In situ synchrotron characterisation of Fe-rich intermetallic formation during the Solidification of Al-Si-Cu-Fe alloys

Abstract

The enrichment of Fe during aluminium recycling increases the quantity of Fe-rich intermetallics formed, particularly ß-Al5FeSi, limiting the usage of recycled Al in many fatigue-sensitive applications. In this study, fast synchrotron x-ray tomography was used to investigate microstructure evolution and defect formation in a commercial A319 alloy (Al-7.5Si-3.5Cu, wt.%) with differing Fe-levels (0.2-0.6 wt.%Fe) during in situ solidification and isothermal uniaxial tensile deformation. The captured dynamic changes were quantified using novel image analysis techniques and analysed using computational fluid dynamics. Together these provide new insights into the mechanisms of intermetallic nucleation and growth, and their influence on flow blockage and defect formation. First, time-resolved qualitative and quantitative characterisation revealed that plate-like ß-intermetallics nucleate both off the primary aluminium dendrites in the bulk of the specimen as well as off the oxide skin on the specimen surface. Second, ß-intermetallic formation is largely complete before the formation of the Al-Si eutectic. Third, ß-intermetallics are geometrically complex, demonstrating fast lateral growth and an ability to grow around and in between the primary dendrite arms. Last, direct impingement and potential branching are observed at a wide range of contact angles, indicating that the growth interaction might not be crystallographically related. The presence of ß-intermetallics contributes to several factors that influence defect formation. They block interdendritic flow, increase pore tortuosity and reduce permeability. Pores were observed to grow preferentially along the solid surface of intermetallics, suggesting that ß-intermetallics may reduce the gas-solid interfacial energy and thus facilitate the pore growth. However, they do not nucleate the porosity. The results also show that failure under mild uniaxial semi-solid tension, simulating hot-tearing, displays a much more brittle-like failure mechanism when large ß-intermetallics are present, compared to the more ductile behaviour of the base alloy with unresolved ß-intermetallics.