Property optimisation of alumina-reinforced aluminium-based surface composites through multi-pass friction stir processing

With increasing demands for lightweight, high-strength materials in aerospace, automotive, and marine applications, conventional aluminium alloys often require surface modifications to enhance wear resistance and mechanical properties. To address this need, Al₂O₃-reinforced Aluminium 5052 surface composites (SCs) were fabricated using five-pass friction stir processing (FSP), and their microstructural and mechanical properties were systematically evaluated. Microstructural analysis revealed a remarkable grain refinement, with the average grain size reducing from ∼100 µm in the base metal (BM) to ∼10 µm in the SC due to dynamic recrystallisation induced by FSP. The uniform dispersion of Al₂O₃ particles further contributed to strengthening mechanisms, leading to a 31 % increase in microhardness, from ∼85 HV in the base alloy to ∼111 HV in the composite. Tensile tests demonstrated substantial improvements, with ultimate tensile strength (UTS) rising by ∼18 % (from ∼254 MPa to ∼298 MPa) and yield strength (YS) increasing by ∼14 % (from ∼238 MPa to ∼271 MPa), while maintaining acceptable ductility. These enhancements are attributed to grain boundary strengthening, Orowan strengthening, and load transfer effects from the Al₂O₃ reinforcements. The study confirms that multi-pass FSP is an effective technique for developing high-performance aluminium SCs with superior mechanical properties for advanced engineering applications.