Shrinkage cavity and shrinkage porosity casting defects are common defects in casting production. They are holes formed due to the volume shrinkage of metal in the solidification process. Concentrated holes are called shrinkage cavities, and small and dispersed holes are called dispersed shrinkage cavities, which are referred to as shrinkage casting defects for short. Due to the existence of shrinkage cavity and porosity casting defects, the effective bearing cross-sectional area of the casting will be reduced, and even the stress concentration will be caused, which will greatly reduce the physical and mechanical properties of the casting. Shrinkage cavity and porosity casting defects destroy the continuity of the casting and significantly reduce the air tightness and corrosion resistance of the casting. Therefore, shrinkage cavity and porosity are one of the main sources of casting failure, which need to be strictly controlled.
Castings with layer by layer solidification and multi-directional heat dissipation tend to form simple concentrated shrinkage cavities. The basic cause of shrinkage cavity in casting is that after the feeding of ingate stops, the volume of liquid shrinkage and solidification shrinkage in the casting shell is greater than that reduced by the solid shrinkage of the shell. The shrinkage cavity occurs at the last solidification position of the casting and the hot spot where the metal gathers.
Shrinkage casting defects can be divided into macro shrinkage and micro shrinkage according to their size. According to its distribution characteristics, it can be divided into three types: dispersed shrinkage, axial shrinkage and local shrinkage. Macroscopic shrinkage can be observed with the naked eye, while microscopic shrinkage can only be observed with a magnifying glass or microscope. The shrinkage porosity dispersed on the whole section becomes dispersed shrinkage porosity; Axis shrinkage occurs at the center of the uniform wall thickness of the casting; Local shrinkage porosity only exists in the local range of the casting, such as the thick part of the casting, the riser root and the vicinity of the ingate.