The Effect of Processing Conditions on Heat Transfer During Directional Solidification via the Bridgman and Liquid Metal Cooling Processes

Abstract

Finite-element-based solidification modeling was used to investigate the thermal characteristics of the Bridgman and liquid metal cooling (LMC) directional solidification (DS) processes. Physically representative boundary conditions were implemented within a finite-element model to test its applicability to a broad range of processing conditions. The dominant heat-transfer step for each case was identified. Relationships between the thermal gradient and the solid–liquid interface position relative to the transition region of the furnace were developed. The solidification rate, the local velocity of the solid–liquid interface, and the cooling rate as a function of withdrawal rate were analyzed. The curvature of the solid–liquid interface varies with the processing conditions and influences the local thermal condition and, therefore, the morphological development of dendritic structure during solidification. An extensive sensitivity analysis of process conditions was conducted for both the Bridgman and LMC techniques. The relative importance of process parameters on the resulting thermal conditions during solidification was identified. A protocol for determination of preferred process conditions was defined. The maximum axial thermal gradient at the surface of the casting occurs when the solid–liquid interface is just above the baffle for both the Bridgman and LMC DS processes, independent of casting geometry or mold-heater temperature.