Effect of the addition of Ni and Cu on solidification microstructure evolution of Al-5Si-0.5Mg alloys

To develop new high-performance hypoeutectic Al-Si-Mg alloys at elevated temperatures, it is often necessary to add alloying elements of Cu and Ni. The high-temperature strength of the most used Al-Si-Mg-Cu-Ni alloy depends on a load transfer from the alpha-Al matrix to the intermetallic compounds. However, there is still a lack of detailed investigation on the effects of the combined addition of Ni and Cu on the solidification microstructure evolution of Al-Si-Mg base alloys with a lower Si concentration. This investigation specifically addresses the effects of the addition of Cu or Cu and Ni on the solidification microstructure evolution of the Al-5Si-0.5Mg alloy. It was found that when Al-5Si-0.5Mg is alloyed with Cu or Cu and Ni, the solidus temperature decreases, while the freezing range of the Al-5Si-0.5Mg alloy increases. With the addition of Cu and even small amounts of Ni, the phases Al7Cu4Ni and Al5Cu2Mg8Si6 were predicted by TC simulations and also observed by SEM. With a higher Ni content, Al3Ni and Al3Ni2 also solidify. More importantly, the presence of Cu and Ni can partition into the alpha-AlSiFeMn phase and change the size and morphology of the alpha-AlSiFeMn phase. This investigation provides a helpful hint to the development of the high-performance Al-Si-Mg-Cu-Ni alloys with a simultaneous improvement of the thermal conductivity and mechanical properties.