Microstructural evolution during semisolid processing of Al-Si-Cu alloy with different Mg contents

A series of Al-6Si-3Cu-(0.3-2)Mg alloys were produced by a conventional casting process. Cooling slope technique was employed to produce feedstocks before they were thixoformed at 50% liquid fraction. The effect of Mg on the microstructure of Al-Si-Cu aluminium alloys under as-cast and semisolid conditions was investigated. It was found that by adding Mg to Al-Si-Cu alloy, some of the Al2Cu phase and silicon were consumed to form Al5Cu2Mg3Si5 and Mg2Si phases. The needle-like beta-Al5FeSi phase transformed to Chinese-script-like pi-Al8Mg3FeSi6 with the addition of Mg. In the as-cast alloys, the primary alpha(Al) was dendritic, but as the Mg content increased, the phase became less dendritic. Moreover, the Mg addition considerably modified the size of the alpha(Al) phase, but it had no significant effect on the silicon morphology. In the thixoformed alloys, the microstructure showed a fine globular primary phase surrounded by uniformly distributed silicon and fragmented intermetallic phases. The eutectic silicon was modified from a flaky and acicular shape to fine fibrous particles. The effect of Mg on eutectic silicon during semisolid processing was evident. The primary Mg2Si particles were modified from big polygonal particles to become smaller and more globular, whereas the morphology of the Chinese-script-like pi-Al8Mg3FeSi6 changed to a compact shape. The results also exhibit that as the Mg content in the A319 alloy increased, the hardness, yield strength and ultimate tensile strength of the thixoformed alloys significantly improved, but the elongation to fracture dropped.