Prediction of Freckle Defects in INCONEL718 VAR Ingots

Abstract

Solidification macro-segregation, particularly in the form of freckles, in nickel base superalloys is the main cause for the limitation of ingot diameter and melting rate during vacuum arc remelting (VAR) and electro-slag remelting (ESR) processes.
The critical solidification conditions for freckle formation in INCONEL718 were experimentally studied by horizontal directional solidification (H-DS) under various solidification conditions, and validated by commercial sized VAR operation. The tendency for freckle formation in this alloy was evaluated by an expression based on modified Rayleigh criterion (Ra^*) 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, simulation method to calculate the Ra^* as an indication for occurrence of freckle in solidification has been developed based on the results of the above experiments. It is found that there is a good agreement between calculation and experiments for distribution of freckle defects in the alloy.