Computational Fluid Dynamic Modelling of Laser Additive Manufacturing Process and Effect of Gravity
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COMPUTATIONAL FLUID DYNAMIC MODELLING OF LASER ADDITIVE MANUFACTURING PROCESS AND EFFECT OF GRAVITY A thesis submitted to The University of Manchester For the degree of Doctor of Philosophy (PhD) In the Faculty of Science and Engineering 2017 Heng Gu School of Mechanical, Aerospace and Civil Engineering List of contents List of contents List of contents ................................................................................................................ 1 List of figures ................................................................................................................... 6 List of tables ................................................................................................................... 12 Nomenclatures ............................................................................................................... 13 Acronyms ....................................................................................................................... 16 Abstract .......................................................................................................................... 17 Declaration ..................................................................................................................... 18 Copyright statement...................................................................................................... 19 Acknowledgement ......................................................................................................... 20 Chapter 1 Introduction ................................................................................................. 21 1.1 Research rationale ............................................................................................. 21 1.2 Aim and objectives of the project ..................................................................... 23 1.3 Thesis structure ................................................................................................. 24 Chapter 2 Literature review of laser additive manufacturing ................................. 27 2.1 Introduction ....................................................................................................... 27 2.2 Laser basics ....................................................................................................... 27 2.2.1 Spontaneous emission and stimulated emission .................................... 28 2.2.2 Population Inversion .............................................................................. 30 2.2.3 Laser Cavity ........................................................................................... 30 2.3 Laser beam characteristics ................................................................................ 30 2.3.1 Monochromaticity .................................................................................. 31 2.3.2 Coherence ............................................................................................... 31 2.3.3 Directionality.......................................................................................... 32 2.3.4 Brightness ............................................................................................... 32 2.4 Types of lasers................................................................................................... 32 2.4.1 Diode Lasers........................................................................................... 33 2.4.2 Fibre lasers ............................................................................................. 34 1 List of contents 2.4.3 Disk lasers .............................................................................................. 35 2.5 Laser additive manufacturing ............................................................................ 36 2.5.1 Pre-placed wire and powder ................................................................... 37 2.5.2 Powder injection..................................................................................... 38 2.5.3 Wire feeding ........................................................................................... 40 2.5.4 Combined wire and powder ................................................................... 41 2.6 Process Parameters ............................................................................................ 42 2.6.1 Laser power and specific energy ............................................................ 42 2.6.2 Process velocity ...................................................................................... 43 2.6.3 Gas-powder flow rate ............................................................................. 44 2.6.4 Overlap ................................................................................................... 45 2.6.5 Deposition pattern .................................................................................. 47 2.7 Mechanical and thermal behaviour ................................................................... 49 2.7.1 Stress ...................................................................................................... 49 2.7.2 Porosity .................................................................................................. 51 2.7.3 Cracking and distortion .......................................................................... 53 2.7.4 Microstructure ........................................................................................ 54 2.7.5 Surface unevenness ................................................................................ 56 2.7.6 Re-melting .............................................................................................. 60 2.8 Melt pool behaviour .......................................................................................... 62 2.8.1 Marangoni flow ...................................................................................... 62 2.8.2 Gravitational force ................................................................................. 64 2.8.3 Melting and solidification ...................................................................... 68 2.8.4 Overhang ................................................................................................ 72 2.9 Modelling .......................................................................................................... 75 2.9.1 Thermal-mechanical modelling ............................................................. 75 2.9.2 Multi-physical modelling ....................................................................... 80 2.10 Summary and discussion ................................................................................. 90 Chapter 3 Formulation of a CFD model for laser additive manufacturing process ......................................................................................................................................... 92 2 List of contents 3.1 Introduction ....................................................................................................... 92 3.2 Previous work ................................................................................................... 92 3.3 Governing equations ......................................................................................... 96 3.4 VOF model ........................................................................................................ 98 3.5 Surface tension .................................................................................................. 99 3.6 Melting and solidification ............................................................................... 101 3.7 Buoyancy force ............................................................................................... 101 3.8 Numerical implementations ............................................................................ 102 3.9 User-defined energy, mass and momentum sources ....................................... 103 3.9.1 Energy source ....................................................................................... 103 3.9.2 Mass source .......................................................................................... 107 3.9.3 Momentum source ................................................................................ 108 3.10 Boundary conditions ..................................................................................... 109 3.10.1 Internal boundary condition ............................................................... 110 3.10.2 External boundary condition .............................................................. 111 3.11 Temperature gradient and solidification rate ................................................ 111 3.12 Summary ....................................................................................................... 113 Chapter 4 Study and prediction of laser metal deposition with various structures ....................................................................................................................................... 114 4.1 Introduction ..................................................................................................... 114 4.2 Material ........................................................................................................... 114 4.3 Experimental equipment ................................................................................. 115 4.3.1 Laser applied ........................................................................................ 115 4.3.2 Powder delivery system