EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Fe is a detrimental impurity element in secondary Al-Si cast alloys leading to the formation of primary, Fe-containing intermetallic particles, e.g. β-Al4.5FeSi, αh-Al7Fe2Si or αc-Al9(Mn,Fe)2Si1.8, which cause a decrease of castability and promote crack formation. The Fe content can principally be reduced by removal of the primary particles. Melt conditioning, i.e. melt treatments closely above the onset-temperature of Al solidification, and addition of specific alloying elements as Mn facilitate their formation. In a previous study, various oxide and carbon-containing filter materials were tested to increase the Fe-removal efficiency revealing an especially promising utilization of carbon-bonded Al2O3 (Al2O3-C). The present study focuses on its interaction with Al-Si melts in view of wettability, chemical reactions and microstructure in the interaction region.

Model sessile-drop and small-scale crucible experiments with Al7.1Si, Al7.1Si1.5Fe and Al7.1Si0.75Fe0.75Mn alloys on Al2O3 and Al2O3-C filter materials were performed at 950°C and 620°C. Cross sections through alloy and filter material were employed for microstructure analysis with SEM/EBSD and SEM/EDS.

The Al-Si melts in contact with Al2O3 represent non-reactive, low-wetting systems acting as reference for the reactive systems with the Al2O3-C substrate. In presence of Fe in the melts, primary αh or, with additional Mn, αc particles form. In case of Al2O3-C, a thin layer of Al4C3 carbide forms at the interface to the Al-Si melts. A comparably larger fraction of primary particles has formed which are specifically attached to the carbide layer. This is associated with a reduced Fe-content in the remaining melt compared to the non-reactive systems. Presence of Mn results in a further reduction of the transition metal content. The particle attachment is associated with oriented growth of the carbide layer and the crystallographic orientation of the particles. Thus, Al2O3-C in combination with Mn can be beneficially utilized to increase the Fe-reduction efficiency in secondary Al-Si alloys.