| Summary: | Heat transfer in vacuum investment casting of a nickel-based superalloy, 1N718, was
studied using a finite-element-based solidification heat transfer code. For the external
radiation boundary, general 2- and 3-dimensional viewfactor calculation codes based on a ray
tracing approach were developed and verified. Heat transfer at the mould-metal interface
may occur by contact conduction between the metal and the mould, and by radiation across
the interface gap areas. A simple, time-dependent model was developed to simulate the
decreasing contact conduction as solidification progresses. Temperature measurements were
made on casting moulds in a series of experiments done in collaboration with Deloro Stellite
Inc. of Belleville, Ontario. The model was applied to the experimental casting
configurations. The model results were most influenced by the value of the mould thermal
conductivity, the interface contact function and the radiation environment surrounding the
mould. The mould thermal conductivity which resulted in the best fit to the data ranged from
0.9 to 1.1 W/m-deg C for the cylindrical castings and 0.8 - 0.9 W/m-deg C for the finned
castings. The interface contact conduction function decreased from 1400 W/m2-deg C at
time t = 0, to a value of 0 at t 700 seconds and t 200 seconds for the cylindrical and
finned castings respectively. The model was used to simulate casting conditions for a tensile
test bar which had been analyzed experimentally by Deloro Stellite Inc. Although the 2-
dimensional model used gave results that were in qualitative agreement with the experiments
in terms of predicting effects on microporosity and secondary dendrite arm spacing, a 3-
dimensional model altered the solidification pattern and time scale for solidification by an
order of magnitude. A 2-dimensional approximation, although requiring less model input
time and computational time, may thus be misleading and result in incorrect conclusions
being drawn. The model developed in this work provides a strong tool which can be used in
conjunction with experiments to develop relationships between heat flow and the
micro structural development of investment castings. === Applied Science, Faculty of === Materials Engineering, Department of === Graduate
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