Principles of Laser Materials Processing
Description
Authoritative resource providing state-of-the-art coverage in the field of laser materials processing, supported with supplementary learning materials
Principles of Laser Materials Processing goes over the most recent advancements and applications in laser materials processing, with the second edition providing a welcome update to the successful first edition through updated content on the important fields within laser materials processing. The text includes solved example problems and problem sets suitable for the readers' further understanding of the technology explained.
Split into three parts, the text first introduces basic concepts of lasers, including the characteristics of lasers and the design of their components, to aid readers in their initial understanding of the technology. The text then reviews the engineering concepts that are needed to analyze the different processes. Finally, it delves into the background of laser materials and provides a state-of-the-art compilation of material in the major application areas, such as laser cutting and drilling, welding, surface modification, and forming, among many others. It also presents information on laser safety to prepare the reader for working in the industry sector and provide practicing engineers the updates needed to work safely and effectively.
In Principles of Laser Materials Processing, readers can expect to find specific information on:
* Laser generation principles, including basic atomic structure, atomic transitions, population distribution, absorption, and spontaneous emission
* Optical resonators, including standing waves in a rectangular cavity, planar resonators, beam modes, line selection, confocal resonators, and concentric resonators
* Laser pumping, including optical pumping, arc/flash lamp pumping, energy distribution in the active medium, and electrical pumping
* Broadening mechanisms, including line-shape functions, homogeneous broadening such as natural and collision, and inhomogeneous broadening
Principles of Laser Materials Processing is highly suitable for senior undergraduate and graduate students studying laser processing, and non-traditional manufacturing processes; it is also aimed at researchers to provide additional information to be used in research projects that are to be undertaken within the technology field.
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Dr. Elijah Kannatey-Asibu, JR., PhD, received his PhD from the University of California at Berkeley, USA, in 1980. He is now Professor Emeritus of Mechanical Engineering at the University of Michigan in Ann Arbor, where he was Professor until 2020. His research focused on multi-sensor monitoring of manufacturing processes, multiple-beam laser processing, acoustic emission investigation of manufacturing processes, and microfabrication using femtosecond lasers.
Content
PREFACE TO THE SECOND EDITION xxi
PREFACE TO THE FIRST EDITION xxiii
ABOUT THE COMPANION WEBSITE xxv
PART I PRINCIPLES OF INDUSTRIAL LASERS 1
1 Laser Background 3
1.1 Laser Generation 3
1.2 Optical Resonators 12
1.3 Laser Pumping 21
1.4 System Levels 24
1.5 Broadening Mechanisms 26
1.6 Beam Modification 29
1.7 Beam Characteristics 35
1.8 Summary 43
2 Types of Lasers 55
2.1 Solid-State Lasers 55
2.2 Gas Lasers 57
2.3 Semiconductor (Diode) Lasers 69
2.4 New Developments in Industrial Laser Technology 80
2.5 Summary 89
3 Beam Delivery 95
3.1 The Electromagnetic Spectrum 95
3.2 Birefringence 96
3.3 Brewster Angle 96
3.4 Polarization 98
3.5 Beam Expanders 101
3.6 Beam Splitters 102
3.7 Beam Delivery Systems 103
3.8 Beam Shaping 116
3.9 Summary 125
PART II ENGINEERING BACKGROUND 133
4 Heat and Fluid Flow 135
4.1 Energy Balance During Processing 135
4.2 Heat Flow in the Workpiece 136
4.3 Fluid Flow in Molten Pool 156
4.4 Summary 161
5 The Microstructure 175
5.1 Process Microstructure 175
5.2 Discontinuities 195
5.3 Summary 202
6 Solidification 209
6.1 Solidification Without Flow 209
6.2 Solidification with Flow 216
6.3 Rapid Solidification 221
6.4 Summary 222
7 Residual Stresses and Distortion 227
7.1 Causes of Residual Stresses 227
7.2 Basic Stress Analysis 232
7.3 Effects of Residual Stresses 237
7.4 Measurement of Residual Stresses 240
7.5 Relief of Residual Stresses and Distortion 250
7.6 Summary 252
PART III LASER MATERIALS PROCESSING 261
8 Background on Laser Processing 263
8.1 System-Related Parameters 263
8.2 Process Efficiency 272
8.3 Disturbances That Affect Process Quality 274
8.4 General Advantages and Disadvantages of Laser Processing 275
8.5 Summary 275
9 Laser Cutting and Drilling 279
9.1 Laser Cutting 279
9.2 Laser Drilling 308
9.3 New Developments 318
9.4 Summary 326
10 Laser Welding 335
10.1 Laser Welding Parameters 335
10.2 Welding Efficiency 344
10.3 Mechanism of Laser Welding 344
10.4 Material Considerations 355
10.5 Weldment Discontinuities 359
10.6 Advantages and Disadvantages of Laser Welding 360
10.7 Special Techniques 360
10.8 Specific Applications 371
10.9 Summary 382
11 Laser Surface Modification 391
11.1 Laser Surface Heat Treatment 391
11.2 Laser Surface Melting 413
11.3 Laser Direct Metal Deposition 414
11.4 Laser Physical Vapor Deposition (LPVD) 419
11.5 Laser Shock Peening 420
11.6 Laser Texturing 427
11.7 Summary 429
12 Laser Forming 437
12.1 Principle of Laser Forming 437
12.2 Process Parameters 439
12.3 Laser-Forming Mechanisms 439
12.4 Process Analysis 443
12.5 Advantages and Disadvantages 447
12.6 Applications 448
12.7 Summary 448
13 Additive Manufacturing 453
13.1 Computer-Aided Design 453
13.2 Part Building 462
13.3 Post-Processing 477
13.4 Applications 478
13.5 Advantages and Disadvantages 480
13.6 Summary 480
14 Medical and Nanotechnology Applications of Lasers 485
14.1 Medical Applications 485
14.2 Nanotechnology Applications 490
14.3 Summary 494
15 Sensors for Process Monitoring 497
15.1 Laser Beam Monitoring 497
15.2 Process Monitoring 504
15.3 Summary 522
16 Processing of Sensor Outputs 527
16.1 Signal Transformation 527
16.2 Data Reduction 532
16.3 Pattern Classification 534
16.4 Summary 550
17 Laser Safety 557
17.1 Laser Hazards 557
17.2 Laser Classification 562
17.3 Preventing Laser Accidents 563
17.4 Summary 569
Appendix 17.A 571
Problem 572
Bibliography 572
Index 573
1 Laser Background
1.1 LASER GENERATION
The term laser is an acronym for light amplification by stimulated emission of radiation, and thus a laser beam is a form of electromagnetic radiation. Light may be simply defined as electromagnetic radiation that is visible to the human eye. It has a wavelength range of about 0.37-0.75 µm, between ultraviolet and infrared radiation. Lasers, on the other hand, may have wavelengths ranging from 0.2 to 500 µm, that is, from X-ray to infrared radiation. In its simplest form, laser generation is the result of energy emission associated with the transition of an electron from a higher to a lower energy level or orbit within an atom.
Figure 1.1 illustrates the electromagnetic spectrum. The colors associated with the various wavelengths in the visible range are shown in Table 1.1.
1.1.1 Atomic Transitions
Under the right conditions, electrons within an atom can change their orbits. Light or energy (photon) is emitted as an electron moves from a higher level or outer orbit to a lower level or inner orbit and is absorbed when the reverse transition takes place. There is a specific quantum of energy (photon), ?E, of specific wavelength or frequency associated with each transition from one energy level to another and is given by:
(1.1)where c is the velocity of light = 3 × 108 (exactly 299,792,458) m/s, hp is Planck's constant = 6.625 × 10-34 J s, ? is the wavelength (m), ? is the frequency of transition between the energy levels (Hz), and ?E is the energy difference between the levels of interest.
1.1.1.1 Population Distribution
For simplicity, let us focus our initial discussion on a single frequency, which corresponds to two specific energy levels, E1 and E2, where E1 is the lower energy level and E2 is a higher energy level, that is, E2 > E1. Furthermore, we let the population or number of atoms (or molecules or ions) per unit volume at level 1 be N1, and that at level 2 be N2. We also assume conditions of non-degeneracy. Degeneracy exists when there is more than one level with the same energy.
Figure 1.1 The electromagnetic spectrum.
Table 1.1 Wavelengths associated with the visible spectrum.
Wavelength range (nm) Color 400-450 Violet 450-480 Blue 480-510 Blue-Green 510-550 Green 550-570 Yellow-Green 570-590 Yellow 590-630 Orange 630-700 RedUnder conditions of thermal equilibrium, the lower energy levels are more highly populated than the higher levels, and the distribution is given by Boltzmann's law that relates N1 and N2 as:
(1.2)where kB is Boltzmann's constant = 1.38 × 10-23 J/K and T is the absolute temperature (K).
This is illustrated in Figure 1.2a. Figure 1.2b illustrates the equilibrium distribution for the more general case. Boltzmann's law holds for thermal equilibrium conditions.
Figure 1.2 Schematic of Boltzmann's law. (a) Two-level system. (b) More general case for a multilevel system.
1.1.1.2 Absorption
When atoms in a ground state, E1, are excited or stimulated, that is, subjected to some external radiation or photon whose energy is the same as the energy difference between E1 and a higher state, E2, the atoms will change their energy level, as shown in Figure 1.3a. This process is called stimulated absorption. The rate at which energy is absorbed by the atoms is given by:
(1.3)where B12 is a proportionality constant referred to as the Einstein coefficient for stimulated absorption, or stimulated absorption probability per unit time per unit spectral energy density (m3 Hz/J s), N1 is the population of level 1 (per m3), e(?) is the energy density (energy per unit volume) at the frequency ?(J/m3 Hz), and nabs is the absorption rate (number of absorptions per unit volume per unit time).
Figure 1.3 Schematic of (a) absorption, (b) spontaneous emission, and (c) stimulated emission.
Once the atom has been excited to a higher energy level, it can make a subsequent transition to a lower energy level, accompanied by the emission of electromagnetic radiation. The emission process can occur by spontaneous emission or stimulated emission. Each absorption removes a photon, and each emission creates a photon.
1.1.1.3 Spontaneous Emission
Spontaneous emission occurs when transition from the excited to the lower energy level is not stimulated by any incident radiation (Figure 1.3b). The transition results in the emission of a photon of energy:
(1.4)where ? is the frequency of the emitted photon. In spontaneous emission, the rate of emission per unit volume, nsp, to the lower energy level is only proportional to the population, N2, at the higher energy level, and is independent of radiation energy density:
(1.5)where Ae is the Einstein coefficient for spontaneous emission, or spontaneous emission probability per unit time.
1.1.1.4 Stimulated Emission
If the atom in energy level 2 is subjected to electromagnetic radiation or photon of frequency ? corresponding to the energy difference ?E = E2 - E1 between levels 1 and 2, the photon will stimulate the atom to undergo a transition to the lower energy level. The energy emitted is the same as the stimulating photon and is superimposed on the incident photon, thereby reinforcing the emitted light (Figure 1.3c). This results in stimulated emission, where the incident and emitted photons have the same characteristics and are in phase, resulting in a high degree of coherence, and the direction, frequency, and state of polarization of the emitted photon are essentially the same as those of the incident photon. The two photons can generate yet another set, with a resulting avalanche of photons. This is illustrated schematically in Figure 1.4. The rate of emission per unit volume, nst, in the case of stimulated emission is given by:
(1.6)where B21 is the Einstein coefficient for stimulated emission, or stimulated emission probability per unit time per unit energy density (m3 Hz/J s).
Figure 1.4 Illustration of the process of stimulated emission.
Source: From Chryssolouris (1991)/Springer Nature.
1.1.1.5 Einstein Coefficients: , ,
Under conditions of thermal equilibrium, the rates of upward (E1 E2) and downward (E2 E1) transitions must be the same. Thus, we have
(1.7)or from Eqs. (1.3), (1.5), and (1.6),
(1.8)This gives the energy density as:
(1.9)This can be compared with the energy density from Planck's law on blackbody radiation:
(1.10)Equations (1.9) and (1.10) are equivalent only if
(1.11a)and
(1.11b)- (b) Repeat Example 1.1a for a transition frequency in the optical region of ? = 1015.
Solution:
indicating that spontaneous emission is then predominant, resulting in incoherent emission from normal light sources. In other words, under conditions of thermal equilibrium, stimulated emission in the optical range is very unlikely.
- (c) What will be the wavelength of the line spectrum resulting from the transition of an electron from an energy level of 40 × 10-20 J to a level of 15 × 10-20 J?
Solution:
From Eq. (1.1), we have
Therefore,
1.1.2 Lifetime
The lifetime, tsp, of atoms in an excited state is a measure of the time period over which spontaneous transition occurs. Strictly speaking, this is how long it takes for the number of atoms in the excited state to reduce to 1/e of the initial value. It can be shown that...
