This study focuses on optimizing precision investment casting parameters for TC4 alloy variable cross-section components using numerical simulation and experimental validation. The centrifugal investment casting process was analyzed through orthogonal experiments, ProCAST simulations, and comprehensive characterization to address shrinkage porosity, stress concentration, and dimensional accuracy challenges.
1. Numerical Simulation and Orthogonal Experiment Design
The casting geometry (580 mm length, 6.5 mm minimum wall thickness) was modeled with 456,058 mesh elements. Key process parameters were evaluated via L9(34) orthogonal experiments:
| Factors | Level 1 | Level 2 | Level 3 |
|---|---|---|---|
| A: Pouring Temp (°C) | 1680 | 1700 | 1750 |
| B: Mold Preheating (°C) | 300 | 350 | 400 |
| C: Pouring Rate (kg/s) | 3 | 5 | 7 |
| D: Centrifugal Speed (rpm) | 350 | 450 | 550 |
The Niyama criterion was employed to predict shrinkage porosity:
$$ M = \frac{G}{\sqrt{\dot{T}}} $$
where G is temperature gradient (K/mm) and Ț is cooling rate (K/s). Centrifugal force was calculated using:
$$ n = 29.9\sqrt{\frac{G}{r_0}} $$
where G = 40-100 (gravity coefficient) and r0 = inner radius (m).
2. Process Optimization Results
The optimized precision investment casting parameters were determined through range analysis:
| Parameter | K1 | K2 | K3 | Range (R) |
|---|---|---|---|---|
| A | 15.53 | 16.08 | 16.87 | 1.34 |
| B | 16.76 | 15.88 | 15.86 | 0.90 |
| C | 15.60 | 16.05 | 16.84 | 1.24 |
| D | 15.92 | 16.45 | 16.11 | 0.53 |
Optimal parameters: A1B3C1D1 (1680°C pouring, 400°C mold preheating, 3 kg/s pouring rate, 350 rpm). This reduced shrinkage porosity by 22.7% compared to initial parameters.
3. Filling and Solidification Behavior
Simulation revealed critical process characteristics:
- Filling completion: 5.2 s with maximum velocity 19.09 m/s
- Solidification sequence: Bottom (9.37 s) → Top (15.65 s) → Middle (20.00 s) → Gating system (49.74 s)
- Microstructure development: Columnar grains (83.2%) with β→α phase transformation at 995°C
The feeding pressure equation explains centrifugal force effects:
$$ P_{feed} = \frac{\omega^2 \rho}{2g}(R^2 – R_0^2) $$
where ω = angular velocity, ρ = melt density (4430 kg/m³), R = rotation radius.
4. Defect Formation Mechanisms
Key findings in precision investment casting defect control:
| Defect Type | Location | Formation Mechanism | Control Strategy |
|---|---|---|---|
| Shrinkage porosity | Top (68%), Middle (22%) | Isolated liquid zones (Niyama < 1.0) | Enhanced thermal gradient (2.4 K/mm) |
| Stress concentration | Gating connections | ΔCTE = 8.7×10-6 K-1 | Radius optimization (R > 5t) |
| Surface defects | Thin-wall sections | Reynolds number > 4200 | Coating viscosity < 2.8 Pa·s |
X-ray analysis confirmed defect reduction effectiveness:
- Shrinkage porosity volume: 4.93 cm³ (simulation) vs 0 cm³ (actual)
- Dimensional accuracy: 98.7% ± 0.3% (Laser scan vs CAD)
5. Microstructure and Mechanical Properties
The precision investment casting process produced typical Widmanstätten structures:
- Prior β grain size: 1.12 ± 0.23 mm
- α lath thickness: 0.76 ± 0.12 μm
- β phase fraction: 18.4% ± 2.1%
HIP-treated specimens showed enhanced mechanical performance:
$$ \sigma_b = 953.5\ \text{MPa},\ \sigma_{0.2} = 835.0\ \text{MPa},\ \delta = 10.0\% $$
Fractography revealed mixed-mode failure characteristics:
- Dimple density: 12,340/mm²
- Cleavage facet size: 18-45 μm
- DBTT: -56°C (Charpy impact)
6. Industrial Implementation
The optimized precision investment casting parameters were validated in production of aerospace components:
| Metric | Before Optimization | After Optimization |
|---|---|---|
| Yield rate | 67.2% | 89.5% |
| UT rejection | 22.4% | 3.8% |
| Lead time | 38 days | 26 days |
This systematic approach demonstrates the effectiveness of combined numerical simulation and experimental validation in precision investment casting optimization for complex titanium components. The methodology provides a framework for addressing similar challenges in high-performance alloy casting applications.