Deformability of white gold alloys and influence of heat treatments on the mechanical properties
In the last decade the gold industry has been subject to a high degree of automation and to a high development in terms of technological devices. However what seems to be lacking is the approach oriented towards the engineering of the sector, intended as a synchronised study of the material characteristics, related to the process and to the production techniques. The technical development of gold industry, when present, does not show innovative peculiarities: the tendency is to adopt and modify alloys and processes developed for other sectors related to "more traditional" industrial areas. At present the approach to design and production is mainly related to four different phases: The identification of a trend or of an historical style The choice of a range of prices The selection of the material alloy, especially as function of market values The selection of the production processes, as consequence of the amount of objects to obtain Dealing with the jewellery manufacturing, only in few cases a defined process can be exactly controlled; even fewer are the cases in which designs are obtained with the aim of exploiting the potentialities of a production process at its best. The work is focused on a Ni based white gold alloy. White gold alloys have been developed during the '20s as substitutes of platinum. Different types of alloys were studied and produced since then, mainly nickel and palladium based ones. Many are the differences: mechanical properties, microstructures, different costs, etc.. The main aim of the research is to determine the best sequence of thermal and mechanical treatments to be carried out on the material in order to optimise its production processes. The characterisation was done through tensile tests, optical microscopy and a specific study offractography using SEM. The added master alloy had the following composition: 56% copper, 30% nickel, 13,941% zinc, 0,05% iridium, 0,009% silicon. A 7 mm diameter bar was used as starting point of the research. It has been produced in continuous casting. Different bars have been cut from the master one for the characterisation of the as cast material. The remaining part has been rolled to obtain a dimension of 90% of starting one (10% deformation). Bars have been obtained for the characterisation also in this case. Subsequent operations have lead the starting bar to dimension from 80% to 30% of the initial value, using step of 10% deformation. For each value different samples have been obtained. All samples have been tensile tested in order to determine the mechanical characteristics as function of the applied deformation. For each degree of deformation different samples for microscopy were obtained, both in the direction of deformation (longitudinal) and in the perpendicular one (radial). Polished samples were etched with a solution of CrO2 in HCl, in order to put in evidence the microstructure. Moreover fracture surfaces of tensile tested samples were observed using SEM. At the end of the characterisation processes of deformed material (from 0 to 70%, as mentioned before), an evaluation of the results to choose the proper level of deformation in order to perform a thermal treatment on the bar was car- ried out. The idea was to choose the maximum level of deformation which did not show any micro-crack on the surface. Different re-crystallisation thermal treatments were identified and carried out. The rolled material were thermally treated at first at 650°C, 700°C or 750°C for 20 minutes in a H2 - N2 atmosphere, water cooled and further rolled starting from a 10% deformation (dimension after rolling = 90% starting dimension) to a 70% deformation, with a step of 10% deformation from one rolling to the following. Once performed the treatments, a new series of analysis was done; the procedure being similar to the previously mentioned one, that is to say an evaluation of mechanical and micro-structural characteristics of rolled samples after thermal treatment, with increasing values of deformations from 0 to 70% of the starting value (considering, in this case, the zero value that after treatment). Microstructural analysis carried out at the beginning of the research pointed out the presence of dendritic structures. This is mainly due to the continuous casting process used for the production of bars, since the slow cooling of the process has influenced the homogeneity of the alloy. The following step has led to the determination of a maximum deformation level of rolling after which the examined alloy has to be thermally treated: it is 50%. The structure of the material after the treatment at 650°C for 20 min in H2 e N2 atmosphere and after the following rolling was then observed. In order to improve the homogeneity of the alloy and to get a final austenitic structure, other treatments were carried out, increasing the temperature of the process. The exam of the results underlined that, among all, the best performing treatment for the examined white gold alloy is a rolling at 50%, a thermal treatment at 750° C for 20 minutes and a following deformation at 30%; this sequence allows to obtain a fair austenitic microstructure and tensile properties reaching approximately 900 MPa; this value is high enough for the jewellery applications.