There are many reasons for casting defects. Gating system, pouring temperature and flow speed of molten liquid, casting size and other factors will affect casting defects. Therefore, it is difficult to find a scientific and reasonable prediction criterion with more factors. The more factors considered will undoubtedly reduce the efficiency of numerical simulation. At the same time, it can be known that the occurrence of casting defects such as shrinkage porosity and shrinkage cavity is closely related to the heat transfer phenomenon in the pouring process. Therefore, from the perspective of heat transfer, researchers have put forward several effective prediction criteria, mainly including flow conduction method, isothermal curve method, solid rate gradient method, temperature gradient method and niyama method. The niyama method is mainly introduced below.
Niyama method was proposed by niyama in 1982. It is found that the distribution of shrinkage cavity and porosity of castings is closely related to the temperature gradient and cooling rate. Therefore, the generation of shrinkage cavity is judged by the value of solidification area by establishing a mathematical function relationship, in which G represents the temperature gradient of a certain area and R represents the cooling rate of a certain area. After this method was put forward, a large number of practical tests found that niyama method has high accuracy in predicting shrinkage casting defects, but there is still a large error in shrinkage porosity. In actual production, the foundry personnel often use the critical solid rate method to assist with the niyama method to improve the accuracy of the prediction of the overall distribution of casting defects. The expression of niyama method is as follows:
Where, G is the temperature gradient; R is the cooling rate; Tupper is liquidus temperature; Tlower is the solidus temperature; Tupper is the time of liquidus temperature; Tlower is the time of solidus temperature.