In the complete annealing process, the purpose of complete annealing of titanium and titanium alloys is to obtain a stable, plastic or microstructure corresponding to certain comprehensive properties. Recrystallization mainly occurs in this process, so it is also called recrystallization annealing. In addition, there are also changes in the composition, morphology, and quantity of a-phase and β-phase. Most a and a+beta titanium alloys are used in fully annealed condition. The two-phase region of all-a-type titanium alloys is very small, and recrystallization occurs mainly during the complete annealing process. The annealing temperature is generally selected to be 120 to 200°C below the a+β/β transformation point. Too high temperature will cause unnecessary oxidation and grain growth, and too low temperature will cause incomplete recrystallization. The cooling rate has little effect on the structure and properties of such alloys, and air cooling is generally used.
In addition to recrystallization during annealing, near-a titanium alloys and a+β titanium alloys also have changes in the composition, quantity and morphology of a-phase and β-phase, and it is also complicated to determine the annealing process. For the delivery state of the metallurgical plant, a stable and plastic structure is mainly required. The annealing temperature is generally selected to be 120-200°C below the a+β/β transformation point, and the cooling method is also air-cooled. However, for the annealing of the product before the final use, it must be determined through experiments according to the influence of the annealing process on the microstructure and mechanical properties.
As for metastable beta titanium alloys, complete annealing is also solution treatment. The annealing temperature of the metallurgical plant before leaving the factory is generally selected to be 80 to 100 °C above the a+β/β transformation point. Within the recommended full annealing process range, the specific process should be determined experimentally based on the processing history of the material, the actual chemical composition, and the equipment used. In order to avoid unnecessary oxidation, the process should be selected with the lowest temperature and shortest time under the premise of meeting the performance requirements.
The beta annealing process has appeared in recent years. The grade 7 titanium alloy plate and a+β titanium alloy are heated in the β phase region and then air-cooled, and a needle (or flake) a is precipitated on the coarse β grains. This structure corresponds to higher fracture toughness, creep resistance and notch sensitivity, but reduces plasticity indicators such as section shrinkage. It can be used as appropriate in situations where this performance is emphasized.
In addition, for some alloys, when it is required to improve the performance stability of long-term operation at the service temperature, a second annealing higher than the service temperature, or isothermal treatment is often used, such as TC9 and tc6 alloys.
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