At present, die-casting aluminum alloy parts have been widely used in the fields of automobile, aerospace and electronic industry. Replacing steel parts with aluminum alloy parts can achieve a weight reduction of 40% to 50%, which is conducive to reducing energy consumption and is an important way to achieve energy saving and emission reduction. Therefore, more and more automotive structural parts are made of aluminum alloy to reduce weight, but its complex structure and high force requirements pose challenges to die-casting process. Many structural parts of automobiles are mostly thin-walled shell parts. Studies have shown that when wall thickness of casting is less than 4mm, Laplace force caused by surface tension of liquid metal will seriously affect flow state of filling liquid, and effect of viscous force is also prominent, which will make it difficult to fill thin-walled part in mold cavity. Die casting technology is to fill cavity with molten metal under pressure, which can not only effectively solve problem of filling mold, but also make molten metal solidify quickly, refine alloy structure, and obtain alloy parts with higher strength.
Due to general shape of these shell parts is relatively complex and local wall thickness is uneven, flow process of metal in mold cavity is also relatively complicated, due to different properties of different casting materials and metal materials, it is also difficult to grasp quality of castings. Nowadays, with development of computer technology, numerical simulation software can more and more accurately reflect flow process of molten metal in die-casting molds, and can accurately predict location of casting defects. Therefore, it is an efficient and cost-effective method to use numerical simulation software to simulate filling and solidification process in advance, then design and optimize die-casting process, analyze quality of parts based on simulation results.
Shock tower of automotive structural part involved in this article is a large and complex aluminum alloy die-casting part. Using FLOW-3D numerical simulation software to simulate, guide design of die-casting process plan of part, verify rationality and feasibility of part manufacturing process. According to simulation results, process plan is improved, finally high-quality die-casting parts are obtained, which improves production efficiency of parts and reduces production costs.

(a) Convex surface Figure 1 3D solid modeling of a shock tower (b) Concave surface
Figure 1 is a schematic diagram of a three-dimensional solid modeling of a shock tower. Maximum outline size of casting is 530mm*345mm*313mm, and average wall thickness of main body is 3mm. Structure of casting is complex, the whole shell is arc-shaped, and surface is designed with criss-cross ribs to improve the overall strength of part; there are many near-cylindrical bosses locally, with a maximum height of 20mm, which makes wall thickness of each part of casting vary greatly. There is a large protruding structure on one side of casting, with a height difference of 195mm from shell of casting. Shock tower is formed by die-casting of A380 aluminum alloy, and net weight of casting is 2.9kg.

Gating system is channel through which molten metal fills cavity under pressure, is an important part of controlling speed, time and flow state of molten metal filling cavity. Therefore, designing a reasonable gating system is an important part of obtaining high-quality die castings. According to characteristics of casting, the largest area of casting contour size is selected as parting surface to facilitate demoulding of part. In order to reduce degree of air entrainment at the beginning of die-casting process, inner gate is set on the side with a relatively straight shape and structure in length direction of part. Calculate cross-sectional area of ingate 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 ingate. According to design manual, filling speed of aluminum alloy at inner gate is 20~60m/s, and value is 40m/s; t is time for molten metal to fill cavity, and 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 area (mm2) of parts with wall thickness b1, b2, b3.... Average wall thickness of shock tower is calculated to be 3 mm, and recommended cavity filling time is 0.05-0.10 s, with a value of 0.07 s. Calculated cross-sectional area Ag of ingate is 391.87mm2; according to design manual, thickness T of ingate is 1.5mm, and the total width of ingate is 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, thickness of runner is D=(8~10)T=15mm; Runner adopts common flat trapezoid with low heat loss of molten metal and convenient processing. Runner adopts common flat trapezoid with low heat loss of molten metal and convenient processing. According to size of pressure chamber of die-casting machine, diameter of sprue (diameter of pressure chamber) is 120mm. Using calculated parameters of sprue, runner and ingate, casting system of shock tower part is designed, as shown in Figure 2.

 

Figure 2 Shock tower gating system

Overflow tank is used to store cold dirty metal liquid mixed with gas and paint residue at the front end of liquid-gas interface. Cooperating with exhaust tank, it can quickly draw out gas in cavity and reduce occurrence of gas entrainment during filling process. At the same time, it can also transfer parts of shrinkage cavity, shrinkage porosity, eddy current entrainment and cold shut. However, in order to play role of overflow groove, overflow must receive front cold dirty metal liquid in a reasonable position and keep it in overflow groove according to flow characteristics of molten metal in cavity. Therefore, overflow groove also needs suitable size. Neither too large nor too small, too large will lead to more waste and increase costs; too small will cause overflow tank to not accept all cold dirty metal, reduce quality of casting. Therefore, it is an efficient design method to carry out numerical simulation on parts with designed gating system first, then determine appropriate overflow system according to flow characteristics of molten metal.
Set simulation parameters in Flow-3D according to actual die-casting process parameters. Molten metal first enters runner and ingate at a slow injection speed of 0.6m/s. When molten metal fills all ingates, injection speed is increased to 5m/s, that is, molten metal fills cavity at a high speed.

 

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.
Simulation results of gating system: temperature field and entrained air
Flow-3D system gives temperature and gas entrainment of molten metal at different time points during filling process. It can be seen that designed gating system can fill cavity more smoothly 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 tends to generate eddy currents when filling here, resulting in an increase in the amount of entrained gas. Therefore, overflow grooves should be designed on both sides of circular structure, so that molten metal in entrained part can be discharged from cavity and enter overflow groove. According to temperature field and characteristics of entrained gas, it can be seen that there is a large area of molten metal with a lower temperature on the right side of part, and there are different degrees of gas entrained in direction extending from the edge to the inside, as shown in circled part in Figure 3(c). Corresponding to structure of shock absorber shown in Figure 1, it can be seen that structure of circled part in figure is relatively complicated. After molten metal enters cavity through rightmost ingate, it directly impacts cavity wall with 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, which can be seen from diagram where molten metal begins to enter cavity (Fig. 3(a)). Parts are filled sequentially from bottom to top. There is a large amount of molten metal with low temperature and severe gas entrainment on upper part of part where molten metal is finally filled, and enough overflow tanks should be set here to receive these molten metals to obtain high-quality castings.
According to simulation results of Flow-3D, there are many molten metals with low temperature and large gas entrainment in some parts, an overflow tank with sufficient volume should be designed, but an overflow tank that is too large will easily lead to backflow of molten metal. Therefore, multiple separate overflow tanks and thin connecting ribs are set at these parts to ensure their strength. Overflow tank mainly adopts a trapezoidal overflow tank that is easy to process, volume of overflow tank is appropriately increased in the part where local air entrainment is serious, and shape is slightly modified according to flow characteristics (circled part in Figure 3 (c)). According to design manual, cross-sectional area of vent groove is set to 30% of cross-sectional area of ingate. Designed overflow tank and exhaust tank are shown in Figure 4.

 

Figure 4 Die-cast shock tower overflow groove and exhaust groove

 

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.
Fig. 5 Simulation results with gating system, overflow groove and exhaust groove: temperature field and air entrainment
Figure 5 shows filling process of molten metal in a die-casting mold with gating system, overflow groove and exhaust groove simulated by Flow-3D system. It can be seen that during filling process of molten metal, part of molten metal located at the front of liquid-gas interface with a lower temperature and severe gas entrainment all enters designed overflow tank, and after molten metal fills cavity (Fig. 5( d)), amount of gas left inside part is minimal. Therefore, designed overflow groove and exhaust groove are suitable for die-casting process of shock tower parts.

 

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 raised structure-concave surface.
Figure 6 shows shape of casting obtained after molten metal is completely solidified. It can be seen that there is a large hole defect in upper part of raised structure in shock tower part. Observing its partial 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 and large part is slow, phenomenon of feeding occurs and holes are formed.
In this regard, method of local chilling is adopted to accelerate solidification speed of this part to obtain dense castings. A chilled copper block is added to mold at this place to achieve purpose of chilling. Flow-3D simulation results are shown in Figure 7, a sound casting with a dense interior and no porosity is obtained. Finally, this process is 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 rate is expected to be further improved.

 

Fig.7 Sound casting obtained after partial quenching

Design and optimization of large and complex automobile structural parts - die-casting pouring system and overflow and exhaust system of aluminum alloy shock tower;
Using method of numerical simulation to analyze location and area of air entrainment of shock tower parts, predict type and location of die-casting defects, and change design of gating system based on this;
Gas entrainment and shrinkage defects are prone to occur at circular structure with a large wall thickness. Local quenching method and other technological measures are adopted to eliminate defects and obtain aluminum alloy shock tower die-casting parts with good overall quality.