Abstract: This paper introduces structural characteristics of a two-in-one housing for a new energy vehicle. An initial die-casting process was designed and pilot-produced. Using CAE software, local defects in housing specimens were studied in depth, their causes analyzed, direction for optimizing and improving gating system determined. Porosity and slag inclusions within casting were resolved.
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 transmissions are core components of new energy vehicles, products that integrate power system and drive system into one unit are increasingly favored by new energy vehicle industry. High-pressure casting can significantly reduce product wall thickness while maintaining structural strength. Moreover, die casting is close to net-shape forming, significantly reducing costs and greatly improving production efficiency. High-pressure casting production of aluminum alloy two-in-one housings composed of motors and transmissions has been recognized by major companies. Because die-casting process uses high-speed, high-pressure filling, gas is easily entangled during filling process, often resulting in porosity and oxide inclusions in die-cast parts.

Aluminum alloy two-in-one housing for a new energy vehicle is shown in Figure 1. Its structure mainly consists of two parts: a motor housing and a gearbox housing. The overall dimensions of two-in-one housing are 468 mm * 312 mm * 286 mm, with a maximum thickness of 29.8 mm. Basic wall thickness of motor housing is 7 mm, and basic wall thickness of gearbox housing is 5 mm. Structure is relatively complex, with a volume of 4251 cm³ and a weight of 11.8 kg. Planned production volume is 100,000 units/year. Material is an Al-Si-Cu alloy, conforming to JIS H5302—2000, with grade ADC12. Its liquidus temperature is 592 ℃ and solidus temperature is 539 ℃. Due to need for motor temperature control, cooling water channels are designed inside housing, thus limiting leakage. Under a pressure test of 300 kPa at room temperature, after holding pressure for 40 seconds, leakage should be within 0.4 cm³/min. In addition, shell requires welding, it is subjected to friction stir welding with water jacket ring and water jacket.

 

Fig. 1 Two-in-one housing

Two-in-one shell casting is large in volume, and flow path of molten aluminum through gating system is long. Therefore, a filling scheme with a shorter flow path needs to be selected. After scheme demonstration, a "Y" type gating and venting scheme was determined, as shown in Figure 2a. To reduce energy loss at gating point during filling, an eagle-beak type gating system is adopted. Slag bags are set at the ends, and slag bags are connected by venting channels, converging into a single vent for vacuuming.

 

Fig. 2 Gating, overflow, and cooling systems
2.1 Gating and Venting System Design
Gating and venting system ensures that each area is rationally configured during casting filling, venting is smooth, and gas entrapment is minimized. Shell consists of two main parts with basic wall thicknesses of 7 mm and 5 mm, respectively. Ingate velocity is 28-35 m/s, ingate cross-sectional area is 1824 mm², ingate thickness is 5 mm, punch diameter is 150 mm, cylinder filling degree is 42%, and ratio of ingate cross-sectional area to piston cross-sectional area is 1:9.7. According to Bernoulli's principle, when ingate flow velocity is 35 m/s, punch velocity is 3.6 m/s. A DCC2500 horizontal cold chamber die-casting machine from L.K. Group is selected, with a clamping force of 25,000 kN.
Overflow system employs a slag bag and vacuum venting. Vacuum venting reduces contact oxidation between molten aluminum and air in mold cavity during filling. Slag bag helps to expel mold release agents, lubricating particles, oxide inclusions in contact with air, gases entrained at flow front from mold cavity and store them in slag bag, ensuring quality of casting. Vacuum system uses a Haiwang HVY800-100SM V5 vacuum machine with a vacuum capacity of 100 m³/h, equipped with a hydraulic vacuum valve. During production, vacuum gauge displays 100 mbar. Table 1 lists process parameters of gating and overflow system.
Table 1 Gating and overflow system parameters

2.2 Cooling System Design
Cooling water is installed in thick-walled areas of casting to ensure effective cooling, prevent shrinkage porosity and cavities. Due to influence of ejector pin and core positions, cooling system design cannot fully cover all thick-walled areas of casting, but it still aims to balance mold thermal balance with cooling of thick-walled areas of casting. Cooling system is shown in Figure 2b; blue represents atmospheric pressure cooling water circuit, green represents mold temperature control oil circuit, red and magenta represent high-pressure point cooling.

Designed die-casting gating process was numerically simulated using Anycasting software to analyze filling effect of molten aluminum alloy and whether design resulted in internal defects in casting.
3.1 Numerical Simulation Analysis
Based on initial die-casting process design, calculation conditions were set in numerical preprocessing: pouring temperature 670 ℃; piston diameter 150 mm; injection speed 0.8 m/s for low speed and 4.1 m/s for high speed; mold material SKD61 steel, preheating temperature 180 ℃; cooling medium set to water, inlet water temperature controlled at 25 ℃; oxide inclusions selected using Anycasting default settings, with 515 ℃ corresponding to dimension 0 and pouring temperature 670 ℃ corresponding to dimension 1; vacuum setting 50 mbar.
Figure 3 shows numerical simulation results of casting filling process. As can be seen from figure, molten aluminum enters cavity simultaneously from both left and right inlets. During process, molten aluminum first fills gearbox section below gating, then motor housing section. Once motor housing section is mostly filled, gearbox section without a gating is filled. Circumference of moving mold side of motor housing is filling end, as is filling end of gearbox housing section without a gating. Filling process is smooth with minimal air entrapment. Filling sequence is basically consistent with expectations, cavity is completely filled without any incomplete filling.

 

Fig. 3 Filling sequence
3.2 Casting Defect Analysis
Based on above analysis, trial production was conducted on Lijing DCC2500 die-casting machine. After trial production, casting was first subjected to X-ray flaw detection. All areas were scanned and enhanced to inspect internal quality of die-casting. After multiple rounds of debugging, internal defects of two-in-one motor housing were mainly concentrated at a suspension hole below gating, as shown in Figure 4. These defects could not be significantly reduced and did not meet customer's acceptance standards.

 

Fig. 4 Blowholes and slag inclusions in casting
Particle tracking function of Anycasting software was used to obtain information such as streamlines and vortices in flow field, as shown in Figure 5. Figure shows that during filling process, molten aluminum fills downwards through gating system at extremely high speed. After encountering resistance below suspension hole, it reverses direction and fills along thick wall of shell, forming a swirling vortex. Gases within mold cavity cannot escape, and granular aluminum slag cooled at the front of molten aluminum also cannot exit cavity. This slag mixes with trapped gas, causing localized porosity and slag inclusions in casting.

 

Fig. 5 Particle tracking analysis during filling process

Gating system of two-in-one motor housing was optimized using UG software, as shown in Fig. 6. First, gating channel near suspension hole was cut off and replaced with a slag bag to remove defects caused by vortex air entrapment during filling process of suspension hole, as Scheme 1. Since blocking one gating channel reduced the overall gating channel area, gating channels on both sides of cut gating channel were enlarged, as Scheme 2. Gating channel on the left side of cut gating channel was not enlarged, as Scheme 3, to verify filling effect.

 

Fig. 6 Improving schemes of gating system
Above three gating system schemes were used to conduct numerical simulation analysis using Anycasting software to observe filling effect of suspension hole area below gating channel. As shown in Figure 7, in Scheme 1, molten aluminum enters slag pot from left side of slag ladle opening, then tends to be squeezed out into mold cavity from right side; in Scheme 2, molten aluminum enters slag pot from right side of slag ladle opening, then is squeezed out into mold cavity in large quantities from left side; in Scheme 3, no backflow into mold cavity was observed after molten aluminum flowed into slag pot.
In summary, Scheme 3 is the most ideal gating system. Based on Scheme 3, increasing slag ladle volume improves filling effect, as shown in Figure 8a, and serves as final improved gating system scheme.

In this study, firstly, through analysis of casting's structural characteristics, an initial die-casting process was designed and trial-produced castings were made; secondly, through defect analysis of trial-produced castings and multiple adjustments to gating system design, the overall gating system scheme was determined; thirdly, further analysis of local defects in casting's suspension holes determined an improvement scheme; finally, based on improved scheme, slag ladle volume was increased, and final optimized gating system structure was determined. Numerical simulation technology was used multiple times in above process to analyze causes of defects and identify directions for process improvement. Mold trial verified effectiveness of gating system. Gas and slag inclusions entered slag bag, eliminating porosity and slag inclusion defects at suspension hole location, as shown in Figure 8b.

 

Fig. 7 Numerical simulation results for different modified gating system schemes

 

Fig. 8 Final gating system scheme and die test verification results

To address forming requirements of a two-in-one aluminum alloy shell for a new energy vehicle, die-casting process was repeatedly improved and perfected using Anycasting software and process experiments. Causes of casting defects due to local eddies and air entrapment during filling process were analyzed and identified. Finally, an optimized gating system design scheme was obtained, and feasibiality of optimized scheme was proven through mold trial verification.