Abstract
The origin of freckles during unidirectional solidification is studied in a transparent, low melting model system, 30 wt pct NH4C1-H2O. In 30NH4Cl-H2O, freckles are caused by upward flowing liquid jets in the mushy zone. The jets erode the mushy zone causing localized segregation and start new grains by producing dendritic debris. It is shown that the jets observed in 30NH4C1-H2O are free convection resulting from a density inversion in the mushy zone. A comparison of driving force, thermal transport effects and solute transport effects in 30NH4C1-H2O and metallic systems shows that jets are possible in metallic alloys where light elements segregate normally or heavy elements segregate inversely. It is concluded that freckles in unidirectionally solidified castings and vacuum consumable-electrode ingots are caused by convective jets. It is shown that the tendency to freckle is greatest in alloys with a large density inversion, high thermal diffusivity, low solute diffusivity, and low viscosity. For a given alloy, the driving force for freckling is proportional to the inverse square of the thermal gradient. Erosion by the jets is decreased by increasing the thermal gradient and growth rate. The location of freckles is influenced by mushy zone curvature.
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