The main defects of the casting products are segregation, porosity, shrinkage and shrinkage of the loose, inclusions, cracks, cold isolation and other defects.
The phenomenon of uneven chemical composition in the casting. Dialysis makes the performance of the casting is not uniform, which will cause serious scrap.
Dialysis can be divided into two categories: micro dialysis and macro dialysis.
Intra-crystalline segregation (also known as dendritic segregation) - is the phenomenon of uneven chemical composition of each part of the grain, is a kind of micro segregation. The crystallization process of alloys that form solid solutions can only be achieved under very slow cooling conditions that allow sufficient diffusion of atoms. Chemically homogeneous grains. In the actual casting conditions, the alloy solidification rate is faster, the atoms do not have time to fully diffuse, so that the dendritic way of growth of grains Internal, its chemical composition is necessarily inhomogeneous. In order to eliminate segregation within the crystal, the casting can be reheated to high temperatures, and after a long time to keep warm, so that the atoms are fully diffused. This heat treatment method is called diffusion annealing.
Density segregation (formerly known as specific gravity segregation) - is the phenomenon of uneven chemical composition of the castings on and under part, is a kind of macro segregation. When the densities of the constituent alloy elements are disparate, pending complete solidification of the castings, the density of small elements mostly concentrated in the upper part, the density of large The elements are more concentrated in the lower part. To prevent density segregation, the pouring should be stirred sufficiently or the cooling of the metal liquid should be accelerated, so that the elements of different densities can not be separated in time.
There are many kinds of macroscopic segregation, in addition to density segregation, there are positive segregation, reverse segregation, V-shaped segregation and band segregation.
In the process of solidification of metals, the solubility of the gas decreases dramatically, and it is difficult for the solid metal to escape and stay in the melt. Formation of stomata. Unlike the shrunken and loosened morphology, pores are generally round, oval, or oblong, distributed singly or in series, and have smooth inner walls. The common gases in the pores are H2, CO, H2O, CO2, etc. The pores are classified into internal pores according to their location in the ingot. According to the location of the pores in the ingot is divided into internal pores, subcutaneous pores and surface pores. The existence of pores reduces the effective volume and density of the ingot, after processing, although it can be deformed by compression, but difficult to weld, resulting in the product The peeling, blistering, pinholes, cracks and other defects.
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