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

Abstract

Abstract. In technical solidification processes like Electro Slag (ESR) or Vacuum Arc Remelting (VAR), freckles present a serious type of defects which limit the maximum ingot size for many grades of steels and superalloys. Therefore, modelling of freckle formation is an important task for optimizing industrial remelting processes. Practically all present freckle models are based on a critical Rayleigh number. They are inspired by the "classical" assumption that freckles are caused by an inversion of the liquid density in the semisolid region. Plumes of the lighter segregated liquid evolve through perturbation of the metastable layering of the melt in the mushy zone, a mechanism which motivates its description via Rayleigh criteria. But these models are not suitable for materials like Alloy 718 which do not show a liquid density inversion, but nevertheless are prone to freckle formation in technical remelting processes. In this paper, a criterion is developed which -instead of using a Rayleigh number -is based on the evaluation of the non-isothermal component of an instantaneous down-hill flow of heavy segregated melt in a melt pool with axial symmetry. With knowledge of the exact pool geometry and the shape and properties of the mushy zone, the occurrence of freckles can be predicted. The model is applied to a technical ESR casting for which temperature fields and microstructural parameters have been obtained using CFD and 3D phase-field modelling, respectively. Furthermore, the implications are discussed which the new model offers for the understanding of freckles in technical remelting processes.

Cross-Permeability of the Semisolid Region in Directional Solidification: A Combined Phase-Field and Lattice-Boltzmann Simulation Approach

“…As stated by several authors, the specific permeability is a determining parameter in fluid flow phenomena like freckles, as well as in defects like porosity and hot cracking among others. [1][2][3][4] Improvements of simulation techniques and the combination of multiple tools working on different length scales through Integrated Computational Materials Engineering (ICME) 5 open up new possibilities of investigations on the impact of permeability on casting defects. Phase-field models have become very popular for the simulation of the microstructure evolution during solidification processes of alloys.…”

“…This allowed for assessment of the relation between temperature, fraction liquid and cross permeability for isothermal fluid flow, which plays a crucial role for freckle formation in technical remelting processes (e.g., of alloy 718). 2 The obtained results are compared with empirical relations provided by Poirier 17 and Madison.…”