The transformation of alloy from liquid to solid is called solidification, during which casting defects are concentrated. In the process of casting transformation from liquid to solid, there will be a certain volume shrinkage. The microscopic reason is that with the decrease of temperature, the distance between metal atoms shortens and the number of holes between atoms decreases. The free shrinkage of the alloy from liquid to solid can be divided into the following three stages.
(1) Liquid shrinkage, the alloy from the beginning of pouring to the liquid temperature range, because the alloy in the cavity is in liquid, when a certain volume shrinkage occurs, because the gating system has feeding function, the liquid level of the cavity after pouring is reduced.
(2) Solidification shrinkage, the alloy from liquid temperature to solid temperature range, when the liquid alloy reaches its crystallization temperature, the alloy will have a certain volume shrinkage, specifically with the increase of crystallization temperature interval, the corresponding volume shrinkage also increases, the solidification process is also the main source of casting size error.
(3) Solid state shrinkage, the alloy from solid temperature to room temperature range, because most of the alloy has been solidified, so it is unable to feed the non solidified area, so solid state shrinkage is the main stage of casting volume shrinkage, internal stress and cracks. At the same time, in the actual pouring process, the alloy is also hindered, including the friction on the surface of the casting, the thermal resistance caused by the shrinkage of different parts of the casting, and the mechanical resistance caused by the shell and core of the casting. Therefore, considering the influence of volume shrinkage and actual obstruction on the final casting error, it is necessary to add the corresponding size compensation coefficient to the casting according to the specific structure of the casting.