Effects of Mg, Fe, Be additions and solution heat treatment on the π-AlMgFeSi iron intermetallic phase in Al–7Si–Mg alloys

Abstract

The presence of magnesium in Be-free Al–7Si–Mg alloys results in the formation of an undesirable iron-intermetallic known as the π-AlMgFeSi phase. The effect of Mg, Fe, and Be on the formation of this phase in both unmodified and Sr-modified Al–7Si–xMg–yFe alloys containing 0.4–0.8-wt% Mg and 0.1–0.8-wt% Fe has been investigated at a dendrite arm spacing of 65 μm. A qualitative microstructural examination was carried out to study the effect of solution heat treatment (540 °C/8 h) on the decomposition of the π-AlMgFeSi phase (“π-phase”) in Al–7Si–xMg–0.1Fe alloys containing 0.4–1.0-wt% Mg. The results indicate that increasing the Mg and Fe content increases the amount of the π-AlMgFeSi phase formed. Quantitative measurements revealed a reduction in the surface fraction of the π-phase after solution heat treatment. Different levels of decomposition of the π-phase into needles of β-Al5FeSi iron intermetallic phase (“β-phase”) were observed at 0.4-, 0.6-, and 0.8-wt% Mg, after solution heat treatment.