The main types of annealing for titanium alloys include stress-relief annealing, simple annealing, isothermal annealing, double annealing, recrystallization annealing, and vacuum annealing.
(1) Stress-relief annealing
An annealing process in which the workpiece is heated to a temperature below the recrystallization temperature to eliminate internal stresses caused by processes such as plastic deformation and welding. The temperature for stress-relief annealing is relatively low; it is a non-recrystallization annealing process where recovery occurs.
(2) Simple annealing
An annealing process in which the workpiece is heated to a temperature slightly below the onset of recrystallization to eliminate residual stresses. This method is commonly used for metallurgical products prior to shipment.
(3) Recrystallization annealing
An annealing process in which the workpiece is heated to a temperature above the recrystallization temperature. Recrystallization is the primary mechanism here, eliminating work hardening, stabilizing the microstructure, and improving plasticity. The annealing temperature falls between the recrystallization temperature and the phase transformation temperature.
(4) Vacuum annealing
This process reduces the hydrogen content in the surface layer of the titanium alloy to a safe level, thereby eliminating the risk of hydrogen embrittlement. Additionally, it reduces residual stresses and ensures the alloy’s mechanical and service properties.
While the purposes and significance of the four heat treatments above are clear, the concepts of isothermal annealing and double annealing are often easily confused. Both isothermal annealing and double annealing are applicable only to α+β type alloys.
(5) Isothermal annealing
Isothermal annealing involves heating the workpiece to a high temperature sufficient for recrystallization, cooling it to a temperature where the β phase is highly stable (generally below the recrystallization temperature), holding it at that temperature, and subsequently cooling it in air.
Compared to simple annealing, the purpose of the holding stage is to ensure the β phase decomposes fully and reaches a more stable state. This stabilizes the alloy’s properties and microstructure, resulting in higher plasticity, thermal stability, and creep-rupture strength. Therefore, isothermal annealing is suitable for α+β type alloys with a high β-phase content. (6) Double Annealing
The distinction between double annealing and isothermal annealing lies in the cooling process following the first stage: in double annealing, the alloy is cooled to room temperature in air before being reheated to the temperature of the second stage (which is lower than that of the first stage).
The advantage of double annealing is that a portion of the metastable phase retained after the first annealing stage undergoes full decomposition—specifically, the decomposition of the β phase—during the second annealing stage. This process induces a strengthening effect and improves the ductility, fracture toughness, and microstructural stability of α+β titanium alloys.
