Effect of interactions between bubbles and graphite particles in copper alloy melts on microstructure formed during centrifugal casting: Part I. Theoretical analysis

Frequently, particles get associated with gas bubbles in a melt and their i nteraction influences the final distribution of particles and porosity in t he casting. An analytical model for the separation of a particle from a bub ble in melts containing dispersed particles and bubbles is proposed. During centrifugal casting of alloys containing dispersed particles, both the par ticles and gas bubbles present in the melt move with the centrifugal forces . Using the force balance between surface tension and net centrifugal force s (centrifugal force minus buoyancy force), the critical rotational speed o f the mold for the separation of the particles and the bubbles during centr ifugal casting is calculated. The critical rotational speed of the mold to separate the particle from the bubble is lower for a small particle attache d to a larger bubble, as compared to the case when a large particle is atta ched to a smaller bubble. For a given bubble size, the critical rotational speed of the mold to separate the bubble from the particle decreases with i ncreasing particle size. For the specific case of spherical 5-mu m radius g raphite particles dispersed in copper alloy melt, it was found that even at a low semiapical angle of about 9 deg, the critical rotational speed needs to be around 5000 rpm for a bubble size of 500-mu m radius and 0.09-m-diam eter mold. The rotational speed decreases to 1000 rpm when the graphite par ticle radius increases to 100 mu m for the same bubble size in copper alloy melt.