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TESTING 1-2-3 Understand the Heat, Improve the Shell

Researchers hope to improve investment casting shell performance by building a database of thermal properties. A MODERN CASTING STAFF REPORT

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eliable and realistic thermal properties data for investment cast- ing shell molds can increase the accuracy of solidification simula-

tions and predictions of shrinkage. Investment casting shells exhibit several phase transformations during firing and pouring that can affect their transient thermal properties. Tese properties depend on time, tempera- ture and process history. Mingzhi Xu, Simon Lekakh and

Von Richards, Missouri Univ. of Sci- ence and Technology, Rolla, Mo., stud-

ADDING IT ALL UP Breaking down the latest research is as easy as 1-2-3.

“Thermal Property Database for Investment Casting Shells,” Mingzhi Xu, Simon Lekakh and Von Richards, Missouri Univ. of Science and Technology, Rolla, Mo.

Background—Thermal properties of investment shells are transient and can be difficult to measure. Considering the difficulties in measuring these properties, researchers may use the inverse method. With this approach, thermocouples are attached to the shell mold, while it is filled with a pure liquid metal with well-defined properties. The thermal properties of the shell then are estimated by running multiple computational fluid dynamic (CFD) simulation iterations by varying thermal conductivity and heat capacity to match the experimental cooling data.

Procedure—Seven different industrial shells were built using the aqueous colloidal silica binder with different minerals as listed in Table 1. Samples were put into an ambient furnace with 15C (27F)/minute heating rate and laser flash tested from 200C (392F) to 1,200C (2,192F) at intervals of 200C (360F). Three runs of each type of specimen were conducted and the average values were reported in the results.

Results and Conclusions—The laser flash method showed values similar to those theoretically calculated, because the thin specimen used in the laser flash method was partially thermally stabilized, which was closer to thermal equilibrium. Nevertheless, the shell in reality was hardly in thermal equilib- rium conditions, so the inverse method provided more realistic effective heat capacity values for modeling. The thermal property data measured from laser flash could be used as starting points in the automatic optimization process, which reduces simulation and limits potential error.

ied the thermal properties (thermal conductivity and specific heat capacity) of seven industrially produced ceramic molds. Tey used an inverse method, where pure nickel was poured into ceramic molds equipped with ther- mocouples. Simulation software then was used to simulate virtual cooling curves that resembled the experimen- tal curves by adjusting the temperature dependent thermal properties of the ceramic mold. Te thermal properties data obtained from this method were compared with measurement results from laser flash in the hope the dataset will serve to improve the accuracy of

investment casting simulation. Te paper, “Termal Property Database for Investment Casting Shells,” provides their analysis of this study.

Question Will the inverse method (comparing

experimental measurements with trial- and-error simulation) provide more accurate measurements for the thermal properties in investment casting shells and help improve simulation?

shells may have 10 to 30% porosity, which can provide air permeabil- ity but also affects mechanical and thermal properties. Termal process- ing history also influences a shell’s thermal properties. Several thermal history stages are involved in the entire process, including pattern removal/ de-waxing (176F-572F [80C-300C]); sintering/firing 1,112F-1,832F [600C-1,000C]); preheating (1,472F-2,192F [800C-1,200C]); and pouring (2,732F-2,912F [1,500C-1,600C]). Colloidal silica binder, flour/filler and ceramic stucco have amorphous structures at signifi- cant extent. Te degree to which the amorphous-to-crystalline transfor- mation takes place during different thermal history conditions affects the shell’s ultimate thermal properties. Te transient nature of the thermal


properties of investment shells make them difficult to measure using clas- sical methods, which require steady state conditions. Considering the difficulties of measuring the thermal property of the non-uniform porous shell, researchers may use the inverse method, which characterizes the thermal properties of the shell during

January 2015 MODERN CASTING | 39

Background Because of the variety in

shell compositions, particle size distribution and pro- cessing parameters, ceramic

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