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Numerical Modeling of Laser-Induced Plumes and Jets

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Numerical Modeling of Laser-Induced Plumes and Jets NUMERICAL MODELING OF LASER-INDUCED PLUMES AND JETS by AUSTIN PALYA ALEXEY N. VOLKOV, COMMITTEE CHAIR DAVID W. MACPHEE SEMIH M. OLCMEN MRUTHUNJAYA UDDI A THESIS Submitted in partial fulfillment of the requirements for the degree of Master of Science in the Department of Mechanical Engineering in the Graduate School of The University of Alabama TUSCALOOSA, ALABAMA 2019 Copyright Austin Palya 2019 ALL RIGHTS RESERVED ABSTRACT The goal of the current work is to perform numerical modeling and identify important phenomena associated with vapor plume and jet flows induced by irradiation of metal targets with short pulse or continuous wave (CW) lasers, discover and explain the mechanisms responsible for vaporized material motion in applications of lasers for material processing and analysis such as deep laser drilling, laser-induced breakdown spectroscopy (LIBS), and selective laser melting (SLM) of metallic powders. The simulations of laser-induced vapor expansion into a background gas are performed with a combined computational model, including the thermal model of irradiated targeted and a kinetic model of multi-component gas flows. The latter is implemented for simulations in the form of the Direct Simulation Monte Carlo method. Based on this model, two major problems are considered. In the first problem, vapor plume expansion under conditions of spatial confinement when the plume, which is induced by irradiation of a copper target by a short-pulse laser, propagates inside a cavity or trench, is considered. The simulations identify two major effects, the focusing effect, appearing due to transient motion of shock waves inside the cavity, and the confinement effect, induced due to overall deceleration of the plume with increasing background gas pressure, as two major mechanisms affecting removal of vaporized material out of the cavity and formation of high-density and high-temperature regions in the plume core. Due to the trade- off between these effects, an optimum background gas pressure exists, when the efficiency of the vapor removal from the cavity is maximized. At later stages of plume expansion, the simulations also reveal a suction effect, when the vapor flow at the cavity throat can be temporarily directed ii into the cavity, inducing a decrease of the overall efficiency of vapor removal. The balance between the focusing and confinement effects is studied in a range of the background gas pressure, for various background gas species, and various geometrical parameters of the cavity and laser beam. It is also shown that application of double laser pulses with short inter-pulse separation can be beneficial for both laser drilling and LIBS. In the second problem, a vapor jet induced by irradiation of a stainless steel target by a CW laser is simulated under conditions specific for SLM of metallic powders. The generated vapor jet is then used to predict motion of powder particles that can be efficiently entrapped into the ambient gas flow induced by the vapor jet. It is shown that powder particles in a broad range of their diameters can be efficiently entrained into the gas flow and, thus, removed from the irradiated surface. These results are in agreement with experimental observations of the surface denudation effect in SLM. iii LIST OF ABBREVIATIONS AND SYMBOLS 2D/3D Two-dimensional/three-dimensional Ar Argon AR Aspect ratio (퐻/퐷) 푐 Specific heat J/(kg∙K) 퐶퐷 Drag coefficient Cu Copper CW Continuous wave 푑 Reference diameter of a gas species (m) 퐷 Cavity/trench width (μm) 퐷퐿 Laser diameter (μm) 퐷/퐷퐿 Cavity diameter by laser diameter ratio DSMC Direct simulation Monte Carlo DZ Denudation zone 퐸퐿 Laser energy (μJ) 퐹 Total number of atoms 2 퐹퐿 Laser fluence (J/cm ) FWHM Full width at half maximum 퐻 Cavity height (μm) He Helium 2 퐼퐿 Incident laser intensity (W/m ) iv J Joule (kg∙m2/s2) K Kelvin 푘퐵 Boltzmann constant (J/K) 퐾푛 Knudsen number 푙푚푓푝(0) Equilibrium mean free path 퐿 Latent heat (J/kg) LIBS Laser induced breakdown spectroscopy 푚 Mass (kg) 푀, 푀푎 Local Mach number 푛 Number density (cm-3) 푛푥,푦,푧/풏푤 Unit normals ns nanosecond 푁 Number flux density (s-1∙cm-3) 푁푃퐶퐿 Number of simulated particles 푁푇 Number of “trajectories” 푝, 푃 Pressure (bar) PBFAM Powder bed fusion additive manufacturing 푃푟 Prandtl number 푞 Heat flux vector (W/m2) 2 푄푙푎푠푒푟 Volumetric energy source (W/m ) 푟 Radial distance (cylindrical coordinates) (μm) ℜ Local gas constant (J/mol∙K) 푅 Reflectivity v 푅푒 Reynolds number SLM Selective laser melting SS316L Stainless steel type 316 L 푡 Time (s, μs, ns) 푇 Temperature (K) 푢 Absolute value of local mixture velocity (m/s) 퐯 Molecule velocity vector (m/s) 푉 Velocity (m/s) VHS Variable hard sphere 푊 Statistical weight 푥 Vertical distance (cylindrical coordinates) (μm) Xe Xenon 푋푤 Surface shape 푧 Cavity height (in diagrams) (μm) Greeks 훼 Linear absorption coefficient (m-1) 훾 Adiabatic index of a monoatomic gas 훿 Dirac delta function 휅 Thermal conductivity W/(m∙K) 휌 Density (kg/m3) 휎 Collisional cross section (m2) 휏퐿 Pulse duration (FWHM) (ns) vi 휏푝푝 Peak-to-peak pulse separation time (ns) 휙 Arbitrary macroscopic quantity -1 -3 휓푐 Condensation flux density (s ∙cm ) -1 -3 휓푒 Evaporation flux density (s ∙cm ) 휔 Viscosity index Subscripts 푎 Ablated 푏 Background gas 푑(퐵) Deposited (bottom of cavity) 푑(푊) Deposited (lateral wall) 푒 Exit/outflow 푔 Gas 푖푛 Inflow 푗 Cell indices 퐿 Laser 푟 Removed (in describing simulated particles, real) 푣 Vapor 푤 Bottom surface vii ACKNOWLEDGEMENTS This work is dedicated to all of the outstanding people that stood with me during my graduate studies. It is through your continued support, guidance and companionship that I am able to pursue my professional and personal goals by completion of this thesis. In particular, I would like to thank my advisor, Dr. Alexey Volkov, for the opportunity to join his group as an undergraduate student. This opportunity inspired my passion for research and led to my continued studies as a Master’s student, where the knowledge and skills I have developed during this journey extend well beyond what I ever thought possible. The experience I have gained during my graduate studies is invaluable, and will serve as building blocks for my next set of adventures and studies. To the other professors at the University of Alabama who took an active role in my development as a student, in particular Dr. MacPhee, Dr. Olcmen and Dr. Uddi who serve on my thesis committee, thank you. It is through your commitment and efforts to students that works spanning a wide range of topics, such as this one, are successful. To my research group, I thank you all for the collaborative efforts on projects and fun conversation over the years in the lab. I am grateful to have worked alongside such smart friends. Finally, to my friends and family that lent their support, time and advice during these challenging studies, I thank you. Without you all, my studies would not have been the same. This work was supported by the National Science Foundation (projects CMMI-1663364 and CMMI-1554589) and Electro Scientific Industries, Inc. (ESI). viii CONTENTS ABSTRACT .................................................................................................................................... ii LIST OF ABBREVIATIONS AND SYMBOLS .......................................................................... iv ACKNOWLEDGEMENTS ......................................................................................................... viii LIST OF TABLES ......................................................................................................................... xi LIST OF FIGURES ...................................................................................................................... xii CHAPTER 1 INTRODUCTION .................................................................................................... 1 1.1 Background and Motivation ................................................................................................. 1 1.2 Pulsed Laser Ablation and Laser-induced Plume Expansion Under Conditions of Spatial Confinement ................................................................................................................................ 5 1.3 Selective Laser Melting and Vapor Jet Expansion ............................................................. 12 1.4 Objectives and Outline ........................................................................................................ 17 CHAPTER 2 COMPUTATIONAL MODEL OF LASER-INDUCED PLUME EXPANSION IN CAVITIES AND TRENCHES ................................................................................................ 22 2.1 Introduction and Computational Setup ............................................................................... 22 2.2 Thermal Model of the Irradiated Target ............................................................................. 24 2.3 Kinetic Model of Plume Expansion .................................................................................... 28 2.4 Direct Simulation Monte Carlo (DSMC) Method .............................................................. 30 2.5 Coupling of Models and Boundary Conditions .................................................................. 34 ix 2.6 Simulation Parameters and Validation of the Computational Model ................................. 37 CHAPTER 3 LASER-INDUCED PLUME EXPANSION UNDER CONDITIONS OF SPATIAL CONFINEMENT ........................................................................................................ 43
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