Creating a Thermal Property Database for Investment Casting Shells
A recent study developed a set of thermal properties for investment casting shells to aid in simulation modeling of solidification and shrinkage prediction. MINGZHI XU, SIMON LEKAKH AND VON L. RICHARDS, MISSOURI UNIVERSITY OF SCIENCE AND TECHNOLOGY, ROLLA, MISSOURI
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eliable and realistic ther- mal properties data for investment casting shell molds are required to cor-
rectly simulate the metal solidification and predict the shrinkage. Investment casting shells exhibit several phase transformations during firing and pouring that affect their transient thermal properties, which affect solidi- fication. Tese properties depend on time, temperature and process history. A recent study collected ther-
mal conductivity and specific heat capacity data from seven industrially produced ceramic molds of various types using an inverse method in which pure nitrogen was poured into ceramic molds equipped with two thermocouples. Software was used to simulate virtual cooling curves that were fitted to experimental curves by adjusting the temperature-dependent properties of the ceramic mold. Te data were compared with measure- ment results from laser flash. Te analysis of the differences will serve to improve the accuracy of investment casting simulation.
Finding a Better Way to Measure In a relatively thin-walled steel
casting, most of the super heat and part of the latent heat of the liquid metal are accumulated in the invest- ment ceramic shell, where specific heat capacity plays an important role. However, excessive heat from a mas- sive casting will transfer through the
shell, in which case, thermal conduc- tivity is a predominant factor. Both are significant in order to have representa- tive simulations for industrial use to control shrinkage defects and optimize casting quality. Because of the wide variety of shell
compositions, particle size distribu- tion and processing parameters, the ceramic shell could have from 10 to 30% porosity, which can provide air permeability but also affect the shell’s mechanical and thermal properties. Termal processing history also influences shell thermal properties. Several thermal stages occur in the investment casting process, including pattern removal/dewaxing, sinter- ing/firing, preheating and pouring. Colloidal silica binder, flour/filler and often ceramic stucco have a signifi- cant amorphous structure. Te degree to which the amorphous crystalline transformation takes place during
different thermal conditions affects the shell’s thermal properties. Te transient nature of the thermal
properties of investment shells make precisely measuring them difficult. Tis study used an inverse method in which a shell mold with installed thermocou- ples is poured with a pure liquid metal with well-defined properties. Shell thermal properties were estimated by running multiple computational fluid dynamic (CFD) simulation iterations, varying the thermal conductivity and heat capacity over a range of values in an effort to fit the calculated cooling curves to the experimental cooling curves for the shell and casting. Te inverse method takes much
effort to achieve an acceptable fit among the cooling curves. In this study, a method to correct the specimen thickness used in the laser flash method was introduced in order to obtain more accurate thermal property data. In a laser flash thermal diffusivity
Fig. 1. Surface topology is used to calculate the effective thicknesss of the test specimen.
test, a small specimen is subjected to a high intensity, short duration radiant laser pulse after thermal equilibration at the test temperature. Te energy of the pulse is absorbed on the front sur- face of the specimen and the resulting rear face temperature rise is recorded by a noncontact infrared radiation thermometer. Te thermal diffusivity is calculated from specimen thick- ness and time required for the rear face temperature to reach 50% of its maximum value. In differential laser flash calorim-
January 2016 MODERN CASTING | 43
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