1. Introduction
The turbine, a crucial high – speed rotating component on superchargers, features a complex curved surface structure. With the increasing demand for high – performance superchargers, especially in the automotive and aerospace industries, the quality requirements for large – size turbine castings have become more stringent. This article delves into the entire process of large – size turbine casting production, from understanding the structure and material requirements to optimizing the casting process through simulation, and finally validating the production process.
2. Structure and Material Characteristics of Large – Size Turbine Castings
2.1 Structural Features
The large – size turbine under study has a contour size of φ400 mm×240 mm. It consists of a trapezoidal thick – walled hub with dimensions of φ220 mm/φ120 mm×202 mm and 12 free – form curved blades. These blades, with a length of 118 mm, are radially distributed around the hub. The long and thin nature of the blades, combined with the large size of the overall turbine, poses significant challenges in the casting process. Table 1 summarizes the key structural parameters of the turbine.
| Structural Component | Dimensions |
|---|---|
| Overall Contour Size | φ400 mm×240 mm |
| Hub Dimensions | φ220 mm/φ120 mm×202 mm |
| Blade Length | 118 mm |
| Number of Blades | 12 |
Figure 1: Schematic Diagram of Large – Size Turbine Structure
2.2 Material Requirements
The turbine is made of the high – temperature alloy K418B, a nickel – based precipitation – hardening equiaxed – grain casting superalloy. This alloy contains a large amount of easily oxidizable elements such as Cr, Al, and Ti. During operation, the turbine has to withstand high centrifugal forces, vibration loads, and exhaust gas temperatures as high as 750℃. Therefore, it requires excellent strength, impact resistance, fatigue resistance, and creep – rupture properties. Any defects such as pores, cracks, shrinkage cavities, and porosity are not allowed in the casting. Table 2 lists the main properties and requirements of the K418B alloy.
| Property | Requirement |
|---|---|
| Oxidation – resistant Elements | High content of Cr, Al, Ti |
| Operating Temperature | Up to 750℃ |
| Mechanical Properties | High strength, impact, fatigue, and creep – rupture resistance |
| Defect Requirements | No pores, cracks, shrinkage cavities, or porosity |
3. Analysis of Turbine Casting Process
3.1 Melting and Pouring Process
To meet the material requirements and the purity of the casting, a two – step melting process is adopted. First, the master alloy rod is melted in a vacuum induction furnace. Then, the parts are poured by vacuum remelting. This process helps to reduce the oxidation of alloy elements, minimize pores and inclusions, and improve the purity of the alloy, thereby ensuring the performance of the casting.
3.2 Risering and Gating System Design
Analysis of the casting structure reveals that there is a single geometric hot spot with a hot – spot circle diameter of φ162 mm at the center of the trapezoidal hub. This thick – walled area accounts for approximately 70% of the turbine’s weight and requires a large amount of feeding. A riser must be set up for proper feeding.
Two main riser position design options are considered: placing the riser at the friction – welded end or the non – friction – welded end of the casting. Figure 2 shows these two options.
Figure 2: Design of the Feeder Head Position
In general, to ensure the welding quality between the turbine and the turbine shaft, the friction – welded end is usually placed at the bottom. Through simulation with ProCAST software, it is found that in Option 1, there is an isolated hot – spot area in the core of the turbine casting, resulting in shrinkage cavity defects (Figure 3).
Figure 3: Casting Solidification Analysis of Option 1
In Option 2, with the friction – welded end facing upwards, although the friction – welded end face is small and does not meet the required riser neck size, by artificially increasing the outer – circle diameter at this position to reach the riser neck size (this area is a machined part), the casting can achieve sequential solidification during the solidification process. Simulation results show that the casting solidifies before the riser neck, and the riser neck solidifies before the riser, effectively preventing shrinkage cavity and porosity defects (Figure 4).
Figure 4: Casting Solidification Analysis of Option 2
3.3 Selection of Casting Shrinkage Rate
For investment casting, factors affecting the casting size include alloy shrinkage, pattern material shrinkage, and shell expansion. Due to the large difference in the wall thickness of the turbine casting, based on the experience of producing similar parts, different shrinkage rates are selected for the axial and radial directions. The axial shrinkage rate is 3.0%, and the radial shrinkage rate is 2.7%. Additionally, a cold wax block, which is 5 – 7 mm smaller than the turbine center hub size on each side, is pre – fabricated and fixed in the mold to ensure the accuracy of the wax pattern size. Table 3 summarizes the key parameters of the casting shrinkage rate.
| Direction | Shrinkage Rate | Supplementary Measure |
|---|---|---|
| Axial | 3.0% | Pre – fabricate a cold wax block |
| Radial | 2.7% | Pre – fabricate a cold wax block |
4. Simulation Analysis of Turbine Casting Process
4.1 Filling and Solidification Coupled Simulation
ProCAST software is used to conduct a filling and solidification coupled simulation of the final casting process. Based on the liquidus temperature (1344.7℃) and solidus temperature (1235.7℃) of the K418B alloy, the filling temperature is set to 1410℃, the choke section is φ40 mm, and the total filling time is 13 s.
4.2 Filling Process Analysis
During the filling process, at 4 s (30% filling), the blade part is completely filled, and the subsequent filling is mainly for the riser part. The filling process is stable, without splashing, gas entrapment, or misruns. When the filling is nearly complete, solidification begins at the blade tips, indicating that the set pouring temperature is appropriate and the turbine casting can be completely filled. Table 4 shows the filling process details.
| Filling Time | Filling Progress | Phenomenon |
|---|---|---|
| 4 s | 30% filling, blade part filled | Stable filling, no splashing, gas entrapment, or misruns |
| 13 s | Complete filling | Solidification starts at blade tips |
Figure 5: Filling Process of the Turbine
4.3 Solidification Process Analysis
Analysis of the solidification process shows that the blade tips, with their thin wall thickness, start to solidify immediately after pouring. The complete solidification time for the blade part is 580 s, for the entire casting body is 2965 s, and for the casting riser is 5865 s.
During solidification, the temperature distribution is non – uniform, with a large temperature difference. The surface temperature is low, while the core and thick – walled parts have higher temperatures, and the riser 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 the blade tips solidify first, followed by the blade roots and then the hub in the radial direction, and from the bottom to the riser in the axial direction. The last part to solidify is the center of the riser. No isolated liquid phase area appears during solidification, ensuring sequential solidification and preventing shrinkage cavity and porosity defects. Table 5 summarizes the solidification process parameters.
| Component | Solidification Time | Temperature Distribution | Solidification Sequence |
|---|---|---|---|
| Blade Tips | Start solidifying immediately after pouring, complete solidification in 580 s | Surface temperature is low, temperature increases towards the core and thick – walled parts | Radially: blade tips → blade roots → hub; Axially: bottom → riser |
| Blade Part | 580 s | – | – |
| Casting Body | 2965 s | – | – |
| Casting Riser | 5865 s | The highest temperature in the riser | – |
Figure 6: Analysis of the Solidification Process of the Turbine Casting Part
Figure 7: Judgement of Shrinkage and Porosity in the Turbine Casting
5. Production Verification
5.1 Production Process
After completing the mold making and gating system design, the trial production is carried out according to the conventional investment casting process. This process includes steps such as pattern making, shell making, shell baking, melting and pouring, and cleaning.
During pouring, the casting mold starts to rotate centrifugally (the rotational speed can be controlled at around 100 r/min). When pouring reaches 1/3 of the riser, the rotation stops to achieve centrifugal filling of the blades. After pouring is completed, the mold is left stationary for 3 min. Once the blades are preliminarily solidified, the mold starts to rotate in the forward and reverse directions according to the set parameters to stir the molten metal. The total stirring time is not less than 45 min.
5.2 Quality Inspection Results
Two furnaces of turbine blanks, a total of 2 pieces, are produced using the above – mentioned casting process. The casting quality is stable. The inspection results are as follows:
- Chemical Composition and Mechanical Properties: All elements are within the required range. Sampling analysis at the connection between the riser and the turbine body shows that the average grain size reaches 4 mm, and the mechanical properties of the samples meet the requirements.
- Nondestructive Testing: Nondestructive testing by X – ray and ultrasonic methods shows that there are no pores, inclusions, shrinkage cavities, or porosity in the turbine blades and interior. Penetrant testing of the surface shows no cracks, inclusions, or other defects exceeding the standard.
- Dimensional Inspection: A digital 3D scanning detector is used to scan and obtain the digital point – cloud data of the turbine and blades. Comparison with the required 3D model of the turbine casting blank, as well as measurement and comparison of key dimensions such as the thickness of the blade tip and surface profile, show that the dimensions of the cast turbine meet the requirements. Table 6 summarizes the quality inspection results.
| Inspection Item | Result |
|---|---|
| Chemical Composition | All elements within the required range |
| Mechanical Properties | Average grain size 4 mm, meets requirements |
| Nondestructive Testing (X – ray, Ultrasonic) | No pores, inclusions, shrinkage cavities, or porosity |
| Nondestructive Testing (Penetrant) | No cracks or inclusions exceeding the standard |
| Dimensional Inspection | Dimensions meet the requirements |
Figure 8: Turbo Castings and its Cross – sections
6. Conclusion
- The use of ProCAST software for the process design and optimization of large – size turbine castings has enabled the production of qualified large – size turbine castings.
- For large – size turbine castings, the casting process design with the friction – welded end facing upwards can be adopted, but relevant fine – grain casting process measures are required.
- To ensure the dimensional accuracy of the casting, different shrinkage rates should be used for the axial and radial directions.
7. Future Perspectives
The continuous development of the supercharger industry calls for further improvement in the quality and performance of turbine castings. Future research can focus on exploring new materials and casting processes to further enhance the mechanical properties and dimensional accuracy of turbine castings. Additionally, the application of advanced simulation technologies, such as multi – physical – field coupling simulation, can provide more in – depth insights into the casting process and help optimize the process parameters more accurately.
