Abstract: Water-cooled motor housing is one of key components of new electric drive assembly for new energy vehicles. It features high heat dissipation, lightweight design, and high mechanical performance, while also placing high demands on internal quality and density of housing. This paper analyzes failure modes of a die-cast inner housing for a new energy vehicle. Numerical simulation was used to analyze aluminum liquid filling state under various gating systems, thereby selecting optimized gating system. Simultaneously, stress was relieved through T5 heat treatment to improve performance. DOE verification revealed optimal hot-fitting and friction stir welding process parameters to minimize their impact on casting performance. Results show that a reasonable gating system, T5 heat treatment, hot-fitting, and friction stir welding processes are key to ensuring high performance of water-cooled motor inner housing.
New energy vehicles represent main direction for transformation, upgrading of global automotive industry and its green development, and are also a strategic choice for high-quality development of China's automotive industry. With increasing production and sales of new energy vehicles year by year, electric drive industry has developed rapidly. As a key component, demand for motor housings has also increased. Current development status of new energy motor housings is characterized by rapid growth and technological innovation. In terms of industrial chain and core technology, quality and performance of new energy motor housings, as one of core components of new energy vehicles, directly affect power, climbing ability and acceleration performance of the whole vehicle. Main cooling methods for new energy drive motors are water cooling and oil cooling. Water-cooled motors have advantages in energy saving and cost reduction. New energy water-cooled motor housings have efficient, reliable and stable thermal management functions. Water-cooled motor housing adopts a sandwich structure with spiral water channels in sandwich. Through connection of inlet and outlet water pipes, efficient water circulation cooling is achieved, which significantly improves heat dissipation effect of motor. In addition, design of water-cooled motor housing also has characteristics of lightweighting. Use of friction stir welding technology further reduces weight of the whole vehicle, making it easier to transport and install. This design not only helps to improve operating efficiency of motor, but also extends service life of motor. Water-cooled motor housings typically employ efficient cooling systems and precision manufacturing processes to ensure excellent performance even at high temperatures.
Water-cooled motor housing is made of A380 die-cast aluminum alloy and consists of two die-cast parts: an outer shell and an inner shell. These parts undergo a series of processes including vertical lathe operation, T5 heat treatment, hot fitting, friction stir welding, and post-weld assembly machining to form a sealed water-cooled cavity motor housing. There are many failure modes for motor inner shell casting, including internal non-density failure, cracking failure, wear failure, corrosion failure, and deformation failure. These failure modes are result of multiple factors, including material properties, design and manufacturing processes, and usage conditions. In this study, the most significant failure mode of water-cooled motor inner shell was internal non-density failure. Internal non-density in motor inner shell casting leads to a decrease in its mechanical properties, making it prone to leakage, cracking, and deformation during use, thus affecting product's service life and safety. Slight internal density defects can lead to coolant leakage, affecting motor efficiency; severe internal density defects can cause inner casing to crack, affecting normal operation of motor.
Factors affecting internal density of motor inner casing castings include composition and melting process of die-cast aluminum alloy, design of die-casting gating system, die-casting process parameters, T5 heat treatment process, hot-fitting press-fitting process, and friction stir welding process. This study, under condition of fixed die-cast aluminum alloy composition, melting process, and die-casting process parameters, focuses on four modules: gating system, T5 heat treatment process, hot-fitting press-fitting process, and friction stir welding process, exploring their impact on performance of motor inner casing.

Motor inner casing has a cylindrical structure, the entire product belongs to leak testing and sealing area; therefore, requirement for its internal density is extremely high. To ensure high internal density of motor inner casing, design of casting gating system must be very reasonable. By rationally designing location, shape, size of ingate, and ensuring smooth molten metal flow, gating system can effectively guarantee internal density of die-cast parts. In the early stages of development, based on structure and performance requirements of motor's inner casing, suitable gating locations were identified, and four different gating systems were pre-designed (see Figure 1). Scheme 1 uses a beak-type single-sided gating system; Scheme 2 uses a flat-overlapping center gating system; Scheme 3 uses a flat-overlapping single-sided gating system; and Scheme 4 uses a beak-type center gating system.

 

Mold flow analysis of four different gating schemes was performed using Magma software (see Figure 2). Option 1 uses a beak-type gate, but when aluminum melt has filled mold cavity, a significant amount of gas remains on the sidewalls of casting, posing a risk of trapped and entangled gas. Option 2 uses a center gate, where aluminum enters mold cavity from center of inner shell. Gate distribution is uniform, resulting in consistent aluminum filling, smooth flow, and no turbulence or entrapment. Gas within mold cavity can escape smoothly, minimizing risk of porosity. Option 3 uses a flat-bottom gate, but due to depth of inner shell cavity, filling is not smooth, leading to a high gas content on the sidewalls after filling. Option 4 uses a beak-type gate, pouring onto the sidewalls of inner shell. While filling is smooth, severe entrapment and entrapment occur, causing rapid temperature drop at entrapment location and a significant risk of poor forming and exposed porosity. Therefore, based on mold flow results, Option 2 (flat-bottom center gate) was ultimately selected for motor inner shell gating system, with gate located on the wall of machined hole at the bottom of inner shell cylinder.

 

After confirming optimal feed position design (flat-type center gating), slag removal position was designed. Final slag removal position design is shown in Figures 3 and 4. Material particle tracking analysis of gating system in Scheme 2 revealed that while a straight-line gating system allows for rapid gas discharge from mold cavity to vent end of motor housing, some gas impacts casting tail and flows back into casting, creating air entrapment and affecting internal density of casting. Therefore, Scheme 2 was further optimized by designing ingate with an angle of <90° to sprue and using a rotary gating method to fill mold cavity. Material particle tracking analysis of mold flow is shown in Figure 4. Molten aluminum is spirally filled inside cylindrical casting of motor housing, expelling gas from mold and preventing significant aluminum backflow. This improves internal density of motor housing casting and enhances its performance.

 

T5 heat treatment of motor inner shell refers to artificial aging treatment of casting after high-temperature die casting and cooling. This process helps to eliminate internal stress generated during casting and improve its mechanical properties. Motor inner shell is made of A380 die-cast aluminum alloy. Mechanical property requirements for sampled casting body are: tensile strength ≥256 MPa, elongation ≥1%, and yield strength ≥150 MPa. Natural aging and T5 heat treatment were performed on this motor inner shell, and mechanical properties were measured at same location under 6 different sample conditions. Results are shown in Table 1. Table 1 shows that mechanical properties of sampled casting body after T5 heat treatment (210 ℃ holding for 75 min + air cooling for 20 min) meet requirements. T5 heat treatment effectively prevents formation of porosity defects by homogenizing internal stress and reducing stress concentration, thereby improving performance of casting.
Tab.1 Mechanical properties of motor shell body sampling

Basic principle of heat fitting for water-cooled motor housings is to utilize characteristics of thermal expansion and contraction to easily press inner housing of motor into outer housing. First, stator holes of motor housing need to be heated to a high temperature, allowing thermal expansion to achieve a larger volume. Then, inner housing of motor is quickly pressed into it. Finally, as heat-fitted part cools, its volume will shrink, generating friction between inner and outer housings, thus achieving a stable assembly.
Heat fitting process for motor housings has characteristics of fast assembly speed and stable connection, but there are also some precautions. For example, heating temperature and heating time need to be controlled during heating to avoid overheating, which could cause fatigue or deformation of motor housing material, leading to cracking of motor housing and affecting its performance.
Principle for setting heat fitting temperature is to minimize temperature while ensuring that no scraping occurs during heat fitting process and that assembly is in place, thus avoiding any impact on tightness of motor housing. To ensure no interference during press-fitting of motor's inner and outer casings, gap between stator holes in motor's inner and outer casings should be controlled within 0.03 to −0.07 mm. Conversely, after thermal expansion of stator holes in motor casing, hole diameter needs to be controlled to be ϕ209.12 to ϕ209.24 mm. Experimental research on relationship between thermal fitting temperature and diameter of thermal fitting holes in motor casing is shown in Figure 5. Inner diameters 1, 2, and 3 are ϕ248.85 mm, respectively. It was found that when diameter of thermal fitting holes in motor casing is within range of ϕ209.12 to ϕ209.24 mm, thermal fitting heating temperature parameter window is 120–140 ℃. Relationship between thermal fitting heating time and temperature is shown in Figure 6. When thermal fitting temperature is between 120 and 140 ℃, heating time parameter window is 15–19 s.

 

In friction stir welding, welding pressure is crucial for obtaining sufficient frictional heat. Simultaneously, pressure limits outflow of plastic fluid and ensures weld formation. Rotation speed and feed rate are main parameters affecting heat input, thus influencing joint performance. For a given shape of friction stir weld joint and assuming good internal quality of casting in welding area, key factors affecting internal density of weld are welding rotation speed, feed rate, and pressure. With other factors remaining constant, optimal process parameters (rotation speed, feed rate, and pressure) for friction stir welding of motor housings were determined through orthogonal experimental research. Levels of three factors are shown in Table 2.
Tab.2 Factors levels of process

Results were verified by DOE experiments and are shown in Table 3. It can be seen that internal quality of motor housing is qualified under both following parameters: rotation speed of 750 r/min, feed speed of 250 mm/min, pressure of 0.3 mm; rotation speed of 850 r/min, feed speed of 350 mm/min, pressure of 0.2 mm. However, latter caused severe wear of friction stir welding head. To ensure internal density of motor housing casting, final selected parameters for friction stir welding were 750 r/min, 250 mm/min, and 0.3 mm.
Tab.3 Results of orthogonal test

(1) Inner housing of a new energy vehicle motor adopts a pouring method from center of bottom of inner housing. Gate is designed with an angle of less than 90° with sprue. Mold cavity is filled by a rotary pouring method, which can solve problem of difficult filling of cylindrical structure of motor inner housing, thereby improving internal density of motor inner housing casting and enhancing its casting performance.
(2) T5 heat treatment (aging temperature 210 ℃, holding time 75 min, air cooling 20 min) effectively prevents formation of porosity defects by homogenizing internal stress and reducing stress concentration, thus improving performance of motor inner shell.
(3) Hot-fitting process can amplify casting defects, thereby reducing internal density of casting. Hot-fitting process has the least impact on performance of motor inner shell when temperature is 120~140 ℃ and heating time is 15~19 s.
(4) Based on a certain shape of stirring head and good internal quality of casting in welding area, the key factors affecting internal density of welding area of motor inner shell are welding speed, feed rate, and pressure. DOE tests verified that internal quality of welding area of motor inner shell is best and casting performance is ideal when speed is 750 r/min, feed rate is 250 mm/min, and pressure is 0.3 mm.