Die casting process design and optimization of automobile water pump shell

Die casting process design and optimization of automobile water pump shell

ABSTRACT: By analyzing the structural characteristics of a certain type of automobile pump shell, the die casting process was designed. Flow-3D software was used to simulate the filling process of castings, and the distribution of entrainment defects was accurately displayed. The process was improved (changing the location and size of the overflow chute), and the improved process was simulated numerically. The results show that the defects are obviously reduced and entrainment gas appears in the overflow tank.
  Shell is an important part of automotive pump used to support bearing, impeller and other parts. This part requires no impurities, flash and cracks, and has a certain degree of hardness. Aluminum alloy has the advantages of weight reduction, energy saving and recycling. With the lightening of automotive parts, most automotive pumps use cast aluminium shell. Pressure casting has compact structure, good surface quality and mechanical properties, and is one of the most widely used processes in automotive aluminium alloy forming process. Because the filling speed of liquid metal is very fast in die casting, it is difficult to exhaust the gas completely in the cavity, and there are often gas holes and oxide inclusions in die castings. For this reason, Flow-3D software is used to simulate the filling process of castings, predict the defect location, and optimize the process in order to improve the quality of castings.
1. Die Casting Process Design
1.1. Structure and Production of Pump Shell
  The shell material of a pump is ADC12 aluminium alloy, which has high strength and hardness and good die casting process performance. The alloy composition is shown in Table 1. The casting volume is 224901 mm3 and the mass is 607.23 G. The castings consist of a stepped round end cover and a cavity cylinder with flanges. The three-dimensional model of the castings is shown in Fig. 1. The maximum wall thickness of the casting is 16.5 mm, the minimum wall thickness is 4 mm and the minimum hole is 6 mm, which meets the casting requirements. DM500 horizontal cold chamber die casting machine is selected for die casting. In order to make full use of the performance of the die casting machine to improve production efficiency, the design and optimization are carried out in one mold and two parts.
Table 1 Chemical constituents of ADC12 (mass fraction%)
Si Cu Mn Fe Zn Ni Sn Al
10.5-11.5 3.0~3.5 0.3-0.5 0.3-0.6 0.6-0.9 0.2-0.5 The Rest
Die casting part 3D model
Fig.1 Die casting part 3D model
1.2. Design of parting surface
  The selection of parting surface directly affects the processing technology of die and the effect of die casting. According to the determination principle of parting surface and considering the coordination of die-casting forming, it ensures that the die castings are left on the side of the moving die when the die is opened, and that the overflow groove is arranged on the parting surface as far as possible. The whole casting exhausts smoothly without forming defects. The parting surface design is shown in the A-A surface in Figure 2.
A-A parting surface
Fig.2 A-A parting surface
1.3. Design of gating system and overflow system
  The gating system consists of straight runner, transverse runner and inner runner. The gating system used in this paper is sector gating system. Taking into account the structure of the castings, the inner runner is placed on the reinforcing ribs of the castings. The cross-section area of the inner runner is calculated according to formula (1):
Formula for calculating inner runner                                        (1)
  In the formula: Ag is the cross section area of the inner gate, mm2; G is the mass of liquid metal passing through the inner gate, 789.4g; Rho is the density of liquid metal, 2.7g/cm3; VG is the filling speed, 40 m/s; t is the filling time of the cavity, 0.06s. According to formula (1), the section area Ag of the inner gate is 121.8 mm2. The thickness of the inner runner is related to the average wall thickness of the castings. The average wall thickness of the castings is about 3.6 mm, and the thickness of the inner runner is 2 mm. The width of the inner runner is divided by the thickness, which is 61 mm. The length of the inner runner is 2 mm. Ar=(3~4)Ag=187.5~250mm2; D=10mm; diameter of straight runner is the same as that of pressure chamber, d=70mm.
  The overflow trough should be able to discharge the gas in the cavity, store the liquid metal mixed with gas and waste residue, control the flow direction of the liquid metal filling, and adjust the thermal balance state of the die. In order to eliminate the gas in the die casting cavity, four trapezoidal overflow grooves are set on the end cover directly impacted by liquid metal filling as shown in Figure 3. Table 2 shows the size of the overflow groove (A, B and H are the length, width and thickness of the trapezoidal overflow groove respectively; a, B and H are the length, width and thickness of the overflow groove respectively).
   In order to balance the pressure of two castings during die casting, the centerlines of the two cavities are perpendicular to each other in the die. Figure 3 shows the castings with gating system and overflow system.
Pump Shell Casting with Pouring system and Overflow system  
Fig.3 Pump Shell Casting with Pouring system and Overflow system
Table 2 Dimensions of overflow chute
Rectangular Overflow Trough A B H a b h
1,3 25 20 10 12 6 1.5
2,4 35 30 15 15 8 2
2. Simulation and analysis of die casting process
  The quality of die castings depends to a large extent on the filling process. Flow-3D software is used to simulate to determine whether the designed process parameters are reasonable. Before the simulation, meshing and simulation parameters need to be set. The simulation parameters are set as follows: the meshing unit size is 0.1 mm. The casting material is ADC12 aluminium alloy, the inner gate speed is 40m/s, the pouring temperature is 680°C, and the die temperature is 230°C.
2.1. Analysis of Filling Process
  Figure 4 is a simulation of the filling process of the castings. When t=0.013s, the liquid metal enters the cavity through the straight runner, the inner runner and the inner runner; when t=0.033s, the flange is filled with liquid metal; when t=0.044s, the filling of the casting is completed. By analyzing the filling conditions of each stage, it can be seen that the temperature far from the straight runner decreases gradually, the temperature distribution before the final solidification of the castings is reasonable, the filling of the castings is stable, and the design of the gating system is reasonable.
2.2. Analysis of entrainment simulation results
  Fig. 5 shows the distribution of entrainment gas in castings. From Fig. 5 (a), we can see that there is a large volume of air entrainment defect in the joint of the inner gate and the casting. Combining with the filling process of the casting, the reason is that this part is the last filling part of the liquid metal, and the gas produced by the liquid metal reflux will form a large volume of air entrainment here when it is near the solidification of the casting; Fig. 5 (b) shows the location of the flange of the casting. In a small amount of exhaust gas, the reason is that the first part of the filling is completed and the gas in the cavity can not be completely discharged. In order to reduce casting defects, the process plan needs to be improved.
Filling Process of Pump shell Casting
Fig.4 Filling Process of Pump shell Casting(Double-click to view the image full size.)
Casting Part Entrained Air Distribution
Fig.5 Casting Part Entrained Air Distribution
3. Die casting process optimization
3.1. Optimization of Overflow System
  There are a lot of entrainment gas in the castings produced by the original scheme. A circular overflow groove is set at the deep cavity of the end cover of the castings to enhance the exhaust gas. The size of overflow groove (diameter *thickness) is quasi-30mm *20mm, and the length *width *thickness of overflow outlet is 12mm *6mm *2.5mm. At the same time, according to the actual situation, the trapezoidal overflow groove with the shape shown in Figure 6 is set at the flange position of the casting. The width and thickness of the overflow groove are 12 mm and 6 mm respectively. Overflow grooves account for about 20% of the total volume of castings. Fig. 6 shows the optimized gating system and overflow system.
Optimized Pouring and Overflow System
Fig.6 Optimized Pouring and Overflow System
3.2. Simulation and result analysis of filling process after optimization
  The optimized filling process of castings is shown in Figure 7. When t = 0.014s, the liquid metal begins to enter the cavity, when t = 0.023s, the liquid metal begins to enter the overflow slot, and when t = 0.046s, the cavity filling is completed. Compared with the original scheme, it can be seen that the temperature distribution in the filling process of the castings is more reasonable after the overflow groove is added, and the temperature difference between the inner gate and the castings combination area is reduced obviously, which helps to reduce the internal defects of the castings. Figure 8 shows the optimized distribution of entrainment gas in castings. It can be seen that the entrainment gas appears in the overflow groove, which shows that the additional overflow groove has played a role in effectively reducing the entrainment gas in the casting.
Pump Shel lCasting Filling Process after Optimization
Fig.7 Casting Filling Process after Optimization(Double-click to view the image full size)
Casting Part Entrained Air Distribution after Optimization
Fig.8 Casting Part Entrained Air Distribution after Optimization
  Figure 9 shows the castings actually produced according to the optimized process plan. According to GB/T 231.1-2009 standard, the hardness of the die-casting is more than 80HBW (5/250/30), no impurities, flash, cracks, etc. The pore size should be less than 1mm on the machined surface and 3mm in the inner part of the casting. No cracks were found in the castings, and the average hardness of the castings was 100 HBW. After X-ray inspection, the porosity of the castings was within the allowable range, and the dimensional accuracy could meet the technical requirements.
Actual production sample
Fig.9 Actual production sample
4. Summary
  In this paper, the die casting process design and simulation optimization of automobile pump shell are carried out by combining traditional process design with numerical simulation. Flow-3D simulation software was used to simulate the filling process of castings. The distribution of entrainment defects in castings was accurately predicted. The improved process was that entrainment gas appeared in the overflow slot after adding an overflow slot in the deep cavity of flange and end cover. Casting quality.


   Copyright © 2019 AIRI technology Ltd. All rights reserved.