1. Surface fine grain zone(Chill zone)
When poured into mould, liquid metal at high temperature touches cold wall of mould, then a layer of tiny nuclei will form rapidly on the surface of wall of mould due to great temperature difference between mould wall and liquid.
2. Columnar zone
After chill zone forms, the temperature difference between the front of crystal and the liquid decreases and it is difficult to form new crystal nuclear, crystallization continues through growing of old nuclei. Because of the limitation of heat releasing direction, growing direction of crystal is toward to the center of mould, and just opposite to mould wall, columnar zone forms.
3. Central equiaxed zone
As columnar crystals grow, temperature difference between the front of crystal and liquid becomes less and less until there is hardly difference, the end result being that columnar crystals stop growing. Further solidification in the center zone needs nucleating and growing independently in the liquid of center zone, as a result, center equiaxed zone forms.