Numerical investigations of freckles in directionally solidified nickel-based superalloy casting with abrupt contraction in cross section

Abstract

Abstract Evolution of grain structure and formation of freckle defects were simulated in directionally solidified nickel-based superalloy casting with abrupt contraction in cross-section area. A two-dimensional (2D) Cellular Automaton (CA) – Finite Element (FE) model was presented. The transport of liquid momentum, heat and solute mass at macroscopic scale, the development of grain envelopes at mesoscopic scale and microsegregation were fully coupled. Influences of alloying elements, cooling rate and geometrical shape of casting on freckle formation were studied. CAFE simulations show that through reducing the initial concentration of destabilizing element (W), the formation of freckle along casting edges was suppressed. Elements such as Co, Ti and Ta are not critical elements that affect density inversion in mushy region. Increasing cooling rate apparently decreased the freckling tendency. Without geometric contraction, there was no obvious formation of freckles along casting edges. It was numerically showed that the contraction in cross section introduced the high-permeability path for the upward fluid flow during directional solidification, which enhanced the freckling tendency.