Friction stir welding of single crystal austenitic stainless steel

The practical success of friction-stir welding (FSW) is necessitating a more fundamental understanding of the underlying physical processes. As a result, an approach involving experiments with single crystals deserves particular attention. In this work, electron backscatter diffraction (EBSD) technique was applied to track grain structure evolution and texture formation during FSW of single crystal austenitic stainless steel. Three initial orientations of the single crystal were investigated: < 100 >// welding direction (WD), < 110 >//WD and < 111 >//WD. In all cases, the microstructural development was found to be a complex process. Far from the tool probe, at relatively low temperatures, the microstructural changes commenced from a formation of deformation-induced boundaries. Their boundary traces were often aligned with close - packed {111} crystallographic planes as well as with tool probe column surface. Approaching to the tool probe, the deformation-induced boundaries decreased in space and increased in misorientation producing a specific lamellar-type grain structure. All these observations suggested a development of continuous recrystallization. In the vicinity of the tool probe, however, temperature raised thus promoting grain boundary migration. This eventually produced fine recrystallized grains containing annealing twins. This process was interpreted in terms of discontinuous recrystallization.