Overview of the heat treatment process of titanium alloys. During the processing of titanium alloys, in order to improve its mechanical properties and part of its organizational structure, it needs to be heat treated frequently. Common titanium alloy heat treatment processes mainly include annealing, complete annealing, solution treatment and aging treatment.
1. Stress relief annealing The main purpose of stress relief annealing is to eliminate the internal stress generated during cold working, cold deformation and welding of titanium alloy. This process is widely used in titanium alloy materials after hot forging, casting, cold deformation processing, cutting, cutting and welding. The selection of annealing temperature and time is crucial to the effect of stress relief annealing. Recrystallization temperature is usually used for annealing, and the recovery process of the material is used to eliminate stress.
2. Complete annealing Complete annealing aims to obtain a recrystallized structure and improve the plasticity of titanium alloy, so it is also called recrystallization annealing. Most alpha titanium alloys and alpha+beta duplex titanium alloys are used in the fully annealed state. For α titanium alloys, the annealing temperature is usually 120 to 200°C below the phase transformation point to avoid grain coarsening and insufficient plasticity. The annealing process of near-α titanium alloys and α+β dual-phase titanium alloys is more complex, involving recrystallization and changes in the α and β phases. Complete annealing of metastable beta titanium alloys usually involves solution treatment.
3. The purpose of solid solution and aging treatment is to obtain metastable phases that can be age-strengthened, such as α′ martensite, α″ martensite or metastable β phase. When these metastable phases decompose, they will produce small equilibrium phases, thereby producing a precipitation strengthening effect and improving the hardness of titanium alloys. Strength and strength. The solid solution temperature is usually 40-100°C lower than the α+β/β transformation point. Aging strengthening has a significant effect in titanium alloys with high β-stabilizing elements, but the effect is weak in near-α alloys and α+β two-phase titanium alloys with smaller β-stabilizing elements.
Structural changes during heat treatment of titanium alloys
1. Structural changes during heating. During the heating process, titanium alloys usually undergo crystalline changes, including the transformation between α phase and β phase. Cold deformed titanium alloys also undergo recovery and recrystallization processes. The recovery process eliminates the second type of internal stress generated during the deformation process through the movement of vacancies and dislocations, while the recrystallization process produces new distortion-free equiaxed grains to replace the deformed grains and restore the plasticity of the material.
2. Structural changes during cooling. Titanium alloys will also undergo structural changes during the cooling process. When cooled slowly, the β phase will transform into the α phase, and the two maintain a specific orientation relationship. Rapid cooling may form structures such as martensite transformation, quenching ω phase, supersaturated α phase, and residual high temperature β phase. The types of these transformation products depend on the content of β-stabilizing elements.
3. Aging transition The metastable phase produced by rapid cooling will transform into an equilibrium phase during the aging process, accompanied by processes such as the decomposition of the metastable phase and the decomposition of the supersaturated α phase. This is the main reason why titanium alloys can be strengthened by heat treatment.
4. Eutectoid transformation The eutectoid transformation of titanium alloys often exists in alloys of titanium and fast eutectoid beta alloys with stabilizing elements, which usually leads to a reduction in the plasticity of the material. Through isothermal treatment, a Beinn-type non-lamellar structure can be obtained to improve the properties of the material.
5. Stress-induced phase transformation The metastable β phase can transform into martensite under the action of strain or stress, including hexagonal martensite α´ and orthorhombic martensite α". This process can produce phase transformation-induced plasticity effects and increase the elongation and strain hardening rate of titanium alloys. In summary, the heat treatment process and structural transformation of titanium alloys are of great significance for the improvement of their mechanical properties and microstructure. Through reasonable heat treatment processes and parameter selection, the performance of titanium alloys can be optimized to meet the needs of different application fields.
