Thermo-mechanical behavior in stationary vortex flow toward friction stir welding of titanium alloy

Vortex flow-based friction stir welding (VFSW), a process modification of conventional friction stir welding, addresses critical challenges such as tool wear and break. The novelty of this study lies in the development of a three-dimensional slip model based on interface friction behavior between identical material to investigate heat generation, temperature fields, and material flow in the stationary plastic vortex of Ti-6Al-4V titanium alloy. This model significantly improves the prediction accuracy of vortex depth and provides new insights into the thermo-mechanical behavior in VFSW. The results identify two heat sources in the plastic vortex: interface friction heat generation (IFHG), governed by the competition between velocity drop and interfacial friction conditions across the T-W interface, and viscous dissipative heat generation (VDHG), governed by heat softening effect induced by heat conducted from the T-W interface. The total IFHG reaches the maximum value of 967.9 W at 700 rpm and 8 mm radius. The plastic vortex exhibits two distinct flow patterns: a central vortex flow and a peripheral shear flow. Adjusting the bar rotation speed promotes the mutual transformation between these two flow patterns, while changing the radius primarily leads to synchronous variations in their distribution. At 700 rpm, the peak vortex temperature significantly reduces flow stress and decelerates velocity attenuation, achieving a maximum vortex depth of 4.3 mm. The findings reveal that by regulating the thermo-mechanical behavior during VFSW, a weld penetration of at least 4.3 mm can be achieved in Ti-6Al-4V alloy, demonstrating the potential of VFSW for high-melting-point metal welding.