Prediction method of shrinkage cavity casting defects

The main function of the casting process numerical simulation software is to simulate and predict the formation of shrinkage porosity and shrinkage cavity casting defects, which has achieved the function of controlling the internal quality of castings. At present, the commonly used numerical simulation software of casting process in the field of casting has many mature standards for judging the shrinkage and shrinkage defects of steel castings or graphite free casting alloys. Because graphite containing cast iron will precipitate graphite with larger specific volume during solidification, its volume change is more complex than that of steel castings and other graphite free casting alloys, and special criteria need to be adopted. The main criteria for predicting the formation of shrinkage porosity and shrinkage cavity casting defects in nodular cast iron are shrinkage expansion dynamic superposition method (DECAM) and micro simulation method (MMM). At present, the most commonly used and accurate prediction methods of shrinkage cavity and porosity casting defects are as follows:

(1) Trichromatic phase zone method. The solidification field is divided into solid phase area, liquid phase area and solid-liquid two-phase area with the critical solid rate as the boundary. The three areas are marked with different colors. Through the color distribution, it can directly judge whether there is an isolated liquid phase area in the solidification process. The critical solid rate defined by the trichromatic phase zone method is generally 0.6 ~ 0.65, that is, when the solid rate is greater than the critical value, it is considered that the feeding channel has been closed and no longer has the feeding capacity; When the solid phase ratio is less than 0.6, feeding can still be continued.

(2) Temporal criterion. By calculating the time taken for each unit of the casting to reach the same reference value, the isochronous distribution cloud map of each part is drawn. According to the difference set by reference value, it can be subdivided into solidus time method, liquidus time method and critical value solid time method. The first two methods are to calculate the time from the pouring of liquid metal into the mold to the temperature cooling to the solidus or liquidus. Practice has proved that the solidus time method is more suitable for eutectic alloys to predict defects. The critical solid rate of the critical solid rate time method is 0.65. The method to judge the casting defects of shrinkage porosity and shrinkage cavity is similar to the solidus time method, that is, the longer the solidification time is, the more defects are likely to occur.

(3) G / โˆš ๐‘… method. Also known as niyama criterion or shinyama criterion, it was proposed by Japanese scholar shinyama Yingfu in 1982. In the formula, G is the temperature gradient and R is the cooling rate. It is mainly to evaluate the feeding capacity of liquid metal through the ratio of cooling rate of each part of the casting at the end of solidification. Compared with the traditional temperature gradient method, g / โˆš ๐‘… method not only considers the volume change during casting shrinkage, but also considers the loss of fluid pressure, so it can more accurately reflect the change of solidification process. The critical value of g / โˆš ๐‘… method is 0.7 ~ 1.3. The specific value is determined according to the size of castings. The upper limit value is taken for large castings and the lower limit value is taken for small castings. In the calculation and simulation, shrinkage porosity and shrinkage cavity casting defects will occur when it is less than the critical value.