Berlin 2005 – wissenschaftliches Programm

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M: Metallphysik

M 25: Material Design

M 25.6: Vortrag

Samstag, 5. März 2005, 16:00–16:15, TU H2038

Investigations on the rheological behaviour of semi-solid metallic alloys processed during Thixoforming — •Lars Pape, Michael Modigell und Markus Hufschmidt — Institut für Verfahrenstechnik der RWTH Aachen, Turmstr. 46, 52056 Aachen

Thixoforming is a shaping process situated between the conventional processes of casting and forging. The metallic alloy is processed as a metallic suspension (globular grains suspended in a liquid matrix), which is characterised by a complex flow behaviour.

This feature is analysed experimentally for Sn15Pb and A356 by performing specific rheological experiments in a Couette rheometer. The metallic suspensions show shear-thinning and time-dependent flow properties and exhibit a yield stress. The isostructural flow behaviour immediately after a change in shear rate indicates shear-thickening attributes. The flow behaviour is strongly influenced by the solid fraction, the particle shape and size and the degree of agglomeration of solid particles.

Based on the experimental results a mathematical model to describe the flow behaviour has been developed to use for numerical simulation of die filling processes. The modelling approach is based on a Herschel-Bulkley law to describe the shear-thinning properties and is extended by a structural parameter k to additionally consider the time-dependent properties of the material. After a change in shear rate, the structural parameter proceeds towards its equilibrium value corresponding to the actual shear rate. This transient variation is modelled by a first order rate equation where the rate constant changes whether a build-up or a destruction of the structure occurs.