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CO2 Lasers: Modern Workhorses in the World of Industrial Manufacturing
For more than 30 years lasers have been successfully used for industrial production. Modern efficient manufacturing would be not imaginable without lasers and laser processes. Since the beginning of laser applications, CO2 lasers dominated the market of continuous cutting and welding applications (referred here as “macro” applications) because of their high power and electrical efficiency, reliability and cost efficiency. A main segment of the laser market is laser cutting - not only of metals but also of wood, plastics, textiles and compounds. The second main application is laser welding. Due to keyhole formation, laser welding is highly efficient, with high welding speeds and low heat load on the welded part resulting in low distortion. Examples are the laser welding of power train components, injection systems, housings of sensors and airbag inflators but also parts of car bodies, tailored blanks, tubes and profiles from 0.5 mm to 15 mm weld penetration. These applications are driven by the well-defined, highly localized huge power density that is obtained by a small focus from lasers of medium and high beam quality. Besides power and wavelength, the laser’s focusability or beam quality plays an important role in the selection of lasers used for a specific application.
Beam quality and focus size A laser beam is not propagating parallel, free of divergence but follows a so-called propagation caustic: The beam size changes with propagation distance by means of a hyperbolic function described by the parameter beam waist radius (ω) and far field divergence (θ). The product of these parameters, called beam parameter product (ω x θ [mm x mrad]), is used to describe the propagation of the laser beam, the size of
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focus diameter and depth of focus. Comparing two lasers that differ in beam quality by a factor of two, will form a focus spot size that differs also by the factor of two, if they are focused under the same conditions. If they are focused to the same spot size, their depth of focus will differ: the beam of lower beam parameter product shows a larger depth of focus. Not only the beam parameter product is used for a description of the beam quality but also the relative quality numbers M² and K that relates the measured beam quality to the theoretical limit λ/π : O S 1 K T 2 Z M
O: Z: T:
THE AUTHOR WOLFRAM RATH Wolfram Rath has studied Physics at Universities Heidelberg and Erlangen - Nürnberg with the degree of Dr. rer. nat. He was working with Siemens in CO2 Laser and excimerlaser development and applications before he became responsible for Rofin-Sinar’s applications in Hamburg using CO2, solid state and diode lasers for macro applications. ●●
Wolfram Rath Rofin-Sinar Laser GmbH Berzeliusstraße 83 22113 Hamburg, Germany Tel.: +49 (0) 40 - 73 36 30 Fax.: +49 (0) 40 - 73 36 31 60 E-mail:
[email protected] Website: www.rofin.com
wavelength waist radius far field divergence
Both focus parameters, spot size and depth of focus, are to be adapted to the application. The smallest spot size, the smallest kerf or weld nugget is not always the best solution for a manufacturing problem. Typical focus spot sizes used for macro applications are in the range between 0.1 mm to 0.6 mm. The depth of focus should be in the order of some mm to obtain sufficient tolerances within the manufacturing process and to obtain the required shapes of the cut kerf or weld cross section. Figure 1 gives a practical idea of the relation between mode shape and beam quality numbers for different resonator modes.
CO2 laser technology
TEM 00
TEM 01*
TEM 10
TEM 20
Multi
After the theoretiK = 0.33 K = 0.2 K = 0.15 K=1 K = 0.5 cal understanding of 2=1 2=2 M2 = 3 M2 = 5 M2 = 6 M M stimulated emission BPP = 10 BPP = 17 BPP = 23 BPP = 3.5 BPP = 7 and the definition of the principles be- FIGURE 1: Different modes and beam quality of CO lasers with 2 tween 1917 and the M², K-number and beam parameter product given in [mm*mrad]. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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FIGURE 2: CO2 slab principle.
1950s, the 1960s was the period of practical demonstration of many common laser materials and most of them are still in use today, such as the CO2 molecule. Within the next decade commercial systems for material processing started to establish themselves and different laser companies started to present their products on the market. Two different technologies were in focus: pulsed solid state lasers (about 1 µm wavelength) pumped by flash lamps for pulsed applications like drilling and spot welding on one side, and CO2 gas lasers pumped by the electric current of a gas discharge in the 10.6 µm wavelength range for continuous wave applications on the other side.
Discharge excitation: DC or RF The CO2 gas laser uses the low-lying vibrational levels of the CO2 molecule. For excitation a very efficient energy transfer from the first excitation of the N2 molecule is used, which is excited by electrons of a gas discharge in a CO2 - N2 - He mixture at pressures up to some hundreds hPa. The discharge can be either a DC gas discharge using electrodes inside the gas or an RF – discharge with several advantages:
THE COMPANY ROFIN-SINAR Laser GmbH With more then 25,000 systems installed worldwide, ROFIN-SINAR has dedicated itself to becoming one of the leading manufacturers of lasers and laser-based solutions for industrial materials processing. For more than 30 years ROFIN-SINAR has developed lasers and laser systems. With a variety of CO2, solid-state, diode and q-switch lasers, ROFIN-SINAR provides a wide range of lasers for almost every industrial application.
© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
FIGURE 3: Beam forming optics.
no electrode wear homogeneous discharge easy power setting by pulse width modulation excellent pulsing capabilities. • For RF power generation and transmission, different technologies are used: Separation between laser head and RF power generator using a matching transfer cable or a free oscillating RF generator directly connected to the electrodes with the advantage of a wider matching range and a more flexible connection between the HV power supply and the RF generator. • • •
Afterwards, the gas is pumped through heat exchangers to cool it down and supplied again to the active zone.
Two types of geometries are used with this flowing technique: The cross flow where the gas is flowing across to the resonator axis (“cross flow lasers”) and the fast axial flow (FAF) where the gas is flowing along the axis of the resonator. The advantage of the cross flow principle is that the length of flow is small and the cross section of the flow is large. There-
Cooling method: cross flow, fast axial flow or diffusion? The main design problem of laser development is the cooling of the active medium, since the efficiency of lasers is small compared to other technical devices or machineries. Two different cooling techniques are used today for industrial CO2 lasers: convection and diffusion. Cooling by convection uses a blower or turbine to pump the gas through the active region with high flow velocities. The entrance of the active region is supplied with cold gas where it is excited by the discharge and heated to its maximum temperature at the exit.
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fore much less effort has to be used to gain the required volume flow for a specific laser power. The advantage of the fast axial flow is that the flow symmetry is the same as the symmetry of the optical resonator. Therefore a higher beam quality can be generated with less influence from the temperature gradient of the discharge and pressure gradient of the gas flow. However, both techniques use an expensive system of flow technique: heat exchangers, blowers or turbines which require relatively high stand-by energy consumption and additionally, all the needs to maintain a clean vacuum within a complex system using rotating fans or turbines. Especially the fast axial flow systems tend to transport small particles onto the surface of optical resonator elements where they can be the cause of increased absorption and restricted lifetime. Because of these technical limitations, a continuous supply of fresh gas is used to maintain the gas quality inside the laser which results in a gas consumption of several tens of norm liters per hour typically and the requirement to connect the laser to a CO2 /N2/ He gas supplying system. Another technological challenge is the high power density on optical components mainly used for stable resonators of fast axial flow lasers with high beam quality. Optical elements that are transparent for the 10.6 µm wavelength are typically made of ZnSe, reflective components are made of copper that can be machined by high precision diamond tools. These water-cooled mirrors are very reliable and resistant to high power densities. However, the transparent ZnSe components that are mainly used as outcoupling mirrors for stable laser resonators of fast axial flow lasers have a restricted lifetime and are an object of maintenance.
Simple design reliable technique: CO2 slab All these technical restrictions are avoided if the cooling mechanism is diffusion. From the beginning this technique was used by low- or no-flow systems where the cooling of the active laser gas was realized by a water jacket surrounding the tubes of the active discharge. The power of these lasers, however, was restricted by approximately 70 W/ m discharge length because of the poor radial cooling capacity of these designs. Therefore the research was focused on different geometries of improved cooling capacity during the 1980s. The result of these efforts was the introduction of the CO2 slab geometry by two inventors in parallel: Prof. Opower, DLR Stuttgart, Germany, and Prof. Tulip, Canada, used a system that excited the gas discharge between two flat electrodes with a gap of only a few mm. Diffusion cooling of the molecules becomes very efficient for small distances and therefore the water-cooled copper electrodes are designed to summarize three basic functions into one element: electrodes for the RF discharge, heat exchanger for the cooling of the gas and waveguide walls able to guide the CO2 laser light inside the resonator. The adaptation of the optical resonator to this slab geometry was solved by a so-called hybrid resonator setup (figure 2). One axis is designed for the oscillation of a fundamental waveguide mode within the small gap between the electrodes; the other axis with the large aperture parallel to the electrode surface uses an unstable resonator, which is well known to enable the oscillation of a fundamental mode from large cross sections. As the beam quality of both resonator branches results from a lowest order fundamental mode, a simple beam forming optics could be designed, that images the beam
waists of the two resonator axis to the same size at the same plane of propagation. A well-collimated beam of high beam quality is generated from this setup. The spatial filter used at an intermediate focus of the unstable resonator axis cleans up the beam from side lobes of the unstable resonator mode, which enables the generation of a CO2 laser beam of very high beam quality. The emitted beam has a Gaussian shape, the propagation and the focusing is described by the laws of Gaussian optics with a beam quality close to one. For applications which require larger focus sizes, a Donut mode can be generated outside the resonator. This mode has an intensity distribution of a TEM 01* and a K-number of K = 0.45 or M² = 2.2. Since the resonator does not use any semitransparent mirror, all resonator mirrors (there are two only) can be made from copper which makes the setup reliable with long life. Resonator components are no longer spare elements. Without exchange of the out coupling mirrors and windows there is also no need for readjustment of the resonator, which can be a very time consuming procedure for folded stable resonators. However, one single transparent component is needed for the transport of the laser beam out of the vacuum vessel. This window, which has no resonator functionality, is made from synthetic grown diamond. This material is highly transparent, has a very high heat conductivity and a high strength. This type of window is running in nearly 4000 high power CO2 slab lasers and shows its reliability day by day and year for year. The total setup is insensitive to thermal load of the high power beam especially because of the diamond window. Many classical lasers change their beam size and the position of the beam waist with the laser power, because the ZnSe out coupling mirror changes its optical properties with the level of transmitted power. The CO2 slab laser has nearly constant beam propagation independent
FIGURE 4: Beam quality measured from 8 kW CO2 slab laser: Gauss mode and Donut mode.
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© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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FIGURE 5: Comparison of cutting speed of mild steel: FAF 4 kW and CO2 slab 2.5 kW.
on the emitted power, due to the fact that only reflective elements and a diamond window are installed into the laser.
Benefits for the customer Within the last 12 years these principles were implemented in approximately 4000 CO2 slab lasers at a power between 1000 W and 8000 W. Long term observation of the service and maintenance costs shows that for this laser a minimum cost of ownership could be realized, because of a minimum gas consumption, small service costs and a long lifetime. The simple design of the CO2 slab laser makes it easy to use and maintain. Rapid power on time, no external gas connection due to an internal gas bottle, 72 h gas exchange interval, no exchange of optical components, built-in RF generator make it a workhorse in material processing.
FIGURE 6: Cutting graph of mild steel by oxygen assist gas for CO2 slab lasers 2 kW to 5 kW. The black line is used for all lasers at reduced laser power of app. 800 W.
Applications Systems for CO2 lasers are using space propagation and copper bending mirrors to guide the beam. These mirrors are fixed if the work piece is moved. So called “flying optics systems” move the focusing head along the Cartesian system of coordinates along the propagating beam. High beam quality lasers emit a beam of low divergence, which can be used without an additional beam expansion telescope in flying optics systems, resulting in a better consistency of the focus parameters within the work area than lasers of worse beam quality would have.
Lasercutting Cutting by lasers is one of the most frequently used laser applications. Wood and plastics are cut by CO2 lasers, mainly because the 10.6 µm wavelength is absorbed very effi-
ciently from those materials, which are often transparent for 1µm solid state laser light. High beam quality lasers can be used with advantages especially for low material thickness up to approximately 6 mm. The smaller spot size of lasers with high beam quality forms a smaller kerf width, which allows cutting either at higher speed at a given power or at reduced power for a given speed. Additionally, scanner based cutting of abrasive paper or airbag textiles uses the high beam quality for larger scanning work areas or smaller focus sizes. The cutting of steel is distinguished between two methods: Oxygen assisted laser flame cutting of low alloyed steel and high-pressure nitrogen assisted laser fusion cutting.
Laser flame cutting The maximum cutting speed of oxygen assisted laser flame cutting is determined not only by the laser power but also by the limitations of the exothermal chemical reaction between oxygen and iron. Therefore the cutting speed is increasing less then linear with the laser power, doubling of power does not result in a doubling of cutting speed. CO2 slab lasers achieve the same cutting speeds as other lasers by using less laser power for a mild steel thickness range up to approx. 6 mm because of the high beam quality. On the other hand this advantage of the better focusability is not limiting the ability and performance of cutting mild steel of high thickness up to 25 mm.
Laser fusion cutting
FIGURE 7: Laser fusion cutting.
© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
The laser power required for laser fusion cutting is higher than for laser flame cutting because no exothermal reaction takes place.
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FIGURE 8: Demonstration of seam shaping by adaptation the focus to the application. Laser power and welding speed is fixed (T: penetration; B: width), Left: Gauss mode, Center: Donut mode, Right: Double focus.
Within a wide range the cutting speed is proportional to the laser power for a given material thickness. Process limits are reached, when the metal vaporization exceeds a certain threshold or when the flow behavior restricts the drive out of the molten material. Lasers of higher beam quality can beat more powerful lasers since at lower kerf width the amount of molten material is smaller, which is to be heated, molten and driven out of the kerf especially for material thickness up to 6 mm. At higher material thickness a certain minimum kerf width is necessary. It is adjusted by the focus parameters in use.
Welding Laser keyhole welding enables a very efficient mechanism to transport the laser power into the material to be joined. At high power densities the material at the surface is evaporated, a keyhole is built into the material filled with metal vapor and the laser power is absorbed very efficiently at the keyhole walls formed by liquid metal. This mechanism allows the generation of narrow welds made at high welding speeds with a minimum of heat load on the parts. An excellent controllability of the laser power in time and space together with possibilities of online process control help to realize laser welding in manifold applications of industrial production where high productivity and reliability of the system including the laser is in the focus of manufacturers. In general the following process rules can be given: • The cross section of the weld is proportional to the energy per unit length cal-
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•
•
FIGURE 9: Basic principle of the remote welding system.
culated by the laser power divided by the welding speed It is distinguished between two regions of welding speed: At low welding speeds (steel: v < 2m/ min) the penetration of the weld is nearly independent on the focus spot size. At high welding speeds the penetration of the weld scales with the ratio of laser power divided by the focus spot size. Lasers of higher beam quality generate welds of larger depth. Depth of focus and the focus position relative to the material surface is of importance for the shape of the weld’s cross section. Therefore lasers of higher beam quality are able to generate weld seams that are more narrow, deeper and generated at higher welding speed.
ing geometry makes use of the width of a laser weld and not of the penetration. For these cases a reduction of weld width would reduce the strength of the weld. Seam shaping can be used to optimize the shape of a weld nugget.
Seam shaping Different methods are used to cover all these requirements of the shape of the weld nugget to a specific application. Figure 8 shows the possible shapes of cross sections at a given welding speed and laser power starting from 6 mm penetration and 0.6 mm width to 2 mm penetration and 2 mm width. Changing focal length and focal position is used to vary the seam’s shape in a smaller range. For the next step the donut mode is
For practical applications the weld of the smallest width is not always the best solution. Very often a compromise is to be found that balances the heat load on the work piece on one hand and the resulting distortion, the bridging of gaps or covering a certain range of geometrical tolerances of the work piece, the tooling or the system on the other hand. Sometimes the join- FIGURE 10: Remote welding system.
© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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used to enlarge the seam width up to 1 mm at 4 mm penetration. The last steps to increase the width of a weld further more is made by using a dual focus optics, which forms two separated individual focus spots either with Gauss or with Donut mode. Welds of 2 mm width can be generated.
Remote Welding Over the last few years, a new technique has developed, called remote laser welding. Many laser welding applications join large components by welding a number of stitches or spots. Laser on-times of those applications are 20 percent only, which makes the cost of ownership high. A huge reduction of the positioning time of the laser weld head can be achieved if a scan-
FIGURE 11: Typical remote welding applications. Left: Welding of back seat components (46 welds in 12 seconds). Right: VW car door.
ning technique is used and the positioning is performed within several ms only. Therefore, the laser on-time is enhanced and the total processing time is reduced.
Basics Remote welding systems are based on a CO2 slab laser with an excellent beam quality and a nominal output power up to 6 kW. The expanded laser beam is focused by an air-cooled lens with a focal length up to 2 m. The focused beam is deflected by a single mirror scanning head, which is designed for high-speed movement and high power laser beams. The rotary movement of the scanning mirror creates the beam deflection in
X- and Y-orientation. The focusing element, which is assembled on a direct driven linear slide, creates the movement of the focus point in Z direction. To increase the working envelope, the scanning head is also fixed on a linear slide, which is able to move parallel to the laser beam. The working envelope with a base area of 1500 mm x 2400 mm and a height of 600 mm is achieved by combining the scanning move of the mirror and the linear motion of the scanning mirror and the lens. The CO2 slab laser is the ideal choice for remote welding applications. The available power range from 1 kW to 6 kW and the excellent beam quality of M2 = 1.1 are features that create the required focus size by long focal length and a long range of focus.
Application examples Typical applications are hang-on parts, doors, hoods, pillars, seats etc. for the car body production. But also other metal sheet welding applications for different industries were realized recently.
Summary From the beginning CO2 lasers were the workhorses of industrial cw laser macro applications because of their reliability, cost efficiency and safety. With the use of the diffusion cooling technique by CO2 slab lasers up to 8 kW it was possible to improve these features.
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