Formation mechanism of shrinkage cavity and porosity during solidification of castings
The volume shrinkage during solidification is the cause of casting defects. Formation mechanism of shrinkage cavity: for the alloy with narrow crystallization temperature range, the casting solidifies layer by layer from the outer surface to the inner surface. When the metal changes from liquid to solid, volume shrinkage will occur, and there is sufficient liquid metal near the solidification front. However, in the final solidification region of the casting, the liquid metal at the solidification front has been exhausted, so shrinkage cavity is produced. Formation mechanism of shrinkage porosity: for the alloy with a wide crystallization temperature range, the coexistence area of liquid and solid phases in the cavity is very large, and the independent small crystals are easy to grow rapidly into dendrites with large scale. Dendrites divide the liquid metal into several small and narrow independent molten pools. There are many small holes in the casting, i.e. shrinkage porosity.
Methods of eliminating shrinkage defects
20CrMoH is a kind of casting alloy with small crystallization range and large solidification shrinkage. Casting alloy with large principle of “sequential solidification”. The principle of “sequential solidification” is applicable to this kind of casting alloy to prevent shrinkage defects.
The significance of the principle of “sequential solidification” is to create favorable conditions for the area farthest from the riser to start solidification first, and then gradually solidify towards the riser. The liquid metal in the riser is responsible for the volume shrinkage caused by the solidification of the feeding casting and finally solidify. Using this principle, a dense and defect free casting can be obtained on the premise of reasonable design of Pouring Scheme and riser chill. In order to reduce the difficulty of cutting riser and improve the usage rate of casting materials, it is necessary to control riser volume reasonably.