Prediction of shrinkage cavity and porosity in sand casting during solidification

Shrinkage cavity and shrinkage porosity are common defects in sand casting production. The main purpose of numerical simulation of sand casting solidification process is to predict the formation of shrinkage cavity and shrinkage porosity in sand casting solidification process, so as to realize the control of internal quality of sand casting.

The casting forming process usually undergoes liquid cooling, solid-liquid transformation and continuous cooling after solidification. Liquid cooling and solid-liquid transformation generally produce large shrinkage. At the initial stage of solidification of sand castings, if the shrinkage caused by the liquid shrinkage of metal and the solid-liquid transformation cannot be effectively compensated, such as the unreasonable riser setting and the premature solidification of the feeding channel is blocked, shrinkage cavities will be generated in the sand castings. Generally, the shrinkage cavities are primary and secondary shrinkage cavities. At the later stage of sand casting solidification, the liquid metal in the final solidified part will be solidified in the way of simultaneous solidification due to the small temperature gradient. At first, many fine grains appear in the whole liquid metal. With the decrease of temperature, the growth of grains and the generation of new grains, many isolated liquid metals separated by grains will appear in the final solidification part, or some liquid metals that are not completely separated by grains but have very small channels with the outside liquid metals.

Because of the high viscosity of liquid metal at this time, it is difficult for the external liquid metal to feed. When these liquid metals are further cooled and solidified, they will produce many small holes and form macroscopic shrinkage porosity, just like the process of shrinkage cavity. At the same time, there will also be many isolated liquid metals left between the dendrites of the grains. These liquid metals will produce more and smaller holes and form microscopic shrinkage porosity during solidification.

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