Effects of tool pin profile in friction stir welding: a review on microstructural evolution and mechanical performance of alloy joints

Friction stir welding (FSW) is an eco-friendly, sustainable, solid-state process that is increasingly being used to join metallic, non-metallic, polymer, and composite materials to create high-quality welds with minimal flaws. The tool pins’ profiles govern material flow, heat generation, and weld integrity. The literature shows that threaded, taper threaded, triangular, and hybrid pins enhance mixing, grain refinement, hardness, and tensile strength, while cylindrical or smooth pins often cause defects. Microstructural investigations confirm that complex pin geometries promote finer grains and higher strength and hardness. A fracture analysis of welded samples reveals that shift in failure location from the nugget to the thermo-mechanically affected zone (TMAZ) depends on the geometry of the tool pin. Despite these advances, only a few studies have examined different materials, and standardised evaluation of tool geometries is lacking. Using computational and machine learning methods for predictive modelling, expanding applicability to lightweight alloys in aerospace and automotive manufacturing, and developing hybrid and adaptive pin profiles are the upcoming research priorities.