The movement of particles in liquid metals due to gravity

Abstract

The buoyancy of a single crystal copper cube in a lead tin melt was examined. A neutral buoyancy melt density of 8828 Kg/m3 at 250 °C was obtained which corresponds to a copper density of 8930 Kg/m3 at 20 °C. The copper cube was found to change from a floating to sinking position with changes in melt density of 1 Kg/m3. Contact with the crucible walls or meniscus prevented the copper cube from moving under buoyancy forces resulting from density differences as high 12 Kg/m3. This is attributed to surface tension forces at the points of contact of the copper, melt, crucible wall, and meniscus. Very small density variations in the copper due to micro-porosity and other imperfections was found to have a very large effect on the movement of the copper under buoyancy forces. In experiments in which grains of tin in partially solidified pure tin were allowed to settle under controlled conditions, it was found that many grains appeared to be separated from neighboring grains. In addition, regions clear of grains were observed in the settling region. It is suggested that this results from clumping of the grains and bridging of the clumps as they settle in the melt.