The rheocasting is a process of creating a semi-solid slurry followed by its pushing into a metal mold for freezing. In thixocasting, the slurry freezes to fully solid phase which is again heated in semi-solid region to achieve viscosity and pushed into the mold.
Thixocasting is related to thixotropic behaviour of macromolecules. In this case viscosity of material increases with increase of shear rate ( shearing force). Rheocasting viscosity of material decreases with decreasing shear rate.
To my understanding difference is in processing route. In the case of rheocasting semisolid state is obtained by cooling from liquid temperature, while for thixocasting one uses preprepared billets and obtains slurry by simply heating the billet (usually by induction heater).
Therefore, by using thixocasting route, initial investment is lowered and one does not need to think about how to obtain spheroidal solid phase (strict process control is essential). On the other hand, it involves intermediate solidification step/billet supplier and is therefore more expensive.
In rheocasting process, metal slurry made directly from liquid state but in thixocasting process the semi-solid feedstock that made by solid/or liquid state processes just heated into mushy state and formd.
schematic illustration of different routs for semi-solid metal pocessing
usually solidification process of conventional casting will give dendritic structure of alpha phase.
Rheocasting process will convert the dendritic microstructure to globular primary alpha surrounded with eutectic mixture, by using different methods such as cooling slope, stirring, electromagnatic stirring, .........
Thixoforming process is reheating the alloy to semisolid state and injection of semisolid metal into a die at specific liquid fraction.
Thixo properties will be improved due many reasons, such as shaping of primary alpha phase and very low level of porosity produced during the solidification process.
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