Forming–Aging Pathways and Damage Tolerance in Al–Cu–Mg–Ag Sheets: Mechanisms, Evidence and Implications for Aircraft Skins

In Al–Cu–Mg–Ag sheets considered for aircraft skins, forming and aging alter the state relevant to subsequent fatigue and fracture. Favorable precipitation and tensile responses alone do not establish retention of a specified fracture or fatigue property. This critical narrative review evaluates composition and inherited state, deformation–aging sequence, and separately defined damage endpoints. Documented supplementary searches and evidence mapping support the authors’ engineering and scientific assessment. Chemistry and thermal history jointly control competing precipitates; a smaller Ω volume fraction can coexist with higher yield strength, while resistance to stable tearing can decrease after thermal exposure even when yield strength changes little. Responses to small tensile prestrain, rolling, loading during aging and warm compression depend on the initial state and intervention timing. Tear tests, three-dimensional damage observations, fatigue-life distributions and fatigue-crack-growth measurements reveal distinct roles for particles, boundaries, texture and initiation sites. Cross-over alloys offer bounded comparisons of forming–hardening strategies. The synthesis identifies controls for comparing complete processing routes and locating susceptible regions. Process-retained damage tolerance provides a reporting framework that preserves absolute performance alongside relative change and distinguishes mechanism identification from route optimization. Future work should connect mapped local histories to matched crack endpoints and representative curved structures.