The effects of varying Mg and Si levels on the microstructural inhomogeneity and eutectic Mg<inf>2</inf>Si morphology in die-cast Al–Mg–Si alloys

Abstract

The effects of varying Mg and Si levels on the microstructural inhomogeneity and eutectic Mg2Si morphology in die-cast Al–Mg–Si alloys have been investigated. It was found both Mg and Si additions decreased the microstructural inhomogeneity by producing more well distribution of primary α-Al and Al–Mg2Si eutectics, but had contrary effects on eutectic Mg2Si morphology. The increasing Mg level transformed eutectic Mg2Si from rod or lamellae to curved flake with larger eutectic spacing λ, while the increasing Si level promoted the formation of rod-like or lamellar eutectic Mg2Si with smaller λ. The reason for the above evolutions can be traced back to alloys’ solidification behaviour. Thermodynamic calculation indicates that both Mg and Si decrease the liquidus temperature and suppress the precipitation of coarse primary α-Al grains (which tend to agglomerate in centre zone of samples) during the first solidification in shot sleeve, thus reducing the microstructural inhomogeneity. Mg addition shifts the eutectic point to lower Mg2Si concentration and induces a slower eutectic growth rate, causing a lower Mg2Si volume fraction in Al–Mg2Si eutectic cell. On the contrary, Si addition increased the Mg2Si volume fraction in eutectic cell by raising the Mg2Si eutectic concentration and the eutectic growth rate. To minimize the interfacial energy, Al–Mg2Si eutectics with different Mg2Si volume fractions exhibit various morphologies. The tensile test results show that both Mg and Si improved the strength at the cost of ductility. However, due to the formation of fine Al–Mg2Si eutectics, Si induced less ductility sacrifice than Mg when achieving the same strength improvement.