Improving comfort and economy of cars has always been an important goal of automotive industry. As one of three major components of a car's drive, transmission is very important. Gear set of 9AT transmission adopts a nested structure. The total length of transmission is controlled within a certain range and speed ratio intervals are small. This not only improves driving comfort, but also allows engine to run in the most economical area, greatly improving fuel efficiency. Compared with 6AT transmission, it can save 10% to 16% of fuel. However, air hole defects often occur in high-pressure casting production of 9AT transmissions with complex structures. Achieving stable and high-quality main housing production is an urgent problem that needs to be solved. At present, there are few domestic research reports on production of 9AT transmission main housings, but a lot of research has been conducted on processes of other die-casting parts and impact on defects such as pores and shrinkage cavities. Some researchers have reduced porosity defects of die-cast aluminum alloy clutch housings by improving gating system and using central gates. It was found that increasing position of high-low speed switching point is beneficial to reducing hole defects in castings, but is detrimental to density, tensile strength and elongation of castings. High vacuum die-casting technology is used to improve internal pore state and mechanical properties of gearbox housing casting. Researchers found that delaying high-low speed switching point can effectively improve jet flow phenomenon during filter housing forming process. In order to explore impact of high and low speed switching position on filling state of metal liquid and subsequent effects, method of simulation + process test was used to study injection high and low speed switching points of ADC12 aluminum alloy 9AT transmission main housing, and actual production verification was carried out based on optimized process parameters obtained from simulation, aiming to provide a reference for its application.

Current mainstream 8AT transmission uses 4 sets of planetary gears and 5 shifting mechanisms. In order to achieve 9 gears, 9AT transmission has designed 4 sets of planetary gears and 6 sets of shifting mechanisms. Through optimization of some parts and structural topology design, the overall volume is equivalent to that of 8AT, with an outer outline size of approximately 470mm*400mm*400mm, and weight is about 12.6kg. The overall structure of casting is complex and wall thickness is uneven. Average wall thickness is about 6mm, the thinnest part is about 4mm, and the thickest part is about 30mm. There are many reinforcing ribs and oil pipes distributed internally, which can easily cause stress concentration during die-casting process, causing problems such as deformation of casting, pores, shrinkage porosity, and shrinkage cavities. Figure 1 is a three-dimensional model diagram of transmission main housing casting. As a part for installing transmission gear support bearing, transmission housing needs to ensure that it can absorb force and torque generated by gear during operation under various complex working conditions without deformation or displacement, and maintain precise relative position between shafts. This requires main shell to have high strength and stiffness, and ADC12 die-cast aluminum alloy has characteristics of low density, high specific strength and specific stiffness, which can meet production requirements of main shell.

 

Figure 1 Three-dimensional view of main housing of transmission

Table 1 Main chemical composition of ADC12 alloy (%)

 

Figure 2 Three-dimensional diagram of pouring system
Usual die-casting production process generally takes position where metal liquid reaches inner gate as high-low speed switching point. However, due to complex shape of main shell and its box structure, conventional high-low speed switching point is not applicable. Combining preliminary work and a large number of simulations, high and low speed switching point is set at 480mm, which is position where molten metal reaches inner gate. High and low speed switching point is set at 520mm, which is position where molten metal enters cavity from middle 5-way branch and merges smoothly. When switching point is 560mm, it is position where molten metal in cavity and metal liquid in right branch meet smoothly. Therefore, in order to study impact of injection high and low speed switching points on filling process of main housing of transmission, three sets of simulation schemes were designed. High and low speed switching points were set at 480, 520 and 560mm respectively, namely scheme 1, scheme 2 and scheme 3.

 

Figure 3 Die casting filling simulation results of Scheme 1

 

Figure 4 Die casting filling simulation results of Scheme 2

 

Figure 5 Die-casting filling simulation results of Scheme 3
Figure 3 shows filling process of Scheme 1. It can be seen that because molten metal enters mold cavity and does not merge smoothly, but directly fills cavity at a high speed, a very obvious jet appears at the front end of molten metal (dotted line box in Figure 3b), which can easily cause backflow and air entrainment. There is an obvious unfilled part at arrow of shallow cavity on the left. Gas that has not yet been discharged from cavity is wrapped in molten metal, causing air hole defects inside transmission main housing. Left side of cavity is filled slowly and air entrainment is serious, which has a great impact on quality of die castings. Figure 4 shows filling process of Scheme 2. It can be seen that due to smooth intersection of molten metal, molten metal can fill mold smoothly when injection speed is converted from low speed to high speed. Compared with Option 1, jet flow at the front end is significantly improved, and air entrainment phenomenon on the left side is also significantly improved. However, since left side is a shallow cavity area, less molten metal needs to be filled. When molten metals merge smoothly and start high-speed injection, molten metal first enters shallow cavity on the left, forming a small amount of jet, and a small amount of gas is wrapped in molten metal in shallow cavity on the left (see arrow in Figure 4b). Figure 5 shows filling process of Scheme 3. It can be seen from Figure 5a that when switching between high and low speeds at high-low speed switching point, molten metal has smoothly merged with molten metal on the right branch. When injection speed is converted from low speed to high speed, due to filling effect of right branch on the right deep cavity, molten metal can fill left shallow cavity area and right deep cavity area at the same time, molten metal fills mold in a laminar flow manner. Jet flow generated during the entire mold filling process is very small, and mold filling is relatively uniform. There is no gas wrapping in shallow cavity on the left side. Filling process is smooth, which is conducive to discharging gas in cavity, thereby reducing occurrence of pore defects.

 

Figure 6 Simulation results of air entrainment probability at three high and low speed switching points

 

Figure 7 Die-casting blank of transmission main housing

 

Figure 8 Microstructure of transmission main housing

Table 2 Mechanical properties of transmission main housing

(1) Simulation of filling process found that when high-low speed switching point is 560mm, the entire filling process is smooth and even. Compared with high-low speed switching point of 480 and 520mm, probability of air entrainment is the smallest.
(2) Select injection low speed value as 0.2m/s, high speed value as 3.5m/s, high and low speed switching point as 560mm for actual production trial production. Trial production transmission main housing has no obvious defects in appearance, and internal structure has fine grains, even distribution and dense structure. Tensile strength and elongation near gate are 272.0MPa and 3.4% respectively, tensile strength and elongation at the end are 230.6MPa and 2.7%, both of which meet mechanical performance requirements of transmission main housing.