During the solidification of a casting, shrinkage cavities often form due to liquid and solidification shrinkage of the alloy. These cavities are categorized into two types: concentrated shrinkage holes (shrink holes) and scattered shrinkage pores (shrinkage holes). Shrinkage cavities are irregular in shape, with rough surfaces and visible dendritic tips, distinguishing them from blowholes. The presence of these cavities significantly reduces the mechanical properties of the casting by reducing the effective load-bearing area and causing stress concentration. Additionally, they compromise the castings’ hermeticity and physical/chemical properties, making shrinkage one of the most critical defects in casting that must be eliminated.
Shrinkage cavities typically form in thick sections or the last areas to solidify, often appearing as inverted cones with rough inner surfaces. The formation process involves the initial solidification of the outer layer, which creates a hard shell around the remaining liquid metal. As cooling continues, the liquid shrinks, and if not compensated, a cavity forms at the top. If a riser is present, the shrinkage cavity may be redirected into it. This phenomenon is common in alloys with high liquid and solidification shrinkage compared to their solid shrinkage.
The occurrence of shrinkage is influenced by several factors, including the alloy composition, cooling conditions, pouring process, and casting design. Alloys with a narrow crystallization range tend to form concentrated shrinkage, while those with a wide range may develop scattered shrinkage. Proper control of the solidification sequence—either through sequential or simultaneous solidification—can help minimize these defects. Sequential solidification directs shrinkage to the riser, ensuring a dense structure, while simultaneous solidification reduces thermal stresses but may result in central porosity.
To prevent shrinkage, casting processes must be optimized based on the alloy’s characteristics and the casting’s geometry. For example, gray iron with high carbon content may benefit from simultaneous solidification, while alloys prone to shrinkage require risers and careful feeding. Understanding the interplay between liquid and solid shrinkage is essential for producing defect-free castings. Ultimately, the goal is to achieve a balanced solidification process that ensures both structural integrity and performance.
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