Rheology and Rheometry of Aluminum Alloys: Influence of Shear and Vibration on Aluminum Flow Properties

Abstract

Understanding of flow properties and flow effects of liquid and semisolid aluminum became a key solution for know-how of casting process. Therefore such properties must be characterized a priory to layout of flow process parameters in order to predict structure formation of aluminum in flow. In order to reach target of material characterization it becomes essential to analyze materials under as close to real process conditions as possible. This task was solved by strong modification of commercial rotational rheometer and application of high-resolution temperature control. Besides understanding the flow properties it is essential to find the way of interactive structure control during flow process. Therefore controllable effects were generated and studied with the help of structure related rheological flow properties. For triggering structure formation an influence of mechanical vibration on flow properties of highly concentrated semisolid alloy is explored in this work. For that experimental set-up was designed and adapted to conventional rotational rheometer with precise rheological characterization capability. Priory to fundamental experiments with highly concentrated aluminum suspension a number of calibration tests were performed. Also prediction of wall slippage in shear flow under vibration was evaluated. Analysis of boundary conditions shows that no considerable side effects were present during shear experiment under vibration. The research reveals precise detection of transition temperatures with the help of steady and transient shear viscosity measurement besides selective measurement of full rheological curves within liquid and semisolid state temperature range. Rheological characterization was performed under shear flow conditions with and without presence of orthogonal to flow direction mechanical vibration. It was found that superposition of mechanical vibration and shear flow radically decreases shear viscosity but only in semisolid state. Liquid state rheological properties shows structural behavior but kept insensitive to application of mechanical vibration. For semisolid alloys, comparison between reference shear viscosities at specified shear rates and those measured under vibration shows considerable differences in flow properties. Conversion of concentrated suspension from strongly shear-thinning to almost Newtonian flow behavior is reported here. It is suggested to relate such phenomenon to non-equilibrium between structure formation and disintegration under vibration and hydrodynamic forces of shear flow. Influence of vibration on structure formation was also well observed during measurement of solidification process. Comparison to reference data shows how sensitive structure of concentrated suspension is to vibration in general and especially during solidification phase. The reveled effects and observations provide a solid bases for further fundamental investigations of structure formation regularities in flow of any highly concentrated systems.