The measurement of interfacial heat transfer coefficient and its application to the solidification model for evaporative pattern casting of A356 aluminum alloy

摘要

This research is to study the solidification heat transfer phenomena for the evaporative pattern casting (EPC) of A356 aluminum alloy. Temperature measurement in an unidirectional heat transfer system along with the Inverse Method are used to measure the interfacial heat transfer coefficient between the casting and the binderfree sand. The measured interfacial heat transfer coefficient is then applied to a solidification model to predict the temperature variation in a EPC casting. A corresponding EPC casting is made and temperature measurements are taken to compare with the simulated results. For the experimental measurement of interfacial heat transfer coefficient, h, a major difficulty is how to design and fabricate an unidirectional heat transfer condition for EPC casting. In this study, a CO{sub}2 sand mold is used to fix the thermocouples and heat insulating material is used to surround the polystyrene pattern to fabricate the unidirectional heat transfer system. Furthermore, temperature measurements are made for two EPC castings; one rectangular and the other one stepwise. The measured temperature data are compared with the simulated ones. Possible reasons besides the interfacial heat transfer coefficient are discussed if discrepancy exists between the two results. For the measurement of interfacial heat transfer coefficient, the value calculated from the Inverse Method is again verified in the unidirectional heat transfer system. With reference to the surface temperature of casting, the values of h can be categorized into three ranges. The value of h is approximately 0.0l2cal /sec.cm{sup}2.℃ for temperature above the liquidus temperature. It becomes 0.006 cal /sec.cm{sup}2. ℃ for temprature below the eutectic temperature. For the temperature in between, the value of h is approximately 0.008ca1 /sec.cm{sup}2.℃. For the simulation of solidification heat transfer in EPC casting, it is found in this study that other than h, the thermal conductivity of sand and the dependence of latent heat released on cooling rate also have significant effects on the results of the simulation.