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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