The purpose of complete annealing process of titanium and titanium alloy is to obtain stable microstructure with good plasticity or corresponding comprehensive properties. Recrystallization occurs mainly in this process, so it is also called recrystallization annealing. In addition, there are changes in composition, morphology, and number of a and beta phases. Most a and a+ beta titanium alloys are used in full annealed condition. The two – phase zone of all a titanium alloy is very small. Generally, the annealing temperature is 120 ~ 200℃ below the phase transition point of a+ beta/beta. Too high temperature will cause unnecessary oxidation and grain growth, and too low temperature will cause incomplete recrystallization. The cooling speed has little effect on the microstructure and properties of this kind of alloy.
In addition to recrystallization of nearly a titanium alloy and a+ beta titanium alloy, there are changes in composition, quantity and morphology of a phase and beta phase during annealing, and the determination of annealing process is also complex. The main requirement for metallurgical plant delivery is stable and plastic structure. Generally, the annealing temperature is 120 ~ 200℃ below the phase transition point of a+ beta/beta, and the cooling mode is also air cooling. However, the effect of annealing process on microstructure and mechanical properties must be determined by experiments before the final use of products.
For metastable beta titanium alloys, full annealing is also known as solution treatment. The annealing temperature before leaving the factory is generally 80 ~ 100℃ above the phase transition point of a+ beta/beta. Within the recommended range of full annealing processes, the specific processes shall be determined by testing based on the processing history of the material, the actual chemical composition, and the equipment used. In order to avoid unnecessary oxidation, the lowest temperature and the shortest time should be adopted when the process meets the performance requirements.
In recent years, the beta annealing process has been developed. A titanium alloy and a+ beta titanium alloy are heated in the beta phase zone and air-cooled, precipitating acicular (or lamellar) a on the coarse beta grains. This kind of structure corresponds to higher fracture toughness, creep resistance and notch sensitivity, but it decreases the plastic indexes such as section shrinkage. Where such performance is emphasized, it may be used as appropriate.
In addition, some alloys, such as TC9 and tc6 alloys, are often treated with a second annealing or isothermal treatment that is higher than the service temperature when the performance stability is required to be improved for a long time at the service temperature.