Shrinkage cavities in precision castings occur during solidification because the volume reduction from liquid shrinkage to solidification shrinkage exceeds the supply capacity of the replenishing molten metal, leading to voids in the final solidified region. This defect significantly reduces the density and mechanical properties of the casting, potentially causing sealing failure or insufficient structural strength, especially in precision castings. Therefore, a comprehensive approach is needed, addressing the solidification principle through process design, material selection, and structural optimization.
The application of the sequential solidification principle is the core strategy for avoiding shrinkage cavities. This principle requires the casting to solidify from the part furthest from the riser, gradually progressing towards the riser, with the riser ultimately completing the final solidification and compensating for shrinkage. Achieving this requires precise design of the riser's location and size: the riser should be placed at the thickest part of the casting or at a hot spot (a locally thick area), with a volume sufficient to store the replenishing molten metal. For example, for precision castings with significant wall thickness variations, multiple small risers can be placed in the thicker sections to form a staged feeding system, preventing the feeding channel from being interrupted by a single riser being too far away. Meanwhile, the cross-sectional area of the riser-casting junction (riser neck) needs to be properly controlled to ensure smooth molten metal flow while preventing premature solidification that could block feeding.
The auxiliary role of chills can enhance local solidification control. Chills accelerate the cooling rate in specific areas, adjusting the temperature gradient of the casting and forcing sequential solidification. For thin-walled to thick-walled transition zones in precision castings where complete feeding through risers is difficult, chills can be placed here to allow the thin wall to solidify first, followed by the thick wall, thus utilizing the riser to concentrate feeding on the thick-walled portion. The chill material must be selected based on the casting material: steel chills are commonly used for steel castings; copper chills are used for aluminum castings to enhance thermal conductivity. Furthermore, the chill surface must be kept clean to prevent oxide layers from affecting cooling efficiency. Before repeated use, the surface condition should be checked to prevent wear that could reduce cooling efficiency.
Optimizing pouring temperature and speed are key parameters for reducing shrinkage defects. Excessive pouring temperature exacerbates liquid shrinkage, increasing shrinkage cavity volume; excessively low temperature may lead to insufficient molten metal flow, failing to fully fill the mold cavity. Precision castings typically employ a moderate pouring temperature to ensure adequate filling capacity while avoiding excessive shrinkage. Pouring speed must be matched to temperature: high-speed pouring reduces the residence time of the molten metal in the mold cavity, minimizing heat loss, but may entrain gas; low-speed pouring facilitates venting, but can cause premature solidification of the first poured portion. In practice, the optimal parameter combination must be determined through experimentation. For example, for complex precision castings, segmented pouring can be used, first filling thin-walled sections at a lower speed, then increasing the speed to fill thick-walled sections.
Adjusting the alloy composition can improve solidification characteristics. Alloys with narrow crystallization temperature ranges (such as eutectic alloys) tend to solidify layer by layer, exhibiting a greater tendency for shrinkage cavities but a smaller tendency for shrinkage porosity; alloys with wide crystallization temperature ranges (such as paste-like solidified alloys) are more prone to shrinkage porosity. For precision castings requiring extremely high density, eutectic or near-eutectic alloys should be prioritized to reduce areas of mushy solidification. Furthermore, adding trace elements to refine the grain size can improve the fluidity and feeding ability of the molten metal; for example, adding titanium to aluminum alloys can significantly reduce shrinkage cavities and porosity.
Optimized design of the casting structure is fundamental to preventing shrinkage cavities. Abrupt changes in wall thickness should be avoided in the design; uniform wall thickness or gradually transitioning structures should be used as much as possible to reduce the number of hot spots. For necessary thick-walled sections, process ribs or reinforcing ribs can be added to disperse shrinkage stress and provide more channels for riser feeding. For example, in precision instrument housing castings, thick-walled sections can be designed as hollow structures with internal reinforcing ribs, reducing weight and lowering the risk of shrinkage cavities.
The application of special casting processes can further improve density. Pressure casting eliminates porosity by compacting the molten metal under high pressure; centrifugal casting enhances the feeding effect using centrifugal force, and is particularly suitable for tubular or symmetrical precision castings. While these processes are more expensive, they significantly improve casting quality and are suitable for applications with stringent performance requirements.
Avoiding solidification shrinkage cavities in precision castings requires coordinated optimization across multiple dimensions, including solidification control, process parameters, material selection, and structural design. By employing sequential solidification principles, chill assistance, pouring parameter control, alloy composition adjustment, structural optimization, and the application of specialized processes, shrinkage cavities can be minimized, ensuring that precision castings meet the requirements for high precision and high performance.
