A Review on the Criteria of Hot Tearing Susceptibility of Aluminum Cast Alloys

The development of new alloys characterized by a high strength-to-weight ratio and low production costs is one of the major objectives of the modern automotive industry with a view to lowering CO2 emissions and obtaining automotive castings that are both lightweight and capable of maintaining their performance under elevated temperature service conditions. The Al-2wt%Cu alloy is one such alloy; it contains 2wt% Cu, 0.05–1wt% Si, 0.4wt% Mg, 0.42wt% Fe, 0.7wt% Mn, and 0.02 wt%Ti (developed by General Motorrs-USA-coded GM220). The low Cu and Si content of the alloy, associated with an acceptable level of tensile strength, make this alloy suitable as a replacement for a number of other alloys such as the 319-type alloys used in engine blocks and cylinder heads. As a new alloy under development, the Al-2wt%Cu alloy has necessitated an in-depth investigation of its castability. Generally, the Al-2wt%Cu-based alloys exhibited higher resistance to hot tearing than 206-based alloys. It was found that an elevated mold temperature is beneficial in reducing the hot tearing susceptibility of the Al-2wt%Cu and 206 alloys in that the hot tearing susceptibility value decreased from 21 to 3, as the mold temperature was increased from 250 to 450 °C. The refinement of the grain structure obtained with Zr-Ti or Ti additions decreased the hot tearing severity as a result of an increase in the number of intergranular liquid films per unit volume and a delay in reaching the coherency point. Increasing the Si content reduced the hot tearing susceptibility of the Al-2wt%Cu alloy considerably; this reduction is attributed to an increase in the volume fraction of eutectic in the structure, and a decrease in the freezing range of the alloy. The addition of Sr caused a deterioration in the hot tearing resistance of the base alloy as a result of the formation of Sr-oxides and an extension of the freezing range of the alloy. It was also observed that α-Fe particles may obstruct the propagation of hot tearing cracks. The 1wt%Si-containing Al-2wt%Cu alloy was judged to be the best composition in view of its low hot tearing susceptibility.