PROPERTIES OF SEMISOLID ALUMINUM-MATRIX COMPOSITES

Steady state viscosities of partially solidified composites have been determined using a high temperature viscometer. The composites are based on the alloy A356, reinforced with MC particles (13 mu m; up to 30 vol.%) or SiC short fibres (14 mu m diameter; 10 vol.%). The average fibre length was 50-100 mu m. All slurries are strongly pseudoplastic at a fraction solidified primary phase of 0.3 for the shear rate range investigated. The reinforcement content or morphology does not influence the pseudoplasticity at shear rates lower than approximately 30 s(-1) and a power law index of between -1.2 and -1.4 is found. At higher shear rates, the reinforcing phase causes the slurries to behave in a more newtonian manner. The viscosity of particle-reinforced slurries shows as a function of concentration a minimum at approximately 20 vol.% particles. This is the result of two counteracting effects of the presence of particles: hindrance of the agglomeration process and an increase in the fraction solid. The fibre-reinforced slurries show steady state viscosities which are similar to or lower than corresponding viscosities of particle-reinforced slurries. Short fibres are more effective in hindering agglomeration and coalescence than are particles.