3] indicates that such a sintering temperature would clearly correspond to solid state sintering conditions. The applied sintering atmosphere was 80 % Ar and 20 % H 2. Like other authors, [2] acknowledges the role of the gas flow and recommends low mass flows in order to warrant equal temperature conditions throughout the samples. Problems with remaining porosity are explained in terms of inhomogeneous packing of the powder particles