Papers by Author: Lorenz Ratke

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Authors: Sonja Steinbach, Johannes Dagner, Marc Hainke, Jochen Friedrich, Lorenz Ratke
Abstract: A quantitative understanding of the effect of fluid flow on the microstructure of cast alloys is still lacking. The application of time dependent magnetic fields during solidification offers the possibility to create defined flow conditions in solidification processing. The effect of rotating magnetic fields (RMF) on the microstructure formation in cast Al-alloys (Al-7wt.%Si, Al-7wt.%Si- 0.6wt.Mg) is studied experimentally and numerically. The forced fluid flow conditions result in pronounced macrosegregation effects and affect microstructural parameters. With increasing fluid flow the primary dendrite spacing decreases whereas the secondary dendrite arm spacing increases. The experimental analysis is supported by a rigorous application of numerical modeling with the software package CrysVUn.
Authors: Lorenz Ratke, D. Uffelmann
Authors: T. Buchholz, J. Alkemper, Kenji Murakami, Lorenz Ratke
Authors: Andreas Orth, Lorenz Ratke
Abstract: The microstructure formation of AlSi alloys is known to be sensitive to specific solidification conditions. In particular, small fractions of heavier alloying atoms can lead to the precipitation of intermetallic phases. Moreover, the mainly dendritic structure is also sensitive to fluid flow in the melt. These two factors and their mutual influence is examined in this paper. The solidification of AlSi7 and AlSi7Mn1 samples was studied while inducing fluid flow by a traveling magnetic field (TMF) of approximately 5 mT strength, traveling up or down the sample axis. All samples were molten and directionally solidified at constant solidification velocities between 0.03 and 0.24 mm/s. The application of two separate heaters allowed the fixation of constant temperature gradients in the solid and liquid parts of the samples, the use of a transparent silica aerogel crucible permitted optical verification of the solidification velocity. Cross sections were cut from the processed samples and the microstructure analyzed using light microscopy and SEM-EDX. From these images, values for the primary, secondary and tertiary dendrite arm spacing were retrieved. Results are presented which show a clear effect of the TMF-induced fluid flow on the binary samples, but almost none for the ternary alloy. Finally, an explanation proposing a process of precipitate particle pushing is given.
Authors: Galina Kasperovich, Sonja Steinbach, Lorenz Ratke
Abstract: Gradient annealing experiments of a near-eutectic AlCu30 alloy with artificial stirring induced by a rotating magnetic field (RMF) of 6 mT were performed. The specific surface area of the primary phase was measured on metallographic sections perpendicular to the sample axis with a fixed amount of fraction solid. The variation of the specific surface area with fluid flow is compared to flow free experiments: the specific surface area varies as the inverse cube root of annealing time if no RMF is applied, but varies as an inverse forth root at 6 mT. The experimental procedure and results are presented in detail and discussed with models of convective coarsening of dendrites.
Authors: Gerhard Zimmermann, Elke Schaberger-Zimmermann, Sonja Steinbach, Lorenz Ratke
Abstract: This paper provides an analysis of the formation of intermetallic phases in AlSi7Fe1 alloy in samples processed onboard the ISS. Based on axial 2D cross-sections obtained from regions of pure diffusive growth and also solidified with forced melt flow, the sizes and distribution of intermetallic β-Al5FeSi phases were determined for different solidification velocities. In diffusive case the phases are larger and more homogeneously distributed than in case of induced melt flow. Additionally, especially for lower solidification velocity, the enrichment of Si and Fe in the centre part of the sample results in a few but rather large β-Al5FeSi particles.
Authors: Jiu Zhou Zhao, Lorenz Ratke
Abstract: A model has been developed by taking into account the common action of the nucleation, the diffusional growth, the collisions and coagulations of the minority phase droplets and the spatial phase segregation to describe the microstructure evolution in an immiscible alloy solidified rapidly under the vertical directional solidification conditions. The model is satisfactorily verified by comparison with an analytically solvable case first, and then applied to predict the microstructure evolution in a directionally solidified Al-Pb alloy. The numerical results show that at a high solidification velocity a constitutional supercooling region appears in front of the solid/liquid interface and the liquid-liquid decomposition takes place there. A higher solidification velocity leads to a higher nucleation rate for a given temperature gradient and, therefore, a higher number density of the minority phase droplets. As a result, the average radius of droplets in the melt at the solid/liquid interface decreases with the solidification velocity.
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