| Summary: | Compared to nonferrous alloy, the microstructure in the nugget zone (NZ) of steel is easy to coarsen during friction stir welding (FSW) because of higher melting point of steel, leading to lower strength and plasticity of the joint. Here, high-strength bainitic steel was subjected to FSW in water + solid CO2 medium, and mechanical properties and deformation behavior of the NZ were studied. The excellent yield strength (617 MPa) being 111% of the basal metal (BM) was obtained in the NZ owing to fine grain strengthening by fine effective grains of 4.3 μm and dislocation strengthening by the high density of dislocations of 6.7 × 1014 m−2. A perfect tensile strength of 664 MPa and uniform elongation of 13.1% were achieved in the NZ depending on the highly sustained work hardening rate, which were up to 112% and 113% of that of the BM. This can be partly explained by the fact that fine effective grains can obviously reduce the mean free path of dislocations. Moreover, dynamic recrystallization of ferrite with low density dislocations provided sufficient space for the multiplication of dislocations during deformation. Furthermore, refined effective grain and dual-phase microstructure (ferrite/bainite) promoted ductile fracture, and the total elongation of sample reached 108% of that of the BM.
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