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.