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

Zhu, Xiangzhen, Yang, Hailin, Dong, Xixi, Ji, Shouxun
Journal of materials science 2019 v.54 no.7 pp. 5773-5787
alloys, energy, magnesium, silicon, solidification, temperature, thermodynamics
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.