1 General Problems of Propagation of Laser Radiation
in Gases and Plasma and Physical Processes on the
Surface of Condensed Media .................................. 1
1.1 Propagation and Focusing of Radiation in Vacuum,
Gases and Plasma ....................................... 2
1.1.1 Focusing of Light in Vacuum ..................... 2
1.1.2 Propagation of Laser Radiation in Gases and
Plasma .......................................... 4
1.2 Absorption, Reflection, and Propagation of Radiation
in Cavities in Condensed Media ......................... 8
1.2.1 Flat Surface .................................... 8
1.2.2 Propagation of Laser Radiation in a Narrow
Channel in a Metal ............................. 12
1.2.3 Waveguide Radiation Propagation Regime ......... 15
1.2.4 Propagation of Plane-Polarized Radiation
in a Cylindrical Keyhole ....................... 17
1.3 Physical Processes on the Surface of Condensed
Media: The Interaction of Vapor with the Surrounding
Gas ................................................... 20
1.3.1 Melting ........................................ 20
1.3.2 Vaporization ................................... 21
1.3.3 Melting-Solidification Dynamics Taking
Vaporization into Account ...................... 22
1.3.4 Stationary Interaction of a Vapor Jet with
the Surrounding Gas ............................ 25
1.4 Vaporization Kinetics and Hydrodynamics ............... 27
1.4.1 Condensation ................................... 33
1.5 Instability of the Laser-Induced Vaporization
of Condensed Media .................................... 36
References ................................................. 43
2 Mechanisms of Laser Processing of Metal Surfaces ........... 45
2.1 Thermal Model of Laser Hardening of the Steel
Surface ............................................... 45
2.1.1 Qualitative Consideration of the Stationary
Thermal Model of Metal Hardening ............... 46
2.1.2 Comparison with Experiments .................... 49
2.1.3 Numerical Calculations ......................... 52
2.1.4 Processing of Metal Surfaces by the
Oscillating Beam of a CO2 Laser ................ 54
2.2 Hydrodynamical Models of Laser-Induced Alloying
of Metal Surfaces ..................................... 57
2.2.1 Analysis of Experimental Data .................. 58
2.2.2 Theoretical Consideration of Melt Motion
During Alloying of Metals ...................... 61
2.2.3 Analytic Consideration of Liquid Metal Motion
Caused by Thermocapilary Forces ................ 62
2.2.4 Numerical Modelling of a Melt Flow During
Alloying ....................................... 64
2.2.5 Nonlinear Effects and the Instability of the
Melt Surface Shape in the Marangoni Flow ....... 68
2.2.6 Development of the Multi-Vortex Structure
of the Melt Flow ............................... 74
2.2.7 Influence of Surfactants on Heat-and-Mass
Transfer During Laser Alloying ................. 78
2.2.8 Mass-Transfer Kinetics During Gas-Phase
Alloying ....................................... 80
2.2.9 Alloying of a Moving Sample Surface by
Stationary Laser Radiation ..................... 84
2.2.10 Melt Flow Upon Pulsed and Repetitively Pulses
Irradiation .................................... 89
2.2.11 Thermocapillary Processes in the Dynamics
of Gas Bubbles in a Melt Pool .................. 91
2.3 Physical Mechanisms of Cladding ....................... 94
2.4 Mechanisms of Laser-Induced Surface Cleaning ......... 108
2.4.1 Cleaning of Surfaces from Microparticles ...... 109
2.4.2 Laser-Induced Solid Surface Cleaning from
Films ......................................... 111
2.4.3 Physical Model of Water Surface Cleaning
from Thin Films of Petroleum Products ......... 114
2.4.4 Laser-Induced Metal Surface Cleaning from
Radionuclides ................................. 118
2.5 Modelling of Selective Laser Melting ................. 122
2.5.1 Structures .................................... 122
2.5.2 Heat Conduction of Powders in Vacuum .......... 123
2.5.3 Calculation of Thermal and Optical Constants
of Initial Materials .......................... 126
2.5.4 Volume and Surface Absorption Coefficients .... 127
2.5.5 Powder Mixtures ............................... 130
2.5.6 Thermal Model of Selective Laser Sintering .... 132
2.5.7 Instability of Selective Laser Melting ........ 135
2.5.8 Thermal Hydrodynamic Model of Selective
Laser Sintering ............................... 136
References ................................................ 139
3 Plasma Phenomena in Laser Processing of Materials ......... 145
3.1 Thermal Properties of the Plasma of Noble and
Molecular Gases and Metal Vapors ..................... 145
3.1.1 Plasma Emission ............................... 149
3.2 Mechanisms of the cw Laser-Induced Breakdown of
Gases Near Solid Surfaces ............................ 150
3.2.1 Stationary Breakdown of Gases in the Absence
of a Target ................................... 150
3.2.2 Nonequilibrium Mechanism of Optical
Breakdown in Gases Near a Target .............. 155
3.2.3 Thermal Model of Optical Breakdown in Gases
Near a Target ................................. 162
3.2.4 Theoretical Model ............................. 163
3.2.5 Numerical Calculation of the Thermal Model .... 166
3.2.6 Optical Breakdown of Chemically Active Gases
Near a Target ................................. 168
3.2.7 Optical Breakdown During Laser Welding ........ 169
3.3 The Numerical Model of an Erosion Plume During
Welding .............................................. 172
3.4 Optical Discharge Burning Near a Sample Surface ...... 173
3.4.1 Theoretical Models of a Continuous Optical
Discharge ..................................... 174
3.4.2 The Heat-Conduction COD Model ................. 177
3.4.3 COD Model Taking into Account the Heat
Conduction and Emission of Plasma ............. 179
3.4.4 Numerical Calculations of Optical Discharge
Parameters .................................... 182
3.4.5 The Radiative-Conductive COD Model ............ 186
3.5 LCWs and a COD in a Gas Flow ......................... 187
3.5.1 Light Combustion Wave ......................... 188
3.5.2 Combustion Wave Supported Due to Thermal
Radiation Transfer ............................ 191
3.5.3 Continuous Optical Discharge in a Gas Flow .... 192
3.5.4 Optical Discharge in a Gas-Vapor Keyhole ...... 202
3.6 Laser Plasmatron and Deposition of Films ............. 204
3.6.1 Physical Processes in Optical Plasmatrons ..... 204
3.6.2 High-Pressure Plasmatron ...................... 206
References ................................................ 208
4 Properties and Mechanisms of Deep Melting of Materials
by a cw Laser Beam ........................................ 211
4.1 Physical Processes Proceeding Upon Deep Melting
of Fixed Samples ..................................... 213
4.1.1 The Thermal Deep-Melting Model ................ 213
4.1.2 Mechanical Limit of Laser Beam Penetration
into Liquid ................................... 215
4.1.3 Peculiarities of Deep Laser Beam Penetration
into Liquid ................................... 221
4.2 Thermal Deep Penetration Melting Model for a Moving
Sample ............................................... 224
4.2.1 Physical Processes in Welding of Materials .... 224
4.2.2 Deep Melting of Various Materials ............. 229
4.2.3 Thermal Efficiency of Laser Welding ........... 231
4.3 Hydrodynamical Processes During Deep Laser-Beam
Penetration into Solids .............................. 237
4.3.1 Experimental Study of Material Melt Flows ..... 237
4.3.2 Models of the Hydrodynamic Flow Upon Deep
Melting ....................................... 243
4.3.3 Influence of Laser Radiation Polarization
and Shield Gas on Laser Welding Properties .... 245
4.3.4 Role of Shield Gases in Deep Melting of
Metals ........................................ 248
4.4 Models of a Gas-Vapor Keyhole of Finite Size ......... 252
4.4.1 Thermal Deep-Melting Model with a Gas-Vapor
Keyhole of Finite Diameter .................... 252
4.4.2 Self-Consistent Stationary Laser Welding
Model ......................................... 254
4.4.3 Stability of a Cylindrical Gas-Vapor
Keyhole ....................................... 261
4.4.4 Instability of the Leading Edge of
a Keyhole ..................................... 267
4.4.5 Melt Pool Instability ......................... 269
4.5 Remote and Hybrid Welding of Metals .................. 270
4.5.1 Features of Laser-Arc Welding of Metals ....... 270
4.5.2 Remote Welding of Metals ...................... 274
4.5.3 Influence of Laser Radiation Quality on
Laser Welding ................................. 276
References ................................................ 283
5 Physics of Remote and Gas-Assisted Cutting with Lasers .... 287
5.1 Mechanism of Remote Cutting with cw Lasers ........... 288
5.1.1 Physics of Melt Removal in Drilling of
Vertical Plates ............................... 288
5.1.2 Drilling of Horizontal Plates ................. 292
5.1.3 Self-Consistent Drilling Model ................ 293
5.1.4 Thermally Thick Limit ......................... 294
5.1.5 Remote Cutting ................................ 296
5.1.6 Experimental Techniques and Results ........... 298
5.1.7 Oscillatory Type of Remote Cutting ............ 300
5.1.8 Comparison of Calculated and Experimental
Results ....................................... 301
5.1.9 Disruption of Cutting Operation ............... 303
5.2 Properties of Gas-Assisted Cutting ................... 304
5.2.1 Gas Dynamics in Laser Cutting ................. 305
5.2.2 Numerical Studies of Gas Dynamics ............. 309
5.2.3 Mechanisms of Melt Removal .................... 313
5.2.4 Instabilities and Nonstationary Mechanisms
of Melt Removal ............................... 317
5.2.5 Modelling of Melting Front and Melt Removal
in Gas-Assisted Cutting of Metals ............. 320
5.2.6 Properties and Efficiency of Gas-Assisted
Cutting ....................................... 325
5.2.7 Beam Polarization ............................. 329
5.2.8 Multiple Reflections .......................... 335
5.3 Physical Processes in Laser Cutting with an Oxygen
Jet .................................................. 336
5.3.1 Model of Stationary Cutting of Steel in an
Oxygen Jet .................................... 339
5.3.2 Instability of Laser Cutting in the Oxygen
Atmosphere .................................... 340
5.3.3 Experimental Studies of High-Quality Laser
6 Cutting of Thick Mild Steels with Oxygen Assist Gas ....... 342
References ................................................ 343
Interaction of Pulsed Laser Radiation with Materials ...... 345
6.1 Physics of Pulsed Laser Ablation and Deposition of
Films ................................................ 346
6.1.1 Initial Stage ................................. 348
6.1.2 Ablation to Vacuum ............................ 352
6.1.3 Ablation to Buffer Gas ........................ 353
6.1.4 Comparison with Experiments ................... 355
6.1.5 Ablation Efficiency ........................... 361
6.1.6 Ablation of Materials Irradiated by
Ultrashort Laser Pulses ....................... 363
6.2 Modelling of Synthesis of Nanoparticles Upon
Pulse Laser Vaporization ............................. 365
6.2.1 Diffusion Model ............................... 367
6.2.2 Results and Discussion ........................ 370
6.2.3 Erosion Jet ................................... 373
References ................................................ 377
7 Pulsed Surface Plasma ..................................... 379
7.1 Pulsed Optical Breakdown Near a Surface .............. 379
7.1.1 Nonstationary Thermal Breakdown ............... 380
7.1.2 Quasi-Stationary Breakdown .................... 381
7.1.3 Optical Breakdown in a Target Vapor Jet ....... 382
7.1.4 Two-Dimensional and Nonequilibrium Effects
in the Pulsed Breakdown ....................... 385
7.2 Nonequilibrium Mechanisms of the Pulsed Breakdown .... 388
7.3 Dynamics of a Plasma Plume and its Interaction
with a Laser Beam .................................... 395
7.3.1 Propagation Mechanisms of the Surface
Plasma ........................................ 396
7.3.2 Propagation of a Laser-Supported Detonation
Wave in the Surrounding Gas ................... 399
7.3.3 Reflecting Properties of a Plasma Plume ....... 406
7.3.4 Numerical Modelling of a Pulsed Optical
Discharge ..................................... 407
7.3.5 Modeling Results .............................. 408
7.3.6 Expansion Mechanisms of Plasmas ............... 409
7.3.7 Plasma Transparency and Transmission
Coefficient ................................... 413
7.3.8 Comparison with Experiments ................... 415
7.4 Plasma Processes in Material Vapors .................. 418
7.4.1 Plasma Processes on a Target Surface .......... 420
7.4.2 Plasma Processes During Vaporization of
Metals in Air ................................. 424
7.4.3 Plasma Phenomena During the Deep Penetration
of a Laser Beam into a Sample and Breakdown
on Microdroplets .............................. 427
References ................................................ 432
8 Physics of the Damage and Deep Melting of Metals by
Laser Pulses .............................................. 435
8.1 Qualitative Hydrodynamical Model of Laser-Induced
Melt Removal ......................................... 435
8.1.1 Removal on a Melt from a Shallow Pool ......... 436
8.1.2 Fountain Wave Regime .......................... 437
8.1.3 Liquid Splash Regime .......................... 439
8.1.4 Specific Damage Energy ........................ 441
8.1.5 Numerical Modelling of Metal Removal from
a Shallow Melt Pool ........................... 443
8.2 Experimental Studies of the Interaction of
Millisecond Laser Pulses with Materials .............. 447
8.2.1 Experimental Study of Shallow Damage of
Materials ..................................... 447
8.2.2 Deep-Penetration Keyhole Damage by a Single
Pulse from a Neodymium Laser .................. 450
8.3 Damage of Materials by Microsecond and Ultrashort
Laser Pulses ......................................... 451
8.3.1 Experimental Studies of the Damage of Metals
by Pulsed СОг Laser Radiation ................. 451
8.3.2 Material Processing by Ultrashort Pulses ...... 454
8.3.3 Theoretical Models of Formation of Deep
Keyholes in Metals by C02 Laser Radiation ..... 458
8.3.4 Waveguide Regime .............................. 460
8.4 Physics of Deep Melting of Metals by Pulsed
Radiation ............................................ 463
8.4.1 Pulsed Welding ................................ 463
8.4.2 Control of the Deep Penetration Melting
Process ....................................... 466
References ................................................ 468
9 Interaction of Repetitively Pulsed Laser Radiation
with Materials ............................................ 471
9.1 Modeling of Thermal Processes During Repetitively
Pulsed Irradiation of a Sample Surface ............... 472
9.1.1 Features of Thermal Processes and Phase
Transitions During Repetitively Pulsed Laser
Irradiation ................................... 473
9.1.2 Thermal Model of Metal-Surface Hardening
by Repetitively Pulsed Laser Radiation ........ 477
9.2 Thermal Model of Deep Melting of Metals by
Repetitively Pulsed Laser Radiation with Low Off-
Duty Ratio ........................................... 480
9.2.1 Thermal Model of Deep Melting of Moving
Samples by Repetitively Pulsed Radiation ...... 480
9.2.2 Thermal Model of Metal Welding with a Pulsed
Laser with Low Off-Duty Ratio ................. 482
9.3 Physical Processes During Welding of Metals by
Repetitively Pulsed Laser Radiation with High
Off-Duty Ratio ....................................... 487
9.3.1 Theoretical Model ............................. 487
9.3.2 Experimental Studies .......................... 493
9.3.3 Dynamics of a Weld Pool Upon Repetitively
Pulsed Irradiation ............................ 495
9.4 Drilling and Cutting of Metals by Repetitively
Pulsed Radiation ..................................... 501
9.4.1 Properties and Mechanism of Metal Cutting
by Repetitively Pulsed CO2 Laser Radiation .... 501
9.4.2 Gas Assisted Laser Cutting of Metals by
Repetitively Pulsed Radiation ................. 504
9.4.3 Modelling of the Instability of Deep Laser-
Beam Penetration into a Moving Target ......... 506
9.5 Damage and Remote Cutting of Metals by
a Repetitively Pulsed Laser .......................... 509
9.5.1 Formulation of the Problem .................... 509
9.5.2 Experimental Results .......................... 510
9.5.3 Numerical Model ............................... 512
9.5.4 Comparison of Numerical Calculations with
Experiment .................................... 515
9.5.5 Remote Damage of Metals by Radiation from
High-Average-Power Lasers ..................... 518
9.5.6 Remote Cutting Model for Thick Plates ......... 519
9.5.7 Thin Plates ................................... 523
References ................................................ 526
Index ........................................................ 529
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