Figure 17 indicates that the average dendrite arm spacing is reduced using movable chills. The average SDAS in Sce- nario B (fixed chill with water cooling) at 52.5 mm (2.06 in) was 75.2 μm. This is higher than the average SDAS in Sce- nario C (movable water cooled chill on demand) and Scenar- io D (movable cooled chill before eutectic) which are found to be 55.7 μm and 51.4 μm respectively. This difference is a result of faster cooling rates using movable chills. There is also a significant difference in SDAS between Scenario C and Scenario D especially at dendrites away from the in- terface. The average SDAS at a distance of 2.5 mm (0.1 in) for Scenario C and D are found to be 12.5 μm and 11.3 μm respectively. Even though this difference is small, it is due to faster cooling before eutectic temperature in Scenario D.
Discussion
Effect of Chill Conditions on Temperature and Contact Time
It can be seen that from Figure 18 the chill contact time for trials in Scenario C is over 70% and for trials in Scenario D is over 90%. This is due to the movable chill indenting between 0.9 - 2.5 mm (0.035 - 0.098 in). Another key observation is that percentage of contact for trials in Scenario B (fixed chill with water cooling) is significantly lower than other chill condi- tions and in particular trials in Scenario A (fixed chill without cooling).
Trials in Scenario B show higher temperatures of about 450C (842F) at 575 sec whereas the movable chills have been reducing the temperature below 300C (572F) for the same time duration. This is sig- nificant since this reduces solidification time thereby increasing productivity for a given casting. Figure 19 shows the combined temperature histories of casting
% of Contact vs Trials
solidification under all different scenarios. Temperature of 540C (1004F) which is slightly below solidus temperature was chosen to compare the thermal histories of Scenario A, C and D with Scenario D. It can be observed that the temperature of casting under Scenario A, C and D reaches 540C (1004F) at 53 sec, 65 sec and 51 sec respectively whereas Scenario B reaches the same temperature only af- ter 227 sec. By comparing the ratio of time to reach this critical temperature we conclude that the castings produced using movable chills solidify over four times faster. This was achieved by 0.9 to 2.5 mm (0.035 -0.098 in) indenta- tion of the chill at the casting face. Impact of this is signifi- cant for improving productivity.
Using 540C as a measure of productivity at first glance shows a tie between Scenarios A, C, & D. Scenario A can- not, of course, dissipate heat nearly as fast as the other two out to 500 sec. Where productivity is concerned, a far more important consideration is the fact that this is a model for
Average SDAS vs Distance
Distance from Interface (mm) Figure 17. Average SDAS vs. Distance.
Temperature After 575 Sec
(a) Figure 18. (a) Percentage of contact vs. trials; (b) Temperature after 575 sec vs. trials. 76 International Journal of Metalcasting/Spring 11
(b)
Percentage of Contact (%)
SDAS (µm) Temperature, C
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