Liquid concentration distribution and planar interface instability at an abruptly changing pulling velocity in directional solidification

Liquid concentration distribution is seriously affected by an abruptly changing pulling velocity under directional solidification. Theoretical and numerical investigations indicate that at the pulling velocity jumping from V0 to V, the solidification system does not achieve the pulling velocity V immediately, and it goes through a non-steady-state transition zone. As the pulling velocity abruptly increases (V/V0 > 1), interface liquid concentration firstly increases to the maximum and then decreases to the steady-state value. The magnitude of interface liquid concentration at the beginning increases with V/V0, the initial pulling velocity V0 and the temperature gradient GL in the liquid. At the same time, solute diffusion length reduces with V/V0 and GL. In contrast, the minimum of interface liquid concentration falls with V/V0 at the pulling velocity decreasing abruptly. As the interface liquid concentration enriched at V/V0 > 1 is more than the value required for the planar interface to keep stable, the solid/liquid interface may become unstable. The analytical results are in agreement with the numerical calculation results of Al-2%Cu alloy.