Dawes Laser Welding

A Practical Guide
1. Auflage 1992
ISBN: 978-1-84569-884-3
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 272 Seiten

Reihe: Woodhead Publishing Series in Welding and Other Joining Technologies

Enables the reader both to understand and to use, in a practical manner, laser welding. The author explains the principles of laser welding and provides examples of industrial applications, examines many aspects of laser welding and devotes a complete chapter to safety.

Autoren/Hrsg.

Dawes, C T

Inhaltsverzeichnis

1;Front Cover;1
2;Laser Welding: A Practical Guide;4
3;Copyright Page;5
4;Table of Contents;6
5;Preface;6
6;Chapter 1. Light and lasers;9
6.1;Light;9
6.2;Lasers;10
6.3;Welding lasers;14
6.4;Nd:YAG lasers;15
6.5;CO2 lasers;18
6.6;Harnessing the laser beam for welding;21
6.7;References;23
7;Chapter 2. Principles of laser welding and its characteristics;24
7.1;Laser welding;24
7.2;Characteristics and considerations;26
7.3;Choosing laser welding;37
7.4;References;38
8;Chapter 3. Industrial applications;39
8.1;Electron guns for television tubes;39
8.2;Razor blades;41
8.3;Heart pacemakers;42
8.4;Opto-electronic transmitters;44
8.5;Food mixer whisks;45
8.6;Tappet housing;47
8.7;Gas hob distribution plates;48
8.8;Passenger car body parts;50
8.9;Seam welded stainless steel tubes;52
8.10;Stainless steel heat exchangers;54
8.11;Saw blades;55
8.12;Solenoid operated hydraulic valves;55
8.13;Viscous couplings and differentials;56
8.14;Specialist fabrication;57
8.15;References;58
9;Chapter 4. Materials;59
9.1;Weldability and alloying elements;61
9.2;Effects of surface coatings and treatments on the weldability of steel;73
9.3;Possible weld defects and solutions;80
9.4;References;85
10;Chapter 5. Laser welding parameters and their effects;86
10.1;Focused spot size, depth of focus and focus position;86
10.2;Shielding gases. shielding devices and plasma suppression;95
10.3;Laser power and welding speed;112
10.4;Material preparation prior to welding;120
10.5;Welding performance troubleshooting checklist;121
10.6;References;124
11;Chapter 6. Welding sheet metal parts;125
11.1;Joint configurations;125
11.2;Laser beam spinning;141
12;Chapter 7. Welding plate, tube and pre-machined parts;147
12.1;Welding conditions;147
12.2;Joint configurations;148
12.3;Joint fit-up and laser beam joint alignment;151
12.4;Joint tacking;153
12.5;Closing the keyhole ofcircumferential welds;154
12.6;Welding partial penetration joints;155
12.7;References;158
13;Chapter 8. Single and multi-pass welding with filler materials;159
13.1;Filler materials;159
13.2;Welding with continuous wire feed;160
13.3;Welding with pre-placed filler materials;169
13.4;References;169
14;Chapter 9. Process performance control and monitoring;170
14.1;Materials;171
14.2;Welding conditions;172
14.3;Periodic post-weld examination;177
14.4;Equipment performance monitoring;179
14.5;In-process weld monitoring;182
14.6;References;184
15;Chapter 10. Work handling, laser beam manipulation and process automation;185
15.1;Workpiece manipulators;185
15.2;Laser beam manipulation;189
15.3;Process automation;193
15.4;Workpiece clamping and weld underbead spatter channels;195
16;Chapter 11. Optical equipment;200
16.1;Focusing optics;200
16.2;Which focusing optic - reflective or transmissive?;207
16.3;Cleaning laser optics and in-process protection;209
16.4;Laser beam transmission systems;212
16.5;References;220
17;Chapter 12. Laser safety;221
17.1;Laser beam hazards;222
17.2;Electrical hazards;223
17.3;Chemical and fume hazards;223
17.4;Practical precautions;224
17.5;Further information on the safe use of lasers;230
18;Chapter 13. Installing and operating a laser - some questions and answers;231
18.1;The laser;232
18.2;Laser beam transmission and focusing equipment;235
18.3;Gas shielding and equipment;236
18.4;Workhandling equipment;237
18.5;Material preparation;238
18.6;Laser safety;238
18.7;Site preparation;239
18.8;Achieving repeatable weld quality;240
18.9;Operator and maintenance engineer training;241
18.10;Planning from concept to production;242
18.11;Epilogue;242
19;Chapter 14. Glossary of common terminology;245
20;Index;260

Leseproben

Chapter 1 Light and lasers
This chapter describes the release of energy from light waves; the difference between laser light and sunlight; the basic principle of how a laser works; the different types of lasers used for welding; and harnessing the laser beam to achieve a high power density. Light
The natural light by which we see is also the essential energy source by which our planet survives; it controls plant growth, the climate and the weather. The light rays from the sun, which travel at about 300 000 km/s and take approximately 8.5 minutes to arrive on the earth’s surface, impart their energy according to the sun’s different light wave lengths and the substances on which they fall. Naturally selected light wave lengths when absorbed by the molecules of certain substances excite the molecules causing them to vibrate and generate heat. A steel plate beside, or even placed behind a plain glass plate in the full sunlight, on a hot summer’s day, will absorb more energy from the light waves than the glass plate. The steel will become so hot that it will be uncomfortable to touch, whereas the glass will not be significantly warmer than the air through which the light rays passed. Glass absorbs less heat because it is transparent to most of the light wave lengths which make up the sun’s rays. The results of this comparison, however, can be changed if a specific wave length is artificially produced. Steel, for example, is partially transparent to certain X-rays. These artificial rays are the same type as light, but of much shorter wave length. The ability of the different light wave lengths to give up their energy in the form of heat when absorbed by different substances, coupled with the fact that light can be transmitted long distances and then be optically focused to a small spot and cause a massive increase in its power density (W/mm2), has led scientists and engineers to develop special light making machines called lasers. Lasers
Lasers produce a collimated and coherent beam of light (coherent: waves of one wave length all in phase). This light is quite different from the incoherent light of the sun, Fig. 1.1, which radiates in all directions from its source. The almost parallel, single wave length, light rays which make up the collimated laser beam have a considerably higher power density and can be focused to a much smaller spot size than the randomly radiated rays of the sun. Consequently, a much more efficient power density is achieved with a laser. 1.1 Coherent and incoherent light: a) The random light waves from the sun are incoherent because they cannot achieve temporal and spatial symmetry, b) The coherent light waves from a laser. The word LASER is an acronym, it stands for: (L) light (A) amplification (S) stimulated by the (E) emission of (R) radiation, and refers to the way in which the light is generated. The basic principle of how a laser works is presented in the following paragraph. Those seeking a more detailed scientific explanation are recommended to read the books by Koechner1 and Duley.2 All lasers are optical amplifiers which work by pumping (exciting) an active medium placed between two mirrors, one of which is partially transmitting, Fig. 1.2. The active medium is a collection of specially selected atoms, molecules or ions which can be in a gas, liquid or solid form and which will lase, i.e. emit radiation as light waves (referred to as photons) when excited by the pumping action. Pumping of liquids and solids is achieved by flooding them with light from a flash lamp and gases are pumped by applying an electrical discharge. 1.2 The basic elements of a laser. The term photon is used instead of light wave when describing the production of laser light, because the photon carries with it a precisely defined amount of energy in relation to its wave length. Whatever the active medium consists of: atoms, molecules or ions there are billions of them and they absorb energy when pumped, which they hold for a very short but random life time. When their life time expires they give up their energy in the form of a photon and return to their former state until pumped again. The release of photons in this manner is called spontaneous emission. The photons released travel in all directions in relation to the optical axis of the laser, Fig. 1.3. If a photon collides with another energised atom, etc, it causes it to release its photon prematurely and the two photons will travel along in phase until the next collision, thus building a stream of photons of increasing density, Fig. 1.4. This action of releasing a photon prematurely is called stimulated emission. Photons which do not travel parallel to the optical axis of the laser are quickly lost from the system. Those which do travel parallel to the axis have their path length considerably extended by the optical feed back provided by the mirrors, before leaving the laser, through the partially transmitting mirror, Fig. 1.5. This action not only serves as an amplifier for photon generation by stimulated emission to achieve the required power level, but also to provide the highly collimated coherent light beam that makes the laser so useful. 1.3 Spontaneous emission of photons from the excited active medium. There are billions of excited atoms, molecules and ions and they release their photons in all directions. 1.4 Stimulated emission of photons. A photon (a) which collides with an excited atom, etc (b) will cause it to release its photon before spontaneous emission can occur and thus two photons (c) will travel on in phase until the next collision (d). By stimulated emission photons which travel parallel to the optical axis build a powerful laser beam. 1.5 Optical feedback of photons by mirrors to increase the path length for stimulated emission and thus amplify the laser power. The power density across the diameter of a laser output beam is not uniform and is dependent on the laser's active medium, its internal dimensions, optical feed back design and the excitation system employed. The transverse cross sectional profile of a laser beam, which shows its power distribution, is called the 'transverse electromagnetic mode' (TEM). Many different TEMs can be designed for and each type is rated by a number. In general, the higher the number the more difficult it is to focus the laser beam to a fine spot to achieve a high power density. Because the latter is paramount when laser welding, lasers with TEM00, TEM01*, TEM10, TEM11*, TEM20 and combinations of these modes are often used. Figure 1.6 shows the basic shape of the beam power profiles of these modes. Some lasers produce several different modes and these are usually referred to as having a multi-mode operation. 1.6 Basic beam modes produced by different lasers; some lasers have combinations of these modes. Welding lasers
The present welding lasers, which can also be used for cutting and surfacing materials, use active mediums which are either in the form of a solid or a gas. Consequently, the two types are referred to as 'solid-state lasers' and 'gas lasers'. Three main types of solid-state laser have been developed: the ruby, neodymium glass and the neodymium yttrium aluminium garnet (Nd:YAG). The Nd:YAG, which has an output wave length of 1.06 µm, has practically replaced the other two types where high production welding is required, as it can achieve higher powers for longer periods, without overheating and thus degrading its performance. Therefore, with respect to solid-state lasers this book will concentrate on the Nd:YAG laser. The gas lasers which are used for welding are currently all 10.6 µm wave length carbon dioxide (CO2) lasers since they have proved to be the most efficient and produce the highest power. However, carbon monoxide (CO) lasers which have a shorter wave length, 5.3 µm, are under development and evaluation. Nd:YAG lasers
Nd:YAG lasers with output powers (average) ranging from approximately 100 W to over 1 kW are commercially available; higher power machines of 2 to 3 kW are being developed. A 1 kW Nd:YAG welding laser and its power supply is shown in Fig. 1.7. Although the average powers developed by Nd:YAG lasers are low when compared with CO2 lasers, they can achieve pulses with peak powers in the order of 10 kW. Performance-wise a 150 W Nd:YAG laser will butt weld 0.5 mm thick steel at 0.3 m/min and a 1 kW machine will easily butt weld 4 mm at the same speed; typical welding performance figures are presented in Chapter 5. 1.7 A1 kW Nd:YAG laser. The beam is transmitted from the laser via an optical fibre to the laser gun, centre foreground. Behind the laser and operator are the laser power supply cabinets. Perhaps the most important feature of the Nd:YAG laser is that the output wave length (1.06 µm) of its beam can be transmitted through a fibre optic cable: beam transmission systems are discussed in Chapter 11. This aspect makes the Nd:YAG laser extremely attractive for high speed welding production and automation, which are discussed in Chapter 10. However, special safety precautions (Chapter 12) have to be taken due to the short wave length. A schematic layout of an Nd:YAG laser is shown in...