Iron-rich intermetallic phases and their role in casting defect formation in hypoeutectic Al?Si alloys

Iron is the most common and detrimental impurity in aluminum casting alloys and has long been associated with an increase in casting defects. While the negative effects of iron are clear, the mechanism involved is not fully understood. It is generally believed to be associated with the formation of Fe-rich intermetallic phases. Many factors, including alloy composition, melt superheating, Sr modification, cooling rate, and oxide bifilms, could play a role. In the present investigation, the interactions between iron and each individual element commonly present in aluminum casting alloys, were investigated using a combination of thermal analysis and interrupted quenching tests. The Fe-rich intermetallic phases were characterized using optical microscope, scanning electron microscope, and electron probe microanalysis (EPMA), and the results were compared with the predictions by Thermocalc. It was found that increasing the iron content changes the precipitation sequence of the β phase, leading to the precipitation of coarse binary β platelets at a higher temperature. In contrast, manganese, silicon, and strontium appear to suppress the coarse binary β platelets, and Mn further promotes the formation of a more compact and less harmful α phase. They are therefore expected to reduce the negative effects of the β phase. While reported in the literature, no effect of P on the amount of β platelets was observed. Finally, attempts are made to correlate the Fe-rich intermetallic phases to the formation of casting defects. The role of the β phase as a nucleation site for eutectic Si and the role of the oxide bifilms and AlP as a heterogeneous substrate of Fe intermetallics are also discussed.