The rheology of a partially solid alloy

Abstract

The effect of various thermomechanical treatments on the structure and rheological behaviour of Sn-15% Pb alloy in its solidifcation range was investigated using a concentric cylinder viscometer. The apparatus was designed to permit wide ranges of cooling rates up to 25° C min⁻¹ and shear rates up to 750 sec⁻¹. Initially, the alloy was continuously sheared as it cooled from above the liquidus to a desired volume fraction solid. In one series of experiments, shear was stopped and the alloy quenched. In a second series, the alloy was held isothermally and subjected to various mechanical treatments. The size and morphology of primary solid particles during continuous cooling is influenced by shear and cooling rates and volume fraction of solid — faster cooling results in finer structures while increased rate of shear reduces the amount of entrapped liquid in individual particles. The viscosity of the slurry, at a given volume fraction solid, decreases with decreasing cooling rate and increasing shear rate. Exercising the full range of shear and cooling rates possible in the viscometer, the apparent viscosity of a 0.55 volume fraction solid slurry varied from 3 to 80P. The structure and viscosity of isothermally held slurries follow the same trends as slowly cooled slurries. However, their viscosity at a given volume fraction solid is consistently lower than that of continuously cooled slurries. The slurries are thixotropic and show a hysteresis loop phenomenon similar to other well known thixotropic systems. Measured areas of hysteresis loops increase with increasing volume fraction solid, initial viscosity and time at rest. The potential applications to improve existing or develop new metal-forming processes are being investigated in a variety of alloys with different solidifcation ranges and temperatures.