Abstract:
This paper presents a comprehensive investigation into the turbine casting process, focusing on the prevention of porosity and shrinkage defects through numerical simulation. The study begins with a detailed analysis of the turbine guide structure and material composition, followed by the design and optimization of the pouring system. Finite element software, specifically ProCAST, is utilized to simulate the filling and solidification processes of the turbine guide investment casting. The simulation results are then validated through non-destructive testing methods. The optimized pouring system, resulting from this study, significantly reduces porosity and shrinkage defects, ensuring compliance with stringent casting standards.
Keywords: turbine casting process, porosity, shrinkage defects, ProCAST, numerical simulation, non-destructive testing
1. Introduction
In recent years, the demand for high-performance turbine components has surged, driven by advancements in the aerospace industry. Turbine guides, being a critical component in aerospace engines, require intricate design and impeccable quality to ensure optimal performance and reliability. The casting of turbine guides poses several challenges, including complex geometries, thin walls, and the need for high-quality surfaces. These challenges are compounded by the fact that porosity and shrinkage defects can significantly compromise the mechanical properties and overall performance of the castings.
1.1 Objectives of the Study
The primary objectives of this study are:
- To analyze the structure and material properties of the turbine guide casting.
- To design and optimize the pouring system for the turbine guide casting process.
- To utilize numerical simulation to predict and minimize porosity and shrinkage defects.
- To validate the optimized casting process through non-destructive testing methods.
1.2 Literature Review
Previous studies have investigated various aspects of turbine casting processes, focusing on improving casting quality and reducing defects. Studies utilizing numerical simulation tools such as ProCAST have gained popularity due to their ability to predict casting defects and optimize casting processes with minimal experimental trials [1-3]. In addition, vacuum investment casting techniques have been shown to produce near-net-shape components with excellent surface finish and dimensional accuracy [4, 5].
2. Turbine Guide Structure and Material Analysis
2.1 Structure Analysis
The turbine guide under investigation is a complex frame structure consisting of an inner ring, blades, and an outer ring. The blades, being the most critical feature, exhibit extreme thinness (as low as 1 mm) and varying thicknesses across the structure. The overall dimensions of the turbine guide are Ø512.80 mm (outer diameter) and Ø334.00 mm (inner diameter), with a thickness of 54.87 mm for the outer ring. The cross-sectional variations and thin walls pose significant challenges during the casting process, necessitating a well-designed pouring system.
Material and Performance Analysis
The turbine guide is cast from K438, a nickel-based superalloy renowned for its high-temperature strength and corrosion resistance. Table 1 summarizes the chemical composition of K438. This alloy undergoes complex solidification and phase transformation processes, making it prone to porosity and shrinkage defects if not properly controlled during casting.
Table 1: Chemical Composition of K438 Nickel-Based Superalloy
| Element | Content (wt.%) |
|---|---|
| C | 0.10 – 0.20 |
| Cr | 15.5 – 16.5 |
| Co | 8.0 – 9.0 |
| W | 2.4 – 2.8 |
| Mo | 1.5 – 2.0 |
| Al | 3.2 – 3.7 |
| Ti | 3.0 – 3.5 |
| Nb | 0.6 – 1.1 |
| Ta | 1.5 – 2.0 |
| B | 0.005 – 0.015 |
| Zr | 0.05 – 0.15 |
| Ni | Balance |
3. Pouring System Design and Optimization
3.1 Initial Pouring System Design
Based on the turbine guide’s structural complexities, a vacuum investment casting process with a top-injection pouring system was chosen. The pouring system comprises a funnel-shaped pouring cup, circular sprue, and gates designed to ensure smooth metal flow and adequate feeding. illustrates the initial pouring system design.
3.2 Numerical Simulation using ProCAST
ProCAST software was employed to simulate the filling and solidification processes of the turbine guide casting. The simulation parameters were set based on the material properties of K438 and the casting conditions. The mesh generation involved dividing the casting and mold into approximately 100,000 triangular surface elements and 700,000 tetrahedral volume elements.
Table 2: Simulation Parameters for K438 Alloy Investment Casting
| Parameter | Value |
|---|---|
| Preheat Temperature | 1050 °C |
| Pouring Temperature | 1480 °C |
| Pouring Time | 5 s |
| Shell-Casting Heat Transfer Coefficient | 800 W/m²·K |
| Shell-Air Heat Transfer Coefficient | 10 W/m²·K |
3.3 Defect Analysis and Optimization
The initial simulation results revealed porosity and shrinkage defects, primarily in the top regions of the inner and outer rings. These defects were attributed to premature solidification of the sprue-gate interface, resulting in inadequate feeding of the top regions. To mitigate these defects, the following optimizations were implemented:
- Incorporation of Insulation Wool: The addition of insulation wool around the sprue and gates reduced heat loss, delaying solidification.
- Design of Feeding Risers: Feeding risers were positioned at strategic locations to enhance feeding of the top regions.
4. Simulation Results and Discussion
4.1 Initial Pouring System Results
The initial pouring system simulation revealed significant porosity and shrinkage defects in the top regions of the turbine guide. the solidification sequence and porosity distribution in the initial design.
4.2 Optimized Pouring System Results
After implementing the optimizations, the simulation results demonstrated a marked improvement in casting quality. The solidification sequence began at the blade tips, progressing towards the thicker regions, ensuring adequate feeding throughout the casting process. depicts the optimized solidification sequence and porosity distribution.
4.3 Temperature Field Analysis
The temperature field analysis revealed a smooth temperature gradient throughout the casting process, with the highest temperatures observed in the thicker regions. The optimized pouring system maintained an adequate temperature difference between the mold and the casting, ensuring uniform solidification and minimal defects.
5. Experimental Validation
5.1 Casting Process
The optimized pouring system was implemented in a vacuum investment casting process. The casting parameters, including pouring temperature, mold preheat temperature, and vacuum level, were strictly controlled to ensure repeatability and consistency.
5.2 Non-Destructive Testing
X-ray computed tomography (CT) scanning was used to validate the casting quality. The CT scans revealed no visible porosity or shrinkage defects, confirming the efficacy of the optimized pouring system. presents a representative CT scan slice of the turbine guide casting.
6. Conclusion
This comprehensive study on the turbine casting process has successfully demonstrated the potential of numerical simulation in optimizing casting designs and mitigating defects. The optimized pouring system, incorporating insulation wool and feeding risers, resulted in a significant reduction in porosity and shrinkage defects. The experimental validation through non-destructive testing further confirmed the effectiveness of the optimized design.
6.1 Key Findings
- Structural and Material Analysis: The turbine guide’s complex structure and thin walls necessitated a careful pouring system design.
- Numerical Simulation: ProCAST simulations predicted porosity and shrinkage defects in the initial design, guiding the optimization process.
- Optimization Strategies: The integration of insulation wool and feeding risers effectively addressed the initial defects.
- Experimental Validation: Non-destructive testing using CT scanning validated the defect-free nature of the optimized castings.
6.2 Future Work
Future studies could focus on:
- Investigating the effects of different pouring temperatures and mold preheat temperatures on casting quality.
- Exploring the influence of gate and riser geometries on feeding efficiency and casting defects.
- Extending the optimization framework to other complex turbine components.
References
- Fan, Z., Li, Y., & Zhang, M. (2019). Numerical simulation of investment casting process based on ProCAST. Journal of Materials Processing Technology, 271, 34-42.
- Liu, J., & Wang, L. (2018). Optimization of the pouring system for investment casting of complex thin-walled parts based on numerical simulation. Journal of Materials Engineering and Performance, 27(4), 1667-1676.
- Zhang, L., Chen, G., & Wang, X. (2020). Study on the formation mechanism of defects in investment casting of a turbine blade and its control measures. Materials, 13(16), 3626.
- Sun, W., Li, X., & Wang, H. (2017). Effects of vacuum level on the microstructure and mechanical properties of investment castings. Materials Science and Engineering: A, 689, 421-429.
- He, B., & Liu, S. (2016). Dimensional prediction and control in investment casting processes. Journal of Materials Processing Technology, 238, 3