Experimental Investigation on the Rheological Behavior of Hypereutectic Al-Si Alloys by a Precise Rotational Viscometer

Determining the viscosity of low solid volume fraction semisolid alloys is important for predicting the gradient of primary particles within functionally graded materials (FGMs), produced by in-situ casting processes. In this study, a new precise rotational viscometer was developed and used to measure the viscosity of Al-22 pct Si and Al-30 pct Si semisolid alloys up to solid volume fractions of 9 and 20 pct, respectively. Three kinds of typical curves, viscosity (η) vs solid volume fraction (f _s ), shear time (t), and shear rate (\( \dot{\gamma } \)), were derived from the results of viscosity measurements for both Al-Si alloys. In the semisolid Al-Si alloys, thixotropic behavior was not detected at low solid volume fractions, but this behavior was obviously observed with increasing solid volume fraction and could be described by an analytical model. Finally, the results showed that the equilibrium viscosity of the semisolid alloys with thixotropic properties decreased by increasing shear rate according to the Ostwald–De Waele power law. A special test, developed in this research, was used to show the effect of agglomeration on the viscosity of the semisolid alloys.