Fig. 5. Illustrations show the core gas surface flux to the metal (a) and the metal solid fraction contour (b) at 1,337 seconds.
cross sections are relatively small, rapid solidification shrinkage mod- eling can produce good results. As can be seen in Fig. 4c, the first
principles model correctly predicted the microporosity in regions A and B, with a smaller porosity area than predicted by the rapid solidification shrinkage model. Tis is understand- able considering the fluid flow in the first principles model. However, like the rapid solidification shrink- age model, the first principles model failed in predicting the porosity in region C. Te porosity in region C might have resulted from gases evolved from the core. To verify this hypothesis, the core gas surface mass flux to the metal and the metal solid
fraction contour at two different times during the solidification of the first principles model were plotted (Figs. 5 and 6). Due to the high pressure of core
gas, it vents into the metal. However, as the metal cools down, part of the core surface is sealed by the solidi- fied metal. As the solidification front closes in, the core gases that vented to the metal at the final stages will be trapped in the metal, forming microporosity. As demonstrated in Fig. 5, core gases vented to the two liquid pockets. Since the two liquid pockets were isolated by the high solid fraction region, core gases that vented to the lower liquid pocket could not have escaped and were
trapped at the solidification front, forming microporosity. Tis location coincided with the trapped loca- tion in region C. Core gases vented to the upper liquid pocket either escaped through the free surface or were trapped inside the metal, form- ing severe microporosity in the last solidified region, which is region A. Even though the core gas was not tracked explicitly in the metal, the possible locations of the porosity can still be extrapolated based on the core gas venting and metal solidifi- cation behavior.
Tis article is based on the paper “Numeri- cal Simulation of Core Gas Defects in Steel Castings” (14-018)presented at the 2014 AFS Metalcasting Congress.
Fig. 6. Illustrations show the core gas surface flux to the metal (a) and the metal solid fraction contour (b) at 2,065 seconds. 30 | MODERN CASTING September 2014
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