Characterisation of the rheological behaviour in the semi-solid state of grain-refined AZ91 magnesium alloys

The results of an experimental investigation carried out with grain-refined AZ91 magnesium-aluminium alloys are presented. The microstructure of the material during partial remelting and holding in the semi-solid state was thus characterised together with its rheological behaviour. For this purpose, various mechanical tests were performed including simple compression, backward extrusion, tension and filtration. The investigated parameters were the shear rate and the temperature which affects the liquid volume fraction. The tests allow the apparent viscosity of the alloy to be determined as a function of the liquid volume fraction and the shear rate together with the densification behaviour of the solid phase. In addition, a strong difference between compression and tension was observed depending on the solid volume fraction. The rheological data are correlated with the solid volume fraction in the alloy, and comparison with other alloys is carried out. Finally the results justify the development of a two-phase model in which the material is considered as a porous solid medium saturated with liquid. (C) 1999 Elsevier Science S.A. All rights reserved.