Common Heat Treatment Methods for Titanium Alloys

Heat Treatment of Titanium Alloys: Purpose and Families

The heat treatments commonly used for titanium alloys fall into two families: annealing, and solution treatment with ageing. Annealing can be applied to all types of titanium alloy; its main purpose is to obtain the best combination of mechanical properties, relieve internal stress, improve plasticity and stabilise the structure. Solution treatment followed by ageing is the principal means of strengthening titanium alloys and is used when a higher strength level than the annealed condition is required.

Stress Relief Annealing

The annealing temperature is set 100-250 °C below the recrystallization temperature of the alloy. The purpose is to eliminate or reduce the internal stresses that arise during processing and to prevent or limit distortion in later operations. The holding time depends on the cross-section of the workpiece, its processing history and the degree of stress relief required. Because the temperature remains below the recrystallization range, the wrought structure is not recrystallized, so strength is essentially retained while residual stress is lowered. In production this treatment is used after welding, after heavy machining of thin sections, and between forming operations.

Complete (Recrystallization) Annealing

Complete annealing, also known as recrystallization annealing, is carried out between the recrystallization temperature and the beta-transition point, that is, at a temperature above the recrystallization temperature of the alloy, so that recrystallization is the dominant process. Its purpose is to reduce hardness, improve plasticity, stabilise the structure and improve machinability. The type of annealing, the annealing temperature and the cooling method differ from one titanium alloy to another, and furnace cooling, air cooling or a controlled cooling rate is selected accordingly. The treatment is used to restore a worked or partly recrystallized structure before final forming or machining.

Double Annealing and Isothermal Annealing

Double annealing consists of a high-temperature stage and a low-temperature stage, followed by air cooling. The high-temperature stage is held 20-160 °C below the beta-transition point, and the low-temperature stage is held below the beta-transition point at the level selected to give the required balance of plasticity and fracture toughness. The purpose of the treatment is to improve the plasticity and fracture toughness of the alloy and to stabilise the structure; it is widely used for forgings and for components that must combine damage tolerance with reasonable strength.

Isothermal annealing is a special form of double annealing. The workpiece is first heated to 20-160 °C below the beta-transition point and held, then transferred to a furnace held at 600-650 °C and held again, and finally air cooled to room temperature. It is suitable for titanium alloys with a high content of beta-stabilising elements, and the slow, uniform cooling is intended to decompose the beta phase completely so that a stable structure is obtained. Isothermal annealing is chosen when section thickness would otherwise produce an uneven cooling rate through the component. Because the transfer step is critical, transfer time and furnace loading are controlled in production. The purpose of the treatment is to obtain a stable structure with uniform properties.

Solution Treatment and Ageing

Solution treatment is carried out above the transformation temperature of the alpha+beta two-phase region, normally 28-83 °C below the beta-transformation point; in special cases the alloy may also be heated above the beta-transformation point. Solution treatment is followed by quenching, which retains a high proportion of metastable beta phase and provides the capacity for subsequent age strengthening.

Ageing treatment is generally carried out at a temperature between 425 °C and 650 °C. Ageing temperature and ageing time are determined from the ageing-hardening curve of the specific alloy, because they control the amount, size and distribution of the precipitated phase. During ageing the metastable beta phase decomposes and a fine second phase precipitates, which raises the strength of the alloy. Solution treatment combined with ageing is the standard route to high-strength titanium alloy components, and both the quench delay and the ageing schedule are held within tight limits so that properties are reproducible from batch to batch.

FAQ: Heat Treatment of Titanium Alloys

Q: What is the difference between annealing and solution treatment?
Annealing softens the alloy, relieves stress and stabilises the structure, and can be applied to every titanium alloy. Solution treatment dissolves the phases at high temperature and is followed by quenching and ageing to raise strength.

Q: How is the stress relief annealing temperature chosen?
It is set 100-250 °C below the recrystallization temperature, so that internal stress is reduced without recrystallizing the wrought structure.

Q: When should isothermal annealing be used instead of double annealing?
When the alloy contains a high level of beta-stabilising elements or when the section is thick enough that direct air cooling would leave the beta phase only partly decomposed. Isothermal annealing uses a 600-650 °C holding stage to ensure complete decomposition.

Q: Why is quenching used after solution treatment?
Quenching suppresses diffusion on cooling and retains a high proportion of metastable beta phase, which is the phase that later precipitates and hardens the alloy during ageing.

Q: How are ageing temperature and ageing time selected?
They are read from the ageing-hardening curve of the specific alloy, because the temperature controls the type and size of the precipitate and the time controls how far the precipitation reaction proceeds.

Q: Can every titanium alloy be annealed?
Yes. Annealing is applicable to all types of titanium alloy, although the annealing type, temperature and cooling method are selected individually for each alloy and product form.