Microstructural evolution and mechanical behavior of TA5 titanium alloy joint in low-temperature friction stir welding with various cooling rates

This study investigates the effects of cooling methods and welding parameters on the microstructural evolution and mechanical properties of TA5 titanium alloy joints fabricated via low-temperature friction stir welding (FSW) with forced cooling. The proposed technique reduces heat input by applying forced cooling to the weld surface, thereby promoting high-quality joint formation under controlled thermal conditions. The results show that forced cooling minimizes grain growth and reduces joint oxidation by lowering peak welding temperatures. Specifically, the application of forced cooling, particularly liquid nitrogen cooling (LNC), enhances grain refinement within the stir zone (SZ), leading to significant improvements in tensile strength, hardness, and impact toughness. Microstructural analysis reveals that the SZ undergoes dynamic recrystallization, producing fine α-phase grains, while the heat-affected zone (HAZ) exhibits partial grain coarsening. Additionally, forced cooling mitigates the temperature gradient along the joint thickness direction, thereby reducing microstructural inhomogeneity. A comprehensive analysis of the welding parameters demonstrates that excessive heat input at high rotation speeds can cause grain coarsening, which negatively impacts the joint's impact toughness. The optimal welding parameters (900 rpm–50 mm/min) under LNC conditions yield the best mechanical properties, with superior joint strength and toughness. These findings provide valuable insights into optimizing FSW for titanium alloys, enhancing their industrial applicability in demanding environments.