Impurity-Tolerance Design and Strengthening–Toughening Strategies in Recycled Aluminium Alloys

Recycled aluminium moves from downcycling towards the automotive sheet, structural castings and high-performance extrusions. The central constraint on closed-loop recycling is no longer the mere presence of impurities. It is the multiscale microstructure that those impurities generate during solidification, heat treatment and deformation, and how that microstructure participates in load bearing and damage. Iron is the most representative cumulative impurity because its equilibrium solubility in solid aluminium is low and its removal by conventional refining is difficult and costly. At a given Fe content, variations in Si, Mn, Cu and Cr, cooling rate and downstream processing can produce plate-like β-Al-Fe-Si, Chinese-script α-Al(Fe,Mn)Si, Al6(Fe,Mn), Al13Fe4 and Al7Cu2Fe intermetallics with distinct size distributions, shape anisotropy, three-dimensional connectivity, phase fractions and interface states. Consequently, Fe content has no alloy-independent monotonic relationship with strength, ductility or damage tolerance. This review follows a composition-phase selection-three-dimensional microstructure-mechanical response evidence chain to examine Fe-rich intermetallic formation and evolution, strengthening-damage competition, alloying and solidification control, thermomechanical processing, solid-state recycling and coupled Fe-Si-Cu-Mn-Zn effects. The available evidence supports defining impurity tolerance as a microstructural acceptance window tied to the alloy system, processing route and target property, rather than as a fixed compositional ceiling. Its assessment must at least account for the hazardous upper tail of particle-size distribution, morphology, three-dimensional connectivity, phase fraction, matrix strength and interface integrity. CALPHAD, in situ synchrotron methods, three-dimensional X-ray computed tomography and machine learning are shifting recycled-alloy design from grade-based impurity limits towards composition-process feasibility domains matched to scrap distributions. Comparable three-dimensional descriptors, quantitative matrix-particle-damage maps and robust design methods for variable feedstocks are now required to complete this transition.