CONVECTION DURING THERMALLY UNSTABLE SOLIDIFICATION OF PB-SN IN A MAGNETIC-FIELD

Convection and macrosegregation in directionally solidified hypoeutect ic Pb-38 wt pct Sn and hypereutectic Pb-64.5 wt pct Sn have been exami ned during upward and downward growth. Temperature fluctuations are ob served along the length of the melt column during downward growth. Wit h increasing Rayleigh number, these fluctuations change from none, to cyclic, to time periodic having multiple harmonics, and finally to ran dom. At the higher convective driving force of 350 K temperature inver sion, the transverse magnetic field decreased convective levels, stron g random temperature fluctuations (flows) becoming smaller and periodi c. The maximum field of 0.45 T was unable to completely eliminate conv ection. For the lower convective driving force of 150 K temperature in version, the 0.05 T magnetic field decreased flows, and at 0.15 T, the field caused a dramatic decrease in the characteristic frequency of t he temperature fluctuations, indicating a change in the nature of the flow, the waveform of the temperature fluctuations changing from sinus oidal to a pulsed wave. Temperature fluctuations and time delays betwe en thermocouples were used to estimate flow velocities. Irrespective o f the convection in the bulk melt (ahead of the mushy zone), longitudi nal macrosegregation occurs only if the interdendritic melt mixes with the bulk melt.