Response of semi-solid Sn-15 pct Pb to rapid shear-rate changes

Semisolid metal alloys, used in thixoforming, are thixotropic. The slurry viscosity is shear-rate- and time dependent provided the microstructure in the semisolid state is nondendritic and consists of solid spheroids surrounded by liquid. Thixoforming takes less than 1 second, during which time the structure of the material breaks down. This breakdown can be studied by viscometry, using rapid data-collection rates, by imposing rapid increases in shear rate on the slurry. An initial rapid (< 1 second) structural breakdown during a shear-rate jump is followed by a more gradual decrease in viscosity, lasting several minutes. The slurry breaks down more rapidly with a higher final shear rate, but the first breakdown time is independent of the initial shear rate. The reverse is found with the shear-rate drops: recovery times increase with increasing final shear rate. Again, this time is independent of the starting shear rate. The shear stress and viscosity during initial breakdown and initial recovery can be described by single exponential equations. Build-up processes were examined by performing jumps after allowing the slurry to rest at a zero shear rate for different times. The increase in peak stress with rest time reflects the evolving degree of particle agglomeration. Microstructural examination confirms this phenomenon.