Aluminum alloy castings play a central role in the manufacture of high-speed centrifugal impellers. The impact of casting defects on the dynamic balancing accuracy of the impeller is directly related to the impeller's operational stability and lifespan. During the casting process, the aluminum alloy undergoes complex physical changes during the mold filling and solidification stages, involving multiple factors such as flow, cooling, shrinkage, and stress fields. These factors can easily lead to defects such as porosity, shrinkage, inclusions, and cracks, which in turn significantly affect the impeller's dynamic balancing performance.
Porosity, a common defect in aluminum alloy castings, arises from the failure of gases to escape promptly during mold filling or the entrainment of impurities by the molten metal. In high-speed centrifugal impellers, the presence of porosity directly contributes to uneven impeller mass distribution. When the impeller rotates at high speed, the loss of mass in the porosity region causes localized centrifugal force fluctuations, disrupting the overall balance of the impeller. This unbalanced force is transmitted through the impeller shafting to the bearings and base, causing increased vibration, noise, and even fatigue fracture of the shafting.
Shrinkage and porosity defects are closely related to the shrinkage characteristics of aluminum alloys during solidification. Liquid metal shrinks during solidification. Inadequate shrinkage compensation can easily lead to shrinkage cavities or pores in thick impeller wall sections or hot spots. These defects reduce the density of the impeller's local structure and lead to uneven mass distribution. Under high-speed rotation, micro-deformation may occur in these shrinkage areas due to insufficient strength, further exacerbating the impeller's imbalance. Over long-term operation, this imbalance accelerates bearing wear and shortens the impeller's service life.
Inclusion defects typically arise from oxide film entrapment or refractory spalling during the aluminum alloy melting process. Inclusions are randomly distributed within the impeller, and their hardness and density differ from those of the base material. When the impeller rotates at high speeds, centrifugal force can cause these inclusions to shift, shifting the impeller's center of mass. This dynamic imbalance can induce periodic vibration, causing alternating stresses in the impeller shafting and increasing the risk of fatigue failure.
Crack defects have a more direct impact on the dynamic balancing accuracy of high-speed centrifugal impellers. Cracks often arise from casting stresses or concentrated thermal stresses, significantly reducing the impeller's structural strength. Under high-speed rotation, crack tips can expand due to stress concentration, leading to localized mass loss in the impeller. This mass loss can trigger violent unbalanced vibrations and even cause impeller fragments to fly out, resulting in serious safety accidents.
The impact of casting defects on the dynamic balancing accuracy of high-speed centrifugal impellers is also reflected in the interaction between defects. For example, the coexistence of pores and shrinkage can create a cumulative effect of localized mass loss. Inclusions adjacent to cracks can accelerate crack propagation, causing a sharp shift in mass distribution. This coupled interaction of multiple defects can significantly reduce the dynamic balancing grade of the impeller and increase operational risks.
To minimize the impact of casting defects on the dynamic balancing accuracy of high-speed centrifugal impellers, multiple approaches are necessary, including casting process optimization, defect detection, and repair. Numerical simulation can be used to optimize the filling and solidification processes to reduce defects such as pores and shrinkage. X-ray or ultrasonic testing can be used to accurately locate and quantify defects. Laser cladding or spray repair techniques can be used to locally strengthen defective areas and restore the uniformity of the impeller's mass distribution. The comprehensive application of these measures can significantly improve the dynamic balancing accuracy of the high-speed centrifugal impeller and ensure its long-term stable operation.
