Experimental study of convective breakdown during solidification of a metal model material

The research reported here examined the convective instabilities that lead to structural and compositional nonuniformities in finished metal castings by the use of the metal model material ammonium chloride - water. The density distribution in front of the semi-solid (mushy-zone) region in the solidifying system was studied using a confocal optical imaging system and the convective breakdown was studied using a particle side scattering technique. It was shown that the convective breakdown is a classic Rayleigh - Benard instability resulting from a density inversion in front of the mushy - zone. The convection was found to be of the three dimensional Benard cell type.