Microstructures and mechanical properties of innovative Al-Si-Mg alloys.

When considering structural applications, the maximum temperature of use of Aluminum alloys is limited by the degrading mechanical properties of Aluminum with temperature and, in the case of age-hardenable alloys, by the thermal stability of the precipitates formed during heat treatments. The addition of elements promoting the precipitation of coarsening-resistant particles, is one of the major and most effective ways to improve mechanical properties at elevated temperature. In this regard, transition elements and rare earths, added in fractions of unity in weight percent in the base alloy, can cause tremendous improvements in the thermal resistance of Al alloys. The present contribution focuses on specific additions made to a conventional Al-7Si-Mg casting alloy (A356). A set of alloys has been designed and cast with different Er and Zr additions. The microstructures of the alloys were characterized by means of optical micrographs, which showed the effectiveness of Er in modifying the Si eutectic, similarly to Sr, already in the as -cast state. Heat treatment cycles were performed and characterized by means of microhardness tests. It was found that, increasing the concentration of Er and Zr, the microhardness of the alloys in the peak hardness and after prolonged exposure to high temperatures was higher than that of the reference alloy.