Impact of gravity on directional solidification of refined Al-20wt.%Cu alloy investigated by in situ X-radiography

Gravity effects such as natural convection in the liquid phase and buoyancy forces acting on the solid phase have a strong influence on the grain structure and microstructure formation dynamics during the solidification of metal alloys. It is thus very useful to undertake experimental studies that will provide benchmark data for a deeper understanding of the role of such gravity effects. In this paper, we study the formation of the equiaxed grain structure during refined Al-20wt.%Cu solidification in a temperature gradient for three different configurations: horizontal, vertical upward and vertical downward solidification. The key grain characteristics, namely grain size, grain elongation and grain growth orientation, were determined for all experiments. A comparative analysis was performed to identify the dominant effects of gravity using the experiment in horizontal configuration as reference case. The impact of buoyancy on the grain structure formation was highlighted for the experiment in vertical upward configuration, inducing a final grain structure with a wider grain size distribution. For the experiment in vertical downward configuration, the final grain structure is composed of thinner and longer grains. The origin of these differences was linked to the impact of grain flotation and solute flows on the equiaxed microstructure development. (C) 2020 Elsevier B.V. All rights reserved.