Effect of initial microstructure on Ti–6Al–4V joint by friction stir welding

Abstract

Ti–6Al–4V alloy plates with three types of initial microstructures, which have different densities of the high angle boundaries, were joined by friction stir welding (FSW), and the microstructure evolution in the stir zone by the FSW was clarified. Concerning the morphology of the microstructure, all three joints show similar characteristics; the top surface of the SZs exhibited a mixture of the lamellar structure with equiaxed α grains and the area fraction of the equiaxed α grain gradually increased with the increasing distance from the top surface due to the temperature gradient. On the other hand, the mean α grain size of the fully equiaxed α structure area decreased with the increasing initial density of high angle boundaries. This accelerated grain refinement was discussed based on the effect of the inhomogeneous microstructure on the recrystallization.