Twin Boundary Induced Grain Coarsening in Friction Stir Welding of Fine- and Ultra-Fine-Grained Commercially Pure Titanium Base Metals

The mechanical properties of commercially pure titanium can further be improved through the grain refinement processes; however, welding fine-grained materials is challenging due to the grain coarsening in the weld area and hence the weakening of the mechanical properties locally. Meanwhile, friction stir welding is a promising process in which the metallurgical bonding is established through the solid-state mechanical mixing of materials to be welded; no studies have reported friction stir welding of the ultra-fine-grained commercial purity titanium to date. In this research, friction stir welding of fine-grained and ultra-fine-grained commercially pure titanium (1.58 and 0.66 mu m, respectively) was conducted. The effect of the microstructural feature of base metals on the microstructural evolution of the stir zone and the feasibility of the friction stir welding process for those materials were discussed. It was found that the fraction of twin boundaries in ultra-fine-grained material was higher than in fine-grained material. It accelerated dynamic recrystallization and recovery in the stir zone, hence inducing the grain coarsening and the loss of ultra-fine-grained structure and character after welding.