Ranadip Acharya

Additive Manufacturing of Single-Crystal Superalloy CMSX-4 Through Scanning Laser Epitaxy: Computational Modeling, Experimental Process Development, and Process Parameter Optimization

This paper focuses on additive manufacturing (AM) of single-crystal (SX) nickel-based superalloy CMSX-4 through scanning laser epitaxy (SLE). SLE, a powder bed fusion-based AM process was explored for the purpose of producing crack-free, dense deposits of CMSX-4 on top of similar chemistry investment-cast substrates. Optical microscopy and scanning electron microscopy (SEM) investigations revealed the presence of dendritic microstructures that consisted of fine γ′ precipitates within the γ matrix in the deposit region. Computational fluid dynamics (CFD)-based process modeling, statistical design of experiments (DoE), and microstructural characterization techniques were combined to produce metallurgically bonded single-crystal deposits of more than 500 μm height in a single pass along the entire length of the substrate. A customized quantitative metallography based image analysis technique was employed for automatic extraction of various deposit quality metrics from the digital cross-sectional micrographs. The processing parameters were varied, and optimal processing windows were identified to obtain good quality deposits. The results reported here represent one of the few successes obtained in producing single-crystal epitaxial deposits through a powder bed fusion-based metal AM process and thus demonstrate the potential of SLE to repair and manufacture single-crystal hot section components of gas turbine systems from nickel-based superalloy powders.

Fabrication of gold nanostructures through pulsed laser interference patterning

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.

Computational Modeling and Experimental Validation of Microstructure Development in Nickel-Base Superalloys Processed through Scanning Laser Epitaxy (SLE)

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.

Modeling of Solidification and Microstructure Evolution in the Scanning Laser Epitaxy (SLE) Process for Additive Manufacturing With Nickel-Base Superalloy Powders

This paper investigates effects of natural and Marangoni convection on the resultant solidification microstructure in the scanning laser epitaxy (SLE) process. SLE is a laser-based additive manufacturing process that is being developed at the Georgia Institute of Technology for the additive manufacturing of nickel-base superalloys components with equiaxed, directionally-solidified or single-crystal microstructures through the laser melting of alloy powders onto superalloy substrates. A combined thermal and fluid flow model of the system simulates a heat source moving over a powder bed and dynamically adjusts the thermophysical property values. The geometrical and thermal parameters of the simulated laser melt pool are used to predict the solidification behavior of the alloy. The effects of natural and Marangoni convection on the resultant microstructure are evaluated through comparison with a pure conduction model. Inclusion of Marangoni effect produces shallower melt pools compared to a pure conduction model. A detailed flow analysis provides insights into the flow characteristics of the powder, the structure of rotational vortices created in the melt pool, and the solidification phenomena in the melt pool. The modeling results are compared with measurements and observation through real-time thermal imaging and video microscopy to understand the flow phenomenon.In contrast to the single weld-bead approach, the raster scan in SLE allows every position in melt pool to be visited twice by the solid-liquid interface as the scan source progresses. To properly address this situation, time tracking is incorporated into the model to correctly couple the microstructure prediction model. An optimization study is carried out to evaluate the critical values of the transition parameters that govern the columnar-to-equiaxed transition (CET) and the oriented-to-misoriented (OMT) transition. This work is sponsored by the Office of Naval Research through grant N00014-11-1-0670.© 2013 ASME