Development of aluminum alloy motor shell casting process

0 Introduction

In recent years, under the demand for energy conservation, emission reduction and environmental protection, the R&D focus of automobile manufacturers is shifting from traditional fuel vehicles to new energy vehicles. The aluminum alloy motor shell is the core casting of the powertrain of new energy vehicles, and its top (open side) is connected to the inverter, and the bottom is connected to the reducer.
And the spindle bearing is connected by the inlaid bearing bushing, and the sidewall is often connected to the subframe by mounting. The structure of the motor housing is more complex. The side wall of the motor shell surrounds the cooling water jacket to ensure 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, shrinkage porosity on the upper and lower end faces and side walls of the motor housing is also a casting defect that needs to be avoided in process development. In this paper, the structural characteristics and common casting defects of motor shells are introduced, and on this basis, the casting process scheme of motor shell castings is discussed and shared design, and the application effect of computer simulation technology in the rapid trial production of motor shell castings.

1 Motor shell product features

1.1 Overview

The motor shell is the core part of the new energy vehicle, it is connected to the reducer at one end, and the inverter is connected to the other end, and the diameter of the motor shell is generally φ350~φ400 mm. The height is generally in the range of 200~300 mm. Figure 1 shows two types of motor housings with different powers, with a main wall thickness of 5~6 mm, the side water jacket structure is varied, but it is often a spiral structure or a semi-helical structure {(b), (c), (d), (e)} in Fig. 2, and the wall thickness of the water jacket is generally 6~7 mm. The weight of the motor shell is generally in the range of 4~10 kg, and the material is generally made of aluminum alloy, which is A356.(ZL101A) alloy (belonging to AlSi7Mg 0.3 casting alloy), and T6 heat treatment is adopted.

Figure 1 Two motor housing products with different powers

Figure 2 The structure of the motor shell water jacket

1.2 Technical Requirements

The mechanical properties generally require the hardness of the bottom surface and the top surface to be not less than 90 HBW, the tensile strength is required to be Rm≥275 MPa and ≥2% elongation with the furnace test rod or the designated body sampling site, and the air tightness requirements are: no bubble leakage in the water jacket at 600 kPa for 10 min, and the surface of the casting and the processing surface are not Casting defects such as porosity, shrinkage porosity, cold separation, cracks, and slag inclusion are allowed, and the internal defects of the casting need to be controlled in ASTM E155 Class III; The casting size meets CT7 requirements.

1.3 Common defects in the casting process

The structure of the motor shell casting is complex, the casting is very difficult, once the process is not reasonable, it is very easy to produce scrap, and the common casting defects are shown in Figure 3. Defects caused by insufficient shrinkage include concentrated shrinkage porosity (Fig. 3(a)}, local shrinkage porosity (Fig. 3(b)}, air bubbles caused by poor exhaust (Fig. 3(c)}, and insufficient pouring or cold separation (Fig. 3(d)} caused by poor filling, which are also the main factors causing the unqualified air tightness of the motor shell {Fig. 3(e)}. In addition, there are defects such as broken core of the water sleeve (Fig. 3(f)}, poor fit of the bearing bushing (Fig. 3(g)}, and severe local sand sticking of the casting (Fig. 3(h)}. Among the above-mentioned defects that are prone to occur in the casting process of motor housing, the defects caused by poor shrinkage are the most important. Therefore, the main concern in the selection of casting process is the shrinkage of castings.

Figure 3 Common forms of casting defects in motor housings

2 Casting process scheme of motor shell

2.1 Core making and assembly plan

The use of core assembly technology for the production of motor shell castings is currently one of the mainstream process solutions in the industry. The core assembly process is used for the casting of motor shells
The art process is shown in Figure 4, and the overall process will not be detailed here.
For the core making and core assembly processes, for the mass production of motor shells, the water jacket generally adopts a hot core box core making method, and the outer contour core adopts a cold core box core making method. In the early trial production stage of the motor casing, the water jacket core can adopt 3D printed sand core due to its complex structure. The outer contour sand core can be 3D printed sand core or The method of manually making cores using handmade core boxes that can process plastic. In the early stage of product trial production, in order to achieve rapid trial production, it is not necessary to Considering the drawing direction of the core box, it is not necessary to follow the parting method of the batch production process {as shown in Figure 5 (a)}, but rather to integrate some sand cores The scheme {is shown in Figure 5 (b)}.

Figure 4 Core Assembly Process Casting Flow Chart of Motor Shell Castings

Figure 5 Core assembly method of motor casing

2.2 Casting process selection and gating system design

In the core assembly process, the specific casting process plan selection mainly depends on the product structure characteristics, workshop production conditions, and then the selection is made based on process reliability, cost, and the convenience of on-site core assembly, pouring, and cleaning operations [3]. The most commonly used casting method for integral motor shells with water jackets on the side walls is low-pressure casting or low-pressure filling and overturning solidification. If the structure of the motor shell is suitable, gravity casting or tilting casting can also be used.

(1) Gravity casting

Gravity casting is the most convenient process method. The biggest advantage of this casting process is its fast pouring process, which can achieve continuous pouring of products with a single beat of 8-12 seconds. It is the fastest pouring beat among these casting processes. Gravity casting is used, and in order to ensure smooth filling, a bottom pouring pouring scheme is often used. The pouring system is shown in Figure 6. The transverse gate using bottom pouring gravity casting can be designed as a circular shape around the outer side of the motor casing casting. The inner gate extends from the bottom of the transverse gate to the flange surface of the casting, with a riser placed on top, and can achieve simultaneous feeding of two pieces in a box. The disadvantage of this casting method is that due to the bottom feeding, the temperature field distribution of the material in the mold cavity after filling is hot at the bottom and cool at the top. This temperature distribution is very unfavorable for the sequential solidification of the casting, and therefore it is very easy to cause shrinkage porosity or even shrinkage defects due to insufficient local shrinkage of the casting, which can lead to the problem of unqualified air tightness of the finished product after the motor shell is machined.

Figure 6 Design of Gravity Casting Pouring System

Figure 7 Gate Form of Low Pressure Casting

⑵ Low pressure casting

The casting scheme of using low-pressure casting to produce motor shells is the most common process method. The biggest difference between low-pressure casting and gravity casting is that low-pressure casting can provide anti gravity feeding to the casting through the inner gate during the solidification process, which can ensure that the feeding below the casting can be effectively solved. The sprue design is shown in Figure 7.

The advantage of this runner design is its high production rate, while the disadvantage is that it is difficult to solve the problem of shrinkage. Due to the relatively thin bottom wall thickness of the motor casing, it will solidify first during the solidification process of the casting, leading to the early closure of the feeding channel connecting the casting area above it to the gate, resulting in The hot spot area above the casting cannot be effectively compensated, as shown in Figure 8 (a) and (b).

Figure 8 Example of problems that are prone to occur in low-pressure casting

Based on the above reasons, according to the structural characteristics of the casting, the bottom of the casting can be locally thickened, as shown in Figure 9 (a), or a runner design can be adopted as shown in Figure 9 (b), (c), (d), and (e). From the perspective of casting filling and shrinkage, the design of the above gating system is feasible.

Figure 9 Optimization design scheme for sprue

The core assembly of (a)~(c) in the runner scheme Figure 9 is relatively simple, but it will increase the burden of the cleaning process and need to be removed through turning processing; The cleaning of schemes (d) and (e) in Figure 9 will be easier, and the pouring system can be directly removed by sawing.

There are also various design schemes for the top riser, as shown in (a), (b), and (c) of Figure 10. The specific scheme chosen mainly depends on the product structure, and the optimal riser scheme can be determined through simulation hot spot analysis and shrinkage porosity analysis.

Figure 10 Different Riser Design Schemes

⑶ Low pressure mold filling, flipping and solidification

The use of low-pressure filling and overturning solidification scheme eliminates the top riser of the low-pressure casting scheme and replaces it with cold iron, while the runner design is consistent with the low
There is no difference in the runner design of die casting (as shown in Figure 11). After filling, flip the sand bag 180 ° using a robotic arm or flipping mechanism,The basic design intention of this plan is that after flipping, the gate and runner serve as risers for feeding, while the cold iron is excited below the casting after flipping The ideal temperature field distribution formed by the cold casting and pouring system is more conducive to sequential solidification. Moreover, due to the cancellation of the riser, the production process of the product Higher yield. However, adopting this process requires a combination of a robotic arm and a flipping mechanism to achieve it.

(4) Tilting pouring

The tilting pouring scheme generally involves setting a pouring cup on one side of the riser and flipping it 90 ° (as shown in Figure 12). The entire flipping process can be controlled within 7-12 seconds, but there may be overheating on one side of the shell during the flipping process, making it difficult to control the temperature field distribution and posing a high risk of product shrinkage and loosening. Moreover, the flipping process is prone to floating and broken cores, which requires high strength and positioning of the sand core. Additionally, the workshop needs a flipping mechanism to achieve the flipping action.

Figure 11 Cold Iron Design for Low Voltage Flipping Scheme

Figure 12 Tilting Pouring Scheme

2.3 Summary of Process Plan Selection

Based on the above analysis, the low-pressure scheme is the most common casting process for motor casing production. As for whether to use standard low-pressure casting or low-pressure flip casting, it needs to be analyzed based on the specific structure of the product and selected according to the production conditions in the workshop.

3 Computer simulation application

3.1 Overview

The computer simulation of the casting process has been widely applied in the development of casting processes. By simulating the casting process of the motor casing, the state of the casting filling and solidification process can be predicted, and the risk of possible casting defects can be evaluated [3,4]. The flow state and temperature analysis during the filling process of the motor casing casting, as well as the liquid phase rate analysis during the solidification process, are shown in Figure 13.

Figure 13 Simulation of the Filling and Solidification Process of the Motor Shell

On the basis of simulation analysis of the casting process, the tracer particle analysis of the filling process of the motor shell {as shown in Figure 14 (a)} can be used to assist in analyzing the smoothness of the filling process, which helps to determine whether the current pouring system design and pouring speed can ensure the smoothness of the motor shell filling process; By comprehensively simulating and analyzing the hot joints and shrinkage porosity of the motor shell {Figure 14 (b) and (c)}, it is possible to predict the possible occurrence of hole defects in the product due to poor shrinkage.

Figure 14 Simulation analysis of particle tracing, hot spot and shrinkage porosity in the motor shell

3.2 Application of Casting Structure Simulation in the Development of Motor Shells

The mechanical properties of the product are closely related to the metallographic structure (grain size) of the casting. By using microstructure simulation, the design of the gating system can be analyzed for its impact on The impact of casting grain size is taken into account from the process design stage, taking into account the impact of the process on product structure and mechanical properties, thereby ensuring that The organization and mechanical properties of the product. For example, in process development, in order to analyze and predict the solution to local shrinkage and porosity of castings, side risers are used
There are two options for feeding or placing cold iron at the hot spot position of the casting for quenching (as shown in Figure 17). Both process options have an impact on the performance and tissue density of the casting The impact can be caused by analyzing the grain diameter λ To analyze tissue density.

Predicting grain size through tissue simulation can be done using a macro scale approach
Analysis methods, such as analyzing the solidification time or cooling at local locations of castings
Analyzing the impact of specific process plans on the grain size of castings from a trend based on the rate
The impact is shown in (a) and (b) of Figure 18. This method is relatively simple and fast
Jie is also relatively more practical. And through the CA-FE method, a more microscopic perspective can be obtained, which is detailed Understand the grain size and morphology under different process conditions. CA-FE is based on The grain density expression for continuous nucleation proposed by Pappaz et al. (such as the formula The CA method shown in (3-2) is combined with the finite element method. Obtain specific processes Under different conditions, the degree of undercooling under the influence of cooling rate leads to the formation of grains Morphology。

Figure 17 Simulation case of internal organization of motor casing castings

Figure 18 Macroscopic Organizational Simulation Analysis

Figures 19 (a) and (b) simulate the microstructure of motor casing castings under the two processes mentioned above using the CAFE (Cellular Automata Finite Element) module of ProCAST software.

3.4 Application significance of simulation

By fully utilizing simulation methods, it is possible to quickly and effectively evaluate and optimize the design of the casting process for motor shells, thereby obtaining the best process plan and achieving rapid development of the casting process for motor shell products.

Figure 19 CAFE simulation of motor casing castings

4 Conclusion

In terms of casting scheme selection and specific casting process design, the main focus is on eliminating the problem of poor casting shrinkage. There are various casting plans, and specific choices need to be made based on product characteristics and workshop production capacity. Computer simulation technology is an effective means of process selection and demonstration. With the help of 3D printing technology, some sand cores can be integrated, simplifying the mold separation of sand cores, and making product process trial production faster and more reliable.

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