Electric power steering system is widely used due to its simple structure, flexible operation, high efficiency and low energy consumption. As an important part of electric power steering system, steering drive motor has a thin-walled die casting with a complex structure, easy to deform, requires good air tightness and high strength. Therefore, it is difficult to form and process is complicated. Numerical simulation of die casting filling and solidification process can effectively predict various defects that may appear in casting and their size, location in die casting process design stage, thereby optimizing casting process design to ensure quality of casting, shorten trial production cycle and reduce production costs.
This study takes thin-walled motor housing parts as object, and numerically simulates filling process based on a mold flow analysis software. In view of complex casting process of electric power steering motor housing and high quality requirements of castings, finite element model is established by using HyperMesh software and ProCAST software respectively, casting process of electric power steering motor housing is simulated and analyzed, location and size of defects inside casting are predicted. Process scheme is trial-produced, and produced castings are observed by X-ray flaw detection. Defects generated in later production of die-casting parts are predicted to provide a reference for die-casting mold design and casting process optimization.
Graphical results
Housing of electric power steering motor is shown in Figure 1. Casting is a thin-walled cylindrical part with an average wall thickness of 4mm. Thickness distribution is relatively uniform, with a minimum wall thickness of 2mm and a maximum wall thickness of 13mm. In the later stage of casting, key parts need to be machined and surface treated to meet requirements of no red rust in salt spray test for 720h. Maximum size of pores in die-casting is required to be no more than φ0.4mm and spacing is more than 8mm. UG software is used to model electric power steering motor housing product, determine parting surface, and design its pouring system, cooling system, overflow system and mold structure. Correctly selecting parting surface is not only conducive to layout of pouring system and ensures that die casting remains in moving mold after mold is opened, but also conducive to simplification of mold structure, improvement of dimensional accuracy and surface quality of die casting. According to structural characteristics of die casting as a cylindrical thin-walled shell part, A-A surface in Figure 2 is selected as parting surface. Selection of this parting surface is conducive to setting up a ring pouring system that meets structural characteristics of casting on basis of meeting requirements of die casting process. Selected pouring system is shown in Figure 3. After molten metal fills annular runner, it is filled along core direction. System has good exhaust conditions and a short process, which allows molten metal to enter cavity smoothly and form good filling conditions.

 

Figure 1 Shell part drawing

 

Figure 2 Parting surface

 

Figure 3 Shell part pouring system
For die casting molds, design of cooling system is conducive to controlling mold temperature, so that internal heat can reach a dynamic balance state, improve mold service life, and ensure quality of casting. Cooling form combining cooling water channels and spot cooling is set in movable mold and fixed mold respectively, which can achieve a higher and more uniform cooling rate. Distribution of cooling system is shown in Figure 4. Core structure is shown in Figure 5. Two sides of opening of thin-walled cylindrical shell are formed by two inserts, top insert 1 of shell contains core 1 and core 2 to form hole structure on both sides of shell.

 

Figure 4 Cooling water channel distribution

 

Figure 5 Core structure diagram

Table 1 Main die casting process parameters

Table 2 Die casting production cycle (s)
Flow field distribution of casting at different times during filling process is shown in Figure 6. Due to high speed and high pressure characteristics of die casting, the entire filling process is 0.06s, and filling time distribution is shown in Figure 7. It can be seen that molten metal enters cavity along annular runner, and last filling part is overflow groove farthest from gate. Filling of molten metal generally meets sequential filling.

 

Figure 6 Flow field diagram of casting filling process

 

Figure 7 Distribution diagram of casting filling time
Die casting scheme of 10 die casting cycles was simulated. Three temperature measuring points located on cavity surface of casting, fixed mold, and movable mold were selected, as shown in Figure 8, and die casting cycle temperature change curve was obtained, as shown in Figure 9. It can be seen that when cycle reaches 7th cycle, temperature of casting, fixed mold, movable mold tends to be stable, and mold and casting reach thermal equilibrium. Then temperature field of 9th cycle in cycle that reaches thermal equilibrium was selected for analysis. Temperature changes of mold and slider at several moments in 9th cycle from beginning of filling to time period before spraying release agent were selected for analysis, and temperature changes are shown in Figure 10. It can be seen that in filling stage, temperature field of mold surges with inflow of molten metal. In pressure holding solidification stage, cooling system of mold takes away most of heat, then enters stage of opening mold to take out part and spraying demoulding agent. Surface temperature of mold cavity drops to average temperature of 200℃ before filling stage. It can be seen that cooling system in scheme can well take away excess heat in mold, so that mold finally reaches thermal balance, and significantly improves temperature field distribution of casting.

 

Figure 8 Casting and mold temperature analysis point selection diagram

 

Figure 9 Temperature change curve

 

Figure 10 Temperature field of mold and slider at different stages

 

Figure 11 Shrinkage prediction and volume measurement results
Designed mold is used for die casting production with selected process parameters, and actual production casting is shown in Figure 12. According to defect prediction results of casting simulation software, casting ears and thick wall (A, B, C and D) are selected for local X-ray flaw detection. Results are shown in Figure 13. It can be seen that there are shrinkage and porosity defects at A, B, C and D to varying degrees. There are a large number of defects at A and B, but they are relatively small. There are also defects at D, but distance between defects is small and the overall range is small. C shows a connected defect state, with obvious shrinkage defects.

 

Figure 12 Die casting production shell parts

 

Figure 13 Local flaw detection diagram of castings
Conclusion
(1) Based on traditional die casting mold design method and theory, pouring system, cooling system and mold structure of electric power steering motor housing die casting mold are designed. Based on die casting characteristics of cylindrical thick wall uneven castings, a reasonable mold is designed.
(2) Based on ProCAST software, metal liquid flow field and mold temperature field of motor housing are simulated. Metal liquid flows smoothly during die casting filling process, which conforms to casting filling order. After 7 die casting cycles, mold reaches a thermal equilibrium state, and temperature fluctuations of each mold component are within a reasonable range.
(3) Compared with X-ray inspection results, locations and areas of shrinkage cavities and shrinkage porosity predicted by numerical simulation are consistent with measured detection results.