Effect of Welding Parameters on Friction Stir Welded Ti–6Al–4V Joints: Temperature, Microstructure and Mechanical Properties

Abstract

In this study, friction stir welding (FSW) of 2-mm-thick Ti–6Al–4V alloy plates was performed using a newly designed friction tool—and the effect of rotation speed and welding speed on microstructure and mechanical properties of the joints were investigated. A simulation model for FSW temperature field calculation was developed, and the effect of rotation speed and welding speed on the temperature field was investigated by experimental and numeric methods. The results show that the rotation speed has a dominant effect on peak temperature, while welding speed determines the dwell time of the weld exposed to high temperatures. In addition, the influence of process parameters on the microstructure of the joints was investigated using optical and scanning electron microscopy. The results revealed that there was a phase transformation in the stir zone during welding. The final microstructure of the stir zone was fully lamellar (α + β) structure, and the heat affection zone had a bimodal microstructure consisting of prior equiaxed α and lamellar (α + β) structure. Both rotation speed and welding speed affect the grain size of the weld. Lower peak temperature with decreasing spindle speed and/or shorter dwell time with increasing feed rate could produce finer grains in the stir zone of the joints, thereby could lead to higher microhardness value and the tensile strength of the joints.