As a crucial component of electric drive system of new energy vehicles, motor housing rear end cover primarily supports motor rotor and fixes motor stator, while preventing external substances such as dust and moisture from entering motor. Therefore, it places certain requirements on product's structural strength and airtightness. Furthermore, bearing hole area uses partial inserts for post-forming, requiring mold to consider positioning and clamping of inserts, increasing difficulty of mold manufacturing and production process debugging. Therefore, ensuring stable mold production is crucial, initial mold design and subsequent process improvement are extremely important.
This paper develops a die-casting process for aluminum alloy motor housing rear end covers. Initially, gating system was rationally arranged according to product structure. Numerical simulation software was used to analyze filling and solidification of gating system. During actual production, it was found that porosity in casting was difficult to eliminate. Different solutions were adopted for different areas based on product structure. For thin-walled areas with porosity where slag pockets cannot be placed at the end of product, inserts for venting and increased wall thickness were used to improve fluidity of molten aluminum. For dense porosity in thick-walled areas at the end of gating system, improved cooling was implemented to accelerate local solidification and increase thickness of dense layer on product surface. For thick-walled porosity at feed inlet, gating system was analyzed, and localized feeding was strengthened. Trial production results show that optimized scheme effectively improved the overall porosity of product and reduced scrap rate.
End cover part for motor housing of a new energy vehicle is shown in Figure 1. Part has dimensions of 397.98 mm * 91.48 mm * 286.46 mm, a die-cast weight of 5.71 kg, an average wall thickness of 8.06 mm, a projected area of 74,759 mm², and is made of ADC12 aluminum alloy. The overall wall thickness of casting is relatively thick, with a maximum wall thickness of 33 mm and a minimum wall thickness of 4 mm, indicating uneven wall thickness. Partial inserts are used in bearing housing of product. Bearing housing inserts are made of 45# steel, heat-treated, with a hardness (HRC) of 24~30. All external dimensions must conform to assembly requirements of drawings. Motor mating surface and junction box cover mounting surface of product have sealing requirements. Bearing holes and half-shaft holes are used to install motor rotor, and there are certain requirements for exposed air holes after machining. In addition, there should be no obvious burns or scratches at any rounded corners, product must be free of burrs and flash. Product requires an airtightness test, specifically: a test pressure of 22 kPa and an allowable leakage rate of <5 mL/min.

 

Figure 1: Schematic diagram of rear end cover of motor housing
1. Motor bearing hole 2. Motor mating sealing surface 3. Half-shaft hole 4. Junction box cover mounting surface
Based on product's structural analysis, a suitable moving and fixed mold parting line is selected. Furthermore, product's side ejection is done in two directions, meaning mold needs to be designed with two core-pulling sliders for parting. Moving and fixed molds and sliders separate product to ensure normal product ejection and production. Figure 2 shows fractal design of rear end cover of motor housing. Figure 3 shows wall thickness analysis of rear end cover of motor housing. Based on product wall thickness, analysis from fixed mold side shows that a small area on the left side has a thicker wall, while right half has a thicker overall wall (see Figure 3a). Analysis from moving mold side shows that moving mold has a ribbed structure, with thick-walled ribs mainly on the left side (see Figure 3b). Therefore, wall thickness on both sides of product is extremely uneven. To ensure sufficient aluminum filling in thicker areas, gate layout in these areas needs to be strengthened.

 

Figure 2 Partition Design of Rear End Cover of Motor Housing

 

Figure 3 Wall Thickness Analysis of Rear End Cover of Motor Housing
The key to gating design lies in selection of gate. Generally, location and form of gate need to be determined based on shape, structure, and precision requirements of casting. Based on product structure analysis, product can be gated on the side of slider 1 and its opposite side. However, considering key locations of product, except for motor mating surface which affects the entire product perimeter, bearing holes and axle holes are located away from slider 1, junction box cover mounting surface is on the back of slider on moving mold side and is affected by material blocking effect of slider 1. Therefore, gate is placed on opposite side of slider 1 to ensure that gate position is close to bearing holes and axle holes, and junction box cover mounting surface is not affected by material blocking effect of slider 1. For internal gate layout, four internal gates are arranged on machined end face opposite slider 1, and one internal gate is arranged on each of non-machined positions on both sides to enhance filling on both sides. Simultaneously, a bridging passage is set in the middle of outermost circular hole of product. Final gating system design is shown in Figure 4.
Magma simulation analysis was used. Initial conditions for mold flow analysis are shown in Table 1, and rationality of relevant processes was checked using a p-Q diagram. Rationality of relevant process settings is shown in Figure 5.

 

Figure 4. Gating System for Rear End Cover of Motor Housing

Table 1. Mold Flow Simulation Settings

 

Figure 5. P-Q² Diagram of Mold Flow Settings
Filling process of the entire casting was analyzed using Magma simulation, as shown in Figure 6. It can be seen that when aluminum liquid fills for 2.604 s, molten metal from two ingates closest to main runner fills cavity first (see Figure 6a); when filling for 2.625 s, all ingates begin to fill cavity (see Figure 6b); when filling for 2.658 s, the entire casting is filled (see Figure 6c). Throughout the entire filling process, molten metal filling time through ingates is 44 ms, and molten metal in thick-walled areas of product fills first, with no underfilling. Gating system meets product filling requirements.
The overall air pressure distribution of casting is shown in Figure 7. Areas with higher air pressure values were found in slag pockets and overflow channels of gating system, as well as in thicker ribs of product, indicating a higher probability of porosity in these locations. Solidification process of product is shown in Figure 8. It can be seen that very few edges solidified 3 seconds after filling (Figure 8a); vast majority of product solidified 12 seconds after filling (Figure 8b); and slower solidification occurred in thicker wall areas of product.
Cooling water was installed in thick-walled areas of casting to ensure effective cooling and prevent shrinkage porosity and shrinkage cavities. Direct cooling water was used around perimeter of product, while point cooling was designed for other areas. Point cooling needles were also used for core needles with a diameter of ϕ5 mm or more. Final cooling system is shown in Figure 9.

 

Figure 6: Casting filling process

 

Figure 7: Simulation of air pressure in cavity of motor housing rear end cover

 

Figure 8: Product solidification process

 

Figure 9: Mold cooling system
Based on mold design, a 16,000 kN Idra die-casting machine was used for production. Punch diameter was selected as 120 mm, effective injection stroke was 620 mm, mass of molten aluminum passing through ingate was 6.15 kg, theoretical high-speed position was set at 380 mm, injection punch height speed was 4 m/s, and pressure boosting position was set at 560 mm. These were used as basic parameters for production debugging. Machine was equipped with fully automated production equipment to effectively ensure stability of production process. During actual production debugging, high-speed position was appropriately adjusted to verify product quality. Ultimately, with a high-speed position of 420 mm, a high-speed speed of 4.2 m/s, and a pressure boosting position of 560 mm, product quality was ideal. However, X-ray flaw detection revealed localized unstable porosity. Porosity of product is shown in Figure 10.
Internal porosity standards: Location 1 has a wall thickness of less than 9.5 mm at the very end of casting; locations 2-4 have a local wall thickness exceeding 9.5 mm, are classified as Level 2 according to aluminum alloy wall thickness and porosity standards. Post-processing appearance standards: Locations 1 and 4 have no limit on number of non-connecting pores smaller than ϕ0.25 mm, and a maximum of 2 non-connecting pores less than ϕ2 mm * 2 mm in depth per 100 mm length; Locations 2 and 3 have no limit on number of non-connecting pores smaller than ϕ0.25 mm, and a maximum of 2 non-connecting pores less than ϕ1 mm * 1 mm in depth per 100 mm length. It can be seen that post-processing appearance standards are stricter than internal standards. Production under these conditions and CNC machining inspection revealed that X-ray inspection showed exposed pores in some locations (see Figure 11). Therefore, further improvement is needed regarding product porosity.

 

Figure 10: X-ray inspection results of motor housing rear end cover

 

Figure 11: Results of exposed pores in product
See Figure 12 for a partial structure of square hole. Square frame hole is located at water outlet of gating system, in the middle of product. Local wall thickness of square frame is 2.8 mm. Because slag pots cannot be placed around square frame hole, localized gas escape is difficult. Simultaneously, wall thickness is relatively thin compared to the overall product, resulting in poor localized flowability of aluminum material and a tendency for cold material accumulation.
Analysis shows that using slag pots for venting and reducing cold material accumulation is the most effective method. Due to product structure limitations, directly adding slag pots is not feasible; therefore, a multi-pronged approach is adopted. Figure 13 shows measures to improve porosity of square frame hole sidewall. Measure 1 involves cutting and inserting parts along exposed sidewalls of porosity (see Figure 13a), utilizing parting surface of mold inserts for venting. Measure 2 involves increasing local wall thickness to 3.8 mm (see Figure 13b), enhancing flowability of molten aluminum and thus improving localized cold material accumulation. After implementing these combined measures, localized porosity was significantly improved, meeting product quality requirements.

 

Figure 12 Local Structure of Square Hole

 

Figure 13 Improvement Measures for Porosity on Side Wall of Square Hole
Porosities at processing positions 2 and 3 are on same side of product, as shown in Figure 14, on water tail side of product's gating system, which is also a relatively thick wall position. For thick-walled areas of product, solidification is generally slower. When surrounding thin-walled areas solidify, local shrinkage compensation channel is broken, making internal shrinkage porosity more likely. Furthermore, production data shows that internal porosity quality meets standard requirements; only exposed porosity caused by processing does not meet standard. Therefore, improvement direction is mainly to address issue of avoiding exposure.
Regarding exposed processed surface of product at water tail, textured surface can effectively vent and cool porosity on floating surface. Improvement method 1 involves adding texture to local end face, but improvement effect is not significant. Therefore, for shrinkage porosity in thick-walled areas, local extrusion should be used. Extrusion pins should be used to compensate for shrinkage before product is completely solidified. This approach is theoretically feasible, but densely packed pores on product are relatively scattered, and a single extrusion pin cannot cover the entire area, while multiple extrusion pins are impractical. Therefore, this solution is not feasible. Considering lowering local mold temperature, improvement method 2 involves adding a cooling point on exposed end face to enhance local cooling and solidification, reducing local shrinkage porosity. Simultaneously, lowering mold surface temperature results in a thicker, denser layer on product surface, thus reducing risk of exposed pores during machining. Figure 15 shows improvement measures for pores on motor mating surface.
Regarding pores at machining location 4, machining allowance is checked and found to be 0.6~0.8 mm, which is within normal machining allowance range. Surrounding wall thickness is approximately 12 mm. Pores are located at gating inlet, so there is no issue of cold material not being able to escape at the end. A water-cooling structure is also arranged inside pores to mitigate shrinkage porosity caused by cooling in thick-walled area. Further analysis of product gating reveals that although there is a gating system at this location, it directly faces material reduction position (see Figure 16), hindering filling of local area by molten aluminum. Therefore, pores in this local location may be caused by insufficient molten aluminum filling.
To address insufficient aluminum molten filling, moving mold was reduced in size, and gating system was modified, with a reinforced feed point at axle hole location selected. Method 1 involved widening gate in a localized runner, exceeding width of corresponding shrinkage groove. This resulted in product directly impacting fixed mold core pin, causing shrinkage cavities around core pin after impact and heating, and frequent core pin breakage, increasing mold failure rate. Method 2 involved directly bypassing fixed mold core pin and moving mold core-pulling pin, adding a runner between them, simultaneously disconnecting feed line to prevent molten aluminum from bridging and causing backflow and coiling. Improvement measures are shown in Figure 17. After improving gate and bridging according to Method 2, porosity of axle hole was significantly reduced.

 

Figure 14: Location and Wall Thickness of Region

 

Figure 15: Measures to Improve Porosity on Motor Mating Surface

 

Figure 16: Local Gating of Half-Shaft Hole

 

Figure 17: Internal Coiling and Gate Modification
Conclusion
Through development of die-casting process for rear end cover of aluminum alloy motor housing, a feasible gating scheme was selected based on product parting structure and gating principles. Numerical simulation analysis was used to assess rationality of product's gating scheme and gating system. Simultaneously, relevant measures were implemented on mold to shorten project development cycle. By comparing problems encountered in actual production with simulated conditions, product's gating system was further optimized, forming process conditions were improved, and casting quality was enhanced.