Precision investment casting is widely employed in manufacturing complex turbine components due to its ability to produce near-net-shape parts with exceptional dimensional accuracy. This study focuses on optimizing the casting process of a turbine nozzle using numerical simulations, with an emphasis on reducing defects and improving metal utilization. The methodology integrates computational modeling, process parameter adjustments, and experimental validation to achieve high-quality castings compliant with aerospace standards.
Process Challenges and Initial Simulation
The turbine nozzle, fabricated from K4169 superalloy, features thin-walled sections (0.5 mm) and thick flanges (24 mm), creating significant challenges in maintaining directional solidification. The initial casting design utilized a combined top-bottom gating system with six oversized risers, resulting in turbulent filling patterns and low metal utilization (12.13%). ProCAST simulations revealed critical issues:
$$
\frac{\partial T}{\partial t} = \alpha \nabla^2 T + \frac{Q}{\rho c_p}
$$
Where \( T \) represents temperature, \( \alpha \) thermal diffusivity, \( Q \) latent heat, \( \rho \) density, and \( c_p \) specific heat. The initial process exhibited:
- Vortex formation during flange filling (40–60% fill stage)
- Isolated liquid zones at blade-root junctions
- Macroporosity in 37% of blade sections
| Parameter | Initial Design | Optimized Design |
|---|---|---|
| Riser Quantity | 6 | 8 |
| Riser Height (mm) | 110 | 40 |
| Gating Type | Top-Bottom | Bottom-Up |
| Metal Utilization | 12.13% | 43.18% |
Optimization Strategy
The redesigned precision investment casting system incorporated three key modifications:
- Gating System Revision: Implemented bottom-fed filling with tapered sprue:
$$
v(t) = v_0 \left(1 – e^{-t/\tau}\right)
$$Where \( v_0 \) = 1.2 m/s initial velocity and \( \tau \) = 0.8 s damping constant.
- Riser Configuration: Distributed eight compact risers (40mm height) around the flange using hexagonal packing:
$$
N_{\text{risers}} = \left\lfloor \frac{\pi D}{d + s} \right\rfloor
$$(\( D \)=366mm flange diameter, \( d \)=40mm riser diameter, \( s \)=15mm spacing)
- Mold Optimization: Inserted zirconia sand cores between blades to enhance cooling uniformity:
$$
\dot{q}_{\text{core}} = k_{\text{core}} \frac{T_{\text{melt}} – T_{\text{mold}}}{\delta_{\text{core}}}
$$
Simulation Results and Validation
The optimized precision investment casting process demonstrated:
- Laminar filling sequence completed in 6.8s (vs. 4s turbulent filling in initial design)
- Directional solidification gradient of 12°C/mm from blade tips to risers
- Zero shrinkage defects in X-ray inspection of 50 production castings
| Quality Metric | Initial | Optimized |
|---|---|---|
| Surface Turbulence Index | 4.7 | 1.2 |
| Solidification Time (min) | 18.5 | 22.3 |
| Microporosity (% area) | 0.15 | <0.02 |
Industrial Implementation
The optimized precision investment casting process achieved 100% compliance with EMS52301/2 specifications while reducing unit cost by 63% through:
$$
\text{Cost Savings} = \left(1 – \frac{m_{\text{optimized}}}{m_{\text{initial}}}\right) \times 100\% = \left(1 – \frac{16.2}{59.8}\right) \times 100\% = 72.9\%
$$
Key production parameters for replicating this precision investment casting methodology include:
- Shell preheat temperature: 1050°C ±15°C
- Vacuum level: 0.08–0.1 mbar
- Cooling rate: 8–12°C/min between 1350–1000°C
This systematic approach to precision investment casting optimization demonstrates how numerical simulation can simultaneously improve quality metrics and production economics in high-performance alloy casting.