In recent years, with demand for energy conservation, emission reduction and environmental protection, research and development focus of automobile manufacturing companies is shifting from traditional fuel vehicles to new energy vehicles. Aluminum alloy motor casing is core casting of new energy vehicle powertrain. Top (open side) is connected to inverter, bottom is connected to reducer, main shaft bearing is connected through an inlaid bearing bushing, side walls are often connected to subframe through suspension.
Structure of motor casing is relatively complex. Side wall of motor casing surrounds cooling water jacket. Ensuring the of water jacket is an important technical requirement for product and the biggest difficulty in casting process. At the same time, shrinkage on upper and lower end surfaces and side walls of motor housing are also casting defects that need to be avoided during process development.
We introduced structural characteristics and common casting defects of motor casing. On this basis, we discussed and shared casting process design of motor casing, application of computer simulation technology in rapid trial production of motor casing castings.

Motor casing is core component of new energy vehicles. One end of it is connected to reducer and the other end is connected to inverter. Diameter of motor casing is generally φ350~φ400 mm, and height is generally in the range of 200~300 mm. Figure 1 shows two common motor casings for different powers. Main wall thickness is 5~6 mm. Side water jacket structures are diverse, but most of them are spiral structures or semi-helical structures (Figure 2). Wall thickness of water jacket is generally 6~7 mm. Weight of motor shell is generally 4~10 kg. Material is generally made of aluminum alloy A356.2 and T6 heat treatment.

 

(a) 140 kW motor housing

 

(b)160 kW motor housing
Figure 1 Two different power motor casings

 

Figure 2 Motor housing water jacket structure

Mechanical properties generally require that hardness of bottom and top surfaces is not less than 90 HBW, and that tensile strength of furnace test rod or designated sampling part of body is ≥275 MPa, and elongation is >2%. Air tightness requirement is: water jacket has no bubble leakage under 600 kPa for 10 minutes. Casting defects such as pores, shrinkage porosity, cold shut, cracks, slag inclusions are not allowed on the surface and machined surfaces of castings. Internal defects of the castings must be controlled to ASTM E155 Level III, and dimensions of castings must meet requirements of CT7 level.

Motor housing casting has a complex structure and is difficult to cast. Once casting process is unreasonable, it is extremely easy to produce waste products. Common casting defects are shown in Figure 3. Defects caused by insufficient feeding include concentrated shrinkage cavities and local shrinkage, air bubbles caused by poor exhaust, insufficient pouring or cold insulation caused by poor mold filling. These defects are also main factors that cause air tightness of motor housing to fail. In addition, there are defects such as water jacket core breakage, poor bearing bushing fit, and severe sand adhesion in parts of casting. Among above-mentioned defects that are prone to occur during motor housing casting process, defects caused by poor feeding are the most important. Therefore, main focus of casting process selection is feeding problem of castings.

 

Figure 3 Common casting defects

Using core assembly process to produce motor housing castings is currently one of mainstream process solutions in industry. Motor housing casting process flow is shown in Figure 4.
For core making and core assembly process, mass production of water jackets for motor casings generally uses hot core box core making, and outer contour core uses cold core box core making. In the early trial production stage of motor housing, due to complex structure of water jacket core, a 3D printed sand core can be used. Outer contour sand core can be 3D printed or made using a processable plastic manual core box. In the early trial production stage of product, in order to achieve rapid trial production, mold removal direction of core box can be ignored, and there is no need to divide mold according to batch production process (Figure 5a), but use partial sand core integration solution (Figure 5b) mold parting method.

 

Figure 4 Motor housing casting flow chart

 

Figure 5 Motor shell core assembly method

In core forming process, choice of specific casting process mainly depends on product structure characteristics and workshop production conditions, then choice is made based on process reliability, cost and convenience of on-site core forming, pouring and cleaning operations. For integral structure motor housings with water jackets on side walls, the most commonly used casting method currently is low-pressure casting or low-pressure mold filling and flip solidification. If structure of motor housing is suitable, gravity casting or tilt casting can also be used.

Gravity casting is the most convenient process. The biggest advantage of this casting process is that pouring process is fast. It can achieve continuous pouring of products with a single-piece cycle of 8 to 12 seconds. It is the fastest production cycle among these casting processes. Gravity casting is used. In order to ensure smooth mold filling, bottom pouring pouring scheme is often used. Pouring system is shown in Figure 6.

 

Figure 6 Design of gravity casting pouring system
Cross runner using bottom injection gravity casting can be designed as a ring around outside of motor housing casting. Inner runner extends from bottom of cross runner to flange surface of casting, with a riser placed on the top, and two pieces in one box can be fed simultaneously.
Disadvantage of this casting method is that since material is fed from the bottom, temperature field distribution of material liquid in cavity after mold filling is completed is hotter at the bottom and colder at the top. This type of temperature distribution is very unfavorable to sequential solidification of casting, so it is very easy for casting to be insufficiently compressed to cause shrinkage defects, which may lead to unqualified air tightness of motor casing after processing.

Low-pressure casting is the most common forming process for producing motor casings. The biggest difference between it and gravity casting is that low-pressure casting can feed casting in anti-gravity direction through inner runner during solidification process, thus ensuring that feeding underneath casting can be effectively resolved. Sprue design is shown in Figure 7.

 

Figure 7 Low pressure casting runner form
Advantage of this runner design is that process yield is high, but disadvantage is that it is difficult to solve feeding problem. Since wall thickness of bottom surface of motor housing is generally relatively thin, it will solidify first during solidification process of casting, causing feeding channel connecting casting area above it to sprue to be closed in advance, resulting in hot section area above casting not being effectively fed, as shown in Figure 8.

 

Figure 8 Low-pressure casting is prone to shrinkage defects
Based on above reasons and according to structural characteristics of casting, bottom of casting can be partially thickened, as shown in Figure 9(a), or a runner design as shown in Figure 9(b)~(e) can be adopted. From aspects of casting filling and feeding, design of above gating system is feasible.

 

 

 

Figure 9 Optimization design plan of sprue
Core assembly of sprue plan in Figure 9(a)~(c) is relatively simple, but it will increase burden of cleaning process and needs to be removed by turning. Cleaning of solutions in Figure 9(d) and (e) will be easier, and gating system can be removed by sawing.
There are also many design options for top riser, as shown in Figure 10. Specific solution to choose mainly depends on structure of product. Optimal riser solution can be determined through simulation analysis of hot node distribution and prediction of shrinkage cavities and shrinkage porosity defects.

 

 

Figure 10 Riser design plan

Low-pressure mold filling flip solidification scheme eliminates top riser of low-pressure casting scheme and replaces it with a cold iron. Runner design is no different from that of low-pressure casting (Figure 11). After mold filling is completed, manipulator or flipping mechanism is used to flip sand bag 180°. Basic design intention of this solution is that after flipping, inner runner and runner will act as a riser for feeding, and cold iron will be under casting for quenching after flipping. Casting and gating system will form an ideal temperature field distribution, which is more conducive to sequential solidification. Moreover, due to elimination of riser, process yield rate of product is higher. However, this process requires cooperation of a manipulator and a flipping mechanism to achieve it.

 

Figure 11 Design scheme of low-voltage flip-chill iron

Tilt pouring generally involves setting a pouring cup on one side of riser and then flipping it 90°, as shown in Figure 12. The entire flipping process can be controlled within 7 to 12 seconds. However, one side of shell may be overheated during flipping process, temperature field distribution is difficult to control, and risk of shrinkage and porosity of product is greater. Moreover, core drift and core breakage are prone to occur during turning process, which requires high strength and positioning of sand core, workshop needs a turning mechanism to realize turning action.

 

Figure 12 Tilt pouring scheme
Based on above analysis, low-pressure casting is the most common casting process for producing motor casings. As for whether to use standard low-pressure casting or low-pressure flip casting, choice must be made based on specific structure of product and workshop production conditions.

Computer simulation technology has been widely used in casting process development. Through simulation analysis of motor housing casting process, casting filling and solidification processes are analyzed, risk of possible casting defects is assessed and predicted. Flow pattern, temperature field and liquid phase ratio analysis of solidification process of motor housing casting are shown in Figure 13.

 

Figure 13 Simulation of filling and solidification process

 

Figure 14 Simulation analysis of tracer particles, hot spots and shrinkage porosity in motor casing
Based on simulation analysis of casting filling and solidification process, Figure 14(a) shows tracer particle analysis of motor housing filling process, which can assist in analyzing smoothness of mold filling, help determine whether pouring system design and pouring speed of current process plan can ensure smooth mold filling of motor casing; Figure 14(b) and (c) shows simulation analysis of hot spots, shrinkage and porosity tendency of motor shell, which can predict hole-like defects caused by poor feeding.

DOE (Design Of Experiments) has been widely used in process intelligent analysis of casting products. As structure of casting products becomes increasingly complex and development cycle becomes shorter and shorter, this brings huge challenges to product casting process design. With the development of computer simulation technology, through DOE block of MAGMA simulation software, influence trend of various process solutions on casting quality can be quickly analyzed, and the best process can be quickly obtained. Method and process of virtual DOE analysis is called "six-step method" (ie: setting goals - defining variables - clarifying standards - improving efficiency - selecting methods - continuous improvement). Simply put, it is to clarify criteria to be analyzed as goal , set multiple process parameters as variables, then conduct a comprehensive evaluation of each process plan through virtual means to find optimal result.
When conducting DOE analysis, the first step is to set goals and define variables (as shown in Figure 15). Upper end and bottom of motor shell are taken as analysis area, four results of reducing shrinkage and porosity, reducing filling flow speed, reducing hot spots, and controlling temperature of filling process are taken as analysis goals, weight-reducing groove structure on the top of motor shell, riser geometry, cold iron scheme, etc. are used as process variables, and above variables are assigned values.
DOE analysis of gating system

 

Then, through DOE calculation and analysis, results of orthogonal test of motor casing are analyzed to evaluate impact of different process variables on the risk of motor casing casting defects in many process plans, and to find optimal process design plan.
By analyzing Correlation Matrix correlation matrix evaluation figure 16(a), a comprehensive evaluation of each target criterion for all process solutions can be obtained.
We can also use target approximation method to select the best process solution from many casting process solutions for motor casing by analyzing influence trend of each factor in DOE results on each target. Analysis is shown in Figure 16(b).

 

Figure 16 DOE analysis

Mechanical properties of product are closely related to metallographic structure (grain size) of casting. Use structural simulation to analyze impact of design of gating system on grain size of casting. From process design stage, impact of process on product structure and mechanical properties can be taken into consideration to ensure product's structure and mechanical properties. For example, in process development, in order to analyze and predict solution to local shrinkage and porosity of castings, two solutions are adopted: side riser feeding or placing cold iron in hot section of casting for quenching, as shown in Figure 17.

 

Figure 17 Simulation analysis of internal structure of casting
Both process solutions will have an impact on mechanical properties and microstructure density of casting. Microstructure density can be analyzed by analyzing grain diameter. According to current research, mechanical properties are directly affected by grain diameter. The most representative one is Hall-Patch equation (1):

 

Therefore, local yield strength of castings under specific process conditions can be indirectly analyzed through grain diameter.
A more macro analysis method can be used to predict grain size through structural simulation. For example, by analyzing solidification time or cooling rate of local parts of casting, and analyzing trend of impact of a specific process plan on grain size of casting (as shown in Figure 18). This method is relatively simple, fast, and relatively practical.

 

Figure 18 Macroscopic organizational simulation analysis
Through CAFE method, we can learn more about grain size and morphology under different process conditions from a more microscopic perspective. CAFE is based on combination of CA method and finite element shown in continuous nucleation grain density expression (2) proposed by Pappaz et al. Under specific process conditions, grain morphology caused by different degrees of supercooling under influence of cooling rate is obtained.

 

In formula: integrating can obtain nucleation density when degree of supercooling is ;  is degree of supercooling;  is maximum nucleation supercooling degree, which is also center value of Gaussian normal distribution curve;  is standard deviation of undercooling;  is initial nucleation base number, that is, the total density of grains.
Figure 19 is a structural simulation of motor housing casting under above two processes using CAFE (cellular automaton-finite element) module of ProOCAST software.

 

Figure 19 CAFE simulation of motor housing casting
By making full use of simulation methods, design of motor casing casting process can be quickly, effectively evaluated and optimized to obtain the best process solution, achieve rapid development of motor casing product casting process.

In terms of casting scheme selection and specific casting process design, main consideration is to eliminate problem of poor feeding of castings. Determination of casting process plan should be based on product characteristics and workshop production capacity. Use of computer simulation technology is an effective means of process selection and process demonstration. With the help of 3D printing technology, part of sand core can be integrated, simplifying mold parting of sand core, making product process trial production faster and more reliable.