Bobbin Tool Friction Stir Welding: A State-of-the-Art Review on Process Mechanics, Material Behavior, Challenges, and Future Perspectives

Bobbin Tool Friction Stir Welding (BT-FSW) is an important variant of friction stir welding that employs a double-shoulder tool configuration to produce fully penetrated welds in thick-section materials. This review explores the mechanics of the BT-FSW process, material behavior, microstructural evolution, and mechanical properties of joints in various aluminum (Al), magnesium (Mg), copper (Cu), and composite systems. Furthermore, the challenges associated with the BT-FSW process and the future research directions are critically evaluated. The thermal cycle and material flow are the two main factors that affect the joint microstructure and mechanical properties, which, in turn, are influenced by the tool geometry and welding parameters. The wealth of experimental investigations reported in the literature, coupled with numerical modeling and statistical optimization techniques, has established robust process-structure-property relationships that guide the development of high-strength, defect-free joints. However, challenges still persist in tool durability, joining of dissimilar materials, and industrial process scalability. On the other hand, there are promising opportunities to expand the application scope and industrial viability of BT-FSW that exist within emerging trends, including hybrid techniques, in situ cooling, and advanced computational frameworks. Future research directions for incorporating BT-FSW in advanced manufacturing sectors should be supported by Multiphysics modeling, machine learning, and process monitoring to enable intelligent, adaptive control systems that enhance joint quality and reliability.