(a) Convex Surface (b) Concave Surface
Figure 1: 3D Solid Model of a Shock Tower
Figure 1 shows a schematic diagram of 3D solid model of a shock tower. Maximum outline dimensions of casting are 530mm*345mm*313mm, and average wall thickness of main body is 3mm. Casting has a complex structure, with the entire shell being curved and surface designed with crisscrossing reinforcement ribs to enhance the overall strength of part. Numerous near-cylindrical bosses, with a maximum height of 20mm, result in significant variations in wall thickness across casting. A large raised structure is located on one side of casting, with a height difference of 195mm from shell. Shock tower is die-cast from A380 aluminum alloy and weighs 2.9kg.

 

Gating system is channel for molten metal to fill mold cavity under pressure. It is an important part to control speed, time and flow state of molten metal filling mold cavity. Therefore, a reasonable design of gating system is an important link to obtain high-quality die castings. According to characteristics of casting, part with the largest area of casting outline is selected as parting surface to facilitate demolding of part. In order to reduce degree of air entrainment in initial stage of die casting process, side with a relatively straight shape structure is selected in longitudinal direction of part to set gate. Cross-sectional area of gate is calculated according to empirical formula (1):

 

Where, V is the total volume of part and overflow and vent system (overflow and vent system volume is calculated as 50% of part volume), which is 1157422mm3; νg is speed of molten metal at gate. According to design manual, filling speed of aluminum alloy at gate is 20~60m/s, and value is 40m/s; t is time for molten metal to fill mold cavity. Its recommended value is determined by average wall thickness. Average wall thickness is calculated according to empirical formula (2):

 

Where b1, b2, b3... are wall thicknesses of a certain part of casting (mm), and S1, S2, S3... are areas of parts with wall thicknesses b1, b2, b3... (mm2). Average wall thickness of shock absorber is calculated to be 3mm, and recommended value of cavity filling time is 0.05~0.10s, with a value of 0.07s. Calculated cross-sectional area of gate Ag is 391.87mm2; according to design manual, gate thickness T is 1.5mm, and the total width of gate L=Ag/T=261.25mm. Die casting machine is a horizontal cold chamber die casting machine, cross-sectional area of runner is Ar=(3~4)Ag=1371.545mm2, and runner thickness D=(8~10)T=15mm; runner is a common flat trapezoidal shape that has low heat loss of molten metal and is easy to process. Based on die-casting machine's pressure chamber dimensions, sprue diameter (pressure chamber diameter) is 120 mm. Using calculated parameters for sprue, runner, and ingate, gating system for shock tower component was designed, as shown in Figure 2.

 

Figure 2 Shock Tower Gating System

Overflow channel is used to store cold, contaminated molten metal mixed with gas and paint residue at the front of liquid-gas interface. Working in conjunction with vent, it quickly removes remaining gas from mold cavity, reducing occurrence of air entrainment during mold filling. It also serves to divert shrinkage cavities, shrinkage porosity, vortex gas entrainment, and cold shuts. However, to fully utilize overflow channel, it must receive and retain cold, contaminated metal at the front of overflow channel at a suitable location, based on flow characteristics of molten metal in mold cavity. Therefore, overflow channel must be appropriately sized. It should be neither too large nor too small. Oversize will result in increased scrap and cost, while undersize will prevent overflow channel from receiving all contaminated metal, reducing casting quality. Therefore, it's an efficient design approach to first perform numerical simulation on components of designed gating system, then determine appropriate overflow system based on flow characteristics of molten metal.
Simulation parameters were set based on actual die-casting process parameters. Molten metal initially entered runner and ingates at a slow injection speed of 0.6 m/s. Once all ingates were filled, injection speed was increased to 5 m/s, allowing molten metal to rapidly fill mold cavity.

 

Temperature field (color scale represents temperature): (a) t=0.190s; (b) t=0.197s; (c) t=0.200s; (d) t=0.204s.

 

Air entrainment (color scale represents volume fraction of entrained air): (a) t=0.190s; (b) t=0.197s; (c) t=0.200s; (d) t=0.204s.
Figure 3. Gating system simulation results: temperature field and air entrainment
Figure 3 shows molten metal temperature and air entrainment at different points during filling process. It can be seen that designed gating system enables molten metal to fill mold cavity relatively smoothly. There are two circular structures on the left side of part. Simulations of filling process show that molten metal is prone to eddy currents during filling, which increases amount of air entrainment. Therefore, overflow troughs should be designed on both sides of circular structures to allow entrained metal to be discharged into overflow troughs. Based on temperature field and air entrainment characteristics, a large area of cooler molten metal exists on the right side of part. Extending inward from edge, varying degrees of air entrainment occur, as shown in circled area in Figure 3(c). Corresponding to shock tower structure shown in Figure 1, circled area is relatively complex. After entering mold cavity through rightmost ingate, molten metal first directly impacts cavity wall at a certain angle. After being blocked, molten metal flows back to fill rightmost area of part, resulting in significant air entrainment. This can be seen in image of molten metal entering mold cavity (Figure 3(a)). Part is filled sequentially from bottom to top. A large amount of cooler molten metal with significant air entrainment exists above part, where molten metal is last to be filled. Sufficient overflow troughs should be provided to receive this molten metal to achieve high-quality castings.
Simulation results indicate that certain areas contain a significant amount of molten metal with low temperatures and high air entrainment. Therefore, overflow troughs of sufficient volume should be designed. However, overly large overflow troughs can easily cause metal backflow. Therefore, multiple separate overflow troughs are provided in these areas, along with thin connecting ribs to ensure strength. Overflow trough primarily utilizes a trapezoidal shape for ease of fabrication. In areas with severe air entrainment, overflow trough volume is appropriately increased, shape is slightly modified based on flow characteristics (see circled area in Figure 3(c)). According to design manual, cross-sectional area of vent channel is set to 30% of cross-sectional area of inner gate. Designed overflow trough and vent channel are shown in Figure 4.

 

Figure 4: Die-cast shock tower overflow trough and vent channel

Gate is connected to casting at the bottom of runner. A bubble collection area is designed in the center of each gate.

 

Temperature field (color scale represents temperature): (a) t=0.190s; (b) t=0.197s; (c) t=0.201s; (d) t=0.215s.

 

Air entrainment (color scale represents volume fraction of entrained air): (a) t=0.190s; (b) t=0.197s; (c) t=0.201s; (d) t=0.215s.
Figure 5 Simulation results with a gating system, overflow trough, and vent: temperature field and air entrainment
Figure 5 shows filling process of molten metal in a die-casting mold equipped with a gating system, overflow trough, and vent. It can be seen that during filling process, lower-temperature, more entrained portion of molten metal at liquid-air interface front enters designed overflow trough. After molten metal fills mold cavity (Figure 5(d)), very little air remains inside part. Therefore, designed overflow groove and exhaust duct are suitable for die casting process of shock tower part.

 

Figure 6. Solidification process simulation
(a) Complete solidification; (b) Enlarged view of upper part of convex structure - convex surface; (c) Enlarged view of upper part of convex structure - concave surface.
Figure 6 shows shape of casting obtained after metal liquid is completely solidified. It can be seen that there is a large hole defect on upper part of convex structure in shock tower part. By observing its local enlarged view, it can be found that there are two large-sized nearly cylindrical bosses with a height of 20mm. During solidification process, solidification speed of this thick part is slow, and shrinkage will occur, forming holes.
In this regard, local cooling method is adopted to accelerate solidification speed of this part to obtain a dense casting. A copper block is added to mold at this part to achieve purpose of rapid cooling. Simulation results are shown in Figure 7, a high-quality casting with a dense interior and no pores is obtained. Finally, this process is used to actually produce qualified aluminum alloy shock tower parts, with a yield rate of more than 90%. By controlling other conditions such as mold temperature, yield rate is expected to be further improved.

 

Figure 7: High-quality castings obtained after localized cooling

1. Die-casting gating system, overflow, and exhaust systems for a large, complex automotive component—an aluminum alloy shock absorber tower—were designed and optimized.
2. Numerical simulation was used to analyze locations and regions where air entrapment occurs in shock absorber tower, predicting types and locations of die-casting defects. Based on this, gating system design was modified.
3. Air entrapment and shrinkage defects are common in thick circular structures. Localized cooling and other process measures eliminated these defects, resulting in an aluminum alloy shock absorber tower die-casting with excellent overall quality.