In the production of aluminum alloy plunger pump castings, casting defects such as shrinkage porosity and porosity often occur. This article analyzes the causes of defects based on the structural characteristics of the castings, optimizes and improves the casting process plan, and conducts numerical simulation analysis using AnyCasting software. The results show that the optimized casting process plan for the plunger pump shell completely solves casting defects such as shrinkage porosity and porosity, and the produced castings fully meet the quality requirements.
1. Introduction
The plunger pump is a core component that provides flow and power in the hydraulic system of an aero-engine, with strict requirements for the casting process and quality. In addition to requiring the material properties of the casting to be qualified, the dimensional accuracy requirements are also high, and internal casting defects such as shrinkage porosity and porosity are not allowed. Due to the complex structure of the plunger pump, the middle tank is a thin-walled structure, and the two sides are thick flange plates, so the process plan is difficult to design.
This study analyzes the existing process defects and casting structure characteristics of the plunger pump shell, redesigns the casting gating system and feeding system, and combines the AnyCasting simulation software for simulation analysis, in order to provide a reference for producing castings that meet the design technical requirements.
2. Casting Process Structure Analysis and Technical Requirements
2.1 Original Process Plan
The internal quality requirements of this casting are high, and the metal mold tilting casting process is adopted. Since the oil circuit interface end of the oil pump housing is the thickest, according to the solidification principle of sequential feeding, the feeding riser is set at the oil circuit interface end, and a horizontal riser is set on each of the three thick bosses on the bottom flange for feeding (see Figure 3). The horizontal riser is painted with thermal insulation coating to improve the feeding efficiency. The casting pouring temperature is 710 – 730 °C, the mold preheating temperature is 280 – 300 °C, and the pouring time is 11 s, with uniform tilting casting.
| Pouring Temperature (°C) | Mold Preheating Temperature (°C) | Pouring Time (s) |
|---|---|---|
| 710 – 730 | 280 – 300 | 11 |
2.2 Defect Cause Analysis
The three-dimensional structure of the plunger pump housing is shown in Figures 1 and 2. The casting mass is 16 kg, its contour size is ϕ180 mm × 177 mm, the minimum wall thickness is 6 mm, the maximum wall thickness is 55 mm, and the average wall thickness is 15 mm. The casting material is ZL101 alloy, and the heat treatment state is T5 treatment. The casting is required to have a porosity level of 2, and 100% of the castings are inspected by real-time imaging. Cracks, shrinkage holes, and penetrating defects are not allowed. The casting difficulties of this casting lie in that one side of the oil pump is the oil circuit interface end, and the other side is the flange plate. The local position of the oil circuit interface end of the casting is thick, while the flange plate surface on the other end has three thick bosses. However, the overall wall thickness of the middle tank of the casting is 6 mm, which belongs to a thick wall at both ends and a thin middle, making it difficult to feed during solidification. Secondly, due to the complex shape of the casting, the liquid flow is unstable, and turbulence is prone to occur during the filling process, resulting in gas entrainment.
The qualified rate of the castings produced by the original process plan is less than 1/3, and the main defects are shrinkage porosity and porosity defects. The shrinkage porosity defects are mainly concentrated on the three bosses on the flange plate surface of the casting. After non-destructive testing, many shrinkage porosity and shrinkage hole defects are found (see Figure 4). This defect accounts for more than 68% of the casting rejection rate. After analysis, it is found that due to the limitations of the mold structure and the casting processing surface, although the volume of these horizontal risers is large, the solidification time of the boss position is still longer than that of the root of the blind riser, resulting in the early closure of the feeding channel and failing to achieve the ideal feeding effect. For the porosity defect, the main reason is that this casting adopts metal mold tilting casting. The molten metal enters from the oil circuit interface end, and the area of the gate is small, but the internal cavity of the mold is large. The molten metal is prone to turbulence and entrains gas to produce porosity defects. The porosity defect accounts for more than 25% of the casting rejection rate.
| Defect Type | Location | Proportion in Rejection Rate |
|---|---|---|
| Shrinkage Porosity | Flange Plate Bosses | > 68% |
| Porosity | – | > 25% |
3. Optimization and Improvement of the Process Plan
3.1 Process Plan Improvement
According to the defect types of the casting and the analysis of the causes of the defects, the process plan of the casting is optimized and improved. First, the feeding system of the casting is optimized and improved (see Figure 5). The flange plate surface of the casting is set upward, and the oil circuit interface end is set downward. A open riser is set on the flange plate. There are many thick parts at the bottom of the oil circuit interface end, and there are multiple isolated hot spots. Therefore, a blind riser is set in the middle for feeding, and two feeding channels are set on the side cavity walls to feed the positions that cannot be fed by the middle blind riser through the open riser. Secondly, to improve the porosity defect of the casting, the flange surface of the casting is set upward, and there are more choices for the gate. The new plan selects a relatively flat cavity surface as the pouring surface, so that the molten metal can fill steadily and avoid gas entrainment due to turbulence.
3.2 Process Plan Simulation Analysis
The AnyCasting simulation software is used to simulate and analyze the design plan. The pouring temperature of the molten metal is 720 °C, the mold preheating temperature is 280 °C, the coating thickness is 500 μm, and the casting tilting casting takes 11 s. The simulation results are shown in Figures 6 and 7. It can be seen from Figure 6 that the solidification sequence is relatively reasonable, and the temperature of the open riser and the blind riser is the highest. It can be seen from Figure 7 that the shrinkage porosity defects of the casting are mainly concentrated in the open riser and the blind riser. During the solidification process of the casting, from the beginning of solid phase precipitation to complete solidification, the liquid phase gradually shrinks from the outer wall of the casting to the riser, and the shrinkage porosity defects on the casting are eliminated. The final solidification time is 4 min.
| Simulation Parameter | Value |
|---|---|
| Pouring Temperature (°C) | 720 |
| Mold Preheating Temperature (°C) | 280 |
| Coating Thickness (μm) | 500 |
| Tilting Casting Time (s) | 11 |
4. Production Verification
According to the improved process plan, 30 oil pump housing castings were poured in the first batch (see Figure 8). Through real-time imaging, the three thick bosses on the flange plate were mainly inspected, and no shrinkage porosity defects were found. The internal quality meets the requirements. After sandblasting the casting surface, no porosity defects were found. After machining, 2 pieces were scrapped, and the qualified rate reached 93.3%, with obvious improvement. The production situation before and after the improvement is shown in Table 1.
| Process | Production Quantity (pieces) | Qualified Quantity (pieces) | Qualified Rate (%) |
|---|---|---|---|
| Before Improvement | 163 | 54 | 33.1 |
| After Improvement | 30 | 28 | 93.3 |
5. Conclusions
5.1 Shrinkage Defect Elimination
According to the structure of the plunger pump shell, by using 补贴 and blind risers to adjust the sequential solidification of the casting, the shrinkage porosity defect can be effectively eliminated.
5.2 Porosity Defect Reduction
By reasonably selecting the feed inlet of the oil pump shell, the stable filling of the molten metal can be ensured, and the porosity defect of the casting can be effectively reduced.
5.3 Feeding Effect of Horizontal Side Risers
In the aluminum alloy metal mold casting process, the horizontal side riser can play a feeding role for the isolated hot spots at special positions, but the effect is limited.
5.4 Feeding Effect of Inner Blind Risers
The blind riser inside the oil pump shell tank is equivalent to a heating blind riser due to its location in the center of the casting, and has a good feeding effect.
