Elucidation of Solidification Behavior of Ti–6Al–4V under Convection and a Temperature Gradient Using Levitation Methods

To understand the effects of convection and a temperature gradient on solidification behavior, Ti–6Al–4V alloy samples were melted and solidified using three levitation methods: electrostatic levitation (ESL), electromagnetic levitation (EML) with severe convection, and aerodynamic levitation (ADL) with a large temperature gradient. Titanium carbide (TiC) particles were added to Ti–6Al–4V as heterogeneous nucleation site particles to make differences in microstructures obvious. The cross sections of the levitated samples were analyzed using electron backscatter diffraction (EBSD) to obtain prior-β grain maps. The grains in the ESL sample were relatively isotropic in shape, whereas the grains and dendrite structures of the EML sample were radially arranged throughout the cross section. Therefore, under significant convection, such as in the EML sample, as grain growth is suppressed downstream, grain growth in the direction perpendicular to the melt flow is accelerated. In the ADL sample with a large temperature gradient in a vertical direction, columnar and equiaxed grains were formed in the lower and upper parts of the sample, respectively. Therefore, during unidirectional solidification due to a large temperature gradient, the shape of the grains changes from columnar to equiaxed as constitutional undercooling increases.