The columnar-to-equiaxed transition in Pb-Sn alloys affected by electromagnetically driven convection

An experimental study with respect to the influence of a rotating magnetic field (RMF) on the unidirectional solidification of Pb-Sn alloys is reported. The magnitude of the bulk flow in the melt generated by the RMF varies with the magnetic Taylor number Ta = σωB2R4/2ρν2. The forced convection causes distinct modifications of the temperature and concentration field such as a reduction of the temperature gradient ahead of the solidification front. Without electromagnetic stirring, the alloy solidifies solely in form of dendrites aligned parallel to the heat flow direction. In contrast, a transition from columnar-to-equiaxed growth (CET) is observed if the solidifying ingot is exposed to an RMF. The position of the CET is shifted towards the bottom of the casting by increasing the Ta number. The CET was found to occur for a cooling rate of about 0.4 K/s and temperature gradients in the range between 0.6 and 1.0 K/mm.