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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 μm, 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.

Ti–6Al–4V alloy is a typical 3D printing metal, and its application has been expanded to various fields owing to its excellent characteristics such as high specific strength, high corrosion resistance, and biocompatibility. In particular, direct energy deposition (DED) has been actively explored in the fields of deposition and the repair of large titanium parts. However, owing to the complicated thermal history of the DED process, the microstructures of the fusion zone (FZ), heat-affected zone (HAZ), and base metal (BM) are different, which results in variations of their mechanical characteristics. Therefore, the process reliability needs to be optimized. In this study, the microstructure and hardness of each region were investigated with respect to various DED process parameters. An artificial neural network (ANN) model was used to correlate the measured characteristics of the FZ, HAZ, and BM of Ti–6Al–4V components with the process parameters. The variation in the mechanical characteristics between the FZ, HAZ, and BM was minimized through post-heat treatment. Heat treatment carried out at 950 °C for 1 h revealed that the microstructure and hardness values throughout the component were homogeneous.

Step rolling can be used to mass-produce ultrafine-grained (UFG) Ti-6Al-4V sheets. This study clarified the effect of subsequent annealing on the tensile properties of step-rolled Ti-6Al-4V at room temperature (RT) and elevated temperature. The step-rolled alloy retained its UFG structure after subsequent annealing at 500–600 °C. The RT ductility of the step-rolled alloy increased regardless of annealing temperature, but strengthening was only attained by annealing at 500 °C. In contrast, subsequent annealing rarely improved the elevated-temperature tensile properties. The step-rolled Ti-6Al-4V alloy without the annealing showed the highest elongation to failure of 960% at 700 °C and a strain rate of 10−3 s−1. The ambivalent effect of annealing on RT and elevated-temperature tensile properties is a result of microstructural features, such as dislocation tangles, subgrains, phases, and continuous dynamic recrystallization.

Rapid electric current heat treatment has been successfully applied to a cold-rolled sheet of commercially pure titanium (CP Ti). The electric current heat treatment was conducted at various temperatures (400, 500, 600 and 700 °C) by altering the current density (A/mm2). The detailed microstructure and texture evolution was studied using electron backscatter and X-ray diffraction analysis. For comparison, conventional heat treatment at 400, 500 and 600 °C were also applied to the cold-rolled sheets. The electrically heat-treated sample showed a much smaller and uniform grain size with a relatively weak texture than the conventionally heat-treated one. As a result, the electrically heat-treated samples exhibited better tensile properties than conventionally heat-treated samples. Furthermore, the electric current treatment produced minimum sheet distortion and good oxidation resistance compared with the conventional heat treatment.