Evolution of Fe bearing intermetallics during DC casting and homogenisation of an Al-Mg-Si Al alloy

The evolution of iron (Fe) bearing intermetallics (Fe-IMCs) during direct chill (DC) casting and homogenisation of a grain refined 6063 aluminium-magnesium-silicon (Al-Mg-Si) alloy has been studied. The as-cast and homogenised microstructure contained Fe-IMCs at the grain boundaries and within Al grains. The primary α-Al grain size, α-Al dendritic arm spacing (DAS), IMC particle size and IMC three dimensional (3D) inter-connectivity increased from the edge to the centre of the as-cast billet, both αc-AlFeSi and β-AlFeSi Fe-IMCs were identified, and overall αc-AlFeSi was predominant. For the first time in industrial billets, the different Fe-rich IMCs have been characterised into types based on their 3D chemistry and morphology. Additionally the role of β-AlFeSi in nucleating Mg2Si particles has been identified. After homogenisation, αc-AlFeSi predominated across the entire billet cross-section, with marked changes in the 3D morphology and strong reductions in inter-connectivity, both supporting a recovery in alloy ductility.