1. Introduction
ZM6 magnesium alloy is widely used in various industries due to its excellent properties such as low density, high specific strength, and good castability. The ZM6 magnesium alloy shell, with its complex structure, has strict requirements for dimensional accuracy, surface roughness, and internal quality. Sand casting is a common method for manufacturing magnesium alloy shells, and coated sand casting is gradually becoming an important shaping process. However, the existing coated sand casting process often leads to porosity defects in magnesium alloy castings, which seriously affect the quality and performance of the products. Therefore, it is of great significance to optimize the coated sand casting process for ZM6 magnesium alloy shells.
2. Research Significance
The optimization of the coated sand casting process for ZM6 magnesium alloy shells can improve the quality and performance of castings, reduce production costs, and enhance the competitiveness of products in the market. By studying the influencing factors of the casting process and finding the optimal process parameters, it is possible to effectively solve the problem of porosity defects, improve the dimensional accuracy and surface quality of castings, and meet the high – quality requirements of modern industries for magnesium alloy components.
3. Research Methods
3.1 Orthogonal Test
Orthogonal test is a scientific experimental method that can comprehensively consider multiple factors and their levels. In this study, orthogonal test was used to optimize the curing process of coated sand. Three factors, namely curing temperature, curing time, and coated sand category, were selected, and each factor had three levels. A total of 9 groups of orthogonal tests were carried out. The orthogonal test factor – level table is shown in Table 1.
| Level | Factor | ||
|---|---|---|---|
| Curing Temperature /℃ (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 |
Three types of coated sand raw materials, namely ordinary silica sand coated sand, high – strength silica sand coated sand, and low – gas – evolution silica sand coated sand, were selected and named as Ⅰ, Ⅱ, and Ⅲ respectively. “8” – shaped specimens were prepared using the RXH – Ⅱ coated sand sample – making machine. The tensile strength, gas evolution, and gas permeability of the specimens were tested using 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 orthogonal test scheme is shown in Table 2.
| Test Number | Curing Temperature /℃ | 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 |
3.2 Numerical Simulation
The ProCAST software developed by the Shanghai Aerospace Precision Machinery Research Institute was used for casting numerical simulation. First, a finite – element model of the gravity – casting ZM6 magnesium alloy shell was established. Then, the initial process parameters were set, with the pouring temperature of 700℃ and the pouring speed of 3.25 kg/s. Finally, the flow field and temperature field were solved to calculate the filling and solidification processes.
4. Coated Sand Casting Process for ZM6 Magnesium Alloy Shell
4.1 Preparation Process of Coated Sand Mold
As mentioned above, three types of coated sand raw materials were selected, and orthogonal tests were carried out on the curing process parameters of coated sand. The purpose was to find the optimal combination of curing temperature, curing time, and coated sand category to obtain coated sand molds with excellent comprehensive performance.
4.2 Casting Process Design
The ZM6 magnesium alloy shell is similar to a box – like structure, with a small height dimension of about 120 mm, but a large wall – thickness difference. The maximum wall – thickness of the top boss is 35 mm, and the main wall – thickness of the thin – wall on both sides is only 10 mm. In the initial casting process design, a one – sided gap gating system was adopted on the thin – wall side to avoid overheating in the area facing the gate. To ensure sufficient feeding and bottom – up sequential solidification, conformal risers were set on the top, and chills were set in thick – wall areas. The three – dimensional model of the casting and the process model are shown in Figure 1.
4.3 Numerical Simulation and Process Optimization
The ProCAST software was used to simulate the filling and solidification processes of the initial casting process. The simulation results showed that when using the one – sided gap gating system for filling, the filling on both sides was inconsistent, the molten metal flow was disordered. When the filling rate reached 40% – 50%, gas entrainment occurred in the thin – wall on the side far from the gap gate, and there were local areas that were not filled in time. The predicted results of the initial process defects showed that the shrinkage porosity was concentrated in the gating system and risers, and the shrinkage porosity tendency inside the casting was small. Gas entrainment was mainly distributed in the risers, but a large – area gas – entrainment area appeared in the thin – wall on the side far from the gap gate, with a maximum gas – entrainment amount of about \(0.00144 g/cm^{3}\).
To avoid gas entrainment and ensure stable and consistent filling of the thin – wall on the side, the casting process was optimized. The optimized process adopted a two – sided gap gating system structure, and the risers at the top of the thin – wall on the side were removed to facilitate the discharge of gas in the thin – wall area through the good gas permeability of the coated sand mold itself, reducing the invasion of gas into the molten metal. In addition, 10 – mm – thick chills were set between the thin – wall vertical cylinders on the side to adjust the solidification sequence and strengthen the feeding of the vertical cylinders to the thin – wall on the side. The optimized casting process model and the finite – element model are shown in Figure 2.
4.4 Pouring Verification
The coated sand mold was prepared using the optimal forming process, and pouring tests were carried out according to the simulation results of the optimized casting process. The dimensions, roughness, and internal quality of the castings were detected.
5. Experimental Results and Analysis
5.1 Results of the Orthogonal Test of Coated Sand Molds
The orthogonal test results of coated sand molds are shown in Table 3.
| Test Number | Tensile Strength /MPa | Gas Evolution /mL·g⁻¹ | Gas 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 | 2.22 | 14.10 | 251 |
The range analysis of the test results shows that the influence degree of the coated sand forming process parameters on the tensile strength, gas evolution, and gas permeability is: coated sand category (C)>curing temperature (A)>curing time (B). The main – effect diagrams of the coated sand mold forming process parameters on the average tensile strength, average gas evolution, and average gas permeability are shown in Figure 3, Figure 4, and Figure 5 respectively.
It can be seen from the figures that the coated sand raw material has a significant impact on the tensile strength, gas evolution, and gas permeability. Coated sand Ⅲ has the lowest gas evolution and the best gas 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 stabilizes, and the gas permeability basically remains unchanged. According to the principle of process selection in actual production, the coated sand mold with a room – temperature tensile strength of not less than 1.6 MPa, low gas evolution, and high gas permeability is selected. Finally, coated sand Ⅲ with a curing temperature of 220℃ and a curing time of 120 s is determined. The tensile strength of the coated sand mold is 1.74 MPa, the gas evolution is 11.17 mL/g, and the gas permeability is 255.
5.2 Results of Casting Process Simulation
The filling process simulation results of the initial casting process of the ZM6 magnesium alloy shell are shown in Figure 6.
The simulation results show that the filling on both sides is inconsistent, and the molten metal flow is disordered. When the filling rate reaches 40% – 50%, as shown in the box in Figure 6(d), gas entrainment occurs in the thin – wall on the side far from the gap gate, and there are local areas that are not filled in time. The predicted results of the initial process defects are shown in Figure 7.
The shrinkage porosity is concentrated in the gating system and risers, and the shrinkage porosity tendency inside the casting is small. Gas entrainment is mainly distributed in the risers, but a large – area gas – entrainment area appears in the thin – wall on the side far from the gap gate.
5.3 Results of Pouring Verification
The coated sand mold, the physical casting, and the X – ray detection results after the 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 qualification rate of subsequent batch – produced castings reaches 90%, verifying that the optimized coated sand casting process meets the production requirements.
6. Optimization of Coated Sand Casting Process
6.1 Optimization of Coated Sand Mold Forming Process
Based on the orthogonal test results, the optimized coated sand mold forming process parameters are: high – permeability and low – gas – evolution coated sand Ⅲ, the best curing temperature of 220℃, and the curing time of 120 s. Under these parameters, the coated sand mold has lower gas evolution and higher gas permeability, which can effectively reduce the occurrence of porosity defects in castings.
6.2 Optimization of Casting Process
The optimization of the casting process mainly includes the improvement of the gating system, the adjustment of the riser and chill positions. The symmetric gap gating system effectively solves the problem of porosity defects in the thin – wall on the side of the shell casting. By removing the risers at the top of the thin – wall on the side and adding 10 – mm – thick chills on the side, the gas in the thin – wall area can be discharged smoothly, and the solidification sequence can be adjusted to ensure the quality of the casting.
7. Conclusion
Through orthogonal tests and numerical simulations, the coated sand mold material system, curing process, and coated sand casting process of the ZM6 magnesium alloy shell were studied, and the results were verified by pouring tests. The main conclusions are as follows:
(1) The forming process of the coated sand shell mold was optimized based on the orthogonal test. High – permeability and low – gas – evolution coated sand Ⅲ was selected, with the best curing temperature of 220℃ and the curing time of 120 s. The coated sand mold had lower gas evolution and higher gas permeability.
(2) The ProCAST software was used to predict the gas entrainment and shrinkage porosity casting defects inside the shell casting, and the casting process was optimized. The symmetric gap gating system effectively solved the porosity defects in the thin – wall on the side of the shell casting.
(3) The pouring verification proved the reliability of the optimized results of the coated sand shell mold forming process and the casting process. The dimensional accuracy of the magnesium alloy shell casting reached CT8, the surface roughness was less than Ra12.5, and the qualification rate reached 90%.
8. Future Research Directions
Although the optimization of the coated sand casting process for ZM6 magnesium alloy shells has achieved certain results in this study, there are still some aspects that can be further explored in the future:
(1) Further study the influence of different coated sand additives on the performance of coated sand molds, aiming to develop more high – performance coated sand materials.
(2) Explore the application of new casting technologies in combination with coated sand casting, such as the integration of 3D printing technology and coated sand casting to achieve more complex casting shapes.
(3) Strengthen the research on the solidification mechanism of ZM6 magnesium alloy during the casting process to provide a more in – depth theoretical basis for process optimization.
