Prediction of shrinkage cavity and porosity casting defects in sand castings

The numerical simulation of sand casting defects is based on the temperature field and shrinkage cavity and porosity criteria during solidification. The temperature field of sand casting during solidification is determined through numerical simulation, the isotherm distribution and isochron distribution on the section during solidification are calculated, and a series of parameters of sand casting section are calculated, such as solidification time, temperature gradient, solidification speed, etc, Through a series of relations to connect the solidification parameters, alloy properties, mechanical properties and microstructure parameters of sand castings, shrinkage cavity, shrinkage porosity, hot crack, cold shut defects, microstructure and mechanical properties of sand castings can be predicted, and the process can be improved to obtain the optimal scheme, so as to improve the quality of sand castings.

In order to reduce the internal defects of sand casting, the location of possible defects in sand casting process is predicted by numerical simulation technology. Different materials have different solidification properties, and the mechanism of shrinkage porosity and shrinkage cavity in sand castings is also different. Therefore, different basis should be used to judge the defects of different materials.

1) Formation mechanism of shrinkage cavity and porosity

During solidification, due to the shrinkage of liquid metal, voids appear in sand castings in the final stage of solidification process. Large and concentrated voids are called shrinkage voids, and small and dispersed voids are called shrinkage porosity. During metal solidification, the volume shrinkage of liquid metal from liquidus temperature to solidus temperature is one of the main reasons for the formation of shrinkage cavity and porosity of sand castings.

In the solidification process, fine grains first appear, the temperature decreases and the grains gradually increase to form new grains. When the dendrite becomes the skeleton, the last solidified part will be isolated into an isolated liquid phase area, or separated into a metal liquid phase area with a small channel connected with the external metal liquid. Due to the high viscosity of the metal liquid, it is difficult for the external metal liquid to feed. With the progress of cooling and solidification, There will be many small cavities, namely macro shrinkage and looseness; At the same time, with the further solidification of the small isolated liquid phase region between dendrites, many smaller cavities, namely micro shrinkage, will be produced.

2) Prediction of shrinkage cavity and porosity of sand castings

At present, the prediction methods of shrinkage cavity and porosity of sand castings include equal temperature curve method, equal temperature gradient method and equal solid ratio curve method.