In order to produce a certain automobile engine oil pump cover efficiently and with high quality, three theoretically feasible pouring and overflow system schemes were designed based on characteristics of pump cover parts, die casting theory and experience, then filling and solidification process of casting was simulated using ProCAST software. Simulation of filling process found that air entrainment is easy to occur at the top of casting, so an optimization scheme of adding an overflow groove here was proposed. Simulation of casting solidification found that eliminating overflow groove on the inside of runner and shortening length of inner runner had no effect on shrinkage distribution of casting, could also improve heat dissipation of casting inside runner and reduce energy loss of metal liquid flow. Pouring and overflow system of Scheme 3 reduces air entrainment. X-ray flaw detection of trial mold casting shows that there are no shrinkage cavities in important positions. Metallographic analysis shows that structure of each part of casting is dense, grain size grade is 4, and microhardness is greater than HV85, which meets product requirements.
High pressure die casting (HPDC) is an efficient near-net forming process. Generally, pouring temperature of aluminum alloy castings is about 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 has characteristics of high temperature, high pressure and high speed, can produce various types of complex and thin-walled parts. The thinnest part of aluminum alloy can reach 0.5 mm or even lower. It is precisely because of "high temperature, high pressure and high speed" of die casting process that any unreasonable aluminum alloy die casting mold and process, and improper operation will significantly affect casting quality, production efficiency and mold life. Among them, there are many factors that affect quality of die castings, the most important of which are: first, composition of alloy and quality of melt; second, design of mold pouring, overflow and exhaust; third, die casting process parameters; fourth, factors such as spraying and mold opening time. Alloy composition determines size of alloy liquid-solid phase interval, thereby affecting alloy fluidity and shrinkage compensation ability, while melt degassing and slag removal effects determine performance of alloy; die casting mold pouring system not only determines pouring filling direction, overflow exhaust conditions, pressure transmission, filling speed and filling time, but also affects mold temperature distribution and mold life; die casting process parameters are equally important, and pouring temperature, injection speed and pressure, mold preheating temperature, mold retention time will affect density and internal stress of casting structure. Under premise of qualified alloy composition and standardized operation, design of pouring, overflow and exhaust system of die casting mold and selection of process parameters become decisive factors for quality of die casting.
Although pump cover is a small part, it has some thick walls and curved surfaces, and is generally formed by aluminum alloy die casting. During die casting production, defects such as surface black spots, cracks, pores, flow marks and cold shut usually occur on cover parts. As an important part of oil pump, pump cover is related to airtightness and heat dissipation of oil pump, and its forming quality is very critical. Tang Wenzhuo et al. significantly improved air entrainment, casting pores and surface quality of cavity by changing shape of upper cover runner, adjusting position and size of overflow groove. Li Xianjun et al. optimized die casting mold of ZL102 alloy pump cover, which not only made mold structure compact and improved efficiency, but also optimized exhaust and slag removal capacity of mold and stabilized product quality. It can be seen that influence of die casting mold design and process parameter selection on quality of pump cover parts is complex and significant. This paper takes aluminum alloy oil pump cover as object, conducts theoretical and empirical design and Procast simulation analysis to obtain the best design scheme for mold and process.

Outer dimensions of oil pump cover part are 149 mm×110 mm×32 mm. It is modeled using UG and its volume is 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 can be seen from Figure 1, casting has an asymmetric shape, with complex curved surfaces and thin walls, and uneven local thickness distribution. There are two thicker parts, as shown in circled area in figure. Wall thickness from top to bottom is approximately 10.5 mm and 10.8 mm, respectively.

 

Figure 1 Analysis of oil pump cover structure and wall thickness
Die-casting material of oil pump cover uses ADC12 aluminum alloy, which belongs to Al-Si-Cu alloy system. Chemical composition is shown in Table 1.

 

Table 1 Chemical composition of ADC12 alloy wB/%
In order to avoid impact of molten metal on core, to avoid complex thin-walled parts and reduce flow distance of molten metal, runner is set at uniform thickness of outer wall, and two side runners are used. Cross-sectional area of inner runner is calculated as follows:

 

Where: Ag is cross-sectional area of inner runner; ρ is density of molten metal; vg is flow rate of molten metal at inner runner; t is time for molten metal to fill cavity; G is weight of molten metal passing through inner runner (including overflow groove and exhaust groove).
Mold is designed according to experience and mold design theory. Design parameters are: filling speed (v) 2 m/s, inner runner speed (vg) 40 m/s, filling time (t) 0.22 s, inner runner thickness (d) 2.8 mm, casting weight (m) 240 g, calculated inner runner cross-sectional area (Ag) is 109 m㎡.
According to shape characteristics of casting and design theory of gating system, three gating schemes were designed. Gating and overflow system designs are shown in Figure 2. First, each scheme sets a protruding part at the end of cross runner, which can store cold and dirty metal liquid, paint residue and gas in protruding part, and at the same time stabilize flow state of metal liquid; in order to discharge gas, cold and dirty metal liquid in front of metal liquid, stabilize flow state and reduce eddy currents, overflow grooves are set at circular holes on both sides close to inner runner; at the same time, in order to facilitate heat dissipation, scheme 2 eliminates overflow groove on inside of cross runner; scheme 3 shortens length of inner runner compared with scheme 2, reduces energy loss during flow of metal liquid, and reduces waste of metal liquid. Considering that bottom of casting is where two liquid flows converge and there is a large tendency to roll gas, scheme 1 adds an overflow groove here.

 

Figure 2 Design scheme of oil pump gating and overflow system

Die casting filling can regard metal liquid as an incompressible fluid, and its flow process obeys law of conservation of weight and momentum. Control equations of filling and solidification process are:

 

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

 

Calculation of convective heat transfer is described by Newton's cooling law:

 

Radiative heat transfer follows Stefen-Boltzman law:

 

Where: cρ is constant pressure specific heat capacity; 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 surface absolute temperature; ε is radiation blackness; σ 0 is Stefen-Boltzman constant.

Visual-Mesh module of ProCAST is used to divide tetrahedral mesh. In order to shorten calculation time and ensure simulation accuracy, casting and pouring and overflow systems are divided into meshes according to different mesh densities. Mesh size of biscuit, runner and casting is 1 mm, mesh of pressure chamber is 2 mm, and mesh of the other thick parts of die casting mold is 4 mm. Number of body meshes of the entire casting and mold is 4.3 million. Initial conditions and boundary conditions of simulation calculation of oil pump cover are shown in Table 2.

Table 2 Initial and boundary conditions of die casting

It can be seen from die casting filling process that molten metal flows into cavity from both sides in sequence during three schemes, as shown in Figure 3. For Scheme 1, when mold is filled to 50%, flow rate near ingates is fast, and molten metal impacts core from both sides. Left side has a relatively flat shape at this time, and molten metal flows more slowly. Molten metal on the right side is blocked by complex convex parts not far from ingates, forming turbulence locally, which is easy to cause air entrapment, and flow rate of convex parts slows down. When mold is filled to 70%, two streams of molten metal converge on upper and lower sides respectively, especially top of upper side is blocked here because it is consistent with flow direction of molten metal, and mold is hit harder, but because three overflow grooves are set on the top, molten metal can be discharged, thereby reducing impact on the top, discharging inclusions and gas; at this time, left and right convex parts are not filled, and air rolls are formed in cavity. When mold is filled to 90%, it can be seen that the lower right corner of casting is last filling area. Scheme 1 sets an overflow groove here. At this time, the rest of casting is basically full, and filling effect is good.

 

Figure 3 Flow field speed of die casting filling process in different schemes
As can be seen from Figure 3a, molten metal flows faster on the left and right sides of circular core and near inner runner. In order to speed up cooling of casting, Scheme 2 removed overflow groove on the inside of runner, and filling process was basically same as Scheme 1. Scheme 3 shortened length of branch runner and added an overflow groove on the top of casting. Compared with Scheme 2, filling process was not much different, but compared with molten metal flow rate at 70% and 90% filling, the faster flow area of Scheme 3 was larger than that of Scheme 2, indicating that shortening length of branch runner did play a role in reducing energy loss of molten metal flow. Comparing three casting schemes, in general, design of Scheme 3 is more reasonable.

Complete solidification time of castings of schemes 1, 2, and 3 is 9.96 s, 9.61 s, and 9.56 s, respectively. Scheme 3 solidifies the fastest, as shown in Figure 4. Three schemes are basically same when solidified for 5 s. Thin walls of casting and position far from ingode solidify first, thick walls and position close to ingode solidify later. Overflow trough solidifies later than casting, cross runner and residual material solidify last. Pouring system plays a shrinkage compensation role on casting, as shown in Figure 4. During solidification process of casting, local area solidifies in advance due to shrinkage compensation channel, and pressure cannot be transmitted to hot spot of casting, resulting in solidification shrinkage defects in hot spot of casting. If hot spot of casting is cooled (such as spot cooling), shrinkage defects can be avoided or reduced. As can be seen from Figure 4, hot node distribution of Scheme 1 and 2 is not much different. Scheme 2 reduces an overflow slot and reduces waste of molten metal. Compared with Scheme 2, Scheme 3 has a shorter entrant length, which reduces waste of molten metal, reduces temperature and speed loss of molten metal flow.

 

Figure 4 Solidification of different schemes for 5s

Distribution of shrinkage holes in Scheme 1 and Scheme 2 is basically same, as shown in Figure 5. It can be seen that defects are mainly distributed in overflow slot and the thicker part of casting. Comparing Figures 5a and 5b, overflow slot between two entrants exists or is cancelled, there are no shrinkage holes and shrinkage here. It can be considered that no overflow slot is set here. Removing overflow slot between two entrants in Scheme 1 has two functions. One is to shorten entrant, reduce melt pressure and temperature loss. The other is to reduce metal filling amount near high-temperature runner and achieve basic sequential solidification with metal at the far end of pouring system, thereby improving the overall cooling efficiency of casting. Compared with Scheme 1 and 2, Scheme 3 has smaller defects inside circle, indicating that slag bag added at the top of Scheme 3 has played a certain role in reducing internal defects of casting.

 

Figure 5 Prediction results of shrinkage and shrinkage of different die-casting mold schemes

Through analysis of simulation results of filling, solidification, shrinkage of three die design schemes of oil pump, it is found that Scheme 3 is the most reasonable. Mold is opened according to Scheme 3, and the overall structure of mold is shown in Figure 6a. Mass-produced die-casting casting (including pouring and overflow system) is shown in Figure 6b. As can be seen from Figure 6b, surface of casting is smooth and free of defects such as flow marks, cracks, and insufficient pouring. It can be seen that pouring and overflow system designed by Scheme 3 and supporting process parameters are guaranteed for surface quality of casting.

 

Figure 6 Die-casting mold and die-casting sample
In order to ensure that pump cover casting does not leak, X-ray flaw detection is performed on the inside of casting, as shown in Figure 7. Compared with simulation results of shrinkage cavity (Scheme 3 in Figure 5), it can be seen that shrinkage cavity and shrinkage position of the entire casting (including overflow groove) are basically consistent with simulation prediction results. Two internal microporous defects were detected in casting area, as shown in A and B positions in figure. According to casting wall thickness analysis of Figure 1c, A and B are exactly one of positions with maximum wall thickness of casting. Although a large overflow groove is set in this area, shrinkage cavity is not completely transferred to slag bag. However, tiny shrinkage cavities at A and B are located on the outside of casting and close to slag bag, indicating that slag bag has a certain sequential solidification effect on casting. It should be pointed out that two micropores appear on assembly surface of casting and are located on the outside of casting, which has no effect on sealing of oil pump cover. Shrinkage at D and E is located in slag bag, indicating that two slag bags play a role in transferring shrinkage of casting to slag bag.
According to filling simulation analysis, casting C in Figure 7 is the last filling part of molten metal and area where two liquid flows converge. It is not only easy to form air entrapment, but also may produce cold shut. However, no holes or cracks were found in X-Ray test, indicating that mold design without overflow grooves at casting C position and selection of corresponding process parameters are reasonable.

 

Figure 7 X-ray overall flaw detection results
In order to analyze microstructure of oil pump cover, samples were taken from thick part A of oil pump cover casting and last filling area C, and observed under a metallographic microscope, as shown in Figures 8a and b. Grains at A, where wall thickness is the largest, are coarse and structure is uneven. There are small and dispersed shrinkage defects. Structure is mainly composed of blocky α-Al phase and needle-shaped eutectic Si phase. Because there is micro-shrinkage formed by solidification shrinkage between α phase and Si phase, strength and plasticity of needle-shaped eutectic Si are very low, mechanical properties of structure at A are poor. Considering that this is thick wall area of casting, tiny shrinkage defects inside organization will not affect sealing of oil pump cover, and shrinkage is acceptable.

 

Figure 8 Metallographic, SEM and grain morphology of oil pump cover
Comparatively, C part with a smaller wall thickness of casting has fine grains, uniform organization, and no obvious defects, which confirms solidification results of Scheme 3. Wall of casting C is thin and filled with metal at the end of filling, and solidification speed is the fastest, so grains at C are the finest and evenly distributed. Figure 8c is same magnification scanning electron microscope (SEM) organization of sample at C of casting. Framed area is further enlarged to more clearly see that there are also two holes in organization, but grains are finer overall, there are α-Al matrix and lamellar Al2Si eutectics at this location. Figure 8d is result of backscatter diffraction (EBSD) of structure at location C of casting, showing grain morphology of C region. It can be seen that grains are small and uniform in size. By comparing with national standard GB/T6394-2017, it can be determined that grain size is level 4.
Samples were taken from casting C for tensile test, and fracture of tensile specimen was observed by scanning electron microscope. Morphology is shown in Figure 9. It can be seen that fracture surface is rough, there are a large number of cleavage planes and tearing edges on cross section, cleavage surface is small, and there are a small number of dimples. It has mixed fracture characteristics of cleavage fracture and ductile fracture, which belongs to quasi-cleavage fracture, which is consistent with low toughness of ADC12 aluminum alloy.

 

Figure 9 Casting fracture morphology
Five samples were taken from thick wall A and thin wall C of casting for microhardness test, and results are shown in Figure 10. Average microhardness of thick wall A is HV94.2, and average microhardness of thin wall C is HV101.6. Hardness requirement of ADC12 aluminum alloy pump cover is not less than HV85, so hardness of oil pump cover die casting meets product requirements.

 

Figure 10 Microhardness at different positions in A and C regions

(1) Simulation of die casting filling and solidification process found that pouring and overflow system design scheme 3, compared with schemes 1 and 2, eliminated slag bag inside ingrowth, shortened ingrowth length, reduced energy loss of molten metal flow, and achieved sequential solidification of casting, which is the most reasonable.
(2) Surface quality of casting was good by die casting scheme 3. No holes or cracks were found in important areas by internal flaw detection. Two small shrinkage defects at assembly position had no effect on airtightness of oil pump cover. Microstructure analysis found that thin-walled area of casting had a dense structure, fine grains and uniform distribution, and grain size could reach level 4.
(3) Fracture at thin-walled position of casting was a quasi-cleavage fracture. Average hardness of thick wall and thin wall could reach HV94.2 and HV101.6 respectively, and hardness met hardness requirements of pump cover. Qualified die-castings can be produced by adopting gating and overflow system design scheme three, which shows that die-casting mold designed based on theory and experience can greatly improve design efficiency after verification by simulation analysis.