This paper focuses on the coated sand casting process of ZM6 magnesium alloy shell. By using orthogonal tests and numerical simulations, the study aims to optimize the process parameters, improve the quality of castings, and solve common problems such as porosity defects. The research results provide valuable references for the production of ZM6 magnesium alloy shell and similar components in the foundry industry.
1. Introduction
ZM6 magnesium alloy is widely used in various industries due to its excellent properties such as low density, high strength – to – weight ratio, and good castability. The ZM6 magnesium alloy shell, with its complex structure, has strict requirements for dimensional accuracy (CT8), surface roughness (less than Ra12.5), and internal quality (meeting HB7780 – 2005 Class I casting requirements). Sand casting is a common forming method for magnesium alloy shells, among which coated sand casting has become an important process for manufacturing complex magnesium alloy shells because of its advantages in producing high – precision and complex sand molds/cores. However, the existing coated sand casting process for magnesium alloy castings often leads to porosity defects, which seriously affect the quality and performance of the castings. Therefore, it is of great significance to optimize the coated sand casting process for ZM6 magnesium alloy shells.
2. Experimental Materials and Methods
2.1 Coated Sand Mold Preparation Process
- Selection of Coated Sand Raw Materials: Three types of coated sand raw materials were selected, namely ordinary silica sand coated sand, high – strength silica sand coated sand, and low – gas – evolution silica sand coated sand, named as I, II, and III respectively.
- Orthogonal Test Design: An orthogonal test was carried out to optimize the curing process of the coated sand. The factors and levels of the orthogonal test are shown in Table 1. The curing temperature was set at 210 °C, 220 °C, and 230 °C, and the curing time was set at 90 s, 120 s, and 150 s. A total of 9 groups of orthogonal tests were conducted.
| Level | Factor | | |
| — | — | — | — |
| | Curing Temperature /°C (A) | Curing Time /s (B) | Coated Sand Category (C) |
| 1 | 210 | 90 | Coated Sand I |
| 2 | 220 | 120 | Coated Sand II |
| 3 | 230 | 150 | Coated Sand III | - Sample Preparation and Testing: “8”-shaped samples were prepared using the RXH – II coated sand sample – making machine. The tensile strength, gas evolution, and permeability of the samples were tested by the JT – SJZ – 2 core sand detection workstation. Each condition was tested 3 times, and the average value was taken as the final result. The test scheme is shown in Table 2.
| Test Number | Curing Temperature /°C | Curing Time /s | Coated Sand Category |
| — | — | — | — |
| 1 | 210 | 90 | Coated Sand I |
| 2 | 210 | 120 | Coated Sand II |
| 3 | 210 | 150 | Coated Sand III |
| 4 | 220 | 90 | Coated Sand II |
| 5 | 220 | 120 | Coated Sand III |
| 6 | 220 | 150 | Coated Sand I |
| 7 | 230 | 90 | Coated Sand III |
| 8 | 230 | 120 | Coated Sand I |
| 9 | 230 | 150 | Coated Sand II |
2.2 Casting Process
- Process Design: First, a 3D model of the casting was established. The magnesium alloy shell is similar to a box – like structure with a small height dimension (about 120 mm) but a large difference in wall thickness. The maximum wall thickness of the top boss is 35 mm, and the main wall thickness of the thin – walled parts on both sides is only 10 mm. Then, a casting process model was designed. To avoid overheating in the area facing the gate, a slit – type gating system was adopted on the thin – walled side. To ensure sufficient feeding and bottom – up sequential solidification, conformal risers were set at the top, and chills were set in the thick – walled parts. The process design is shown in Figure 1.
- Numerical Simulation: The ProCAST software of the Shanghai Aerospace Precision Machinery Research Institute was used for casting numerical simulation. First, a finite – element model of gravity casting of the magnesium alloy shell was established. Then, the initial process parameters were set: the pouring temperature was 700 °C, and the pouring speed was 3.25 kg/s. Finally, the flow field and temperature field were solved to calculate the filling and solidification processes.
- Process Optimization: According to the results of numerical simulation, the causes and mechanisms of defects were analyzed. Then, the gating system, the positions and numbers of chills and risers were optimized. The ProCAST software was used again to simulate the optimized process model until there were no defects inside the casting.
- Pouring Verification: The coated sand mold was prepared by the best forming process. Pouring tests were carried out according to the results of the optimized casting process simulation, and the dimensions, roughness, and internal quality of the castings were detected.
3. Experimental Results
3.1 Results of Orthogonal Tests for Coated Sand Molds
- Influence of Process Parameters: The results of the orthogonal tests are shown in Table 3. Through range analysis, it can be seen that the influence degree of coated sand forming process parameters on tensile strength, gas evolution, and permeability is as follows: coated sand category (C)>curing temperature (A)>curing time (B). The main – effect diagrams of the forming process parameters of the coated sand mold shell on the average value of tensile strength, gas evolution, and permeability are shown in Figure 2, Figure 3, and Figure 4 respectively. It can be seen from the figures that the raw materials of coated sand have a significant impact on tensile strength, gas evolution, and permeability. Coated sand III has the lowest gas evolution and the best permeability. With the increase of curing temperature and the extension of curing time, the tensile strength of the coated sand first increases and then decreases, the gas evolution first decreases and then tends to be stable, and the permeability basically remains unchanged.
| Test Number | Tensile Strength /MPa | Gas Evolution /mL·g⁻¹ | Permeability |
| — | — | — | — |
| 1 | 1.25 | 14.60 | 243 |
| 2 | 2.17 | 16.76 | 254 |
| 3 | 1.60 | 12.00 | 251 |
| 4 | 2.46 | 16.27 | 252 |
| 5 | 1.74 | 11.17 | 255 |
| 6 | 1.83 | 13.50 | 247 |
| 7 | 1.73 | 11.44 | 257 |
| 8 | 1.79 | 13.52 | 244 |
| 9 | 2.01 | 14.78 | 250 |
- Determination of Optimal Process Parameters: According to the principle of process selection in actual production, under the premise of ensuring safe pouring, the product quality and production efficiency need to be improved. Generally, the room – temperature tensile strength of the coated sand mold should not be lower than 1.6 MPa, and it should have a low gas evolution and high permeability. To reduce porosity defects, coated sand III was finally selected, with a curing temperature of 220 °C and a curing time of 120 s. Under these conditions, the tensile strength of the coated sand mold is 1.74 MPa, the gas evolution is 11.17 mL/g, and the permeability is 255.
3.2 Results of Casting Process Simulation
- Initial Process Simulation: The filling process simulation results of the ZM6 magnesium alloy shell with the initial casting process are shown in Figure 5. The simulation results show that when the unilateral slit – type gating system is used for initial filling, the filling on both sides is inconsistent, and the molten metal is disordered. When the filling rate reaches 40% – 50%, as shown in the box in Figure 5(d), air entrainment occurs in the thin – walled part on the side far from the slit gate, and there are local areas that are not filled in time. The defect prediction results of the initial process are shown in Figure 6. The shrinkage porosity is concentrated in the gating system and risers, and the shrinkage porosity tendency inside the casting is small. The air entrainment is mainly distributed in the risers, but a large – area air – entrainment area appears in the thin – walled part on the side far from the slit gate, with a maximum air – entrainment amount of about \(0.00144 g/cm^{3}\).
Process Optimization and Simulation: To avoid air entrainment and ensure stable and consistent filling of the thin – walled parts on the side, the casting process was optimized. The optimized model is shown in Figure 7. The optimized process adopted a slit – type gating system structure with pouring from both sides. At the same time, the risers at the top of the thin – walled parts on the side were removed to facilitate the discharge of gas in the thin – walled area through the good permeability of the coated sand mold itself and reduce the invasion of gas into the molten metal. In addition, 10 – mm – thick chills were set between the thin – walled vertical cylinders on the side to adjust the solidification sequence and strengthen the feeding of the vertical cylinders to the thin – walled parts on the side. The filling process simulation results of the optimized casting process for the ZM6 magnesium alloy shell are shown in Figure 8. It can be seen that the filling process of the alloy liquid tends to be stable, and no air – entrainment phenomenon occurs. The defect prediction results are shown in Figure 9. There is no shrinkage porosity tendency inside the casting, and the air – entrainment amount in the thin – walled part on the side is extremely low.
Pouring Verification Results: The results of the pouring test after process optimization are shown in Figure 10. The surface of the casting is smooth, the roughness is lower than Ra12.5, and the dimensional accuracy meets CT8. There are no obvious porosity, shrinkage porosity, and other defects inside the casting, and the internal quality meets the requirements of HB7780 – 2005 Class I castings. The pass rate of subsequent batch – produced castings reached 90%, verifying that the optimized coated sand casting process scheme meets the production requirements.
4. Discussion
4.1 Influence of Coated Sand Raw Materials on Process Performance
The type of coated sand raw materials has a crucial impact on the performance of the coated sand mold. Coated sand III, with its low gas – evolution and high – permeability characteristics, is more conducive to reducing porosity defects in the casting process. The resin and curing agent content in different coated sands affect the gas – evolution amount during the casting process. High – gas – evolution coated sands are more likely to cause gas – related defects such as porosity. In addition, the permeability of coated sand determines the ease of gas discharge. Good permeability can ensure that the gas generated during the casting process can be discharged in time, reducing the chance of gas invading the molten metal.
4.2 Optimization of Curing Process Parameters
The curing temperature and time have a significant impact on the strength, gas evolution, and permeability of the coated sand. Appropriate curing temperature and time can ensure the complete curing of the resin in the coated sand, improve the strength of the sand mold, and reduce the gas evolution. When the curing temperature is too low or the curing time is too short, the resin cannot be completely cured, resulting in a large amount of gas evolution during the casting process. On the contrary, when the curing temperature is too high or the curing time is too long, the sand mold may become too brittle, affecting its strength and performance.
4.3 Influence of Casting Process Optimization on Defect Reduction
The optimization of the casting process, including the improvement of the gating system, the adjustment of the positions of chills and risers, effectively reduces the casting defects. The symmetrical slit – type gating system ensures the uniform filling of the molten metal, avoiding the occurrence of air entrainment and incomplete filling. The removal of the risers at the top of the thin – walled parts on the side and the addition of chills on the side can adjust the solidification sequence, strengthen the feeding, and reduce the shrinkage porosity and air – entrainment defects in the thin – walled parts.
5. Conclusion
- Optimization of Coated Sand Mold Forming Process: Through orthogonal tests, the forming process of the coated sand shell mold was optimized. High – permeability and low – gas – evolution coated sand III was selected, with an optimal curing temperature of 220 °C and a curing time of 120 s. The coated sand mold prepared under these conditions has a low gas – evolution and high – permeability, which provides a good foundation for obtaining high – quality castings.
- Prediction and Optimization of Casting Defects: The ProCAST software was used to predict the air – entrainment and shrinkage – porosity casting defects inside the shell casting, and the casting process was optimized. The symmetrical slit – type gating system effectively solved the porosity defects in the thin – walled parts on the side of the shell casting, improving the internal quality of the casting.
- Reliability Verification of the Optimization Results: Through pouring verification, the reliability of the optimized results of the coated sand shell mold forming process and the casting process was proved. The magnesium alloy shell casting has a dimensional accuracy of CT8, a surface roughness of less than Ra12.5, and a pass rate of 90%, meeting the production requirements.
In general, this research provides a feasible solution for the coated sand casting process of ZM6 magnesium alloy shells, which can be extended to the production of similar magnesium alloy components, and promotes the development of the foundry industry for magnesium alloy products.
6. Future Research Directions
- Further Optimization of Process Parameters: Although the current research has obtained relatively optimal process parameters, there is still room for further improvement. Future research can focus on finer – grained adjustment of process parameters, such as exploring the impact of small – range changes in curing temperature and time on the performance of coated sand and casting quality.
- Study on New Coated Sand Materials: With the continuous development of materials science, exploring new types of coated sand materials with better performance, such as higher strength, lower gas evolution, and better thermal stability, can further improve the quality of magnesium alloy castings.
- Combined Application of Multiple Casting Technologies: Considering the combination of coated sand casting with other casting technologies, such as combining with vacuum casting technology to further reduce porosity defects and improve the internal quality of castings.
