THERMOSOLUTAL CONVECTION DURING CELLULAR/DENDRITIC ARRAYED GROWTH OF Pb-Sn ALLOYS: INFLUENCE OF MAGNETIC FIELD

Abstract

Directional solidification of hypoeutectic lead-tin alloys, with melt on top, solid below and gravity pointing down, results in thermosolutal convection in the interdendritic melt in the mushy zone and also in the bulk melt ahead of the cellular/dendritic array. Convection in the interdendritic melt during dendritic arrayed growth produces macrosegregation along the length of the samples. The extent of macrosegregation increases with increasing tin content of the alloy, becomes maximum for 33 wt% Sn, and decreases with further increase in tin. Longitudinal macrosegregation also results from the thermosolutal convection due to the solutal build up ahead of the cellular arrays. Growth under a transverse magnetic field (0.45 T) has no influence on the morphology of dendritic arrays. However, for cellular arrays it causes extensive channel formation in the mushy zone, as opposed to the well aligned and uniformly distributed cells formed in the absence of the field. The channels are produced due to the anisotropy in the thermosolutal convection caused by the magnetic field. Macrosegregation along the length of the directionally solidified samples is however, not influenced by the magnetic field for either the cellular or dendritic arrays.