| An apparatus was built for dispersing ceramic particles into semi-liquid aluminum to produce metal-matrix composite castings. A variety of reinforcing material and matrix alloy combinations were prepared. It was observed that even though reinforcing particles may be wetted and uniformly dispersed in a liquid metal, they can become segregated into dendrite interstices during solidification. In some cases, this segregation causes severe agglomeration and interparticle contact, but in others, it is not observed at all. The factors believed to influence this effect are the dendrite arm spacing of the matrix (proportional to the dendrite growth rate), the size of the particles, the relative thermal conductivities of the matrix and particles, and the difference in contact angles between a particle/liquid and a particle/solid interface.In this study, a set of experiments was performed to determine what combinations of these factors produce this effect. Particles of five different materials in five different size classes were used to prepare composites. A mold was designed which produced a range of solidification rates. The composites cast in this mold were sectioned, polished and photographed in an SEM. The photographs were digitized into a computer, and the inter-particle spacings were statistically analyzed. The results indicate wide variations in segregation behavior among different types of particles, probably due to differences in particle/solid contact angle. |
