Effect of Chill Conditions on SDAS
The (SDAS) has a direct relationship with cooling. As expected, the secondary dendrites are observed to be finer at the chill interface and gradually increasing away from the interface. Figures show microstructure of castings taken at a fixed distance from the chill. In order to clearly identify the effectiveness of different chill conditions on SDAS, a graph was plotted between SDAS and distance
from the interface for each casting sample. A logarithmic trend line of best fit was drawn over the measured den- drite value as shown in Figure 17. From this figure it can be seen that the averaged dendrites in Scenario B (fixed chill with water cooling) is much higher than the rest of the trials. This is because the cooling rates of castings produced under Scenario B trials are much lower than other scenarios as a result of sudden air gap formation near the beginning of the cooling cycle.
“Scenario A” Fixed Chill Without Water Cooling
11.5 μm at 2.5mm
22.3 μm at 7.5mm 41.2 μm at 15 mm
57.7 μm at 37.5 mm
Figure 13. Microstructure at different distances from the chill face for Scenario A. “Scenario B” Fixed Chill With Water Cooling
64.1μm at 52.5 mm
Figure 14. Microstructure at different distances from the chill face for Scenario B. 13.3 μm at 2.5mm
49.6 μm at 7.5mm 51.7 μm at 15 mm
68.2 μm at 37.5 mm
75.2 μm at 52.5 mm “Scenario C” Movable Chill With Water Cooling (Displacement On Demand)
12.5 μm at 2.5mm
24.7 μm at 7.5mm
42.1 μm at 15 mm 47.3 μm at 37.5 mm 55.7 μm at 52.5 mm
Figure 15. Microstructure at different distances from the chill face for Scenario C. “Scenario D” Movable Chill With Water Cooling (Displacement Before Eutectic)
11.3 μm at 2.5mm
20.3 μm at 7.5mm International Journal of Metalcasting/Spring 11
32.1 μm at 15 mm
41.2 μm at 37.5 mm 51.4 μm at 52.5 mm Figure 16. Microstructure at different distances from the chill face for Scenario D. 75
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