Shrinkage of cast alloy and factors affecting shrinkage

1.Shrinkage in the process of solidification and cooling, the reduction of volume and size is called shrinkage. Shrinkage is the physical property of casting alloy itself and the basic cause of many defects (such as shrinkage cavity, shrinkage cavity, crack, deformation, residual internal stress, etc.) in casting. In order to obtain a perfect casting with compact structure and the shape and size in accordance with the technical requirements, the regularity of shrinkage must be studied. The structure of the liquid metal close to the melting point is composed of atomic groups and holes, and the atomic spacing is much larger than that of the solid. During the cooling process from pouring metal into the mold to solidification, due to the decrease of temperature, the number of holes and the distance between atoms decrease, and the volume of liquid metal decreases. After the metal crystallizes, the holes disappear completely, and the distance between metal atoms is further reduced. After the metal solidifies, it will continue to cool until the room temperature, and the atomic spacing will be reduced. It can be seen that metal cooling from pouring temperature to room temperature goes through three interrelated shrinkage stages:

① The shrinkage of liquid state from the cooling of pouring temperature to the starting temperature of solidification (liquidus temperature);

② The shrinkage from the beginning of solidification to the end of solidification (solid line temperature);

③ The shrinkage of solid state from the end temperature of solidification to room temperature.

The liquid shrinkage and solidification shrinkage of the alloy show the volume reduction of the alloy, which is usually expressed by the volume shrinkage. They are the basic reasons for the shrinkage cavity and porosity of the casting. Although the solid shrinkage of alloy is also the volume change, it only causes the change of the external dimension of the casting. Therefore, it is usually expressed by the linear shrinkage. Solid shrinkage is the main cause of internal stress, crack and deformation. The shrinkage of cast steel is the biggest among the common alloys. Table shows the volume shrinkage of carbon steel.

2. The factors influencing shrinkage include chemical composition, pouring temperature, casting structure and mold conditions.

① With the increase of carbon content, the solidification shrinkage increases, while the solid shrinkage decreases slightly. In gray cast iron, carbon is the element to form graphite, and silicon is the element to promote graphitization, so the more carbon silicon content is, the smaller the shrinkage is. Sulfur can prevent the graphite from folding out and increase the shrinkage of the casting, but the appropriate manganese content, but the appropriate manganese content, can be combined with sulfur to synthesize MNS, which offsets the inhibition of sulfur on graphitization and reduces the shrinkage. If the manganese content is too high, the shrinkage of cast iron will increase.

② The higher the pouring temperature is, the higher the superheat is and the higher the liquid shrinkage is.

③ Cast structure and cast condition alloy are not free shrinkage but blocked shrinkage. The resistance comes from the following two aspects: 1) the resistance to shrinkage caused by the different cooling rate of each part of the casting, and 2) the mechanical resistance of the mold and core to shrinkage. Obviously, the actual linear shrinkage of the casting is smaller than the free linear shrinkage of the alloy. Therefore, when designing the model, the appropriate shrinkage must be selected according to the variety of the alloy, the specific shape, size, molding process and other factors of the casting.

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