Rheological behaviour of partially solidified aluminium matrix composites

Abstract The rheology of an aluminium matrix composite A356 containing 18 vol.% of 12 μm SiC particles and the corresponding matrix alloy is examined after partial solidification using a high temperature Searle-type viscometer. Both slurries are shear-thinning. At a solidified primary phase fraction of 0.2–0.4 a power law index of −1.3 is found for the shear rate dependence of the steady state viscosity of the matrix. The presence of SiC particles in the composite does not change the value of the power law index at shear rates lower than about 30s −1 , but the absolute value of the viscosity is reduced by a factor of 2. The presence of SiC particles diminishes the agglomeration state of primary aluminium globules, resulting in a lower viscosity at the same temperature. At higher shear rates the composite slurry becomes more newtonian and the power law index becomes −0.8. After a sudden change in shear rate the agglomeration state adjusts. The rate of agglomerate building up in the slurry is much lower than that of breaking down through stirring. The presence of SiC particles slows down the agglomeration rate but does not significantly alter the deagglomeration rate. It is argued that agglomeration and deagglomeration are governed by diffusion and shear forces respectively.