(a) Convex surface (b) Concave surface
Figure 1 3D solid model of a certain shock-absorbing tower
Figure 1 is a schematic diagram of a 3D solid model of a shock-absorbing tower. Maximum outline size of casting is 530mm*345mm*313mm, and average wall thickness of main body is 3mm. Casting structure is complex, the entire shell is arc-shaped, and surface is designed with criss-crossing reinforcement ribs to improve the overall strength of part; there are many nearly cylindrical bosses in parts, with maximum height reaching 20mm, which makes wall thickness of various parts of casting vary greatly. There is a larger convex structure on one side of casting, with a height difference of 195mm from casting shell. Shock tower is made of A380 aluminum alloy die-casting, with a net weight of 2.9kg.

Gating system is a channel for molten metal to fill mold cavity under pressure. It is an important part of controlling speed, time and flow state of molten metal filling mold cavity. Therefore, a properly designed gating system is an important part of obtaining high-quality die castings. According to characteristics of casting, area with the largest contour size of casting is selected as parting surface to facilitate part demoulding. In order to reduce degree of air entrainment at the beginning of die-casting process, an ingate is set on the side with a straighter shape and structure in length direction of part. Calculate cross-sectional area of inner gate according to empirical formula (1):

 

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

 

In formula, b1, b2, b3... are wall thickness (mm) of a certain part of casting, and S1, S2, S3... are areas (mm2) of parts with wall thickness b1, b2, b3.... Average wall thickness of shock tower is calculated to be 3mm, and recommended mold cavity filling time is 0.05~0.10s, with a value of 0.07s. Calculated cross-sectional area Ag of inner gate is 391.87mm2; according to design manual, thickness T of inner gate is 1.5mm, and the total width of inner 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 thickness of runner is D=(8~10)T=15mm. Horizontal runner is a common flat trapezoid that has low heat loss of molten metal and is easy to process. According to die-casting machine pressure chamber size, sprue diameter (pressure chamber diameter) is 120mm. Using calculated parameters of sprue, lateral runner and inner gate, gating system of shock tower parts is designed, as shown in Figure 2.

 

Figure 2 Shock tower pouring system

Overflow tank is used to store cold and dirty metal liquid mixed with gas and paint residue at the front end of liquid-gas interface. Cooperating with exhaust channel, it can quickly lead out gas in mold cavity and reduce occurrence of air entrainment during filling process. At the same time, it can also remove shrinkage cavities, shrinkage porosity, vortex air inclusions and cold insulation parts. However, to play role of overflow tank, overflow must accept front cold dirty metal liquid at a reasonable position and retain it in overflow tank based on flow characteristics of molten metal in mold cavity. Therefore, overflow tank also needs to be appropriately sized. It should be neither too large nor too small. If it is too large, it will lead to an increase in scrap and increase costs; if it is too small, overflow tank will not be able to accept all cold metal, which will reduce quality of casting. Therefore, it is an efficient design method to first perform numerical simulation on designed parts of gating system, then determine appropriate overflow system based on flow characteristics of molten metal.
Simulation parameters are set according to actual die-casting process parameters. Molten metal first enters lateral runner and inner gate at a slow injection speed of 0.6m/s. When molten metal fills all inner gates, injection speed is increased to 5m/s, allowing molten metal to fill mold cavity quickly.

 

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

 

Entrained gas (color scale represents volume fraction of entrained gas): (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 conditions
Figure 3 shows temperature and gas entrainment of molten metal at different time points during mold filling process. It can be seen that designed pouring system can achieve a relatively smooth filling of mold cavity with molten metal. There are two circular structures on the left side of part. According to simulation of filling process, it can be seen that molten metal easily generates eddy currents when filling here, resulting in an increase in air entrainment. Therefore, overflow grooves should be designed on both sides of circular structure so that molten metal in air-entrained part can be discharged from mold cavity and enter overflow groove. According to temperature field and gas entrainment characteristics, it can be seen that there is a large area of lower temperature molten metal on the right side of part, and there are varying degrees of gas entrainment in direction extending inward from the edge, as shown in the area circled in Figure 3(c). Corresponding to shock tower structure shown in Figure 1, it can be seen that structure of circled part in figure is relatively complex. Molten metal enters mold cavity through inner gate on far right and directly impacts mold cavity wall at a certain angle. After being blocked, molten metal flows back to fill rightmost part of part, thus causing a large amount of gas to be involved. This can be seen from figure where molten metal begins to enter mold cavity (Figure 3(a)). Parts are filled in sequence from bottom to top. There is a large amount of molten metal with lower temperature and serious entrainment on the part where molten metal is last filled. Enough overflow tanks should be set up here to accept molten metal to obtain high-quality castings.
According to simulation results, there are many molten metals with low temperature and large air entrainment in some parts. An overflow tank with sufficient volume should be designed. However, an overflow tank that is too large can easily cause liquid metal to flow back. Therefore, multiple individual overflow channels are provided at these locations and thin connecting ribs are provided to ensure their strength. Overflow tank mainly uses a trapezoidal overflow tank that is easy to process. Volume of overflow tank is appropriately increased in parts with severe local air entrainment and shape is slightly modified according to flow characteristics (circled in Figure 3(c)). According to design manual, cross-sectional area of exhaust channel is set to 30% of cross-sectional area of inner gate. Designed overflow tank and exhaust channel are shown in Figure 4.

 

Figure 4 Die-cast shock tower overflow tank and exhaust channel

Gate is connected to casting by bottom of runner, and a bubble gathering area is designed in the middle 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.

 

Entrained gas (color scale represents volume fraction of entrained gas): (a)t=0.190s; (b)t=0.197s; (c)t=0.201s; (d)t=0.215s.
Figure 5 Simulation results with gating system, overflow tank and exhaust duct: temperature field and air entrainment conditions
Figure 5 shows filling process of molten metal in a die-casting mold with a gating system, an overflow tank, and an exhaust channel. It can be seen that during process of filling mold with molten metal, part of molten metal with lower temperature and serious air entrainment located at the front edge of liquid-gas interface all enters designed overflow tank. After molten metal fills mold cavity (Figure 5(d)), amount of gas remaining inside part is very small. Therefore, designed overflow groove and exhaust channel are suitable for die-casting process of shock tower parts.

 

Figure 6. Solidification process simulation
(a) Completely solidified; (b) Magnified view of upper part of convex structure - convex surface; (c) Magnified view of upper part of convex structure - concave surface.
Figure 6 shows shape of casting after molten metal has completely solidified. It can be seen that there is a large hole defect in upper part of raised structure in shock tower part. Looking at partial enlargement, it can be found that there are two large 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 is used to speed up solidification rate in this part to obtain a dense casting. A copper block is added to mold here to achieve rapid cooling. Simulation results are shown in Figure 7, a high-quality casting with a dense and non-porous interior is obtained. Finally, this process was used to actually produce qualified aluminum alloy shock tower parts, with a yield of more than 90%. If other conditions such as mold temperature are controlled, yield is expected to be further improved.

 

Figure 7 High-quality castings obtained after partial cooling

1. Design and optimize die-casting pouring system, overflow and exhaust systems of large and complex automotive structural parts - aluminum alloy shock towers.
2. Use numerical simulation methods to analyze air entrainment generation parts and areas of shock tower parts, predict types and locations of die-casting defects, and change design of gating system based on this.
3. Air entrainment and shrinkage cavity defects are prone to occur in circular structures with large wall thickness. Process measures such as local cooling methods are used to eliminate defects and obtain aluminum alloy shock tower die castings with good overall quality.