The phase transformation of titanium alloy is the basis of the heat treatment of titanium alloy, in order to improve the performance of titanium alloy, in addition to the use of reasonable alloying, but also with the appropriate heat treatment in order to achieve. Titanium alloy heat treatment is more varied, commonly used annealing treatment, aging treatment, deformation heat treatment and chemical heat treatment.
(1) Stress relief annealing. In order to eliminate casting, cold deformation and welding and other processes produced in the internal stress, can be used to stress relief annealing. Stress relief annealing temperature should be lower than the recrystallisation temperature, generally 450 ~ 650 ℃, the time required depends on the cross-section size of the workpiece, processing history and the degree of stress relief required.
(2) Ordinary annealing. Its purpose is to make the titanium alloy semi-finished products to eliminate the basic stress, and has a high strength and meet the requirements of the technical conditions of plasticity. Annealing temperature is generally comparable with the recrystallisation temperature or slightly lower, this annealing process is generally used when metallurgical products are shipped from the factory, so it can also be called factory annealing.
(3) complete annealing. The purpose is to completely eliminate work hardening, stable organisation and improve plasticity. This process occurs mainly recrystallisation, so also known as recrystallisation annealing. Annealing temperature is preferably between the recrystallisation temperature and the phase change temperature, if the phase change temperature is exceeded, the formation of Weiss tissue and the deterioration of alloy properties. For a variety of different types of titanium alloys, the type of annealing, temperature and cooling are different.
(4) Double annealing. In order to improve the plasticity of the alloy, fracture toughness and stable organisation can be used double annealing. The annealed alloy organisation is more uniform and close to the equilibrium state. Heat-resistant titanium alloy in order to ensure the stability of the organisation and properties at high temperatures and under long-term stress, often using such annealing. Double annealing is a double heating and air cooling of the alloy. The first high-temperature annealing heating temperature is higher than or close to the end temperature of recrystallisation, so that recrystallisation is fully carried out, but does not make the grain grow significantly, and control the volume fraction of ap phase. After air cooling, the organisation is not stable enough, the need for a second low-temperature annealing, annealing temperature lower than the recrystallization temperature, insulation for a longer period of time, so that high-temperature annealing of the substable β-phase decomposition.
(5) Isothermal annealing. Isothermal annealing can obtain the best plasticity and thermal stability. This type of annealing is suitable for dual-phase titanium alloys with high content of β-stable elements. Isothermal annealing adopts a graded cooling method, i.e., after heating to above the recrystallisation temperature and holding, it is immediately transferred to another furnace with lower temperature (generally 600-650℃) and then air-cooled to room temperature.
