Microstructure and texture distribution of Ti–6Al–4V alloy joints friction stir welded below β-transus temperature

Abstract

Ti–6Al–4V alloy plates with a thickness of 5mm were successfully joined by friction stir welding (FSW) at a significantly lower than the β-transus temperature, and the microstructure and texture evolution in the stir zone (SZ) along the thickness and transverse directions of the joint were clarified. The fully equiaxed α structure was observed even near the top surface in the 50rpm joint, while a mixture of the lamellar structure with equiaxed α grains was found near the top surface in the 150rpm joint. The existence of the lamellar structure resulted in a strong [0001]α concentration in this area due to the variant selection during the phase transformation. A bimodal structure was formed in the retreating side (RS) of the SZ obtained at 50rpm, and a higher area fraction of the coarse grain structure was found in the RS of the SZ center than in the RS located 1.0mm from the top surface due to the gradients in the strain, strain rate and temperature. The local texture measured by EBSD clarified that [0001]α was perpendicular to the welding direction on the advancing side (AS), and gradually rotated toward to the welding direction when the measurement positions were moved to the RS in the SZ of the Ti alloy joints. The difference in the distribution of the [0001]α concentration direction in the SZ along the transverse direction between the FSWed Mg and Ti alloy joints should be due to the difference in the plastic behavior caused by higher work hardening ability of the Ti alloy.