Microstructure characteristics and mechanical properties improvement of gravity cast Al-7Si-0.4Mg alloys with Zr additions

Abstract

Al-7Si-0.4Mg alloys are one of the most commonly applied aluminum alloys in various casting processes. In this study, 0.06%, 0.14% and 0.20% Zr are added to gravity cast Al-7Si-0.4Mg alloys. The microstructural analysis of the as-cast alloys with Zr shows the formation of (Al,Si)3(Zr,Ti) and π-Fe phases. Compared with alloys without Zr, the alloys with Zr have finer grains; additionally, a small secondary dendrite arm spacing (SDAS) is obtained with a proper Zr content. After T6 heat treatment, the Mg in Fe-rich intermetallics diffuse into the matrix, decreasing the size of the intermetallic phases. Nanoscale precipitates of the (Al,Si)3(Zr,Ti) phase are found in α-Al grains. Due to the finer grains and smaller SDAS as well as the presence of (Al,Si)3(Zr,Ti) precipitates, the tensile strength and elongation of the alloys with 0.06% Zr are 332 MPa and 8.7%, respectively, whereas those of the Al-7Si-0.4Mg alloys without Zr are 302 MPa and 4.6%, respectively.