# Prediction of several main casting defects

1) Prediction of shrinkage porosity and porosity

Shrinkage cavity and shrinkage porosity are the holes that appear when the temperature of liquid metal decreases and the volume shrinkage of liquid metal cannot be compensated. The main prediction methods of shrinkage cavity and porosity are critical solid rate method and g / R method.

① Critical solid rate method

Critical solid fraction method means that when the solid fraction of liquid metal reaches the critical value, it will stop flowing and lose the ability of feeding other regions. This characteristic is often used to study the shrinkage porosity and porosity of casting. The observation method in the numerical simulation is that when the isoline of critical solid rate of liquid metal forms a closed region, defects will occur in the closed region. This method is simple and intuitive to judge defects, but it is difficult to supplement when the critical solid rate curve is in the shape of “U” and has not been closed yet, so the accuracy of this method is not enough.

② G /R method

G is the temperature gradient (℃ / cm), R is the cooling rate of the calculation unit (℃ / min)

2) Prediction of stomata

The air hole is a curled up air hole related to the filling state. From the distribution of velocity vector in the process of casting filling, it can be roughly determined whether it is possible to roll up porosity. If the casting wall size is small and solidification is fast, this method will be very effective.

3) Prediction of inclusions

According to the state and velocity of the high temperature liquid metal colliding with each other in the mold cavity, we can roughly describe whether the whole filling process will produce defects such as inclusion oxidation. When the velocity of liquid metal exceeds the critical velocity of 0.35-0.5m/s, the liquid metal and the oxide inclusions are easy to form. If it is necessary to judge whether there are serious inclusions in the casting, it is necessary to judge whether the parts that produce scum in the solidification process belong to non part positions, such as riser. Among them, Stokes equation can be used to calculate the floating time of inclusions.

4) Prediction of cold insulation

In the process of filling high temperature liquid metal, the unit solid phase ratio at the flow front reaches the critical solid phase ratio, which causes the liquid metal to stop flowing and produce cold shut. In the process of numerical simulation, when the critical solid ratio of casting is about 0.45, the flow velocity can be set to 0. The size of the solid phase ratio is related to the way to deal with the latent heat of solidification. At present, the equivalent specific heat capacity method, temperature rising method and enthalpy method can be used to deal with the precipitation of latent heat of solidification crystallization. When the latent heat of solidification is treated by the method of equivalent specific heat, the relationship between temperature and solid phase rate in the phase diagram can be used to calculate the solid phase rate, which is assumed to be linear. 