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 (β" phases). Also, the addition of 1.0wt.% Mg results in the formation of evenly distributed fine Mg 2 Si 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 Mg 2 Si phases. This causes significant loss of ductility, and the alloys show poor elongation to failure of ~2%. [ABSTRACT FROM AUTHOR]