Liquid channel segregation and morphology and their relation with hot cracking susceptibility during columnar growth in binary alloys

The segregation and morphology of liquid channels at the last stage of solidification are closely related to hot cracking formation. We use Phase-Field (PF) simulations to investigate liquid channel characteristics during columnar growth of single-crystal and bi-crystal Al-Cu alloys. It is found that the back-diffusion is weak in Al-Cu alloys. Liquid channels in Al-2.0, 3.0 wt%Cu alloys coalesce into droplets above the eutectic temperature while the reduced Hot Cracking Susceptibility of Al-4.0 wt%Cu can be correlated to eutectic formation at the root of liquid channels. For bi-crystal growth, we demonstrate that non-equilibrium solute segregation at intergranular liquid channels (prospective Grain Boundaries (GBs)) depends on the convergent/divergent growth conditions between grains, and their misorientation angle. Divergent growth leads to stronger solute segregation as more solute diffuses laterally into divergent intergranular channels. It is thus expected that for the same GB energy, intergranular channels with higher solute composition can extend to lower temperatures, thus promote hot cracking. We introduce an approach for combining solid fraction vs. temperature curves calculated from microstructurally complex PF simulations with hot cracking models in the literature to predict the nominal composition with the highest HCS. Preliminary results from this work agrees well with experiments. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.