Rheological behaviour and microstructure of stir-casting zinc-aluminium alloys

Abstract

The influence of processing variables on the rheological and microstructural behaviour of stir-cast (rheocast) ZA-27 alloy (Zn-27 wt % Al-2wt % Cu) has been investigated experimentally. A concentric cylinder viscometer with shear rate range up to 650 sec−1 was used to measure the apparent viscosity of the slurries. During continuous cooling and at high shear rates (300–640 sec−1), non-dendritical materials obey the power-law fluid model, i.e.\(\eta _{\text{a}} = k\dot \gamma ^n \) where ηa is the apparent viscosity and\(\dot \gamma \) the shear rate. At lower shear rates (125 sec−1), the slurries display dendritical-liquid mixture with viscosity up to 50 poises. Microstructural studies of continuously cooled materials reveal a clear tendency of primary particles to cluster. This phenomenon could be explained by the reduction of the amount of entrapped liquid in the particles.