During the solidification of a casting, shrinkage cavities often form due to the liquid and solidification shrinkage of the alloy. These cavities are classified into two types: concentrated shrinkage holes, which are large and localized, and scattered shrinkage pores, which are smaller and more dispersed. Shrinkage cavities are typically irregular in shape, with rough surfaces and visible dendritic structures, distinguishing them from blowholes.
The presence of shrinkage cavities significantly reduces the mechanical properties of the casting by decreasing the effective load-bearing area and causing stress concentrations. Additionally, these defects can compromise the casting’s integrity, reducing its hermeticity and physical/chemical properties. As a result, shrinkage is one of the most critical defects in casting, requiring careful control during the process.
Shrinkage cavities generally occur in thick sections or at the final solidification areas of the casting, often forming an inverted cone shape. The formation process involves the initial solidification of the outer shell, followed by the contraction of the inner liquid metal. If the shrinkage cannot be compensated, a cavity forms at the top of the casting. In cases where a riser is used, the shrinkage cavity may be redirected into it.
The occurrence of shrinkage depends on several factors, including the alloy’s properties, cooling conditions, pouring process, and the design of the casting structure. Alloys with high liquid and solidification shrinkage, as well as those with a wide crystallization range, are more prone to shrinkage. On the other hand, alloys with narrow temperature ranges tend to form concentrated shrinkage holes.
To prevent shrinkage, proper casting techniques such as sequential or simultaneous solidification are employed. Sequential solidification ensures that the thickest parts of the casting solidify last, allowing the riser to feed the shrinking areas. This method helps produce dense castings but may increase the risk of thermal cracking. Simultaneous solidification, on the other hand, aims to minimize temperature differences across the casting, reducing stress and deformation but potentially leading to central shrinkage.
Choosing the appropriate solidification method depends on the alloy type, casting geometry, and desired properties. For example, gray cast iron with high carbon-silicon content is often suited for simultaneous solidification, while alloys with significant shrinkage benefit from sequential methods. In complex castings, a combination of both approaches may be used to balance the advantages of each.
Understanding the mechanisms behind shrinkage and implementing suitable strategies during casting design and process planning is essential to producing high-quality, defect-free castings.
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