The effect of Mg on the structure and properties of Type 319 aluminum casting alloys

The precipitation hardening behavior of a Type 319 aluminum alloy (Al–6.7 wt.% Si–3.75 wt.% Cu) with and without 0.45 wt.% Mg has been investigated. It is shown that both θ and Q phase exist in the as-cast samples and are dissolved during solution heat treatment. Subsequent artificial aging results in the precipitation of both the metastable θ′ phase typical of aged alloy 319 and, in the case of the Mg-containing alloy, an additional and extremely small phase, which was identified as Q′ based on previous literature. Both types of precipitates have been characterized using transmission electron microscopy (TEM) and three-dimensional atom probe (3DAP) tomography. The Mg-containing alloy exhibits higher strength and lower ductility, presumably due to both solid solution strengthening and the presence of the fine Q′ phase.