Response of semi-solid Sn-15 pct Pb to rapid shear-rate changes
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I. INTRODUCTION
VIGOROUS agitation of metal alloys during solidification can produce near-spheroidal solid particles suspended in the liquid matrix. These interacting solid particles, held together by microscopic bonds (i.e., metallic bonds formed by the collision of solid particles) when the alloy is left at rest, enable the material to be handled like a solid. When a shear stress is imposed, the bonds between the particles are broken down and the material flows in a laminar way. This behavior, known as thixotropy, has been shown extensively in semisolid metal slurries.[1–9] Thixotropy is defined, according to the British Standards Institution, as the “decrease in viscosity under stress, followed by gradual recovery when the stress is removed.” This effect is time dependent. Figure 1 shows a schematic diagram of the change in viscosity with time following changes in shear rate. Semisolid metal processing (or thixoforming), where the nondendritic metal slug is heated to its semisolid temperature before forming it into a die, exploits this behavior and, hence, lower power/energy is needed compared to solid forging. Compared with the turbulent flow in die casting of fully liquid metal the flow is laminar and there is less shrinkage porosity and/or gas entrapment. Thixoforming is a viable industrial process.[10–14] However, the rheological behavior of the alloy slurries requires further investigation. In particular, the slurry undergoes a sudden increase in shear rate from rest to 100 s⫺1 or more as it enters the die. This change takes place in less than 1 second. Hence, measuring the T.Y. LIU, Ph.D. Student, and P.J. WARD, Postdoctoral Research Fellow, are formerly with the Department of Engineering Materials, University of Sheffield. H.V. ATKINSON, formerly Reader with the Department of Engineering Materials, University of Sheffield, is Professor, Department of Engineering, University of Leicester, Leicester, LE1 7RH United Kingdom. Contact e-mail: [email protected] D.H. KIRKWOOD, Honorary Senior Lecturer, is with the Department of Engineering Materials, University of Sheffield, Sheffield, S1 3JD United Kingdom. Manuscript submitted April 15, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
rheological response under rapid changes in shear rate is critical to the development of modeling of die filling and successful die design.[15,16] The majority of previous work[1–5] has centered on measuring the steady-state viscosity ( ) of a Sn15 pct Pb semisolid metal slurry achieved over a period of time after a change in shear rate (␥˙ ). The microstructural characteristics of the Sn15 pct Pb alloy are known to be similar to those of the thixoformable aluminum alloys used in the manufacturing industry. An additional advantage of using the Sn15 pct Pb alloy for viscometry experiments is its low melting temperature and wide semisolid temperature range. The results for steady-state viscosity (e) from previous works,[3,6,7] together with results obtained from the authors’ earlier work,[17] have been fitted to the Cross equati
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