Charakterisierung und Modellierung der Fließeigenschaften des flüssigen und teilerstarrten Metalls ohne und mit der Anwesenheit von Fremdpartikeln
Final Report Abstract
The influence of the particle properties on the flow behaviour of the aluminium alloy A356 in semi-solid and liquid state has been determined at temperatures between 590 and 630 °C (solid fraction 40-0 %). The particle size has been determined for varying shear history (shear rate and shear duration). A model of the particle coarsening due to shearing of the material has been set up. The viscosity of this material generally decreases with increasing shearing time. During this report period, a quantitative relationship between shear history and flow properties has been determined. Further, a modelling approach for the influence of particle size on viscosity has been set up. The wall slip effect (due to particle segregation) has been determined as a function of shear stress for varying solid fractions. The results of the wall slip velocity investigations indicated that wall slip occurs in semi solid A356 for solid fraction of 20 % and above. For 40 % solid fraction and above measurements could not be performed successfully, hence 20-30 % was measured. The experiments were performed in a concentric rotational cylinder (Searle-type) rheometer. The wall slip velocity was found as a function of shear stress employing the method by Kiljanski. For the investigated ranges, the wall slip velocity increases linearly with increasing shear stress. The gradient of the wall slip velocity increases with increasing solid fraction. The results from the mathematically calculated wall slip velocities have been taken into account to determine the real viscosity of the material bulk phase. This was compared to experimental results obtained with grooved wall rheometer rotational rod and cup. The results indicate that the application of a grooved rotational rod eliminates the wall slip, whereas grooving of the cup is redundant. Qualitatively, the influence of oxide particles was shown to be similar to the influence of solid A356 particles. Difficulties were encountered regarding the determination of the influence of oxide particles. The oxidized samples did not reach a significant volume fraction of oxidized particles and samples with a range of known oxide levels were not available. This hindered the development of a special model of the influence of oxide particles.
Publications
- Modeling of Shear Induced Coarsening Effects in Semi-Solid Alloys, Trans. Nonferrous Met. Soc. China, Volume 20 (9), 2010, pp. 1696-1701
M. Modigell, A. Pola
- Wall Slip of semi-solid A356 in Couette Rheometers, AIP Conf. Proc. 1353, 2011, pp. 1075-1080
S. Harboe, M. Modigell
- The influence of particle size on viscosity in Thixomaterials, Key Engineering Materials, Volumes 504 - 506, 2012, pp. 333-338
S. Harboe, M. Modigell
