Formation of primary intermetallic phases in the interaction of Fe-containing melts of recycled Al-Si alloys with Al₂O₃ and Al₂O₃-C filter materials: A µ-CT study

Fe is the most detrimental impurity element in recycled Al-Si alloys due to the formation of brittle, primary, Fe-containing, intermetallic particles during solidification. Their removal from the Al-Si melts e.g. by filtration can reduce the Fe content. New active filter materials can facilitate the formation of these particles for their removal and contribute to the production of high-quality, recycled Al-Si alloys. The interaction of the alloy with the filter material can lead to modification of the thermodynamics of the alloy or of the kinetics of the particle formation. Time-resolved, three-dimensional microstructural investigations have been carried out to study the formation of primary intermetallic particles in Al7.1Si1.5Fe and Al7.1Si0.75Fe0.75Mn alloy melts in contact with Al₂O₃ and Al₂O₃-C filter substrate material during a melt conditioning treatment at 620°C. The microstructures, in particular the primary intermetallic particles α_c and α_h, have been characterized by computed tomography (CT) and supplementary scanning electron microscopy (SEM). As expected by thermodynamics, the total volume fraction of primary particles remains unchanged by the interaction with the substrate materials. However, kinetic advantages for Fe-removal efficiency can be achieved by an accelerated and preferred selective particle formation in contact with the Al₂O₃-C material. Furthermore, particle formation is discussed in view of its different stages: nucleation, growth, and ripening.