Through the numerical simulation software, the temperature field of the casting solidification process can be determined, and the isotherm and isochrone distribution of the casting on the solidification section can be obtained. In addition, the parameters of solidification time t, solidification speed r, temperature gradient grad T and feeding efficiency f can also be calculated. Then through a series of related formulas and judgment basis, the macroscopic defects such as shrinkage porosity, shrinkage cavity, cold shut and segregation can be predicted. By improving and optimizing the, the casting defects can be eliminated or improved, so as to improve the casting life. The following is the prediction of common defects of castings:
(1) Prediction of shrinkage cavity of casting
Shrinkage cavity occurs in the last stage of the solidification process of the casting. Because the liquid metal shrinks in the casting during the final solidification stage, the closed part can not be fed by the liquid metal. Finally, a cavity is formed, which is large in volume and gathered together. It is called shrinkage cavity, which is small and dispersed. One of the key points of numerical simulation of casting solidification process is to predict the defects of shrinkage cavity, and to know the location and size of shrinkage cavity, so as to improve the casting process parameters and casting process. Niyama method, isothermal curve method and solid rate gradient method are used to predict the shrinkage porosity of castings.
(2) Prediction of segregation
Segregation is a phenomenon of uneven crystal distribution during solidification, that is, redistribution of solute occurs. If segregation occurs in the solidification process, it has a great influence on the mechanical properties of the casting, and it is difficult to eliminate it by post heat treatment. Solidification process is a process of unstable heat conduction. We should consider the flow and other factors in the solidification process of molten metal. Through numerical simulation, we can predict the location and size of casting segregation. The prediction methods of casting segregation include: thickening time method, L / h method, solidification speed method, etc.
(3) Prediction of microstructure
The formation of microstructure is a complex process. There are macro heat and mass transfer, micro grain nucleation and growth process. By simulating the temperature field and concentration field of solidification process, the thermal parameters of solidification process can be calculated, and then associated with the solidification structure and properties of the casting, the formation state of the microstructure of the casting can be predicted, and then the mechanical properties of the casting can be predicted. The simulation technology of casting microstructure formation is mature. We can predict it by determinism, stochastic theory and phase length method.