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behavior of the liquid metal is somewhat non-linear up until about 200C (392F). The non-linearity can be attributed to non-isothermality of the GED, as measured by the various thermocouples. The height measurement data in Figure 4 was used in Equation 2 to calculate an effective volumetric expansion coefficient of the liquid metal


temperature. This parameter describes the metal expansion behavior specific to the present setup.


The liquid metal volumetric expansion coefficient obtained from the separate metal-only expansion tests employing the borosilicate bulb was found to be constant with respect to temperature. This constant volumetric expansion coefficient of the liquid metal,


, is equal to 1.12x10-4 5 1/°F). Using this value of 1/°C (6.22x10- , the predicted metal height


change in the GED is plotted alongside the measured metal height change in Figure 4. It can be seen that the measured and predicted height change curves are parallel above 200C (392F), implying that is indeed the true metal expansion coefficient. However, the actual expansion behavior of the liquid metal, inclusive of any effects due to non-isothermality below 200C (392F), is directly described by . Subsequent- ly, was interpolated at intervals of 0.1°C from the GED measurements for use in the pure gas expansion and binder gas evolution calculations. The average density of the liquid metal at room temperature was found to be 6.297 g/cm3


. Pure Gas expansion tests


Figure 5 shows the ratio of the measured to known molecu- lar weight of (a) pure argon gas and (b) pure hydrogen gas as a function of temperature during heating at low rates of


as a function of


2°C/min (3.6°F/min) and 3°C/min (5.4°F/min). For both gases and most temperatures, the measured dimensionless molecular weights are within 5% of unity. This indicates that the present experimental setup allows for reasonably accu- rate gas molecular weight measurements over a large tem- perature range. The dramatic increase in the dimensionless molecular weight for hydrogen above 560C (1040F) is pre- sumably due to dissolution of hydrogen into the liquid metal.


As previously noted, the true gas temperature was assumed to be equivalent to the measured metal temperature. Figure 5 shows that good agreement between the measured and true gas molecular weights is achieved when the gas temperature is approximated as the measured metal temperature. Addi- tional calculations were performed assuming that the gas temperature was equivalent to the measured glass tempera- ture, but the resulting molecular weights were very inaccu- rate. Therefore, equating the gas and metal temperatures was deemed acceptable.


Figure 5 shows that the dimensionless molecular weight measurements draw closer to the ideal value of unity with in- creasing temperature (aside from the hydrogen dissolution) and eventually become essentially constant. This indicates that, as the gas volume increases during heating, the sensi- tivity and accuracy of the molecular weight measurements increase. This is also reflected by the differences between the curves in Figure 5 (a). The argon molecular weight curves corresponding to lower initial volumes experience greater deviation from unity and are more erratic than the curves


(a) Argon gas molecular weight results.


(b) Hydrogen gas molecular weight results. The dramatic increase in ϕ above 560C (1040F) is presumably due to dissolution of the hydrogen gas into the liquid metal.


Figure 5. Ratio of measured to known molecular weight of (a) argon gas and (b) hydrogen gas as a function of temperature for low heating rates.


32 International Journal of Metalcasting/Spring 2012


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