Process development of aluminum alloy motor casing castings for new energy vehicles

In recent years, under the requirements of energy saving, emission reduction and environmental protection, the research and development focus of automobile manufacturers is shifting from traditional fuel vehicles to new energy vehicles. The aluminum alloy motor casing is the core casting of the new energy vehicle's powertrain. Its top (open side) is connected to the inverter, the bottom is connected to the reducer, and the main shaft bearing is connected through the inlaid bearing bush. The sidewalls are often connected to the subframe by suspensions.

The structure of the motor casing is relatively complicated. The side wall of the motor housing surrounds the cooling water jacket. Ensuring the tightness of the water jacket is an important technical requirement of the product, and it is also the biggest casting difficulty of the product. At the same time, the shrinkage porosity of the upper and lower end surfaces and side walls of the motor casing is also a casting defect that needs to be avoided in process development.

1. Casting process scheme of motor housing

One end of the motor casing is connected to the reducer, and the other end is connected to the inverter. The diameter of the motor casing product is generally 350-400mm, and the height is generally 200-300mm. The main wall thickness is 5-6mm, and the side water jacket structure is various, but most of them are helical structure or semi-helical structure, and the wall thickness of the water jacket is generally 6-7mm. The product quality is 4-10kg. The commonly used aluminum alloy is A356.2 (ZL101A) (belonging to AlSi7Mg0.3 casting alloy), which adopts T6 heat treatment.

Using the core assembly process to produce motor casing castings is one of the mainstream process solutions in the industry.

For the core making and core assembly process, in the mass production of motor housings, the water jacket is generally made of hot box cores, and the outer contour core is made of cold box cores. In the preliminary trial production stage of the motor shell, the water jacket core can use 3D printing sand core due to its complex structure. The outer contour sand core can be 3D printed sand core or manual core box with machinable plastic.

2. Casting process selection and gating system design

Under the core assembly process, the selection of the specific casting process plan mainly depends on the characteristics of the product structure, the production conditions of the workshop, and then the process reliability, cost, and the convenience of on-site core assembly, pouring and cleaning operations. For the integral structure motor housing with water jacket on the side wall, the most commonly used casting method is low-pressure casting or low-pressure filling and turning and solidification. If the structure of the motor housing is suitable, gravity casting or tilting casting can also be used. Way.

(1) Gravity casting

 Gravity casting is the most convenient process. The biggest advantage of this casting process is that the pouring process is fast, and it can achieve continuous pouring of products with a single beat of 8-12s. It is the fastest pouring beat among these casting processes. Fast. Gravity casting is adopted, in order to make the filling stable, the pouring scheme of bottom pouring is often used.

The runner using bottom injection gravity casting can be designed as a ring around the outside of the motor casing casting, and the inner runner extends from the bottom of the runner to the flange surface of the casting. A riser is placed on the top, and a box of two pieces can be fed at the same time.

The disadvantage of this casting method is that, due to the bottom feeding, the temperature field distribution of the molten metal in the cavity after the filling is completed is hotter at the bottom and cooler at the top. This distribution is not conducive to the sequential solidification of castings, so It is prone to shrinkage porosity or even shrinkage cavity defects caused by insufficient feeding of parts of the casting, and thus leads to the problem that the airtightness of the finished product after the motor casing is machined is unqualified.

(2) Low-pressure casting 

The casting scheme of producing motor casings by low-pressure casting is the most common process. The biggest difference between it and gravity casting is that low-pressure casting can supplement the casting in the direction of anti-gravity through the inner runner during the solidification process. shrinkage, thus ensuring that the feeding under the casting can be effectively resolved.

The advantage of this runner design is that the process yield is high, and the disadvantage is that it is difficult to solve the problem of shrinkage. Since the wall thickness of the bottom surface of the motor casing is generally relatively thin, it will solidify first during the solidification process of the casting, resulting in the early closure of the feeding channel connecting the casting area above it with the gate, resulting in the hot section area above the casting not being able to get Effective feeding.

(3) Low-pressure filling and turning and solidification 

The scheme of low-pressure filling and turning and solidification is to cancel the top riser of the low-pressure casting scheme and replace it with cold iron, and the design of the runner is the same as that of the low-pressure casting. After the filling is completed, the sand bag is turned over 180° by the manipulator or the turning mechanism. The basic design intention of this scheme is that after turning over, the sprue and runner act as risers for feeding, and the cold iron is placed under the casting after turning over. Chilling, castings and gating system form an ideal temperature field distribution, which is more conducive to sequential solidification. Moreover, due to the cancellation of the riser, the process yield rate of the product is higher. However, the use of this process requires the cooperation of manipulators and or turning mechanisms.

(4) Tilting pouring 

The tilting pouring scheme is generally to set a pouring cup on one side of the riser, and then turn it over 90°. The whole turning process can be controlled within 7-12s, but during the turning process, one side of the shell will be overheated, the temperature field distribution is not easy to control, and the risk of product shrinkage is high. Moreover, floating cores and broken cores are prone to occur during the turning process, and the requirements for the strength of the sand core and the positioning of the sand core are high. Moreover, the workshop needs a turning mechanism to realize the turning action.

Based on the above analysis, the low-voltage process is the most common casting process for motor casings. As for the standard low-pressure casting or low-pressure inversion casting, it should be analyzed according to the specific structure of the product, and the choice should be made according to the production conditions of the workshop.