Industrial Laser Materials Processing - Wiley Online Library

Laser Materials Processing Review

Industrial Laser Materials Processing

10 Years

Laser Technik Journal

A review of the origin, current status and an outlook Wolfram Rath and Corinna Brettschneider

For more than fourty years, laser technology writes a story of success and a huge number of different applications would not be possible without it. This article reflects the development of the laser technologies for macro materials processing in view of the continued improvement of laser sources and application techniques.

The first step is always the hardest Laser materials processing started in the second half of the sixties with pulsed solid-state lasers used for drilling and spot welding on the one hand and with continuous wave CO2 lasers used for cutting and continuous wave welding on the other hand. Macro laser applications, namely laser flame cutting (1967 TWI), laser fusion cutting, laser welding, hardening and cladding were introduced into the industrial production practice within the late seventies and early eighties of the last century. CO2 lasers were the favored beam source, as they had the highest electro-optical efficiency and the required power for industrial throughput was available. Not all techniques and components were available in these early days. Lasers, beam delivery optics, focusing elements like lenses and off-axis parabolic copper mirrors, gas nozzles, process techniques and gas supply systems had to be developed and optimized for high-power and industrial environment. Additionally, all safety regulations needed to be developed and standardized internationally as well as the development of standards for beam quality definition and measurement.

Fig. 1 Left: CO2 cross-flow laser (1973); right: CO2 slab laser (2014).

Industrial breakthrough With larger industrial interest in laser processes the demand for reliable laser processing systems increased. For CO2 laser sources and -systems the development of diamond turned copper optics and low-loss ZnSe transmissive components equipped with reflective or antireflective thin film coatings was of high importance for resonator optics, beam delivery and focusing. One of the major improvements at this time was the development of reliable highly robust molybdenum coated focusing mirrors for welding applications. Cutting heads with automated nozzle distance control and optimized gas flow were developed for consistent cut qualities. Three different technologies competed in the CO2 laser source market, distinguished by the type of gas cooling: slow-flow for high beam quality medium power lasers up to approx. 150 W and cross-flow and fast-axial flow lasers with powers up to 20 kW. An additional differentiating technology was the type of the gas discharge excitation. The gas can be excited either by a DC discharge, which is more efficient or by a RF discharge, which has the advantage that the

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electrodes can be located outside the gas tubes which helps to keep the gas clean. The following positions determine the cost of ownership of these laser sources: n Gas consumption n Consumption of electrical power especially during standby n Consumption of optics (mainly beam outcoupling elements) n Consumption of other spares for blower or turbines, gas mixing elements, electrodes etc. Although these components were continuously improved, a new principle is reducing the Total Cost of Ownership clearly in one-step without disadvantages: the diffusion-cooled RF excited CO2 slab laser. Due to this new technique of diffusion cooling, the CO2 slab laser does not need any complex setup of a gas venting system resulting in a negligible gas consumption, minimum standby power and no requirements for maintenance of any venting technique. With the additional introduction of a resonator made of reflective copper mirrors only and the use of a diamond window for beam transmission the periodic replacement of optical components was not required anymore. Laser Technik Journal

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Fig. 2 On the left: 4 kW diode-pumped solid-state laser; on the right: 4 kW fiber laser.

The excellent fundamental transversal mode beam quality allows additionally the use of smaller kerf width in cutting and narrower weld seams in welding applications, which result in lower power requirement or higher processing speeds. Rofin introduced the first slab laser into the market in 1993 and developed the technology up to 8 kW laser power in 2005, which covers the main range of industrial laser applications. As the long wavelength of around 10 µm is advantageous for several processes, these lasers are an excellent choice today and in future. Another type of laser that has conquered its place in industrials materials processing are the solid-state lasers emitting in the wavelength range of 1 µm. A major benefit of using solid-state lasers is the possibility of guiding the beam by the use of highly flexible optical fibers, which have entered the market at the second half of the eighties. Year 1980 1985 1990 1995 2000 2005 2010 2015

Available cutting Maximum mild power CO2 steel thickness 500 W 6 mm 1500 W 12 mm 2000 W 15 mm 2500 W 20 mm 3500 W 25 mm 4500 W 25 mm 4500 W 25 mm 6000 W

Together with its subsidiary Optoskand, Rofin has developed optical fibers for the pulsed solid-state lasers with an output power of 500 watts. During this time the core diameter of the fibers was in the range of 800 to 1000 µm resulting in a beam quality of 80 mm mrad. In the middle of the nineties, an important step in the development of the fiber technology came with the use of a quartz block at the fiber end. The larger surface gives lower power densities, improves the mechanical mount and reduces the losses due to an AR coating. Fiber diameters of 600 µm became state of the art for welding of car body applications. Because of the significant improvement in beam quality, diodepumped lasers requested reduced fiber diameters in the range of 300 to 400 µm. Today, low loss, reliable Optoskand fibers from 50 µm to 1 mm core size are widely used. The oldest known design for a solidstate laser is the use of Nd:YAG crysAvailable cutting power 1µm

1000 W 300 µm 2200 W 300 µm 1500 W 100 µm 2000 W 50 µm 6000 W 100 µm

Maximum cutting speed 1mm N2 6 m/min 12 m/min 18 m/min 25 m/min 37 m/min 14 m/min 40 m/min 55 m/min

Tab. Chronological development to higher power & higher material thickness. Color: red: CO2 slab, blue: CO2 plasma cut, green: rod / disk / fiber laser

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tal in a rod geometry. Until the end of the nineties, this crystal was optically pumped by flash lamps. These lasers had an efficiency of 3 % only and the flash lamp life time was around 1000 to 2000 hours. All laser experts expected the replacement of these lamps by laser diodes with two technical advantages: higher efficiency and lower thermal load resulting in a better total efficiency and an improved beam quality. Rofin acquired the diode laser company Dilas in 1997 and introduced the first multikW diode-pumped solid-state laser only one year later. These lasers had the benefit of attaining a much higher efficiency of approx. 10 % and a beam quality of 12 mm mrad combined with a longer service life of the excitation modules. The next technology step was the change from the rod to a disk-shape of the laser-active material offering advantages such as an overall efficiency of up to 20 % and an increased beam quality of about 5 mm mrad. Beside of the life time of the pumping modules, an important issue in the field of solid-state lasers was the efficient heat removal within the laser-active medium. In case of classic solid-state lasers with a rod-shaped laser medium the beam quality is significantly influenced by a radial thermal gradient, the so-called “thermal lens”. Using a thin disk as a laser-active medium reduces the disturbing thermal lens effect due to an efficient large area cooling on the back side of the disk. This yields a high, continuously consistent beam quality. In the time from 2003 until 2010, Rofin delivered high-power disk lasers used i. e. for industrial cutting and welding applications. The most important step in the field of solid-state laser technology followed after the turn of the millennium with the development of the high-power fiber lasers offering fiber delivered single-mode beam quality. Fiber lasers profit from the development of large mode area, rare-earth doped double clad fibers of large cladding diameters, fiber integrated pump couplers and high-power laser diode pumps. This techniques enables the design of high-power fiber laser modules realized as “integrated fiber design” which makes the modules simple, robust and efficient, ideal for the use in lasers for industrial production. Power


Laser Materials Processing

Fig. 3 Left: Messer cutting system in the early seventies: The Laser source is tilt by 45°. This enables the user to obtain circular polarisation at the workpiece by use of one phase shift retardation 90° beam bender. Right: Messer cutting system in 2014: Large scale system equipped with 4.5 kW CO2 Slab laser. All Rofin CO2 lasers emit a polarized beam tilt by 45° to the horizontal plane.

scaling is obtained by combining these modules by all-fiber combiners with high beam quality multi-mode output. Rofin introduced this fiber laser technology in 2007 after a number of strategic acquisitions of specialized companies like Corelase, Nufern, m2k Laser and PMB. Components like diodes, active double clad large mode area fibers, fiber pump couplers, highpower fiber combiners and fiber beam deliveries, as well as power supplies are developed and manufactured by these specialists within the Rofin group. That enables a generation of different single-mode and multi-mode fiber laser products with powers of up to 6 kW offering the following features: n Simple modular design compared to thin disk laser designs n Robust, no mechanical stability required n Beam quality adapted to different applications from single-mode (0.4 mm mrad) to multi-mode with high beam parameter products (40 mm mrad). n Efficient: due to an optical conversion efficiency of more than 80 %, the wall plug efficiency exceeds 30 % . These fiber lasers are universal processing tools for industrial production. They show a wider range of applicability than all other lasers before because of its wide range of power and beam qualities and due to their flexible beam switching, beam splitting and scanning possibilities. Furthermore, their efficiency, the wavelength and minimum maintenance requirements allow highly economical processes in a large variety of industries. An important aspect in the success story of industrial lasers in general was the development from a scientific labo-

ratory setup to an industry-proven product in view of maintenance and service requirements. The first industrial lasers required maintenance cycles in the range of 1000 hours and about one week of maintenance work. The customers have accepted this without objection as they were only interested in this new promising processes enabling new improved products at reduced production times and higher quality. In the following years, issues such as “Total Cost of Ownership” and maintainability became more and more relevant. In the field of CO2 lasers, maintenance cycles changed with the development from cross-flow lasers to fast-axial flow lasers from 1000 to 2000 hours but also with reduced maintenance work. With the development of the diffusion-cooled CO2 slab laser, Rofin was able to offer a laser with very low service and maintenance expenses and a maintenance cycle of 3000 hours with only 6 hours of working time. In the area of solid-state lasers, service and maintenance expenses changed significantly through the technology step from lamp-pumped to diodepumped lasers whereas fiber lasers as the latest technology are virtually maintenance free. A further important issue is the availability of the laser sources in production plants. In the field of CO2 lasers, the availability increased significantly from cross-flow lasers to the CO2 slab technology and modern fiber lasers. Today, the laser is seen as a machine tool with all its requirements and industrial standards like an extended spare parts service worldwide, 24/7 hotline as well as e-service capability. In addition, a quick local response and support in all strategic markets are fun-


damental requirements for a successful business.

From the source to the solution – today´s industrial practice Since more than fourty years now, lasers are successfully employed in various industrial applications. Cutting and welding are most predominant and are described hereafter. However, also other applications like hardening, cladding and surface treatment are important as well but will not be discussed in detail. TWI demonstrated laser cutting as early as 1967. Today, it is still the predominant laser macro application used for sheet metal cutting, tube and profile cutting, 3D cutting of car body compo-

Company ROFIN-SINAR Laser GmbH Hamburg, Germany

With more than 35 years of experience, ROFIN is a leading developer and manufacturer of lasers and laser-based technologies for industrial material processing applications. The company focuses on developing key innovative technologies and advanced production methods for a wide variety of industrial applications. In terms of fiber lasers, ROFIN is equipped with in-depth technology know-how and components due to specialized group members like DILAS, Corelase, Nufern, Optoskand, m2k Laser and PMB. Components like diodes, fibers and fiber beam deliveries, power supplies as well as high-power fiber optics are developed and manufactured by these specialists within the ROFIN group leading to high quality fiber lasers for industrial production. www.rofin.com


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Laser welding solutions need to take into account part preparation, mechanical joining, clamping and laser welding which is fundamentally different to laser cutting. Therefore more system manufactures offer special solution to their specific markets. First applications in the automotive industry

Fig. 4 Scanner welding of automotive parts with a 4 kW fiber laser.

nents but also cutting of nonmetals like airbags, plastic displays, abrasive paper, die boards etc. The CO2 laser was the ideal laser source for this application: high cw power at high beam quality (3.5 mm mrad – approx. 8 mm mrad), able to be pulsed at frequencies higher than 1 kHz, three times more efficient than lamp-pumped solid-state lasers and reliable. In the beginning, fast-axial flow lasers were preferred due to more symmetric and higher beam quality compared to cross-flow lasers, but both laser types are still available in the market. CO2 slab lasers profit from their beam quality: For thin and medium sheet thickness, they achieve higher cutting speeds than lasers of lower beam quality. After the availability of fiber delivered high-power Nd:YAG lasers some of these applications were realized with 1 µm lasers in combination with robots. First by use of high average power pulsed lamp-pumped Nd:YAG lasers later by cw lamp pumped Nd:YAG lasers. A major breakthrough accured by the long time proposed and expected diode-pumped Nd:YAG rod laser that could be designed with a CO2 laser`s typical efficiency of approx. 10 %. With beam qualities of 12 mm mrad and 300 µm fibers cut qualities and speeds became interesting for several cutting applications. The newest developments of diodepumped fiber and disk lasers achieved an electric optical efficiency of 30 % typically at beam qualities equal or better than fundamental mode CO2 lasers.

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Fiber lasers are state of the art today with “all-in-fiber” beam generation and delivery up to the cutting head. When metal sheet cutting was investigated using these lasers, several unexpected observations caused an increased interest to cutting process investigations: n Single-mode beam quality is not useful for general metal sheet cutting applications n Thin sheets can be cut very efficiently without the CO2 laser`s typical plasma effects n Aluminum can be cut fast and reliable, but CO2 slab laser cuts are better in terms of quality n Stainless steel of intermediate and high thickness cannot be cut with the typical CO2 laser quality n Any contamination of optical components effects the cutting result strongly – operators and system manufacturers need to handle the cleanliness demands n Higher demands of safety enclosure increase the costs for the systems Due to the fast and accurate control of the laser power cutting systems benefit from the CNC control development quasi automated design to part process which enables the market to deliver a cut sample just in time. Today, flat bed (flying optics) systems, punch laser combi systems, tube and profile cutting systems, 5-axis 3D cutting systems and robot based solutions are reliable industrial production tools available with either CO2 or fiber lasers.

n 1973 – Ford underbody welding system with 3 kW CO2 laser n 1976 – Fiat develops process to laser weld synchronous gears n 1979 – GM Truck & Bus uses CO2 laser to weld stainless steel n 1980 – Toyota and Honda use a Fuji 3 axis system with a CO2 laser for trim die replacement n 1983 – Thyssen Steel welds two pieces of galvanized steel with 1 kW CO2 laser. First “tailor welded blank” n 1984 – Ford (Saline) uses a 500 W CO2 laser to trim plastic dashboards n 1986 – BMW uses 5-axis gantry with CO2 laser to weld roof Further examples of the late eighties was the welding of valve lifters, fuel filters, vibration dampers, and gears established in the automotive industry as rotatory symmetric parts. In the early 90ties, roof welding was realized with CO2 lasers, articulated mirror arms and robots first, later by use of flexible fibers and lamp-pumped Nd:YAG lasers. The development of the laser sources to higher reliability, efficiency and beam quality as well as new application techniques enabled the market to expand and to grow into new applications (selection of some examples): n Differentials, axles (filler wire, induction pre and post heat treatment) n Diamond saw blades n Tailor welded blanks – one dimension – 2-dimensions – press hardening steels (ablation of AlSi coating) n Laser welded tubes, endless continuously welded mainly by CO2 slab laser (fast camera based seam tracking) n Heat exchangers (clamping techniques) n Aluminum welding: Fuel filters, car body, hybrid laser welding (beam quality, power stability) n Aluminum weld of fuselage parts (Airbus CO2 slab laser) n Complex sheet metal designs, joint


Laser Materials Processing

by laser welding like exhaust components (automation) n Laser welding of injection nozzles (beam quality) n Scanner based welding techniques (beam quality) – Remote welding system (seat structures, CO2 slab laser) – Remote scanner welding on the fly (body in white applications, fiber/ disk laser) – Scanner welding of heat exchangers Most of the applications can be realized by either CO2 or fiber laser systems. Today the CO2 laser weld process has a much lower splatter formation which makes it the tool of choice for selected powertrain applications or for tube welding applications. The efficiency, the wavelength and minimum maintenance requirements of fiber lasers allow highly economical processes added by the flexible process fiber beam delivery.

The future of industrial lasers The laser techniques for macro applications are well developed and established today. The character of improvements will be more an evolution than a revolution and will follow the demands of the market. Some experts expect a new generation of high-efficient diode lasers coupled to powerful beams for general cutting applications. These concepts need to be superior in terms of efficiency, cost, serviceability and reliability to the modern fiber lasers which raised the bar to a notable level. The market grows in the established applications, expands into new fields especially for welding and into new applications like laser additive manufacturing, remote cutting et cetera. DOI: 10.1002/latj.201400037

Authors Corinna Brettschneider studied physical engineering at the University of Applied Sciences in Wedel, Germany, and joined ROFIN for her diploma thesis in 1996. Since 2007, she is responsible for marketing and public relations.

Wolfram Rath has

studied Physics at Universities Heidelberg and Erlangen-Nürn berg with the degree of Dr. rer. nat. He was working with Siemens in CO2 laser and excimer laser development and applications before he became responsible for RofinSinar’s applications lab in Hamburg for Macro applications. Since 2011 he is responsible for the Product Management for all Rofin Macro laser sources.

Corinna Brettschneider, ROFIN-SINAR Laser GmbH, Berzeliusstr. 87, 22113 Hamburg, Germany, Tel.: +49-40-73363-4380, Fax: +49-40-73363-4138, E-mail: [email protected]



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