Recent advancements in friction stir welding of dissimilar alloys: An in-depth and systematic analysis

Friction stir welding (FSW) is an advanced solid-state joining technique that has transformed the fabrication of both similar and dissimilar metallic materials. Unlike conventional fusion welding, FSW joins materials below their melting temperatures, thereby minimizing common welding defects such as porosity, hot cracking, excessive distortion, and the formation of undesirable intermetallic compounds (IMCs). Owing to these advantages, FSW has emerged as a preferred joining process for dissimilar metal combinations, including aluminum–magnesium (Al–Mg), aluminum–copper (Al–Cu), aluminum–titanium (Al–Ti), and aluminum–steel (Al–steel), which are increasingly used in aerospace, automotive, marine, railway, and defense applications to produce lightweight, high-performance structures. Extensive research has been conducted to overcome the challenges associated with dissimilar FSW. For Al–Mg and Al–Cu joints, the primary concerns include excessive IMC formation, galvanic corrosion, and accelerated tool wear. In contrast, Al–Ti and Al–steel combinations are particularly difficult to weld because of their large differences in hardness, melting temperature, and thermal conductivity, which adversely affect material flow, interfacial bonding, and tool life. To address these challenges, several innovative approaches, such as double-pass FSW, stationary shoulder tools, interlayer-assisted welding, and hybrid processing techniques incorporating interfacial coatings, have been developed. These advancements have significantly improved weld quality, mechanical performance, and process reliability. Computational models have been widely used to simulate heat generation, material flow, strain distribution, and residual stress evolution, enabling better prediction of defect formation, IMC growth, and process optimization. Furthermore, advanced characterization techniques, including scanning electron microscopy (SEM), X-ray diffraction (XRD), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and nanoindentation, have provided valuable insights into microstructural evolution, phase formation, mechanical behavior, and fracture mechanisms of dissimilar joints. This review presents a comprehensive and critical assessment of recent developments in the FSW of dissimilar metals. It systematically discusses the influence of material combinations, welding parameters, interlayer materials, and post-processing treatments on joint performance. The review also highlights persistent challenges, including brittle intermetallic formation and thermal and mechanical property mismatches, while evaluating current strategies for mitigating these issues. Comparative figures and tables summarize the reported mechanical properties, microstructural characteristics, and corrosion behavior under various processing conditions. By consolidating recent advances and identifying existing research gaps, this review provides valuable guidance for future investigations aimed at developing reliable, defect-free, and high-strength dissimilar metal joints to meet the evolving demands of next-generation manufacturing and lightweight engineering applications.