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the casting process. Te shell mold, with a number of thermocouples, is filled with a pure liquid metal with well defined properties. Te thermal properties of the shell then are esti-


obtain more accurate thermal prop- erty data. Afterward, the physically measured thermal property data was applied to the inverse method as the starting points to reduce the time and errors induced from extrapolating the optimization algorithm. Seven indus- trial shells were evaluated. A thermal property database was developed to help increase the accuracy of the investment casting simulations. Pattern and Shell A 3 x 3 x 1-in. (76.2 x 76.2 x 25.4mm) expandable polystyrene (EPS) foam pattern was attached to a pouring cup. Patterns were sent to sev- eral metalcasting facilities for shelling.


2 Procedure


Te research team intro- duced a method to correct the specimen thickness used in the laser flash method to


mated by running multiple computa- tional fluid dynamic (CFD) simula- tion iterations by varying the thermal conductivity and specific heat capacity to match the calculated cooling curves


Pattern removal, firing and properties analyses were done at Missouri Univ. of Science and Technology. Shells were pre-fired according to requirements from each individual casting facility. Seven different industrial shells were built using the aqueous colloidal silica binder with different mineral fillers as listed in Table 1. Improved Laser Flash Method In a laser flash thermal diffusiv-


ity test, a small specimen is subjected to a quick, intense radiant laser pulse after thermal equilibration at the test temperature of interest. Typical speci- men disc dimensions were 0.5 in. x 0.5 x 0.07 in. (12.7 x 12.7 x 2 mm). Te energy of the pulse is absorbed by the front surface and the temperature of the rear face is recorded. Samples were put into an ambi-


ent furnace with 27F (15C)/minute heating rate and laser flash tested from 392F (200C) to 2192F (1,200C) at intervals of 360F (200C). Tree runs of each type of specimen were conducted and the average values were reported in the results. Inverse Method: Setup and


Simulation After firing, one thermocouple


Fig. 1. The graph shows the inverse calculated thermal curves after fitting to experimentally obtained results.


(protected by a 0.08-in. [2-mm] diam- eter OD quartz sheath) was installed in the center of the mold cavity, and the other thermocouple was buried 0.04 in. (1 mm) below the external shell surface. Te shells then were


Table 1. Composition of Industrial Shells Used in this Study Prime coat


Slurry


Shell #1 Fused silica + Zircon Shell #2 Fused silica + zircon Shell #3 Shell #4 Shell #5


Shell #7 40 | MODERN CASTING January 2015 Stucco


Fused silica Zircon


Slurry


Fused silica Fused silica


Fused silica


Alumina + silica Alumina


Shell #6 Fused silica + zircon Aluminosilicate


Aluminosilicate + fused silica


Aluminosilicate + fused silica


Zircon + aluminosilicate (rapid shelling process)


Aluminosilicate + fused silica


Backup coat Stucco


Fused silica Fused silica


Fig. 2. The graphs show (a) specific heat capacity and (b) thermal conductivity val- ues determined by the inverse method.


with the experimental cooling curves for the shell and casting. Tis inverse method can require a lot of effort to achieve an acceptable match between the two curves.


entirely wrapped with 0.5-in. (12.7- mm) thick insulation to thermally iso- late the shell and limit the influence of the external cooling environment. Te shell then was filled with 99.5% nickel at an initial pouring temperature of 2,768F (1,520C). Te temperature curves were collected with a 24-bit data acquisition system. CFD inverse modeling was done using the optimization module of the simulation software. Initially, a base simulation was completed to repre- sent the actual casting conditions by using initial properties. Te process- ing information for initial shell and liquid metal temperatures, pouring time and insulating wrap locations were used in the simulation defini- tion (Fig. 1). Te nickel dataset was


Seal coat Slurry


Fused silica Fused silica


Firing


temperature, ºC 850


982 850 850 850 850


850


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