Influence of Fe-rich intermetallics and their segregation on anodising properties of Al-Si-Mg rheocast alloys

During rheocasting of Al alloys, the component's surface can be enriched in Fe-rich intermetallics due to the surface liquid segregation or the interaction with the die material. Although this precipitates enrichment affects only the surface, it can have a big impact on the quality of the surface treatments or durability. In this paper, the effect of Fe-rich intermetallics on the hardness and corrosion resistance of the anodized layer of AlSiMg alloys was examined by using nanoindentation and electrochemical impedance spectroscopy (EIS) techniques. A sulfuric acid anodising process was performed on unground and mechanical ground surfaces of TX630 substrates produced by the rheocasting process. During anodising, Fe-rich intermetallics can be dissolved, leaving voids in the oxide during its growth. The anodised samples with two different oxide layer thickness (4 and 7 μm) were tested by EIS in 3 wt-% NaCl solution for 12 h. A comparison of the EIS results of unground and mechanically ground surfaces demonstrated that the mechanical grinding successfully removed the Fe enriched layer, improving corrosion protection of the anodised oxide. Moreover, the hardness measurements indicate that the oxide layer on the mechanically ground surface with low Fe content shows higher hardness than on the unground surface with high Fe content.