Thixotropy in Semisolid Steel Slurries under Rapid Compression
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INTRODUCTION
THE initial work that led to the interest in semisolid metal processing (also known as thixoforming) can be traced back to the work at the Massachusetts Institute of Technology in the early 1970s.[1] This work was originally directed at the problem of hot tearing in alloy castings, but it was later realized that a new technology of near net shaping of complex shapes was discovered. Spencer et al.[1] originally obtained alloys with a nondendritic microstructure by cooling a fully liquid alloy to its semisolid state while applying vigorous mechanical agitation throughout solidification. The vigorous agitation of the metal prevented a dendritic structure from forming, resulting in a suspension of small spheroidal solid particles in liquid. The mixture behaved like a liquid slurry with an effective low viscosity and flowed at much lower shear stresses than would be expected for a M.Z. OMAR, formerly PhD Student, Department of Engineering Materials, University of Sheffield, is Associate Professor, Department of Mechanical and Materials Engineering, Universiti Kebangsaan Malaysia (The National University of Malaysia), 43600 UKM Bangi, Selangor, Malaysia. Contact e-mail: [email protected]; zaidi@ eng.ukm.my H.V. ATKINSON, formerly Reader, Department of Engineering Materials, University of Sheffield, is Professor and Head of Mechanics of Materials Group, with the Department of Engineering, University of Leicester, Leicester LE1 7RH, United Kingdom. P. KAPRANOS, Senior Research Fellow and Short Courses Director, is with the Department of Engineering Materials, The University of Sheffield, Sheffield S1 3JD, United Kingdom. Manuscript submitted August 17, 2010. Article published online April 2, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
dendritic material of the same solid fraction (solidification under conventional conditions results in dendritic microstructures[2–4]). It is this spheroidal microstructure that is a requirement for thixotropic behavior and for semisolid processing. When such a semisolid microstructure is allowed to stand, the spheroids agglomerate and the viscosity increases with time. If the material is sheared, the agglomerates are broken and the viscosity falls; the material behaves like a pseudo-plastic fluid (i.e., showing shear thinning behavior). In practice, an alloy with a spheroidal microstructure and having up to 50 pct liquid, not only will retain its shape and support its own weight, but also can still be handled like a solid. As soon as it is sheared, it can flow with a substantially lower viscosity. It is this behavior that is exploited in thixoforming.[5–7] As such, it is thought that thixoforming can rival other conventional manufacturing routes such as forging and casting for military, aerospace, and most notably automotive components.[7–9] There are various routes to thixoformable feedstock material and types of thixoforming process.[10] Generally, for the thixocasting type processes, the fraction liquid is quite high (40 pct or above), whereas for the thixoforging routes, the fractio
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