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Papers by Ranadip Acharya

John Wiley & Sons, Inc. eBooks, Sep 6, 2013

This paper focuses on computational modeling of Scanning Laser Epitaxy (SLE), an additive manufac... more This paper focuses on computational modeling of Scanning Laser Epitaxy (SLE), an additive manufacturing technology being developed at Georgia Tech for the creation of equiaxed, directionally-solidified or single-crystal structures in nickel-based superalloys. The thermal modeling of the system, carried out in a commercial CFD software package, simulates a heat source moving over a powder bed and dynamically adjusts the property values for consolidating CMSX-4 alloy powder. For any given position of the beam, the geometrical parameters of and the temperature gradient in the melt pool are used to estimate the resulting solidification microstructure. Detailed study of the flow field also revealed formation of rotational vortices in the melt pool. Microstructural predictions are shown to be in good agreement with experimental metallography. This work is sponsored by the Office of Naval Research through grant N00014-11-1-0670.

Springer eBooks, 2013

This paper focuses on computational modeling and experimental validation of microstructure evolut... more This paper focuses on computational modeling and experimental validation of microstructure evolution in Scanning Laser Epitaxy (SLE), an additive manufacturing technology aimed at the repair and production of turbine engine hot-section components made of for superalloys. A coupled thermal and fluid flow model is developed to simulate the melt pool created by the scanning laser’s heat source. The detailed effects of natural and Marangoni convection on the flow field are studied and the results are analyzed in terms of the temperature gradient, the vorticity parameter, the melt pool dimensions and the mushy region extent. The geometrical parameters and the temperature gradient of the melt pool then are used to estimate the resulting solidification microstructure in alloy CMSX-4 for any given position of the beam. The critical parameter values for columnar-to-equiaxed and the oriented-to-misoriented transitions are identified. The microstructural predictions show excellent agreement with experimental metallography and process observations. This work is sponsored by the US Office of Naval Research through grant N00014-11-1-0670.

This paper focuses on simulation-based optimization of the Scanning Laser Epitaxy (SLE) process a... more This paper focuses on simulation-based optimization of the Scanning Laser Epitaxy (SLE) process applied to gas turbine hot-section components made of nickel-base superalloys. SLE creates equiaxed, directionally-solidified and single-crystal microstructures from superalloy powders melted onto like-chemistry substrates using a fast scanning, high power laser beam. In this paper, a transient coupled flow-thermal approach is implemented to accurately simulate the melting and solidification process in SLE. The laser movement is modeled as a Gaussian moving heat source, and the thermophysical properties of the alloys are adjusted based on the thermal field. Simulations for different superalloys such as IN100, René 80 and MAR-M247 are performed and the instantaneous melt pool characteristics are recorded. Comparisons of the simulations with experimental results show reasonably good agreement for the melt depth. Feedback control is implemented, and demonstrated to produce superior quality SLE deposits. This work is sponsored by the ONR through grants N00014-11-1-0670 and N00014-14-1-0658.

Physical Review Materials, 2021

We investigate precipitation dynamics in the presence of a local solute gradient using phase-fiel... more We investigate precipitation dynamics in the presence of a local solute gradient using phase-field simulations. During the homogenization heat treatment of the solidified Inconel 718 alloy, high Nb concentration within the Laves phases or at the core of the secondary arms results in Nb diffusion into the γ matrix. The volume fraction and spatial distribution of precipitation during subsequent annealing can be controlled by tailoring the Nb concentration gradient in the matrix during homogenization. We use a surrogate Ni-Fe-Nb alloy for Inconel 718 to explore the growth dynamics of δ precipitates related to the local Nb concentration levels. The simulations indicate that in the presence of a Nb concentration gradient the growth rate of δ precipitates is higher than in a matrix of uniform average Nb concentration. The higher growth rate is a result of the higher local thermodynamic driving force at the interface between the solute-rich matrix and the δ interface. We propose a phenomenological model to describe the diffusion-controlled growth kinetics of the δ phase under a solute concentration gradient.

This paper reports on the experimental development and the theoretical analysis of the scanning l... more This paper reports on the experimental development and the theoretical analysis of the scanning laser epitaxy (SLE) process that is currently being investigated and developed at the Georgia Institute of Technology. SLE is a laser-based manufacturing process for deposition of equiaxed, directionally solidified and single-crystal nickel superalloys onto superalloy substrates through the selective melting and re-solidification of superalloy powders. The thermal modeling of the system, done in a commercial CFD software package, simulates a heat source moving over a powder bed and considers the approximate change in the property values for consolidating CMSX-4 nickel superalloy powder. The theoretical melt depth is obtained from the melting temperature criteria and the resulting plots are presented alongside matching experimental micrographs obtained through cross-sectional metallography. The influence of the processing parameters on the microstructural evolution, as evidenced through ob...

This paper focuses on microstructure evolution in single-crystal alloys processed through scannin... more This paper focuses on microstructure evolution in single-crystal alloys processed through scanning laser epitaxy (SLE); a metal powder-bed based additive manufacturing technology aimed at the creation of equiaxed, directionally-solidified or single-crystal structures in nickelbase superalloys. Galvanometer-controlled movements of the laser and high-resolution raster scanning result in improved control over the melting and solidification processes in SLE. Characterization of microstructural evolution as a function of the complex process physics in SLE is essential for process development, control and optimization. In this paper an ANSYS CFX based transient flow-thermal model has been developed to simulate microstructure characteristics for single-crystal superalloys such as CMSX-4 and René N5. Geometrical parameters and melt pool properties are used to estimate the resulting solidification microstructure. Microstructural predictions are compared to experimental metallography and reas...

Nastac/CFD, 2012

Abstract This paper focuses on modeling of the scanning laser epitaxy (SLE) process that is curre... more Abstract This paper focuses on modeling of the scanning laser epitaxy (SLE) process that is currently being investigated and developed at the Georgia Institute of Technology. SLE is a laser-based manufacturing process for the creation of equiaxed, directionally solidified and single-crystal deposits of nickel superalloys onto superalloy substrates through melting and resolidifícation of alloy powders using a scanning laser beam. The thermal modeling of the system, done in a commercial CFD software package, simulates a heat source moving ...

Applied Physics Letters, 2013

ABSTRACT In this Letter, we report on the experimental development and computational modeling of ... more ABSTRACT In this Letter, we report on the experimental development and computational modeling of a simple, one-step method for the fabrication of diverse 2D and 3D periodic nanostructures derived from gold films on silicon substrates and over areas spanning 1 cm2. These nanostructures can be patterned on films of thickness ranging from 50 nm to 500 nm with pulsed interfering laser beams. A finite volume-based inhomogeneous multiphase model of the process shows reasonable agreement with the experimentally obtained topographies and provides insights on the flow physics including normal and radial expansion that results in peeling of film from the substrate.

Additive Manufacturing (AM) refers to a process by which digital three-dimensional (3-D) design d... more Additive Manufacturing (AM) refers to a process by which digital three-dimensional (3-D) design data is converted to build up a component by depositing material layer-by-layer. United Technologies Corporation (UTC) is currently involved in fabrication and certification of several AM aerospace structural components made from aerospace materials. This is accomplished by using optimized process parameters determined through numerous design-of-experiments (DOE)-based studies. Certification of these components is broadly recognized as a significant challenge, with long lead times, very expensive new product development cycles and very high energy consumption. Because of these challenges, United Technologies Research Center (UTRC), together with UTC business units have been developing and validating an advanced physics-based process model. The specific goal is to develop a physics-based framework of an AM process and reliably predict fatigue properties of built-up structures as based on detailed solidification microstructures. Microstructures are predicted using process control parameters including energy source power, scan velocity, deposition pattern, and powder properties. The multi-scale multi-physics model requires solution and coupling of governing physics that will allow prediction of the thermal field and enable solution at the microstructural scale. The state-of-the-art approach to solve these problems requires a huge computational framework and this kind of resource is only available within academia and national laboratories. The project utilized the parallel phase-fields codes at Oak Ridge National Laboratory (ORNL) and Lawrence Livermore National Laboratory (LLNL), along with the high-performance computing (HPC) capabilities existing at the two labs to demonstrate the simulation of multiple dendrite growth in threedimensions (3-D). The LLNL code AMPE was used to implement the UTRC phase field model that was previously developed for a model binary alloy, and the simulation results were compared against the UTRC simulation results, followed by extension of the UTRC model to simulate multiple dendrite growth in 3-D. The ORNL MEUMAPPS code was used to simulate dendritic growth in a model ternary alloy with the same equilibrium solidification range as the Ni-base alloy 718 using realistic model parameters, including thermodynamic integration with a Calphad based model for the ternary alloy. Implementation of the UTRC model in AMPE met with several numerical and parametric issues that were resolved and good comparison between the simulation results obtained by the two codes was demonstrated for two dimensional (2-D) dendrites. 3-D dendrite growth was then demonstrated with the AMPE code using nondimensional parameters obtained in 2-D simulations. Multiple dendrite growth in 2-D and 3-D were demonstrated using ORNL's MEUMAPPS code using simple thermal boundary conditions. MEUMAPPS was then modified to incorporate the complex, time-dependent thermal boundary conditions obtained by UTRC's thermal modeling of single track AM experiments to drive the phase field simulations. The results were in good agreement with UTRC's experimental measurements. 2. STATEMENT OF OBJECTIVES The main objectives of the project are (1) to incorporate the phase field model developed by UTRC in the phase field simulation codes at ORNL and LLNL to demonstrate massively parallel simulations of multiple dendrites in three dimensions (3-D), (2) to extend the model to simulate dendritic solidification in the Ni-base alloy 718 by coupling with AM-process-specific thermal boundary conditions provided by UTRC and (3) to compare simulation predictions with experimental input from UTRC. In order to realize the above objectives, it was necessary to modify / enhance the existing phase field codes at ORNL and LLNL to adapt the codes to handle alloy and process parameters characteristic of the AM process. Specifically, the following project tasks describe the technical problems that were undertaken.

Journal of the Atmospheric Sciences

The drop freezing process is described by a phase-field model. Two cases are considered: when the... more The drop freezing process is described by a phase-field model. Two cases are considered: when the freezing is triggered by central nucleation and when nucleation occurs on the drop surface. Depending on the environmental temperature and drop size, different morphological structures develop. Detailed dendritic growth was simulated at the first stage of drop freezing. Independent of the nucleation location, a decrease in temperature within the range from ~ −5 to −25°C led to an increase in the number of dendrites and a decrease in their width and the interdendritic space. At temperatures lower than about −25°C, a planar front developed following surface nucleation, while dendrites formed a granular-like structure with small interdendritic distances following bulk nucleation. An ice shell grew in at the same time (but slower) as dendrites following surface nucleation, while it started forming once the dendrites have reached the drop surface in the case of central nucleation. The formed...

International Journal of Computational Methods and Experimental Measurements

The approach to obtain a specific user-defined/as-desired or conformal/epitaxial microstructure i... more The approach to obtain a specific user-defined/as-desired or conformal/epitaxial microstructure in additive manufacturing (aM) is a challenging and expensive iterative process. Modeling and validation of solidification microstructure and residual stresses can be leveraged to reduce iteration cost in obtaining as-desired microstructure, minimize residual stress and prevent hot cracking. In the present study, computational fluid dynamics analysis is used to predict melt pool characteristics, and phase-field modeling is employed to simulate solidification with corresponding microstructure evolution in the as-deposited state for laser powder bed fusion (lPBf) process. Different features of lPBf microstructure such as segregation of secondary elements, dendrite sizes, dendritic orientation and dendritic morphology are predicted. The methods are further extended to predict orientation change as a function of number of layers. a constitutive materials model coupled to solidification is used to predict the stress in as-built part as well as the effect of stress on microstructural features. The model encompasses the effect of thermo-mechanical and shrinkage stresses and considers creep flow due to the presence of liquid phases in the mushy region. a phase-field-based methodology is proposed that can solve for hot cracking starting from the intrinsic defects such as porosity in lPBf process. Depending on the residual stress, crack propagation can be predicted from the unified model. The model was incorporated in a finite element code and used to predict crack growth phenomena such as values of critical stress, crack path, etc. Phase-field models of crack growth reduce the computational complications associated with singularities and allow finite element predictions of crack propagation without remeshing. This work intends to develop a unified phase-field framework that can sequentially predict solidification microstructure, residual stresses and structural cracking.

Journal of Mechanical Design

Real-world engineering design and calibration often become slow, intractable and reduced in scope... more Real-world engineering design and calibration often become slow, intractable and reduced in scope due to frequent iterations over high-dimensional expensive black-box (HEB) class of models. One way to mitigate this challenge is to incorporate multi-fidelity models with variable complexity and accuracy into the design framework. This paper proposes a machine learning based multi-fidelity modeling (MFM) and information-theoretic sequential sampling strategy for optimization, where the associated models can have complex discrepancies among each other. From the perspective of statistical learning, the advantages of MFM based optimization over a single high fidelity surrogate, specifically under complex constraints, are discussed with benchmark optimization problems involving noisy data. The proposed framework, based on modeling of the varied fidelity information sources via Gaussian processes, is augmented with efficient active learning strategies which involve sequential selection of o...