Enhanced Microstructural Refinement, Mechanical Performance, and Tribological Properties of Al-9.2Si-4.9Cu-0.7 Mg 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 Al2Cu (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 > 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.