Grain Structure Development During Friction Stir Welding of Single-Crystal Austenitic Stainless Steel
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INTRODUCTION
THE very successful practical implementation of the friction stir welding (FSW) technique during the last decade has necessitated a more fundamental understanding of its underlying physical processes. As a result, microstructural and textural studies are now becoming a research hotspot in FSW. In this context, an approach involving experiments with single crystals seems to be particularly attractive. This method may significantly simplify microstructural and textural observations because any grain boundary development as well as crystallographic rotations can easily be traced and interrelated with each other. The first works in this field were performed with aluminum alloys. Fonda et al.[1,2] have provided an excellent illustration of how initial single-crystal orientations transform into a simple shear texture during FSW. Shibayanagi et al.[3] have reported the formation of compression texture components and have attributed this effect to the vertical component of material flow during FSW. In all these studies, the observed crystal rotations have been clearly demonstrated to be closely linked with grain boundary development which ultimately broke up the initial single-crystal structure into a polycrystalline aggregate.[1–3] Generally, these studies have essentially contributed to our current understanding of the process of grain structure evolution during FSW. JONG JIN JEON, Ph.D. Student, SERGEY MIRONOV, Postdoctoral Fellow, YUTAKA S. SATO, Associate Professor, and HIROYUKI KOKAWA, Professor, are with the Department of Materials Processing, Graduate School of Engineering, Tohoku University, 6-6-02 Aramaki-aza-Aoba, Sendai 980-8579, Japan. Contact e-mail: [email protected] SEUNG HWAN C. PARK, Researcher, and SATOSHI HIRANO, Senior Researcher, are with the Hitachi Research Laboratory, Hitachi Ltd., 7-1-1, Omika-cho, Hitachi 319-1291, Japan. Manuscript submitted December 12, 2012. Article published online March 14, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
The recent progress in FSW tooling has driven expansion of FSW technology to high melting point alloys. As a result, the structural response of these materials to FSW is of interest. In our previous work,[4] grain boundary development and texture evolution in single-crystal austenitic stainless steel were studied during the tool plunging step of FSW. It was demonstrated that the grain structure evolution during the pin plunging was dominated by continuous recrystallization, which broke down the single crystal into an ultrafine-grained microstructure with a mean grain size of ~0.2 lm. During the subsequent shoulder contacting step, however, the associated increase in heat input induced discontinuous recrystallization, which significantly coarsened the grain structure and promoted annealing twinning. In both steps, the crystal rotations were shown to be related to simple shear deformation and the process of grain boundary development was deduced to be closely linked to texture evolution. As an extension of this work, in the current study, gr
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