The columnar to equiaxed transition during the horizontal directional solidification of Sn–Pb alloys

Experiments have been carried out to analyze the columnar to equiaxed transition during the horizontal directional solidification of Sn–Pb hypoeutectic alloys as a function of solidification parameters, heat transfer coefficients, temperature gradients, growth rates, cooling rates and composition Co. For this purpose, a water-cooled solidification experimental apparatus was developed, and specimens were solidified under unsteady state heat flow conditions. A combined theoretical and experimental approach is developed to quantitatively determine the solidification thermal variables considered. The results have supported a criterion proposed based on a critical value of cooling rate which for a particular binary alloy system is independent of the solute concentration in the hypoeutectic range. The observation of the macrostructures has indicated that the columnar-to-equiaxed transition occurred in a zone rather than in a sharp plane parallel to the chill wall and that resulting thermo-solutal convection seems to favor the structural transition which occurs for a critical cooling rate of about 0.048 K/s and temperature gradients in the range between 0.25 and 0.90 K/mm for any of four alloy compositions examined. A comparative analysis between some results of this work and those from the literature proposed to analyze the CET during upward and downward vertical solidification of Sn–Pb hypoeutectic alloys is conducted.