In the manufacturing of gas turbine blades, traditional lost wax investment casting processes often require iterative trials to determine mold dimensions, leading to extended timeframes and high costs. This study explores a rapid lost wax investment casting approach that integrates additive manufacturing to streamline production. By utilizing fused deposition modeling (FDM) to create blade patterns from ABS material and combining them with wax gating systems, the method accelerates the prototype development phase. The assembly undergoes a series of coating and stuccoing steps to form a ceramic shell, followed by dewaxing, firing, and metal pouring to yield the final casting. However, challenges such as shell cracking during production were identified and addressed through deformation compatibility analysis and finite element simulations. This article details the process optimization, including material adjustments and structural enhancements, to achieve high-quality blade castings.
The conventional lost wax investment casting process involves multiple stages: initial process planning, mold fabrication, wax pattern creation, assembly, shell building, dewaxing, firing, pouring, and inspection. Any defects detected may necessitate revisions, prolonging the cycle to 1–3 months. In contrast, rapid lost wax investment casting leverages 3D printing to produce patterns directly, reducing dependency on physical molds and enabling faster iterations. This approach is particularly advantageous for complex components like turbine blades, which feature intricate airfoil profiles and internal cooling channels that are difficult to machine. The integration of ABS patterns with wax systems forms the basis of this method, emphasizing agility and cost-efficiency in prototyping and small-batch production.
The rapid lost wax investment casting process begins with designing a three-dimensional digital model of the blade, which is converted to STL format for FDM printing. ABS thermoplastic is extruded layer-by-layer to form the blade pattern, replicating the precise geometry required for casting. Subsequently, the ABS pattern is attached to a wax gating system using adhesive compounds, forming a complete investment mold assembly. This assembly is then subjected to shell building, where it is coated with ceramic slurries and stuccoed with refractory materials to create a multi-layered shell. The shell is dried under controlled conditions to ensure uniformity and strength.
Key parameters in shell construction include slurry viscosity, binder composition, and stucco particle size. For instance, the shell typically consists of five layers, each with specific formulations to enhance thermal resistance and dimensional stability. The table below summarizes a typical coating and stuccoing recipe used in the process:
| Layer Number | Binder | Powder Material | Stucco Type | Grit Size (Mesh) |
|---|---|---|---|---|
| 1 | Silica Sol | White Alumina | Alumina | 70 |
| 2 | Silica Sol | Alumina | Alumina | 30/60 |
| 3 | Silica Sol | Alumina | Coal Gangue | 30/60 |
| 4 | Silica Sol | Alumina | Coal Gangue | 16/30 |
| 5 | Silica Sol | Alumina | Coal Gangue | 16/30 |
After shell building, the assembly undergoes dewaxing and ABS removal through thermal processes, where the wax and ABS are melted or burned out to create a hollow cavity. The shell is then fired at temperatures between 970°C and 1,030°C to enhance its mechanical properties and prepare it for metal pouring. During firing, thermal stresses can lead to shell cracking, particularly in regions with high curvature, such as blade edges. To analyze this, a deformation compatibility equation is derived for a simplified model of the ABS pattern and ceramic shell. Assuming an infinitely long cylindrical geometry, the radial displacements due to thermal expansion and elastic deformation are expressed as:
For the ABS pattern:
$$X_1 = 2\pi b \alpha_1 \Delta t – \frac{2\pi b \sigma_1}{E_1}$$
For the ceramic shell:
$$X_2 = 2\pi b \alpha_2 \Delta t + \frac{2\pi b \sigma_2}{E_2}$$
The compatibility condition requires:
$$X_1 = X_2$$
Thus:
$$2\pi b \alpha_1 \Delta t – \frac{2\pi b \sigma_1}{E_1} = 2\pi b \alpha_2 \Delta t + \frac{2\pi b \sigma_2}{E_2}$$
where \(b\) is the outer radius, \(\alpha\) is the coefficient of thermal expansion, \(\Delta t\) is the temperature change, \(\sigma\) is stress, and \(E\) is Young’s modulus. This equation highlights how mismatched thermal expansion between ABS and ceramic materials induces stress, potentially causing shell failure.
Finite element analysis (FEA) is employed to simulate thermal stress distributions in the shell during heating. A two-dimensional model of the blade cross-section is created, assuming plane stress conditions and bonded contact between the ABS pattern and shell. The mesh is generated using four-node PLANE13 elements, and the temperature rise from 22°C to 200°C is modeled with a linear function:
$$K = 22 + a t$$
where \(K\) is the temperature at time \(t\), and \(a\) is a constant. Material properties for ABS include an elastic modulus of 1.0 MPa, Poisson’s ratio of 0.43, and thermal expansion coefficient of \(92.0 \times 10^{-6} /^\circ\text{C}\). The shell has an elastic modulus of 630 MPa, Poisson’s ratio of 0.26, and thermal expansion coefficient of \(4.0 \times 10^{-6} /^\circ\text{C}\). The equivalent stress contours from FEA reveal that maximum stresses concentrate at the leading and trailing edges of the blade, where curvature is highest. This aligns with observed crack sites in experimental trials, confirming the need for geometric and process optimizations.
To mitigate shell cracking, several improvements are implemented in the rapid lost wax investment casting process. First, the blade design is modified to increase the radius of curvature at the edges, reducing stress concentrations. Second, the internal structure of the ABS pattern is altered to a low-density grid, which decreases thermal mass and minimizes expansion-induced stresses. Third, the shell thickness is increased by extending the number of coating layers from five to six, enhancing mechanical robustness without compromising dimensional accuracy. The table below compares key parameters before and after optimization:
| Parameter | Initial Process | Optimized Process |
|---|---|---|
| Number of Shell Layers | 5 | 6 |
| ABS Internal Structure | Solid | Grid |
| Edge Radius | Baseline | Increased |
| Firing Temperature | 970–1,030°C | 970–1,030°C |
Validation trials with the optimized parameters demonstrate a significant reduction in shell defects, leading to the successful production of blade castings with improved surface finish and dimensional accuracy. The rapid lost wax investment casting method, combined with FEA-driven insights, proves effective for accelerating development cycles and reducing costs in turbine blade manufacturing. Future work could explore advanced materials for patterns and shells, as well as real-time monitoring during firing to further enhance process control.
In conclusion, the integration of 3D printing with lost wax investment casting offers a viable path for rapid prototyping of complex components. By addressing thermal stress challenges through deformation analysis and finite element simulations, this study contributes to the optimization of casting processes, ensuring reliability and efficiency in industrial applications. The iterative nature of lost wax investment casting is thus streamlined, underscoring the value of computational tools in traditional manufacturing domains.