Elucidating the origin of microstructural gradients in AFSD 7475 alloy through a synergistic monitoring-modeling-validation approach

7xxx-series high-strength aluminum alloys are susceptible to hot cracking. Consequently, they are difficult to process by selective laser melting (SLM) or wire-arc additive manufacturing (WAAM). Additive friction stir deposition (AFSD) offers a solid-state alternative that avoids melting and solidification. However, its layer-by-layer nature imposes complex thermal cycles, generating pronounced microstructural and property gradients whose origins remain unclear. A closed-loop strategy integrating process monitoring, model refinement, and experimental validation, combined with multiscale characterisation, elucidates gradient evolution in AFSD 7475 aluminum alloy. Cumulative thermal cycles govern microstructural transformations and produce a distinct through-thickness mechanical gradient. At the top, dynamic recrystallization (DRX) forms equiaxed grains, while rapid cooling retains dense η′ precipitates, yielding UTS 498 MPa and 18.5% elongation. At the bottom, prolonged high-temperature exposure drives grain growth with dynamic recovery (DRV); precipitates nucleate on E-phase dispersoids, coarsen by clustering, and an interrupted-aging history with increasing dislocation density accelerates secondary precipitation. These effects reduce UTS to 325 MPa. The results link thermal-history gradients, microstructural evolution, and macroscopic properties, providing a mechanistic basis for microstructural control and property uniformity in AFSD aluminum components.