The moving coil skeleton is a crucial component of the electric shaker, which is mainly used for transmitting vibrations and carrying specimens. Therefore, it is required to have characteristics such as light weight, high stiffness, and high resonant frequency. However, traditional gravity casting processes for the moving coil skeleton have many defects, poor performance, and a low finished product rate.
Low-pressure casting is a casting process where the liquid metal fills the mold under reverse gravity conditions and solidifies under certain pressure. Compared with gravity casting, it has the advantages of smooth liquid metal filling, less tendency to entrap gas and inclusions, and the ability to improve the feeding of the liquid metal during solidification under pressure, reducing defects such as shrinkage cavities and porosity, and significantly enhancing the mechanical properties of the castings.
In this study, the ProCAST software was used to conduct a numerical simulation of the low-pressure casting process of the ZL302 aluminium alloy moving coil skeleton. The effects of the pouring temperature, sand mold preheating temperature, and holding pressure on the low-pressure casting formation of the aluminium alloy moving coil skeleton were investigated.
1. Model Establishment
The material of the moving coil skeleton is ZL302 alloy, and its chemical composition is shown in Table 1. The part structure is depicted in Figure 1, with an overall size of 320mm × 360mm × 320mm, a maximum wall thickness of 23mm, and a minimum wall thickness of only 8mm. Based on the analysis of the part structure according to the low-pressure casting process, all the surfaces of the part were set as non-cast holes, and a 4mm machining allowance was set on the upper and lower surfaces. The UG three-dimensional modeling software was used to perform geometric modeling of the moving coil skeleton casting, as shown in Figure 1b.
The casting adopted a slit gating system, as shown in Figure 1c. The double riser and cross runner at the lower part of the casting determined the filling state of the liquid metal during the pouring process. The connection between the casting and the slit inner runner allowed for smooth filling of the liquid metal, reducing the occurrence of gas entrapment and slag inclusion. The top part of the casting was a riser, which played a role in feeding and slag collection for the casting, reducing the shrinkage cavity and porosity rate of the casting and improving the casting quality.
The established geometric model was imported into the ProCAST software in the X_T file format for mesh generation. Surface and volume checks were performed on the casting model, and defective models were repaired. Meshing was carried out for the sand mold and casting parts, with cell sizes of 10mm and 5mm respectively. The final number of surface meshes and volume meshes was 195,496 and 3,926,891, respectively, as shown in Figure 1d.
2. Low-Pressure Casting Process
In low-pressure casting, the main parameters affecting the casting quality include the pouring temperature, sand mold preheating temperature, and holding pressure. For the aluminium alloy moving coil skeleton casting, low-pressure sand mold casting was adopted based on the principles of low-pressure casting and production experience. The casting process parameters are presented in Table 2, and the designed numerical simulation schemes are shown in Table 3.
3. Numerical Simulation Calculation
For the numerical simulation, the following settings were made in the CAST module of the ProCAST software: the gravity direction was set opposite to the filling direction of the liquid metal; the virtual sand box material was set as Resin Bonded Sand and configured as a solid, with a mechanical type of Linear-Elastic; the material of the casting was set as ZL302, with a mechanical type of Rigid; the heat transfer coefficient between the liquid metal and the sand mold was set as h = 500 W/(m^2 · K); the outer surface of the sand box was set as Air cooling, and the inlet pressure was set at the riser mouth (this pressure parameter value should be 0.1 MPa higher than the pressure value of the casting process parameters); and LPDC Filling was selected in the Simulation Parameters for simulation.
4. Simulation Results Analysis
4.1 Effect of Pouring Temperature on the Low-Pressure Casting Process
Four schemes, namely Scheme 1 to Scheme 4, were selected for numerical simulation analysis, as shown in Figure 2. It can be observed that the filling process of the castings under the four schemes was relatively consistent, with the liquid metal smoothly filling the mold from the riser mouth to the interior of the mold, without any gas entrapment or splashing phenomenon. According to the numerical simulation results, as the pouring temperature increased from 690°C to 750°C, the filling time of the castings decreased from 22s to 20s, indicating that with the increase in the pouring temperature, the filling efficiency of the castings improved, and the filling time of the castings shortened. The main reason is that when the pouring temperature is low, the flow performance of the liquid metal deteriorates, making it difficult to fill the cavity, which may lead to defects such as cold shut and insufficient pouring.
Figure 3 shows the distribution of shrinkage cavities and porosity in the castings at different pouring temperatures. It can be seen that the distribution positions of the shrinkage cavities and porosity under the four schemes were relatively consistent, mainly located at the connection between the rib plate and the base of the casting, where the wall thickness of the casting was relatively thick, easily forming a hot spot to generate an independent liquid phase area, so that the pressure could not make the liquid metal feed the hot spot area during the pressure solidification of the casting, resulting in the formation of shrinkage cavities and porosity defects. The volumes of the shrinkage cavities and porosity of the castings in Scheme 1 to Scheme 4 were 0.224, 0.202, 0.128, and 0.140 cm^3 respectively, among which the volume of the shrinkage cavities and porosity in Scheme 3 (with a pouring temperature of 730°C) was the smallest, indicating that appropriately increasing the pouring temperature can reduce the volume of the shrinkage cavities and porosity.
| Scheme | Pouring Temperature (°C) | Volume of Shrinkage Cavities and Porosity (cm^3) |
|---|---|---|
| 1 | 690 | 0.224 |
| 2 | 710 | 0.202 |
| 3 | 730 | 0.128 |
| 4 | 750 | 0.140 |
4.2 Effect of Sand Mold Preheating Temperature on the Low-Pressure Casting Process
To explore the influence of the sand mold preheating temperature on the low-pressure casting process of the moving coil skeleton, Scheme 3 and Scheme 7 were selected for numerical simulation analysis, and the results are presented in Figure 4. It can be seen that under different sand mold preheating temperatures, the filling process of the liquid metal remained stable without any splashing phenomenon. With the increase in the sand mold preheating temperature, the filling time of the liquid metal did not change significantly, indicating that at a higher pouring temperature, the sand mold preheating temperature had a relatively small impact on the filling rate.
Figure 5 shows the distribution of the shrinkage cavities and porosity in the castings at different sand mold preheating temperatures. It can be observed that the distribution positions of the shrinkage cavities and porosity were basically the same under different sand mold preheating temperatures. However, when the sand mold preheating temperature reached 100°C, the volume of the shrinkage cavities and porosity generated inside the casting decreased. The main reason is that with the increase in the sand mold preheating temperature, the temperature of the liquid metal was high upon the completion of filling, and during the pressure holding stage, the liquid metal could improve the feeding effect on the hot spot area under the action of pressure. Therefore, in actual production, an appropriate increase in the sand mold preheating temperature can enhance the feeding effect of the liquid metal.
| Scheme | Sand Mold Preheating Temperature (°C) | Volume of Shrinkage Cavities and Porosity (cm^3) |
|---|---|---|
| 3 | 30 | – |
| 7 | 100 | Decreased |
4.3 Effect of Holding Pressure on the Low-Pressure Casting Process
To investigate the influence rule of the pouring temperature on the low-pressure casting process of the moving coil skeleton, Scheme 3, Scheme 5, and Scheme 6 were selected for numerical simulation analysis, and the results are shown in Figure 6. It can be seen that upon the completion of filling, the solid phase rate at the thin-walled part of the rib plate of the casting was the highest, while the solid phase rate near the inner runner and the thick part at the top was the lowest. From the distribution of the solid phase rate during the solidification process of the casting, it can be seen that the overall solidification process of the casting tended towards sequential solidification. Figure 7 shows the distribution of the shrinkage cavities and porosity in the castings after solidification. It can be seen that with the increase in the holding pressure, the volume of the shrinkage cavities and porosity in the casting decreased from 0.13 cm^3 to 0.11 cm^3, so an appropriate increase in the holding pressure can improve the casting quality.
| Scheme | Holding Pressure (kPa) | Volume of Shrinkage Cavities and Porosity (cm^3) |
|---|---|---|
| 3 | 23 | 0.13 |
| 5 | 28 | – |
| 6 | 33 | 0.11 |
5. Conclusions
(1) With the increase in the pouring temperature, the fluidity of the liquid metal enhanced, the filling time reduced, and the volume of the shrinkage cavities and porosity in the castings first decreased and then increased with the increase in the pouring temperature. Appropriately increasing the pouring temperature can improve the casting defects.
(2) The increase in the sand mold preheating temperature had a relatively small impact on the filling rate of the castings, but when the sand mold preheating temperature increased from 30°C to 100°C, the volume of the shrinkage cavities and porosity in the castings decreased accordingly.
(3) During the pressure holding and solidification stage, with the increase in the holding pressure, the flow performance of the liquid metal improved, the feeding performance enhanced, and the volume of the shrinkage cavities and porosity in the castings decreased from 0.13 cm^3 to 0.11 cm^3. Therefore, in actual production, under the premise of ensuring the strength of the sand mold, an appropriate increase in the holding pressure can improve the casting quality.
In summary, through the numerical simulation of the low-pressure casting process of the aluminium alloy moving coil skeleton, the effects of different parameters on the casting process were analyzed, providing valuable references for the actual production of the moving coil skeleton. By optimizing the pouring temperature, sand mold preheating temperature, and holding pressure, the quality of the castings can be effectively improved, reducing the occurrence of defects such as shrinkage cavities and porosity.
In addition, it is also worth noting that in the actual production process, other factors may also affect the quality of the castings. For example, the quality and properties of the raw materials, the accuracy and stability of the equipment, and the operation skills and experience of the workers. Therefore, in order to obtain high-quality castings, comprehensive consideration and control of various factors are required.
Furthermore, continuous research and innovation are necessary to further optimize the low-pressure casting process. For example, exploring new materials or alloy compositions that can better meet the performance requirements of the moving coil skeleton; developing more accurate numerical simulation methods and software to improve the prediction accuracy of the simulation results; and studying the interaction and coupling effects between different process parameters to achieve more precise control and optimization of the casting process.
In conclusion, the low-pressure casting process of aluminium alloy moving coil skeleton is a complex process that requires in-depth research and continuous optimization. By understanding and mastering the influence of various factors, and through scientific design and reasonable control of the process parameters, it is possible to produce high-quality moving coil skeleton castings, meeting the needs of different applications in various fields.