The rheological behavior of metallic alloys containing both solid and liquid phases is investigated in the low solid fraction range (<50%). This behavior depends on both the solid fraction and the shear rate. The concept of effective volume fraction (EVF) is used to decorrelate the influence of these two parameters. At high shear rate the slurry behaves like a suspension of hard spheres, whereas at lower shear rate particles tend to aggregate in clusters, entrapping liquid and thus increasing the EVF and the viscosity. A lattice model is introduced to simulate the aggregation/break-up processes within a slurry under shear. When the steady state is reached, the entrapped liquid fraction is calculated, leading to a viscosity estimation. Simulation results for the viscosity and structure of the three-dimensional cluster are in good agreement with experimental results.