Nilkanta Barman

Studies on macrosegregation and double diffusive convection during directional solidification of binary mixture

Abstract Experimental and numerical studies on macrosegregation during directional solidification of an aqueous ammonium chloride solution in a rectangular cavity are presented. Depending on the initial concentration and boundary temperature conditions, three distinct situations in the liquid during the solidification process are studied: mass diffusion only, solutal convection and double diffusive convection. The time dependent concentration of the solution inside the cavity is measured online for the above three situations using a laser based technique. This technique measures refractive index of the solution, which indirectly gives the concentration variation at a point in the solution during solidification. The interface growth is photographed using a shadowgraph technique, and the flow is visualised using a sheet of laser light scattered by neutrally buoyant, hollow glass spheres seeded in the fluid. Corresponding numerical studies are performed using a macroscopic model based on a fixed grid, enthalpy based, control volume approach. A good agreement between the experimental and computational results is observed.

Heatline Visualization During Solidification of a Eutectic Solution in a Rectangular Cavity

The present work is focused on the visualization of heatline during solidification. The investigation considers a rectangular cavity filled with eutectic aqueous ammonium chloride solution. In order to develop Rayleigh-Bennard convection in the cavity, the temperatures of the top and bottom walls are maintained below and above eutectic temperature of the solution, respectively, whereas the vertical walls are assumed to be adiabatic. The solidification process is modelled by a set of mass, momentum and energy conservation equations coupled with boundary conditions. The governing differential equations are solved using semi-implicit finite volume method according to SIMPLER algorithm with the help of line-by-line TDMA solver. The evolution of heatline pattern during solidification is presented. From the evolution of solid–liquid interface, it is found that the solidified layer undergoes remelting at the solid–liquid interface during later stages of solidification as a consequence of thermal superheat. In the present work, the cause of the remelting phenomenon during solidification is investigated in details using energy flux vectors. It is observed that convection loop pattern i.e. energy flow below the remelted interface changes at the time of remelting.

Studies on Transport Phenomena during Continuous Casting of an Al-Alloy in Presence of Electromagnetic Stirring

In the present work, a numerical study is performed to predict the transport phenomena during continuous casting of an aluminum alloy (A356) in presence of weak stirring. A set of volume averaged single phase conservation equations (mass, momentum, energy and species) is used to represent the casting process. The electromagnetic forces are incorporated in the momentum equations. The governing equations are solved based on the pressure-based finite volume method according to the SIMPLER algorithm using TDMA solver along with an enthalpy update scheme. The simulation predicts the temperature, solid fraction and species in the computational domain. A parametric study is also performed.

Thermal Behavior of a Hot Moving Steel Strip Under Multi-Cooling Jets

In the present work, the thermal behavior during cooling of a steel strip is predicted numerically. Multiple jets of pressurized water are considered for cooling purposes; they impinge on the surface of the metal strip. The cooling of the metal strip is represented by a steady-state energy conservation equation, where an effective heat transfer coefficient is considered based on jet and strip velocities for jet–surface heat interaction. The numerical simulation is performed using a finite-volume method, and distribution of temperature within the strip is predicted. Subsequently, the effect of strip velocity on the temperature distribution and corresponding cooling rate is studied.

Modelling of heat transfer in submerged arc welding process

In this work, an analytical model for transient temperature distribution during submerged arc welding for joining two steel plates is presented. The conservation of energy equation is used to represent the thermal behaviour of the submerged arc welding process. A three-dimensional double-paraboloid shape for volumetric heat source with Gaussian distribution is considered for electric arc during welding, and a parabolic-shaped cross section for the weld pool is considered. A set of experiments is conducted to determine the geometric parameters. The final analytical solution considers effect of the electric arc, convective heat transfer from the exposed surface and heat of molten electrode material. Subsequently, the prediction is compared with experimentally predicted temperatures where a good agreement is found.

Evolution of Microstructure During Solidification of an Aluminium Alloy Under Stirring

In the present work, the evolution of microstructure during solidification of A356 alloy under stirring is performed experimentally in a high temperature concentric viscometer. The stirring during solidification results a semisolid slurry in the annular space between the cylinders. This slurry is removed periodically during processing using a vacuum removal quartz tube and quenched in water for micrograph analysis. From the micrograph analysis, the shape, stacking arrangement and corresponding microstructural evolution of the suspended primary particles in the slurry are studied. The work also predicts the fraction of solid present in the extracted slurry. Finally, the effect of microstructure and the solid-fraction on the slurry viscosity is presented.

Study on Rheological Behavior of Semisolid A356 Alloy During Solidification

In the present work, a model is developed to predict the rheological behavior of an Al-alloy (A356) in semisolid state where the alloy is sheared between two parallel plates during continuous cooling. The flow field is represented by the momentum conservation equation where the non-Newtonian behavior of the semisolid alloy is incorporated considering the Herschel–Bulkley model. In the slurry, the agglomeration and de-agglomeration phenomena of the suspended particles under shear are represented using a time dependent structural parameter. The temperature field during cooling is predicted considering the transient energy conservation equation, and hence the fraction of solid and the yield stress of the semisolid alloy are continuously updated. Considering an apparent viscosity of the semisolid alloy as a function of structural parameter, shear stress and shear rate, the governing equations are solved analytically. Finally, the work predicts the variation of the apparent viscosity of the semisolid A356 alloy with fraction of solid. At first, the present prediction is validated against an available experimental data and, thereafter, the work predicts the effect of process parameters such as shear rate and cooling rate on the apparent viscosity.

Rheological Characterization of Semi-Solid A356 Aluminium Alloy

Rheological behavior of semi-solid slurries forms the backbone of semi-solid processing of metallic alloys. In particular, the effects of several process and metallurgical parameters such as shear rate, shear time, temperature, rest time and size, distribution and morphology of the primary phase on the viscosity of the slurry needs in-depth characterization. In the present work, rheological behaviour of the semisolid aluminium alloy (A356) slurry is investigated by using a high temperature Searle type Rheometer using concentric cylinders. Three different types of experiment are carried out: isothermal test, continuous cooling test and steady state test. Continuous decrease in viscosity is observed with increasing shear rate at a fixed temperature (isothermal test). It is also found that the viscosity increases with decreasing temperature for a particular shear rate due to increasing solid fraction (continuous cooling test). Thixotropic nature of the slurry is confirmed from the hysteresis loops obtained during experimentation. Time dependence of slurry viscosity has been evaluated from the steady state tests. After a longer shearing time under isothermal conditions the starting dendritic structure of the said alloy is transformed into globular grains due to abrasion, agglomeration, welding and ripening.

Rheology of A356 Alloy During Solidification Under Stirring

This work presents an experimental investigation on the rheology of A356 alloy in semisolid state using a high temperature Couette type viscometer. The molten liquid, resides in the annular space between the cylinders, is stirred and cooled continuously during experiments. The stirring results in fragmentation of dendrites which are transported into bulk liquid and form a semisolid slurry. The viscosity of the slurry is distinct in nature, which depends on microstructure of the suspended dendrites after coarsening. Hence, in the work, the variation of viscosity and microstructure is captured during cooling under different process parameters such as shear rate and cooling rate. Angular velocity of the inner cylinder and torque applied to stir the slurry are recorded to determine the apparent viscosity of the slurry. Temperature of the slurry is recorded to calculate the fraction of solids present in the slurry. For micrograph analysis, a vacuum quartz tube is used to remove the slurry-samples during experiments and they are quenched them in water.

Study on Thixotropic Property of A356 Alloy in Semi-Solid State

In the present work, the thixotropic property of a semisolid aluminium alloy (A356) under deformation is investigated numerically where the Couette flow between two parallel plates is considered. The flow field is represented by momentum conservation equations where the non-Newtonian behavior of the semisolid material is represented by the Herschel-Bulkley model. The agglomeration and the de-agglomeration phenomena of the suspended particles under shear are represented using a time dependent structural parameter influenced by the rate of strain and shear stress. The simulation predicts the flow field, rate of strain and apparent viscosity of the semisolid materials under transient and steady state conditions. It is found that the apparent viscosity shows a transient nature during sudden change in the shear rate, and its value decreases with increasing shear rate and vice-versa. It is also found that the present prediction shows a good agreement with prior work.