CONTROL OF THE SOLIDIFICATION OF Al-Ni ALLOYS USING A TRAVELLING MAGNETIC FIELD: MACROSEGREGATION

摘要

The dendritic microstructure is the most common microstructure found during the elaboration of metals and their alloys. Dendrites can be columnar or equiaxed according to the elaboration conditions (heat gradient, pulling rate, and solute concentration for an alloy, presence or not of refining particles). The control of their form and their distribution is of primary importance for the resulting materials properties. Additionally, segregations left during the solid formation at different scales determine also the properties of the final material, so that a precise mastering of growth processing is essential to reproducibly tailor products of specified quality. Segregations are mainly due to fluid flow in the bulk liquid and at the level of the mushy zone. To study and control the macrosegregation, we perform experiments using a travelling magnetic field (TMF). Electromagnetic field allows controlling the fluid flow velocity. Moreover, for the specific case of the travelling magnetic field, we can change the direction of the Lorentz force so that forced flow is opposed or not to the natural convection. When the forced flow is in the same direction as natural convection, we obtain an eutectic layer at the periphery whereas, for a forced flow in the direction opposed to natural convection, we obtain a central segregated channel. Moreover, a forced flow in the same direction as natural convection yields a decrease of the radial macrosegregation as well as an increase of the longitudinal macrosegregation. On the other hand, the mean grain size decreases in the zone where fluid flow is entering in the mush.