The Effect of Eutectic Undercooling on Microsegregation of Rapidly Solidified Al-Cu Droplets

Abstract : Al-Cu alloys of (nominal) compositions 4.3 wt%Cu, 5 wt%Cu, 10 wt%Cu and 1 7 wt%Cu were atomized using a drop tower type technique under N2 and He gas. The atomized droplets were characterized using X-Ray tomography, Neutron Diffraction and Stereology calculations on Scanning Electron Microscope images. From the results of these experiments, the position of nucleus within an individual droplet, the volume percent of initial growth, recalescence and eutectic fraction were determined. Some of this data was used to initialize the solidification piodel developed for microsegregation during rapid solidification. The model solves the heat balance equation as the droplet loses heat to the gas in the atomizing chamber and phase transformation occurs during solidification. Starting with an off-center nucleation site and an initial user-defined undercooling for the nucleation of the primary phase, the model predicts the thermal history, fraction solidified and microsegregation in the droplet as a function of time. The calculated amount of eutectic in the solidified droplet is compared with the measured values. It will be shown that accounting for eutectic undercooling is critical to the agreement between experimental and predicted results.