Effect of Mg content on age-hardening response, tensile properties, and microstructures of a T5-treated thixo-cast hypoeutectic Al-Si alloy

Effect of Mg content on age-hardening response, tensile properties, and microstructures of a T5-treated thixo-cast Al-7wt.%Si-0.01wt.%Sr alloy has been investigated. The strengths of the T5-treated alloy increase with increasing the Mg content due to the dispersion of high density of needle-like precipitates (beta '' phases). Also, the addition of 1.0wt.% Mg results in the formation of evenly distributed fine Mg2Si phases and Fe-rich compounds in the eutectic Si regions, and therefore, the Al-7wt.%Si-1.0wt.%Mg-0.01wt.%Sr alloy exhibits high 0.2% proof stress of 241 MPa and moderate elongation to failure of 5.1%, which are comparable to those of T6-treated gravity-cast Al-7wt.%Si-0.3wt.%Mg (ASTM A356) alloys. Although the addition of 1.5wt.% Mg or more further improves the strengths, the excess additions of Mg lead to the formation of coarse Mg2Si phases. This causes significant loss of ductility, and the alloys show poor elongation to failure of similar to 2%.