1. Problem Statement
In a trial production run of TC4 forgings, several forged-performance indicators failed, including "notched stress fracture" life below 5 hours. The analysis should start from TC4's metallurgical structure and morphology, then examine the forging process.
2. TC4 Microstructure Characteristics
TC4 titanium alloy is an alpha+beta alloy (Ti-6Al-4V). The annealed structure is alpha+beta phase, containing about 6% of the alpha-stabilizing element aluminum; vanadium stabilizes beta weakly, so the annealed beta fraction is small - about 7-10%. Depending on heat treatment and thermal working conditions, the phase proportions, nature and morphology vary greatly. The beta transus is about 1000°C:
| Heating & Deformation | Structure | Characteristics |
|---|---|---|
| 950°C, air cool | Primary alpha + transformed beta | Balanced |
| 1100°C, air cool | Coarse fully transformed beta (Widmanstatten) | Lower plasticity and impact toughness; better creep resistance |
| Above beta transus, large deformation | Basket-weave (net) structure | Plasticity/toughness better than Widmanstatten; good high-temperature and creep performance |
| Below beta transus, sufficient deformation | Equiaxed structure | Best overall performance - high plasticity and impact toughness |
3. Root Causes of Performance Decline
From the microstructural analysis, TC4 performance decline is usually caused by two links in forging: (1) heating temperature too high - reaching or exceeding the beta transus, which coarsens beta grains and reduces elongation/section shrinkage; (2) insufficient or excessive single-blow deformation. Titanium's deformation resistance is high but thermal conductivity is poor; during forging, localized temperature can exceed the beta transus, and oversized or undersized deformation can coarsen grains.
4. Process Corrections and Results
Keep billet heating below the beta transus (furnace and start-forging temperatures not too high).
Use light-hit-fast hammering - reduce the deformation per single blow to avoid local overheating and recrystallization aggregation.
Set post-forge heat treatment at 760-770°C (lower than the original parameter) - furnace-temperature deviation above 780°C degrades performance.
With these corrections, all performance indicators passed, including notched stress fracture life greater than 5 hours. Production verification with the same practice confirmed the results.
5. Conclusion
In production of TC4 titanium alloy forgings, strictly control forging process parameters: first, light-hit-fast forging with reduced single-blow deformation; second, post-forge heat treatment in the 760-770°C range. This ensures forging quality.
6. Broader Context
Titanium alloy forging is widely used in aviation and aerospace manufacturing; isothermal forging is used for engine parts and aircraft structural parts, and increasingly for automotive, power and naval industries. Higher-temperature titanium-aluminide and intermetallic alloys are under active research along with their deformation processes.
