Interaction between moving cellular solidification front and graphite particles during centrifugal casting

Particle pushing phenomena during solidification in the melt containing a particle lighter than the melt under the influence of centrifugal forces is described. This phenomena is observed in centrifugally cast copper alloys originally containing an average of 7 vol% graphite particles, cast in a 9.5-cm outer diameter mold rotating at 800 and 1900 rpm. A model to calculate the critical interface velocity, depending on the centrifugal force and the location of the cellular interface where the particle interacts with the interface, is proposed, and the particle pushing criterion is discussed. It is found that as the particle velocity increases with increasing the rotational speed of the mold, the effect of the particle velocity on the critical interface velocity increases; in addition the critical interface velocity is lowest at the tip of the cellular interface, and increases with distance from the tip of the interface. The critical interface velocity calculated in copper alloy containing 5 μm graphite cast in a 9.5-cm outer diameter mold rotating at 800 rpm (1.5×10−3 m s−1) is higher than the interface velocity calculated for casting in a mold rotating at 1750 rpm (5×10−4 m s−1) obtained numerically by Kang and Rohatgi (C.G. Kang, P.K. Rohatgi, Met. Trans. 27B (1996) 277). The fact that the actual interface velocity is lower than the critical interface velocity leads to pushing of particles by the growing dendrites in the last freezing interdendritic regions, as observed experimentally.