- Email: [email protected]
IR lasers as tools for the future
ln/?ared Phys. Technol. Vol. 36, No. I, pp. 401~,06, 1995
Pergamon
Copyright ~ 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 1350-4495/95 $9.50 ~- 0.00
1350-4495(94)00103-0
IR
LASERS
AS TOOLS
GERD HERZIGER and
FOR
THE
FUTURE
R O L F SCHLOMS
F r a u n h o f e r - l n s t i t u t ffir Lasertechnik, Steinbachstrasse 15, D - 5 2 0 7 4 Aachen, G e r m a n y
(Received 2 August 1994)
I. I N T R O D U C T I O N In laser production technology the title of this paper is more a question than a statement. After 30 years of development laser technology has penetrated nearly all fields of economic significance, but compared to the world market of production technology it covers still niches. On the time scale of machine tool development laser technology is a young technology, and significance and dynamics of laser production technology is influenced by parameters, like • • • •
global economy, dissimination of know how, development of established laser processes, development of new processes.
The paper deals with general trends of these paramaters of significance, and indicates that laser technology is one of the key elements of flexible manufacturing and therefore a tool for the future. I1. T H E LASER M A R K E T Looking through the annual Review and Forecast of Laser Markets 19XX in the journal Laser Focus Worm it can be seen that world laser market is strongly growing since 1975 up to a volume of over one billion U.S. $ (Fig. 1). Having in mind that laser technology is a key technology, that means the costs for a laser based system are an order of magnitude higher than the laser source itself, it is comprehensible that at the beginning of the next century the laser system market will reach a volume of 50 billion U.S. $.1'~ Approximately one-third of this market is dedicated to laser sources for materials processing, which essentially means IR lasers like CO2-, Nd : YAG-, or diode lasers (Fig. 2). The prediction of the laser market for the next century results in a clear answer to the above-mentioned question. Unfortunately this prediction was done in 1991 on the basis of an extrapolation of the market data. Due to the onset of the world wide recession in 1990 there has been a decline of the market which makes an extrapolation of the market data of a young technology difficult. Nevertheless there are indications that exponential growth is only postponed. • Compared to the machine tool industry, which has especially in Japan and Europe a drop in turnover of approx. 30-60%, the negative development of the laser market has to be evaluated as moderate. • In terms of units the world laser market of the most expensive CO:-, N d : Y A G lasers with output powers higher than 500 W is still growing. 1:3~ • The recession shows the need of technology to open new markets and compete world wide. The market data also allow an additional conclusion. The geographical distribution of the world laser market shows that 25% of this market is located in Japan. ~3~This concentration is correlated JNV ~,~ AA
401
402
GERD
HERZIGER
and
ROLF SCHLOMS
2000 --
//,y/ ///
1000
//
7,7 //
// // //
~// / / 500
// //
///
(/" ~ / /// //, ///
200
100
~.. ;//
y~
;)¢
//
;d<
//
~d
50
79 80 81
~// / / ~// // // // //
// //
//
// // //
//
// //
//
// //
// //
75 76 77 78
//
//
82 83 84 85 86 87
Year Fig. 1. Development of laser market source: Laser
88
89 90
91 92 93 94
Focus WorM.
to the world leadership o f the Japanese industry in flexible manufacturing and shows that laser technology is a key element o f m o d e r n manufacturing technology. Nevertheless these conclusions are dangerous. The dynamics o f the market o f such a y o u n g technology is strongly driven by the development o f the technology which passes in leaps and b o u n d s and defies simple methods like extrapolation.
III. GENERAL
TRENDS
IN LASER DEVELOPMENT
In the development o f high power laser systems for materials processing some general trends can be pointed out. (4'5) First, there is a trend to develop low cost laser systems by high volume production lots and cheaper c o m p o n e n t s like turbine blowers, switch-mode power supplies, etc. Furthermore, new laser concepts, e.g. diffusion cooled CO2-1asers are able to reduce the costs. A n o t h e r very important trend is that a higher beam power is requested by the customers. Welding applications which have been done two or three years ago with a 5 k W laser system should be done CO2 56.50%
Div. 3.08% ~l~on Solid state 24.31%
3.08%
9.24% Di°de
Excimer 3.76% Fig. 2. World market of laser materials processing global sales in million U.S. $ (g = 292 million U.S. $).3
IR lasers as tools for the future
403
now with 10 kW beam power. One reason is that the improved production speed influences the production costs significantly. Working with higher processing speeds requires a more stable beam power and position. This leads to closed control loops to control process parameters like beam power. As multi-kilowatt N d : Y A G - l a s e r systems are on the market now, there is a strong trend to use robot systems with fibers, even if the CO2-1aser with similar power is cheaper than the Nd:YAG-laser. The whole system including laser source, beam guiding and robot (gantry) could be cheaper when using a N d : Y A G - l a s e r with a fiber for the beam guiding. In addition to these trends concerning CO2- and Nd :YAG-lasers, high power diode laser systems are being developed at the moment. Diode laser systems can be used for pumping Nd:YAG-lasers or for direct applications in material processing. In the last case they will mainly be used for hardening, soldering, melting surfaces and maybe spot welding in the next years. Table 1 shows a comparison of laser diodes, N d : Y A G - and CO2-1asers. Important is the very good overall efficiency of the diodes. A problem at the moment are the high watts per lasing volume. This leads to a heating of the diodes themselves. Therefore, the most important work which is carried out at the moment is to increase the cooling efficiency. If the price of the diodes is reduced as we could expect from the extrapolation of the actual development, it will be about $10 per Watt beam power in the next years. The change from CO2- or N d : Y A G - l a s e r s to diode lasers can be compared with the change from valve to transistor technology in the electrical industry. IV. N O N F L O W CO 2 L A S E R S New approaches to the diffusion cooled CO2-1aser have been investigated. Slab and annular geometry are promising laser designs in the kW range, thus exceeding the power limit of wall stabilized lasers considerably. The active medium of the diffusion cooled CO2 slab laser with thickness of a few millimeters is located between two water cooled electrodes. Due to the small gap size, the waste heat is transferred efficiently to the water cooled discharge walls. With an approximate discharge size of 60 cm length and 10 cm width an output power of 1.1 kW with an efficiency of 11% has been demonstrated with an output beam close to diffraction limit. ~6'7~ With an annular geometry a maximum laser power of 1.5 kW with an efficiency of 12% was measured. The beam quality is close to diffraction limit. 19~ V. S O L I D S T A T E L A S E R S Compared to CO2-1asers, solid state lasers in the kW range are frequently preferred for materials processing due to: • higher absorption in particular for metals, • lower beam disturbance by laser-induced plasma, • beam-guiding by fibers.
Efficiency Wavelength Power Lifetime Maintenance Price/Watt Fiber delivery Voltage Watts per lasing volume
Table 1. Comparison of laser beam sourcesTM Laser diodes Nd:YAG 30-60% 1-3% 0.78-0.83 # m 1.064/~m Multi-kW Up to 3 kW 20.00(O100.000 h 10.000 h Maintenance free Each 200 h (lamps) 200-400 D M / W 300-1000DM/W Possible Possible Up to 100 V Up to 1000V 1000 W/cms 50 W/cm3
CO: 5-10% 10,6 g m Up to 20 kW 10.000 h Each 500 h 200-400 DM/W Not possible Up to 10 kV I W/cm~
404
GERD HERZIGERand ROLF SCHLOMS
Different approaches have been investigated to increase the power of rod and slab lasers. Common are the oscillator-amplifier-design and the incoherent superposition of several independent lasers via optical fibers or beam guiding techniques. Both methods result in high power with a typical beam quality of > 150 mm × mrad. An approach to achieve improved beam quality with high power turns out to be beam multiplexing. Beam multiplexing conserves the beam quality of the single laser module and offers optional operation mode from cw to a high frequency duty cycle. Several low power laser modules of high beam quality have been multiplexed to an output power of 3 kW with a beam quality of 15 mm × mrad. A third approach is the annular solid state laser medium combining large active medium volume with efficient waste heat removal. Such annular systems have been investigated with different types of active media (Nd:YAG, Nd:glass, N d : G G G ) and various excitation techniques (DC-flash lamps and RF-excited lamps). DC-flash lamp excited lasers were operated with an output power of 1 kW at a total efficiency of 7.5%. "°'~'1 Thermal lensing of annular lasers is below 20% of an equivalent rod laser, which means that annular lasers have a big potential for high beam quality. VI. DIODE LASER SYSTEMS To generate high power laser diode radiation, there are mainly three different ways possible: first stacking of diodes to a closed packed two-dimensional array, second lens multiplexing and third fiber multiplexing. To conduct the diode laser radiation into a fiber, micro-optics have to be used. These micro-optics can be corrected micro-optics with 3 4 elements, grin lenses, spherical optics or aspherical optics. Using a cylindrical grin lens, a maximum about 20% beam power can be focused into a fiber of 100/~m diameter, tS~If a higher percentage of beam power should be coupled into a fiber of 100/~m diameter, the micro-lens has to change from quartz to a high index material and the best possibility is to use aspherical optics. When using a lot of fibers (fiber bundle) and for each diode array (focused into one fiber) a separate power supply, the laser diode offers a totally new possibility in laser technology. By focusing the laser light coming out of the different fibers to the workpiece, it is possible to shape the integral beam profile by controlling and changing the output power of the different diode units. The laser diode intensity is limited at the moment to 5 × 105W per cm 2 and this means all applications below this power density like hardening, remelting, soldering, etc. can be done with a controlled beam profile adapted to the process which opens new possibilities in future. Among laser materials processing surface refinement with laser radiation is not very common, but the use of diode laser radiation promises to get increasing shares of the market. Another application for laser diodes is pumping the Nd:YAG-laser. The advantage is higher overall efficiency and, resulting from this, the heat input into the Nd:YAG-crystal is lower and this leads to a better beam quality. However, this good beam quality will be reduced again if the beam guiding fiber is used. VII. LASER BEAM C U T T I N G Laser beam cutting is a well-established process in Europe. Laser cutting devices including the whole periphery are available on the market and mainly used in job-shops for metal processing. Modelling investigations of laser beam cutting indicates the existence of two cutting regimes. A low speed cutting regime as commonly encountered with laser cutting and a high speed cutting regime distinguished by a temperature distribution across the kerf front which induces a melt flow perpendicular to the incident laser beam. By controlling the melt removal according to the results of the numerical simulations the cutting speed could be considerably improved. With a prototype of an industrial slitting line developed at Fraunhofer metal sheets are cut with a speed of up to 250m/min, "2~ the maximum value
IR lasers as tools for the future
405
depending on sheet thickness and material, respectively. Excellent cutting qualities are achieved with CO2-1asers. In the case of electrical steel laser cutting induces less disturbance in the magnetic properties than mechanical slitting. Processing sensitive materials as electrical steel or coated metal sheets laser cutting turns out to be the superior manufacturing method. The effort to achieve a better understanding of the cutting process also opens new ways in the low speed cutting regime. For instance, with 1.2 kW CO:-lasers 80 mm thick mild steel (St 37) is cut at a speed of 0.2 m/min, t~3) The average roughness Rz of 45/~m is well below the roughness typical for autogeneous cutting, whereas the cutting speed is comparable. The laser controls the exothermal reaction by stabilizing the temperature distribution across the kerf front. From experimental work in progress it can be concluded that laser cutting of sheets with a thickness up to 100 mm can be performed economically with high quality.
VIII. LASER BEAM WELDING Maybe laser beam welding is the process with the strongest demand in industry. New ideas concerning the design can be realized due to the unique features of the laser welded seam. For example, lightweight cars should be produced using the laser for beam welding of aluminium. As in case of laser beam cutting the processing speed in sheet metal welding is significantly enlarged. The onset of instability depending mainly on welding speed and laser power is characterized by a sequence of humps and holes within the welding seam (humping-effect). The humping formation for a sheet thickness of, e.g. 200 pm starts at about 18 m/min. By controlling the melt flow according to the results of theoretical investigations it is possible to enlarge the welding speed up to 80 m/rain without the onset of any instability. Hence simulation makes it possible to use high speed welding for industrial application. A prototype of a high perfomance laser welding body maker for can production operates at welding speeds of up to 70 m/min.
IX. EXPLOITATION OF HIGH POWER LASERS High power lasers are frequently applied for upscaling of processing geometries in particular in the case of heat treatment or increasing of processing speed, e.g. for welding. Deep penetration laser welding of 25 mm thick mild steel is performed with 16 kW laser power and a welding speed of 0.6m/min. Fiftymm thick steel can be welded by double side weld pass, with the same parameters. Deep penetration welding of joints in T-configuration opens new ways in steel construction or ship building. When welding a 15 mm T-joint by one side weld pass, a welding speed of 1.2 m/rain is realized with a laser power of 16 kW.
X. RAPID PROTOTYPING Stereolithography, laser assisted laminated object manufacturing, laser caving and selective laser sintering are new processes for freeform manufacturing which require laser technology. In automotive, household, electrical industry and their suppliers there are approx. 60 applications in Europe. It can be expected that rapid prototyping technologies will establish themselves in the near future. Actually, these processes are mainly restricted to plastics, wax or paper. In order to fulfil the demands of industry, especially of the automotive industry, scientists in Europe are working on the development of direct manufacturing of metallic prototypes. Keywords in this field are sintering, three-dimensional welding, generating and bending.
406
GERD HERZIGERand ROLFSCHLOMS XI,
PROCESS
CONTROL
AND
QUALITY
ASSURANCE
Every innovative tool has to improve and assure the quality of the process. Process control is considered crucial for the long-term success of laser materials processing as regards its contribution to broaden and overcome the existing limits of production technology. It is one of the outstanding features of laser processing that the state of tool and process can be monitored on-line. Another aspect is the precise and fast control of the laser beam action with regard to intensity, duration and position. The combination of these two features will allow for a process which is completely monitored and controlled. Laser processing offers a great potential for on-line process optimization and quality assurance. As an example, 10 mm thick aluminium can be welded with a laser power of 10 kW, measuring the electron temperature of the plasma on-line during the process. A close correlation between the electron temperature and the irregularities in the upper and lower bead of the seam are recognized. Even irregularities in the melt pool, which occur at the bottom of the seam, are detectable by monitoring the electron temperature of the laser induced plasma. These benefits, in combination with the indisputable flexibility of laser processing, make laser technology attractive for the application on the shop floor. XII. CONCLUSION It can be expected that laser technology still undergoes a rapid development and therefore new applications with high net product and new (niche) markets will come up. The well-being of the industrial nations depends on the development of intelligent products, tools and processes. There is no doubt that laser technology is an important representative of this development and is a tool for the future. REFERENCES 1. G. Herziger, Lasertechnik--Anwendungen und Perspektiven Produktion, pp. 23/24 (1991). 2. Laser Focus Worm 1/94, Review and forecast of laser markets (1994). 3. Optech consulting, Markt- und Technologiestudie iiber Laser und Lasersysteme in der industriellen Materialbearbeitung (1994). 4. P. Loosen, Conf. Proc. L A M P "92, Nagaoka, Japan, pp. 61~6 (1992). 5. E. Beyer, V. Krause and P. Loosen, Conf. Proc. LASER M2P (1993). To be published. 6. K. M. Abramski, A. D. Colley, H. J. Baker and D. R. Hall, Appl. Phys. Lett. 54, 1833-1835 (1989). 7. R, Nowack, H. Opower, U. Sch/ifer, K. Wessel, Th. Hall, H. Kriiger and H. Weber Proc. ECO 3, CO, Lasers and Application 11, SPIE 1276, pp. 18-28 (1990). 8. R. Nowack, H. Opower, K. Wessel, H. Kr/iger, W. Haas and N. Wenzel, Laser Optoelektr. 3, 68-81 (1991). 9. U, Habich, P. Loosen, H.-D. Plum and D. Ehrlichmann, EQEC 91, Edinburgh, 27-30 August (1991). 10. H. Hodgson, Q. L/i, S. Dong, B. Eppich and V. Wittrock, Laser Optoelektr. 23 (3) (1991). 11. G. Albrecht, J. Eggleston and J. Ewing, IEEE Jl Quantorn Electron. 11, 1999-2105 (1986). 12. K.-U. Preil3ig, J. Albrecht, D. Bingener, D. Petring, A. Gillner and E. Beyer Bander, Bleche, Rohre 33 (10), 79-88 (1992). 13. J. Franke, W. Schulz and G. Herziger, VDI-Z-Special Blechbearbeitung, Okt. '93, p. 48 (1993).
Pergamon
Copyright ~ 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 1350-4495/95 $9.50 ~- 0.00
1350-4495(94)00103-0
IR
LASERS
AS TOOLS
GERD HERZIGER and
FOR
THE
FUTURE
R O L F SCHLOMS
F r a u n h o f e r - l n s t i t u t ffir Lasertechnik, Steinbachstrasse 15, D - 5 2 0 7 4 Aachen, G e r m a n y
(Received 2 August 1994)
I. I N T R O D U C T I O N In laser production technology the title of this paper is more a question than a statement. After 30 years of development laser technology has penetrated nearly all fields of economic significance, but compared to the world market of production technology it covers still niches. On the time scale of machine tool development laser technology is a young technology, and significance and dynamics of laser production technology is influenced by parameters, like • • • •
global economy, dissimination of know how, development of established laser processes, development of new processes.
The paper deals with general trends of these paramaters of significance, and indicates that laser technology is one of the key elements of flexible manufacturing and therefore a tool for the future. I1. T H E LASER M A R K E T Looking through the annual Review and Forecast of Laser Markets 19XX in the journal Laser Focus Worm it can be seen that world laser market is strongly growing since 1975 up to a volume of over one billion U.S. $ (Fig. 1). Having in mind that laser technology is a key technology, that means the costs for a laser based system are an order of magnitude higher than the laser source itself, it is comprehensible that at the beginning of the next century the laser system market will reach a volume of 50 billion U.S. $.1'~ Approximately one-third of this market is dedicated to laser sources for materials processing, which essentially means IR lasers like CO2-, Nd : YAG-, or diode lasers (Fig. 2). The prediction of the laser market for the next century results in a clear answer to the above-mentioned question. Unfortunately this prediction was done in 1991 on the basis of an extrapolation of the market data. Due to the onset of the world wide recession in 1990 there has been a decline of the market which makes an extrapolation of the market data of a young technology difficult. Nevertheless there are indications that exponential growth is only postponed. • Compared to the machine tool industry, which has especially in Japan and Europe a drop in turnover of approx. 30-60%, the negative development of the laser market has to be evaluated as moderate. • In terms of units the world laser market of the most expensive CO:-, N d : Y A G lasers with output powers higher than 500 W is still growing. 1:3~ • The recession shows the need of technology to open new markets and compete world wide. The market data also allow an additional conclusion. The geographical distribution of the world laser market shows that 25% of this market is located in Japan. ~3~This concentration is correlated JNV ~,~ AA
401
402
GERD
HERZIGER
and
ROLF SCHLOMS
2000 --
//,y/ ///
1000
//
7,7 //
// // //
~// / / 500
// //
///
(/" ~ / /// //, ///
200
100
~.. ;//
y~
;)¢
//
;d<
//
~d
50
79 80 81
~// / / ~// // // // //
// //
//
// // //
//
// //
//
// //
// //
75 76 77 78
//
//
82 83 84 85 86 87
Year Fig. 1. Development of laser market source: Laser
88
89 90
91 92 93 94
Focus WorM.
to the world leadership o f the Japanese industry in flexible manufacturing and shows that laser technology is a key element o f m o d e r n manufacturing technology. Nevertheless these conclusions are dangerous. The dynamics o f the market o f such a y o u n g technology is strongly driven by the development o f the technology which passes in leaps and b o u n d s and defies simple methods like extrapolation.
III. GENERAL
TRENDS
IN LASER DEVELOPMENT
In the development o f high power laser systems for materials processing some general trends can be pointed out. (4'5) First, there is a trend to develop low cost laser systems by high volume production lots and cheaper c o m p o n e n t s like turbine blowers, switch-mode power supplies, etc. Furthermore, new laser concepts, e.g. diffusion cooled CO2-1asers are able to reduce the costs. A n o t h e r very important trend is that a higher beam power is requested by the customers. Welding applications which have been done two or three years ago with a 5 k W laser system should be done CO2 56.50%
Div. 3.08% ~l~on Solid state 24.31%
3.08%
9.24% Di°de
Excimer 3.76% Fig. 2. World market of laser materials processing global sales in million U.S. $ (g = 292 million U.S. $).3
IR lasers as tools for the future
403
now with 10 kW beam power. One reason is that the improved production speed influences the production costs significantly. Working with higher processing speeds requires a more stable beam power and position. This leads to closed control loops to control process parameters like beam power. As multi-kilowatt N d : Y A G - l a s e r systems are on the market now, there is a strong trend to use robot systems with fibers, even if the CO2-1aser with similar power is cheaper than the Nd:YAG-laser. The whole system including laser source, beam guiding and robot (gantry) could be cheaper when using a N d : Y A G - l a s e r with a fiber for the beam guiding. In addition to these trends concerning CO2- and Nd :YAG-lasers, high power diode laser systems are being developed at the moment. Diode laser systems can be used for pumping Nd:YAG-lasers or for direct applications in material processing. In the last case they will mainly be used for hardening, soldering, melting surfaces and maybe spot welding in the next years. Table 1 shows a comparison of laser diodes, N d : Y A G - and CO2-1asers. Important is the very good overall efficiency of the diodes. A problem at the moment are the high watts per lasing volume. This leads to a heating of the diodes themselves. Therefore, the most important work which is carried out at the moment is to increase the cooling efficiency. If the price of the diodes is reduced as we could expect from the extrapolation of the actual development, it will be about $10 per Watt beam power in the next years. The change from CO2- or N d : Y A G - l a s e r s to diode lasers can be compared with the change from valve to transistor technology in the electrical industry. IV. N O N F L O W CO 2 L A S E R S New approaches to the diffusion cooled CO2-1aser have been investigated. Slab and annular geometry are promising laser designs in the kW range, thus exceeding the power limit of wall stabilized lasers considerably. The active medium of the diffusion cooled CO2 slab laser with thickness of a few millimeters is located between two water cooled electrodes. Due to the small gap size, the waste heat is transferred efficiently to the water cooled discharge walls. With an approximate discharge size of 60 cm length and 10 cm width an output power of 1.1 kW with an efficiency of 11% has been demonstrated with an output beam close to diffraction limit. ~6'7~ With an annular geometry a maximum laser power of 1.5 kW with an efficiency of 12% was measured. The beam quality is close to diffraction limit. 19~ V. S O L I D S T A T E L A S E R S Compared to CO2-1asers, solid state lasers in the kW range are frequently preferred for materials processing due to: • higher absorption in particular for metals, • lower beam disturbance by laser-induced plasma, • beam-guiding by fibers.
Efficiency Wavelength Power Lifetime Maintenance Price/Watt Fiber delivery Voltage Watts per lasing volume
Table 1. Comparison of laser beam sourcesTM Laser diodes Nd:YAG 30-60% 1-3% 0.78-0.83 # m 1.064/~m Multi-kW Up to 3 kW 20.00(O100.000 h 10.000 h Maintenance free Each 200 h (lamps) 200-400 D M / W 300-1000DM/W Possible Possible Up to 100 V Up to 1000V 1000 W/cms 50 W/cm3
CO: 5-10% 10,6 g m Up to 20 kW 10.000 h Each 500 h 200-400 DM/W Not possible Up to 10 kV I W/cm~
404
GERD HERZIGERand ROLF SCHLOMS
Different approaches have been investigated to increase the power of rod and slab lasers. Common are the oscillator-amplifier-design and the incoherent superposition of several independent lasers via optical fibers or beam guiding techniques. Both methods result in high power with a typical beam quality of > 150 mm × mrad. An approach to achieve improved beam quality with high power turns out to be beam multiplexing. Beam multiplexing conserves the beam quality of the single laser module and offers optional operation mode from cw to a high frequency duty cycle. Several low power laser modules of high beam quality have been multiplexed to an output power of 3 kW with a beam quality of 15 mm × mrad. A third approach is the annular solid state laser medium combining large active medium volume with efficient waste heat removal. Such annular systems have been investigated with different types of active media (Nd:YAG, Nd:glass, N d : G G G ) and various excitation techniques (DC-flash lamps and RF-excited lamps). DC-flash lamp excited lasers were operated with an output power of 1 kW at a total efficiency of 7.5%. "°'~'1 Thermal lensing of annular lasers is below 20% of an equivalent rod laser, which means that annular lasers have a big potential for high beam quality. VI. DIODE LASER SYSTEMS To generate high power laser diode radiation, there are mainly three different ways possible: first stacking of diodes to a closed packed two-dimensional array, second lens multiplexing and third fiber multiplexing. To conduct the diode laser radiation into a fiber, micro-optics have to be used. These micro-optics can be corrected micro-optics with 3 4 elements, grin lenses, spherical optics or aspherical optics. Using a cylindrical grin lens, a maximum about 20% beam power can be focused into a fiber of 100/~m diameter, tS~If a higher percentage of beam power should be coupled into a fiber of 100/~m diameter, the micro-lens has to change from quartz to a high index material and the best possibility is to use aspherical optics. When using a lot of fibers (fiber bundle) and for each diode array (focused into one fiber) a separate power supply, the laser diode offers a totally new possibility in laser technology. By focusing the laser light coming out of the different fibers to the workpiece, it is possible to shape the integral beam profile by controlling and changing the output power of the different diode units. The laser diode intensity is limited at the moment to 5 × 105W per cm 2 and this means all applications below this power density like hardening, remelting, soldering, etc. can be done with a controlled beam profile adapted to the process which opens new possibilities in future. Among laser materials processing surface refinement with laser radiation is not very common, but the use of diode laser radiation promises to get increasing shares of the market. Another application for laser diodes is pumping the Nd:YAG-laser. The advantage is higher overall efficiency and, resulting from this, the heat input into the Nd:YAG-crystal is lower and this leads to a better beam quality. However, this good beam quality will be reduced again if the beam guiding fiber is used. VII. LASER BEAM C U T T I N G Laser beam cutting is a well-established process in Europe. Laser cutting devices including the whole periphery are available on the market and mainly used in job-shops for metal processing. Modelling investigations of laser beam cutting indicates the existence of two cutting regimes. A low speed cutting regime as commonly encountered with laser cutting and a high speed cutting regime distinguished by a temperature distribution across the kerf front which induces a melt flow perpendicular to the incident laser beam. By controlling the melt removal according to the results of the numerical simulations the cutting speed could be considerably improved. With a prototype of an industrial slitting line developed at Fraunhofer metal sheets are cut with a speed of up to 250m/min, "2~ the maximum value
IR lasers as tools for the future
405
depending on sheet thickness and material, respectively. Excellent cutting qualities are achieved with CO2-1asers. In the case of electrical steel laser cutting induces less disturbance in the magnetic properties than mechanical slitting. Processing sensitive materials as electrical steel or coated metal sheets laser cutting turns out to be the superior manufacturing method. The effort to achieve a better understanding of the cutting process also opens new ways in the low speed cutting regime. For instance, with 1.2 kW CO:-lasers 80 mm thick mild steel (St 37) is cut at a speed of 0.2 m/min, t~3) The average roughness Rz of 45/~m is well below the roughness typical for autogeneous cutting, whereas the cutting speed is comparable. The laser controls the exothermal reaction by stabilizing the temperature distribution across the kerf front. From experimental work in progress it can be concluded that laser cutting of sheets with a thickness up to 100 mm can be performed economically with high quality.
VIII. LASER BEAM WELDING Maybe laser beam welding is the process with the strongest demand in industry. New ideas concerning the design can be realized due to the unique features of the laser welded seam. For example, lightweight cars should be produced using the laser for beam welding of aluminium. As in case of laser beam cutting the processing speed in sheet metal welding is significantly enlarged. The onset of instability depending mainly on welding speed and laser power is characterized by a sequence of humps and holes within the welding seam (humping-effect). The humping formation for a sheet thickness of, e.g. 200 pm starts at about 18 m/min. By controlling the melt flow according to the results of theoretical investigations it is possible to enlarge the welding speed up to 80 m/rain without the onset of any instability. Hence simulation makes it possible to use high speed welding for industrial application. A prototype of a high perfomance laser welding body maker for can production operates at welding speeds of up to 70 m/min.
IX. EXPLOITATION OF HIGH POWER LASERS High power lasers are frequently applied for upscaling of processing geometries in particular in the case of heat treatment or increasing of processing speed, e.g. for welding. Deep penetration laser welding of 25 mm thick mild steel is performed with 16 kW laser power and a welding speed of 0.6m/min. Fiftymm thick steel can be welded by double side weld pass, with the same parameters. Deep penetration welding of joints in T-configuration opens new ways in steel construction or ship building. When welding a 15 mm T-joint by one side weld pass, a welding speed of 1.2 m/rain is realized with a laser power of 16 kW.
X. RAPID PROTOTYPING Stereolithography, laser assisted laminated object manufacturing, laser caving and selective laser sintering are new processes for freeform manufacturing which require laser technology. In automotive, household, electrical industry and their suppliers there are approx. 60 applications in Europe. It can be expected that rapid prototyping technologies will establish themselves in the near future. Actually, these processes are mainly restricted to plastics, wax or paper. In order to fulfil the demands of industry, especially of the automotive industry, scientists in Europe are working on the development of direct manufacturing of metallic prototypes. Keywords in this field are sintering, three-dimensional welding, generating and bending.
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PROCESS
CONTROL
AND
QUALITY
ASSURANCE
Every innovative tool has to improve and assure the quality of the process. Process control is considered crucial for the long-term success of laser materials processing as regards its contribution to broaden and overcome the existing limits of production technology. It is one of the outstanding features of laser processing that the state of tool and process can be monitored on-line. Another aspect is the precise and fast control of the laser beam action with regard to intensity, duration and position. The combination of these two features will allow for a process which is completely monitored and controlled. Laser processing offers a great potential for on-line process optimization and quality assurance. As an example, 10 mm thick aluminium can be welded with a laser power of 10 kW, measuring the electron temperature of the plasma on-line during the process. A close correlation between the electron temperature and the irregularities in the upper and lower bead of the seam are recognized. Even irregularities in the melt pool, which occur at the bottom of the seam, are detectable by monitoring the electron temperature of the laser induced plasma. These benefits, in combination with the indisputable flexibility of laser processing, make laser technology attractive for the application on the shop floor. XII. CONCLUSION It can be expected that laser technology still undergoes a rapid development and therefore new applications with high net product and new (niche) markets will come up. The well-being of the industrial nations depends on the development of intelligent products, tools and processes. There is no doubt that laser technology is an important representative of this development and is a tool for the future. REFERENCES 1. G. Herziger, Lasertechnik--Anwendungen und Perspektiven Produktion, pp. 23/24 (1991). 2. Laser Focus Worm 1/94, Review and forecast of laser markets (1994). 3. Optech consulting, Markt- und Technologiestudie iiber Laser und Lasersysteme in der industriellen Materialbearbeitung (1994). 4. P. Loosen, Conf. Proc. L A M P "92, Nagaoka, Japan, pp. 61~6 (1992). 5. E. Beyer, V. Krause and P. Loosen, Conf. Proc. LASER M2P (1993). To be published. 6. K. M. Abramski, A. D. Colley, H. J. Baker and D. R. Hall, Appl. Phys. Lett. 54, 1833-1835 (1989). 7. R, Nowack, H. Opower, U. Sch/ifer, K. Wessel, Th. Hall, H. Kriiger and H. Weber Proc. ECO 3, CO, Lasers and Application 11, SPIE 1276, pp. 18-28 (1990). 8. R. Nowack, H. Opower, K. Wessel, H. Kr/iger, W. Haas and N. Wenzel, Laser Optoelektr. 3, 68-81 (1991). 9. U, Habich, P. Loosen, H.-D. Plum and D. Ehrlichmann, EQEC 91, Edinburgh, 27-30 August (1991). 10. H. Hodgson, Q. L/i, S. Dong, B. Eppich and V. Wittrock, Laser Optoelektr. 23 (3) (1991). 11. G. Albrecht, J. Eggleston and J. Ewing, IEEE Jl Quantorn Electron. 11, 1999-2105 (1986). 12. K.-U. Preil3ig, J. Albrecht, D. Bingener, D. Petring, A. Gillner and E. Beyer Bander, Bleche, Rohre 33 (10), 79-88 (1992). 13. J. Franke, W. Schulz and G. Herziger, VDI-Z-Special Blechbearbeitung, Okt. '93, p. 48 (1993).