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 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
Seal coat Slurry
Fused silica Fused silica
Firing temperature, ºC
850 982 850 850 850 850
850
etry, a reference specimen and the test specimen are mounted together under the same condition at the same temperature and irradiated uniformly with a homogenized laser beam. To
ensure similar emissivity, a graphite spray coating covers the front and rear faces of both the reference and test specimens. Te temperature rise of the reference with known specific
heat capacity and the specimen are measured. If the density of the shell is known, then specific heat capacity can be calculated.
Te laser flash method was designed for dense specimens, while measurement of highly porous materi- als has associated difficulties in defin- ing the applicable specimen thickness. To evaluate the effective specimen thickness and density in this study, the researchers used a 3-D high resolution optical profiler to obtain the specimen surface topology (Fig. 1). Ten the effective thickness and density were determined and those data used to cal- culate thermal diffusivity and specific heat capacity. Specimens were taken separately
from prime coats and backup coats. For comparison, the rule of mixtures was used to estimate the thermal property of the entire shell based on the thickness ratio between the prime coats and backup coats. Tree runs of each type of specimen were conducted and the average values calculated. Te physically measured thermal
property data was applied to the inverse method as the starting points to reduce a significant amount of computational time and avoid errors induced from extrapolation in the optimization algorithm.
Discussing the Results Te specific heat capacities and
Fig. 2. Graphs show (a) specifici heat capacity and (b) thermal conductivity values of studied shells determined by the inverse method.
44 | MODERN CASTING January 2016
thermal conductivity of the shell and insulating material as well as external heat transfer coefficient are the main parameters that influence the tempera- ture curves of the casting and shell. Preliminary modeling showed that solidification time and the coordinate of the point where the shell reached the highest temperature were mainly influenced by the specific heat capac-
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