With rapid development of automobile industry, issues such as energy conservation and environmental protection have become increasingly prominent. On the premise of ensuring strength of car body, reducing body mass as much as possible to reduce emissions is one of important directions for automobile development. Two main trends in production of lightweight automotive parts are structural weight reduction and application of material weight reduction. Aluminum alloys are widely used in various engineering fields, especially automotive parts, due to their high strength, low density, corrosion resistance and easy recycling. At present, die-casting is one of main methods for producing aluminum alloy parts. However, due to characteristics of high speed and high pressure in die casting process, defects such as air entrainment and pores will inevitably occur in die castings during forming process, which will reduce mechanical properties of castings and even scrap them.
Engine end cover is an important part of car's power system. Its material is usually aluminum alloy or cast iron. It is subject to body vibration during car operation, so it needs sufficient strength. This study is based on AnyCasting software to simulate filling and solidification process during die-casting process of engine block rear end cover, analyze defects and causes during die-casting process, add cooling water channels on mold according to product structure, and optimize die-casting process parameters through orthogonal experiments to provide reference for actual production.
Figure 1 is a three-dimensional structural diagram of end cover die casting. This part is rear end cover of a domestic automobile engine cylinder. Material is ADC12 aluminum alloy, and mold material is H13 steel. Table 1 shows thermophysical parameters of two materials. Outer contour dimensions of casting are approximately 241mm * 335mm * 79mm. Shape is complex and wall thickness is uneven. Main wall thickness is 9mm, maximum wall thickness is 12mm, and weight is 2.7kg. Middle boss part is thicker and has many reinforcing ribs, which is prone to shrinkage and shrinkage defects. According to structural characteristics of casting, a 1-cavity die-casting process is used. Circular boss in the center of casting has reinforcing ribs and holes, and wall thickness is relatively large. Outer area solidifies before this, and it is easy to form an isolated liquid phase area, which is a key filling part during mold filling process. Using a multi-gate design, cross-sectional area of inner gate is 416mm2 and thickness is 2mm. A fan-shaped lateral runner is used, cross-sectional shape is trapezoidal, and flow channel area maintains a convergent change to reduce amount of air entrainment generated during filling. At the same time, multiple overflow grooves are designed at filling end and sides of die casting to discharge gas and molten metal in cavity. Figure 2 is a three-dimensional diagram of a die-casting part with a pouring and draining system.

 

Figure 1 Three-dimensional structure diagram of die casting

Table 1 Thermophysical parameters

 

Figure 2 3D view of die casting with pouring and draining system
Three-dimensional solid modeling of gating system and castings was completed in UG software, and imported into AnyCasting software in .stl format to complete mesh division. After division, there were a total of 11948594 cells. Casting material is ADC12 alloy, mold material is H13 steel, and high-pressure die casting is used. Pouring temperature is 660℃, mold preheating temperature is 200℃, slow injection speed is 0.2m/s, and fast injection speed is 4.5m/s, ambient temperature is 25℃, heat transfer coefficient between mold and outside world is 1000W/(m2·K).

 

(a)t=0.3659s (b)t=0.3854s (c)t=0.3875s (d)t=0.3914s
Figure 3 Filling sequence simulation results

 

(a)t=5.5s (b)t=7.4s (c)t=10.6s (d)t=101.6s
Figure 4 Solidification sequence simulation results

 

(a) Shrinkage cavities and porosity on outer surface (b) Shrinkage cavities and porosity on internal surface
Figure 5 Defect simulation results
In view of internal defects of product, it is necessary to speed up cooling rate of ring part before filling. Therefore, two sets of cooling water pipes with a diameter of 12mm are added to movable and fixed mold parts of mold to speed up cooling rate of casting through cooling water. Figure 7 shows optimized defect prediction. Comparing Figure 7 with Figure 5b, it can be seen that shrinkage cavity defects inside casting are reduced compared with original plan, but there are still some areas with a high probability of defects, and process parameters need to be further optimized.

 

Figure 6 Optimized cooling solution

 

Figure 7 Casting defect prediction results of optimized solution
In die-casting production process, three factors, injection speed, pouring temperature and mold preheating temperature, have a significant impact on product quality. Using orthogonal test method, injection speed, pouring temperature and mold preheating temperature are selected as factors, and shrinkage porosity (number of meshes with shrinkage cavities as a percentage of the total mesh of casting) is calculated as evaluation index through quantity analysis module of AnyCasting. The smaller shrinkage porosity, the better quality of casting. Table 2 is orthogonal factor level table. We designed L9 (33) orthogonal table and conducted 9 sets of tests. Table 3 shows results of orthogonal test. Pouring temperature is 680℃, injection speed is 4m/s, mold preheating temperature is 220℃ for die-casting production of aluminum alloy engine end covers.

Table 2 Orthogonal factor level table

Table 3 Orthogonal test results

 

Figure 8 Actual picture of casting

 

Figure 9 X-ray non-destructive testing results
In conclusion

(1) Numerical simulation of die-casting process of aluminum alloy rear end cover and analysis of filling and solidification processes can predict location of defects in castings. Use of optimized cooling water channels can make solidification process of castings more uniform and sufficient, reducing probability of shrinkage cavities and shrinkage porosity.

(2) Apply orthogonal experimental design to optimize die-casting process parameters. Optimized die-casting process parameter combination of aluminum alloy rear end cover: pouring temperature is 680℃, injection speed is 4.0m/s, and mold preheating temperature is 220℃.
(3) Based on numerical simulation analysis and process parameter optimization results, die-casting test was conducted, and product quality was significantly improved. Rationality of optimal process parameters was verified through testing.