Mechanical Properties of Al-Si-Cu-Mg Cast Alloys: Effects of Tramp Elements

Critical automotive applications using heat-treatable alloys are designed for high values of mechanical properties, which can be improved using a specified heat treatment. Castings were prepared from both experimental and industrial 319 alloy melts containing 0 to 0.6wt% Mg. Impact test bars were cast in two different cooling rate molds, namely a star-like permanent mold and an L-shaped permanent mold, with DASs of 24 mu m and 50 mu m, respectively. Tensile test bars were cast in an ASTM B108 permanent mold with a DAS of 25 mu m. The bars were tempered at 180C (356F) and 220C (428F) for 2-48 h. The results showed that Mg content, aging conditions and cooling rate have a significant effect on the microstructure of both experimental and industrial alloys and, consequently, on the mechanical properties. The addition of Mg resulted in the precipitation of the beta-Mg2Si, Q-Al5Mg8Cu2Si6, pi-Al8Mg3FeSi6 and of the block-like theta-Al2Cu phases. The presence of Mg and Cu, as well as the higher cooling rates improved the alloy strength, ductility and hardness values, especially. in the T6 heat-treated condition. As a result of this treatment, the alloys show hardening after up to 24 h of aging time because of the influence of several hardening phases. The activity of Cu significantly lowers the impact properties, which are determined mainly by the Al2Cu phase and not by the eutectic Si particles. The addition of Mg was also seen to diminish the effects of impact toughness. The crack initiation energy in these alloys is greater than the crack propagation energy, reflecting the high ductility of Al-Si-Cu-Mg base alloys. Applying aging at 220C, (428F) causes overaging and alloy softening after 2 h of aging time. The experimental alloys demonstrate higher values of mechanical properties than the industrial alloys.