Convection during thermally unstable solidification of Pb-Sn in a magnetic field

Abstract

Convection and macrosegregation in directionally solidified hypoeutectic Pb-38 wt pct Sn and hypereutectic Pb-64.5 wt pct Sn have been examined during upward and downward growth. Temperature fluctuations are observed along the length of the melt column during downward growth. With increasing Rayleigh number, these fluctuations change from none, to cyclic, to time periodic having multiple harmonics, and finally to random. At the higher convective driving force of 350 K temperature inversion, the transverse magnetic field decreased convective levels, strong random temperature fluctuations (flows) becoming smaller and periodic. The maximum field of 0.45 T was unable to completely eliminate convection. For the lower convective driving force of 150 K temperature inversion, the 0.05 T magnetic field decreased flows, and at 0.15 T, the field caused a dramatic decrease in the characteristic frequency of the temperature fluctuations, indicating a change in the nature of the flow, the waveform of the temperature fluctuations changing from sinusoidal to a pulsed wave. Temperature fluctuations and time delays between thermocouples were used to estimate flow velocities. Irrespective of the convection in the bulk melt (ahead of the mushy zone), longitudinal macrosegregation occurs only if the interdendritic melt mixes with the bulk melt.

Suppression of channel convection in solidifying Pb-Sn alloys via an applied magnetic field

“…rosegregation, even when it is strong enough to suppress channel flow. However, Song et al [27] also found that,…”

Effect of melt convection and solid transport on macrosegregation and grain structure in equiaxed Al–Cu alloys

“…The buoyancy induced melt flow and transport of solid that occurs during solidification generates compositional and structural nonuniformities at the macroscopic level [3,4]. It is the goal of models, such as that by Beckermann and Wang [5], to predict such compositional and structural nonuniformities in an attempt to minimize their negative effect on the final properties of a casting.…”

“…These studies have resulted in the determination of the interfacial drag between equiaxed dendrites and the melt over a wide range of solid volume fractions. A study that illustrated the effects of convection was done by Hellawell et al [15] using an aqueous solution of NH 4 Cl. In this study [15] channel plume flow and convective transport of solid fragments were observed causing macrosegregation.…”

A Numerical Method of Macrosegregation Using a Dendritic Solidification Model, and Its Applications to Directional Solidification via the Use of Magnetic Fields

“…Figure 10(a) is the superposed diagram of the C distribution, g S contours, and fluid flow vectors. Figure 10(b) is T vs g S relationships at the element number (I = 20, K = 58; I and K are the element numbers in X and Z directions, respectively) marked by a white diamond and at element (20,56), which is 2 elements below element (20, 58) with negative segregation. Figure 10(c) shows how the C concentration and g S at the element (20, 58) vary along with the segregation parameter** as time proceeds.…”

“…However, most of them are concerned only with bulk liquid zone or with negligible mushy zone. With respect to the DS of alloys with an extensive mushy zone, only limited research has been devoted to study the effects of the magnetic field: Tewari et al [19] and Song et al [20] have carried out the DS experiments on Pb-Sn alloys to investigate the effects of magnetic field (maximum 0.45 Tesla) on the mushy zone morphology and macrosegregation. These authors stated, as far as the macrosegregation is concerned, that the magnetic field had no influence on the formation of it (freckle).…”