MEASUREMENTS OF METAL-MOLD INTERFACIAL HEAT TRANSFER COEFFICIENTS DURING SOLIDIFICATION OF Sn AND Sn-Pb ALLOYS

A study is reported on the evolution of the thermal resistance at the interface between the mold and the metal during solidification of metal castings. Experiments were performed to measure the interfacial heat transfer coefficient h(i)) for solidification of Sn and Sn-Pb alloys from a vertical wall. The experiments with pure Sn were simulated numerically using a constant h(i), and good agreement was found. The variation of h(i) in the presence of a mushy zone at the interface was studied. In the case of a Sn-10wt% Pb alloy it was found thas h(i) varied as a function of the liquid fraction at the interface. The interfacial heat transfer coefficient had a higher value during the initial stage of solidification, when the liquid fraction at the interface was high. However, as the temperature at the interface approached the eutectic temperature and the liquid fraction decreased to zero, hi dropped to a lower value. Once the temperature at the interface fell below the eutectic temperature, h(i) remained approximately constant. In the case of a eutectic Sn-Pb alloy, the interfacial resistance increased slightly due to thermal contraction of the metal as solidification proceeded. The observed variation in hi for the three different metals examined in this study is explained by volumetric contraction upon solidification.