Experimental analysis of the columnar-to-equiaxed transition in directionally solidified Al–Ni and Al–Sn alloys

Experiments were carried out to investigate the columnar-to-equiaxed transition (CET) during the upward vertical directional solidification of hypoeutectic Al–Ni and Al–Sn alloys. For the growth conditions examined both the thermal and solutal profiles inside the mushy zone and in the overlying melt ahead of the dendrite tips are expected to be stable, since the solute enrichment increases the melt density, and no solute movement induced by convection will affect the CET. From the experimental thermal profiles, tip cooling rates, tip growth rates and thermal gradients have been determined. It was found that the CET occurs for a critical value of cooling rate, with the columnar growth prevailing throughout the casting for cooling rates higher than such critical value, which for a particular binary alloy system is independent of the solute concentration in the hypoeutectic range. For Al–Sn and Al–Ni alloys the critical cooling rates were found to be 0.30 K/s and 0.16 K/s, respectively.