This paper focuses on the die-casting process of the 9AT automotive transmission main housing. Through filling simulation, the influence of different high and low speed switching points of injection on the flow and filling of the metal liquid is analyzed. The results show that when the low-speed injection value is 0.2 m/s, the high-speed value is 3.5 m/s, and the switching point is 560 mm, the mold flow is stable and the probability of gas entrapment is the lowest. The optimized process parameters are used for trial production, and the castings have dense structure and good mechanical properties. This research provides important guidance for improving the quality of transmission main housing production.
1. Introduction
In the automotive industry, improving the comfort and economy of vehicles is a crucial goal. The transmission, as one of the three major components of the vehicle drive system, plays a significant role. The 9AT transmission has a nested gear structure, which not only improves driving comfort with a small speed ratio interval but also significantly enhances fuel efficiency as the engine often operates in the most economical region, saving 10% – 16% of fuel compared to the 6AT transmission. However, the high-pressure casting production of the complex 9AT transmission main housing often suffers from porosity defects. Achieving stable and high-quality production of the main housing is an urgent problem to be solved. Although there are extensive studies on the process of other castings and the influence of defects such as pores and shrinkage holes, there is relatively little research on the production of the 9AT transmission main housing in China. Therefore, this study aims to investigate the impact of the high and low speed switching point of injection on the filling process of the 9AT transmission main housing through simulation and experimental methods to provide a reference for its application.
2. Casting Model and Material
2.1 Transmission Main Housing Model
The 9AT transmission is designed with 4 sets of planetary gears and 6 shifting mechanisms to achieve 9 gears. Its overall volume is comparable to that of the 8AT through part optimization and structural topology design. The outer dimensions are approximately 470 mm × 400 mm × 400 mm, and the mass is about 12.6 kg. The casting has a complex overall structure with uneven wall thickness, an average wall thickness of about 6 mm, the thinnest part is about 4 mm, and the thickest part is about 30 mm. There are many reinforcing ribs and oil pipes inside, which are prone to stress concentration during the die-casting process, resulting in casting deformation, porosity, shrinkage porosity and other problems. Figure 1 shows the 3D model of the transmission main housing casting.
2.2 Material Properties
The transmission housing needs to ensure that it can absorb the forces and moments generated by the gears during operation without deformation and displacement under various complex working conditions, requiring high strength and stiffness. ADC12 die-casting aluminum alloy, with its characteristics of low density, high specific strength, and high specific stiffness, can meet the production requirements of the main housing. Table 1 shows its chemical composition.
3. Die-Casting Process and Scheme
3.1 Gating System Design
The gating system is designed considering the irregular shape and different cavity depths of the transmission main housing. The inner gate is set on the deep cavity side to facilitate filling the deep cavity, and a branch is set on the right valve plate surface (indicated by the dotted line) to fill the complex valve plate surface. See Figure 2 for the 3D diagram of the gating system.
3.2 Process Parameter Determination
Based on the die-casting process design manual and practical production experience, the initial process parameters are determined as follows: the initial temperature of the metal liquid is 680 °C, the initial temperature of the mold is 200 °C, the total mass is about 18.95 kg, and the total projected area is 265,327 mm². The injection pressure is selected as 80 MPa with a safety factor of 1.2. The calculated clamping force is about 25,470 kN, so the Bühler CATAT305 die-casting machine with a clamping force of 30,500 kN is selected. The punch diameter is 150 mm, the low-speed injection speed is 0.2 m/s, and the high-speed injection speed is 3.5 m/s. The total length of the barrel is 800 mm.
Since the main housing has a complex shape and is a box structure, the conventional high and low speed switching point is not applicable. Through previous work and extensive simulation, when the high and low speed switching point is 480 mm, it is the position where the metal liquid reaches the inner gate; when it is 520 mm, it is the position where the metal liquid enters the cavity from the middle 5 branches and intersects smoothly; when it is 560 mm, it is the position where the metal liquid in the cavity and the metal material on the right branch intersect smoothly. Therefore, three simulation schemes are designed with the high and low speed switching points set at 480, 520, and 560 mm respectively, namely Scheme 1, Scheme 2, and Scheme 3.
4. Simulation Results and Analysis
4.1 Filling Process Simulation
The filling process of the three schemes is simulated using Flow – 3D software.
- In Scheme 1 (see Figure 3), the metal liquid enters the cavity and fills it at a high speed directly without a smooth intersection, resulting in obvious jetting at the front end of the metal liquid (indicated by the dotted box in Figure 3b), which is prone to reflux and gas entrapment. There is an obvious unfilled part at the arrow on the left shallow cavity, and the gas that fails to be discharged from the cavity is wrapped by the metal liquid, causing internal porosity defects in the transmission main housing. The filling on the left side of the cavity is slow, and the gas entrapment is severe, which has a great impact on the quality of the casting.
- In Scheme 2 (see Figure 4), due to the smooth intersection of the metal liquid, when the injection speed changes from low to high, the metal liquid can fill the cavity smoothly. Compared with Scheme 1, the jetting at the front end is significantly improved, and the gas entrapment on the left side is also improved. However, since the left side is a shallow cavity area and requires less metal liquid to fill, when the metal liquid intersects smoothly and starts high-speed injection, it first enters the left shallow cavity part, forming a small amount of jetting, and a small amount of gas is wrapped in the metal liquid of the left shallow cavity (see the arrow in Figure 4b).
- In Scheme 3 (see Figure 5), when the high and low speeds are switched at the switching point, the metal liquid has already intersected smoothly with the metal liquid on the right branch. When the injection speed changes from low to high, due to the filling effect of the right branch on the right deep cavity, the metal liquid can fill the left shallow cavity area and the right deep cavity area simultaneously, and the metal liquid fills in a laminar flow manner. The jetting generated during the entire filling process is very small, and the filling is relatively uniform. There is no gas entrapment phenomenon in the left shallow cavity part. The filling process is stable, which is conducive to discharging the gas in the cavity and reducing the occurrence of porosity defects.
4.2 Gas Entrapment Probability Analysis
Figure 6 shows the simulation results of the gas entrapment probability of each part after filling in the three schemes. When the high and low speed switching point is 480 mm, the gas entrapment probability in the left shallow cavity and the end is about 30%, and a small part reaches 50%. When it is 520 mm, the left shallow cavity area is significantly improved, and only the end part has a gas entrapment probability of 50%. When it is 560 mm, the overall gas entrapment probability is the smallest, the gas entrapment probability in the left shallow cavity area is less than 13%, and that in the end part is less than 30%. Compared with Scheme 1 and Scheme 2, Scheme 3 has the most stable overall filling process and the smallest probability of gas entrapment. This indicates that the high and low speed switching point has a significant impact on the die-casting filling process. When the switching point is too early, the metal liquid sprays under the action of high-speed injection, increasing the probability of porosity defects. Increasing the low-speed injection stroke and appropriately delaying the high and low speed switching point can make the metal liquid flow into the cavity smoothly and disperse evenly. When the low injection speed changes to the high injection speed, the force brought by the punch is evenly dispersed in each part of the metal liquid instead of concentrating on a certain part, thus improving the jetting situation during the filling process.
5. Microstructure and Mechanical Properties
5.1 Microstructure Observation
The castings are obtained by actual production using the parameters of Scheme 3. The transmission main housing castings have a clear outline and high dimensional accuracy, and the surface is smooth without pores and oxide inclusions. Microstructure observation is carried out by sampling from the near-gate position and the end position of the casting. As shown in Figure 8, the shell structure is mainly composed of α-Al and α-Al + Si eutectic phases. The primary α-Al phase partly presents a dendritic crystal morphology and partly a spherical or granular morphology, and the eutectic Si exists in the form of acicular flakes. At the same time, there are a small amount of blocky precipitated phases in the matrix. Due to the faster cooling rate at the end, the primary α-Al and eutectic Si cannot continue to grow, and a large number of primary α-Al grains are spherical. It can be seen that the microstructure of the 9AT main housing is relatively dense, and the grains are fine and evenly distributed.
5.2 Mechanical Property Analysis
Mechanical property analysis is carried out on the near-gate and end positions of the actual production transmission main housing. The results are shown in Table 3. The tensile strength and elongation at the near-gate are 272.0 MPa and 3.4% respectively, and those at the end are 230.6 MPa and 2.7% respectively. The mechanical property requirements of the transmission main housing are that the tensile strength reaches 190 MPa and the elongation is greater than 1%. It can be seen that the mechanical properties of the transmission main housing produced by Scheme 3 meet the requirements.
6. Conclusion
(1) Through the filling process simulation, it is found that when the high and low speed switching point is 560 mm, the entire filling process is stable and uniform, and the probability of gas entrapment is the smallest compared with when the switching points are 480 and 520 mm.
(2) When the low-speed injection value is 0.2 m/s, the high-speed value is 3.5 m/s, and the high and low speed switching point is 560 mm for actual production trial, the trial-produced transmission main housing has no obvious defects in appearance, and the internal microstructure has fine grains, uniform distribution, and dense structure. The tensile strength and elongation at the near-gate are 272.0 MPa and 3.4% respectively, and those at the end are 230.6 MPa and 2.7% respectively, all meeting the mechanical property requirements of the transmission main housing.
This research provides valuable insights into the optimization of the die-casting process of the transmission main housing, which is beneficial to improving the production quality and performance of the transmission main housing and has certain guiding significance for the actual production of similar castings.
