Abstract: This paper introduces development and improvement process of die-casting water pump housing. Based on structural characteristics of water pump housing and sealing requirements of key areas, gating system was initially designed. According to theoretical calculation process parameters, filling and solidification were numerically simulated and analyzed using Magma software to optimize gating system, identify theoretical defect locations in advance, and take measures in advance. Based on actual production problems, gating system and cooling system were improved, and local extrusion was added to reduce defects.
Aluminum alloys are widely used in automobiles, aerospace, transportation and other fields due to their low density, high strength and corrosion resistance. As core component of automobile cooling system, water pump housing has a complex geometric structure and a circulating water channel inside housing wall. It requires good mechanical properties and air tightness. Considering its working environment, water pump housing needs to have high temperature and high pressure resistance. Aluminum alloys have good heat dissipation and high strength, which can fully meet requirements of working environment of water pump housing. Due to its unique structure, water pump housings are prone to shrinkage cavities and other defects during aluminum alloy die casting, leading to increased leakage risk under certain pressure conditions. This presents challenges for die casting process design and defect mitigation. To address this issue, this study proposes and analyzes relevant measures, aiming to provide a reference for related production.

Part is a water pump housing, with its three-dimensional structure shown in Figure 1. Part's dimensions are 162 mm * 141.5 mm * 167.5 mm, its weight is 1.38 kg, and casting material is ADC12 alloy with an average wall thickness of 3 mm. Product surface and parting surface must be free of burrs and flash defects, all external dimensions must conform to drawings and assembly requirements. Product has strict technical requirements for 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.

 

Fig.1 3D diagram of water pumping shell

Considering impact of aluminum molten material filling stroke length, main runner is designed to ensure that aluminum molten material flows from the thickest wall area to the thinnest wall area. Simultaneously, to prevent premature blockage of metal molten material feeding channel, width and thickness of main runner are appropriately increased. Since filling direction of main runner is nearly perpendicular to filling direction of pin post holes, pin posts become difficult to fill. Therefore, a gate needs to be set at this difficult-to-fill location to facilitate complete filling; hence, a side runner is added directly opposite pin posts. Due to material flow from main runner and side runners, central column surrounded by them becomes last filling position. Considering that cold shut and porosity defects caused by column being last to fill have a significant impact on product quality, a small branch runner is set between two runners to improve situation where the column is filled at the end, thus affecting product quality. Based on above analysis, pre-designed gating system is shown in Figure 2.

 

Fig.2 Pre-design of gating-exhausting system

This part is a small to medium-sized part with a complex structure. It adopts a one-mold-one-cavity casting method. According to product parting result, mold main 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 overall product structure.

 

Fig. 3 Main structure of mold
1. Angled core puller 2. Left core puller 3. Fixed mold core 4. Right core puller 5. Moving mold core
Core-pulling action of mold needs to be noted that left core-pulling 2 and oblique core-pulling 1 have a sequential relationship: after mold is opened, oblique core-pulling pin 1 must be pulled out first before left core-pulling 2 can be pulled out; before mold is closed, left core-pulling 2 must be pulled in first and then oblique core-pulling pin 1.

Magma software was used to perform high-pressure casting simulation on preset part gating system, and results were analyzed.
(1) Aluminum Liquid Filling
Aluminum liquid filling process is shown in Figure 4. It can be seen that aluminum liquid filling process is stable, and molten metal from multiple ingates converges at water channel opening. Flow channels are clearly layered, with no obvious phenomena such as counter-current or coiling. Central pin column is filled by main flow channel. Although pre-set counter-current pin column side flow channel does not fill pin column position, it has a very obvious effect on filling of water tail. Therefore, side flow channel is retained first.

 

Fig.4 Filling field of aluminum melt
(2) Aluminum Liquid Temperature
Figure 5 shows temperature field of aluminum liquid filling. It can be seen that aluminum liquid temperature is 650 ℃, which is above ADC12 liquidus temperature of 580 ℃. The overall temperature is uniform and within a reasonable range.

 

Fig.5 Temperature field of aluminum melt
(3) Gas Pressure Analysis
Figure 6 shows simulation results of filling gas pressure. It can be seen that most of gas is mainly distributed in slag pot position. Main problem point of product is in the central pin column position, where there is a serious gas coiling.

 

Fig.6 Gas pressure distribution
(4) Speed Analysis
Figure 7 is a schematic diagram of simulation results of aluminum liquid filling speed. Based on production experience, under condition that there is no sticking to mold when ingate impact occurs, ingate speed is generally controlled at 40~70m/s for castings with porosity requirements. From speed analysis, it can be seen that ingate speed of main runner of product is 65 m/s, which is within a reasonable range.

 

Fig.7 Schematic diagram of filling velocity
(5) Thermal point analysis
Figure 8 shows simulation results of product thermal points. It can be seen that thermal point is the largest at product oil passage position opposite main runner. This position is also the position with the largest product wall thickness and the slowest solidification. Risk of shrinkage is extremely high. Cooling should be added at corresponding position to ensure sequential solidification of casting.

 

Fig.8 Schematic diagram of hot spot in shell
Through analysis of simulation results, it was found that preset gating system can generally meet actual production requirements of parts. For still existing risk positions, such as serious gas entrapment of intermediate pin column, firstly, mold at pin column hole position is equipped with a core pin, which has a certain venting effect. For gas entrapment at pin column position, a needle sleeve is added to enhance venting effect. At the same time, core pin is set as a spot cooling needle so that an oxide film hard layer is formed on contact surface between pin hole and needle. Oxide layer generally has good porosity, and above measures are used to improve pneumatic wrapping of pin column. In addition, for product position of main runner punching is relatively hot, direct cooling and spot cooling are set near hot spot to ensure product solidifies sequentially, reduce impact of hot spot. By optimizing mold structure in advance, defects are prevented, trial production is carried out in this state. Gating system, part structure and mold structure are further optimized in actual production state to improve quality of die casting products.

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 pressure of 80 MPa, and a slider wedge angle of 8°, expansion force is calculated to be 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.

 

Fig.9 Defects in shell at different high velocity positions
Comparing actual high-speed position to theoretical calculation, moving high-speed position forward by 20mm resulted in severe cold shut-off at water tail position. Moving actual high-speed position backward by 20mm reduced porosity compared to theoretical calculation. Further shifting high-speed position backward resulted in poor forming of feed inlet pillar. Adjustments to processes such as pressurization to address issues like water tail porosity and pin hole porosity were ineffective. Ultimately, it was decided to initially produce at a high-speed position of 440mm to obtain actual processing data for targeted improvements to mold structure.

After continuously producing 200 pieces using final confirmed parameters and completing subsequent deburring and machining processes, production data is shown in Table 1 (individual defects are statistically analyzed separately; some products exhibit multiple defects).
Tab.1 Defects in machined shell and proportion

Statistical analysis revealed that the most significant problems leading to product scrap were porosity, shrinkage cavities, and shrinkage porosity. External porosity was mainly concentrated at X-ray display location shown in Figure 9c, with pin hole porosity being the most prevalent. Shrinkage cavities and shrinkage porosity were all located at product's hot spots. In addition, product burn issues can be improved by adjusting spray coating or mold cooling, and chipping can be resolved by adding a radius (R-angle) at slag outlet. Further improvements are needed for main issues leading to product scrap.

Internal porosity of pin pillars is prone to external leakage, indicating that pre-designed solution is not significantly effective in improving pin hole porosity. To address this issue, following improvement plan is proposed:
Option 1: Strengthen side gating. Since side gating faces pin pillar filling direction, a strengthened side gating is adopted to enhance aluminum molten filling of pin pillar structure and improve its internal quality. Width of side gating is increased to twice its original size. See Figure 10 for side gating before and after modification. After strengthening side gating, 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 percentage. Porosity leakage status is shown in Figure 11. Strengthening side gating system did not significantly improve porosity of pin holes. Main reason is that side gating system path is long. When aluminum liquid in side gating system does not completely fill pin column, it is pushed back by material flow of main channel from connecting rib on the other side to form a roll-up, causing porosity. Strengthening side gating system significantly improves porosity of water tail. Modified flow channel is retained to continue to verify other schemes.

 

Fig.10 Altering side-pouring channel

 

Fig.11 Gas hole in machined pin
Option 2: Add local extrusion. After molten metal is filled, it is cooled and solidified for a certain period of time. When molten metal is in a semi-solid state in cavity, pressure is applied to thick wall of last solidified part to force shrinkage, making structure more compact, thereby reducing or eliminating shrinkage cavities and porosity defects in that area. Pin column has enough space in fixed mold area to arrange extrusion cylinder, and core pin of pin column is changed to an extrusion pin. After increasing local extrusion, porosity of pin column is slightly improved. X-ray inspection results are shown in Figure 12. It can be seen that under same process conditions, there are still scattered pores on the outside of pin column of two products, and porosity needs to be further improved. In addition, extrusion pin is unstable during use and is prone to sticking to mold. Even if slope of front end of extrusion pin and slope of some product positions are increased, improvement effect on sticking to mold is not obvious.

 

Fig.12 X-ray detection for castings after adding bridge
Option 3: Add material bridging. On the one hand, addressing problem of pin sticking to die during use, sticking occurs after side ribs break, indicating that two side ribs can no longer withstand force applied to pin by extrusion pin. Adding material bridging can balance force on extrusion pin. On the other hand, adding material bridging in main flow direction allows molten aluminum to fill middle pin faster along main flow direction, ensuring internal quality of pin. See Figure 13 for before and after adding material bridging.

 

Fig.13 Adding transport bridges
As can be seen, extrusion effect is stable after adding material bridging. X-ray inspection of pin shows results in Figure 14, revealing no pores in pin.

 

Fig.14 X-ray detection of castings by local squeezing and adding bridges
By adding extrusion pins and material bridging, two solutions were implemented simultaneously. Fifteen pieces were continuously produced for trial processing and verification. All pin post holes were successfully machined, and improved machining state is shown in Figure 15. Pin post porosity was completely resolved.

 

Fig.15 Appearance of machined pin after altering measures

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. In subsequent process of improving pin hole porosity, strengthening side gating had no effect on 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 relatively obvious porosity in the middle of casting, as shown in Figure 16b. Improvement is still needed for this location.

 

Fig.16 Gas hole in machined pin
This location is in a thick section of product. Considering insufficient shrinkage due to overheating, a spot cooling needle solution was designed. X-ray inspection showed no significant 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.

 

Fig.17 Improved effects after adding slag

Oil channel at inlet is a thick section of product. Due to mold structure limitations, casting cannot be needled. Machining oil channel from casting to finished product 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 after machining.

 

Fig.18 Shrinkage cavity in oil channel
Solution 1: Using 3D printing conformal cooling. Figure 19 shows oil channel location before and after conformal cooling. 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, and 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 opened 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, as shown in Figure 19b.

 

Fig.19 Conformal cooling at oil channel position
After insert was changed to conformal cooling, 15 pieces were produced for verification. Oil passages of products were machined. Machining state of oil passages after change to conformal cooling is shown in Figure 20. Compared with Figure 18, shrinkage cavity is significantly reduced, but it still exists, indicating that 3D printed conformal cooling insert reduces shrinkage porosity caused by solidification of product, and further improvement is needed.

 

Fig.20 Reduction of shrinkage cavity in oil channel
Solution 2: Thinning ingate at anti-gating position of oil passage. To enable faster cooling of oil passages, it is necessary to reduce number of channels continuously supplying heat to these passages. A thinning of ingate at anti-sumption position of oil passage is adopted. Ingate thickness at anti-sumption oil passage position is reduced, accounting for half of the overall ingate thickness of main runner. Figure 21 shows a schematic diagram before and after ingate thinning. By thinning ingate and using 3D-printed inserts for conformal cooling, 50 parts were produced continuously at high speed, and all were machined for verification. Shrinkage cavity in oil passages was significantly improved, and improved machining state is shown in Figure 22.

 

Fig.21 Schematic diagram of thinned ingate

 

Fig.22 Images of altered measures

Numerical simulation can provide a good reference for quality control and defect improvement of aluminum alloy die castings, shortening development time. Reasonable speed switching point selection 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, including casting structure, gating system, and venting system.