Einfluss von Strömungen auf die Entwicklung des Mikrogefüges bei der gerichteten Erstarrung von Al-Si- und Al-Si-Mg-Legierungen

The effect of magnetically controlled convective conditions on the microstructure formation in cast Al-alloys is investigated. A technical Al-7wt.%Si-0.6wt.%Mg alloy (A357) and the binary counterpart Al-7wt.%Si have been directionally solidified upwards over a wide range of constant solidification velocities (0.0075 - 0.15mm/s) under medium temperature gradient (3K/mm) in the furnace facility ARTEMIS under natural convective and forced fluid flow conditions. This kind of furnace facility utilizes the extreme properties of transparent nanostructured silica aerogels as a crucible material, leading to flat isotherms and allowing the direct optical observation of the solidification process. Three pairs of Helmholtz coils around the cylindrical sample (120mm length, 8mm in diameter) induce a homogeneous rotating magnetic field (3mT and 6mT at 50Hz) being able to generate a controlled fluid flow in the melt close to the growing solid-liquid interface.The application of rotating magnetic fields during directional solidification results in pronounced segregation effects, leading to a deformation of the solidification front. For high magnetic field strengths a change to pure eutectic solidification at the axis of the sample is observed. The investigations show that the microstructural features like the primary dendrite, the secondary dendrite arm spacing, the eutectic spacing and the fraction solid change in a unique manner with solidification speed and rotating magnetic field strength. The scientific results indicate a significant decreasing of the primary dendrite spacing, whereas the secondarydendrite arm spacing increases when a convective solute transport regime is approached. The ripening exponent changes from 1/3 toward a value of 1/2. The results are describable with the available theories of Hunt and Lu and Lehmann respectively for the primary spacing and Ratke and Thieringer and Diepers and Beckermann for the secondary dendrite arm spacing. The results point to a possible pitfall of laboratory experiments: there seems tobe no sufficient control on the fluid flow within laboratory facilities for directional solidification and thus a comparison of the results with theoretical predictions, considering diffusive heat and mass transport conditions, seems to be difficult and not free enough from fit parameters. Consequently, one of the most fruitful approaches to study convection induced effects has been the utilization of microgravity solidification experiments. Therefore aerogel based furnace facilities with and without a coil system for experiments under microgravity conditions on sounding rockets were developed and tested succesfully. The experimental results indicated that the primary spacing of Al-6wt.%Si samples directional solidified in microgravity increased and the secondary dendrite arm spacing decreased, when compared with similar samples solidified under earth conditions