α/β phase transformation and dynamic recrystallization induced microstructure development in fine-grained Ti-6Al-4V friction stir weld

Highlights: • Quenched and hot rolled fine-grained (1.21 μm) Ti-6Al-4V alloy was friction stir welded. • Stir zone composed of a mixture of α + β lamellar structure and equiaxed α and β grains. • Homogeneous misorientation increase in low angle boundaries was dominant mechanism of continuous dynamic recrystallization. • βα phase transformation was governed by the Burgers orientation relationship. • No softening in the stir zone though a little grain coarsening occurred. Fine-grained materials possess great industrial significance as they have exceptional strength, ductility, and superplastic formability. However, the poor thermal stability of these materials leads to an excessive grain growth and property deterioration during welding. In this regard, friction stir welding (FSW) is considered as a potential candidate for the welding of fine-grained materials as it is a solid-state welding process with a lower process temperature. In this work, the microstructure development in the stir zone of a fine-grained Ti-6Al-4V alloy during FSW was investigated. The microstructure, misorientation distribution and crystallography of the base metal and stir zone were characterized using the electron backscatter diffraction technique to explain the microstructural evolution. The microstructure of different layers of stir zone composed of either fine α + β lamellar structure or fine equiaxed α and β grains or the mixture of both. The combination of heating/cooling induced α/β phase transformations and plastic deformation induced continuous dynamic recrystallization (CDRX) was responsible for such microstructural developments in the stir zone. Homogeneous misorientation increase in low angle boundaries was found as the dominant mechanism of CDRX in the stir zone of fine-grained Ti-6Al-4V during FSW. The hardness measurement confirmed the absence any plausible softening though there was a little grain coarsening in the top layer of the stir zone.