The effects of varying Mg and Si levels on the microstructural inhomogeneity and eutectic Mg2Si morphology in die-cast Al–Mg–Si alloys

The effects of varying Mg and Si levels on the microstructural inhomogeneity and eutectic Mg 2 Si 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–Mg 2 Si eutectics, but had contrary effects on eutectic Mg 2 Si morphology. The increasing Mg level transformed eutectic Mg 2 Si 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 Mg 2 Si 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 Mg 2 Si concentration and induces a slower eutectic growth rate, causing a lower Mg 2 Si volume fraction in Al–Mg 2 Si eutectic cell. On the contrary, Si addition increased the Mg 2 Si volume fraction in eutectic cell by raising the Mg 2 Si eutectic concentration and the eutectic growth rate. To minimize the interfacial energy, Al–Mg 2 Si eutectics with different Mg 2 Si 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–Mg 2 Si eutectics, Si induced less ductility sacrifice than Mg when achieving the same strength improvement.