In situ temperature measurement during spray forming of A2-tool steel and axisymmetric two-dimensional analysis
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D.R. White Materials Systems Reliability Department, Ford Research Laboratory, Dearborn, Michigan 48121-2053
E.J. Lavernia Department of Chemical and Biochemical Engineering and Materials Science, University of California—Irvine, Irvine, California 92697 (Received 4 December 1999; accepted 19 April 2000)
An in situ temperature measurement was performed during spraying of A2-tool steel, and the results were used to verify an axisymmetric two-dimensional computer simulation program, which was developed for the prediction of shape and temperature variation in a spray-forming process. A thin thermocouple was placed inside of the chamber in advance and brought to the surface of the deposit during spraying. The temperature was then recorded. The surface temperature of the deposit was also measured by an infrared video camera. The emissivity of the surface of A2-tool steel during spraying was determined to be 0.23 through comparison of the temperatures measured by the thermocouple with the ones measured by the infrared video camera. The heat transfer coefficient at the top surface was estimated by comparing the calculated results with the experimental data. The cooling curve predicted on the basis of the numerical simulation showed good agreement with the experimental data. I. INTRODUCTION
The morphology and integrity of the material produced by spray forming are governed by the various operating parameters, as well as by the thermophysical characteristics of the material. Therefore, the knowledge of the effect of each parameter is very important to control the microstructure. Unfortunately, detailed experimental study of the influence of each individual parameter is precluded by the time and expense that would be required to properly isolate all possible effects. Accordingly, numerical simulation of spray-forming phenomena may be implemented as an alternative to gain insight into the possible influence of each parameter.1–7 It is very difficult to measure the temperature variation of the deposit during spray forming because the direct measurement of the temperature of droplets is almost impossible and the shape of the deposit changes during the process. Numerical simulation of the spray forming can also be used to study the physical state, the solidification and thermal history, and the residual stress of the deposit. So far, measurement of the surface temperature of the deposit during spraying has been tried by positioning thermocouples through the substrate at various heights before spraying.9 The thermocouple bead is much bigger than the falling droplets and a deposit is formed also on J. Mater. Res., Vol. 15, No. 8, Aug 2000
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top of the thermocouple just like the snow on electrical cables. Therefore, we attempted in this study to measure in situ the top surface temperature by placing a thin thermocouple on top of the deposit during spraying. The temperature of the deposit surface can also be measured by an optical pyrometer.8 This requires the exact emissivit
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