Development of Dendritic Structure in the Liquid-Metal-Cooled, Directional-Solidification Process

A single-crystal nickel-base superalloy was directionally solidified over a range of withdrawal rates to assess the benefits of using liquid-metal cooling (LMC) for small-scale castings. Cylindrical bars of 1.6-cm diameter were solidified at a rate of 3.4 mm/min using conventional (Bridgman) radiation cooling and at rates of 8.5, 12.7, and 21.2 mm/min using LMC. PROCAST modeling was used to predict dendrite arm spacings based on local thermal conditions. The LMC process exhibited higher thermal gradients and finer primary and secondary spacings of up to 50 and 70 pct, respectively, in comparison to the Bridgman process. The maximum refinement in dendritic spacings using the LMC process occurred at a withdrawal rate of 12.7 mm/min. Variability in axial and lateral dendrite spacings decreased with increasing withdrawal rate, up to the point of maximum refinement. Withdrawal rates exceeding 12.7 mm/min increased the variability in spacings and produced lateral overgrowth of the primary dendrites by secondaries and promoted formation of high-angle grain boundaries.