The phenomenon that the volume and size of the liquid alloy continue to decrease during the cooling and solidification process is called contraction (shrinkage). Shrinkage is the physical property of the casting alloy itself, and it is the basic cause of many defects (shrinkage cavity, shrinkage porosity, internal stress, deformation and cracks, etc.) in the casting. The alloy liquid goes through three stages from pouring into the cavity to cool down to room temperature:
1. Liquid shrinkage: the shrinkage from the cooling of the pouring temperature to the liquidus temperature at which crystallization begins.
2. Solidification shrinkage: the shrinkage from the temperature of the beginning of crystallization to the solidus temperature at which the crystallization is completed.
3. Solid state shrinkage: the shrinkage from the temperature at which the crystallization is completed to the room temperature.
The liquid shrinkage and solidification shrinkage of the alloy are manifested as the volume reduction of the alloy, which is usually expressed by the volume shrinkage rate. They are the basic reasons for the shrinkage cavity and shrinkage defects of the casting. Although the solid-state shrinkage of the alloy is also a volume change, it only causes a change in the outer dimensions of the casting. Therefore, it is usually expressed by the linear shrinkage rate. Solid state shrinkage is the root cause of defects such as internal stress, deformation and cracks in castings.
The chemical composition of the alloy, pouring temperature, mold conditions and casting structure are the main factors affecting alloy shrinkage. The shape, size and process conditions of the casting are different, and the actual shrinkage is also different.
In addition, the unevenness of the chemical composition of each part in the process of cooling the alloy liquid into a casting, that is, segregation, gas absorption and oxidation have an adverse effect on the casting performance.