(440i) Rheology of Metal Alloys in the Solid-Liquid Phase

Unlike in normal casting processes, thixo- and rheocasting processes employ liquid-solid mixtures that are pressed into dies. One of the main advantages of solid-liquid material (semi-solids) castings in contrast with conventional casting is the reduced porosity in the cast part and significantly lower temperature and solidification time. The knowledge of the rheology of the solid-liquid mixture is critical for optimization of the thixocasting process.

The rheological behaviour of multiple alloys in the liquid and liquid-solid state was investigated applying rotational and oscillatory rheometry using a high temperature rheometer from Anton Paar.

In addition when oxidized the alloy forms a thin layer of metal/metal-oxide colloidal dispersion that can influence the behaviour.

Different shear rates/shear stresses showed a significant influence on the rheological behaviour and thus the viscosity of the alloy. This was true for the liquid material as well as in the solid-liquid phase between liquidus and solidus, which is crucial for the casting processes. The viscosity at high applied shear stresses like they are used in thixo- and rheocasting processes, is significantly lower than when measured at rest. Structural changes on metallographic microsection were observed linking the loss of dentritic structure with the rheological data.

Also strategies to minimize oxidation and therefor loss of accurate signal both by the employment of a flux as well as inert and reducing atmosphere with the accompanying rheological data will be discussed.