Semisolid Metals: A Suspension with Non-Newtonian Liquid Matrix

The improvement of mathematical models for semisolid alloy flow properties requires profound understanding of the underlying physical nature. To date, it is commonly accepted that the shear thinning behaviour of these suspensions is caused by the solid phase microstructure, while the liquid phase is assumed to be Newtonian with a viscosity in the lower mPas -range. Recent measurements however, demonstrate non-Newtonian behaviour of fully liquid metals with pronounced shear thinning and high viscosities (multiple Pas ) in the low shear-rate range. By gathering and analysing rheological measurement data of various alloys (Sn14.2%Pb, A356 and X210CrW12), the relevance of the new findings for semisolid metals is investigated. The results indicate that the previously unexamined non-Newtonian flow behaviour of the liquid matrix has, besides the solid fraction, the most dominant influence on the shear thinning behaviour of semisolid alloys. The influences of shear-rate and solid fraction are nearly independent of each other which allow the construction of master-curves; a general flow curve for the suspension where the solid fraction is considered by a scaling factor. Consequently, a modelling approach is suggested in which the dependency of solid fraction is considered independently of the shear-rate.