Compared with sand casting, die casting production has characteristics of dense casting structure and high production efficiency, and is widely used in automobiles, communications and other fields. As core component of water pump, water pump casing needs to have certain anti-leakage and anti-corrosion capabilities, can meet mechanical properties and low-temperature impact resistance under specific conditions in water. Therefore, it has high requirements for air tightness and mechanical properties. This requirement can be met by using die castings to deal with defects such as shrinkage cavities and shrinkage porosity. Combining computer numerical simulation technology with actual production can greatly reduce production costs.
Taking ADC12 aluminum alloy shell die-casting as research object, die-casting process design is carried out based on its structural characteristics. Determine location and form of casting parting surface, gating system, and overflow system as well as die-casting process parameters, and initially formulate a die-casting process plan. Use Flow-3D software to numerically simulate filling process and filling results of initial plan, determine location and causes of casting defects based on pressure, temperature, air entrainment, and surface quality. Based on analysis results, original process plan is optimized, optimized plan is simulated and analyzed again to obtain a process plan that meets production requirements.

Research object is a water pump shell die-cast with a volume of 185cm3, a maximum wall thickness of 10mm, an average wall thickness of 3.27mm, and a weight of about 450g. It is selected to have good fluidity, medium air tightness and good thermal crack resistance. In particular, ADC12 aluminum alloy with high wear resistance and low thermal expansion coefficient is used as a die casting material. Figure 1 is a three-dimensional structural diagram of water pump housing. According to selection principle of parting surface and combined with structural characteristics of research object, parting method is shown in Figure 2.

 

 

Figure 1 Solid 3D model of die casting

 

Figure 2 Solid parting method of shell castings
Figure 3 shows size of inner gate. Two inner gates are set up, and width of each inner gate is 20mm. Cross-sectional area Ar of cross runner is 3 to 4 times that of inner gate, and Ar is determined to be 180mm2. Appropriateness of overflow system affects quality of castings to a great extent. Shape of overflow tank mainly used in this plan is shown in Figure 4. Among them, b, a, h, and A are the width, length, thickness, and length of overflow port respectively. B and H are width and thickness of overflow tank respectively.

 

Figure 3 Inner gate thickness, width and length

 

Figure 4 Overflow tank shape

 

Figure 5 Original process plan
Original die-casting plan was imported into Flow-3D software for numerical simulation. Figure 6 shows flow field simulation results of original plan. According to simulation results, maximum temperature of die casting is 680℃ and minimum temperature is about 603℃. Temperature of molten metal in mold cavity generally decreases gradually as distance from inner gate increases. Locations with lower temperatures solidify first, and locations with higher temperatures solidify more slowly. It can be seen from Figure 6 that filling process is not stable, and droplet splashing occurs at t=0.027s. Comprehensive analysis shows that during simulation process, solidification method of die casting is layer by layer, and order of solidification in direction away from inner gate is followed, so shrinkage can be obtained in time, so serious shrinkage and shrinkage holes will not occur.

 

a. t=0.015s

 

b. t=0.027s

 

c. t=0.030s

 

(b)t=0.038s
Figure 6 Flow field distribution simulation

 

a. Side A view

 

(b) Side B view
Figure 7 Simulation results of air entrainment distribution in initial scheme

 

a. Side A view

 

(b) Side B view
Figure 8 Surface defect simulation results and analysis
For castings with complex shapes, multi-strand ingates are often used, and three ingates are set up. Widths of ingates are 8, 10, and 20mm respectively, and length is mm. Cross-sectional area of inner gate is increased to increase injection speed. In order to improve exhaust conditions, three annular overflow grooves are added on both sides of casting. Dimensions of overflow groove I are A=5mm, H=8mm, B=15mm; dimensions of overflow groove II are A=10mm, H=10mm. B=15mm, overflow tank III dimensions are A=5mm, H=8mm, B=15mm; an overflow tank is added at lifting lug, dimensions are A=15mm, H=10mm, B=20mm, and a cylindrical riser is also installed, R=10mm, H=15mm. In order to increase exhaust effect and improve air entrainment, an exhaust slot is added to the end cover of casting. Exhaust slot is used to exhaust gas generated from air and paint volatilization in cavity. In order to exhaust as much gas as possible in mold cavity during injection, exhaust slot is set at last filled position of molten metal. Optimization plan is shown in Figure 9.

 

Figure 9 Three-dimensional diagram of optimization plan

 

a. t=0.016s

 

b. t=0.033s

 

c. t=0.044s

 

(b)t=0.045s
Figure 10 Flow field simulation of optimization plan

 

 

Figure 11 Air entrainment simulation results of optimization scheme

 

 

Figure 12 Defect simulation results of optimization plan

 

Figure 13 Actual production castings

Based on Flow-3D software, water pump housing die-casting process was numerically simulated, causes of air entrainment and defects were analyzed, and process was optimized. Multiple internal gates were added, overflow grooves and exhaust grooves were added. Simulation showed that there were no defects. Casting begins to solidify from inner gate. During filling process of casting, molten metal is filled relatively smoothly, and no splashing of molten metal occurs, indicating that optimized solution has a good filling effect. Places with serious air entrainment are concentrated in overflow groove and exhaust groove, which shows that overflow groove and exhaust groove set up have a good effect. There is no air entrainment on casting, and no defects appear in casting. Maximum surface defect is 2 % or less, serious surface defects appear on overflow groove, and overflow groove can be removed after die-casting is completed. Optimization plan is reasonable and feasible, and castings that meet requirements are obtained after production verification.