Effect of rotation rate on microstructure and texture evolution during friction stir welding of Ti-6Al-4V plates

Ti-6Al-4V alloy plates with a thickness of 2 mm were successfully friction stir welded (FSW) and the microstructure change in the weld was clarified. The FSW was conducted at the tool rotation rates ranging from 225 to 300 rpm under the constant tool traverse speed of 50 mm/min. The microstructure of the stir zone exhibited inhomogeneous distribution along the plate thickness due to the temperature gradient caused by the lower thermal conductivity. The microstructure of the stir zone consisted of two types of morphologies. The fully lamellar structure was observed near the top surface, while the fully equiaxed structure was observed near the bottom surface. At the intermediate area between the top and the bottom regions, a mixed microstructure having both lamellar and equiaxed α grain morphologies was observed. The change in texture is also quite consistent with the microstructure distributions. This inhomogeneity was mainly due to the change in the microstructure development processes that involve phase transformation or plastic deformation.