Effect of V and Zr microalloying, and heat treatment on microstructure and mechanical properties of secondary Al-7Si-3Cu-0.3Mg alloy

The Al–Si–Cu alloys, which are widely used in automotive powertrains, exhibit limited high-temperature strength properties; the high diffusivity of the main strengthening elements Cu and Mg in a-Al at temperatures between 200 and 300 8C is a dominating factor in alloy softening. In this study, effects of slow-diffusing elements (Zr and V) and heat treatment on the microstructure and mechanical properties of secondary Al-7Si-3Cu-0.3Mg alloy were investigated. Majorities of both Zr and V were retained inside the a-Al matrix during solidification. T6 heat treatment induced the solid-state precipitation of multiple, nano-sized particles in a-Al grain interiors. Unlike Cu/Mg-rich strengthening precipitates that form during aging, the Zr/V-rich precipitates formed during solution heat treatment, which indicates high potential for high-temperature strengthening in Al–Si alloys via transition metal addition. Other transition metals, such as Mn, Fe, Cr and Ti, which were present as impurities in the base alloy significantly promoted the formation of nano-sized Zr/V-rich precipitates inside a-Al grains. These thermally more stable precipitates were credited for the enhanced high-temperature strength properties of Al-7Si3Cu-0.3Mg alloy by 20 %.