Layer–dependent microstructure–property relationships in Al–Mg–Zn–Cu alloy fabricated by additive friction stir deposition at the optimal rotational speed

Layer–dependent microstructural heterogeneity is an inherent characteristic of additively manufactured aluminum alloys because cumulative thermal cycling and strain accumulation vary along the build height. However, quantitative understanding of how rotational speed governs build–height–dependent microstructural evolution and its correlation with mechanical behavior in additive friction stir deposition (AFSD) remains limited. This work systematically investigates the layer–dependent microstructure–property relationships of an Al–Mg–Zn–Cu alloy fabricated via AFSD under optimized rotational speed. In-situ process monitoring reveals that increasing rotational speed enhances heat generation and upsetting force, thereby influencing deposition stability. Spatial variations in thermal cycling and plastic deformation produced pronounced microstructural heterogeneity throughout the deposit. At 400 rpm, a balanced thermo–mechanical condition, characterized by a peak temperature of 402.4 °C and an average upsetting force of 7.6 kN, ensures stable deposition and achieved the highest tensile properties (ultimate tensile strength of 528.7 ± 3.7 MPa, yield strength of 353.0 ± 4.3 MPa, elongation of 15.7 ± 1.0%). Strength originates from combined contributions of second–phase precipitation, solid–solution strengthening, and dislocation strengthening. Layer–resolved characterization revealed that the lower layers contained finer recrystallized grains (bottom layer (L1): 1.5 ± 0.4 μm; top layer (L5): 2.4 ± 1.1 μm) but coarser and more abundant precipitates (L1: 249.2 ± 153.8 nm; L5: 165.2 ± 86.1 nm), predominantly Mg(Zn, Cu, Al) 2 and Al 2 Mg 3 Zn 3 . Correspondingly, mechanical properties varied along the build height, with the top layer exhibiting the best strength–ductility product.