Settling and Growth of Fe-bearing Intermetallic Particles in Recycled Al-Si Alloys

Abstract To reduce the carbon footprint of aluminium production while meeting steadily increasing demand, aluminium recycling is increasingly pursued. However, one of the main limitations of aluminium recycling, which significantly reduces the production of recycled aluminium, is the high impurity content of the melt, particularly iron (Fe). A practical approach to reduce Fe content in the melt is to add Fe-bearing intermetallic particle formers, such as manganese (Mn) and chromium (Cr). These additions play a crucial role in neutralising and converting the harmful β -Al 5 SiFe phase to the thermodynamically stable α -Al 15 Si 2 M 4 phase ( M = Mn, Cr). The present work investigates the growth and sedimentation rates of Fe-bearing intermetallic particles under different Mn-Cr additions and cooling rates. Therefore, a physics-based growth-sedimentation model was developed to calculate the sedimentation rate of Fe-bearing intermetallic particles by accounting for particle growth, settling behaviour, sedimented height, and particle volume fraction. The model was calibrated and evaluated against experimental data for two different cooling rates obtained from small-scale experiments using Al-11Si-0.5Fe alloys. The model reproduced the measured interface height and captured the transition from growth-assisted to equilibrium-limited settling. Accurate calculations across cooling rates required cooling rate-dependent effective resistance parameters since different cooling rates affect the morphology of the Fe-bearing intermetallic particles (with slower cooling exhibiting higher effective resistance). In addition, Mn addition, in combination with Cr, further hinders particle settling, consistent with increased particle population and more complex morphologies that enhance hindered settling.