Structural characteristics of a two-in-one casing of a new energy vehicle are introduced. An initial die-casting process was designed and trial production was carried out. With the help of CAE software, we conducted in-depth research on local defects of shell specimen, analyzed their causes, determined direction of optimization and improvement of gating system, solved defects of internal air holes and slag holes in castings.
In recent years, China's new energy vehicle industry has developed rapidly, and new energy vehicles are becoming a "new force" in automotive industry. Drive systems such as motors and gearboxes are core components of new energy vehicles. Products designed by integrating power systems and drive systems such as motors and gearboxes are increasingly favored by new energy vehicle industry. High-pressure casting production can significantly reduce product wall thickness while maintaining structural strength. Moreover, die casting is close to net shape, cost is significantly reduced, and production efficiency is greatly improved. High-pressure casting production of aluminum alloy two-in-one housings consisting of motors and gearboxes has been recognized by major companies. Since die-casting process uses high-speed, high-pressure filling, gas is easily involved during die-casting filling process, resulting in problems such as pores and oxidized inclusions in die-casting parts.

Aluminum alloy two-in-one housing of a new energy vehicle is shown in Figure 1. Structure mainly consists of two parts, one part is motor housing and the other part is gearbox housing. Outline dimensions of two-in-one housing are 468 mm * 312 mm * 286 mm, the thickest part is 29.8 mm, basic wall thickness of motor housing part is 7 mm, and basic wall thickness of gearbox housing part is 5 mm. Structure is relatively complex, housing volume is 4 251 cm³, weight is 11.8 kg, and planned output is 100,000 pieces/year. Material is Al-Si-Cu alloy, implementation standard is JISH5302-2000, brand is ADC12, its liquidus temperature is 592 ℃ and solidus temperature is 539 ℃. Due to need for motor temperature control, a cooling water channel is designed in housing, so there is a limit on leakage. Test at a pressure of 300 kPa at normal temperature, and leakage amount should be within 0.4 cm³/min after maintaining pressure for 40 s. In addition, shell needs to be welded, it is friction stir welded with water jacket ring and water jacket.

 

Figure 1 A certain two-in-one housing

Two-in-one shell casting is large in size, and process after aluminum liquid passes through pouring system is long. It is necessary to choose a filling scheme with a shorter process. After program demonstration, the overall "Y" type pouring scheme was determined, as shown in Figure 2a. In order to reduce energy loss at sprue during filling process, an olecranon-shaped sprue was adopted. Set slag bags at ends respectively, connect slag bags through exhaust duct, and collect them into one exhaust port for vacuuming.

 

Figure 2 Pouring and cooling system

Pouring and drainage system can ensure reasonable arrangement of various areas during casting filling, smooth exhaust, and minimize involvement of gas. Basic wall thickness of the two parts of shell is 7 mm and 5 mm respectively, gate speed is 28~35 m/s, gate cross-sectional area is 1824 mm2, gate thickness is 5 mm, and punch diameter is 150 mm, filling degree of material cylinder is 42%, cross-sectional area of gate and piston is 1:9.7. According to Bernoulli's principle, when flow velocity in gate is 35 m/s, punch speed is 3.6 m/s. DCC2500 horizontal cold chamber die-casting machine of Lijin Group was selected, with a clamping force of 25 000 kN.
Overflow system uses a slag bag and vacuum exhaust. Use of vacuum reduces contact oxidation between aluminum liquid and air in cavity during filling process. Slag bag helps to discharge release agent, lubricating particles, oxidized slag in contact with air, gas involved in flow front end that are mixed into aluminum liquid during die casting from mold cavity and store them in slag bag to ensure quality of casting. Vacuum system uses Haiwang Company's HVY800-100SM V5 vacuum machine, which has a vacuum capacity of 100 m³/h and is equipped with a hydraulic vacuum valve. During production process, vacuum gauge shows 100 mbar. Table 1 lists process parameters of pouring and drainage system.

Table 1 Pouring and drainage system parameters

Set up cooling water in wall thickness area of casting to ensure cooling effect in wall thickness area, avoid shrinkage and shrinkage holes in wall thickness area. Design of cooling system is affected by position of ejector pin and core, and it is difficult to completely take into account all wall thickness areas of casting. However, it is still necessary to take into account thermal balance of mold and cooling of thick wall area of casting as much as possible. Cooling system is shown in Figure 2b. Blue is normal pressure cooling water circuit, green is mold temperature machine oil circuit, red and magenta are high-pressure point cooling.

Anycasting software was used to numerically simulate designed die-casting pouring process plan to analyze filling effect of aluminum alloy liquid and whether design caused internal defects in casting.

According to initial die-casting process design, calculation conditions are set in numerical preprocessing: pouring temperature 670 ℃; piston diameter 150 mm; low injection speed is 0.8 m/s, high speed is 4.1 m/s; mold material is SKD61 steel, preheating temperature is 180 ℃; cooling medium is set to water, and inlet water temperature is controlled to 25 ℃; default setting of Anycasting is selected for oxidation slag inclusion, 515 ℃ corresponds to dimension 0 and pouring temperature of 670 ℃ corresponds to dimension 1; vacuum setting is 50 mbar.
Numerical simulation results of casting filling process are shown in Figure 3. It can be seen from figure that aluminum liquid enters mold cavity from left and right inner runner at the same time. During process, aluminum liquid first fills gearbox part below runner, then fills motor shell part. When motor housing part is basically filled, gearbox part without sprues is filled. Circumference of movable mold side of motor shell is filling end, and gearbox shell part without sprues is also filling end. Filling process was smooth, with less air entrainment. Filling sequence was basically consistent with expectations. Mold cavity was completely filled, and there was no under-pouring.

 

Figure 3 Filling sequence

Based on above analysis, trial production was carried out on Lijin DCC2500 die-casting machine. After die-casting trial production, castings were first subjected to X-ray flaw detection, all areas were scanned and enhanced to detect internal quality of die-castings. After multiple rounds of debugging, it was found that internal defects of two-in-one motor housing were mainly concentrated at a suspension hole below sprue, as shown in Figure 4, which could not be significantly reduced and failed to meet customer's acceptance standards.

 

Figure 4 Defects of pores and slag holes inside castings
Particle tracking function of Anycasting software during mold filling process is used to obtain information such as streamlines and vortices in flow field, as shown in Figure 5. It can be seen from figure that during mold filling process, aluminum liquid is filled downward at a very high speed through runner, then fills in reverse direction after encountering resistance below suspension hole, and filled along thick wall of shell, forming a rewinding vortex. Gas in mold cavity cannot be discharged. In addition, granular aluminum slag cooled at the front end of aluminum liquid cannot be discharged from mold cavity. It mixes with undischarged gas to cause local pores and slag holes in casting.

 

Figure 5 Particle tracking analysis of mold filling process

Use UG software to optimize two-in-one motor housing gating system. As shown in Figure 6, first cut off sprue near suspension hole and change it to a slag bag to discharge defects caused by eddy current entrainment during filling process of suspension hole to outside of casting, first option; due to blocking of one runner, the overall runner area is reduced, so sprues on both sides of cut runner are enlarged as second option; runner on the left side of cut runner is not enlarged as the third option to verify mold filling effect.

 

Figure 6 Improvement plan for gating system
Above three pouring system solutions were used, and Anycasting software was used to perform numerical simulation analysis to check filling effect of suspended hole below sprue. As shown in Figure 7, in the first option, aluminum liquid enters slag bag on the left side of slag bag opening and then squeezes into mold cavity on the right side; in second option, aluminum liquid enters slag bag on the right side of slag bag opening, then is squeezed in large quantities on the left side and discharged into mold cavity; in third plan, after aluminum liquid was introduced into slag package, no backflow into cavity was found.

 

Figure 7 Numerical simulation results for different gating system improvement plans
In summary, pouring system of Plan 3 is the most ideal; based on Plan 3, increase volume of slag bag and improve mold filling effect, as shown in Figure 8a as final pouring system improvement plan.

 

Figure 8 Final pouring system scheme and test mold verification results

In the process of this research, firstly, by analyzing structural characteristics of castings, initial die-casting process was designed and trial production of castings was carried out; secondly, by analyzing defects of trial production castings and adjusting design of gating system multiple times, the overall scheme of gating system was determined; thirdly, local defects in suspension holes of castings were further analyzed and an improvement plan was determined. Finally, based on improvement plan, volume of slag bag was increased to determine final optimized pouring system structure. In above process, numerical simulation technology was used many times to analyze causes of defects and find direction for process improvement. Mold trial verified that gating system was effective, gas and slag inclusions entered slag bag, eliminating air holes and slag hole defects in suspension holes, as shown in Figure 8b.

In response to forming requirements of a new energy car's aluminum alloy two-in-one shell, Anycasting software and process experiments were repeatedly used to improve die-casting process. Causes of casting defects caused by local eddy currents and air entrainment during casting filling process were analyzed and discovered. Finally, an optimized gating system design was obtained. After mold testing, feasibility of optimized plan was proved.