Buoyancy-driven convection and channel segregation during solidification in metallic, aqueous and organic systems.

摘要

This investigation concerns the interaction between density driven convective flow during solidification and a particular form of macrosegregation known as channels or freckles. These defects, found in directionally solidified alloys or large billet castings, manifest themselves as rods of material rich in lighter solutes, usually found near the top of the casting.; Previous work concerning channel segregation is reviewed which has lead to a model for the formation of channels during solidification vertically upward of an alloy containing a less dense solute. This configuration results in a metastable density inversion within the mushy zone surmounted by a thermally stabilized bulk liquid. The model predicts breakdown of metastability near the dendritic front by thermosolutal convection when a critical thermosolutal Rayleigh number is exceeded with subsequent coupling with flow of interdendritic liquid to form channels.; In this study, channel formation is examined in three model systems, the metallic lead-tin and transparent aqueous-salt analogue, ammonium chloride-water eutectic systems and the transparent organic succinonitrile-ethanol monotectic system. The commonality of the phenomena is demonstrated despite wide variation in materials parameters.; Critical Rayleigh numbers have been hypothesized to depend on the primary dendrite spacing. Primary dendrite spacings are varied for the Pb-10% Sn alloys by control of the solidification conditions and the incidence of channels noted in order to test the hypothesis.; The composition difference between plumes and bulk liquid or channels and matrix are quantified in the aqueous and metallic systems. Temperature differences between plumes and bulk liquid and plume flow rates also determined in the analogue to evaluate the relative magnitudes of buoyant and restraining forces which dictate the observed density difference, channel radius and flow rate relationships.; An expression is developed based on a balance of buoyancy and viscosity which satisfactorily predicts the flow velocity from established channels in the analogue systems and permits extrapolation to determination of flow velocities in the metallic alloy.; Finally, the composition range for channel formation during unidirectional solidification of succinonitrile-ethanol is determined and quantification of channel formation in the system begun.