Recent advances in mechanical and tribological enhancement of aluminium-based friction stir surface composites: A focused review

Friction Stir Surface Composite (FSSC) processing has emerged as an effective solid-state surface-modification technique for enhancing the mechanical and tribological performance of aluminium alloys and aluminium-based surface composites. It is a derivative of friction stir welding in which a rotating pinned tool severely plastically deforms the surface, refines the grain structure, and disperses hard reinforcement particles into the surface matrix. This focused review presents a critical, methodologically transparent assessment of recent developments in aluminium-based FSSCs reported between 2010 and 2026, with particular emphasis on studies published during 2020–2026. Following a PRISMA-style search protocol, the influence of matrix alloys, reinforcement strategies, process parameters and multi-pass processing on hardness, tensile strength, ductility, and wear behaviour is analysed. The roles of the major strengthening mechanisms — Hall–Petch grain refinement, Orowan strengthening, load transfer, dislocation strengthening and tribofilm formation — are examined together with the conditions under which each dominates and their approximate quantitative contributions. FSP/FSSC is also compared with competing surface-modification and fabrication routes, and its limitations, the hybrid ceramic–lubricant trade-off, and open research gaps are discussed. Hybrid, nano and sustainable reinforcement systems are highlighted as the principal current trends.