Rheological model of semi-solid A356-SiC composite alloys. Part I: Dissociation of agglomerate structures during shear

This study investigates the rheological behavior of semi-solid A356 alloys reinforced with SiC particles of 10–15 μm. The interaction between the primary phase and the reinforcing particles plays a major role in characterizing the thixotropic nature of these alloys. Viscosities were determined using a Couette type viscometer by measuring the torque on a calibrated spindle rotating in a cylindrical alumina crucible containing the alloy composite. The thixotropic behavior of these composites was described by an analytical model where the dissociation of the primary phase particle structure is expressed in terms of a structural parameter λ, which depends on shear rate, duration of shear and effective solid fraction. The analytical model was applied successfully to the results of this study as well as to the results of similar studies in the literature. Observations showed that the kinetics of dissociation of this structure is strongly influenced by increasing shear rate, and decreasing fraction of primary solid phase. It was also demonstrated that the effective solid fraction composed of agglomerates of primary particles with entrapped liquid alloy has a larger influence than the form factor or mean particle size. The presence of reinforcing particles was found to oppose the tendency of primary particles to agglomerate so that the kinetics of dissociation of the particle structure is enhanced.