In the above experiments, aimed at measuring the boundary conditions between the liquid metal and the solid metal sur- face, it is not easy to see why such physical conditions, such as liquid metal raising against a chill surface, should need to increase the value of boundary condition, which rise to a maximum and decline.
To verify whether the peak in the estimated heat flux transients is an artifact of the in- verse modeling technique, two sets of ex- periments were carried out. In the first set of experiments the chill surface is brought in contact with hot water at about 80C (176F). Heat flux transients are plotted for copper, aluminum and stainless steel chills against hot water (Fig. 7). The figure clearly shows that the peak is not obtained during heating of the chill by hot water.
An experiment was also carried out for as- sessment of heat transfer from a hot chill at 500C (932F) to a chill material at 27C (81F). Experimental set-up consisted of cold and hot chills held one above the other at known temperature, each being instru- mented with thermocouples. With a sheet of insulation held between the two chills, they are kept at the respective temperatures, until the insulation was removed. The re- moval of the insulation layer leads to a gap of 1mm (0.1cm, 0.039 inch) at the inter- face. This would set up an instantaneous heat flux within the chill interface. With the use of temperature history in the chill, heat flux transients are estimated by inverse analysis. Figure 8 shows the heat flux tran- sients estimated for the copper chill against the aluminum chill.
In both types of experiments, the peak in the heat flux transients was not observed. How-
ever, such a peak is obtained in all experiments involving so- lidification against chills. This clearly indicated that the peak in the heat flux transients during the solidification of alloys against chills or in metallic molds is not an artifact of the experiment.
Figure 4. Thermal plot of aluminum chill at different time intervals during solidification.
Figure 5. Heat flux transients during solidification against Al and graphite chills.
66
chill surface temperature (T/Tmax) for the alloy solidifying against graphite chill.
Figure 6. Normalized heat flux (q/qmax International Journal of Metalcasting/Fall 2011
) vs normalized
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