Dealing with Fe in Secondary Al-Si Alloys Including Metal Melt Filtration

AbstractFe is a detrimental impurity element in secondary, i. e. recycled, Al–Si cast alloys (Zhang et al. in Miner. Process. Extr. Metall. Rev. 33:99, 2012;Raabe et al. in Prog. Mater. Sci. 128, 2022;). It leads to decrease of castability and promotes crack formation due to formation of primary, Fe-containing intermetallic particles, e.g. plate-shaped β-Al–Fe–Si, coarse αh-Al–Fe–Si or αc-Al–(Fe,Mn,Cr)–Si in presence of further transition metal elements e.g. Mn and Cr. Successfully, dealing with such secondary Al–Si cast alloys contributes to sustainability, circular economy and reduction of energy consumption (Raabe et al. in Prog. Mater. Sci. 128, 2022;Taylor in Mater. Sci. Forum 689:429, 2011;). In the present chapter, a systematic understanding is provided for dealing with Fe impurities in secondary Al–Si alloys by. removal of Fe on the basis of melt conditioning and metal melt filtration and modification of Fe-containing phases into harmless microstructural components. In this context new insight is obtained into. the crystal structures of some relevant intermetallic phases occurring in secondary Al–Si alloys, thermodynamics and kinetics of phase formation during solidification and the interaction of different filter materials with the transition metal containing Al–Si alloys. The crystal structures of the β-Al–Fe–Si and δ-Al–Fe–Si phases and of their ordered variants were investigated. This allowed reliable distinction of occurring intermetallic phases, the αh-Al–Fe–Si, the αc-Al–(Fe,Mn,Cr)–Si, the β-Al–Fe–Si and the δ-Al–Fe–Si phase, especially by electron backscatter diffraction. While modification of the alloy composition by the Mn, Cr content and presence of other transition metal elements affect the thermodynamic properties of the phases, these elements also significantly affect the kinetics of phase formation during solidification at high cooling rates including the resulting phase morphology. The formation of primary, intermetallic phases during melt conditioning closely above the solidification temperature of the (Al)-solid solution can be utilized for the removal of Fe by separating the primary, Fe-containing, intermetallic particles from the Fe-depleted Al melt. Removal of such particles by application of filters to increase the Fe-removal efficiency extends the filters’ use beyond the removal of nonmetallic inclusions, contributing to production of high-quality, recycled Al–Si alloys. Evaluation of wettability, chemical reactions and microstructure in the interaction region between the filter material and Al–Si melts and the Fe-depleted melt reveals a beneficial effect of C-bonded Al2O3 filter material.