Holographic And Microscopic Study Of Morphology And Velocity Distribution Of Solidifying Particles In Rapidly Stirred Melts

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Abstract
A technique is described to observe shape development and particle displacements of fast moving solidifying particles during stir casting. The optical set-up consists of a Ruby la-ser (generating pulse pairs), a frequency doubled Nd3+:YAG laser (generating single pulses at a 10 Hz repetition rate), a model device of an actual metal stir casting apparatus filled with a transparent organic alloy and three different recording systems. Double exposure holograms and microphotographs are used to analyse the three- and two-dimensional particle displacements, respectively. Simultaneously recorded videotapes, microphotographs and holograms provide the opportunity to study the shape development at a repetition rate of 10 Hz over periods of 103 seconds in an imaging volume of 3 x 2 x 2 mm3. We found that a NPA-water alloy nucleates initially as equiaxed dendritic particles. After a period of stirring, the dendrite tips become more rounded. Finally the particles obtain the shape of a bunch of grapes. This final state is in good agreement with the morphology of stir casted metal alloys. Further we found that the flow behaviour at the start of the experiment can be decisive for the solidification process. We observed Taylor vortices and measured particle displacements, that occur in the direction of rotation and also along the axis of rotation (although there was no flow pressure excerted in that direction) and in the radial direction, too.

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