How to Control Titanium Sheet Hot Forming Parameters

Titanium alloys are difficult to form at room temperature because they have high yield strength σ_0.2, a high yield-to-tensile-strength ratio, a low elastic modulus E, and a low E/σ_0.2 ratio. Sheet therefore requires high forming force, cracks readily, and exhibits severe springback. Above about 500°C, ductility and elongation improve substantially. Hot forming is consequently used for aircraft skins, thermal frames, fairings, access panels, and parts made by bending, flanging, or deep drawing.

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Hot-Forming Principle

Hot forming uses the high ductility and low deformation resistance of heated titanium to raise forming limits, reduce springback, and improve accuracy. The process combines isothermal hot pressing with creep and stress relaxation:

1. Preheat the dies and heat the blank uniformly to the specified forming temperature, then place it on the die.
2. Close the dies at the specified speed to form the part.
3. Keep the dies closed under constant pressure and temperature. Creep and high-temperature stress relaxation convert elastic strain into plastic strain and reduce residual stress.
4. After the specified hold, release the tooling, remove the part, and air-cool it.

Isothermal Hot Pressing

The punch, die, and sheet are heated to the same temperature before forming. True stress-strain behavior shows that stress falls as temperature rises and strain rate decreases. Titanium hot forming should therefore use elevated temperature and a relatively low strain rate.

Stress Relaxation

After pressing, keep the dies stationary at constant pressure and temperature. The sheet relaxes, elastic strain converts to plastic strain, residual stress falls, and die conformance improves. At elevated temperature, longer relaxation time reduces internal stress until it approaches a stable value.

Process Parameters

Forming temperature and speed, closing pressure, and hold time determine part quality. Temperature lowers deformation resistance and improves ductility. Pressure balances forming resistance. Time allows creep and stress relaxation.

Forming Temperature

Select temperature from the material’s temperature-property behavior while accounting for oxidation, hydrogen pickup, and other adverse effects. Some titanium alloys lose ductility between 300 and 500°C and improve only around 500°C. At 800-850°C, oxidation, hydrogen absorption, grain growth, and microstructural changes become significant.

Representative forming-temperature ranges for annealed alloys are:

Forming speed determines strain rate. Its influence on ductility is small at low temperature but increasingly important at medium and high temperatures. Very long cycles thicken the contaminated surface layer, so every temperature has a maximum practical forming time and the strain rate must not be excessively low.

Closing Pressure and Hold Time

Higher closing pressure can remove wrinkles but has little effect on springback after unloading. Use the lowest pressure that works at the selected temperature to avoid tool deformation and unnecessary equipment load.

Holding the closed dies at constant temperature and pressure allows stress relaxation, lowers residual stress, and reduces springback. Longer holding generally lowers residual stress, and relaxation is stronger at higher temperature. Excess temperature or time, however, increases oxidation and reduces productivity. Typical holds for Ti-2Al-1.5Mn, Ti-4Al-1.5Mn, and Ti-6Al-4V parts are 8-20 minutes.