Effects of Magnesium on the Microstructure of Hypereutectic Al–20Si Alloys

A detailed analysis of the effects of magnesium on the microstructure of hypereutectic Al–20Si alloys is provided in this study. Experimental results show that the addition of Mg significantly refines the primary silicon phase relative to the unmodified Al–20Si alloy, transforming its morphology from a complex form to a singular plate-like structure. Notably, for the first time, equiaxed aluminum grains appear in the aluminum matrix under conventional solidification conditions. The generation of these grains is closely related to the quenching effect caused by rapid cooling during metal mold casting, which promotes the generation of equiaxed aluminum grains within tightly constrained temporal and spatial parameters. The Al–Si eutectic structure exhibits a regular lamellar morphology, with an average eutectic silicon spacing of 930.97 nm. The phase analysis shows that the alloy mainly consists of Al, Si, and Mg2Si phases after the addition of Mg. With the increase in Mg concentration, the diffraction peaks for Al(200) and Si(220) first shift to lower angles and then move to higher angles, along with significant peak broadening. Ambient temperature mechanical testing indicates that tensile strength first increases with increasing Mg concentration, then declines, with the highest tensile strength of 235.1 MPa at 3 wt.% Mg in the Al–20Si alloy. The fracture mechanism of the testing specimens changes from cleavage fracture to ductile fracture. Microhardness testing indicates a continuous increase in the hardness of the aluminum matrix with rising Mg concentration; the hardness of primary silicon declines first and then increases, whereas the hardness of the eutectic structure exhibits a first increase followed by a decline.