Mechanism of grain refinement in cast alloy

1. Homogeneous nucleation is not possible for actual production

In order to realize homogeneous nucleation in homogeneous liquid phase, the formation of crystal nucleus needs a lot of surface energy, so it needs a lot of supercooling. The melting point and supercooling required for homogeneous nucleation of some metals commonly used in foundry industry.

It is difficult to crystallize and solidify in this way, even in high-end laboratory. Many people have also considered adding iron powder to the molten steel to promote homogeneous nucleation, but the stability of small iron powder in high temperature molten steel is very poor and difficult to control. In fact, the crystallization and solidification process of various cast alloys generally depend on heterogeneous nucleation.

2. Heterogenesis

The basis of grain refinement is to strengthen heterogeneous nucleation. The inoculation treatment of cast iron and the modification treatment of aluminum alloy and magnesium alloy are to introduce a large number of micro heterogeneous nuclei by adding inoculant and modifier.

Effective heterogeneous nuclei should have the following four characteristics:

(1) It’s a solid at the temperature of the liquid metal.

(2) Good stability in liquid metal.

(3) The degree of mismatch with solidified metal crystal lattice is small.

(4) Can be wetted by liquid metal.

3. Way of introducing heterogeneous nuclei

The best way to introduce heterogeneous crystal nucleus into liquid casting alloy is to add metallic or non-metallic materials which can combine with the constituent elements or elements such as carbon, oxygen, nitrogen and sulfur in the liquid alloy, so as to form compounds in the liquid alloy, produce a large number of fine particles with high melting point and become heterogeneous crystal nucleus.

It is basically not feasible to directly add the powder material with the same composition as the target heterogeneous crystal nucleus, because:

(1) Although the material is fine powder, but as the crystal nucleus is too thick, so it is invalid.

(2) As the heterogeneous nucleation material, the melting point is very high, and it is difficult to dissolve in the alloy liquid, so it is easy to aggregate and float or sink after adding into the alloy liquid, or to wrap into the alloy to become inclusions, which is difficult to control.