Commissioning of a large number of new energy vehicles can alleviate problems such as energy consumption and environmental pollution to a certain extent. However, unlike traditional fuel vehicles, new energy vehicles are mainly driven by electricity, and long-range endurance has become a focus issue. According to relevant data, for every 100kg reduction in vehicle weight, fuel consumption and CO2 emissions of traditional fuel vehicles can be reduced by 6mL/km and 8.4g/km respectively, and cruising range of new energy vehicles can be increased by 9%~12%. Therefore, it is very important to make vehicles lightweight. Transmission control housing controls degree of freedom of connecting rod in vehicle transmission. Continuous shifting during driving of vehicle will cause wear of transmission control housing, which will seriously affect driving of vehicle, so quality requirements of transmission control housing parts are relatively high. Because uneven wall thickness of transmission control housing is prone to shrinkage during production process, its production process needs to be optimized to improve product quality.
This study takes an aluminum alloy transmission control housing as object and uses numerical simulation software to perform numerical simulation on its low-pressure casting. Taguchi method is used to optimize casting process. Minimum secondary dendrite arm spacing value (SDAS) and shrinkage volume value are used as optimization targets. Mold preheating temperature, pouring temperature and cooling temperature are theoretically matched and optimized, purpose of improving product forming quality and production efficiency is finally achieved.

Figure 1 is a three-dimensional model of an aluminum alloy transmission control housing. Outline size of product is 196mm*108mm*90mm, and weight is 0.664kg. Its structure is relatively complex and wall thickness is relatively uneven. Maximum wall thickness is 31.5mm and minimum wall thickness is less than 9mm. According to structure of transmission control housing casting and characteristics of low-pressure casting process, a bottom pouring closed pouring system is established using Catia software, which can not only ensure smooth filling of mold when metal liquid rises, but also reduce oxidation slag caused by gas and turbulence. Model is imported into ProCAST software for pre-processing and meshing. Material of transmission control housing is A356 aluminum alloy, and its main chemical composition is shown in Table 1. Basic information of A356 is queried through ProCAST software material database. Its thermophysical parameters are shown in Table 2. Mold material is H13 steel. According to Pascal's principle and empirical formula, process parameters of low-pressure casting are calculated and shown in Table 3. Initial pouring temperature is selected as 700℃, and mold preheating temperature is 300℃. Heat transfer coefficient between molten metal and mold is set to 1000W/(m2·K), heat transfer coefficient between molds is set to 3500W/(m2·K), heat transfer coefficient between mold and atmosphere is set to 20W/(m2·K). Initial cooling method is air cooling.

 

(a) Transmission control housing

 

(b) Casting system
Figure 1 Three-dimensional model of casting and casting system

Table 1 Chemical composition of A356 aluminum alloy (%)

Table 2 Thermophysical parameters of A356 aluminum alloy

Table 3 Low-pressure casting casting process parameters of transmission control housing
During solidification process of casting, due to thickness and heat transfer area of casting itself, overly thick part will cut off transmission path of sequential solidification, so that molten metal cannot be effectively compensated in time, shrinkage defects will be formed. Figure 2 shows shrinkage position of initial process scheme. Original process results show that casting cannot achieve a top-down solidification sequence at room temperature 20℃. Therefore, a cooling system needs to be added around casting to improve solidification temperature gradient of casting so that it can solidify sequentially.

 

Figure 2 Distribution of shrinkage cavities

 

Figure 3 Layout of cooling system pipelines

Table 4 Cooling process parameters for low-pressure casting of transmission control housing
In low-pressure casting process of aluminum alloy, pouring temperature and mold preheating temperature have a direct impact on casting forming. At the same time, temperature of cooling water will affect temperature of casting during solidification and thus size of defect volume. These three factors are controllable in low-pressure casting process. According to model after cooling, above three factors were selected as research objects, and Taguchi orthogonal test was used to perform multi-objective optimization of process parameters to study their influence on quality of castings. In this test, each factor selected 4 levels within appropriate range. Factor levels are shown in Table 5. Standard orthogonal table L16 (43) was used to arrange test plan. According to process and mechanism of metal crystallization, metal properties are related to secondary dendrite arm spacing (SDAS). As an important structural parameter of cast aluminum alloy, secondary dendrite arm spacing greatly affects mechanical properties of casting. The smaller secondary dendrite arm spacing value, the higher strength of casting. Size of secondary dendrite arm spacing is directly affected by alloy composition and local solidification time. In addition, shrinkage defects are also an important factor affecting quality and fatigue life of castings. Therefore, secondary dendrite arm spacing and shrinkage volume are used as indicators to evaluate quality of castings.

Table 5 Orthogonal test factor level table

Table 6 Numerical simulation results and signal-to-noise ratio

Table 7 Variance analysis table
In order to obtain influence weight of each factor on evaluation index and optimal level of each evaluation index, signal-to-noise ratio of four indicators is analyzed by mean and range, and results are shown in Table 8. Analysis shows that signal-to-noise ratio of secondary dendrite arm spacing (SDAS) decreases with increase of pouring temperature and mold preheating temperature, indicating that the lower pouring temperature and mold temperature, the higher signal-to-noise ratio, which means that the smaller secondary dendrite arm spacing value, the higher quality of casting. In general, influence of each factor on SDAS is mold preheating temperature> pouring temperature> cooling water temperature; influence on shrinkage volume is cooling water temperature> pouring temperature> mold preheating temperature.

Table 8 Mean and range analysis table

 

Figure 4 Effect of different process parameters on SDAS and signal-to-noise ratio of shrinkage and shrinkage cavity volume

Table 9 Comparison of process scheme results

 

Figure 5 Trial product of transmission control housing

(1) Through variance analysis, it is found that preheating temperature of die-casting mold of transmission control housing has the most significant effect on secondary dendrite arm spacing (SDAS); cooling water temperature has the most significant effect on shrinkage cavity volume value.
(2) Through mean and range analysis, optimal combination scheme of die-casting process of transmission control housing is casting temperature of 690℃, mold preheating temperature of 310℃, and cooling water temperature of 40℃. Feasibility of process scheme is verified through trial production.