10. Laser Beam Welding 2003 10. Laser Beam Welding 129 The term laser is the abbreviation for 1917 postulate of stimulated emission by Einstein ,,Light Amplification by Stimulated 1950 work out of physical basics and realisation of a maser (Microwave Amplification by Stimulated Emission of Radiation”. The laser is Emission of Radiation) by Towens, Prokhorov, Basov 1954 construction of the first maser the further development of the maser 1960 construction of the first ruby laser (Light Amplification (m=microwave), Figure 10.1. Al- by Stimulated Emission of Radiation) 1961 manufacturing of the first HeNe lasers and Nd: glass lasers though the principle of the stimulated 1962 development of the first semiconductor lasers emission and the quantum- 1964 nobel price for Towens, Prokhorov and Basov for their works in the field of masers mechanical fundamentals have al- construction of the first Nd:YAG solid state lasers and CO gas lasers 2 ready been postulated by Einstein in 1966 established laser emission on organic dyes the beginning of the 20th century, the since increased application of CO2 and solid state laser 1970 technologies in industry first laser - a ruby laser - was not 1975 first applications of laser beam cutting in sheet fabrication industry implemented until 1960 in the Hughes 1983 introduction into the market of 1-kW-CO lasers 2 Research Laboratories. Until then 1984 first applications of laser beam welding in industrial serial production numerous tests on materials had to br-er10-01e.cdr be carried out in order to gain a more History of the Laser precise knowledge about the atomic structure. The following years had Figure 10.1 been characterised by a fast devel- opment of the laser technology. Already since the beginning of the Seventies and, increasingly since the Eighties when the first high-performance lasers were available, CO2 and solid state lasers have been used for production metal working. The number of the annual sales of la- 3 ser beam sources 109 € has constantly in- 2 creased in the 1.5 course of the last 1 few years, Figure 0.5 10.2. 0 1986 1988 1990 1992 1994 1996 1998 2000 Japan and South East Asia The application ar- North America West Europe br-er10-02e.cdr eas for the laser beam sources sold Figure 10.2 10. Laser Beam Welding 130 in 1994 are shown in Figure 10.3. The main application areas of the laser in the field of production metal working are joining and cutting jobs. The availability of more efficient laser drilling welding 1,8% 18,7% beam sources opens up new ap- others inscribe 9,3% 20,5% plication possibili- ties and - guided by financial con- siderations - makes cutting micro 44,3% electronics the use of the laser 5,4% also more attrac- br-er10-03e.cdr tive, Figure 10.4. Figure 10.3 Figure 10.5 shows the characteristic properties of the laser beam. By reason of the induced or stimu- 40 lated emission the kW CO radiation is coher- 20 2 10 ent and mono- r 5 chromatic. As the e w o 4 p divergence is only r e s 3 a l 1/10 mrad, long 2 Nd:YAG 1 transmission paths diode laser 0 without significant 0 5 0 5 0 5 0 7 7 8 8 9 9 0 9 9 9 9 9 9 0 1 1 1 1 1 1 2 beam divergences br-er10-04e.cdr are possible. Figure 10.4 10. Laser Beam Welding 131 Inside the resonator, Figure 10.6, the laser-active medium (gas molecules, ions) is excited to a higher energy level (“pumping”) by energy input (electrical gas dis- charge, flash lamps). During retreat to a lower level, the energy is released in the form of a light quantum (photon). The wave length depends on the energy difference between both excited states and is thus a characteristic for the respective laser-active medium. A distinction is made between spontaneous and induced transition. While the spon- taneous emission is non-directional and in coherent (e.g. in fluorescent tubes) is a laser beam gener- light bulb Laser ated by induced induced emission emission when a E particle with a 2 higher energy level 0, E1 46" exited ground state state is hit by a photon. 0 ,9 4" The resulting pho- polychromatic monochromatic ton has the same (multiple wave length) coherent incoherent (in phase) properties (fre- (not in phase) small divergence large divergence quency, direction, br-er10-05e.cdr © ISF 2002 phase) as the excit- Characteristics of Laser Beams ing photon (“coher- ence”). In order to Figure 10.5 maintain the ratio of the desired induced resonator energy source emission I sponta- neous emission as high as possible, m a active laser medium e the upper energy b r e s a level must be con- l stantly over- crowded, in com- fully reflecting partially reflecting mirror mirror parison with the R = 100% energy source R < 100% br-er10-06e.cdr © ISF 2002 lower one, the so- Laser Principle called “laser- Figure 10.6 10. Laser Beam Welding 132 inversion”. As result, a stationary light wave is formed between the mirrors of the resonator (one of which is semi-reflecting) causing parts of the excited laser-active medium to emit light. In the field of production metal working, and particularly in welding, especially CO2 and Nd:YAG lasers are applied for their high power outputs. At present, the devel- opment of diode lasers is so far advanced that their sporadic use in the field of mate- rial processing is also possible. The industrial standard powers for CO2 lasers are, nowadays, approximately 5 - 20 kW, lasers with powers of up to 40 kW are available. In the field of solid state lasers average output powers of up to 4 kW are nowadays obtainable. In the case of the 0,6 thrust of second type 2 002 CO2 laser, Figure eV transition without transmission of emission 10.7, where the vibration energy 0,4 y g r resonator is filled e n thrust of first type e 0,3 0,288 eV 1 001 0,290 eV DE = 0,002 eV LASER l = 10,6 µm with a N2-C02-He 0,2 100 gas mixture, pump- discharge through 0,1 thrust with helium ing is carried out 0 0 000 over the vibrational N2 CO2 br-er10-07e.cdr © ISF 2002 excitation of nitro- Energy Diagram of CO Laser 2 gen molecules Figure 10.7 which again, with thrusts of the sec- radio frequency high voltage exitaion ond type, transfer their vibrational laser beam energy to the car- bon dioxide. During cooling water cooling water the transition to the lower energy level, laser gas CO2 molecules laser gas: CO2: 5 l/h vakuum pump He: 100 l/h emit a radiation N2 : 45 l/h gas circulation pump with a wavelength br-er10-08e.cdr of 10.6 µm. The helium atoms, fi- Figure 10.8 10. Laser Beam Welding 133 nally, lead the CO2 Cooling water molecules back to laser gas: CO2: 11 l/h He: 142 l/h turning their energy level. N: 130 l/h mirrors 2 gas circulation pump laser beam The efficiency of up mirror (partially reflecting) to 15%, which is gas discharge achievable with CO2 high perform- laser gas end mirror ance lasers, is, in cooling water comparison with br-er10-09e.cdr other laser sys- tems, relatively Figure 10.9 high. The high dis- sipation component is the heat which must be discharged from the resonator. This is achieved by means of the constant gas mixture circulation and cooling by heat exchangers. In dependence of the type of gas transport, laser systems are classified into longitudi- nal-flow and transverse-flow laser systems, Figures 10.8 and 10.9. With transverse-flow laser systems of a compact design can the multiple folding ability of the beam reach higher output powers than those achievable with longitudi- nal-flow systems, the beam quality, however, is worse. In d.c.-excited systems (high voltage), the f2,57" electrodes are po- d0 sitioned inside the QF unfocussed beam focussed beam resonator. The in- dF teraction between the electrode mate- rial and the gas molecules causes K = 2l 1 p d.Q 0<K<1 ss electrode burn-off. with dd.Q=.Q=Qd. =c. ss00FF In addition to the br-er10-10e.cdr © ISF 2002 wear of the elec- Laser Beam Qualitiy trodes, the burn-off Figure 10.10 10. Laser Beam Welding 134 also entails a contamination of the laser gas. Parts of the gas mixture must be there- fore exchanged permanently. In high-frequency a.c.-excited systems the electrodes are positioned outside the gas discharge tube where the electrical energy is capacitively coupled. High electrode lives and high achievable pulse frequencies characterise this shutter beam divergence mirror kind of excitation resonator partially reflecting principle. In diffu- end beam transmission mirror mirror absorber tube sion-cooled CO2 LASER systems beams of a high quality are generated in a beam creation focussing system minimum of space. work piece Moreover, gas ex- work piece manipulator change is hardly br-er10-11e.cdr ever necessary. Figure 10.11 The intensity distribution is not constant across the laser beam. The intensity distribu- tion in the case of the ideal beam is described by TEM modes (transversal elec- tronic-magnetic). In the Gaussian or basic mode TEM00 is the peak energy in the centre of the beam weakening towards its periphery, similar to the Gaussian normal distribution. In practice, the quality of a laser beam is, in accordance with DIN EN 11146, distinguished by the non- dimensional beam quality factor (or propagation factor) K (0...1), Figure 10.10.