CHARACTERIZATION OF MICROSTRUCTURAL EVOLUTION IN A Ti-6Al-4V FRICTION STIR WELD

摘要

The aim of this study was to characterize the microstructures present in a titanium alloy friction stir weld in an attempt to later relate them to mechanical properties and ultimately optimze weld performance. The allotropic phase transformation that occurs in titanium alloys, coupled with deformation and continuous cooling, produces complex stir zone microstructures compared with those observed in aluminum alloy friction stir welds. In this study, a mill annealed Ti-6Al-4V base material microstructure was completely transformed into a Widmanstaetten microstructure during processing. Equiaxed α grains on the order of 1μm in diameter were also present in the stir zone at triple points and α lath boundaries. The α laths contained basal faults similar to those observed previously in quenching experiments in the absence of deformation. There was no evidence of gross plastic deformation in the stir zone microstructure however curved misfit dislocations in the α/β interphase boundaries in the stir zone indicate grain boundary sliding. The thermomechanically affected zone was characterized by "fingers" of base material that appeared to be drawn into the stir zone. These "fingers" scaled closely with microtexture patterns observed the base material. Microhardness results showed no heat affected zone softening that is typically present in titanium alloy fusion welds.