Experiment
Experiments were designed to reveal the effectiveness of movable water cooled chills. The order of operations was:
• carry out casting trials using different cooling con- ditions,
• record the temperature histories from the thermo- couples,
• record the corresponding air gap signature from contact measurements of each casting under differ- ent cooling scenarios,
• relate the temperature and air gap data to micro- structure by measuring the secondary dendrite arm spacing (SDAS) at various distances in the casting from the chill,
• iteratively develop boundary conditions to approx- imate the heat transfer coefficients at the casting/ chill interface of the computer model to simulate a close representation of the solidification under each cooling scenario,
• relate the temperature, air gap, and heat transfer coefficient and microstructure data to validate the effectiveness of the chill under different sce- narios.
The casting trials were carried out under four scenarios and repeatability trials were performed for each.
Experiment Setup
To understand the heat transfer effect of the chill, labora- tory trials were performed by casting aluminum in an in- sulating mold (made from dense fused silica - DFS) and cooling it from one end using a copper chill block under varying conditions. The reusable DFS mold was created to produce one dimensional heat transfer through the casting. Approximately 20µm coating of boron nitride was applied to the walls of the mold (but not the chill face) for easy release of the casting from the mold. The chill was made up of copper with a push screw mechanism. The chill was designed to push a maximum of 3 mm (0.12 in) into the casting. Two thermocouple holes of 1.75 mm (0.07 in) di- ameter were made in the copper chill at a distance of 5 mm each from the interface. The screw was threaded with a pitch of 2.54 mm (0.1 in). The chill was pushed forward by 2.54 mm per revolution of the screw. The screw was coated with copper gel to prevent jamming resulting from thermal expansion at higher temperatures.
Two type-K thermocouples were inserted into the holes drilled in the copper chill and one thermocouple was insert- ed into the casting cavity. The thermocouples were 1.5 mm (0.06 in) diameter and press fit and the tips were coated with copper paste to ensure contact between the tip (hot junction) and the chill. The thermocouples were located at a distance of 5 mm (0.2 in) in the casting and at 5 mm and 10 mm (0.2
Table 2. Physical and Thermal Properties of Casting Materials
Trial A (Average) (Fixed Chill) (Without Cooling)
Trial A (Average) (Air Gap Signature)
Time, sec
Figure 3. (a) Temperature profile; (b) Air gap signal for Scenario A. (a)
International Journal of Metalcasting/Spring 11
Time, sec (b)
67
Temperature, C
Voltage, V
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