Timing chain cover of automobile engine is produced by high-pressure casting of aluminum alloy and installed on the side of engine. It is engine timing gear and chain protection cover. Lightweighting of automobile parts has always been direction and goal of automobile parts research and development, so aluminum alloy high-pressure casting parts also tend to be thin-walled parts with complex shapes. Due to particularity of structure and shape of timing chain cover, high requirements for size, performance and surface quality of product, it brings great difficulty to casting process. Therefore, in the early stage of development, with the help of AnyCasting software, numerical simulation analysis of gating system of different structures can be carried out to quickly determine a reasonable process plan, reduce cost and time of mold development.
Graphic results
Figure 1 is a 3D image of a newly developed timing chain cover. From appearance, part is horn-shaped, with a large hollow area in the middle, reinforcing ribs designed at thin wall, and irregular narrow strip sealing surfaces around it. Casting has an outline size of 590 mm * 356 mm * 46 mm, a weight of 2.135 kg, and a general wall thickness of 2 mm in the middle of casting. Casting is made of ADC12 alloy, and sealing performance requires a maximum allowable leakage of 10 mL/min under a pressure of 100 kPa. Since timing chain cover is installed on the side of engine and is also an appearance part, casting is not allowed to have surface defects such as flow marks and cold shuts.
Wall thickness and draft angle of timing chain cover are analyzed, as shown in Figure 2. Middle wall thickness of casting is 2 mm, and maximum wall thickness of edge is 21.9 mm; draft angle is ≥1.5°, which meets process requirements. Parting line is determined according to draft angle of casting. Main parting surface is selected at maximum contour of casting, which is a flat parting surface. Local parting surface is designed according to shape of parting line for local special-shaped parts. Since casting has no lateral core pulling and depression, there is no need to set a side drawing parting surface.
Location design of inner gate is the key to design of gating system. Inner gate is generally designed in thick wall area of casting on parting surface, which is conducive to transmission of injection pressure. Inner gate of this timing chain cover can be arranged at thick wall of outer edge. In order to prevent inner gate from impacting core, casting is suitable for filling in different areas with multiple branch runners.

 

Figure 1 3D diagram of timing chain cover

 

Figure 2 Determination of casting parting surface
Projection area of casting is 980 cm2, projection area of gating system is 30% of projection area of casting, and the total projection area is about 1 274 cm2. Since casting is a thin-walled seal, injection pressure ratio is selected as 100 MPa, safety factor is 1.2, and a 16 000 kN die-casting machine is selected after calculation.
According to wall thickness and structural characteristics of casting, inner gate filling speed is selected as 50 m/s, filling time is selected as 0.03 s, and cross-sectional area of inner gate is calculated to be 658 mm2. Chamber diameter is φ105 mm, and fast injection speed of injection punch is calculated to be 3.8 m/s.
Commonly used gating system forms of horizontal cold chamber die casting machines are shown in Figure 3. Horizontal gating system has a stable and controllable casting process and is suitable for most parts. Vertical gating system is suitable for parts with complex structures or special parts, especially two structures in Figure 3c and Figure 3d. Because structural form of gating leads to inconsistent gate heights, initial casting process adjustment process is complicated. Therefore, numerical simulation is required before scheme is determined to determine feasibility of process.
Due to particularity and complexity of structure of timing chain cover, two gating system schemes of Figure 3a and Figure 3c were initially selected in design stage, as shown in Figure 4. Horizontal gating enters from one side of timing chain cover, other three sides are opened with slag bags and exhaust channels; vertical gating arrangement is to turn irregular "ram's horn" shape of part upward, gating is filled from both sides of casting, slag bags are set on upper and lower sides of filling end. AnyCasting software is used to analyze its filling, exhaust, solidification, temperature and other processes to determine optimal process plan.

 

Figure 3 Common gating system structure for horizontal die castings
1. Casting 2. Runner

 

Figure 4 Timing chain cover structure diagram with gating system
1. Runner 2. Casting 3. Slag bag
Before using AnyCasting software for mold flow analysis, modeling entity in original 3D file needs to be exported in STL format and imported into simulation software, then variable grids are divided. When dividing grid, ensure that each section of part in three directions of X, Y, and Z has at least three layers of grids, including inner gate and overflow groove. Only when there are enough grids can accuracy of simulation calculation be guaranteed. Minimum size of automatic grid division of timing chain cover is set to 1, and maximum size ratio is set to 2; automatic transition smoothing factor is 1.1, and maximum size ratio is set to 3. A total of 5.67 million grids are divided for horizontal runner and 4.82 million grids for vertical runner.
Material of timing chain cover is ADC12 alloy, liquidus temperature is 595 ℃, solidus temperature is 540 ℃, pouring temperature is set to 645 ℃, and solidification shrinkage volume change is 7.14%; die casting mold in contact with aluminum alloy is imported hot working die steel W350 according to design requirements. The total weight of casting with pouring system is 4.77 kg, diameter of material cylinder is φ10 5mm, length is 780 mm, the first-level injection speed of injection punch is 0.15 m/s, and second-level fast injection speed is 3.8 m/s.
Timing chain cover is simulated and analyzed according to horizontal single-side pouring system in Figure 4a, and results are shown in Figure 5. It can be seen that: ① Filling process is relatively stable, high-speed switching starts at a filling time of 0.454 2 s, and filling is completed at 0.497 2 s. Filling time of casting is 0.043 0 s. Since wall thickness of middle area B is thin and there are many cavities that hinder filling of aluminum liquid, and boost pressure is difficult to be transmitted from B area to C area with a larger wall thickness, it will cause casting defects such as cold shut and poor filling in the area C far away from runner. ②Analysis of air entrainment during filling process shows that there is no obvious turbulence and air entrainment in casting cavity. Overflow and exhaust grooves are reasonably arranged at the end of filling air entrainment, which can assist discharge of end gas. ③Speed of gate is unstable during filling process. As shown in Figure 5c, instantaneous speed of gate in the middle part is greater than 80 m/s, and a jet flow will appear, causing poor filling of C area. At the same time, mold will be severely scoured, affecting quality of casting and life of mold. ④During solidification process, due to uneven wall thickness of casting and thin wall thickness in the middle (wall thickness is 2 mm), B area solidifies first, wall thickness of C area is thicker (local wall thickness is 21.9 mm) and far away from runner, and it solidifies last, forming a partially isolated liquid phase area, which will cause shrinkage holes in C area.

 

Figure 5 Numerical simulation of horizontal single-sided filling
According to analysis of numerical simulation of horizontal gating system of timing chain cover, gating system has a high risk of casting defects during filling and solidification process, and will reduce service life of mold. This gating system is not suitable for die casting of timing chain cover.
Timing chain cover is simulated and analyzed according to longitudinal double-sided filling gating system in Figure 4, and results are shown in Figure 6. It can be seen that ① Filling process realizes sequential filling of casting, high-speed switching starts at 0.447 9 s, and filling is completed at 0.477 6 s. Casting filling time is 0.029 7 s, which can realize rapid filling of thin-walled castings; at the same time, bidirectional filling is adopted, and inner gate enters cavity from thick wall of both sides, which is conducive to transmission of injection pressure. ②Analysis of air entrainment during filling process. Lower branch runner corresponds to internal cavity of casting. After molten metal enters cavity, it impacts cavity wall, causing local turbulence and air entrainment, as shown in Figure 6d. Overflow and exhaust grooves are reasonably arranged at upper air entrainment part at the end of filling, which can assist exhaust of end gas. Exhaust duct cannot be set at lower air entrainment part. ③Speed of inner gate during filling is relatively stable, and instantaneous speed is less than 60 m/s, which is suitable for filling thin-walled aluminum alloy parts. ④Middle thin-walled area solidifies first during solidification process, and thick-walled areas on both sides solidify later. During solidification process, thick walls on both sides are pressurized and compensated through inner gate. There is no isolated liquid phase area, and there is no risk of shrinkage.

 

Figure 6 Numerical simulation of longitudinal double-sided filling
According to analysis of numerical simulation of longitudinal gating system of timing chain cover, gating system is suitable for this casting, but there are problems in analysis of air entrainment process. There is vortex air entrainment in the middle of cavity during filling process. At the same time, exhaust duct cannot be opened at lower air entrainment part. Porosity defects will occur inside casting, which will affect sealing of casting in severe cases. Longitudinal double-sided pouring system is further optimized, as shown in Figure 7. Slag collection bag at confluence of lower metal liquid is changed to a branch runner to speed up filling speed in this area, making the entire filling process more stable and smooth. According to flow direction of metal liquid and position of air entrainment, two slag collection bags are added at central through hole to improve air entrainment state. Optimized pouring system is filled in a "U" shape. Numerical simulation is shown in Figure 7. It can be seen that filling process is stable, sequential filling, and exhaust is smooth; the two added overflow grooves play an auxiliary exhaust role in filling process. There is no isolated liquid phase area in solidification analysis, and there is no risk of shrinkage. Through temperature field analysis, temperature field of casting is balanced, which can greatly reduce risk of shrinkage and deformation during temperature reduction of casting. According to results of above simulation analysis, longitudinal U-shaped pouring system scheme is suitable for die casting of timing chain cover parts.

 

Figure 7 Numerical simulation of longitudinal U-shaped filling

 

Figure 8 Timing chain cover parts
Based on results of numerical simulation analysis of longitudinal pouring system of timing chain cover, longitudinal U-shaped pouring system was used to develop and trial-produce mold of new product. A 16,000 kN die-casting machine was used, and die-casting parameters optimized by numerical simulation were adopted: casting pressure was 100 MPa, slow injection speed was 0.15 m/s, and fast injection speed was 3.8 m/s. Appearance of casting had no obvious casting defects such as flow marks and cold shuts. Timing chain cover parts after cleaning are shown in Figure 8. X-ray flaw detection and sealing tests were performed on them. Product quality is excellent, and OTS sample passed at one time. It has now entered mass production, and product qualification rate is above 96%, which is higher than similar products.