Effect of conventional and rheocasting processes on microstructural characteristics of hypereutectic Al–Si–Cu–Mg alloy with variable Mg content

Hypereutectic A390 alloy (Al–17%Si–4.5%Cu–0.5%Mg (wt%)) and similar alloys with the addition of 6 wt% and 10 wt% Mg were investigated using conventional and rheocast processes. Conventional castings were produced with two different cooling rates: −0.15 ± 0.05 °C s−1 and −1.0 ± 0.2 °C s−1. The rheocast samples were subjected to a rotation speed of 260 rpm during solidification with a cooling rate of −0.15 ± 0.05 °C s−1. The microstructure of the conventional casting for the high cooling rate samples was found to be finer than for the low cooling rate samples. With high Mg content, the primary silicon transforms to primary Mg2Si and the nature of the eutectic reaction is also changed from a binary (Al + Si) to a ternary (Al + Si + Mg2Si) reaction. The morphology of eutectic silicon distributed in matrix microstructure significantly changes from large individual flakes to a fine, skeleton network with Chinese script morphology, similar to the eutectic Mg2Si. This is caused by the decrease of the eutectic formation temperature resulting from the addition of Mg. The matrix microstructure of the rheocast samples was found to contain fragmented eutectic silicon together with globular alpha aluminium phase particles. The morphology of eutectic silicon in rheocast samples becomes finer with increasing Mg content.