Dynamic recrystallization during solid state friction stir welding/processing/additive manufacturing: Mechanisms, microstructure evolution, characterization, modeling techniques and challenges

Dynamic recrystallization (DRX), a resultant of severe plastic deformation occurring at moderately high temperatures, plays a pivotal role in deciding the structural integrity of the material being processed via solid-state friction stir-based techniques such as friction stir welding, processing and additive manufacturing (FS-W/P/AM). This article presents insight into the types, features, and characterization of DRX, as well as a comprehensive review of factors affecting DRX phenomena in terms of microstructural evolution and mechanical properties. An extensive literature review reflects the initial grain size and second phase particles as major material characteristics, while tool-related factors, additional cooling environment, pattern of stirring, and reinforcement are important process parameters that control the extent of DRX. Moreover, this article summarizes the insights about popular numerical and computational modeling developed for modeling and simulating the dynamics of heat generation, grain evolution, and material flow in FS-W/P/AM. A critical review of the major characteristics, capabilities, and limitations of each category of modeling techniques shows that integration of machine learning, like artificial intelligence techniques, will definitely improve the capabilities of these tools. The present worldwide research scenario reveals the aerospace and electronics industry as the major sectors reporting maximum application of FS-W/P/AM. However, the research trend over last decade shows that sectors like medical and construction have also started to adopt these techniques. Additionally, the major challenges and future outlooks of DRX and FS-W/P/AM are summarized. Key concerns include processing high-strength materials, heterogeneity in microstructure, orientation, size limitations, and pre/post-processing requirements. Future directions demand database of FS-W/P/AM and its outcome, robust modeling and simulation tools, integrating artificial intelligence and other Industry 4.0 practices to increase process efficiency. In totality, this article paves the way for researchers and practitioners of friction stir technologies to understand and take advantage of DRX to get a customized solution for industrial needs.