Rapid compression of aluminum alloys and its relationship to thixoformability
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20/6/03
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Rapid Compression of Aluminum Alloys and Its Relationship to Thixoformability T.Y. LIU, H.V. ATKINSON, P. KAPRANOS, D.H. KIRKWOOD, and S.C. HOGG Shaping of metals by thixoforming relies on the unusual flow behavior of semisolid slurries containing nondendritic solid phase. The microstructure of an alloy stirred during freezing consists of rounded particles of solid, as opposed to the dendrites associated with conventional solidification. In the semisolid state, these slurries are thixotropic, in that their apparent viscosity is dependent on shear rate and time. Here, a technique of rapid compression testing is outlined, carried out under conditions similar to normal industrial thixoforming, to assess slurry flow behavior and to examine the correlation between feedstock production routes, microstructure, and resistance to flow. Samples are heated to the desired temperature in the semisolid state with various soaking times and rammed at constant velocity against a platen backed by a load cell. The load-displacement curves produced from the tests may show an initial peak, believed to originate from a skeletal structure which rapidly breaks down under shear. The load signal during flow decreases with increasing soaking time and with temperature, and the initial peak eventually disappears in all alloys investigated. Quantitative metallography indicates that the lower loads correspond to greater spheroidicity of the solid particles within the slurry. The curves have been analyzed to derive the viscosity as a function of average shear rate and demonstrate that the semisolid slurries exhibit pseudoplastic flow behavior which is dependent on the compression velocity and is far removed from steady-state conditions.
I. INTRODUCTION
SEMISOLID processing is a novel method of forming complex-shaped components in the semisolid state. This near-netshape method was first introduced in the 1970s, and the key to this process is the internal nondendritic microstructure, which gives the alloys thixotropic properties in the semisolid state.[1,2] Originally, researchers obtained alloys with a nondendritic microstructure by cooling a fully liquid alloy to its semisolid state while applying vigorous agitation by mechanical stirring throughout solidification. Upon reaching the desired solid fraction, the semisolid alloy slurry was then injected into a shaped die (in a process also known as Rheocasting[3]). Mechanical testing of rheocast components showed that they possessed better properties than those produced by liquid casting.[3] Furthermore, shaping in the semisolid state meant that the work required to generate a component is less than that for solid forging.[3,4] The vigorous agitation of metal alloys during solidification leads to the formation of spheroidal solid particles suspended in the liquid matrix through a process of dendrite-arm detachment and coarsening. Such semisolid microstructures exhibit thixotropic behavior (i.e., a decrease in viscosity during shearing, followed by gradual recovery with
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