Papers by Author: Georg Müller

Abstract: A global numerical model of a Bridgman furnace is developed and validated. The model includes heat transfer due to radiation and conduction in a stationary setup. The rotating magnetic field (RMF) device mounted on the facility is implemented into the model also. A numerical study on the flow field caused by the axial non-uniform RMF and natural convection is performed, helping to get a better understanding of the convective conditions during the directional solidification experiments.

Abstract: The solidification microstructure is the consequence of a wide range of process parameters, like the growth velocity, the temperature gradient and the composition. Although the influence of these parameters is nowadays considerably well understood, an overall theory of the influence of convection on microstructural features is still lacking. The application of time dependent magnetic fields during directional solidification offers the possibility to create defined solidification and flow conditions. In this work, we report about solidification experiments in the ARTEMIS and ARTEX facilities including rotating magnetic fields (RMF). The effect of the forced melt flow on microstructural parameters like the primary and secondary dendrite arm spacing is analyzed for a wide range of magnetic field parameters. The experimental analysis is supported by a rigorous application of numerical modeling. An important issue is hereby the prediction of the resulting macrosegregation, i.e., differences in the composition on the scale of the sample (macroscale) due to the RMF. For the considered configuration and parameters an axial enrichment of Si is found beyond a certain magnetic field strength. The results are compared to available theories and their applicability is discussed.

Abstract: has commissioned the Crystal Growth Laboratory in the framework of the Materials Science Laboratory (MSL) User Support Program to develop the MSL furnace inserts and samplecartridge assemblies Thermal Modeling Tool (TMT). The TMT assists the definition, preparation and analysis of the experiments onboard the International Space Station (ISS). The tool is based on the CrysVUn software, which was especially designed for global simulation of heat and mass transport processes during crystal growth and alloy solidification in high tem perature furnaces with complex (axi-symmetric) geometries. The main features of CrysVUn are briefly presented in this paper. The preliminary thermal model of the Low Gradient Furnace is illustrated and the model optimization strategy using genetic algorithms is briefly explained. Initial correlation results show that good agreement between simulated and measured axial temperature distributions is obtained. Direct modeling of the radiative heat transfer through the heater Multi-Layer Insulation (MLI) using view factors appears to be necessary for an accurate predict on of the resulting heater power. An example for the application of the TMT is presented.

Abstract: The MICAST research program focuses on a systematic analysis of the effect of convection on the microstructure evolution in cast Al-alloys. The experiments of the MICAST team are carried out under well defined thermally and magnetically controlled, convective boundary conditions and analyzed using advanced diagnostics and theoretical modeling, involving phase field simulation, micro-modeling and global simulation of heat and mass transport. The MICAST team uses binary, ternary and technical alloys of the Al-Si family. This paper gives an overview on recent experimental results and theoretical modelling of the MICAST team.