Shrinkage cavity and porosity defect is one of the most common defects in the casting process. The prediction of the formation, size, position and shape of shrinkage cavity and porosity defect is not only an important content of numerical simulation research, but also the most important part of numerical simulation software at present.
Shrinkage porosity and shrinkage cavity defects are the holes caused by the lack of feeding of foreign liquid metal at the last solidification part of the casting due to liquid shrinkage and solidification shrinkage during the solidification process of the casting. The reason for the shrinkage defect is that the solidified solid metal has formed a closed shell on the hot spot of the casting, so that the liquid metal at the hot spot is separated from the external feeding source, and the later solidification shrinkage and liquid shrinkage can not be supplemented by the external metal. At this time, the shrinkage defect occurs. The reason of shrinkage porosity defect is that the alloy with a certain solidification temperature range will produce developed dendrites and form a skeleton structure with certain mechanical strength When the volume feeding is stopped at a certain time, the foreign liquid metal can only supplement the solidification shrinkage defects of castings through the seepage between dendrites. The seepage resistance between dendrites is relatively large. Shrinkage porosity defects will occur when the seepage liquid metal is not enough to supplement the solidification shrinkage of castings.
For the prediction of shrinkage defects, Japanese scientist Yoshio ohnaka believes that the flow and pressure drop of liquid metal are the main reasons for the formation of primary shrinkage defects, and the concentration of solute components and bubbles generated by dissolved gas and reaction gas are the main causes of shrinkage defects and micro shrinkage defects. Qianqianyan and Jinfu put forward the solid rate gradient method and critical solid rate method. These methods are based on the calculation of temperature field and do not consider the liquid phase flow caused by solidification shrinkage, which is very different from the mechanism described by Dazhong Yixiong, so their critical value swings greatly. Dazhong Yixiong and others considered both the temperature field and the flow field generated by solidification shrinkage, correctly evaluated the flow resistance, and also considered the consequences of pressure drop caused by increasing the amount of solidification. They truthfully simulated the generation of shrinkage defects, but the influence of gas precipitation was not considered in the prediction of shrinkage defects. The influence of gas precipitation on the formation of shrinkage defects is considered, but it is only limited to plate castings, and the effect of solidification shrinkage is ignored. It can be seen that it is also quite different from the mechanism. The flow analysis of solidification process is carried out, the temperature field is calculated, the solidification speed and the width of solid-liquid coexistence zone are obtained, and the central shrinkage defect is predicted. This method is only limited to plate castings.