Quench sensitivity of A379 alloy under high-pressure die-casting and gravity-casting processes

Abstract

Adding Cr to Al–Si–Mg alloys can eliminate the adverse effects of Fe, but can increase the quench sensitivity of the alloys. The influence of gravity-casting and high-pressure die-casting on the quench sensitivity of A379 (Al–7Si–0.5Mg–0.2Cr) alloys were studied using time–temperature–property curves. The nose temperatures of the gravity-casting- and high-pressure die-casting-fabricated specimens are near 350 °C and 355 °C, respectively. The temperature regions with high quench sensitivity characteristics of the former and latter occur from 310 °C to 390 °C and from 290 °C to 400 °C, respectively. The time–temperature–property curves of the high-pressure die-casting-fabricated samples are farther to the left than that of gravity-casting-fabricated samples, indicating that the high-pressure die-casting-fabricated specimens are more sensitive and are easier to generate precipitates than the gravity-casting-fabricated specimens during quenching because of the following reasons. For both specimens, the quench-induced precipitates (Mg xSi) nucleate on the Cr-containing dispersions in the grains, but the Cr-containing dispersions in the high-pressure die-casting-fabricated specimens are finer and more dispersible than in the gravity-casting-fabricated specimens. Meanwhile, the high-pressure die-casting-fabricated specimens exhibited finer grain sizes than the gravity-casting-fabricated specimens; thus, they can provide more grain boundary nucleation sites for forming the Mg xSi phase than the gravity-casting-fabricated specimens during isothermal treatment.