
1. Introduction
Investment casting is a precision casting process that has been widely used in the manufacturing of complex and high-precision components, especially in the aerospace and power generation industries. Gas turbine blades, with their complex geometries and high-performance requirements, are typically manufactured using investment casting techniques. However, the traditional investment casting process often involves time-consuming and costly trial-and-error procedures to determine the optimal mold dimensions. This has led to the exploration of more efficient and cost-effective methods, such as the combination of additive manufacturing and investment casting.
In this study, we focus on the rapid investment casting process for gas turbine blades, aiming to optimize the process and overcome the challenges associated with mold cracking and dimensional accuracy. By integrating 3D printing technology with investment casting, we seek to reduce the lead time and cost while maintaining or improving the quality of the final product.
2. Rapid Investment Casting Process for Gas Turbine Blades
2.1 Process Overview
The rapid investment casting process investigated in this study involves several key steps. Firstly, the gas turbine blade is designed and modeled using computer-aided design (CAD) software. The 3D model is then converted into a format suitable for 3D printing. In this case, an FDM printer is used to fabricate the blade prototype from ABS resin. The ABS blade prototype is then assembled with a traditional wax casting system to form the investment mold assembly. This assembly is then dipped in a slurry and coated with refractory materials multiple times to build up the ceramic shell. After drying and dewaxing, the shell is sintered at a high temperature to enhance its strength and dimensional stability. Finally, molten metal is poured into the shell cavity to obtain the final blade casting.
2.2 Experimental Procedure
- ABS Blade Prototype Fabrication: The CAD model of the gas turbine blade is sliced into layers and printed using an FDM 3D printer. The printing parameters, such as layer thickness, nozzle temperature, and printing speed, are carefully controlled to ensure the dimensional accuracy and surface quality of the prototype. The printed ABS blade is then post-processed to remove any support structures and improve its surface finish.
- Investment Mold Assembly: The ABS blade prototype is attached to a wax sprue and runner system using a suitable adhesive. The assembly is designed to ensure proper filling of the molten metal during casting. The wax pattern is then inspected for any defects or imperfections and repaired if necessary.
- Shell Building: The investment mold assembly is dipped into a ceramic slurry, typically composed of a binder, refractory powder, and additives. After each dip, the assembly is coated with a layer of refractory sand. The process is repeated multiple times to build up the desired shell thickness. The coating and drying conditions are strictly controlled to prevent cracking and ensure uniform shell quality.
- Dewaxing and Sintering: The shell with the wax pattern is heated in a furnace to melt and remove the wax. This is followed by a high-temperature sintering process to strengthen the shell and improve its resistance to thermal shock. The sintering temperature and time are optimized based on the shell material and the requirements of the casting process.
- Casting and Finishing: Molten metal, usually a high-temperature alloy, is poured into the sintered shell cavity under carefully controlled conditions. After solidification, the shell is removed, and the casting is subjected to a series of finishing operations, including machining, heat treatment, and surface treatment, to achieve the final dimensions and surface quality requirements.
3. Analysis of Shell Cracking in Rapid Investment Casting
3.1 Causes of Shell Cracking
- Thermal Stress: During the sintering and casting processes, the ceramic shell experiences significant temperature changes, which lead to thermal expansion and contraction. If the thermal expansion coefficients of the shell and the blade prototype are not well-matched, or if the temperature gradients within the shell are too large, thermal stresses can develop and cause cracking.
- Shell Thickness and Structure: The thickness and uniformity of the shell play a crucial role in its mechanical strength and resistance to cracking. Insufficient shell thickness or uneven coating can result in weak points where cracks are more likely to initiate. Additionally, the internal structure of the shell, such as porosity and grain size, can affect its mechanical properties.
- Process Parameters: The parameters used in the shell building and sintering processes, such as slurry viscosity, dipping speed, drying time, and sintering temperature, can all influence the quality of the shell. Improper settings of these parameters can lead to defects in the shell, such as cracking, warping, and delamination.
- Blade Design: The complex geometry of the gas turbine blade can also contribute to shell cracking. Sharp corners, thin sections, and sudden changes in cross-section can cause stress concentrations in the shell during the casting process.
3.2 Finite Element Analysis of Shell Stress
- Modeling and Meshing: A finite element model of the blade prototype and the ceramic shell is created using commercial software. The model takes into account the geometry, material properties, and boundary conditions of the system. The blade and shell are meshed with appropriate element types to accurately represent the stress distribution.
- Thermal and Mechanical Analysis: The model is subjected to a thermal analysis to simulate the temperature distribution during the sintering and casting processes. The resulting thermal stresses are then calculated using a mechanical analysis. The effects of different process parameters and blade designs on the stress distribution are investigated by varying the input parameters in the model.
- Stress Concentration Areas: The analysis reveals that the areas of highest stress concentration are typically located at the blade edges, where the curvature is the greatest. These areas are more prone to cracking due to the higher stress levels. The results also show that the stress distribution is affected by the shell thickness, with thinner shells experiencing higher stresses.
3.3 Deformation Coordination Equation
- Derivation: To better understand the interaction between the blade prototype and the ceramic shell, a deformation coordination equation is derived. Assuming the blade prototype as an infinite long circular cylinder and the shell as a concentric cylinder surrounding it, the equation relates the thermal expansion and elastic deformation of both components. The equation is based on the principle of equilibrium of forces and displacements at the interface between the blade and the shell.
- Application: The deformation coordination equation is used to analyze the stress distribution in the shell under different temperature conditions. By solving the equation, the relationship between the thermal expansion coefficients, elastic moduli, and dimensions of the blade and shell can be determined. This helps in identifying the critical factors that contribute to shell cracking and provides a basis for optimization.
4. Optimization Measures for Rapid Investment Casting Process
4.1 Blade Model Optimization
- Radius Modification: Based on the analysis of stress distribution, the radii of the blade edges are increased to reduce stress concentrations. This is achieved by modifying the CAD model of the blade before 3D printing. The modified blade design is expected to result in a more uniform stress distribution in the shell during casting.
- Internal Structure Optimization: The internal structure of the blade prototype is redesigned to have a grid-like or porous structure. This helps in reducing the overall thermal expansion of the blade and improving its compatibility with the ceramic shell. The porous structure also allows for better gas escape during the casting process, reducing the risk of porosity in the casting.
4.2 Shell Building Process Improvement
- Coating Thickness Adjustment: The number of coating layers is increased from 5 to 6.5 to enhance the shell thickness and strength. The additional layers provide better protection against thermal and mechanical stresses during the casting process. The coating formulation and application process are optimized to ensure good adhesion and uniformity of the layers.
- Process Parameter Optimization: The parameters related to shell building, such as slurry viscosity, dipping speed, and drying time, are fine-tuned based on experimental results and finite element analysis. The goal is to achieve a more uniform shell thickness and better mechanical properties. For example, the slurry viscosity is adjusted to improve the wetting and adhesion of the refractory materials to the wax pattern.
4.3 Sintering and Casting Process Optimization
- Sintering Temperature Optimization: The sintering temperature is carefully controlled within a narrow range to balance the shell strength and dimensional stability. Too low a sintering temperature may result in insufficient shell strength, while too high a temperature can cause excessive shrinkage and cracking. The optimal sintering temperature is determined through a series of experimental trials and finite element simulations.
- Casting Parameter Optimization: The casting parameters, such as pouring temperature, pouring speed, and mold preheating temperature, are optimized to ensure proper filling of the shell cavity and minimize thermal shock. The pouring temperature is adjusted to achieve good fluidity of the molten metal while avoiding overheating, which can lead to defects in the casting.
5. Experimental Results and Validation
5.1 Comparison of Original and Optimized Processes
- Shell Quality: The optimized process results in a significant improvement in shell quality. The shells produced are more uniform in thickness and have fewer cracks and defects. The increased shell thickness and improved coating adhesion contribute to better resistance against thermal and mechanical stresses.
- Casting Dimensional Accuracy: The dimensional accuracy of the castings obtained from the optimized process is within the required tolerances. The modifications in the blade design and process parameters help in reducing the distortion and shrinkage of the castings. The comparison of the dimensions of the original and optimized castings is shown in Table 1.
Parameter | Original Casting | Optimized Casting |
---|---|---|
Blade Length (mm) | 100.5 ± 0.5 | 100.2 ± 0.2 |
Blade Width (mm) | 50.3 ± 0.3 | 50.1 ± 0.1 |
Blade Thickness (mm) | 10.2 ± 0.2 | 10.1 ± 0.1 |
- Mechanical Properties: The mechanical properties of the optimized castings, such as tensile strength and hardness, are tested and compared with the original castings. The results show that the optimized process leads to an improvement in the mechanical properties, which is attributed to the better quality of the shell and the reduced internal defects in the casting.
5.2 Microstructural Analysis
- Grain Structure: The microstructure of the castings is examined using optical microscopy and scanning electron microscopy. The results show that the optimized process results in a finer and more uniform grain structure. The modified blade design and casting parameters help in promoting a more favorable solidification process, leading to improved mechanical properties.
- Porosity and Defects: The porosity and other internal defects in the castings are significantly reduced in the optimized process. The improved shell quality and the better control of the casting process parameters contribute to a reduction in gas entrapment and shrinkage porosity.
5.3 Performance Testing of Cast Blades
- High-Temperature Oxidation Resistance: The cast blades are subjected to high-temperature oxidation tests to evaluate their performance in a simulated gas turbine environment. The results show that the optimized blades have better oxidation resistance, which is attributed to the improved surface quality and the reduced porosity. The weight gain of the blades after oxidation is measured and compared, as shown in Table 2.
Blade Type | Weight Gain after Oxidation (mg/cm²) |
---|---|
Original Blade | 5.2 ± 0.5 |
Optimized Blade | 3.5 ± 0.3 |
- Fatigue Life: The fatigue life of the cast blades is tested using a rotating bending fatigue testing machine. The results indicate that the optimized blades have a longer fatigue life compared to the original blades. The improved microstructure and reduced internal defects contribute to better fatigue resistance. The fatigue life data is presented in Table 3.
Blade Type | Fatigue Life (cycles) |
---|---|
Original Blade | 10^5 ± 10^4 |
Optimized Blade | 1.5 × 10^5 ± 10^4 |
6. Conclusion
In this study, a rapid investment casting process for gas turbine blades was developed and optimized. The combination of 3D printing technology and traditional investment casting techniques enabled the production of complex blade geometries with improved efficiency and reduced cost. The analysis of shell cracking using finite element methods and the derivation of the deformation coordination equation provided valuable insights into the stress distribution and failure mechanisms in the casting process.
The optimization measures implemented, including blade model optimization, shell building process improvement, and sintering and casting process optimization, resulted in significant improvements in shell quality, casting dimensional accuracy, mechanical properties, and performance characteristics of the cast blades. The experimental results validated the effectiveness of the proposed optimization strategies.