Direct microscopy of alloy nucleation, solidification and ageing (coarsening) during stir casting

Abstract

In order to explain the slurry thixotropy and spherical particle morphology observed during the stir casting of metal alloys, we performed real time microscopic observations during the stir casting process using as a model substance the transparent organic alloy neopentyl alcohol with water. An optical set-up was developed consisting of two pulsed lasers, an optically adopted model of a stir casting apparatus, a microscope system and three different recording system to take (simultaneously) holograms, microphotographs and video recordings of the fast moving (up to 10 m s −1) small solidifying particles (10-10 3 μm). Our preliminary observations indicate that several mechanisms occur (nearly) simultaneously. Initially these are: both primary and secondary nucleation, rapid growth of the first nuclei into equiaxed dendrites, fragmentation of dendrite arms, clustering and coarsening. The development of the distribution of particle size and dendrite tip diameter and of the distribution of particle flow velocity and direction is shown quantitatively. We found that: (i) The increase of the fraction solid was mainly due to the formation of new nuclei (even after partial remelting), instead of outgrowth of existing nuclei. (ii) Not the fragmentation or clustering mechanisms but coarsening of the (branches of) equiaxed dendrites leads to the typical spherical particle shapes after stir casting. (iii) The rate of coarsening increases with stirring rate. (iv) Complex flow patterns at higher stirring rates can be attributed to the presence of Taylor vortices. Thixotropy is discussed in terms of coarsening and (local) variations of process parameters. The observation techniques used to determine the development of size and shape distributions, interactions and flow patterns of alloy particles during solidification seem also suitable to study industrial bulk crystallization.