Abstract: To achieve efficient and high-quality production of an automotive engine oil pump cover, three theoretically feasible gating and overflow system schemes were designed based on characteristics of pump cover part and die-casting theory and experience. Filling and solidification processes of casting were then simulated using ProCAST software. Simulation of filling process revealed that air entrapment easily occurs at the top of casting; therefore, an optimization scheme of adding an overflow groove at this location was proposed. Solidification simulation showed that eliminating overflow groove inside sprue and shortening ingate length had no impact on distribution of shrinkage porosity and shrinkage cavities in casting, could also improve heat dissipation in casting area inside sprue and reduce energy loss during molten metal flow. Scheme three, with its gating and overflow system, reduced air entrapment. X-ray flaw detection of trial casting showed no shrinkage cavities in key locations. Metallographic analysis showed that microstructure of all parts of casting was dense, with a grain size grade of 4 and a microhardness greater than HV85, meeting product requirements.
High-pressure die casting (HPDC) is a highly efficient near-net-shape forming process. Typical pouring temperature for aluminum alloy castings is approximately 670-700℃, filling and holding pressure is usually 400-500 MPa, filling speed is 0.5-120 m/s, and filling time is only 0.01-0.2 s. It is characterized by high temperature, high pressure, and high speed, can produce various complex, thin-walled parts; the thinnest aluminum alloy can reach 0.5 mm or even lower. Due to "high temperature, high pressure, and high speed" of die casting, even slight imperfections in die casting mold and process, or improper operation, will significantly affect casting quality, production efficiency, and mold life. Among factors influencing quality of die castings are many, the most important being: first, alloy composition and melt quality; second, mold pouring, overflow, and venting design; third, die casting process parameters; and fourth, factors such as spraying and mold opening time. Alloy composition determines size of liquidus-solidus zone, thus affecting alloy fluidity and feeding ability, while degassing and slag removal effects of melt determine alloy's performance. Die-casting mold gating system not only determines pouring direction, overflow and venting conditions, pressure transmission, filling speed, and filling time, but also affects mold temperature distribution and mold life. Die-casting process parameters are equally important; pouring temperature, injection speed and pressure, mold preheating temperature, mold dwell time all affect density and internal stress of casting. Assuming alloy composition is qualified and operating procedures are followed, design of die-casting mold's gating, overflow, and venting systems, along with selection of process parameters, become decisive factors in die-casting quality.
Although pump covers are small parts, they often have thick walls and curved surfaces, and are generally die-cast from aluminum alloy. Defects such as surface black spots, cracks, porosity, flow marks, and cold shuts commonly occur during die-casting production of cover-type parts. As a crucial component of oil pump, pump cover is related to pump's airtightness and heat dissipation; its forming quality is extremely critical. Tang Wenzhuo et al. significantly improved air entrapment in mold cavity, casting porosity, surface quality by changing shape of top cover's horizontal runner, adjusting position and size of overflow groove. Li Xianjun et al. optimized design of die-casting mold for ZL102 alloy pump covers, which not only made mold structure more compact and improved efficiency, but also optimized mold's venting and slag removal capabilities, stabilizing product quality. It is evident that design of die-casting molds and selection of process parameters have a complex and significant impact on quality of pump cover parts. This paper takes aluminum alloy oil pump covers as object, through theoretical and empirical design and Procast simulation analysis, aims to obtain optimal design scheme for mold and process.

External dimensions of oil pump cover part are 149 mm * 110 mm * 32 mm. Using UG modeling, its volume was measured to be 63,826.1 mm³, with a maximum thickness of 10.8 mm, a minimum wall thickness of 1.5 mm, and an average thickness of 3.07 mm, as shown in Figure 1. As shown in Figure 1, casting has an asymmetrical shape, with complex curved surfaces and thin walls. Thickness distribution is uneven, with two particularly thick sections (circled in figure), with wall thicknesses of approximately 10.5 mm and 10.8 mm from top to bottom, respectively.

 

Fig. 1 Analysis of structure and wall thickness of oil pump cover
Oil pump cover is die-cast using ADC12 aluminum alloy, which belongs to Al-Si-Cu alloy system. Chemical composition is shown in Table 1.
Table 1 Chemical compositions of ADC12 alloy wB /%

To avoid impacting core with molten metal and to avoid complex thin-walled sections, minimizing flow distance of molten metal, gating system is located on the outer side where wall thickness is uniform, employing two side gating systems (left and right). Cross-sectional area of ingate is calculated as follows:

 

Where: Ag is cross-sectional area of ingate; ρ is density of molten metal; vg is flow velocity of molten metal at ingate; t is time for molten metal to fill cavity; G is mass of molten metal passing through ingate (including overflow and venting channels).
Based on experience and mold design theory, mold is designed with following parameters: filling speed (v) 2 m/s, ingate speed (vg) 40 m/s, filling time (t) 0.22 s, ingate thickness (d) 2.8 mm, casting mass (m) 240 g. Calculated cross-sectional area of ingate (Ag) is 109 mm².
Based on shape characteristics of casting and gating system design theory, three gating schemes are designed. Gating and overflow system designs are shown in Figure 2. Firstly, each design incorporates a protruding section at the end of runner to store molten metal, paint residue, and gas, while also stabilizing molten metal flow. To expel gas and molten metal from front of molten metal, stabilize flow, and reduce turbulence, overflow channels are installed at two circular openings near ingate. Additionally, to facilitate heat dissipation, design two eliminates overflow channel inside runner. Compared to design two, design three shortens length of ingate, reducing energy loss and molten metal waste during flow. Considering that bottom of casting is where two liquid flows converge, creating a significant tendency for air entrapment, design one adds an overflow channel at this location.

 

Fig. 2 Design scheme of oil pump casting and overflow system of oil pump cover

In die casting, molten metal can be considered as an incompressible fluid, and its flow process obeys laws of conservation of mass and momentum. Governing equations for filling and solidification processes are:

 

Where: Φ is general vector; xj is coordinate component; uj is velocity component; T is thermodynamic temperature; DΦ is generalized diffusion coefficient; SΦ is source term.
Heat conduction is described using Fourier's law and the heat conduction differential equation:

 

Convective heat transfer is described using Newton's cooling law: 

 

Radiative heat transfer follows the Stefen-Boltzman law:

 

Where: cρ is specific heat capacity at constant pressure; λ is thermal conductivity; Q is heat source term; α is convective heat transfer coefficient; Tf is fluid characteristic temperature; Tw is solid boundary temperature; Ts is absolute surface temperature; ε is emissivity; σ0 is Stefen-Boltzman constant.

Tetrahedral meshes were generated using Visual-Mesh module of ProCAST. To shorten computation time while ensuring simulation accuracy, meshes were generated for casting, gating, and overflow systems according to different mesh densities. Mesh size for sprue, runner, and casting was 1 mm, pressure chamber mesh was 2 mm, and mesh size for other thicker parts of die casting mold was 4 mm. The total number of volumetric meshes for the entire casting and mold was 4.3 million. Initial and boundary conditions for simulation of oil pump cover are shown in Table 2.
Table 2 Initial and Boundary Conditions of Die Casting

From die casting filling process, it can be seen that in all three schemes, molten metal flows into cavity sequentially from both sides during die casting, as shown in Figure 3. For Scheme 1, when mold is 50% filled, flow velocity near ingate is relatively fast. Molten metal impacts core from both sides. On the left side, due to relatively flat shape, molten metal flows more gently. On the right side, molten metal, after flowing a short distance from ingate, is obstructed by complex protruding parts, creating localized turbulence and easily causing air entrapment. Flow velocity at protruding parts slows down. When mold is 70% filled, two streams of molten metal converge on upper and lower sides respectively. Especially at the top, where flow direction is aligned with molten metal, molten metal is blocked, resulting in a greater impact on mold. However, because three overflow channels are set at the top, molten metal can be discharged, thus reducing impact on the top, expelling inclusions and gas. At this point, neither left nor right protruding parts are completely filled, resulting in air entrapment in cavities. When mold is 90% filled, it can be seen that lower right corner of casting is last area to be filled. Scheme 1 includes overflow channels at this location. At this point, remaining areas of casting are basically filled, resulting in a better filling effect.
As shown in Figure 3a, molten metal flows faster on both sides of circular core and near ingate. To accelerate casting cooling, Scheme 2 removed overflow groove inside gating system; filling process was essentially same as Scheme 1. Scheme 3 shortened length of sprue and added an overflow groove at the top of casting. Compared to Scheme 2, filling process was also similar. However, comparing molten metal flow rates at 70% and 90% filling, Scheme 3 showed a larger area of faster flow rate than Scheme 2, indicating that shortening sprue length did indeed reduce energy loss in molten metal flow. Comparing three casting schemes, Scheme 3 was generally the most reasonable design.

 

Fig. 3 Flow field velocities of filling process of die casting with different schemes

Complete solidification times of castings under schemes 1, 2, 3 were 9.96 s, 9.61 s, and 9.56 s, respectively. Scheme 3 solidified the fastest, as shown in Figure 4. Solidification times of three schemes after 5 seconds were basically consistent: thin-walled sections and areas far from ingate solidified first, followed by thick-walled sections and areas near ingate. Overflow channel solidified later than casting, sprue and remaining material solidified last. Gating system played a feeding role in casting, as shown in Figure 4. During solidification process, in some areas, feeding channels solidified prematurely, preventing pressure from being transmitted to hot spots, leading to solidification shrinkage defects at these hot spots. Strengthening cooling at hot spots (e.g., using spot cooling) could avoid or reduce shrinkage defects. As shown in Figure 4, distribution of hot spots in Schemes 1 and 2 is not significantly different. Scheme 2 reduces one overflow channel, thus reducing molten metal waste. Compared to Scheme 2, Scheme 3 has a shorter ingate length, further reducing molten metal waste, lowering temperature and velocity losses during molten metal flow.

 

Fig. 4 Solidification of different schemes for 5 s

Distribution of shrinkage porosity and cavities in Schemes 1 and 2 is basically same, as shown in Figure 5. It can be seen that defects are mainly distributed in overflow channel and thicker parts of casting. Comparing Figures 5a and 5b, overflow channel between two ingates is present or absent, and no shrinkage cavities or porosity appear in this area. Therefore, it is advisable to omit overflow channel here. Removing overflow channel between two ingates in Scheme 1 has two effects: firstly, it shortens ingate, reducing melt pressure and temperature losses; secondly, it reduces amount of metal filling near high-temperature gating, allowing for sequential solidification with metal at far end of gating system, thereby improving the overall cooling efficiency of casting. Compared to Schemes 1 and 2, Scheme 3 exhibits fewer defects within circle, indicating that added slag pocket at the top of Scheme 3 effectively reduces internal defects in casting.

 

Fig. 5 Prediction results of shrinkage porosity and shrinkage cavity of different die casting schemes

Analysis of filling, solidification, shrinkage porosity, and shrinkage cavity simulation results for three mold designs of oil pump reveals that Scheme 3 is the most reasonable. Mold was opened according to Scheme 3, and the overall mold structure is shown in Figure 6a. Mass-produced die-cast parts (including gating and overflow systems) are shown in Figure 6b. As can be seen from Figure 6b, casting surface is smooth, without flow marks, cracks, incomplete filling, or other defects. This demonstrates that gating and overflow system designed using Scheme 3, along with corresponding process parameters, ensures surface quality of casting. This design scheme has already entered mass production.

 

Fig. 6 Die casting die and die casting sample
To ensure pump cover casting is leak-proof, X-ray flaw detection was performed on the inside of casting, as shown in Figure 7. Comparing this with simulation results of shrinkage porosity and shrinkage cavity (Scheme 3 in Figure 5), it can be seen that location of shrinkage cavities and shrinkage porosity in the entire casting (including overflow groove) is basically consistent with simulation prediction results. Two internal micropore defects were detected in casting area, as shown in locations A and B in figure. Based on casting wall thickness analysis in Figure 1c, locations A and B are precisely among areas of maximum wall thickness. Although a large overflow channel was installed in this area, it did not completely transfer shrinkage cavities to slag collection pot. However, small shrinkage cavities at locations A and B are located on outer side of casting and close to slag collection pot, indicating that slag collection plays a certain role in sequential solidification of casting. It should be noted that these two micropores appear on casting assembly surface and are located on outer side of casting, and therefore do not affect sealing performance of oil pump cover. Shrinkage cavities at locations D and E are both located within slag collection pot, indicating that these two slag collection pots play a role in transferring shrinkage cavities from casting to slag collection pot.

 

Fig. 7 Results of X-ray global flaw detection
According to filling simulation analysis, location C in Figure 7 is final filling point of molten metal and also the area where two liquid flows converge. This area is prone to air entrapment and may also produce cold shuts. However, X-ray inspection did not detect any pores or cracks, indicating that mold design and selection of corresponding process parameters for not installing an overflow channel at location C are reasonable. To analyze microstructure of oil pump cover, samples were taken from the thickest part of casting, area A, final filling area, area C, and observed under a metallographic microscope, as shown in Figures 8a and 8b. Area A, with the thickest wall, has coarse grains and an uneven microstructure, exhibiting small, dispersed shrinkage defects. Microstructure mainly consists of blocky α-Al phase and acicular eutectic Si phase. Due to microscopic shrinkage defects formed by solidification shrinkage between α and Si phases, fact that acicular eutectic Si has very low strength and plasticity, mechanical properties of microstructure at area A are poor. Considering that this is a thick-walled area of casting, small shrinkage defects within microstructure will not affect sealing performance of oil pump cover; therefore, shrinkage is acceptable.

 

Fig. 8 Metallography, SEM and grain morphology of oil pump cover
Comparatively, cast part at location C, with its thinner wall, exhibits finer grains, a more uniform microstructure, and no obvious defects. This corroborates solidification results of Scheme 3. Location C has a thin wall and is filled with metal from mold filling end, resulting in the fastest solidification rate. Therefore, grains at location C are the finest and most uniformly distributed. Fig. 8c shows microstructure of sample at location C of cast part at same magnification using scanning electron microscopy (SEM). Further magnification of selected area reveals two pores within microstructure, but overall, grains are relatively fine. This location contains an α-Al matrix and a lamellar Al₂Si eutectic. Figure 8d shows backscattered diffraction (EBSD) results of microstructure at region C of casting, revealing grain morphology of C region. It can be seen that grains at this location are fine and uniform in size. Comparison with national standard GB/T 6394—2017 confirms a grain size of grade 4.
A sample was taken from region C of casting for a tensile test. Fracture surface of tensile specimen was observed using a scanning electron microscope, and morphology is shown in Figure 9. It can be seen that fracture surface is relatively rough, with numerous cleavage planes and tearing edges. Cleavage planes are small, and there are also a few dimples, exhibiting mixed fracture characteristics of cleavage fracture and ductile fracture, belonging to quasi-cleavage fracture, consistent with low toughness characteristic of ADC12 aluminum alloy.

 

Fig. 9 Fracture morphology of casting
Five samples were taken from both thick-walled region A and thin-walled region C of casting for microhardness testing. Results are shown in Figure 10. Average microhardness at thick-walled section A is HV94.2, and average microhardness at thin-walled section C is HV101.6. Hardness requirement for ADC12 aluminum alloy pump cover is not lower than HV85, therefore hardness of oil pump cover die casting meets product requirements.

 

Fig. 10 Microhardness at different locations in areas A and C

(1) Simulation of die casting filling and solidification process revealed that, compared to sections one and two, section three of gating and overflow system design, by eliminating slag pockets on the inner side of ingate and shortening ingate length, reduced energy loss of molten metal flow, achieving sequential solidification of casting, making it the most reasonable.
(2) Using section three for die casting, surface quality of casting is good. Internal flaw detection did not reveal any holes or cracks in important areas. Two minor shrinkage cavities at assembly location had no impact on sealing performance of oil pump cover. Microstructural analysis revealed that thin-walled area of casting has a dense microstructure, fine and uniformly distributed grains, with a grain size reaching level 4.
(3) Fracture at thin-walled location of casting is a quasi-cleavage fracture. Average hardness of thick-walled and thin-walled parts can reach HV94.2 and HV101.6 respectively, which meets hardness requirements of pump cover. Use of third design scheme for gating and overflow system can produce qualified die castings, indicating that die casting mold designed based on theory and experience can greatly improve design efficiency through simulation analysis.