Effect of interactions between bubbles and graphite particles in copper alloy melts on microstructure formed during cent
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I. INTRODUCTION
THE stir casting technique has been widely used to synthesize metal-matrix composites due to its simplicity. However, this technique can lead to a nonuniform distribution of the reinforced particles in the matrix due to flotation or settling of the particles in the liquid and the rejection of the particles by the solidifying interface. In addition, there is a possibility of increasing in the porosity for the final castings due to the incorporation of air into the melt during mixing, especially if stir mixing is done in an air environment.[1–4] In addition, the presence of particles near the growing solid/liquid interface can facilitate the formation of gas bubbles, especially in the locations between the particles and the solid/liquid interfaces.[5,6] Ghomy and Campell[7] also show that the porosity in metal-matrix composites is influenced by the surface tension between the particle and the melt. Since porosity has a detrimental effect on the properties, it is important to understand the formation and the stabilization of porosity in the presence of particles in the melt. The problem is of importance in achieving porosity-free castings during both static and centrifugal casting processes. Unlike gravity casting, the utilization of the centrifugalcasting technique to cast shapes from metal-matrix composite melts has an advantage of concentrating the particles near the inner or the outer periphery, depending mainly on the differences in the density between the particle and the
J.K. KIM, Postdoctoral Fellow, and P.K. ROHATGI, Wisconsin Distinguished Professor, are with the Materials Department, University of Wisconsin-Milwaukee, Milwaukee, WI 53211. Manuscript submitted June 13, 1997.
METALLURGICAL AND MATERIALS TRANSACTIONS B
melt. Particles such as graphite, which are lighter than aluminum melts, segregated to the inner periphery of the centrifugal castings of aluminum-graphite alloys;[8,9] whereas zircon and SiC particles, which are heavier than the molten aluminum alloys, segregated near the outer periphery. The segregation of graphite particles near the inner periphery of aluminum castings gives antifriction properties to the inner periphery of the casting.[10] In metal-matrix composites in liquid form, bubbles are frequently present in addition to particles. During centrifugal casting, both the bubbles and particles move under the influence of centrifugal forces. If the particles and bubbles are associated with each other, they can move together and influence the porosity and particle distribution in the casting. As an example, in centrifugal castings of copper-matrix alloys containing graphite particles, which are lighter than the melt, the particles were found to be segregated near the inner periphery and were also associated with higher amounts of porosity.[11] In this article, the interaction between a bubble and a particle in the melt has been investigated. The specific case of gas bubbles and graphite particles moving to the inner periphery of a horizontal centrifugal casting of molten
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