The probe route as well as the material flow around the keyhole during weld termination were tracked and used to determine the probe position just prior to weld termination. This approach enables analysis of in-situ microstructure distribution and the nature of material flow during friction stir welding (FSW). A FSW flow model was established. During steady state FSW, the material at the advancing side (AS) recrystallized rapidly when it was swept by the hot rotating probe. The recrystallized

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... ains were driven to rotate around the probe along the tool rotation direction, forming the material flow zone (MFZ). As the base material approached the probe, it was compressed initially, forming a compression zone (CPZ) mainly consisted of coarse deformed grains. When the coarse deformed grain in CPZ collided with the hot material in the MFZ, it recrystallized and added into the MFZ, thickening the MFZ ahead of the probe. During FSW, the material intersecting the path of probe was displaced to rotate around the probe and then deposited behind the probe at roughly the same transverse position as its initial position.