MODELING AND IN-SITU X-RAY VIDEO MICROSCOPY OF CONFINED EQUIAXED GRAIN GROWTH AND BUOYANT MOTION IN AL-CU Pierre Delaleau 1 , Ragnvald H. Mathiesen 2 , Paul L. Schaffer 1 , Lars Arnberg 1 , Christoph Beckermann 3 1 Department of Materials Science and Engineering; NTNU, N-7491 Trondheim, Norway 2 Department of Physics; NTNU, N-7491 Trondheim, Norway 3 Department of Mechanical and Industrial Engineering; The University of Iowa, IA 52242, U.S.A. Keywords: Equiaxed grains, Buoyancy, Synchrotron radiation, Solidification, Aluminium alloys. Abstract Equiaxed dendritic growth in grain refined Al-x wt% Cu (x=15-25) has been studied in-situ during directional solidification by means of synchrotron X-ray video microscopy. At these compositions, the α-Al grains have a lower density than the surrounding melt and experience buoyant forces which affect their growth rates and morphologies. As the samples are concealed within a thin container, the walls severely influence grain motion. A model for one single grain has been derived taking into account the influence of sample confinement on the drag force exerted on the free grain. The model is compared with the in-situ experiments to evaluate its present merits and to devise possible routes for further improvement in order to develop it to describe α-Al dendritic growth during buoyant motion. Introduction The microstructure determines the mechanical properties of a casting, and chemical inhomogeneities that arise during processing can result in serious defects. Accordingly, understanding, modeling, and controlling the various physical phenomena that occur during liquid to solid transformation are core issues in applied and fundamental solidification science. Generally, solidification may lead to two types of dendritic grain morphologies: columnar and equiaxed. In Al-based alloys where inoculation can be promoted with the addition of TiB 2 particles to the melt, equiaxed dendritic growth is the prevalent mode for primary phase formation. The governing phenomena are dynamical and, as a consequence, it is of major interest to investigate solidification processing in-situ and in real time. Several of the Al-based alloys are excellent candidates for in-situ X-radiographic investigations of solidification microstructure evolution and such experiments have been carried out previously to understand the growth of equiaxed dendritic grains [1-3]. The results presented in this paper will focus on detailed measurements and analysis of the buoyant motion and growth of equiaxed grains with different alloy compositions. 643