Rheology of semisolid alloys under rapid change in shear rate
Industrial thixoforming processes are carried out within a second. Characterisation of the rheological behaviour of the semisolid metal slurry during this short space of time is therefore important. Rheological experiments were carried out on Sn 15%Pb alloy (fraction solid 0.2-0.5) in a HAAKE cylindrical rotational viscometer using ~ 1 kHz data collection rate to obtain the shear stress response to a rapid change in shear rate. Results and conclusions based on them are valid over a limited range of 0 s·I-200 S·l. The slurry undergoes an initial rapid structural breakdown during a jump upwards in shear rate, followed by a more gradual breakdown. The former occurs within a second of the start of the jump. The metal slurry breaks down more rapidly with a higher final shear rate, but is independent of the initial shear rate. The reverse is found with downward jumps in shear rate: recovery times increase with increasing final shear rate. Again, it is independent of the starting shear rate. The rheological behaviour of aluminium alloys with solid fraction >0.5 at 1 to 100 s'\ was studied by rapidly compressing cylindrical slugs against a load cell. The slurry exhibits a near-Newtonian behaviour at the thixoforming temperature, after an appropriate soaking time is employed and a near-spheroidal microstructure develops. Use of image analysis to quantify the spheroidicity indicates that particle shape affects the flow of the slurry, a higher spheroidicity corresponding to a lower load. Using solid-state mechanical deformation to break up the dendrites, produces thixoforming feedstock with lower resistance to flow than magnetohydrodynamic stirring. Power law index values were obtained for both rapid compression and viscometry experiments. Results from this work have shown the importance of understanding the rheological behaviour of a semisolid slurry in the transient state.