The Tendency for Freckle Formation in Alloy 718

Abstract

Solidification macro-segregation, particularly in the form of freckles, in nickel base superalloys is the main cause for the limitation of the ingot diameter and melting rate during vacuum arc remelting (VAR) and electro-slag remelting (ESR) process.The critical solidification conditions for freckle formation in Alloy 71 8 were studied by horizontal directional solidification (H-DS) experiments under various solidification conditions, and validated by commercial sized VAR ingots. The tendency for freckle formation in this alloy was evaluated by an expression based on the Rayleigh criterion including solidification rate, thermal gradient in the mushy zone, and solidification front angle to the horizontal plane.In order to systematize the methods to predict proper VAR operating conditions for any alloy and ingot diameter, a solidification simulation approach has been developed based on the results of the above trials.

Effects of instabilities on the properties of ingots made by the ESR and VAR processes

“…It is suggested that the initiation of freckle channel formation is the result of local irregularities in the dendrite network which lead to local increases in the network permeability. The random nature of the irregularities accounts for the formation of a small number of freckles in an ingot with no indicated regions of high Ra number [18]; also for the demonstrated possibility of producing an ingot without freckles under process parameters which would produce a high Ra value in the ingot [17]. The irregularities are the result of perturbations in the heat balance of the solidifying dendritic front brought about principally by lack of control of the arc heating process.…”

Development and application of a new freckle criterion for technical remelting processes

“…However, freckle type defects have also been observed for alloy 718 in industrial castings [12,20,[27][28][29], although heavy elements like Nb lead to an increasing density of the segregated melt during solidification of this alloy which should result in the formation of a stable layering. Similar to buoyancy driven freckles, these defects appear as dark spots in the cross-section, preferentially at mid-radius of the ingot.…”

“…The lack of density inversion for alloy 718 was either ignored [20,31] or rejected, pointing out that the high silicon contents of older grades of alloy 718 could have led to a decreasing density of the melt during solidification [12,27,30].…”

“…Much work has been devoted to clarify the mechanisms underlying such convective instabilities. Fundamental investigations on convective instabilities during directional solidification of binary alloys [1][2][3][4][5] are complemented by composition dependent models [6], numerical approaches [7], experiments with forced convection/high gravity [8][9][10] and by numerous observations and results for superalloys and steels in view of their technical importance [11][12][13][14][15][16][17][18][19][20].…”