Probing the mechanisms of morphological evolution and phase selection of intermetallic compounds for impurity-tolerant processing of recycled Al alloys

Fe-rich intermetallic compounds (IMCs) are a persistent challenge in the recirculation of secondary aluminium alloys. Despite significant research effort, largely via post-solidification studies, the mechanisms governing IMC phase selection in higher-Fe ($>1$ wt.%), recycled Al alloys and how they can be controlled to facilitate more benign IMC species and/or morphologies remain poorly understood. This creates barriers to compositional and process design for more Fe-tolerant alloys. In this paper, we present a systematic real-time investigation of IMC formation, phase selection and morphological evolution in recycled 3xx series Al alloys with elevated Fe concentrations (up to 2.5 wt%), using in situ synchrotron X-ray radiography. Coupled with thermodynamic simulations, we develop a method to reliably estimate the formation temperatures of primary $\alpha$-AlFeSi and $\beta$-AlFeSi IMCs, and show direct insights into their formation sequence and kinetics. Contrary to widely held assumptions based on low Fe-containing ($<$0.6 wt%) primary alloys, we show that in recycled alloys containing higher Fe concentrations, increased cooling rate significantly promotes the formation of the more anisotropic $\beta$-AlFeSi (over the more compact $\alpha$-AlFeSi), which however can be fully suppressed at slow cooling. We propose how a solute-suppression mechanism kinetically controls the $\alpha$/$\beta$ IMC phase evolution. Further, we reveal and quantify a faceted-to-non-faceted morphological transition of $\alpha$-AlFeSi from a faceted polyhedral to non-faceted near-equiaxed dendritic morphology. This transition is governed by an interplay between solidification velocity and liquid undercooling at the local IMC/liquid interfaces. This study provides insights into how solidification conditions may be leveraged to improve microstructural control in high Fe-containing recycled alloys.