Overview of the heat treatment process of titanium alloy
In the process of processing, titanium alloy often needs to be heat treated to improve its mechanical properties and microstructure. Common titanium heat treatment processes of titanium alloy mainly include stress relief annealing, full annealing, solution treatment, and aging treatment.
1. Stress relief annealing
The main purpose of stress relief annealing is to eliminate the internal stress generated by titanium alloy during cold working, cold deformation, and welding. This process is widely used in titanium alloy materials after hot forging, casting, cold deformation, 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 material recovery process is used to eliminate stress.
2. Full annealing
Full annealing aims to obtain a recrystallized structure and improve the plasticity of titanium alloy, so it is also called recrystallization annealing. Most α titanium alloys and α+β dual-phase titanium alloys are used in a fully annealed state. For α titanium alloys, the annealing temperature is usually 120-200°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 complicated, involving recrystallization and changes in α and β phases. The complete annealing of metastable β titanium alloys is usually a solid solution treatment.
3. Solution treatment and aging treatment
The purpose of solution treatment is to obtain metastable phases that can be strengthened by aging, such as α′ martensite, α″ martensite, or metastable β phase. These metastable phases will produce fine equilibrium phases when decomposed, thereby producing a precipitation-strengthening effect and improving the hardness and strength of titanium alloys. The solution temperature is usually 40-100℃ lower than the α+β/β phase transformation point. Aging strengthening has a significant effect in titanium alloys with high β-stabilizing element content, but the effect is weaker in near-α alloys and α+β two-phase titanium alloys with low β-stabilizing element content.
Microstructure changes during heat treatment of titanium alloys
1. Microstructure changes during the heating process
During the titanium heat treatment process, titanium alloys usually undergo crystal 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 vacancies and dislocation movement, while the recrystallization process produces new distortion-free equiaxed grains to replace the deformed grains and restore the plasticity of the material.
2. Changes in the cooling process
Titanium alloys also change their structure 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 martensitic phase transformation, quenched 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 transformation
The metastable phase produced by rapid cooling will transform into an equilibrium phase during the aging process, accompanied by 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 titanium heat treatment.
4. Eutectoid transformation
The eutectoid transformation of titanium alloys often exists in alloys of titanium and fast eutectoid β alloy stabilizing elements, which usually leads to a decrease in the plasticity of the material. Isothermal treatment can obtain a non-lamellar structure of the Bainite type to improve the performance of the material.
5. Stress-induced phase transformation
The metastable β phase can transform into martensite under strain or stress, including hexagonal martensite α´ and orthorhombic martensite α". This process can produce phase transformation-induced plasticity effect, and increase the elongation and strain hardening rate of titanium alloy.
In summary, the titanium heat treatment process and organizational transformation of titanium alloy are of great significance to the improvement of its mechanical properties and microstructure. Through a reasonable heat treatment process and parameter selection, the performance of titanium alloy can be optimized to meet the needs of different application fields.
