Variation in mushy zone length during upward directional solidification in Pb-36mass%Sn alloy

Vertical directional solidification experiments have been carried out on Pb-36mass%Sn binary alloy with the melt on the top of solid to investigate the variation in the length of mushy zone. Carbon crucibles with an inside diameter of 6 mm and a length of 150 mm were used. The crucible was not moved but the furnace was moved upward at a speed in a range from 0.3 cm/h to 190 cm/h. Temperature gradient along the crucible was 36.5 K/cm or 62.5 K/cm. The carbon crucible was dropped into water bath to quench the solid-liquid interface. The structures in longitudinal and transverse sections were examined by optical metallography. It is noted that the length of mushy zone in the upward directional solidification was not constant even if the steady-state dendritic growth was maintained. The length of mushy zone decreased with increasing fraction solidified due to the solute convection occurred in the mushy zone and bulk melt. The greater the temperature gradient is, the greater the decrease in length of mushy zone is. Effect of solute convection on the length of mushy zone is responsible for constitution change of Sn in front of mushy zone.