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.

High resolution energy dispersive spectroscopy mapping of planar defects in L12-containing Co-base superalloys

“…Four alloy compositions were investigated and the compositions are given in Table 1. The 2Ta, CrTa, and 6Ti alloys were directionally solidified via the Bridgman process, and the CoNi-C alloy was cast via the Liquid Metal Cooling process [11,23,24]. The single crystal bars were cast using withdrawal rates similar to those used for Ni-base superalloys [23,25].…”

Stability of dendrite growth during directional solidification in the presence of a non-axial thermal field

“…Utilization of the LMC process has demonstrated benefits with respect to the refinement of dendritic structure and suppression of defect-formation processes due to enhanced heat extraction [6][7][8][9][10][11][12][13][14][15][16][17]. However, the presence of enhanced lateral heat extraction under some processing conditions can lead to a significant inclination of the solidification front [6][7][8][9]. Increases in the inclination angle of the solidification front can be attributed to (1) sharp changes in geometry or (2) non-axial thermal conditions during solidification.…”

Evaluating Local Primary Dendrite Arm Spacing Characterization Techniques Using Synthetic Directionally Solidified Dendritic Microstructures

“…[3][4][5][6][7] The solidification morphology associated with dendrite arm spacing has been described in the literature. [8,9] Historically, the primary dendrite arm spacing (PDAS) has been found to correlate with processing (e.g., solidification/cooling rate) [7,[10][11][12][13][14][15][16][17] as well as with properties (e.g., creep strength, fatigue properties). [6,16,18] Lamm and Singer [6] produced single crystal nickel-based microstructures (PWA 1483) with a varied range of different dendrite arm spacings (250 to 600 lm).…”

“…Similar observations of a correlation between dendrite arm spacings and microporosity or solute segregation has been observed by researchers as well. [7,15,17] On the processing side, manufacturing turbine blades via the liquid metal cooling (LMC) process [4,14,19,20] enables a greater degree of control over the size of the primary dendrite arm spacing because of the higher thermal gradient associated with this process. For example, Franke et al [21] summarizes several recent studies comparing the LMC process and the conventional high rate solidification process.…”

“…[22][23][24] The use of various processing techniques such as LMC may allow thermal gradients, and hence primary dendrite arm spacing, to be tailored towards a (more desirable) lower mean primary dendrite arm spacing and more homogeneous primary dendrite arm spacing. [14] The identification of the primary dendrites in these studies is required to quantify the effect of processing/ alloying on the microstructure. In many experimental studies of dendrite arm spacing, the dendrite core is typically identified manually in the experimental micrographs.…”