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. Fig. 1 shows two kinds of motor shells with different powers, the main wall thickness is 5~6 mm, and the side water jacket structure is diverse, but it is often a spiral structure or a semi-helical structure {(b), (c), (d), (e)} in Figure 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 core assembly scheme

The production of motor shell castings by the core assembly process is one of the mainstream process schemes in the industry at present, and the process flow of the motor shell casting by the core assembly process is shown in Figure 4, and the overall process is not described in detail here. For the core making and core assembly process, the mass production of the motor housing, the water jacket is generally made of a hot core box, and the outer contour core is made of a cold box. In the early trial production stage of the motor shell, the water core can be 3D printed due to its complex structure. The outer contour core can be 3D printed or used The method of hand-making core making using a handmade core box that can process plastics. In the early trial production stage of the product, in order to achieve rapid trial production, it is not necessary to follow the parting method of the batch production process (as shown in Fig. 5(a)}, but to integrate part of the sand core (as shown in Fig. 5(b)} without considering the draft direction of the core box.

Figure 4 Flow chart of core casting process for motor shell castings

Figure 5 The way the motor shell is cored

2.2 Casting process selection and gating system design

Under the coring process, the selection of the specific casting process scheme mainly depends on the characteristics of the product structure and the production conditions of the workshop, and then the trade-offs are made from the process reliability, cost and whether the operation of on-site core-building, pouring and cleaning is convenient. For monolithic motor housings with water jackets on the sidewalls,
At present, the most commonly used casting method is to use low-pressure casting or low-pressure filling to flip and solidify, if the structure of the motor shell is suitable, it can also be used gravity casting or tilt casting.

(1) Gravity casting
Gravity casting is the most convenient process, and this casting process has the greatest advantages
The point is that the pouring process is fast, and the continuous pouring of products with a single piece cycle time of 8~12 s can be achieved, which is the fastest pouring cycle time among these casting processes. Gravity casting is adopted, and in order to make the filling smooth, the bottom-injection pouring scheme is mostly adopted, and the gating system is shown in Figure 6. The crossrunner with under-injection gravity casting can be designed as an annular shape around the outside of the motor shell casting, the inner sprue extends from the bottom of the cross-sprue to the flange face of the casting, the riser is placed at the top, and two pieces can be fed at the same time in one box. The disadvantage of this casting method is that, because it is the bottom feeding, the temperature field distribution of the material liquid in the cavity after the filling is completed, the lower part is hot, the upper part is cool, and this temperature distribution form is very unfavorable to the sequential solidification of the casting, so it is very easy to have the shrinkage porosity or even shrinkage defects caused by the partial failure of the casting to be fully replenished, and thus lead to the problem that the air tightness of the finished product is unqualified after the motor shell is machined.

Figure 6 Design of a gravity casting gating system

(2) Low-pressure casting
The casting scheme of using low-pressure casting to produce motor shells is the most common process
The biggest difference between it and gravity casting is that low-pressure casting can carry out anti-gravity direction reinforcement of the casting through the inner gate during the solidification process, so as to ensure that the reinforcement under the casting can be effectively solved. The runner design is shown in Figure 7.

Figure 7 Sprue form for low-pressure casting

The advantage of this sprue design is that the process yield is high, and the disadvantage is that it is difficult to solve the problem of shrinkage. Because the wall thickness of the bottom surface of the motor shell is generally relatively thin, it will be the first to solidify during the solidification process of the casting, resulting in the early closure of the feeding channel connected to the gate by the casting area above it, resulting in the ineffective replenishment of the hot joint area above the casting, as shown in Figure 8(a) and (b).

Figure 8 shows an example of a problem that can 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 partially thickened, as shown in Figure 9 (a), or the sprue design shown in Figure 9 (b), (c), (d) and (e) can be adopted. From the perspective of casting, filling and shrinkage, the design of the above gating system is feasible.

Figure 9 Runner optimization design

The core assembly of (a)~(c) of the sprue scheme diagram 9 is relatively simple, but it will increase the burden of the cleaning process, and it needs to be removed by turning; Schemes (d) and (e) in Figure 9 are easier to clean up and remove the gating system directly by sawing. The design scheme of the top riser is also various, as shown in (a), (b) and (c) of Figure 10, which scheme is mainly selected according to the structure of the product, and the optimal riser scheme can be determined by simulation hot joint analysis and shrinkage porosity analysis.

Figure 10 Different riser designs

(3) Low-pressure filling flip solidification
The low-pressure filling flip solidification scheme is to eliminate the top riser of the low-pressure casting scheme and replace it with cold iron, and the runner design is no different from the runner design of low-pressure casting (as shown in Figure 11). After the filling is completed, the sand bag is flipped 180° by the manipulator or the turning mechanism, so that the basic design intent of the scheme is that the gate and the sprue act as a riser for shrinkage after flipping, and the cold iron is cooled below the casting after flipping, and the casting and the gating system form an ideal temperature field distribution, which is more conducive to sequential solidification. Moreover, due to the elimination of the riser, the process of the product is out Higher product yield. However, the adoption of this process requires the cooperation of manipulator and turning mechanism to achieve.
(4) Tilt pouring
The tilt-turn pouring scheme typically involves setting the pouring cup on one side of the riser and then flipping it 90° (as shown in Figure 12). The whole turning process can be controlled at 7~12 s, but the tilting process will cause overheating on one side of the shell, the temperature field distribution is not easy to control, and the risk of shrinkage and looseness of the product is large. Moreover, the flipping process is prone to floating core and broken core, which has high requirements for the strength of the sand core and the positioning of the sand core, and the workshop needs to have a turning mechanism to realize the action of flipping.

Figure 11 Cold iron design for a low-pressure flip scheme

Figure 12 Tilt pouring scheme

2.3 Summary of process scheme selection

Based on the above analysis, the low-voltage scheme is the most common casting process in the production of motor shells, as for the use of standard low-pressure casting or low-pressure flip casting, it is necessary to analyze the specific structure of the product and choose according to the production conditions of the workshop.

3 Computer simulation applications

3.1 Overview

Computer simulation of the casting process has been widely used in casting process development. By simulating the casting process of the motor housing, it is possible to predict the state of the filling and solidification process of the casting, and to assess the risk of possible casting defects. Figure 13 shows the fluid and temperature analysis of the filling process and the liquid phase rate analysis of the solidification process of the motor shell casting.

Figure 13 Simulation of motor shell filling and solidification process

On the basis of the simulation analysis of the casting process, the tracer particle analysis of the motor shell filling process (shown in Fig. 14(a)) can be used to assist in the analysis of the filling stability, which is helpful to determine whether the gating system design and pouring speed of the current process scheme can ensure the stability of the motor shell filling. Through the comprehensive simulation analysis of the hot joints and shrinkage porosity of the motor shell {Fig. 14(b), (c)}, it is possible to predict the possible hole defects caused by poor shrinkage.

Fig. 14 Simulation of tracer particles, hot joints, and porosity in a motor shell

3.2 Application of Virtual DOE Intelligent Optimization

DOE (Design Of Experiments) has been widely used in the process intelligence analysis of casting products. Due to the increasingly complex structure of casting products and the shorter and shorter development cycles, this brings great challenges to product process design. With the development of computer simulation technology, the MAGMA simulation DOE module can quickly analyze the trend of the influence of multiple processes on the quality of castings, and quickly obtain the optimal process. The method and process of virtual DOE analysis is called the “six-step method” (i.e., setting goals, defining variables, clarifying standards, improving efficiency, selecting methods, and continuous improvement), which simply means clarifying the criteria to be analyzed as goals, setting multiple process parameters as variables, and then carrying out each process through virtual means The plan is comprehensively evaluated to find the best result. When performing a DOE analysis, the first step is to set goals and define variables (figures15): Take the upper end face of the motor housing, and the bottom end as the analysis area, and will fall Low shrinkage porosity, reduced flow velocity, lower hot joints, and control of filling process temperature The degree of 4 results as the analysis target, the top of the motor housing weight reduction groove structure, rising Mouth geometry, cold iron scheme, etc. are used as process variables, and the above variables are advanced Row assignment. Then, after DOE analysis and calculation, the results of the orthogonal test of the motor shell were analyzed, so as to evaluate the impact of different process variables on the motor shell in many processes the impact of the risk of casting defects and find the optimal process design solution. Through the line chart analysis of each process variable, we can understand the degree of influence of each process variable on the specific criteria of the casting. For example, as shown in Figure 16(a), the effect of the process variables of the chilled iron and the inner gate on the porosity tendency of the top is very small, and the effect of the riser variable is very significant. From the scatter plot {Fig. 16(b)}, the ranking of the impact of each process scheme on the DOE analysis objective criterion arranged by the specified process variables can be obtained. Through the analysis of the Correlation Matrix (Fig. 16(c)}, a comprehensive evaluation of each target criterion can be obtained for all process options. The target approximation method can also be used to select the best process scheme from the many casting process schemes of the motor shell (as shown in Fig. 16(d)) by using the influence trend of each factor of the DOE results on each target.

Figure 15 MAGAM Interface for Setting Targets and Process Variables

Figure 16 DOE analysis

3.3 Application of casting structure simulation in the development of motor housing The mechanical properties of the product are closely related to the metallographic structure (grain size) of the casting, and the influence of the design of the gating system on the grain size of the casting can be analyzed by using the microstructure simulation, so as to ensure that the influence of the process on the product structure and mechanical properties is taken into account from the process design stage Tissue and mechanical properties of the product. For example, in process development, in order to analyze and predict the solution to the local shrinkage porosity of the casting, two options for quenching by using side riser shrinkage or placing a chilled iron at the hot joint of the casting (as shown in Figure 17). Both process options will have an impact on the performance and microstructure compactness of the casting To cause an impact, tissue compactness can be analyzed by analyzing the grain diameter λ. According to the existing research, the mechanical properties are directly affected by the grain diameter, the most representative of which is the Hall-Patch equation.

Figure 17 Simulation of the internal structure of a motor shell casting

Grain size prediction through microstructure simulation can be done using a more macroscopic analysis method, such as analyzing the solidification time or cooling rate of the local position of the casting, and trending the effect of a specific process scheme on the grain size of the casting (as shown in (a) and (b) of Fig. 18). This method is relatively simple, quick, and relatively more practical. With the CA-FE method, you can get a more detailed view from a more microscopic perspective Solve the grain size and morphology under different process conditions. CA-FE is a CA method based on the grain density expression of continuous nucleation proposed by Pappaz et al. (as shown in equation (3-2)) combined with finite elements.

Figure 18 Macro organizational simulation analysis

(a) and (b) of Figure 19 are CAFEs (formerly) utilizing ProCAST software Cellular automata-finite element) module on motor shell castings under the above two processes group Weaving simulation.

Figure 19 CAFE simulation of a motor shell casting

3.4 Significance of simulation application

By making full use of simulation methods, the design of the motor shell casting process can be quickly and effectively evaluated and optimized, so as to obtain the best process scheme and realize the rapid development of the casting process of the motor shell product.

4 Conclusion

In terms of casting scheme selection and specific casting process design, it mainly revolves around
Eliminate the problem of poor shrinkage of castings. There are a variety of casting schemes, specifically according to the characteristics of the product and the production capacity of the workshop to make a choice, with the help of computer simulation technology is an effective means of process selection and process demonstration, with the help of 3D printing technology, you can enter part of the sand core
Row integration, simplifying the parting of the sand core, making the product process trial production faster and more reliable.