UBC Theses and Dissertations
Mathematical modelling of heat transfer in the vacuum investment casting of superalloy IN718 Dominik, Barbara Eva
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.
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