Analytical modeling of the rheological behavior of semisolid metals and composites
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TRODUCTION
BOTH, the thermodynamic and rheological properties of alloys and composites are of primary importance for their processing by semisolid forming procedures. A good knowledge of these properties is necessary for the selection of compositions that will be suitable for semisolid applications.[1] In addition, the experimental determination and modeling of the rheological behavior is necessary to determine the nature of the material flow in order to carry out simulations and optimization of semisolid processing. It is now well established that the thixotropic characteristic of rheocast structures is due to a structural breakdown upon shearing. Different approaches have been proposed to describe and measure the thixotropic nature of semisolid slurries of alloys and of composites at medium and high solid fractions.[1–4] The degree of thixotropy of semisolid slurries can be measured by shearing the semisolid mixture at a constant temperature and shear rate until the viscosity reaches an equilibrium value. Shear step change experiments have shown that a sudden decrease of shear rate will produce a sudden increase in viscosity followed by a gradual increase corresponding to a restoration of the structure under the new shear rate. On the other hand, a sudden increase of shear rate will produce a sudden decrease in viscosity followed by a gradual decrease corresponding to the degradation of the structure under the higher shear rate. Another way to measure the degree of thixotropy is to subject the semisolid slurry to a linear cycle of increasing and decreasing shear rate at a constant temperature. This cycle of the variation of the shear stress vs shear rate results in a hysteresis loop. This approach is more practical from L. AZZI, Research Officer, is with the Industrial Materials Institute, National Research Council Canada, Boucherville, PQ, Canada J4B 6Y4. Contact e-mail: [email protected] F. AJERSCH, Professor, is with the De´partement de Ge´nie chimique, E´cole Polytechnique, Montre´al, PQ, Canada H3C 3A7. Manuscript submitted March 16, 2006. METALLURGICAL AND MATERIALS TRANSACTIONS B
an experimental point of view because it is not restricted by the equipment inertia as in the step change experiments. In this article, we will present the rheological behavior of semisolid alloys and composite mixtures subjected to such shear cycles. The results show how the initial state of the microstructure of the slurries affects the nature of the hysteresis loop shape. An analytical model describing the experimental results is presented.
II.
EXPERIMENTAL
A. Materials Investigated and Experimental Setup Two different alloys were investigated. Unreinforced commercial A356 alloy (Al 6.98 wt pct Si-0.37 wt pct Mg) and reinforced 6S3G GrA-NiÒ alloy (Al 7.7 wt pct Si-2.4 wt pct Ni-0.37 wt pct Mg) containing 6 vol pct of SiC and 3 vol pct of graphite particles, developed by INCO.[5]* *INCO is a trademark of INCO Alloys International, Huntington, WV.
The GrA-Ni composites consist of an aluminum matrix reinforced by SiC (10 to 1
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