includes latent heat effects from phase changes. However, a small mass specimen is equilibrated at an environmental test temperature in the laser flash. Consequently, the transformation occurring in the inverse method may have already taken place prior to the measure- ment by the laser flash method. Similarly, when comparing the total enthalpy change from room temperature (68F [20C]) to 2,885F (1,420C) among the values from theoretical calculation of the inverse method and the laser flash method, the laser flash method shows similar values, because the thin specimen used in the laser flash method was under partially thermally stabilized condition which is closer to ther- mal equilibrium. Te shell in reality is hardly in thermal equilibrium conditions, thus the inverse method provided more realistic effective heat capacity values for modeling the
pouring and solidification processes. Thermal property data mea-
sured by laser flash could be used as the starting point in the automatic optimization process, which greatly reduces the num- ber of simulation cases needed to approach a well fitted case and reduces the potential extrapolation error in iteration step estimates. Te theoretical thermal conduc-
tivity of pure silica with 33% porosity was plotted in Fig. 5 as well as ther- mal conductivity values of shell #1 and shell #3. Tose industrial shells had similar measured and theoreti- cal values of thermal conductivity at a lower temperature but were more heat conductive at a higher tempera- ture. Tis could result from different particle and porosity size distribu- tions, since smaller particle size with higher grain boundary to volume ratio will lower thermal conductiv- ity. Tis theoretical model may not
consider the photon conductivity of the pore phase at higher temperature. Obtaining the data from laser flash and then applying the data in the inverse method can be time con- suming and costly. Te researchers recommend that industries develop- ing their own investment casting shells pick the thermal property data of shells from given investment casting facilities in the study with the closest composition and utilize those estimates in their simulations. Whoever uses the data must measure the bulk density and porosity of their shells, because bulk density is used in most simulations and porosity is needed to adjust the value of thermal conductivity.
Tis article was based on the paper, “Termal Property Database for Investment Casting Shells,” (Paper No. 14-020, presented at the 118th alcasting Congress.
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January 2016 MODERN CASTING | 47
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