Casting Defects of Metal Materials, part 1

The main defects of casting products are segregation, pores, shrinkage and shrinkage, inclusions, cracks, cold shut and other defects.

1. Segregation

Segregation - the phenomenon of uneven chemical composition in the casting. Segregation makes the performance of the casting uneven, and in severe cases it will cause waste.

Segregation can be divided into two categories: microsegregation and macrosegregation.

Intracrystalline segregation (also known as dendrite segregation) - refers to the phenomenon of uneven chemical composition in various parts of the grain, which is a kind of microsegregation. All alloys that form solid solutions can only obtain grains with uniform chemical composition under very slow cooling conditions to allow atoms to fully diffuse.

Under actual casting conditions, the solidification rate of the alloy is relatively fast, and the atoms do not have time to fully diffuse. In this way, the chemical composition of the grains that grow in a dendritic manner must be uneven. In order to eliminate intracrystalline segregation, the casting can be reheated to a high temperature and kept warm for a long time to allow the atoms to fully diffuse. This heat treatment method is called diffusion annealing.

Density segregation (formerly known as specific gravity segregation) refers to the phenomenon of uneven chemical composition between the upper and lower parts of the casting, which is a type of macro segregation.

When the density of the alloying elements differs greatly, after the casting is completely solidified, the elements with low density are mostly concentrated in the upper part, while the elements with high density are more concentrated in the lower part. To prevent density segregation, the metal liquid should be fully stirred or cooled quickly during pouring so that elements with different densities do not have time to separate.

There are many types of macro segregation. In addition to density segregation, there are also positive segregation, reverse segregation, V-shaped segregation and banded segregation.