Improving the comfort and economy of automobiles has always been an important goal of the automotive industry, and the transmission, as one of the three major driving components of automobiles, is very important. The gear set of the 9-speed automatic transmission adopts a nested structure, and the total length of the transmission is controlled within a certain range. The speed ratio interval is small, which not only improves driving comfort but also allows the engine to operate in the most economical area, greatly improving fuel efficiency. Compared to the 6-speed automatic transmission, it can save fuel by 10% to 16%. However, the high-pressure casting production of complex 9AT transmissions often results in porosity defects, and achieving stable and high-quality transmission housing production is an urgent problem that needs to be solved. At present, there are few research reports on the production of 9AT transmission casings in China, while a large amount of research has been conducted on the processes of other high-pressure castings and their effects on defects such as porosity and shrinkage. Wan Xiaomeng et al. reduced porosity defects in high-pressure cast aluminum alloy clutch shells by improving the pouring system and using a central gate. Bo Bing found that increasing the position of high and low speed switching points is beneficial for reducing porosity defects in high-pressure castings, but it is not conducive to the density, tensile strength, and elongation of high-pressure castings. Zhao Weihong et al. improved the internal porosity and mechanical properties of high-pressure castings for gearbox shells using high vacuum and high-pressure casting technology. Zhang Yu et al. found that delaying the switching points between high and low speeds can effectively improve the jet phenomenon during the forming process of filter shells. In order to explore the effect of high and low speed transition positions on the filling state of metal liquid and its subsequent effects, a simulation and process experiment method was used to study the injection high and low speed transition points of the ADC12 aluminum alloy 9AT transmission housing. Based on the optimized process parameters obtained from the simulation, practical production verification was carried out, aiming to provide reference for its application.
1. High pressure casting model and materials
At present, the mainstream 8-speed automatic transmission uses 4 sets of planetary gears and 5 shifting mechanisms, while the 9-speed automatic transmission has designed 4 sets of planetary gears and 6 shifting mechanisms to achieve 9 gears. Through optimization of some parts and structural topology design, the overall volume is equivalent to the 8-speed automatic transmission, with an outer contour size of about 470 mm x 400 mm x 400 mm and a mass of about 12.6 kg. The overall structure of high-pressure casting parts is complex and the wall thickness is uneven, with an average wall thickness of about 6 mm. The thinnest part of the wall thickness is about 4 mm, and the thickest part is about 30 mm. There are many reinforcing ribs and oil pipes distributed inside, which can easily cause stress concentration during the high-pressure casting process, resulting in deformation, porosity, shrinkage, and other problems of the high-pressure casting parts. Figure 1 shows a three-dimensional model of the high-pressure cast part of the gearbox housing.
As a component for installing support bearings for variable speed gears, the transmission housing needs to ensure that it can absorb the forces and moments generated by the gears during operation under various complex working conditions, without deformation and displacement, and maintain accurate relative positions between the shafts. This requires the transmission housing to have high strength and stiffness, and ADC12 high-pressure cast aluminum alloy has the characteristics of low density, high specific strength and specific stiffness, which can meet the production requirements of the transmission housing. Table 1 shows its chemical composition.
| Si | Ni | Fe | Zn | Cu | Pb | Mn | Sn | Mg | Ti | Cr | Al |
| 9.6～12.0 | ≤0.55 | ≤1.3 | ≤1.0 | 1.5～3.5 | ≤0.2 | ≤0.5 | ≤0.2 | ≤0.3 | ≤0.3 | ≤0.05 | allowance |
2. High pressure casting process and scheme
2.1 Pouring system design
The design scheme of the pouring system is shown in Figure 2. The shape of the gearbox housing is irregular, and the depth of the internal cavity varies. The internal gate of the high-pressure casting is set on one side of the deep cavity, which is conducive to filling the deep cavity area. A branch (dashed line) is set on the right valve plate surface, which is conducive to filling the complex shaped valve plate surface.
2.2 Determination of process parameters
Based on the high-pressure casting process design manual and actual production experience, the process parameters have been preliminarily determined: the initial temperature of the molten metal is 680 ℃, the initial temperature of the mold is 200 ℃, the total mass is about 18.95 kg, and the total projected area is 265 327 mm2; Select a specific injection pressure of 80 MPa and a safety factor of 1.2. Through calculation, the locking force is about 25470 kN, so the high-pressure casting machine of the Buhler CATAT305 model is selected, with a locking force of 30 500 kN and a punch diameter of ϕ 150 mm, low-speed injection speed is 0.2 m/s, high-speed injection speed is 3.5 m/s, and the total length of the material barrel is 800 mm.
The usual high-pressure casting production process usually takes the metal liquid to the position of the inner gate as the high and low speed switching point. However, due to the complex shape of the gearbox housing and the box structure, the conventional high and low speed switching points are not suitable. Based on previous work and extensive simulation, when the high and low speed switching point is set at 480mm, it is the position where the metal liquid reaches the inner gate. When the high and low speed switching point is set at 520mm, it is the position where the metal liquid enters the mold cavity from the middle 5 branches and smoothly intersects. When the high and low speed switching point is set at 560mm, it is the position where the metal liquid in the mold cavity smoothly intersects with the metal liquid in the right branch. Therefore, in order to study the influence of high and low speed switching points on the filling process of the gearbox housing, three simulation schemes were designed. The high and low speed switching points were set at 480, 520, and 560 mm, respectively, as Scheme 1, Scheme 2, and Scheme 3.
3. Simulation results and analysis
To understand the filling situation during the high-pressure casting process of the transmission housing, Flow-3D software was used for numerical simulation. Figure 3 shows the filling process of Scheme 1. It can be seen that due to the fact that the metal liquid does not smoothly intersect into the mold cavity, but directly fills the mold cavity at high speed, there is a very obvious jet flow at the front end of the metal liquid (dashed box in Figure 3b), which is prone to backflow and air entrainment. There is a clear unfilled part at the arrow on the left shallow cavity, and the gas in the future exhaust cavity is wrapped in metal liquid, causing defects in the internal pores of the transmission housing. The left side of the mold cavity is filled slowly and has severe air entrapment, which greatly affects the quality of high-pressure castings.
Figure 4 shows the filling process of Scheme 2. It can be seen that due to the smooth intersection of the metal liquid, when the injection speed changes from low speed to high speed, the metal liquid can smoothly fill the mold. Compared with Scheme 1, the jet flow at the front end has been significantly improved, and the air entrainment phenomenon on the left side has also been significantly improved. However, due to the shallow cavity area on the left, less metal liquid needs to be filled. When the metal liquid smoothly intersects and high-speed injection begins, the metal liquid first enters the left shallow cavity, forming a small amount of jet flow, and a small amount of gas is wrapped in the left shallow cavity metal liquid (see arrow in Figure 4b).
Figure 5 shows the filling process of Scheme 3. From Figure 5a, it can be seen that during the high and low speed switching point, the metal liquid has smoothly intersected with the right branch metal liquid. When the injection speed changes from low speed to high speed, due to the filling effect of the right branch on the right deep cavity, the metal liquid can simultaneously fill the left shallow cavity area and the right deep cavity area. The metal liquid is filled in a laminar flow manner. The jet generated during the entire filling process is very small and the filling is relatively uniform. There is no gas wrapping in the shallow cavity on the left side. The filling process is smooth, which is conducive to expelling the gas inside the mold cavity, thereby reducing the occurrence of porosity defects.
Figure 6 shows the simulation of the probability of air entrainment in each part after the completion of filling with three different schemes. The results indicate that the switching points of high and low speed injection have a significant impact on the filling process of high-pressure casting. When the switching point between high and low speeds of injection is too early, the metal liquid produces a jet under the action of high-speed injection, forming a large amount of splashing and increasing the probability of porosity defects. Increasing the low-speed injection stroke and appropriately delaying the switching point between high and low speeds can ensure that the metal liquid flows smoothly into the mold cavity and is evenly dispersed. When the low-pressure injection rate changes to the high-pressure injection rate, the force brought by the punch is evenly distributed in various parts of the metal liquid, rather than concentrated in a certain part, thereby improving the jet flow situation during the filling process. According to the simulation results, it can be seen that when the switching point between high and low speeds is 480 mm, the probability of air entrainment in the left shallow cavity and the end is about 30%, with a small portion reaching 50%; When the switching point between high and low speeds is 520 mm, there is a significant improvement in the left shallow cavity area, and only the probability of air entrainment in the end area reaches 50%; When the switching point between high and low speeds is 560 mm, the overall probability of air entrainment is the smallest. The probability of air entrainment in the left shallow cavity area is less than 13%, and the probability of air entrainment in the end section is less than 30%. Compared to Scheme 1 and Scheme 2, Scheme 3 has the most stable overall filling process and the lowest probability of air entrainment.
4. Microstructure and mechanical properties
Using the parameters of Scheme 3 for actual production, high-pressure cast parts of the transmission housing with clear external contours and high dimensional accuracy were obtained, as shown in Figure 7. It can be seen that the surface of high-pressure castings is smooth, without pores and oxide inclusions. Samples were taken from the near gate and end positions of high-pressure castings for microstructure observation and mechanical property analysis. Figure 8 shows the microstructure of the 9AT transmission housing.
From Figure 8a, it can be seen that the structure of the transmission housing is mainly composed of α- Al and α- Al+Si eutectic phase composition, primary α- Part of the Al phase exhibits dendritic morphology, while others exhibit spherical or granular morphology. Eucrystalline Si exists in needle like form. At the same time, there are a small amount of blocky precipitates in the matrix. As shown in Figure 8b, due to the fast cooling rate at the end, the initial growth α- Al and eutectic Si cannot continue to grow, resulting in a large amount of primary growth α- The Al grains are spherical in shape. It can be seen that the microstructure of the 9AT transmission housing is relatively dense, with small and uniformly distributed grains. Mechanical performance analysis was conducted on the near gate and end parts of the actual production gearbox housing, and the results are shown in Table 2. It can be seen that the tensile strength and elongation near the gate are 272.0 MPa and 3.4%, respectively, and the tensile strength and elongation at the end are 230.6 MPa and 2.7%. The mechanical properties of the gearbox housing require a tensile strength of 190 MPa and an elongation greater than 1%. From this, it can be seen that the mechanical properties of the transmission housing produced by Plan 3 meet the requirements.
| Sampling location | Near the gate | Terminal |
| Tensile strength/MPa | 272.0 | 230.6 |
| Elongation rate/% | 3.4 | 2.7 |
5. Conclusion
(1) Simulation of the filling process in high-pressure casting revealed that when the switching point between high and low speeds was 560 mm, the entire filling process was stable and uniform, with the lowest probability of air entrapment compared to when the switching points were 480 and 520 mm.
(2) The low-speed value of injection molding is 0.2 m/s, the high-speed value is 3.5 m/s, and the switching point between high and low speeds is 560 mm for actual production trial production. The appearance of the trial produced gearbox housing is free of obvious defects, and the internal structure has fine grains, uniform distribution, and dense structure. The tensile strength and elongation near the gate are 272.0 MPa and 3.4%, respectively, and the tensile strength and elongation at the end are 230.6 MPa and 2.7%, both meeting the mechanical performance requirements of the transmission housing.