The Effects of Mg and Si Contents on the Microstructure and Solidification Behavior of Dilute Al-Mg-Si-Fe Alloys

The effects of Mg and Si contents on the microstructure and solidification behavior of dilute Al-Mg-Si alloys with about 0.1 wt.% Fe impurities were investigated using optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), energy dispersive X-ray spectroscopy (EDS), differential scanning calorimeter (DSC) and thermodynamic simulation. The results show that the grain size and secondary dendrite arm spacing of as-cast dilute Al-Mg-Si-Fe alloys decrease with the increase in Mg and Si content, and the grain size can be predicted using growth restriction factor Q. The increase in Mg content suppresses the transformation from α-AlFeSi to β-AlFeSi. However, the increase in Si content promotes α-AlFeSi converted to β-AlFeSi. In addition, the increase in either Mg or Si content decreases the melting point. The Mg/Si ratio can influence the formation of the eutectic structure as well as the type of Fe-bearing phase in it. The Fe-bearing phases in the eutectic structures of excess Mg and excess Si alloys are α-AlFeSi and β-AlFeSi, respectively. The results of thermodynamic simulation of solidification behavior are in good agreement with experiments.