Abstract
Underwater friction stir welding (FSW) has been demonstrated to be an effective method to improve the mechanical properties of joints. To illuminate the characteristics of underwater FSW, the microstructural evolution and its effect on mechanical performance of an underwater joint were investigated in the present paper. The weld nugget zone (WNZ) is characterised by the homogeneity of refined grain structures in the direction of thickness. The precipitate deterioration is gradually strengthened from the heat affected zone (HAZ) to the WNZ. The dislocation movement featured by different types of grain boundaries suggests that continuous dynamic recrystallisation has occurred in the WNZ during FSW. The evolutions of strengthening precipitates and grain structures have significant influence on hardness distributions and the tensile fracture features of the underwater joint.
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