Biomedical Engineering Reference
In-Depth Information
6.4.1
Ex-situ
routes
The major purpose of the different ex-situ processes is to disperse nanoscale
reinforcement homogeneously into the metal matrix material at different
process temperatures.
Liquid-phase MMNC ex-situ fabrication processes
In liquid-phase processes, ceramic nanoreinforcements are dispersed into a
molten metallic matrix. Many stirring techniques have been used in the
casting process to fabricate MMCs. However, when the size of the
reinforcements reaches nanolevel, the reinforcements tend to agglomerate
and are hard to separate using traditional stirring methods (Li et al., 2004;
Yang et al., 2004). A larger energy input is then needed to separate clusters
and homogenously disperse the reinforcements into the melt. Ultrasonic
cavitation aided casting is a modified method of mechanical stir casting and
has been proven to be a successful method to fabricate MMNCs. In
ultrasonic cavitation based solidification processing, nanoreinforcements
are placed on the surface of molten metal. Ultrasonic vibration is then
applied to disperse particulates in the melt matrix. In the process, transient
micro 'hot spots' with a temperature of about 5000
C and pressure above
1000 atm can be formed by ultrasonic cavitations. The strong heating and
cooling rates during the formation process can break the nanoparticle
clusters and also clean particle surfaces (Lan et al., 2004; Li et al., 2007).
After applying ultrasonic vibration for a certain amount of time, the melt
with nanoparticles is cast into a mould to make bulk MMNC samples.
The advantages of this technology are significant (Yang and Li, 2007; Cao
et al., 2008a).
8
.
Cast MMNCs with complex shapes can be formed directly.
.
It is one of the most cost-effective MMNC fabrication processes.
.
Application of this technology into fabricating Al- and Mg-based
nanocomposites has shown improved mechanical properties.
The challenges for casting MMNCs are obvious.
.
Methods are needed to improve the wettability of liquid metal and solid
ceramic particles.
.
Even though this technology has proven effective, the effect of
nanoparticles on the solidification of structures is unknown. Zhou and
Xu (1997), in an experiment on casting SiC reinforced MMC, reported
that SiC particles acted as substrates for heterogeneous nucleation of Si
crystals. More research is needed to reveal the mechanism of the process
