mmc 16
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12 PROCESSING
processes while the alloy temperature is between liqui-
dus and solidus Flemings 1991 ). In both cases, that is,
above liquidus temperature or between liquidus and
solidus temperature, the metal-particulate slurries ex-
hibit a non-newtonian behavior. Their viscosity de-
creases up to an order of magnitude as the shear rate
increases see Figure 1 .6). Mo reover, it has been exper-imentally observed that comp osite slurries are thixo tro-
pic, mean ing that when the sh ear rate is abruptly
changed , the viscosity of the slurry changes only pro-
gressively to reach the steady-state value of the new
shear rate. Such behavior has been reported by Moon
on SiC/Al-6.5 w 94 Si Moo n et al. 1991 ) in both
semisolid and liquid metal cases, and by Mada and
Ajersch 1990 ) on similar systems in the sem isolid state
only, for a smaller range of shea r rates. Th e mechanisms
underlying the pseudoplastic behavior of composite
slurries are n ot yet clearly understood. Th e existence ofsuch behavior in slurries above the m etal liquidus sug-
gests that clustering and declustering of particles plays a
major role. Interfacial chemical reaction between rein-
forcemen t and m etal has also been repo rted to increase
the slurry viscosity, apparently because the reaction
produ cts have a different density, break loose, or chang e
the morphology of the reinforcement Lloyd 1991).
In the semisolid metal temperature range, the behav-
ior of composite slurries is generally explained on the
same basis as is their unreinforced counterparts. Th at is,
there is a change in morphology of the solid phaseunder shear as a result of dendrite fragmentation, rip-
ening and abrasion, and collision and coalescence of
solid particles Fleming s 19 91 ). M ada and Ajersch
1990) developed an analytical model of thixotropicbehavior in semisolid composite slurries, based on an
analogy between rate constants for chemical reactions
and the rates of dissociation and formation of solid
aggregates. From their analysis and experimental re-
sults, they concluded that particle addition has little
influence on the thixotropic nature of th e m elt, and thatthe primary m etal solid phase is the m ajor factor. Yet, an
effect of particle addition o n the pseudoplastic behavior
has been observed by M oon M oon et al. 1 991); at a
given shear rate, th e viscosity of slurries composed of
Sic particles combined with Al-6.5 w Si in the
semisolid range was lower than that of unreinforced
semisolid slurry of the same total v olume fraction solid.
This result is tentatively explained by the preferential
location of reinforcement particles between dendrites
arms, limiting contact and agglomeration of dendritic
solid particles.
1 2 2 3 Par t icle Migrat ion
After m ixing, the semisolid compo site may be at rest
before complete solidification. The issue of particle
migration because of gravity differences between
metal and reinforcement t hen arises. For high volume
fractions, the sedimentation is influenced by interac-
tions between particles an d by the ir size distribution
Lloyd 1991).When solidification takes place, particle migration
caused by solidification effects competes with migration
caused by gravity. Understanding and controlling the
interaction between growing solid metal and the parti-
cles is crucial to p roducing homo geneously reinforced
comp osites. Thus, when encountering a m oving liquid/
solid interface, particles may be engulfed in the solid
metal, or they may be pushed by the interface and
consequen tly migrate into areas that solidified last, for
example, interdendritic regions s illustrated in F igure
1 7 Work on this issue has recently been reviewed by
several authors Mortensen and Jin 1992; Rohatgi et al.
1990b; Stefanescu and Dhindaw 1988). From experi-
mental observations, some particle/metal systems fea-
ture a critical interface velocity, V below which theparticles are pushed and above which they are trapped.
The value of V depends on a number of factors, in-
cluding particle size, shape and composition, metal
composition, differences in thermal conductivity be-
tween particle and metal, and cooling rate. Several
models have been developed to quantify particle-pushing phenomena, but they are somewha t difficult to
compare with experiments because the parameters d ono t all vary independently, and the castings generally do
no t solidify at steady-state.
Figure 1 6 nfluenceof shear rate on the vis osity of
A-356-15volYo Sic Reprinted y permission from D. J
Lloyd 99
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