Rheology in Thixo- and Rheocasting Processes: A Rheological Characterization of Aluminum Alloys
Thixo- and rheocasting processes employ liquid-solid mixtures (semisolids) that are pressed into dies. Knowledge of the rheology of the liquid-solid mixture is critical for the optimization of the thixo- and rheocasting processes. The rheological behavior of aluminium alloys in the liquid and liquid-solid state was investigated by applying rotational and oscillatory rheometry using a high temperature rheometer from Anton Paar.
Different shear rates/shear stresses showed a significant influence on the rheological behavior and thus the viscosity of the alloy. This was true for the liquid material as well as the liquid-solid phase between liquidus and solidus temperatures, which is crucial for the casting processes. The viscosity at high applied shear stresses (like those 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 dendritic structure with the rheological data.
Thus high temperature rheology is shown to be a powerful tool to investigate metal alloys in modern casting processes.
Unlike in normal casting processes, thixo- and rheocasting processes employ liquid-solid mixtures (called semisolids within metallurgy) containing non-dendritic solid fractions that are pressed into dies. One of the main advantages of liquid-solid material (semi-solids) castings as opposed to conventional castings is the reduced porosity in the cast part and the significantly lower temperature and solidification time. This is commonly associated with the shear-induced destruction of the dendrites (from Greek: dendritês → treelike) forming in a non-stirred metal while crystallizing. In a stirred alloy, crystallization instead forms a more globular structure influencing both the condensed structure as well as the viscosity of the semisolid. This is by virtue of both the globular structure as well as the lack of hook-like dendrites. The reduced particle-particle tribological interaction greatly reduces the viscosity.
In contrast to the rheocasting process, thixocasting processes are heated to the semi-solid temperature range and injected into a die utilizing extremely high shear rates. The thixotropic behavior of the alloy (which is also thought to be a function of semi-solid shape and sphericity) is taken advantage of to utilize better processing parameters. In contrast to this, in the rheocasting process, molten alloy is treated into a slurry by stirring directly before casting. Production of those slurries is achieved via mechanical stirring (1).
The main effect is reduced porosity as a result of the lower viscosity of the semi-solid melt. Further, the lower temperature and solidification time are thought to be a direct result of the supercooling that is achievable through the influx of kinetic energy via shearing, which is used to modify the solidification process.
The knowledge of the rheology of the alloy during cooling and in the semi-solid temperature range is critical for optimization of the thixocasting and rheocasting processes. Both of these processes require the alloy to be considered a (quite) complex fluid, which necessitates the employment of rheology to be fully understood.
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