Composite Materials Research Progress

Recent Advances in Discontinuously Reinforced Aluminum… 279
(c)
Figure 2. Microstructure of the PM 6061 Al /20% Al 2O 3 composite: (a) as-extruded condition. The
reinforcing alumina size is ~ 1-5 μm, (b) after four ECAP passes and (c) after seven ECAP passes [24].
Figure 3. Experimental setup of ultrasonic assisted melting [10].
In PM nanocomposites, clustering of nanoparticles often occurs during processing and
the degree of agglomeration increases with increasing filler content [36] (Fig. 5). Through a
solid-state cryomilling route, better dispersion of nanoparticles in aluminum matrix can be
achieved. In the process, collisions between the grinding media lead to repeated fracture and
welding of the raw powders in a high-energy ball mill. Low temperature (liquid nitrogen)
environment suppresses the recovery and recrystallization of matrix grains during milling,
thereby yielding finer grain structures. The nature of the process allows the incorporation of
large volume fractions of reinforcement into aluminum matrix with a homogeneous
distribution [12,13, 15, 37, 38]. Consolidation of cryomilled powders via hot pressing, cold
isostatic pressing (CIP), hot isostatic pressing (HIP), extrusion, spark plasma sintering, etc. is
necessary to produce bulk composites with full density, useful shapes and sizes for practical