The conventional lost wax casting process, a cornerstone for manufacturing complex components like gas turbine blades, involves a lengthy and iterative cycle of tooling design, pattern production, and trial castings to finalize mold dimensions. This iterative process, essential for achieving dimensional accuracy, often spans one to three months, incurring significant time and economic costs, particularly for high-value, low-volume parts. This research project was initiated to explore and develop a rapid prototyping methodology that integrates additive manufacturing (AM) with the fundamental principles of lost wax casting. The primary objective was to drastically reduce lead time and cost for prototyping and small-batch production of turbine blades while maintaining the geometric fidelity and integrity inherent to the investment casting process. Our focus was on a specific process route utilizing Fused Deposition Modeling (FDM) for pattern creation and a subsequent analysis of critical failure modes, specifically shell cracking, encountered during the process. Through a combination of experimental trials, theoretical stress analysis, and finite element simulation, we identified root causes and implemented targeted optimizations to successfully produce sound castings.
Rapid Lost Wax Casting Process Methodology and Experimental Procedure
The core of our rapid lost wax casting approach lies in substituting the traditional wax injection mold and wax patterns with digitally-driven, directly 3D-printed patterns. The selected technical route is summarized in the following sequence: 1) Digital modeling and FDM printing of the blade pattern using ABS (Acrylonitrile Butadiene Styrene) polymer; 2) Assembly of the printed ABS pattern with a conventional wax gating system to form the complete investment cluster; 3) Building of a ceramic shell around the cluster via multiple slurry dipping and stuccoing steps; 4) Simultaneous removal of both the wax gating and the ABS pattern (the “lost” phase) followed by shell firing; 5) Molten metal pouring; and 6) Post-casting processes. This method eliminates the need for hard tooling (the die) for the blade itself, enabling a direct transition from CAD model to metal part.
For the experimental validation, a 3D CAD model of a gas turbine compressor blade was created and exported in STL format. This file was used to fabricate the blade pattern on an FDM printer using ABS filament. The printed ABS pattern exhibited good surface quality for the initial shell-building step. The pattern was then attached to a prefabricated wax pouring cup, runner, and ingates using a specialized adhesive, forming the sacrificial cluster for investment. The gating system was designed to introduce metal from the blade root (the larger cross-sectional area).
The shell-building process is critical in lost wax casting for achieving the necessary strength and thermal stability. A five-layer ceramic shell was applied. Each layer involved dipping the cluster into a ceramic slurry, draining the excess, and subsequently raining a refractory stucco material over the wet coating. The primary coat used a high-refractory slurry, while subsequent backup coats utilized different formulations. The composition for each layer is detailed in Table 1. After each coating and stuccoing step, the cluster was placed in a controlled environment (constant temperature and humidity) to allow for complete and uniform drying of the binder, which is essential to prevent shell defects like cracks or peeling.
| Layer Number | Slurry Binder | Slurry Refractory Flour | Stucco Material | Stucco Mesh Size |
|---|---|---|---|---|
| 1 (Primary) | Colloidal Silica | White Fused Alumina | Fused Alumina | 70 |
| 2 | Colloidal Silica | Fused Alumina / Others | Molochite / Coal Gangue | 30 / 60 |
| 3 | Colloidal Silica | Fused Alumina / Others | Molochite / Coal Gangue | 30 / 60 |
| 4 | Colloidal Silica | Fused Alumina / Others | Molochite / Coal Gangue | 16 / 30 |
| 5 | Colloidal Silica | Fused Alumina / Others | Molochite / Coal Gangue | 16 / 30 |
Following the shell-building and drying, the next critical phase is pattern removal and shell firing. The entire assembly was subjected to a thermal cycle to melt out the wax gating system and thermally decompose the ABS polymer pattern. This process must be carefully controlled to avoid excessive pressure build-up from rapidly vaporizing pattern material, which can crack the shell. After the pattern material was removed, the hollow ceramic shell underwent a high-temperature firing (between 970°C and 1030°C). This firing sinters the ceramic particles, burns out any residual organics, and prepares the shell to withstand the thermal shock of molten metal. It was at this stage that we observed a significant issue: cracking of the ceramic shell, predominantly at the thin, high-curvature regions of the blade’s leading and trailing edges. Despite attempts to patch these cracks, subsequent pouring of stainless steel yielded a casting with significant fins (flash) and dimensional inaccuracies, confirming the shell failure.
Thermo-Mechanical Analysis of Shell Cracking in Rapid Lost Wax Casting
The failure of the ceramic shell during the dewaxing and firing stage represents a major challenge in this variant of lost wax casting. Unlike traditional wax, which melts and flows out, ABS decomposes over a temperature range, potentially generating gases and exerting different thermal expansion stresses on the constraining shell. To analyze this, we developed a simplified analytical model followed by a more detailed finite element analysis (FEA).
We model the system as a thick-walled cylinder (representing the ABS pattern) perfectly bonded to an outer cylindrical shell (the ceramic investment). Let the inner radius of the ABS be \(a\), its outer radius (equal to the shell’s inner radius) be \(b\), and the outer radius of the ceramic shell be \(c\), so the shell thickness is \(x = c – b\). Both materials are initially at a uniform temperature \(T_1\) and are heated to \(T_2\), with a temperature change \(\Delta T = T_2 – T_1\). The thermal expansion coefficients are \(\alpha_{ABS}\) and \(\alpha_{ceramic}\), and the Young’s moduli and Poisson’s ratios are \(E_{ABS}, \nu_{ABS}\) and \(E_c, \nu_c\), respectively.
Upon heating, both materials want to expand freely. However, because they are bonded at the interface, they constrain each other, inducing interfacial pressure \(P\). The ABS cylinder is constrained from expanding outward, effectively placing it under compressive stress. Conversely, the ceramic shell is forced to expand more than it would alone, placing it under tangential tensile stress—the primary mode of failure for brittle ceramics.
The radial displacement at the interface (\(r = b\)) for the ABS cylinder, due to both thermal expansion and the compressive stress from pressure \(P\), is given by:
$$ u_{ABS}(b) = b \cdot \alpha_{ABS} \Delta T – \frac{P b}{E_{ABS}} \cdot \frac{a^2 + b^2}{b^2 – a^2} $$
For a solid ABS pattern (\(a = 0\)), this simplifies significantly.
The radial displacement at the inner surface of the ceramic shell (\(r = b\)) is:
$$ u_{c}(b) = b \cdot \alpha_{c} \Delta T + \frac{P b}{E_{c}} \cdot \frac{c^2 + b^2}{c^2 – b^2} $$
The compatibility condition at the bonded interface requires that these displacements be equal:
$$ u_{ABS}(b) = u_{c}(b) $$
This allows us to solve for the interfacial pressure \(P\). The resulting hoop (tangential) stress in the ceramic shell at the critical inner surface (\(r = b\)) is tensile and given by:
$$ \sigma_{\theta, ceramic}(b) = P \cdot \frac{c^2 + b^2}{c^2 – b^2} $$
Substituting the expression for \(P\) from the compatibility equation shows that this stress is proportional to the difference in thermal expansion coefficients \(\Delta \alpha = \alpha_{ABS} – \alpha_{c}\) and the temperature change \(\Delta T\). Given that \(\alpha_{ABS} ( \approx 90 \times 10^{-6} /°C)\) is an order of magnitude larger than \(\alpha_{c} ( \approx 4-6 \times 10^{-6} /°C)\), a large tensile stress is inevitable during heating. This simplified model clearly indicates that the mismatch in thermal expansion is the fundamental driver of shell stress in this rapid lost wax casting process.
To obtain a more realistic and spatially resolved stress distribution, particularly for the complex geometry of a turbine blade, we employed Finite Element Analysis. A 2D plane-strain model was created, representing a cross-section of the blade. Two key design variables were analyzed: the internal structure of the ABS pattern (solid vs. a low-density, porous infill) and the thickness of the ceramic shell. The material properties used for the simulation at an elevated temperature relevant to ABS decomposition (~150°C) are listed in Table 2.
| Material | Young’s Modulus (E) | Poisson’s Ratio (ν) | Coefficient of Thermal Expansion (α) |
|---|---|---|---|
| ABS Pattern | 1.0 MPa | 0.43 | 92.0 × 10-6 /°C |
| Ceramic Shell | 630 MPa | 0.26 | 4.0 × 10-6 /°C |
The model was constrained appropriately, and a temperature load from 22°C to 150°C was applied. The FEA results visualized the von Mises stress distribution (a representative stress measure) within the ceramic shell. The simulation confirmed the analytical prediction: the maximum tensile stresses were highly concentrated at regions with the smallest radius of curvature—specifically the leading and trailing edges of the blade airfoil. These are the precise locations where cracking was observed experimentally. The analysis further showed that a solid ABS pattern generated higher interfacial pressures and shell stresses compared to a pattern with a porous, low-density internal structure. Shell thickness also played a role, with slightly thicker shells distributing the stress somewhat better but not eliminating the concentration at sharp edges.
Process Optimization and Validation
Based on the root cause analysis from both the analytical model and FEA, we proposed and implemented a multi-faceted optimization strategy for our rapid lost wax casting process. The goal was to reduce the peak tensile stress in the ceramic shell during the critical pattern removal and firing stage.
1. Geometric Optimization of the Digital Pattern: Since stress concentration is inversely proportional to the radius of curvature, the most effective design change was to modify the 3D CAD model of the blade. We deliberately increased the fillet radii at the leading and trailing edges. This simple modification spreads the thermal strain over a larger area of the shell, significantly reducing the peak stress. This is a unique advantage of the digital process chain, as such a change would require costly and time-consuming mold modification in traditional lost wax casting.
2. Optimization of the 3D-Printed Pattern Structure: To reduce the effective thermal expansion force exerted by the ABS pattern on the shell, we altered the FDM printing parameters. Instead of printing a solid, high-density pattern, we used a sparse internal infill structure (e.g., a grid or honeycomb). This reduces the effective stiffness and the volumetric expansion of the pattern assembly during heating, thereby lowering the interfacial pressure \(P\) as suggested by the theoretical model.
3. Shell Building Process Enhancement: While FEA indicated that merely increasing thickness was not a complete solution, a stronger shell is better equipped to resist tensile stresses. We increased the number of ceramic layers from five to six-and-a-half, using the same slurry and stucco progression as before. This increased the overall shell thickness and its mechanical strength at the firing temperature.
The impact of these optimizations is summarized conceptually in Table 3. A final validation trial was conducted incorporating all these measures: the blade CAD model with increased edge radii, an ABS pattern printed with low-density internal structure, and a shell built with 6.5 layers. The result was a significant improvement: the ceramic shell survived the dewaxing and firing stages without visible cracking. Subsequent pouring of molten stainless steel yielded a blade casting with a clean surface, free of the fins caused by previous shell fractures, and with markedly improved dimensional accuracy.
| Optimization Measure | Mechanism of Action | Effect on Shell Stress |
|---|---|---|
| Increase Leading/Trailing Edge Radii | Reduces stress concentration factor (increases radius of curvature). | Directly lowers peak tensile stress at critical locations. |
| Use Low-Density Internal Infill in ABS Pattern | Reduces effective stiffness and volumetric expansion force of the pattern. | Lowers the interfacial pressure (P) driving shell stress. |
| Increase Number of Ceramic Shell Layers | Increases shell thickness and cross-sectional area to bear load. | Increases shell’s load-bearing capacity and may slightly lower stress. |
Conclusion
This study successfully demonstrated a viable rapid prototyping and manufacturing route for turbine blades by hybridizing FDM-based 3D printing with established lost wax casting principles. The process effectively bypasses the need for hard tooling for the wax pattern, offering a path to produce functional metal prototypes or small batches in a fraction of the time required by conventional methods. However, the inherent material mismatch between common 3D-printing polymers like ABS and the ceramic shell introduces a significant thermo-mechanical challenge, manifesting as shell cracking during pattern burnout.
Through a combination of experimental investigation and theoretical analysis—employing a deformation compatibility model and finite element simulation—we quantitatively linked this failure to the high tensile stress induced in the shell due to differential thermal expansion. The stress was found to concentrate at geometric features with small radii of curvature. The derived deformation equation, $$ 2\pi b \alpha_{ABS} \Delta T – \frac{2\pi b \sigma_{ABS}}{E_{ABS}} = 2\pi b \alpha_{c} \Delta T + \frac{2\pi b \sigma_{c}}{E_{c}} $$, underpins this understanding. The FEA results provided a clear visual confirmation via stress contour plots.
The proposed optimizations—geometric filleting of the digital model, using a low-density printed pattern structure, and building a slightly thicker ceramic shell—were derived directly from this analysis. Their implementation in a subsequent trial proved effective, resulting in the production of a sound, high-quality blade casting. This work confirms that rapid lost wax casting is a potent technique for agile manufacturing, and its successful application hinges on a fundamental understanding and proactive management of the thermal stresses involved in the process adaptation.