Enhanced Microstructural Refinement, Mechanical Performance, and Tribological Properties of Al‐9. <scp>2Si</scp> ‐4. <scp>9Cu</scp> ‐0. <scp>7 Mg</scp> Alloy via Friction Stir Processing

ABSTRACT In this study, the microstructural evolution, mechanical performance, and tribological behavior of an Al‐9.2Si‐4.9Cu‐0.7 Mg alloy subjected to friction stir processing (FSP) were systematically investigated. Plates were processed at tool traverse speeds of 40, 60, and 80 mm/min (P‐40, P‐60, and P‐80), while extreme conditions (20 and 100 mm/min) were excluded due to tunnel defect formation. FSP refined the as‐cast coarse microstructure, reducing average grain size from ∼5.2 μm at 40 mm/min to ∼2.6 μm at 80 mm/min through dynamic recrystallization and particle‐stimulated nucleation. Simultaneously, fragmentation and uniform redistribution of eutectic Si and Cu/Fe‐rich intermetallics improved phase homogeneity, while nanoscale Al 2 Cu (Θ‐phase) precipitates contributed to precipitation strengthening. Hardness measurements revealed an increase from 108.6 ± 2.8 HV in the base alloy to 157.1 ± 1.2 HV (P‐40) and 171.8 ± 2.4 HV (P‐80). Tensile strength improved markedly from 273.5 ± 10.1 MPa in the as‐cast alloy to 492.1 ± 10.2 MPa (P‐40) and 596.5 ± 9.7 MPa (P‐80), accompanied by enhanced elongation (from 1.14% ± 0.4% to 2.8%–3.1%) and toughness (from 257.9 ± 8.9 MJ·m −3 to &gt; 1400 MJ·m −3 ). Wear resistance was also significantly enhanced, as the wear rate decreased from 6.9 ± 0.2 μg/m in the base alloy to 4.1 ± 0.3 μg/m (P‐40) and 3.5 ± 0.2 μg/m (P‐80), while the friction coefficient was reduced from 0.54 ± 0.03 to 0.31 ± 0.02.