Why Titanium Forging Is Difficult
The deformation resistance of titanium alloy is large and its chemical properties are active, so the forging process has special problems: the microstructure of titanium forgings is very sensitive to forging thermal parameters; the forging temperature range is narrow; and the deformation resistance increases significantly with the deformation rate (strong strain-rate sensitivity). In addition, the thermal conductivity is poor, so local overheating during forging causes a large internal-external temperature difference, aggravating uneven deformation and cracking. Selecting a reasonable forging process is therefore critical.
TC4 (Ti-6Al-4V): The Most Widely Used α+β Alloy
TC4 titanium alloy - Ti-6Al-4V - is the most widely used α+β two-phase titanium alloy, applied in aerospace, automotive and medical fields. Its phase-transition point (α+β)/β is about 990 °C (measured metallographically).
The Three Forging Processes
In a typical study, a TC4 billet (Φ100 mm × 450 mm) is divided into three sections and forged with 50% deformation using different routes on a 3-ton free-forging hammer, followed by a double heat treatment (900 °C × 1 h/AC + 600 °C × 4 h/AC):
α+β forging (Tβ - 60 °C): forging in the two-phase field.
Near-β forging (Tβ - 20 °C): forging close to the beta-transus temperature.
β forging (Tβ + 40 °C): forging above the transus in the single-beta field.
Microstructure Results
After forging, α+β forging produces an equiaxed structure, near-β forging a mixed (bimodal) structure, and β forging a lamellar (Widmanstätten) structure.
Mechanical Property Comparison
Strength: similar for all three processes.
Plasticity: higher after α+β and near-β forging than after β forging.
Impact toughness: best after β forging.
Overall performance: TC4 bars show the best combination of properties after near-β forging.
Fracture: all three show ductile fracture; α+β and near-β forgings have deep, evenly distributed equiaxed dimples, while β-forged alloys show flatter, elongated dimples.
Practical Guidance
- Choose near-β forging when a balance of plasticity, toughness and strength is required (typical for structural forgings).
- Choose β forging when maximum impact toughness and creep performance are needed (for example turbine discs in some designs).
- Choose α+β forging for fine equiaxed microstructures with good fatigue performance.
FAQ
Q: What is the difference between β forging and α+β forging of TC4?
A: β forging is performed above the beta-transus (about 990 °C), giving a lamellar structure with the best impact toughness; α+β forging is performed below it, giving an equiaxed structure with better plasticity and fatigue performance.
Q: Why is near-β forging considered optimal for TC4?
A: It produces a mixed (bimodal) structure combining good plasticity and high impact toughness, giving the best overall mechanical properties.
Q: Why is TC4 difficult to forge?
A: Narrow forging temperature range, strong strain-rate sensitivity, poor thermal conductivity and active chemical properties cause local overheating and cracking if the process is not controlled.
Q: What heat treatment follows TC4 forging?
A: Typically a double treatment such as 900 °C × 1 h/air cool plus 600 °C × 4 h/air cool, which stabilizes the structure and properties.
