Comparison of Rheological Measurement Techniques for Semi-Solid Aluminium Alloys

Abstract

Alloys of a specific microstructure exhibit a shear-rate history-dependent flow behaviour in the semi-solid state. Rheological studies have been carried out using various experimental techniques such as concentric cylinder rheometers of Couette or Searle type as well as capillary rheometers and parallel plate compression tests. With the Searle type rheometer and the capillary rheometer two different approaches of rheological measurement techniques are discussed and compared regarding advantages, potentials and its distinct characteristics. Experiments using the Searle type rheometer have been conducted under steady-state or oscillating conditions and have been used to verify time-dependency of step responses due to discontinuous or linear variances of the shear rate. These experiments provide all types of flow curve parameters as implemented in the numerical simulation. However the variances of the commonly used Couette and Searle type rheometers are limited to shear-rates which are significantly lower than that appearing in the technical thixo- or rheocasting processes. For that reason an additional experimental set-up with a horizontal slot capillary integrated in a shot-controlled high-pressure die-casting machine is used in order to examine flow properties at shear rate conditions similar to those in real thixocasting processes. After an initial examination of low temperature model alloys, such as PbSn15, the experimental set-up is then adjusted in order to obtain rheological data for light metal alloys such as Aluminium AlSi7Mg0.3. The Aluminium alloy is processed within each rheometer either as partially solidified or partially remelted in order to represent conditions of both, the conventional thixocasting and the lately introduced rheocasting process. Finally the appearance of physical phenomena which affects the accuracy of rheological experiments such as wall slip is considered for both types of rheometers.