In the production of aluminum alloy plunger pump shells, we frequently encounter metal casting defects such as shrinkage porosity and gas holes, which significantly impact product quality and performance. These metal casting defects are particularly challenging due to the complex geometry of the pump shell, which features thin-walled sections in the central barrel and thick flanges on both ends. This structural imbalance often leads to uneven solidification and inadequate feeding, exacerbating the formation of metal casting defects. Through a detailed analysis of the root causes and the implementation of optimized casting techniques, we have developed a comprehensive approach to mitigate these issues and enhance the overall integrity of the cast components.
The plunger pump shell, made of ZL101 aluminum alloy, has a mass of 16 kg and dimensions of approximately ϕ180 mm × 177 mm, with wall thicknesses ranging from 6 mm to 55 mm. The stringent quality requirements, including Grade 2 pinhole standards and 100% real-time radiographic inspection, necessitate a defect-free internal structure. Common metal casting defects like shrinkage porosity often occur in the thick flange areas, while gas holes arise from turbulent flow during mold filling. Our initial process involved a tilt-pouring method with metal molds, where the feeding system included risers at the thick oil passage end and horizontal risers for the flange凸台s. However, this setup resulted in a high rejection rate, with over 68% of defects attributed to shrinkage and 25% to gas entrapment, highlighting the persistent challenges in controlling metal casting defects.
To address these metal casting defects, we first analyzed the solidification behavior using principles of sequential solidification. The original process positioned the pump shell with the oil passage end up, relying on a top feeder and side risers. However, the early closure of feeding paths in the thick凸台s led to isolated hot spots and shrinkage porosity. Moreover, the small ingate area caused turbulent metal flow, entrapping air and forming gas holes. We identified that the key to reducing metal casting defects lies in optimizing the feeding and gating systems to promote directional solidification and minimize turbulence. The following table summarizes the main parameters and defect statistics from the initial process:
| Parameter | Value | Defect Type | Rejection Rate (%) |
|---|---|---|---|
| Pouring Temperature | 710–730 °C | Shrinkage Porosity | 68 |
| Mold Preheating Temperature | 280–300 °C | Gas Holes | 25 |
| Pouring Time | 11 s | Other Defects | 7 |
The solidification time for a casting can be estimated using Chvorinov’s rule, which relates the solidification time \( t \) to the volume \( V \) and surface area \( A \) of the casting: $$ t = k \left( \frac{V}{A} \right)^2 $$ where \( k \) is a constant dependent on the mold material and casting conditions. In our case, the thick sections had a higher \( V/A \) ratio, leading to longer solidification times and increased risk of metal casting defects like shrinkage. For the flange凸台s, the solidification time exceeded that of the riser roots, causing premature feeding channel closure. To quantify the defect probability, we used a simplified model for shrinkage formation: $$ P_s = 1 – e^{-\lambda (t_f – t_c)} $$ where \( P_s \) is the probability of shrinkage, \( \lambda \) is a material constant, \( t_f \) is the local solidification time, and \( t_c \) is the critical time for feeding. This model helped us identify hotspots prone to metal casting defects.
In the optimized process, we repositioned the casting to have the flange face upward and the oil passage end downward. This allowed for the implementation of top open risers on the flange and blind risers at the bottom thick sections. Additionally, we introduced feeding channels along the barrel walls to enhance补缩. The gating system was redesigned with a larger, smoother ingate to reduce turbulence and minimize gas entrapment, directly targeting the gas-related metal casting defects. The pouring temperature was maintained at 720 °C, with a mold preheating temperature of 280 °C, and a consistent tilt-pouring time of 11 s. The following equation describes the fluid flow velocity \( v \) in the gating system to avoid turbulence: $$ v = \frac{Q}{A} $$ where \( Q \) is the volumetric flow rate and \( A \) is the cross-sectional area. By increasing \( A \), we reduced \( v \), thereby decreasing the Reynolds number and minimizing vortex formation that contributes to metal casting defects.
We utilized AnyCasting software for numerical simulation to validate the optimized design. The simulations accounted for heat transfer, fluid flow, and solidification phenomena. The temperature distribution showed that the risers remained the hottest regions, ensuring proper feeding until the casting solidified completely. The defect probability analysis indicated that shrinkage was confined to the risers, with no significant metal casting defects in the casting body. The total solidification time was approximately 4 minutes, with the liquid phase retreating progressively toward the risers. The simulation results confirmed that the optimized process effectively eliminates metal casting defects by maintaining a controlled temperature gradient and feeding path. The table below compares key parameters between the original and optimized processes:
| Aspect | Original Process | Optimized Process |
|---|---|---|
| Orientation | Oil passage end up | Flange face up |
| Riser Type | Top feeder and horizontal risers | Top open risers and blind risers |
| Gating System | Small ingate, turbulent flow | Large ingate, smooth flow |
| Simulated Shrinkage Defects | High in flange凸台s | Confined to risers |
| Gas Hole Probability | Significant due to turbulence | Minimal due to laminar flow |
Production trials with the optimized process involved casting 30 pump shells. Real-time radiographic inspection revealed no shrinkage porosity or gas holes in the critical areas, demonstrating a substantial reduction in metal casting defects. The castings underwent shot blasting and machining, with only two rejections out of 30, yielding a qualification rate of 93.3%. This marks a significant improvement over the initial 33.1% qualification rate. The success of this optimization underscores the importance of a holistic approach in addressing metal casting defects through design modifications and simulation-backed validation. The following formula was used to calculate the feeding efficiency \( \eta_f \) of the risers: $$ \eta_f = \frac{V_f}{V_c} \times 100\% $$ where \( V_f \) is the volume of fed metal and \( V_c \) is the volume of the casting section. In the optimized setup, \( \eta_f \) exceeded 95% for the thick sections, ensuring adequate compensation for solidification shrinkage and minimizing metal casting defects.
In conclusion, the optimization of the aluminum alloy plunger pump shell casting process has successfully mitigated metal casting defects such as shrinkage porosity and gas holes. By reorienting the casting, enhancing the riser system, and improving the gating design, we achieved directional solidification and reduced turbulence. Numerical simulations played a crucial role in predicting and eliminating metal casting defects, leading to a high qualification rate in production. This approach can be extended to other complex castings to address similar metal casting defects, emphasizing the value of integrated process design and analysis in achieving superior casting quality.