Recent Advances in Friction Stir Welding of Aluminum Alloys and Aluminum Metal Matrix Composites: Process Developments, Microstructural Evolution, and Mechanical Performance

Friction stir welding (FSW) is an established solid-state joining process for aluminium alloys and aluminium metal matrix composites (Al-MMCs), offering advantages over conventional fusion welding by reducing melting-related defects, distortion, and thermal degradation. However, weld quality and mechanical performance are governed by the coupled effects of process parameters, tool characteristics, thermal conditions, material flow, and, in Al-MMCs, reinforcement behaviour. Although several recent reviews have examined individual aspects of FSW, a comprehensive comparative synthesis linking process parameters and thermal/deformation mechanisms with microstructural evolution, defect formation, tool wear, and mechanical performance across conventional and advanced FSW of both aluminium alloys and Al-MMCs remains limited. This review addresses this gap by critically examining conventional FSW and selected advanced variants, including underwater/submerged FSW, vibration- and ultrasonic-assisted FSW, laser-assisted FSW, and electrical current-assisted FSW, from an integrated process–structure–property perspective. The central contribution of this review is a mechanism-oriented comparative framework that identifies how the different FSW approaches modify heat generation, heat dissipation, material plasticization, and material flow, and how these changes subsequently influence microstructure, defects, tool wear, residual stress, and mechanical performance. Particular emphasis is placed on rotational speed, traverse speed, cooling conditions, tool geometry and material, external energy assistance, dynamic recrystallization, reinforcement redistribution, and reinforcement–tool interactions. The comparison demonstrates that the effectiveness of each FSW variant is strongly dependent on the material system and processing conditions, limiting the applicability of universal process guidelines. For Al-MMCs, reinforcement fraction, particle size and distribution, particle fragmentation or clustering, and tool–reinforcement interactions introduce additional challenges compared with conventional aluminium alloys. Despite significant advances, challenges remain in controlling thermal conditions, achieving uniform material flow, detecting internal defects, minimizing tool wear, and translating advanced FSW variants from laboratory-scale investigations to industrial production. Future research should therefore focus on systematic comparative studies, in-process monitoring, data-driven optimization, sustainable manufacturing, and scalable process development to improve the reliability and industrial applicability of FSW for aluminium alloys and Al-MMCs.