Yield behavior of commercial Al-Si alloys in the semisolid state
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NTRODUCTION
SEMISOLID metal processing of aluminum alloys is a relatively new technology having distinct advantages over traditional near-net-shaping technologies, such as low cycle time, increased die life, reduced porosity, and improved mechanical properties. As most novel technologies progress, commercial applications precede the development of the science base. Although there have been several successful commercialization campaigns with 356 and 357 Al–based alloys in the automotive industry (i.e., automobile wheels, engine brackets, brake master cylinders, etc.),[1–5] semisolid metal processing has not been widely adopted in the metal-casting industry. A fundamental knowledge base of the flow behavior of semisolid materials will allow one to establish the critical measures for process control. Process robustness and quality-assurance measures are based on a thorough understanding of the rheological behavior of semisolid metal slurries. However, the sensitivity of the semisolid slurry to temperature variations, coupled with its history-dependent rheological properties, makes process control a challenge. In recent years, considerable efforts have been devoted to understanding the complex rheology of semisolid metal slurries.[6–10] The current understanding is based on the fact that under steady-state conditions, the semisolid
Q.Y. PAN, Research Associate Professor, and D. APELIAN, Howmet Professor and Director, are with the Metal Processing Institute, WPI, Worcester, MA 01609. Contact e-mail: [email protected] A.N. ALEXANDROU, Professor, is with the School of Engineering, University of Cyprus, Nicosia, 1678, Cyprus. Manuscript submitted January 22, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS B
slurry behaves as a shear-thinning (pseudoplastic) fluid, wherein the effective viscosity decreases with increasing shear rate. However, under rapid transient conditions, the semisolid slurry behaves as a shear-thickening fluid, i.e., where the effective viscosity increases with increasing shear rate.[6–9] Specifically, at low shear rates, the semisolid slurry has a finite yield stress, similar to Bingham-type fluids. A semisolid metal slurry is a mixture of liquid metal and solid particles attained either by lowering the temperature of the alloy melt into the two-phase range, or by heating the semisolid billet into the two-phase range. The latter is named the thixocasting route, and the former is termed the rheocasting route. The difference is that in the rheocasting route, the slurry is available at the instant the melt temperature is lowered; however, in the thixocasting route, one needs to apply a certain amount of stress for the semisolid slurry to flow. The solid particles have, predominantly, a globular shape during the reheating stage. In essence, the rheological behavior of semisolid metal slurries is similar to that of viscoplastic materials such as concentrated suspensions, pastes, foodstuffs, emulsions, and foams. This class of materials is characterized by the existence of a yield stress, which implies that the
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