Ultrafine-grained (UFG) and a coarse-grained (CG) 304L stainless steels were produced by a combination of cold rolling and annealing. Friction stir welding (FSW) was applied at the same welding conditions to study the effects of the grain size of the base metal on the grain structure of the weld nugget and its recrystallisation mechanism. The results showed that the recrystallisation mechanism changes with the grain size of the base metal. Continuous dynamic recrystallisation (CDRX) was found to be dominant at the FSW of the UFG sample while discontinuous dynamic recrystallisation is the main recrystallisation mechanism in the FSW of the CG sample. Although the weld nugget grain size of the UFG welded sample is slightly higher than that of the CG welded sample, its nanohardness was found to be higher. This behaviour is attributed to the higher dislocation densities of the austenite grains in the UFG welded sample produced through CDRX.
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