Syntactic functionally graded metal matrix composites (SFGMMC) are a class of
metallic foams in which closed porosity results from the presence of hollow ceramic microspheres
(microballoons), whose spatial distribution varies continuously between the inner and the outer
section of the part, thus resulting in a continuous variation in properties. In this work, aluminiumbased
SFGMMC rings were fabricated by radial centrifugal casting. The graded composition along
the radial direction is controlled mainly by the difference in the centrifugal forces which act on the
molten metal matrix and the ceramic particles, due to their dissimilar densities. In this case where
the density of the SiO2-Al2O3 microballoons is lower than that of molten aluminium, the particles
show a tendency to remain closer to the inner periphery of the ring. Thus the microballoon volume
fraction increases along the radial direction of the ring from the outer to the inner periphery; in
other words, the particle-rich zone is limited to an inner layer of the ring. Precursor conventional
MMCs were prepared by stir-casting from the constituent materials, by homogeneously dispersing
commercial SiO2-Al2O3 microballoons (particle size: 50 µm; particle volume fraction: 5 and 10 %)
within a molten commercial Al-7Si-0.3Mg (A356) alloy. The resulting MMCs were then re-melt
and centrifugally cast in order to produce the functionally graded composites. Particle gradients in
the centrifugally cast composites were investigated by quantitative image analysis of optical
micrographs (for the estimation of the particle volume fraction, mean particle diameter and porosity
volume fraction).
Effect on microstructures, hardness and SDAS of primary Mg2Si/Al-Si eutectic phases of centrifugally cast functionally graded Al-(Mg2Si)p in-situ composites