Thermal dependence of large-scale freckle defect formation

Abstract

The fundamental mechanisms governing macroscopic freckle defect formation during directional solidification are studied experimentally in a Hele–Shaw cell for a low-melting point Ga-25 wt.% In alloy and modelled numerically in three dimensions using a microscopic parallelized Cellular Automata Lattice Boltzmann Method. The size and distribution of freckles (long solute channels, or chimneys) are shown to be strongly dependent on the thermal profile of the casting, with flat, concave and convex isotherms being considered. For the flat isotherm case, no large-scale freckles form, while for concave or convex isotherms, large freckles appear but in different locations. The freckle formation mechanism is as expected buoyancy-driven, but the chimney stability, its long-term endurance and its location are shown to depend critically on the detailed convective transport through the inter-dendritic region. Flow is generated by curved isopleths of solute concentration. As solute density is different from that of the bulk fluid, gravity causes ‘uphill’ or ‘downhill’ lateral flow from the sample centre to the edges through the mush, feeding the freckle. An excellent agreement is obtained between the numerical model and real-time X-ray observations of a solidifying sample under strictly controlled temperature conditions. This article is part of the theme issue ‘Heterogeneous materials: metastable and non-ergodic internal structures’.

Heterogeneous materials: metastable and non-ergodic internal structures

“…A new approach continuing this review paper on thermodynamics of rapid solidification and crystal growth kinetics in glassforming alloys is developed in the next paper given by Peter Galenko et al [9], where a deviation from thermodynamic equilibrium is introduced via gradient flow relaxation of the phase field. Another type of heterogeneity in the form of solute channels (so-called freckles) is considered by Andrew Kao et al [10]. These freckles are common defects which appear during the solidification of metal alloys.…”

A Phenomenological Analysis of Freckling in Directional Solidification of Ni-Base Superalloy: The Role of Edge and Curvature in Casting Components

“…Freckles remain as one of main casting defects in single-crystal (SC) or directionally solidified (DS) Nibase components. [1][2][3][4] They appear to be long trails of equiaxed grains aligning roughly parallel to the direction of gravity and are often found on the surface of SC or DS superalloy castings. [5][6][7] It is generally acknowledged that freckles are formed as a consequence of thermossolutal convection, which is driven by density inversion in the mushy zone.…”

“…• In addition, larger casting size leads to severe freckling. [1,4,5,[26][27][28] This is because an increased cross section of a casting has a wider mushy zone, providing a sufficient reservoir for the interdendritic convection, and the convention favors the freckle formation. [29][30][31][32] In contrast, freckles are normally not found in components with small cross-sectional area.…”

Moment‐based boundary conditions for straight on‐grid boundaries in three‐dimensional lattice Boltzmann simulations

“…Spatial locality, a linear advection term, no Poisson solver for pressure, and relative ease of imposing complex boundaries and coupling with other numerical solvers are some of the LBM advantages over conventional computational fluid dynamics methods and are the reason for its use in many applications 4-6 . Because of the locality property, the method can be massively parallelized for speed‐up on modern computing architectures 7-11 …”

“…The most popular one is the bounce‐back scheme, where particles collide with the wall and reverse their momentum. It is widely and effectively used in applications with complex geometries 7-11,17-19 due to its efficiency, simplicity, and second‐order accuracy when the boundary is placed midway between gridpoints, and first‐order otherwise. However, it has some drawbacks.…”