Regarding the solidification patterns of the three optimized designs, no isolated liquid areas are forming in the bot- tom area as in the original layout, depicted in Figure 7. The bottom chill plate and the stair-type chill around the conical section induced directional solidification towards the thermal axis and the riser. Solidification patterns were checked over the entire solidification interval. It was found that none of these designs exhibit apparent isolated liquid areas although solution 3 is really on the edge later in so- lidification in an area right below the riser-neck, see Figure 17. The reason is that the riser is too small and the heavi- est section of the casting slowly begins to be the last to solidify. If one should fully rely on the numerical results, solution 3 would be good enough for production. However, simulation does not take into account all crucial factors that occur in practice. For instance the quality of the melt can be compromised by a dirty ladle with residuals from the previous batch. Next could be the human factor, which of- ten compromises the quality of a casting process. Having all this in mind it was decided together with the foundry not to go for solution 3 to avoid failure in production. But this solution is still shown and discussed here.
In Figure 18, the centerline porosity is expressed by the Niyama criterion. The light blue areas stand for values of 0,4 and lower which will contain macroscopic shrinkage. Everything above the 0,45 up to 1 will most likely be microporosity, not detect- able by the radiography techniques. It is seen that despite solu- tion 3 being on the edge, it still shows no occurrence of porosity in the casting body. Only the bottom pins contain small porous areas. The reasonable remedy for this was addressed earlier.
A similar situation applies for shrinkage porosity shown in Figure 19. The casting body appears to be porosity free in all three cases, except for the pins again.
The last assessment concerns the casting yield. The aim of the entire project has been primarily to eliminate the presence of various casting defects. Once this was achieved, the next step was to optimize the riser volume for the casting yield im- provement. The results of this assessment are given in Table 4. Compared to the original design, the casting yield could be increased by approximately 25% if the optimized solution 3 was applied. Due to a high risk of shrinkage occurrence below the riser neck, solution 3 was not approved for production.
Table 4. Casting Yield Assessment
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(b) International Journal of Metalcasting/Fall 10 73
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