Abstract: This article focuses on the research of the investment casting process of impellers using additive manufacturing technology. It includes the design of pouring systems, numerical simulation analysis, and the actual casting process. The combination of additive manufacturing and investment casting shows great potential in producing high-quality impeller castings.
1. Introduction
Investment casting is a special casting method widely used in manufacturing precise and complex parts, especially in the fields of aviation and industrial gas turbines. It involves several steps, starting from creating a wax pattern, which determines the dimensional accuracy of the final casting. The wax pattern is then coated with a ceramic slurry and hardened. After melting the wax, a mold is obtained, which needs to be dried and preheated before pouring the molten metal. Finally, after the metal solidifies, the mold is broken, the pouring system is cut, and the final casting is obtained through grinding.
Additive manufacturing technology has emerged as a revolutionary approach in recent years. By combining it with the investment casting process, 3D printed models can replace traditional wax patterns as investment molds. This integration can significantly shorten the part development cycle and reduce production costs. In this study, we aim to explore the application of additive manufacturing in the investment casting of impellers and optimize the casting process through numerical simulation and practical experiments.
2. Impeller Structure and Materials
2.1 Impeller Structure
The impeller has a bottom diameter of 104 mm, a top diameter of 25 mm, and a side width of 49 mm. The blade wall thickness is determined to be 2.5 mm based on the workpiece size and relevant standards. The complex shape and curved blades of the impeller result in a complicated mold cavity, which can affect the pouring process of the molten metal and the quality and precision of the casting.
2.2 Materials
- 3D Printing Material: Polyactic acid (PLA) is used for 3D printing the impeller model.
- Pouring Metal: ZL104 aluminum alloy is chosen as the pouring metal with a pouring temperature of 750 °C.
- Mold Shell Material: A high-temperature-resistant gypsum with good fluidity, low thermal conductivity, and high replication accuracy is used as the mold shell material.
3. Pouring System Design and Simulation
3.1 Pouring System Scheme Design
Three pouring system schemes are designed according to the impeller structure, namely top-casting, side-casting, and bottom-casting systems, as shown in Figure 4.
3.2 Simulation Analysis
- Top-casting System: The mesh of the top-casting system is generated, and the simulation process shows that during the filling process, the molten aluminum alloy enters the bottom of the cavity from the main runner. Under the action of gravity, the flow rate increases, resulting in turbulence and unstable filling. This may cause defects in the casting. Additionally, the gas in the cavity may not be completely discharged as the molten metal fills from top to bottom, leading to shrinkage porosity at the bottom of the casting, as shown in Figure 6.
- Side-casting System: The simulation of the side-casting system indicates that the filling process is stable, and the solidification occurs in an upward direction. However, the combination defects appear at the junction of the runner and the casting, and shrinkage porosity may occur on the runner and blades. This may be due to the rapid filling that prevents the air in the cavity from being completely exhausted, as shown in Figure 7.
- Bottom-casting System: The simulation of the bottom-casting system shows that the solidification process satisfies the sequential solidification principle. There are no combination defects on the casting surface, and the shrinkage porosity defects are concentrated at the corners of the runner, while there are almost no defects on the blade surface, as shown in Figure 8.
The simulation results are summarized in Table 1.
| Pouring System | Filling Stability | Solidification Order | Defect Location | Main Defects |
|---|---|---|---|---|
| Top-casting | Unstable | Bottom-up | Bottom of casting | Turbulence, shrinkage porosity |
| Side-casting | Stable | Bottom-up | Junction of runner and casting | Shrinkage porosity on runner and blades |
| Bottom-casting | Stable | Sequential solidification | Corners of runner | Shrinkage porosity at runner corners |
Based on the simulation analysis, the bottom-casting system is selected as the optimal pouring system for the experiment.
4. Investment Casting Experiment
4.1 Mold Preparation
The bottom-casting system impeller mold is printed using an FDM 3D printer. The gypsum slurry is prepared according to a ratio of 100:45 (gypsum to water) and stirred thoroughly. The impeller mold is coated with the gypsum slurry, ensuring that the blades are also filled with gypsum. The gypsum slurry is then poured into a steel crucible for bottoming, and the surface-dried impeller model is placed in the crucible to ensure it is centered in the gypsum. When the gypsum slurry is about to cover the pouring riser, the pouring of gypsum is stopped.
4.2 Casting Process
The steel crucible with the wrapped impeller mold is placed in a test resistance furnace. The furnace temperature is raised to 400 °C and held for 15 minutes, then to 600 °C and held for 30 minutes, and finally to 750 °C and held for 2 hours to ensure the complete vaporization of the PLA material. After the furnace cools to 600 °C, the furnace is opened. The aluminum alloy is melted, and the molten aluminum is poured into the crucible with the impeller mold through the pouring riser. When the molten metal overflows from the pouring riser, the pouring is completed.
4.3 Post-treatment
After the steel crucible cools, the gypsum in the crucible is broken and removed, and the casting is taken out. The surface gypsum and pouring riser are removed to obtain the impeller casting. The impeller casting surface has a layer of oxide scale but no welding marks, pores, or shrinkage porosity defects. The casting is then polished with sandpaper to obtain a smooth surface.
5. Results and Discussion
5.1 Casting Quality
The obtained impeller casting shows good quality with no obvious defects such as welding marks, pores, and shrinkage porosity. This indicates that the combination of additive manufacturing and investment casting can produce high-quality impeller castings.
5.2 Numerical Simulation Verification
The numerical simulation results accurately predict the defects and solidification behavior of the investment casting process. This verifies the effectiveness of using numerical simulation software in optimizing the pouring system and predicting casting defects.
5.3 Significance of the Study
This study solves the problem of casting defects caused by the cumbersome manufacturing process of impeller molds. It provides a practical method for the efficient production of impeller castings, which has important implications for the manufacturing industry.
6. Conclusion
In this study, the investment casting process of impellers based on additive manufacturing was investigated. The following conclusions can be drawn:
- The impeller structure was analyzed, and appropriate materials were selected for 3D printing, pouring metal, and mold shell.
- Three pouring system schemes were designed and simulated. The bottom-casting system was determined to be the optimal scheme based on the simulation results.
- The investment casting experiment was successfully carried out using the 3D printed impeller mold and the optimized pouring system. The obtained impeller casting had good quality.
- The numerical simulation software effectively predicted the casting defects and solidification behavior, verifying the importance of numerical simulation in the casting process.
- The combination of additive manufacturing and investment casting effectively solved the problem of casting defects caused by the cumbersome impeller mold manufacturing process, providing a valuable reference for the manufacturing industry.
In the future, further research can be carried out to explore the application of other additive manufacturing technologies and optimize the investment casting process for different types of impellers and other complex parts. This will contribute to the continuous development and improvement of the manufacturing industry.