in and 0.4 in) in the copper chill as shown in Figure 1. To sense the air gap formation at the interface a simple elec- tronic circuit was used as shown in Figure 2.
A small amount of voltage of 1.5 V was passed from the casting to the chill. The circuit was connected to a com- puter through a data logger which measures voltage and display using strip chart display software. This enabled in process monitoring of the air gap formation at the cast- ing chill interface. This information was required in order to squeeze the chill closer to the casting. This system is based on the assumption that the surface of the chill and the casting are perfectly flat. The chill was squeezed for- ward on complete cut-off (i.e. no electrical conductivity). Precautions were taken to avoid any short circuit through the cooling channels.
The alloy used for casting trials was A356. The eutectic temperature of aluminum silicon alloy is 577C (1070F). The table below shows the thermophysical property of the alloy.
Trial B (Average) (Fixed Chill) (With Cooling)
Trial B (Average) (Air Gap Signature)
Experimental Results “Scenario A” Fixed Chill without Water Cooling
Figure 3 shows temperature and air gap profiles of a fixed chill without any cooling. It is evident from the voltage drop that the first complete signal “cut-off” between the casting and the chill took place between 224-233 seconds over the entire Sce- nario A trials. After this period the chill became ineffective in removing heat from the casting. The air gap indicator in the present experiment only indicates complete isolation of the casting from the chill; however there is every possibility of isolated and smaller air gaps forming prior to this time. For simplicity and symmetry, it is assumed in this work that the entire flat surface of the casting shrinks away uniformly from the chill. Also an effort was made to measure the total air gap between the castings and chill at the end of each trial (i.e. after 600 seconds). This was done by moving the chill closer to the casting until the circuit was restored. Hence the total air gaps found at the end of the Scenario A trials was 0.9 mm (0.04 in).
Time, sec (a)
Figure 4. (a) Temperature profile; (b) Air gap signal for Scenario B. Trial C1
(Movable Chill) (With Cooling) (Displacement on Demand)
Trial C (Air Gap Signature)
Time, sec (b)
Time, sec
Figure 5. (a) Temperature profile; (b) Air gap signal for Scenario C. (a)
68
Time, sec (b)
International Journal of Metalcasting/Spring 11
Temperature, C
Voltage, V Voltage, V
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