Effect of isothermal ageing on the semi-solid microstructure of rheoprocessed and partially remelted of A390 alloy with 10% Mg addition

The semi-solid microstructure of commercial A390 (Al—17%Si—4.5%Cu—0.5%Mg) hypereutectic Al–Si alloy with an addition of 10% Mg was investigated for two different processing routes: 1) rheocasting after stirring with rotation speed of 260 rpm and 2) partial remelting after fast cooling in a steel mould. The results show that the morphology of α-Al grains becomes globular during isothermal holding time for both cases. However, at the same isothermal condition, the size of the α-Al phase particles for rheocast samples are larger and their morphology are more globular than for the samples examined after the partial remelting process. The microstuctural evolution, size and shape of the primary Mg₂Si as well as the silicon particles during isothermal ageing in the semi-solid region was also investigated for the two processing conditions.