Castability of alloy

Jan 13,2023

The casting property of alloy refers to the technological performance of alloy during casting, mainly refers to the fluidity and shrinkage of 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 the casting with clear outline and complete size. On the contrary, if the fluidity of the alloy is poor, it is easy to produce defects such as insufficient pouring, cold shut, air hole and slag inclusion.

Among the commonly used alloys, gray cast iron and silicon brass have good fluidity, while cast steel has poor fluidity.

There are many factors affecting 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 the casting alloy itself, and is the basic reason for many defects (shrinkage cavity, shrinkage porosity, internal stress, deformation and crack) in the casting. The alloy liquid goes through three stages from pouring into the mold cavity to cooling to room temperature:

1. Liquid contraction: the contraction from the cooling of pouring temperature to the liquidus temperature at which crystallization begins.

2. Solidification contract: the contraction from the cooling of the initial crystallization temperature to the solidus temperature after crystallization.

3. Solid contraction: contraction from the temperature after crystallization to room temperature.

The liquid shrinkage and solidification shrinkage of the alloy are represented by 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 shrinkage porosity 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, so it is usually expressed by linear shrinkage. Solid shrinkage is the root cause of defects such as internal stress, deformation and crack in castings.

The chemical composition, pouring temperature, casting conditions 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 during the cooling process of casting, that is, segregation, aspiration and oxidation have adverse effects on the casting performance.