Aluminum alloys, due to their low density, high strength, corrosion resistance, are widely used in automotive, aerospace, transportation industries. As a core component of automotive cooling systems, water pump housings have complex geometries and internal circulation channels, requiring good mechanical properties and airtightness. Considering their working environment, water pump housings need to withstand high temperatures and high pressures. Aluminum alloys, with their good heat dissipation and high strength, fully meet requirements of working environment of water pump housings. Due to their special structure, water pump housings are prone to shrinkage porosity and shrinkage cavities in aluminum alloy die-casting production, leading to increased leakage risk under certain pressure conditions. This poses a challenge to die-casting process design and defect improvement. To address this issue, this study proposes relevant measures and analyzes them, aiming to provide a reference for related production.
Part is a water pump housing, and its three-dimensional structure is shown in Figure 1. Part's dimensions are 162 mm * 141.5 mm * 167.5 mm, its weight is 1.38 kg, and it is made of ADC12 alloy with an average wall thickness of 3 mm. Product surface and parting surface must be free of burrs, flash defects, all dimensions must conform to drawings and assembly requirements. Product has strict technical requirements regarding leakage: no leakage in oil passages at 0.2 MPa pressure, and no leakage in water passages at 0.2 MPa pressure. Therefore, internal cavities of oil and water passages must not have internal quality defects such as shrinkage cavities or porosity.

 

Figure 1: Three-dimensional schematic diagram of water pump housing
Main flow channel is designed to consider impact of aluminum liquid filling stroke length, ensuring that aluminum liquid flows from the thickest wall area to the thinnest wall area. Simultaneously, to prevent premature blockage of metal liquid replenishment channel, width and thickness of main flow channel are appropriately increased during design process. Because main runner's filling direction is nearly perpendicular to pin post hole's filling direction, pin post becomes a difficult area to fill. Therefore, a gate needs to be installed at this difficult location to facilitate complete filling. Thus, a side runner is added directly opposite pin post. Due to material flow from main runner and side runner, central pillar surrounded by them becomes last area to be filled. Considering that pillar is prone to cold shuts and porosity defects caused by last-filling, which significantly impact product quality, a small branch runner is added between two runners to mitigate issue of pillar being filled at the end and affecting product quality. Based on above analysis, pre-designed gating system is shown in Figure 2.
This part is a small to medium-sized part with a complex structure. It uses a one-mold, one-cavity gating method. Based on product parting results, main mold structure design is shown in Figure 3. In addition to normal moving and fixed mold parting, two core-pulling sliders and one core-pulling pin are also required to ensure normal forming of the overall product structure. Core-pulling action of mold requires attention to sequential relationship between left core-pulling pin 2 and oblique core-pulling pin 1: after mold opens, oblique core-pulling pin 1 must be inserted first before left core-pulling pin 2 can be inserted; before closing mold, left core-pulling pin 2 must be inserted first, followed by oblique core-pulling pin 1.

 

Image 2 Preset gating system

 

Image 3 Mold main structure
1. Oblique core-pulling pin; 2. Left core-pulling pin; 3. Fixed mold core; 4. Right core-pulling pin; 5. Moving mold core
Preset part gating system was simulated for high-pressure casting using Magma software, and results were analyzed. Aluminum liquid filling process is shown in Figure 4. It can be seen that aluminum liquid filling process is smooth, with multiple ingate molten metal flows converging at water channel openings. Each flow channel is clearly layered, with no obvious anti-jamming or coiling phenomena. Central pin pillar is filled by main flow channel. Although preset anti-jamming pin pillar side gating flow shows that it does not fill pin pillar position, it has a very obvious effect on filling water tail, so side gating is retained first. Figure 5 shows temperature field of molten aluminum filling. It can be seen that aluminum temperature is 650 ℃, above ADC12 liquidus temperature of 580 ℃, the overall temperature is uniform and within a reasonable range.
Figure 6 shows simulation results of filling gas pressure. It can be seen that most of gas is mainly distributed at slag pocket, and main problem area of product is at central pin pedestal, where there is severe gas entrapment.
Figure 7 is a schematic diagram of simulation results of molten aluminum filling speed. Based on production experience, under condition that there is no sticking to mold during ingate impact, ingate speed for castings with porosity requirements is generally controlled at 40~70 m/s. Speed analysis shows that ingate speed of main runner is 65 m/s, which is within a reasonable range.
Figure 8 shows simulation results of product's thermal bottleneck. It can be seen that thermal bottleneck is largest at product oil passage opposite main runner. This location also has the largest wall thickness and the slowest solidification, posing a significant risk of shrinkage cavities. Cooling should be increased at this location to ensure sequential solidification of casting.

 

Figure 4 Schematic diagram of molten aluminum filling

 

Figure 5 Temperature field of molten aluminum filling

 

Figure 6 Gas pressure distribution

 

Figure 7 Schematic diagram of filling speed

 

Figure 8 Schematic diagram of product thermal zone
Based on designed casting, product's projected area is 22,490 mm², gating projected area is 20,164 mm², left slider's projected area is 10,135 mm², and right slider's projected area is 10,462 mm². Taking a safety factor of 1.25, an injection specific pressure of 80 MPa, and a slider wedge angle of 8°, expansion force is 456 kN. Measured mold thickness is 815 mm. Based on existing model, an 8,000 kN die-casting machine is selected for production.
Verification was conducted using 8,000 kN die-casting machine in actual production. Theoretically calculated high-speed position is 420 mm. Three high-speed switching positions (400 mm, 420 mm, and 440 mm) were selected for actual production verification. Results are shown in Figure 9. After producing 200 units continuously using final confirmed parameters, completing subsequent deburring and machining processes, production data is shown in Table 1 (individual defects are statistically analyzed separately, and some products have multiple defects). Statistical analysis revealed that the most significant issues leading to product scrap were: porosity, shrinkage cavities, and shrinkage looseness.

 

Figure 9: Defects in shell produced at different high-speed positions

Table 1: Defects and percentages of products after machining
Internal porosity of pin pillar is prone to leakage, indicating that preset solution is not significantly effective in improving porosity of pin holes. To address this issue, following solutions are proposed:
Solution 1: Strengthen side gating. Since side gating faces filling direction of pin pillar, a strengthened side gating is adopted to enhance filling of molten aluminum into pin pillar structure, thereby improving its internal quality. Width of side gating is increased to twice its original size. Side gating before and after modification is shown in Figure 10. After strengthening side gating system, 10 pieces were continuously produced at high speed and sent for trial processing verification. Six of these pieces showed varying degrees of pin hole porosity leakage, a relatively high proportion. Porosity leakage is shown in Figure 11. Strengthening side gating system did not significantly improve pin hole porosity, mainly because side gating path was too long. When molten aluminum did not completely fill pin posts in side gating system, it was pushed back by main flow from connecting ribs on the other side, causing coiling and resulting in porosity. Strengthening side gating system significantly improved porosity at water tail. Modified flow path was retained for further verification of other solutions.

 

Image 10 Modified side gating system

 

Image 11 Pin hole porosity leakage after processing
Solution 2: Increase local extrusion. After molten metal is filled, cooled and solidified for a certain period, when molten metal is in a semi-solid state in mold cavity, pressure is applied to the thickest part of last solidified section using extrusion pins for forced shrinkage, making microstructure denser, thus reducing or eliminating shrinkage cavities and porosity defects at that location. Pin column has sufficient space in fixed mold area to accommodate extrusion cylinder, and core pin of pin column is replaced with an extrusion pin. After adding local extrusion, porosity of pin column is slightly improved, as shown in Figure 12. It can be seen that under same process conditions, there are still scattered pores on outer side of pin column in both products, and porosity needs further improvement. Furthermore, extrusion pin is unstable during use, easily causing column to stick to mold. Even increasing angle of extrusion pin tip and angle of some product positions has little effect on improving sticking effect.

 

Figure 12 X-ray image after adding bridging
Solution 3: Add material bridging. On the one hand, addressing issue of pin sticking to die during extrusion, which typically occurs after side ribs break, indicating that ribs are unable to withstand force exerted on pin by extrusion pin, adding a material bridging mechanism can balance force on extrusion pin. On the other hand, adding a material bridging mechanism in main flow direction allows molten aluminum to fill central pin more quickly, ensuring internal quality of pin. Figure 13 shows results before and after adding material bridging mechanism. It can be seen that extrusion effect is stable after adding material bridging mechanism. X-ray inspection of pin, as shown in Figure 14, reveals no porosity.
By simultaneously implementing both extrusion pin addition and material bridging mechanism, 15 pieces were continuously produced for trial processing and verification. All machined pin holes were qualified/up to standard, and improved processing state is shown in Figure 15, completely resolving the porosity issue in pin.

 

Figure 13 Adding material bridging

 

Figure 14 X-ray image after local extrusion and adding material bridging

 

Figure 15: Improved pin hole machining state
During numerical simulation analysis, side gating mainly filled water tail section. Side gating was retained, but previously opened side gating was slightly narrow, limiting its filling effect on water tail area. Later, in improving pin hole porosity, strengthening side gating had no effect on improving pin hole porosity, but it significantly improved water tail porosity. This indicates that reserving and strengthening side gating has a significant effect on improving water tail porosity. X-ray inspection results are shown in Figure 16. The overall porosity state at water tail area is improved compared to Figure 9c, but there is still a noticeable porosity in the middle of casting, as shown in Figure 16b. Improvement is still needed for this location.
This location is in the thickest part of product. Considering insufficient shrinkage due to overheating, a spot cooling needle solution was designed, but X-ray inspection showed little improvement. This location is at the end of filling process, where final cold material is difficult to expel. A slag bag was added at corresponding parting line, as shown in Figure 17. After adding slag bag, X-ray inspection showed a significant improvement in porosity at this location.

 

Image 16 Porosity leakage from pin hole after machining

 

Image 17 Improvement result from adding slag bag]
Oil channel at inlet is a thick part of product. Due to mold structure limitations, casting cannot exit needle. Machining this casting to finished oil channel is equivalent to machining a ϕ14 mm * 26 mm hole in a solid aluminum block. A large amount of aluminum is processed, resulting in severe shrinkage cavities after machining.
Solution 1: Use 3D printing with conformal cooling. Figure 19 shows difference between before and after conformal cooling at oil channel location. In Figure 19a, irregularly shaped insert on the right side is too small to accommodate two point cooling joints. Therefore, a point cooling hole is drilled on the right side of insert, two channels are drilled in the middle of insert to connect to water pipe on the left. Three process holes are sealed with screw plugs. After connecting water pipe, water flow in point cooling hole on the right side of irregularly shaped insert is difficult, and adjusting water flow time has little effect. Irregularly shaped insert on the right side was changed from point cooling and existing series point cooling hole to conformal cooling. After connecting water pipe, cooling water flows inside insert, making it easier to remove heat and improving cooling effect (see Figure 19b).
After changing insert to conformal cooling, 15 pieces were produced for verification. Oil passages were machined, and machining status of oil passages after changing to conformal cooling is shown in Figure 20. Compared with Figure 18, shrinkage cavities are significantly reduced, but still exist, indicating that 3D printed conformal cooling insert reduces shrinkage porosity caused by solidification in product, and further improvement is needed.

 

Figure 18 Cavity reduction at oil passage location

 

Figure 19 Conformal cooling at oil passage location

 

Figure 20 State after reducing oil passage cavity
Solution 2: Thinning ingate at opposing position of oil passage. To enable faster cooling at oil passage location, it is necessary to reduce number of channels continuously supplying heat to oil passage. A solution is to thin ingate at opposing position of oil passage. Ingate thickness is mainly reduced at opposing oil passage location, accounting for half of the overall ingate thickness of main runner. A schematic diagram before and after thinning ingate is shown in Figure 21. By thinning ingate and using 3D printed inserts for conformal cooling, 50 parts were produced continuously at high speed, all were processed and verified. Oil passage cavity was significantly improved. Processed state after improvement is shown in Figure 22.

 

Figure 21 Schematic diagram of thinned ingate

 

Figure 22 Post-improvement machining diagram
Conclusion
Numerical simulation can provide a good reference for quality control and defect improvement of aluminum alloy die castings, shortening development time; reasonable selection of speed switching points is an important basis for ensuring good internal quality and surface finish of castings. When adjusting process parameters cannot affect product quality, causes of defects should be considered from multiple aspects such as casting structure, gating system, and venting system.