“Scenario B” Fixed Chill with Water Cooling
Figure 4 shows the temperature and air gap profiles of a fixed chill with water cooling. Most “spot” cooling circuits in in- dustry use water as the medium for aggressive cooling. It is evident from the voltage vs. time graph in this scenario that a complete air gap occurs at 30-45 seconds after pouring. This is a significantly faster formation of air gap than observed in Scenario A (i.e. between 224-233 seconds). While this may not seem intuitive at first, an explanation is made here that as the molten metal comes into contact with the water cooled chill, a surface layer (skin) of metal freezes instantly and starts to shrink away from the chill. This significantly faster formed air gap makes the remaining solidification inefficient or less than potentially optimum. Repeated temperature re- sults show that the fixed chill with water cooling (Scenario B) is less effective than the fixed chill without cooling (Sce- nario A). In Scenario B, the air gap was formed at an earlier stage and the total air gap size found at the end of the solidi- fication cycle (i.e. after 600 seconds) was 0.74 mm (0.03 in).
“Scenario C” Movable Chill with Water Cooling (Displacement on Demand)
Figure 5 shows the temperature and air gap profiles of a mov- able chill with water cooling where the displacements were made on demand. In this series of trials the chill was moved forward to maintain contact as the air gap evolved. It is evident from the corresponding voltage vs. time graph that the first cut- off took place between 18-30 seconds over all trials in Scenario
C. The faster times were due to the chill being immediately brought into contact with the casting to facilitate optimum heat transfer throughout the solidification cycle. The correspond- ing temperature graphs also reveal high cooling rates and rapid losses of temperature in the measured part of the casting. The chill was displaced to 0.9 mm (0.04 in) for Scenario C.
“Scenario D” Movable Chill with Water Cooling (Displacement Before Eutectic Temperature)
From the literature review it is expected that a faster cooling before the eutectic temperature will lead to significant improve- ments in the dendrite arm spacing thereby improving the qual- ity and mechanical properties of the casting.3-5
In Scenario C, it
was found that the air gap evolves before the eutectic tempera- ture (577C) thereby reducing further heat transfer between the chill and casting. To counter this in Scenario D trials, it was decided to give an initial displacement of 0.35 mm (0.0137 in) before the eutectic temperature thereby increasing heat transfer over this time. This delayed the evolution of the first air gap to between 185-300 seconds. Also a constant force of 5 N (New- ton) was applied at the end of a chill to maintain a close contact with the chill. In Scenario D experiments, the chill was pushed to a total distance of 2.5 mm (0.1 in) into the casting to maintain a constant contact. This is due to the fact that the chill was easier to push into the aluminum in a mushy state before the eutectic temperature was reached. It can be seen from the temperature profile that the casting has been cooled at a faster rate by this technique compared with the other scenarios. The table below summarizes the experimental findings.
Table 3. Experimental Conditions
Trial D1
(Movable Chill) (With Cooling) (Displacement Before Eutectic Temperature)
Trial D (Air Gap Signature)
Time, sec (a)
Figure 6. (a) Temperature profile; (b) Air gap signal for Scenario D. International Journal of Metalcasting/Spring 11 69
Time, sec (b)
Temperature, C
Voltage, V
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