Stationary shoulder friction stir welding–low heat input joining technique: a review in comparison with conventional FSW and bobbin tool FSW

Invention of friction stir welding (FSW) is revolutionarily redefined solid-state materials joining process for lightweight constructions. With numerous commercial applications, FSW has been classified as a matured joining process with some key issues, such as high shoulder heat input on top surface, high process downforce, weld thinning, and relatively poor surface asperity. Stationary shoulder friction stir welding (SSFSW) is one of the most important variants derived from the conventional FSW (CFSW) possessing almost uniform and balanced heat input through the thickness of plates to be welded. Thus, the SSFSW eliminates or suppresses the above key issues of the CFSW process with improved microstructural and mechanical properties. Numerous reviews are available summarizing the development of CFSW, while not such on SSFSW. With the advancement of SSFSW in recent years, sufficient literature of SSFSW deserves a review to help researchers from both academia and industry gaining process aspects and unexplored areas. The present paper summarizes the research progress on SSFSW critically reviewing microstructural evolution, mechanical properties, and derivatives to cope with particular problems. Moreover, this review provides a detailed comparison of CFSW, SSFSW, and bobbin tool friction stir welding (BTFSW) on different aspects, such as process principle, tooling system, heat generation, joint features, and joint performance. To put more emphasize on commercialization of SSFSW, the different variants of SSFSW along with their industrial applications are also presented. Finally, the process challenges and future scopes of SSFSW are proposed.