Castability (casting property) of alloy refers to the technological properties of the alloy during casting, mainly including the fluidity and shrinkage of the alloy. These properties are very important for obtaining sound castings.
mobility
Liquidity refers to the ability of liquid alloy to fill the mold.
The fluidity of the alloy liquid is good, and it is easy to fill the mold cavity to obtain castings with clear contour and complete size. On the contrary, if the fluidity of the alloy is poor, defects such as insufficient pouring, cold shut, porosity and slag inclusion are easy to occur.
Among the commonly used alloys, grey cast iron and silicon brass have the best fluidity, while cast steel has the worst fluidity.
There are many factors that affect the fluidity, including the chemical composition of the alloy, pouring temperature and filling conditions of the mold.
Contractility
The phenomenon that the volume and size of liquid alloy decrease continuously during cooling and solidification is called contraction. Shrinkage is the physical property of casting alloy itself and the basic cause of many defects (shrinkage cavity, porosity, internal stress, deformation and crack, etc.) in castings. The cooling of alloy liquid from pouring into the mold cavity to room temperature goes through three stages:
1. Liquid contraction: the contraction between cooling at pouring temperature and liquidus temperature at which crystallization begins.
2. Solidification contraction: the contraction from the starting crystallization temperature to the solidus temperature after crystallization.
3. Solid contract: the contraction from the temperature after crystallization to room temperature.
The liquid shrinkage and solidification shrinkage of the alloy are shown as the volume shrinkage of the alloy, which is usually expressed by the volume shrinkage rate. They are the basic reasons for the shrinkage cavity and porosity defects of the casting. Although the solid shrinkage of the alloy is also a volume change, it only causes changes in the external dimensions of the casting. Therefore, it is usually expressed by linear shrinkage. Solid shrinkage is the root cause of casting defects such as internal stress, deformation and crack.
The chemical composition, pouring temperature, mold condition and casting structure of the alloy are the main factors affecting the shrinkage of the alloy. The actual shrinkage varies with the shape, size and process conditions of castings.
In addition, the phenomenon that the chemical composition of each part of the alloy liquid is not uniform in the process of cooling into the casting, namely, segregation, gas absorption and oxidation, has a negative impact on the casting performance.
