As a researcher in the field of advanced manufacturing, I have focused on the critical aspects of producing high-performance components for aerospace applications. In this study, I investigate the vacuum investment casting process for aerospace engine brakes, which are essential parts in jet engines. The reliability and strength of these aerospace casting parts directly impact engine performance, making it vital to optimize their production. These castings aerospace components often feature complex geometries, large diameters, and thin walls, leading to inevitable casting defects during manufacturing. Variations in pouring techniques and solidification processes result in different defect distributions, underscoring the practical significance of refining the casting process for aerospace casting parts. Through numerical simulation and experimental validation, I aim to enhance the quality of these critical castings aerospace elements.
In developing the physical model for the brake castings, I considered the entire assembly within a vacuum furnace environment. The铸件 exhibits rotational symmetry, but this symmetry is disrupted by radiative heat transfer effects from the furnace walls, necessitating a full-scale model. I constructed geometric models for the sand mold, ceramic shell, and the brake铸件 itself, assembling them into a cohesive system. The gaps between the sand mold and shell were filled with insulating felt, which was also placed above the shell to minimize heat loss. Prior to pouring, the shell and sand mold were preheated to 830°C and then placed in a vacuum furnace with a diameter of 3 meters. The pouring process lasted between 9 to 13 seconds at a temperature of 1500°C, followed by controlled cooling. The interfaces between components—such as the sand mold and insulating felt, the felt and shell, and the shell and铸件—involve complex heat transfer relationships with variable thermal resistances that change with temperature. Since cooling occurs under vacuum conditions with water cooling, gray-body radiation governs the heat exchange between the sand mold outer wall, the insulating felt upper surface, and the furnace walls. The emissivity of the insulating felt varies with temperature, while the furnace wall temperature remains constant. Given the extensive temperature range in this process, material properties cannot be treated as constants; instead, I defined them as functions of temperature.
The铸件 material is K4169 alloy, specifically designed for aerospace applications, with a density of 8193 kg/m³, a solidus temperature of 1198°C, and a liquidus temperature of 1362°C. The ceramic shell, made of alumina, has a density of 3096 kg/m³. The thermal conductivity and specific heat capacity for both the铸件 alloy and shell material are temperature-dependent, as summarized in the tables below. These properties are crucial for accurate simulations of aerospace casting parts, as they influence heat flow and solidification behavior in castings aerospace components.
| Temperature (°C) | Thermal Conductivity (W/m·°C) | Specific Heat Capacity (J/kg·°C) |
|---|---|---|
| 300 | 15.02 | 479 |
| 400 | 16.73 | 504 |
| 500 | 18.33 | 518 |
| 600 | 20.01 | 552 |
| 700 | 21.29 | 569 |
| 800 | 22.33 | 588 |
| Temperature (°C) | Thermal Conductivity (W/m·°C) | Specific Heat Capacity (J/kg·°C) |
|---|---|---|
| 26 | 6.93 | 547 |
| 593 | 4.72 | 1103 |
| 790 | 3.98 | 1137 |
| 1180 | 3.88 | 1198 |
| 1350 | 3.76 | 1229 |
To model the casting process accurately, I established a mathematical framework that couples temperature and flow fields. This is essential for analyzing the temperature variations of the molten metal during filling and the resulting solidification patterns. Assuming the alloy melt is an incompressible Newtonian fluid, the governing equations include the momentum conservation equation, energy conservation equation, volume fraction equation, and continuity equation. These equations form the basis for simulating the behavior of aerospace casting parts under varying conditions.
The momentum conservation equation accounts for fluid flow and is given by:
$$ \rho \left( \frac{\partial \mathbf{v}}{\partial t} + \mathbf{v} \cdot \nabla \mathbf{v} \right) = -\nabla p + \mu \nabla^2 \mathbf{v} + \mathbf{f} $$
where \( \rho \) is the density, \( \mathbf{v} \) is the velocity vector, \( t \) is time, \( p \) is pressure, \( \mu \) is the dynamic viscosity, and \( \mathbf{f} \) represents body forces such as gravity. This equation helps predict flow patterns that affect defect formation in castings aerospace components.
The energy conservation equation describes heat transfer during the process:
$$ \rho c_p \left( \frac{\partial T}{\partial t} + \mathbf{v} \cdot \nabla T \right) = \nabla \cdot (k \nabla T) + Q $$
Here, \( c_p \) is the specific heat capacity, \( T \) is temperature, \( k \) is the thermal conductivity, and \( Q \) represents heat sources or sinks. This equation is critical for tracking temperature changes in aerospace casting parts, especially during solidification.
The continuity equation ensures mass conservation:
$$ \frac{\partial \rho}{\partial t} + \nabla \cdot (\rho \mathbf{v}) = 0 $$
Additionally, the volume fraction equation tracks the interface between different phases, which is vital for modeling the filling and solidification of castings aerospace parts. During solidification, I compute the instantaneous solid fraction at each point to determine volume shrinkage, which predicts the formation of shrinkage porosity and cavities. The porosity volume fraction \( \phi \) can be expressed as:
$$ \phi = \frac{V_{\text{porosity}}}{V_{\text{total}}} = f(T, g_s) $$
where \( V_{\text{porosity}} \) is the volume of porosity, \( V_{\text{total}} \) is the total volume, \( T \) is temperature, and \( g_s \) is the solid fraction. This relationship allows for the identification of defect-prone areas in aerospace casting parts.
I employed the finite element method to solve these equations numerically, discretizing the domain into elements as detailed in the mesh statistics. The mesh was refined to capture complex geometries, ensuring accurate simulations for these high-precision castings aerospace components.
| Component | Number of Elements (×10⁴) | Maximum Element Axes (Long × Short) | Minimum Element Axes (Long × Short) |
|---|---|---|---|
| 铸件 | 209.3 | 12 × 12 | 4 × 1 |
| Shell | 280.2 | 12 × 12 | 4 × 2 |
| Insulating Felt | 109.8 | 36 × 36 | 12 × 12 |
| Sand Mold | 4.7 | 36 × 6 | 36 × 6 |
In the initial casting process simulation, I analyzed the temperature and flow fields during filling. The results showed that vent channels above the斜支板 played a crucial role in ensuring proper filling. The filling phase was relatively short compared to solidification, with molten metal temperatures ranging between 1470°C and 1500°C. Areas with narrow passages or sharp corners experienced the most significant temperature drops. Under the preheating condition of 830°C, rapid cooling occurred primarily in thin-walled sections, while the risers in thicker regions provided insufficient control over solidification. This led to an irregular solidification sequence and the formation of shrinkage defects. A comparison with X-ray inspection of actual castings aerospace parts confirmed that the simulation accurately predicted defect locations, validating the model’s effectiveness for optimizing aerospace casting parts.
To address these issues, I proposed an optimized process. I replaced the insulating felt at the bottom of the sand mold with quartz sand, slightly elevating the central portion to contact the lower surface of the shell at the inner and outer flanges. Additionally, I increased the preheating temperature to 1090°C while keeping other parameters unchanged. This modification aimed to enhance riser efficiency and promote a more controlled solidification pattern for aerospace casting parts.
The improved process simulation revealed a more uniform temperature distribution, with edge sections cooling later than the central斜支板 and outer thin walls. In thicker areas, regions near risers cooled slower than those farther away, reducing the likelihood of defects. The prediction indicated that only the mid-sections of the outer thin walls were prone to minor shrinkage porosity, a significant improvement over the original process. This demonstrates how precise control of thermal parameters can enhance the quality of castings aerospace components.
In conclusion, my research highlights the importance of coupled temperature and flow field simulations in optimizing the vacuum investment casting process for aerospace engine brakes. By increasing preheating temperatures and improving solidification control, I effectively minimized defects in these critical aerospace casting parts. The mathematical model proved reliable for predicting and addressing issues in castings aerospace production, offering a scientific basis for further工艺 refinements. This work underscores the potential of numerical methods to advance the manufacturing of high-integrity aerospace components, ensuring better performance and reliability in demanding applications.