The Effect of Convection on Microstructures and Interface Stability of Al-4.5wtpctCu Alloy Made by Directional Solidification

The effect of convection and diffusion introduced by sample size and curvatures on microstructures, stability of solid/liquid (S/L) interface, and solute distribution in an Al-4.5wtpctCu alloy was studied with growth velocity between 1 and 96 μm/s. The experiments were undertaken via directional solidification within coaxial tubes of a variety of diameters. The intent was to achieve the same temperature gradient and growth velocity but different magnitudes of convection. The results indicate that with respect to planar growth, the growth front of the smallest diameter samples (1 mm) in the inner tube is less stable than that of large diameter (7.3 mm). The samples with the smallest diameter have weak convection and show no planar growth. Because of the reduced convection, the transition from cellular-to-dendritic growth occurs at relatively slow growth rate (4 μm/s). Increasing sample sizes results in formation of a planar growth front. The interface position of the inner smaller samples is located behind that of the outer larger samples, and more interfacial Cu segregation is found in smaller samples than in larger samples because of convection reductions in small samples. The change of the curvature of the samples affects the convection. At certain conditions, small curvature results in reduced convection. Samples with smaller sizes and therefore reduced convections have larger S/L curvatures than the larger samples.