Stir zone microstructure of commercial purity titanium friction stir welded using pcBN tool

In the present study, friction stir welding was applied to commercial purity titanium using a polycrystalline cubic boron nitride tool, and microstructure and hardness in the weld were examined. Additionally, the microstructural evolution during friction stir welding was also discussed. The stir zone consisted of fine equiaxed alpha grains surrounded by serrate grain boundaries, which were produced through the beta ->alpha allotropic transformation during the cooling cycle of friction stir welding. The fine a grains caused higher hardness than that in the base material. A lath-shaped alpha grain structure containing Ti borides and tool debris was observed in the surface region of the stir zone, whose hardness was the highest in the weld. (C) 2007 Elsevier B.V. All rights reserved.