This article focuses on the process simulation design and verification of large-size turbine castings. It begins with an introduction to the importance and challenges of turbine casting, followed by a detailed analysis of the turbine casting process, including structure characteristics, material selection, and process design considerations. The use of ProCAST software for simulation optimization is described, along with the results of the simulations for filling and solidification. Production verification steps and results are presented, demonstrating the effectiveness of the designed process. The article concludes with a summary of the key findings and their implications for the manufacturing of large-size turbine castings.
1. Introduction
Turbines are crucial components in many engineering applications, especially in the field of power generation and automotive industries. The casting process of turbine components is a complex task that requires precise control of various parameters to ensure the quality and performance of the final product. In recent years, with the increasing demand for larger and more efficient turbines, the development of advanced casting techniques and process optimization has become essential.
The large-size turbine castings considered in this study have unique challenges due to their complex geometry, high performance requirements, and the use of special materials. The aim of this research is to develop an optimized casting process through simulation and verification to produce high-quality turbine castings that meet the strict standards of the industry.
2. Turbine Casting Process Overview
2.1 Structure Characteristics of Turbine Castings
The large-size turbine under study has a contour size of . It consists of a trapezoidal thick hub with dimensions of and 12 free-curved surface blades with a length of wound around the hub surface. The blades are radially distributed and are long and thin. The main structure-related challenges are as follows:
| Challenge | Description |
|---|---|
| Complex Geometry | The combination of a thick hub and thin, long blades with free-curved surfaces makes the casting geometry complex. |
| Size Requirements | Larger size compared to traditional turbines, with specific requirements for blade stability and overall dimensional accuracy. |
| Thin Sections | The thinnest part of the blade is only at the outer circle, which poses difficulties in the casting process. |
2.2 Material Requirements
The turbine is made of a high-temperature alloy K418B, which is a nickel-based precipitation-hardened equiaxed crystal casting high-temperature alloy. This alloy contains a significant amount of easily oxidized elements such as Cr, Al, and Ti. During operation, the turbine has to withstand high centrifugal forces, vibration loads, and an exhaust gas temperature of up to . Therefore, the alloy must have high strength, impact, fatigue, and endurance properties, and the casting must be free from defects such as pores, cracks, shrinkage cavities, and porosity.
2.3 Casting Process Selection
Considering the material and purity requirements of the high-temperature alloy, the following casting process steps are adopted:
- Melting: Vacuum induction furnace is used to melt the master alloy rod, followed by vacuum remelting for part pouring. This helps to reduce the oxidation of alloy elements, as well as the formation of pores and inclusions, thereby improving the purity of the alloy and ensuring the performance of the casting.
- Pouring Scheme: After analyzing the structure of the casting, it is determined that there is only one geometric hot spot with a hot spot circle diameter of located at the center of the trapezoidal hub. A feeding riser is required due to the large thickness and weight proportion of this area. Turbine castings usually adopt a center riser top-pouring scheme. The riser can be set at either the friction welding end or the non-friction welding end of the casting. Through simulation optimization using ProCAST software, it is found that the scheme with the friction welding end facing up is more favorable for achieving sequential solidification and avoiding defects.
3. Process Simulation using ProCAST Software
3.1 Simulation of Filling Process
For the final process scheme, a filling and solidification coupling simulation analysis is carried out using ProCAST software. To ensure the filling of the blades, based on the liquidus temperature () and solidus temperature () of the K418B alloy, the filling temperature is set to , the choke section is set to , and the total filling time is 13s.
| Filling Parameter | Value |
|---|---|
| Filling Temperature | |
| Choke Section | |
| Total Filling Time | 13s |
During the filling process, at 4s (30% filling), the blade part is filled completely, and the remaining is the filling of the riser part. The pouring is stable during filling, without any splashing, entrained air, or insufficient pouring phenomena. As the filling nears completion, the blade tip starts to solidify, indicating that the set pouring temperature is appropriate and the turbine casting can be filled completely.
3.2 Simulation of Solidification Process
The solidification analysis shows that the blade tip of the turbine has a thin wall thickness and starts to solidify after pouring. The time required for the blade part to completely solidify is 580s, for the turbine casting body to completely solidify is 2965s, and for the casting riser to completely solidify is 5865s. During the solidification process, the temperature distribution is uneven, with a large temperature difference. The surface temperature is low, while the core and thick parts have higher temperatures, and the riser part has the highest temperature. The temperature gradient increases in the radial direction from the blade tip to the blade root and then to the hub, and in the axial direction from the bottom to the riser. The solidification sequence is that in the radial direction, the blade tip solidifies first, followed by the blade root and then the hub; in the axial direction, it solidifies from the bottom to the riser, and the last part to solidify is the center of the riser. During the solidification process, there is no isolated liquid phase area, which fully realizes the sequential solidification of the casting and ensures that there are no shrinkage cavities and porosity defects in the casting.
| Solidification Parameter | Value |
|---|---|
| Blade Solidification Time | 580s |
| Casting Body Solidification Time | 2965s |
| Riser Solidification Time | 5865s |
4. Production Verification
4.1 Casting Process Steps
After completing the mold making and pouring system design, the trial production is carried out according to the conventional investment casting process, including mold making, shell making, shell baking, melting and pouring, and cleaning. During the pouring process, the centrifugal rotation of the casting mold can be started as soon as the pouring begins (the rotation speed can be controlled at about ). When pouring to 1/3 of the riser, the rotation is stopped to achieve the centrifugal filling of the blades. After pouring is completed, the casting is left to stand for . After the blades have initially solidified, the casting mold is rotated forward and backward according to the set parameters to stir the molten metal, and the total stirring time is not less than .
4.2 Inspection and Results
Two furnace runs of turbine blanks are produced according to the above casting process, and the casting quality is stable. The following inspections and results are obtained:
- Chemical Composition and Mechanical Properties: All elements are within the required range. Samples taken from the connection part between the riser and the turbine body show an average grain size of , and the mechanical properties of the samples meet the requirements.
- Non-destructive Testing: Through X-ray and ultrasonic non-destructive testing, it is found that there are no pores, inclusions, shrinkage cavities, and porosity defects in the turbine blades and inside. The surface is inspected by penetration testing, and there are no cracks and excessive inclusions.
- Dimensional Detection: The digital three-dimensional scanning detector is used to scan the digital point cloud data of the turbine and blades, and then compared with the required three-dimensional model of the turbine casting blank. At the same time, the key dimensions such as the thickness of the blade tip and the surface profile are measured and compared. The results show that the size of the cast turbine meets the requirements.
5. Conclusions
- The ProCAST software is effectively used to optimize the process design of large-size turbine castings, enabling the production of qualified products.
- For large-size turbine castings, the casting process design with the friction welding end facing up is a viable option, but it requires the application of relevant fine-grained casting process measures.
- To ensure the dimensional accuracy of the casting, different shrinkage scales in the axial and radial directions need to be adopted for the casting comprehensive shrinkage rate.
In summary, this study provides a comprehensive approach to the simulation design and verification of the casting process for large-size turbine castings, which can serve as a reference for the manufacturing industry to improve the quality and performance of turbine castings.
6. Detailed Analysis of Process Optimization
6.1 Optimization of Pouring Scheme
The choice of pouring scheme is crucial for the quality of turbine castings. In the initial analysis, two schemes were considered. Scheme one had certain drawbacks as it led to the formation of isolated hot spots and shrinkage cavities in the turbine casting core, as demonstrated by the ProCAST simulation. Scheme two, with the friction welding end facing up, was more favorable. Although the friction welding end was relatively small and required adjustments to the outer diameter to meet the riser neck size requirements (this being a machined part), it achieved sequential solidification. This ensured that during the solidification process, the casting solidified before the riser neck, and the riser neck solidified before the riser, effectively eliminating shrinkage cavities and porosity defects.
6.2 Determination of Shrinkage Rate
The determination of the casting comprehensive shrinkage rate is essential for ensuring the dimensional accuracy of the turbine casting. For this large-size turbine casting, considering the significant differences in wall thickness and previous production experience of similar parts, an axial shrinkage rate of 3.0% and a radial shrinkage rate of 2.7% were selected. Additionally, a cold wax block with a size smaller than the turbine center hub by 5 – 7mm on one side was prefabricated during mold making and fixed in the mold. This measure further enhanced the accuracy of the wax model size and, consequently, the dimensional accuracy of the final casting.
6.3 Fine-Grained Casting Process
Due to the large size of the turbine casting and the thick hub and riser, the solidification time is long. Using conventional refining agents often results in a decline in effectiveness and fails to achieve sufficient grain refinement in the turbine core. To address this issue, a positive-negative centrifugal stirring fine-grained process was adopted. The parameters of this process were determined through a combination of literature review and experimental verification. The stirring speed was set at 150r/min, the positive and negative stirring time was 40s, and the positive and negative rotation conversion time was less than 5s. This process effectively refined the grains in the turbine hub, ensuring that the grain size met the strict requirements of the casting.
7. Importance of Simulation in Turbine Casting
7.1 Predicting Defects
Simulation using software like ProCAST allows for the prediction of potential defects in the turbine casting process. By accurately modeling the filling and solidification processes, it is possible to identify areas where shrinkage cavities, porosity, or other defects may occur. For example, in the initial simulation of scheme one, the presence of isolated hot spots and subsequent shrinkage cavities was clearly identified. This enables the casting engineer to make necessary adjustments to the process design before actual production, saving time and resources.
7.2 Optimizing Process Parameters
The simulation also helps in optimizing various process parameters. The determination of the filling temperature, choke section, and total filling time is based on the simulation results to ensure proper filling of the blades. Similarly, the analysis of the solidification process provides insights into the temperature distribution and solidification sequence, which in turn helps in setting the appropriate riser size and location. The optimization of these parameters is crucial for achieving a high-quality turbine casting with no defects and accurate dimensions.
7.3 Cost and Time Savings
By predicting defects and optimizing process parameters in advance, simulation significantly reduces the cost and time associated with the casting process. Without simulation, multiple trial and error attempts may be required in actual production to achieve a satisfactory result. This not only consumes a large amount of raw materials and energy but also prolongs the production cycle. Simulation allows for a more efficient and targeted approach to process design, reducing the need for repeated trials and minimizing waste.
8. Future Trends in Turbine Casting Technology
8.1 Advanced Simulation Techniques
As computer technology continues to advance, more sophisticated simulation techniques are expected to emerge in the field of turbine casting. These techniques may include more detailed models of material behavior during solidification, such as the consideration of microstructural evolution and phase transformations. This will provide a more accurate prediction of the casting quality and enable further optimization of the casting process.
8.2 New Materials and Alloys
The development of new materials and alloys is another trend in turbine casting technology. With increasing demands for higher performance and efficiency, researchers are exploring new materials that can withstand higher temperatures, pressures, and mechanical stresses. These new materials may require different casting techniques and process optimizations, presenting both challenges and opportunities for the casting industry.
8.3 Additive Manufacturing in Turbine Casting
Additive manufacturing, or 3D printing, is gradually being explored in the context of turbine casting. Although currently in its early stages, it has the potential to revolutionize the way turbine components are manufactured. Additive manufacturing allows for more complex geometries to be produced with greater design freedom, potentially reducing the need for traditional casting molds and associated processes. However, there are still many technical challenges to be overcome, such as ensuring the quality and mechanical properties of the printed components.
9. Challenges and Solutions in Turbine Casting
9.1 Challenges in Casting Complex Geometries
The complex geometries of turbine castings, such as the combination of thick hubs and thin, long blades with free-curved surfaces, pose significant challenges in the casting process. Achieving uniform filling and solidification across such complex shapes is difficult. The thin sections of the blades, especially at the outer circle, are prone to incomplete filling or premature solidification, leading to defects. Additionally, the presence of geometric hot spots in the hub requires careful consideration of the feeding system to ensure proper solidification and avoid shrinkage cavities and porosity.
9.2 Solutions for Complex Geometries
To address the challenges of casting complex geometries, several solutions have been implemented. The use of centrifugal force during pouring, as described in the production process, helps in achieving better filling of the blades. The optimization of the pouring scheme and the determination of appropriate riser sizes and locations based on simulation results also contribute to more uniform solidification. The fine-grained casting process helps in improving the mechanical properties of the casting, especially in areas with thick sections where grain coarsening may occur.
9.3 Challenges in Meeting Material Requirements
The high-temperature alloy K418B used in turbine castings has strict material requirements. The presence of easily oxidized elements requires careful control of the melting and pouring processes to avoid oxidation and ensure the purity of the alloy. The high-performance requirements of the turbine, such as withstanding high centrifugal forces, vibration loads, and high temperatures, necessitate that the casting has excellent mechanical properties and is free from defects.
9.4 Solutions for Meeting Material Requirements
To meet the material requirements, a vacuum induction furnace is used for melting and a vacuum remelting process is employed for pouring. This helps in reducing the oxidation of alloy elements and improving the purity of the alloy. The fine-grained casting process and the optimization of the casting process through simulation also contribute to ensuring that the casting has the required mechanical properties and is free from defects.
10. Conclusion
In conclusion, the casting process of large-size turbine castings is a complex but essential task in the manufacturing of high-quality turbines. Through a comprehensive approach that includes an understanding of the structure characteristics and material requirements of the turbine, the use of ProCAST software for process simulation and optimization, and production verification, it is possible to produce turbine castings that meet the strict requirements of the industry. The optimization of the pouring scheme, determination of shrinkage rates, and implementation of fine-grained casting processes are all crucial steps in achieving high-quality castings. The importance of simulation in predicting defects, optimizing process parameters, and saving costs and time cannot be overstated. Looking ahead, future trends in turbine casting technology, such as advanced simulation techniques, new materials and alloys, and additive manufacturing, offer both challenges and opportunities for the casting industry. By addressing the challenges in casting complex geometries and meeting material requirements, the casting industry can continue to improve the quality and performance of turbine castings and contribute to the development of more efficient and reliable turbines.
