Effects of cobalt content on microstructure and mechanical properties of hypereutectic Al–Si alloys

Cobalt is generally considered as the element that can promote the high-temperature mechanical properties of Al–Si alloys. In order to develop new hypereutectic Al–Si alloys that can be used at high temperature, the changes of microstructure of Al–20Si–2Cu–1Ni–0.6Mg–xCo alloy with different contents of Co were studied in this paper. The results show that the volume fraction of intermetallic compounds was increased with the increasing of Co content. The Co-bearing compounds presented as small block shape at first and gradually turned into dendritic or fish-bone shape. After Co addition, the Fe-bearing compounds such as Al9(Fe, Co, Ni)2 could be dissolved and spheroidized to a certain degree through heat treatment. The addition of Co has a positive effect on tensile strength of the Al–20Si–2Cu–1Ni–0.6Mg alloys that added equal or less than 0.7% Co. The maximum tensile strength at room temperature and elevated temperature (300 °C) was 290 MPa and 182 MPa respectively with a Co content of 0.7%. Furthermore, the content of Co varying from 0 to 1.5% had little influence on the refining effect of primary Si and modification effect of eutectic Si, under P modification of the alloy melt.