Nucleation control strategy for sustainable aluminium: Improving Fe removal efficiency or increasing Fe-tolerance

Abstract

Achieving ultra-low Fe level or refining Fe-containing intermetallic compounds (Fe-IMCs), both governed by heterogeneous nucleation, remains a major challenge for development of high-performance recycled Al alloys. This study demonstrates that the sensitivity of Fe-IMC formation to casting conditions is dictated by nucleation difficulty, which is controlled by both kinetic factors (diffusion time) and thermodynamic driving forces (nucleation and continuous undercooling). We provide the first direct evidence for dual-size primary Fe-IMCs and their distinct nucleation pathways: large P1-α-Al₁₅(Fe,Mn)₃Si₂ particles originating from non-equilibrium θ-Al₁₃Fe₄ nucleated at higher temperatures, and nanoscale P2-α-Fe particles nucleating heterogeneously on MgAl₂O₄ oxides at lower temperatures with larger nucleation undercooling. Building on this new mechanistic understanding, two casting strategies were developed: (1) promoting Fe-IMC nucleation to enhance Fe removal down to 0.3 wt%, and (2) suppressing Fe-IMC formation to increase Fe tolerance and refine second-phase particles, enabled by tuning pouring temperature, cooling rate, and casting routes. A comprehensive process map linking Fe-IMC formation to cooling rate and pouring temperature is established, providing a predictive framework for process optimization. These insights position nucleation-control-based design as a powerful approach for sustainable aluminium production.