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Influence of TMF-induced fluid flow on the microstructure of Al-Si7-Mn1 wt.% alloy

The precipitation of Mn-rich intermetallics in AlSiMn alloys during solidification ahead of the mushy zone affects the solidification microstructure, especially if fluid flow is present. Recently Steinbach and Ratke reported a barrier effect of α-AlMnSi, meaning these intermetallics prevent fluid flow to enter the mush. To investigate this effect further we studied the solidification of AlSi7Mn1 with a fluid flow field induced by a traveling magnetic field (TMF). Samples were molten and directionally solidified within a silica aerogel crucible at various 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 aerogel as crucible material permitted direct optical verification of the desired solidification velocity using an infrared line camera. Three collinear coils induced a TMF of approximately 5 mT strength, traveling either up or downward in the direction of the sample axis. The microstructures of the processed samples were studied using light microscopy and SEM-EDX and characterised by the primary and secondary dendrite arm spacing, the distribution of intermetallic phases as well as the radial segregation of primary phase and eutectic. Results are presented which show differences between samples with and without TMF-induced fluid flow. We noticed a radial macro-segregation dependent on the orientation of the TMF and the effects of the induced fluid flow on the primary and secondary dendrite arm spacing are examined.