1. Introduction
Investment casting, also known as lost-wax casting, is a precision casting process that has been widely used for manufacturing complex and high-precision components. It offers excellent dimensional accuracy and surface finish, making it suitable for a variety of applications, especially in the aerospace, automotive, and medical industries. Additive manufacturing, on the other hand, has emerged as a revolutionary technology in recent years, enabling the rapid production of complex geometries with reduced lead times and costs. The combination of investment casting and additive manufacturing has the potential to further enhance the capabilities of both processes and open up new opportunities for the production of high-quality components. In this study, we focus on the application of additive manufacturing in the investment casting of impellers, which are critical components in many fluid-handling systems.
1.1 Background of Investment Casting
Investment casting has a long history dating back to ancient times. The process involves creating a wax pattern of the desired component, which is then coated with a ceramic shell. The wax is melted out, leaving a cavity in the shell, and molten metal is poured into the cavity to form the final casting. The key advantages of investment casting include its ability to produce complex shapes with high precision, excellent surface finish, and the ability to use a wide range of materials. However, the traditional investment casting process can be time-consuming and costly, especially for small batch production or complex geometries.
1.2 Additive Manufacturing and Its Potential in Investment Casting
Additive manufacturing, or 3D printing, is a process of building parts layer by layer from a digital model. It offers several advantages over traditional manufacturing methods, such as the ability to produce complex geometries without the need for specialized tooling, reduced lead times, and lower costs for small batch production. In the context of investment casting, additive manufacturing can be used to produce the wax patterns directly, eliminating the need for traditional pattern-making processes. This can significantly reduce the production time and cost, while also enabling greater design flexibility.
1.3 Significance of Studying Impeller Casting
Impellers are crucial components in pumps, turbines, and other fluid-handling systems. They are typically designed with complex geometries to optimize fluid flow and performance. The quality of the impeller casting directly affects the efficiency and reliability of the entire system. Therefore, it is essential to develop efficient and reliable casting processes for impellers. By combining additive manufacturing and investment casting, we aim to explore a new approach for producing high-quality impeller castings with improved performance and reduced costs.
2. Impeller Structure and Design Considerations
2.1 Geometric Features of the Impeller
The impeller used in this study has a specific geometric design. The average thickness of the blades is 2.0 mm, and the impeller has a three-dimensional structure with a certain degree of complexity. The blades have a twisted shape, which poses challenges for the casting process as it can lead to unstable filling of the molten metal and potential defects such as shrinkage porosity and cavities.
2.2 Design Requirements for Investment Casting
In order to produce a high-quality impeller casting using investment casting, several design requirements need to be considered. These include the design of the pouring system, the selection of appropriate materials for the wax pattern, the ceramic shell, and the casting alloy, as well as the determination of the casting parameters such as pouring temperature, shell preheating temperature, and pouring speed.
3. Pouring System Design and Simulation
3.1 Design of Different Pouring Systems
Three different pouring systems were designed for the impeller casting: top injection, side injection, and bottom injection. The top injection system pours the molten metal from the top of the impeller, the side injection system injects the metal from the side, and the bottom injection system feeds the metal from the bottom. Each pouring system has its own advantages and disadvantages, and the choice of the pouring system can significantly affect the filling process and the quality of the casting.
| Pouring System | Advantages | Disadvantages |
|---|---|---|
| Top Injection | Simple design, good for thin-walled parts | May cause defects at the junction with the riser |
| Side Injection | Can provide a more stable filling process for some geometries | May have gas entrapment and shrinkage issues |
| Bottom Injection | Good for parts with a large base area | Slow metal flow, potential for gas entrapment |
3.2 Simulation of the Filling Process Using AnyCasting
The filling process of each pouring system was simulated using AnyCasting software. The software allows us to predict potential defects such as air entrainment, shrinkage porosity, and cavities during the casting process. Before the simulation, the three-dimensional model of the impeller was meshed using the AnyPRE module. For example, in the case of the top injection pouring system, the mesh number was 998,004. The simulation was set to analyze the filling process and the solidification process after filling. The results of the simulation were then analyzed to evaluate the performance of each pouring system.
3.3 Analysis of Simulation Results
3.3.1 Top Injection Pouring System
The solidification sequence of the top injection pouring system is from top to bottom and from outside to inside. The blades of the impeller, being thinner, cool first, while the main body of the impeller cools last. Through the analysis of combined defect parameters, it was found that there may be a small number of defects at the junction with the riser. The analysis of probability defect parameters for shrinkage porosity and cavities showed that the defects were very small and located on the wall of the central hole of the casting, indicating a high integrity of the impeller.
3.3.2 Side Injection Pouring System
The solidification sequence of the side injection pouring system is also from top to bottom and from outside to inside. However, the solidification trend is not uniform. The blades cool first due to their thinness, and the main body of the impeller cools last. From the simulation results, it was observed that there were shrinkage porosity and cavity defects near the riser and in the middle of the casting. The possible reason is that the solidification time is fast during side injection, and the large number of blade cavities and their twisted shape prevent the gas from being expelled through the riser in a timely manner, resulting in defects in the impeller casting.
3.3.2 Bottom Injection Pouring System
The overall solidification sequence of the bottom injection pouring system is from top to bottom and from outside to inside. The bottom injection system has a problem with poor heat dissipation of the bottom runner, resulting in a slow melt flow velocity. With a large number of blade cavities, the gas cannot be expelled from the cavity in a timely manner, causing casting defects. There are defects near the junction of the pouring system riser and the casting, as well as shrinkage porosity and cavities in the middle of the hole wall and on the plane.
Based on the analysis of the simulation results, the top injection pouring system was selected as the optimal pouring scheme for the experiment. Three risers were placed on the upper surface of the impeller to ensure that the gas can be expelled smoothly and effective shrinkage compensation can be achieved.
4. Experimental Procedure
4.1 Preparation of the Wax Pattern Using Additive Manufacturing
The wax pattern of the impeller was produced using additive manufacturing technology. In this study, the 3D printing material used was polylactic acid (PLA). The 3D printing process allows for the rapid production of the wax pattern with high precision and complex geometries, significantly reducing the production time compared to traditional wax pattern-making methods.
4.2 Preparation of the Ceramic Shell
The ceramic shell was prepared using a specific recipe. The plaster and water were mixed in a ratio of 100:45, and the mixture was stirred thoroughly to allow the bubbles in the plaster slurry to be expelled. The surface of the impeller wax pattern was coated with the plaster slurry, and the blades were also filled with the slurry. The plaster slurry was then poured into a steel crucible for bottoming, and the surface-dried impeller model was placed in the crucible to make full contact with the bottoming plaster. More plaster was poured into the crucible to ensure that the wax pattern was centered in the plaster. When the plaster slurry was about to cover the pouring and riser openings, the pouring of plaster was stopped.
4.3 Melting Out of the Wax and Pouring of the Casting Alloy
The prepared plaster mold was dried for 2 hours and then placed in a resistance furnace and heated to 600 °C for solidification. After solidification, the PLA wax pattern was vaporized, and the molten ZL104 alloy (pouring temperature of 750 °C) was poured into the mold. The mold was then heated to 750 °C for 1 hour of insulation, followed by another 2 hours of insulation at 750 °C, and finally cooled to 650 °C.
4.4 Removal of the Casting and Inspection
After the casting process was completed, the casting was removed from the steel crucible. The pouring and riser openings were removed, and the casting was inspected for quality. The inspection results showed that the casting had a complete filling, clear contours, and met the dimensional accuracy requirements. There were no obvious casting defects such as shrinkage cavities and porosity, indicating a successful casting process.
5. Results and Discussion
5.1 Quality of the Casting
The final impeller casting produced using the combination of additive manufacturing and investment casting exhibited excellent quality. The casting had a complete filling, clear contours, and good surface quality. There were no obvious casting defects such as shrinkage cavities and porosity, indicating that the selected casting process and parameters were appropriate.
5.2 Comparison of Different Pouring Systems
The simulation and experimental results demonstrated the differences between the three pouring systems. The top injection pouring system showed relatively fewer defects compared to the side injection and bottom injection systems. The side injection system had issues with gas entrapment and shrinkage porosity and cavities, while the bottom injection system had problems with slow metal flow and gas entrapment. These results emphasize the importance of selecting the appropriate pouring system based on the geometry of the component and the requirements of the casting process.
5.3 Advantages of Additive Manufacturing in Investment Casting
The application of additive manufacturing in investment casting offered several advantages. Firstly, it significantly reduced the production time of the wax pattern, enabling a faster turnaround for the casting process. Secondly, it allowed for greater design flexibility, as complex geometries could be easily produced. Thirdly, it reduced the cost for small batch production, as there was no need for expensive tooling for pattern-making.
6. Conclusion
6.1 Summary of the Research
In this study, we investigated the application of additive manufacturing in the investment casting of impellers. We designed three different pouring systems and simulated their filling processes using AnyCasting software. Based on the simulation results, the top injection pouring system was selected as the optimal pouring scheme. We then produced the impeller wax pattern using additive manufacturing and carried out the investment casting process. The final casting exhibited excellent quality with no obvious defects.
6.2 Contributions to the Field
This research contributes to the field of investment casting and additive manufacturing in several ways. Firstly, it provides a practical example of the combination of the two technologies for the production of complex components such as impellers. Secondly, it demonstrates the effectiveness of using simulation software to optimize the casting process and select the appropriate pouring system. Thirdly, it highlights the advantages of additive manufacturing in reducing production time, increasing design flexibility, and reducing costs.
6.3 Future Research Directions
Future research could focus on several aspects. Firstly, further optimization of the additive manufacturing process for the production of wax patterns could be explored, such as improving the resolution and surface quality of the printed patterns. Secondly, the study could be extended to other materials and geometries to investigate the generalizability of the proposed approach. Thirdly, the integration of other advanced manufacturing technologies with investment casting and additive manufacturing could be investigated to further enhance the capabilities of the production process.