Abstract: A die-casting process was designed for housing based on its structural characteristics. Numerical simulations of two gating systems using ProCAST software analyzed location and causes of shrinkage cavities and porosity. A more optimal gating system was selected for die-casting process optimization. Results showed that optimized castings were free of shrinkage cavities and porosity defects, production verification confirmed that they met technical requirements.
Die-casting is a highly automated casting technology capable of mass-producing complex parts. Resulting castings exhibit excellent density, precision, minimal machining allowance, and superior mechanical properties, making them widely used in automotive, machinery, and other fields. Chassis housing is a crucial mounting platform for automotive parts. While its walls are relatively thin, high mechanical properties, precision, airtightness requirements are high, and large-scale production is required. Therefore, die casting is the best choice for manufacturing housing.
In this study, casting structure was analyzed, a gating system was designed, and simulations were performed using ProCAST software. By analyzing simulation results, process was optimized to eliminate existing defects such as shrinkage cavities and porosity, resulting in a die-casting process that meets technical requirements for housing.

Casting studied is a chassis housing for automotive parts produced by a certain company. A schematic diagram of its 3D model is shown in Figure 1. Dark areas represent machined surfaces of casting, with a machining allowance of 0.5 mm. Casting's outline dimensions are 103 mm * 98 mm * 89 mm, with a volume of 234 * 108 m³ and a weight of 632 g. The thickest wall is 5.5 mm, the thinnest wall is 2.5 mm, and average wall thickness is 3 mm. Casting material is YL113, an Al-Si-Cu alloy with good fluidity, excellent airtightness, and high wear resistance. Its alloy composition is shown in Table 1. Casting requires a smooth surface, a draft angle not exceeding 1.5°, a casting shrinkage of 0.6%, and no internal defects such as shrinkage cavities or porosity.

 

Figure 1 Schematic diagram of three-dimensional shape of casting

 

Table 1 YL113 alloy composition (wB/%)

In mold design, die-casting process is the most important, directly affecting casting quality, production cost, and mold manufacturing difficulty. Die-casting process includes selection of parting surface, design of gating system, design of overflow and exhaust systems.

Housing has a complex shape and requires a core-pulling mechanism, which makes mold manufacturing difficult. Therefore, casting is cast using a single-mold, single-cavity method. The most basic principle for parting surface selection is to select the area with the largest projected area of casting. There are two methods for setting parting surface for this casting, as shown in Figure 2. Using parting surface a only requires a single core-pulling mechanism, but casting cavity is deep, resulting in a strong core-clamping force and difficulty in removing casting. Furthermore, thin casting wall makes it difficult to install an ejection mechanism. Using parting surface b requires multiple core-pulling mechanisms, making mold manufacturing more complex. However, casting is essentially symmetrical from top to bottom, ensuring smooth filling. Furthermore, it facilitates installation of an ejection mechanism, facilitates placement of overflow and venting grooves, and better meets requirements of die-casting process. Therefore, parting surface b was selected for this casting.

 

Figure 2 Schematic diagram of casting parting surface

A 3D schematic diagram of gating system design is shown in Figure 3.

 

Figure 3 3D schematic diagram of gating system

Ingates can be classified into flat ingates, end-face ingates, center ingates, and ring ingates. This shell is cylindrical. To prevent molten metal from directly impacting core and causing adhesion, both casting systems use an annular gate for tangential feeding. That is, an annular runner is set up next to casting. Molten metal enters cavity after filling annular runner. In this way, molten metal can obtain approximately same speed along annular circumference, making molten metal filling mold smooth and gas in cavity easy to discharge. Secondly, a push rod can also be set on ingates to avoid push rod marks on casting. Cross-sectional area of ingates is calculated according to formula (1).

 

Where: Ag is cross-sectional area of ingates, m2; V is sum of volumes of die casting and overflow tank, m³; v is filling speed of molten metal at ingates, m/s; t is filling time of cavity, s. For aluminum alloys, gate speed is generally 20 to 60 m/s. Filling time for a casting with an average wall thickness of 3 mm is 0.028 to 0.04 s, and gate wall thickness is 1.5 to 2.5 mm. A filling speed of 40 m/s, a filling time of 0.03 s, and a gate wall thickness of 2 mm are assumed. Calculated gate cross-sectional area Ag = 223.6 m2, gate width is 111.8 mm, and with four directional feeds, average gate width is 28 mm. Designed annular runner has an outer diameter of 106 mm, an inner diameter of 86 mm, and a thickness of 20 mm.

Runner is transition channel from sprue to gate. Different runner structures are used for different die castings. For cylindrical castings, a circular converging structure is used. In order to prevent metal liquid from generating negative pressure during its flow, cross-sectional area of runner should be gradually reduced. This study uses a horizontal cold chamber die casting machine, and depth of its runner is calculated according to formula (2). D = (5-8) T (2). 
Where: D is runner depth, mm; T is gate thickness, mm. Take D = 10 mm. Since runner feeds tangentially, minimum runner width is 20 mm. In order to facilitate better demolding of casting, runner draft angle is set to 15°.

Sprue is primary channel for metal liquid to enter cavity from die casting machine. Its size is related to pressure chamber diameter of die casting machine. In this study, pressure chamber diameter is selected as 60 mm, excess material thickness is set to 10 mm, and draft angle is 10°.

According to design principle, overflow trough is generally set at a place where metal liquid converges and it is difficult to exhaust near cavity. This casting is parted from top to bottom, so molten metal converges at parting surface. Therefore, both gating systems require an overflow trough at parting surface. Furthermore, to prevent backflow of molten metal from ring runner, both gating systems require an overflow trough on each side of ring runner. As shown in Figure 2b, wall thickness at the front of casting is relatively thick. To adequately remove gas and inclusions, gating system I features an overflow trough on each side. 3D schematics of overflow troughs for gating systems I and II are shown in Figures 4a and b, respectively.

 

Figure 4 3D Schematic of Overflow Trough

3D model was imported into simulation software ProCAST for meshing. Mesh cell size for casting was set to 2 mm, and mesh cell size for mold was set to 10 mm.

Set simulation process parameters: (1) Casting material is European standard ENAC-41600 (YL113), and mold material is H13; (2) Recommended value of molten metal pouring temperature is shown in Table 2. In this study, 620 ℃ is taken, and mold preheating temperature is 220 ℃; (3) Heat transfer coefficient between mold and casting is 1 000 W/(m2·K); (4) Filling speed of molten metal is 3 m/s, and casting is cooled by air cooling.

Table 2 Aluminum alloy pouring temperature

Filling process of pouring system I is shown in Figure 5. As can be seen, at 32.7% filling, molten metal begins to flow toward ingates; at 52.4% filling, molten metal flows toward bottom of casting, and a small amount begins to flow toward overflow. At 74.2% filling, casting contour is almost completely filled, with a small amount of molten metal flowing toward overflow. At 96.85% filling, casting contour is completely filled, and molten metal flows toward overflow trough. The entire filling process demonstrates smooth metal flow, flowing from sprue to runner, then to annular ingates, and finally to overflow trough, which stores cool, contaminated metal and air. Molten metal flow is correct, and gating system I is properly configured.

 

Figure 5: Schematic diagram of the filling process of gating system I
Figure 6: Filling process of gating system II. As shown in figure, when mold is 34.5% full, molten metal begins to flow from annular inner gate into casting. At 55.6% full, molten metal flows from four directions toward ends of casting, with a small amount beginning to flow toward overflow. At 76.8% full, casting contour is almost completely filled, and molten metal flows into overflow trough. At 96.8% full, molten metal fills overflow trough, storing cool, contaminated metal. Throughout filling process, molten metal flows smoothly without splashing. Molten metal flows from annular inner gate into casting, ultimately filling overflow trough. Correct molten metal flow direction indicates that gating system II is properly configured.

 

Figure 6 Schematic diagram of filling process for gating system II
Temperature fields of castings from gating systems I and II at complete solidification are shown in Figure 7. It can be seen that temperature in the middle of sidewalls of castings from both gating systems is higher. This is because ribs in the middle of castings, coupled with thicker casting walls, dissipate heat more slowly during solidification, making it more susceptible to formation of heat spots. Shrinkage cavities and porosity are predicted to occur in this area.

 

Figure 7: Temperature field after complete solidification of casting
Figure 8 shows shrinkage cavities and porosity defects produced in a casting using gating system I. Defects are concentrated in areas where rib wall thickness is greater. This is because casting solidifies at a higher temperature in this area, resulting in slower solidification of molten metal and uneven solidification rates. This creates isolated small pores, which prevent molten metal from feeding shrinkage during complete solidification, leading to shrinkage cavities and porosity defects.

 

Figure 8: Schematic diagram of defects in gating system I
A schematic diagram of defects in a casting using gating system II is shown in Figure 9. Shrinkage cavities and porosity defects also occur in thicker areas of casting wall, and are more numerous than those produced using gating system I. This is because ingate of gating system II is closer to thicker areas of casting wall, while ingate of gating system I is farther away. Therefore, in thick wall areas of casting, gating system II generates higher temperatures than gating system I. During solidification, casting produces more isolated small pores due to uneven solidification rates. Upon complete solidification, more shrinkage cavities and porosity defects are produced. Therefore, of two gating systems, gating system I produces fewer defects and a superior process.

 

Figure 9: Schematic diagram of defects in gating system II

To ensure uniform solidification in thick wall areas of casting, this study installed cooling water channels directly below shrinkage and porosity areas of casting. Two overflow troughs were also installed where these areas were concentrated to fully remove gas and inclusions, shift location of porosity. Optimized process scheme is shown in Figure 10. After optimization, heat transfer coefficient between cooling water channel and mold was set at 2000/(m2·K), while other parameters remained unchanged. Shrinkage cavities and defects after optimized process are shown in Figure 11. As can be seen, casting has no shrinkage and porosity defects in thick wall areas of ribs, meeting technical requirements. Defects of preliminary design process are caused by uneven cooling of casting.

 

Figure 10 Three-dimensional diagram of optimization scheme

 

Figure 11 Schematic diagram of shrinkage cavities and shrinkage after optimization

Figure 12 shows shell casting produced after optimization of gating system I. Through testing, it was found that there were no shrinkage cavities and shrinkage defects inside, no cracks on the surface, it met mechanical performance requirements, achieved dimensional accuracy and airtightness requirements. It has been mass-produced.

 

Figure 12 Actual picture of casting

(1) According to structure of shell, two gating systems of casting were designed, and numerical simulation was performed using ProCAST software. Results showed that casting had shrinkage cavities and shrinkage defects in wall thickness; shrinkage cavities and shrinkage defects were less when inner gate was set farther away from wall thickness. Analysis found that reason for shrinkage cavities and shrinkage in casting was that casting solidified unevenly in wall thickness, some areas were isolated and could not be compensated for shrinkage.
(2) Through process optimization, casting is free of shrinkage cavities and shrinkage defects, and optimized process is used for production verification. Through testing, it is found that there are no shrinkage cavities and shrinkage defects inside casting, and it meets technical requirements, which can be used to guide design of similar casting die-casting processes.