Friction Stir Welding of Steels—A Localized Thermo-mechanical Processing Technique for Producing Ultrafine-Grained Structures
Producing ultrafine-grained steels offers the prospect of very high strength from standard steel compositions. However, the key industrial issue is the feared loss of work hardening rate (especially of the uniform elongation) in such materials. In addition, the production of ultrafine-grained steels requires further heat treatment to establish the balance between strength and ductility, which is both time- and energy-consuming process. In the present work, it has been shown that in-process external cooling treatment during friction stir welding (FSW) has a potential to develop nanometer-grained or at least sub-micrometer-grained structures. It is believed that the current practice of controlling interface plastic flow and resulting microstructures improves strength and ductility combination. Therefore, the present work describes the fundamental understanding of the scientific approach used for producing ultrafine-grained steels using FSW technique.
KeywordsUltrafine-grained steels Plastic deformation Friction stir welding External cooling
Authors greatly acknowledge the New Energy and Industrial Technology Development Organization (NEDO), the Global COE Program and a Grant-in-Aid for Science Research from the Japan Society for Promotion of Science. The authors are also grateful to Prof. Hidetoshi Fujii at Joining and Welding Research Institute (JWRI) for providing the experimental facility and his useful inputs in this work.
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