Abstract: Taking aluminum alloy transmission housing as the research object, pouring system was designed based on characteristics of die casting and part structure, and initial process was numerically simulated using Magma software. Results showed that mold filling is not smooth, solidification is not sequential, shrinkage cavities and thermal crack defects occur. According to simulation results and causes of defects, gating system is improved, a cooling system is added, a process plan that eliminates defects and meets requirements is finally obtained.
Transmission assembly is a key component in automobile transmission system, and transmission housing, as part that installs transmission gear support bearing, needs to ensure that under various complex working conditions, it can absorb force and torque generated by gear during operation without causing deformation and displacement, maintain precise relative position between shafts. This requires transmission housing to have high strength and rigidity. However, when transmission is manufactured, defects such as shrinkage, shrinkage holes, and thermal cracks are prone to occur, which will greatly affect performance of parts. Pressure casting is a process in which liquid metal is quickly poured into mold cavity and solidified under high pressure, which can effectively reduce casting defects. In order to improve the overall yield of transmission housing of a domestic passenger car brand, Magma software was used to conduct a feasibility study on pressure casting process of transmission housing. First, die-casting scheme is initially designed based on casting manual and empirical formulas, and optimized scheme is obtained based on simulation results. Feasibility of scheme is then verified through trial production.

Transmission housing and gating system model is shown in Figure 1. Shell size is 230 mm * 300 mm * 120 mm, weight is 2.366 kg, material is AlSi9Cu3, shrinkage rate during die casting is 0.5% ~ 0.6%, conventional pressure casting process of one mold with two pieces is used, and die casting machine type is DM1500 Horizontal cold chamber die casting machine. There are some ribs distributed on the upper part of shell, with a hollow cavity in the middle. Two holes are used to install support bearings. Model is relatively complex as a whole. Maximum wall thickness is 26 mm, located at A in the picture, minimum wall thickness is 7 mm, located at B in figure, and average wall thickness is 10 mm. Bottom of shell is relatively flat and parallel to orthographic projection direction, and bottom is selected as parting surface.

 

Figure 1 Transmission housing and gating system model
Since a two-piece process is used in one mold, and a single inner runner is generally used when designing die-casting runner, it is not suitable to change direction too much and reduce process, so side gate type pouring system is selected. Inner gate is set on the side, and molten metal enters from both sides to complete mold filling. Area of inner gate is calculated from formula (1) and is 4.16c㎡; diameter of sprue is determined by type of die-casting machine and is 100 mm.

 

In formula: A is sum of cross-sectional area of inner gate, c㎡; G is the total weight of casting, g; ρ is density of alloy liquid, g/m³; v is linear velocity of alloy liquid at outlet of inner gate, cm /s; t is filling time, s.

Import model STL format into Magma and perform meshing. The total number of generated meshes is 1,084,326, of which number of fluid meshes is 513,722. Casting material is AlSi9Cu3, pouring temperature is 660℃, mold material is H13, and preheating temperature is 225℃. Thermophysical parameters of casting and mold materials are shown in Table 1. Injection specific pressure is 60 MPa, filling speed is 0.5~2 m/s, and pressure holding time is 50 s for simulation.

Table 1 Thermophysical parameters of castings and mold materials

Filling time of the entire casting is 0.06 s. In order to better observe flow of molten metal during filling process, tracer particles are used to observe filling process, as shown in Figure 2. It can be seen from figure that when t=0.02 s, the entire gating system has been filled with molten metal, and molten metal flows relatively smoothly, entering cavity from side and flowing to the other side. When t=0.032 s, molten metal is in high-speed filling stage, significant separation and backflow occurs in runner (circled in Figure 2), which affects stability of the entire flow, easily caused air entrainment and slag inclusion, and ultimately affected quality of parts.

 

Figure 2 Tracer particle path during mold filling

Temperature field changes during solidification process are shown in Figure 3. When t=1.049 s, some relatively thin ribs on transmission housing begin to solidify; when t=5.625 s, solidification rate reaches 50%, some parts of casting with smaller wall thickness begin to solidify, mainly located on upper part of shell and around central hole; when t=11.764 s, solidification rate is 85%. At this time, most of castings have solidified, mainly some parts with larger wall thickness, which have not yet completely solidified. Judging from changes in temperature field during the entire solidification process, sequential solidification is not achieved during solidification. Some thin-walled areas solidify first, while thick-walled areas solidify last, and wall-thickened areas are far away from gate. It is easy to produce an isolated liquid phase during solidification, which cannot be fed, eventually forming shrinkage cavities. At some corners, due to difference in solidification time, shrinkage strain rate is too large, resulting in thermal cracks on the surface.

 

Figure 3 Temperature field changes during solidification process

Based on analysis of results of filling process and solidification process, distribution locations of shrinkage holes and hot cracks in casting are predicted. Results are shown in Figure 4. It can be seen that possible locations of shrinkage cavities are close to those analyzed previously, thermal cracks are also located at circled area in figure at junction of thick and thin walls.

 

Figure 4 Simulated defect prediction

Since initial process cannot obtain castings that meet requirements, it needs to be optimized, which mainly includes two aspects: (1) Improve size of gating system. In initial pouring system, when molten metal enters runner and flows to both sides, runner becomes narrower and flow rate increases, which easily produces jets and turbulence, which in turn leads to separation and backflow. In order to ensure smooth flow, size of runner is optimized; (2 ) Add a cooling system to adjust temperature field distribution during solidification to achieve sequential solidification. Cooling system uses water cooling, and cooling water temperature is 20℃. Improved model is shown in Figure 5, and other cooling process parameters are shown in Table 2.

 

Figure 5 Improved pouring system and cooling system

Table 2 Cooling process parameters

Optimized process plan was numerically simulated. Tracer particle path during filling process and temperature field changes during solidification are shown in Figure 6. It can be seen that during the entire mold filling process, flow of molten metal in runner is smooth, phenomenon of separation and backflow is obviously eliminated; during solidification process, when t=1.209 s, in addition to thin-walled ribs starting to solidify, surface of wall thickness where cooling system is applied also begins to solidify slowly. When t=7.470 s, compared with before cooling, wall thickness area has basically completed solidification, mainly because some areas near gate have not completely solidified, basically meeting principle of sequential solidification. Predict optimized casting defects, as shown in Figure 7. It can be seen from figure that shrinkage holes and hot crack defects are basically eliminated, indicating that optimization scheme can significantly improve quality of castings and meet requirements.

 

Figure 6 Optimized mold filling process and solidification temperature field changes

 

Figure 7 Simulation defect prediction after optimization

In order to further verify feasibility of improved scheme, improved scheme was trial-produced. Actual produced transmission housing is shown in Figure 8. After inspection, the overall quality of castings was good, no casting defects such as cracks, shrinkage cavities, and shrinkage porosity were found, which was consistent with above simulated defect prediction results.

 

Figure 8 Actual picture of transmission housing

Numerical simulation method of forming process is used to improve and optimize die-casting process of transmission housing with purpose of eliminating shrinkage cavities and thermal cracks. Research results show that improved pouring system can effectively solve problem of uneven mold filling, while added cooling system can improve temperature gradient and achieve sequential solidification. Through actual production verification, improved solution can eliminate shrinkage holes and thermal crack defects, improve forming quality of parts.