Figure options
Microstructural evolution of the regions ahead the welding tool was studied to understand the on-going events there before the occurrence of dynamic recrystallization, and to find out the reasons for sigma phase formation in the weld nugget. From EBSD, the microstructure just ahead of the welding tool was identified as austenite grains with a small amount of delta ferrites which are present from the base metal (Fig. 10(a)). It was found that the grain growth has occurred in comparison with the UFG base metal during the heating cycle of FSW (Fig. 10(b)). EBSD results also showed that the actual grain size before the occurrence of dynamic recrystallization during the heating cycle of FSW varies between 3 μm at the welding speed of 20 mm/min to 1.5 μm at the welding speed of 80 mm/min.