Development and Application of an Optimization Protocol for Directional Solidification: Integrating Fundamental Theory, Experimentation and Modeling Tools

Abstract

A protocol for identifying the preferred process conditions for directional solidification has been developed using the axial thermal gradient at the surface of the casting during solidification. Solidification modeling has been utilized to predict local thermal conditions during solidification for a broad range of geometrical configurations, alloy compositions and heat-extraction conditions. Three different mold configurations were evaluated for three alloy compositions using both conventional and high-gradient directional solidification processes. The high-gradient directional solidification process investigated was the Liquid Metal Cooling (LMC) process that utilizes a liquid-metal coolant in the cold zone of the directional-solidification furnace. Process conditions associated with the development of dendritic structure and solidification defects have been analyzed in detail for each configuration. Classical defect maps have been extended to consider the important effects of solid-liquid interface curvature. The utilization of the surface maximum axial thermal gradient as a means to identify preferred processing conditions is applicable to a range of solidification conditions, and accounts for changes in casting geometry, alloy composition or degree of heat-extraction. Experiments have been conducted to validate model predictions and improve the understanding of the role of solid-liquid interface curvature on dendrite-growth morphology. The optimization technique has been demonstrated for an atypical casting configuration and applied to a complex geometry, in which timedependent process conditions were required to maintain desired single-crystal growth.