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The present study describes about the preparation of semisolid metal (SSM) slurry by using an oblique plate. In this process, A356 aluminum alloy melt partially solidifies while flowing over a bottom cooled oblique plate. Melt flow inertia shears columnar dendrites formed on plate wall into equiaxed/fragmented grains by resulting semisolid slurry at plate exit. Effects of plate length providing required shear and cooling rate enabling necessary solidification are investigated. A 3-phase numerical model vis-à-vis transport of mass, momentum, energy and species is developed for prediction of velocity, temperature, macrosegregation and solid fraction. Model uses volume of fluid (VOF) for tracking-metal-air-interface and finite volume method (FVM) with enthalpy based phase change algorithm for tracking-solid-liquid-interface within the metal. Darcy model is used for porous mushy zone. Slurry variable viscosity is represented by Oldenburg model. Stokes model incorporates solid phase movement and gravity effect along the flow. Dendrite fragmentation is considered for generation of moving solid phase. Solid movement is handled by coherency point and characteristic diameter of moving grains. Model neglects nucleation and growth kinetics, solidification shrinkage/expansion and thermo-solutal buoyancy. Slurry solid fractions at plate exit are 16%, 22%, and 27% for plate lengths of 200 mm, 250 mm, and 300 mm, respectively. And, are 5%, 22%, and 27% for heat transfer coefficients of 1000 W/m 2 -K, 2000 W/m 2 -K and 2500 W/m 2 -K, respectively. Numerical predictions agree well with experimental results. [ABSTRACT FROM AUTHOR]

A three-dimensional transient mathematical model (following a fixed-grid enthalpy-based continuum formulation) is used to study the interaction of double-diffusive natural convection and non-equilibrium solidification of a binary mixture in a cubic enclosure cooled from a side. Investigations are carried out for two separate test systems, one corresponding to a typical model “metal–alloy analogue” system and other corresponding to a real metal–alloy system. Due to stronger effects of solutal buoyancy in actual metal–alloy systems than in corresponding analogues, the convective transport mechanisms for the two cases are quite different. However, in both cases, similar elements of three-dimensionality are observed in the curvature and spacing of the projected streamlines. As a result of three-dimensional convective flow patterns, a significant solute macrosegregation is observed across the transverse sections of the cavity, which cannot be captured by two-dimensional simulations. [Copyright &y& Elsevier]

Abstract: The variation in temperature and concentration plays a crucial role in predicting the final microstructure during solidification of a binary alloy. Most of the experimental techniques used to measure concentration and temperature are intrusive in nature and affect the flow field. In this paper, the main focus is laid on in-situ, non-intrusive, transient measurement of concentration and temperature during the solidification of a binary mixture of aqueous ammonium chloride solution (a metal-analog system) in a top cooled cavity using laser based Mach–Zehnder Interferometric technique. It was found from the interferogram, that the angular deviation of fringe pattern and the total number of fringes exhibit significant sensitivity to refractive index and hence are functions of the local temperature and concentration of the NH4Cl solution inside the cavity. Using the fringe characteristics, calibration curves were established for the range of temperature and concentration levels expected during the solidification process. In the actual solidification experiment, two hypoeutectic solutions (5% and 15% NH4Cl) were chosen. The calibration curves were used to determine the temperature and concentration of the solution inside the cavity during solidification of 5% and 15% NH4Cl solution at different instants of time. The measurement was carried out at a fixed point in the cavity, and the concentration variation with time was recorded as the solid–liquid interface approached the measurement point. The measurement exhibited distinct zones of concentration distribution caused by solute rejection and Rayleigh Benard convection. Further studies involving flow visualization with laser scattering confirmed the Rayleigh Benard convection. Computational modeling was also performed, which corroborated the experimental findings. [Copyright &y& Elsevier]

Abstract: Numerical and experimental studies on transport phenomena during solidification of an aluminum alloy in the presence of linear electromagnetic stirring are performed. The alloy is electromagnetically stirred to produce semisolid slurry in a cylindrical graphite mould placed in the annulus of a linear electromagnetic stirrer. The mould is cooled at the bottom, such that solidification progresses from the bottom to the top of the cylindrical mould. A numerical model is developed for simulating the transport phenomena associated with the solidification process using a set of single-phase governing equations of mass, momentum, energy, and species conservation. The viscosity variation of the slurry, used in the model, is determined experimentally using a rotary viscometer. The set of governing equations is solved using a pressure-based finite volume technique, along with an enthalpy based phase change algorithm. The numerical study involves prediction of temperature, velocity, species and solid fraction distribution in the mould. Corresponding solidification experiments are performed, with time–temperature history recorded at key locations. The microstructures at various temperature measurement locations in the solidified billet are analyzed. The numerical predictions of temperature variations are in good agreement with experiments, and the predicted flow field evolution correlates well with the microstructures observed at various locations. [Copyright &y& Elsevier]

Abstract: A numerical scheme with hybrid explicit and implicit time stepping in solidification problems is presented. An explicit time stepping scheme is used for solving coupled temperature and concentration fields, while an implicit scheme is used for solving equations of motion. The explicit approach results in a local point-by-point coupling scheme for the temperature and concentration fields that uses constitutive model for back diffusion in solid. The present method offers distinct advantages of simplicity and flexibility in incorporation of micro-scale models, as demonstrated by the use of a back diffusion parameter in the microsegregation model. Results from the present method are compared with those in the literature using a fully implicit method, and they show significant improvement in final macrosegregation prediction. [Copyright &y& Elsevier]

A two-dimensional transient fixed-grid enthalpy-based numerical method is developed to analyze the effects of turbulent transport during a binary alloy solidification process. Turbulence effects are introduced through standard k–ϵ equations, where coefficients are appropriately modified to account for phase-change. Microscopically-consistent estimates are made regarding temperature-solute coupling in a non-equilibrium solidification situation. The model is tested against laboratory experiments performed using an NH4Cl–H2O system in a rectangular cavity cooled and solidified from the top. Particular emphasis is laid on studying the interaction between Rayleigh–Benard type convection and directional solidification in the presence of turbulent transport. Numerical predictions are subsequently compared with experimental results regarding flow patterns, interface growth and evolution of the temperature field, and the agreement is found to be good. [Copyright &y& Elsevier]