Thixocasting of hypereutectic Al–25Si–2.5Cu–1Mg–0.5Mn alloys using densified powder compacts

Hypereutectic Al–Si alloys with fine and evenly distributed Si grains have superior mechanical characteristics and are usually fabricated by powder metallurgy (PM). However, PM methods cannot easily achieve the net-shape forming of complicated engineering parts. This work presents a novel method, which combines consolidating prealloyed powders with thixocasting the consolidated compacts, for net-shape forming of a hypereutectic Al–25Si–2.5Cu–1Mg–0.5Mn alloy. Experimental results indicate that as-thixocast specimens fabricated with larger powders and greater deformations have higher tensile strengths. Fractographs show that the strengths were strongly affected by the detrimental oxides that originated in the powder compacts, and that the oxides could be effectively eliminated by plastic deformation during thixocasting. Isothermal heat-treatment tests of the powders revealed two distinct ranges of Si grain-coarsening rates in the semi-solid temperature range. The Si grains ripened slowly at temperatures from 540 to 560 °C, considered as the appropriate processing temperature range for obtaining fine Si grain. This low coarsening rate was explained by the diffusion of Si through the semi-solid matrix that only consists of a little liquid, since Si diffuses in solid Al much more slowly then in liquid Al.