Effect of “Movable Chill (Displacement on Demand)” on Heat Transfer “Scenario C”
In the case of the movable chill with water cooling, the heat transfer coefficient falls below 1000 W/m²K because of the air gap. As the chill is squeezed into the casting to restore the contact between the casting and chill, the heat transfer coef- ficient increases to nearly 1900 W/m²K. To simulate the heat transfer at the exact instant of chill displacement, a number of input heat transfer coefficients were applied.
Effect of “Movable chill (Displacement Before Eutectic)” on Heat Transfer “Scenario D”
As in this case the chill is continuously squeezed into the cast- ing and a continuous contact is maintained between the casting and the chill the initial heat transfer is relatively high compared to Scenario C (chill with displacement on demand). The heat transfer coefficient before 100 seconds is maintained above 2200 W/m²K. More importantly the heat transfer is more prom-
inent during the period of dendrite growth. The calculated heat transfer coefficients used to simulate the air gap phenomenon for the various trials are summarised in table 4.
Microstructure
The casting samples under each series were cut at a distance of 5 mm, 15 mm, 30 mm, 45 mm and 60 mm (0.2 in, 0.6 in, 1.2 in, 1.8 in and 2.4 in) respectively from the interface as shown in Figure 12. To determine the changes in SDAS along its length, the castings were cut parallel to the axis. The samples were cut into rectangular pieces of 15 mm X 10 mm (0.6 in X 0.4 in) and observed under an optical microscope.
The samples closer to the chill (less than 5 mm) were ob- served under higher magnification (20 times) whereas sam- ples away from the chill face (over 5 mm) were observed at lower magnification (5 times). This is because the dendrites closer to the chill were much finer compared to the dendrites located away from the chill.
Table 4. Heat Transfer Coefficients (HTC) Obtained from Casting Simulations
Figure 12. Casting cross section for micro structural analysis. 74 International Journal of Metalcasting/Spring 11
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