Phase-field Modeling of the Initial Transient in Directional Solidification of Al-4wt%Cu Alloy

The initial transient in directional solidification of Al–4wt%Cu alloy by cooling-down is investigated by numerical simulation using the phase-field model proposed by Karma (Phys. Rev. Lett., 87 (2001) 115701), which includes solute antitrapping in mass conservation relation and is solved by the adaptive finite element method. The simulated velocity of the unsteady planar solidification interface and the solute profile in the liquid are always close to the predictions of the Warren–Langer analytical model of initial solidification transient (Phys. Rev. E, 47 (1993) 2702) but only in the very beginning of growth in fair quantitative agreement with the experimental data obtained by means of in situ and real-time X-ray radiography at the European Synchrotron Radiation Facility (ESRF). Then, the influence of gravity-driven fluid flow becomes significant in experiments, and increases with time. In the phase-field simulations, once the smooth solidification front has lost morphological stability in the initial solidification transient, the evolution of the non-planar solid–liquid interface microstructure varies with the processing control parameters. It is found that the solid–liquid interface shape changes through transitions from flat to cellular, cellular to dendritic, cellular or dendritic to seaweed depending on the values of the applied cooling rate and temperature gradient.