Pipeline Technology 2006 Conference

The VPL® Fiber Laser System - an Innovation in the Welding Technology for Pipeline Construction E. Vietz 1); H. Kohn 2); C. Thomy 2); F. Vollertsen 2) (1) VIETZ GmbH, Germany (2) BIAS Bremen, Germany Abstract Pipeline girth welding under field conditions always faced severe demands considering quality and cost. To cope with these demands, various welding techniques have been developed in the past. However, all of these technologies have certain drawbacks. For example, all manual welding techniques face cost problems in countries with high wage levels and the weld quality strongly depends on the individual training and performance of the welder. A completely new welding process and system overcoming most of the drawbacks associated with conventional pipe laying techniques for land as well as off-shore pipelines might be laser welding. Although there have been some efforts to apply lasers to girth welding, previous laser sources have proved to be incapable of enduring the rough field conditions ranging from e.g. -50 °C in Alaska to +40 °C and even more in the Arab countries. Only the newly-developed fiber laser is potentially fit for such an application. In this paper, results of studies to prepare the application of fiber lasers to pipeline girth welding with the VPL fiber laser system will be discussed. 1. introduction One field of application for welding technology is pipeline construction. Some decades ago pipes were joined with socket or flange connections, later with screwed connections and nowadays they are welded. The diameter of the pipes goes from 50 mm to 1,500 mm with a wall thickness between 2.5 and 25 mm. Whereas all other industrial branches use stationary welding units or at least carry out the welding process at one stationary product, in the field of pipeline construction the means of production move along the pipeline and are thus exposed to all influences of the changing environment and to changing weather conditions. The welding is done out-of-position (even with horizontal pipe axis) next to the pipe trench to be digged or inside the pipe trench. These construction site conditions � depending on the weather conditions, the bad ergonomic conditions and the necessity of adaptation to changing environments � have more influence on the quality of the welding result than welding problems and welding methods. These severe conditions have brought about the development of several welding techniques but they also cause increased problems for the testing and evaluation of the welding joint. Figure 1 shows a list of different factors, which are of importance for obtaining perfect pipe weldings.

Pipeline Technology 2006 Conference fig. 1 influencing factors and quality control of the welding joints

main influencing factors for welding joints Quality control • tube material, dimensions, welding consumables• machines, equipment, appliances, tools • work preparation, welding parameter • pipe welder, working conditions, supervision • climate and environmental conditions • knowledge, experience, responsibility

• inspection, examination of the

surface • test welds (destructive testing) • ultrasonic examination and radiation

tests • pressure tests with water or air

2. welding methods and welding plants in pipeline construction The above mentioned requirements determine the demands in the welding methods and welding units to be used. Possible welding methods are manual stick welding or partially or completely mechanized methods. Criteria such as materials, dimensions, intended purpose and economic efficiency are decisive for the choice of the welding method. For steel pipes manual stick welding, Innershield welding and lately also MAG orbital welding as fully or partially automated welding technique are most commonly used. Before giving a short description of each of these methods, the demands for a welding unit, which result from the necessity for mobility will be explained in the following. 2.1 demands in the welding machines Welding machines for pipeline construction must have special characteristics. The concept of the welding sets, which have been used during several decades has not changed much until today. The engine is a diesel engine (preferably air-cooled). A brush type generator, which produces linear direct current, is operating as a welding generator. A typical problem is that cellulose-coated or basic electrodes for vertical downhill welding are coated very thinly and are thus very susceptible to fluctuations of current, which may occur due to changing engine speeds. Moreover, if the open-circuit voltage is too low, this may cause problems for the ignition of the arc. The latest development in the world market shows that only now welding machines are being equipped with inverter technology, with which the same welding features are achieved as with brush generators, which are based on 40 years old technology. Still nowadays, in many countries of the world pipelines are welded vertical-down manually, with machines older than 30 years. fig. 2 job site situation in Eastern Europe

In these countries wages do not play an important role, so modern welding technologies, such as Innershield welding or MAG orbital welding are not used due to cost reasons. Moreover, every new development in the sector of pipeline construction is primarily considered from the cost point of view and secondarily from the quality aspect.

Pipeline Technology 2006 Conference 2.2 stick metal arc welding Stick metal arc welding with coated electrodes (vertical downhill) is therefore still the most frequently used welding technology for steel pipes. Prerequisites for a perfect welding operation are the exact alignment of the pipes with the help of the right centring devices, an even air gap, little edge misalignment and the avoidance of too fast falling temperatures between the individual welding layers. In order to economically apply vertical down hill welding technology (which allows a deposition rate of about 1.7 kg per hour if cellulose-coated electrodes are used) the following prerequisites have to be given:

• a large number of welders, very well trained in vertical down hill welding • appropriate alignment devices (no need to tack the pipes) • good quality welding electrodes and • appropriate welding power sources, which produce linear direct current

Especially the first point however has a considerable influence on the economic efficiency of the technology in a country with high wages, like for example Germany. 2.3 Innershield welding The main aim of this further development is to increase the deposition rate. The technology was developed by the American company Lincoln and is nowadays frequently used by Chinese companies, but also by Russian and Indian Pipeline firms. A welding set with changeable characteristics, a rectifier welding set or a welding inverter are required as a power source. Not too long ago, Lincoln Company was the only producer of the special welding wire required for Innershield welding. However, as meanwhile the patent has expired, Hobart Company (USA) as well as ELGA Company (Sweden) produce such wires, too. Until now it was only possible to weld the filling passes and capping passes with this technology, after having welded the root by hand. Thanks to the enhancements of the wires made by Hobart and ELGA, it is now also possible to weld the root perfectly in the overhead position. The welding wires normally have a diameter of 1.9 or 2.0 mm. Metal powder particles are rolled into the whole cross-section of the wire. This creates a stable arc simultaneously to a shielding gas atmosphere. The deposition rate usually amounts to 2.6 kg per hour. Also for this technology, which requires some very strict specifications, welders must be given a special training. The main disadvantage of this technology is based on the fact of fully depending on the welder�s skills and his daily shape, as all manual welding technologies. 2.4 MAG orbital welding 2.4.1 historical development In order to gain independence from the daily shape of the welders, numerous efforts have been made since the 1970ies. Some of the attempts of developing an automatic or semi automatic welding method were more and some less successful. The automatic method was applied for the first time in the construction of the �Druspa� Pipeline, which leads from Russia to former East Germany. Unfortunately all attempts failed, as the prerequisites (modern electronic and control systems) were not given. VIETZ Company applied its first system in a project in Denmark in the late

Pipeline Technology 2006 Conference 1970ies. In this project district heating pipes were laid from one part of Copenhagen to another part of the town through a channel under the harbour. The customer had required the root to be TIG welded and all further filling and final passes to be welded uphill, with an oscillating welding torch, using metal powder wire. The repair rate was 0,5 %. However, all attempts to apply this method on a line construction site failed. Even if the root was welded with cellulose-coated electrodes, whereas MAG orbital uphill welding using metal powder wire was used for the filling and capping runs, no long term success could be achieved. 2.4.2 current status of MAG orbital welding MAG orbital welding applied in pipeline construction is an economic method. Depending on the length and wall thickness of the pipes, different variations are recommendable. In order to choose the right variation, investment costs and welding speed (and thus the economic efficiency) are to be taken into consideration. The exact requirements, which a pipeline construction company has, are established first of all on the grounds of the following questions:

• Length of the pipeline to be welded and period of time required? • Diameter of the pipe? • Wall thickness of the pipe? • Topographic conditions: Plain or mountainous territory? • Outside temperature: High or low temperatures? • Construction period to be realized? • Do you have sufficiently skilled workers, e.g. welding and electronics experts?

At the moment numerous variations are present in the world market, all of which are being applied. One thing, which all these variations have in common is that they all require several stations and that at least the filling passes and the final pass are MAG orbital welded. The main difference between the variations lies in the technology used for the root, in the degree of automatization/mechanization, in the demands in the joint preparation and in the number of welding stations, which are used at the same time. The deposition rate amounts up to 6 kg per hour, depending on the number of MAG-torches used. The repair rates are normally between 3 % and 5 %. In the following one of the more frequently used variations will be described in detail (figure 3): fig. 3:required equipment for the economically efficient use of MAG orbital welding

Pipeline Technology 2006 Conference High investment costs are characteristic for this method, as a bevelling machine with a hydraulic power unit is required. The pipes must be lifted one by one with a side boom on the job site, in order to allow the special preparation of the pipe ends with the bevelling machine. The bevel normally has a narrow gap J-shape with approximately 2 mm root gap, in order to lower the quantity of filling material required. On one of the pipe ends, the guiding band, which leads the welding bugs has to be mounted before welding the root. 2 MAG orbital welding heads weld the root pass from the 12 o� clock position to the 6 o� clock position. A pneumatic internal line-up clamp with copper shoes is required in order to allow good quality root welding from the outside. The function of the copper shoes is to back the liquid welding metal, in order to obtain an adequate root, which joins the internal edges of the pipes and which assures a root sag of not more than 1 mm. The pipe ends are centred without any air gap, so that � beginning at the 12 o� clock position � the first head melts the gap with high electric power, the weld metal being backed by the copper shoes. The second welding head also starts at the 12 o� clock position, as soon as the first welding head reaches the 2 o�clock position. In order to obtain a high-quality root pass, the power supply for the inverters or rectifiers must be very constant to avoid variations of the welding parameters while starting the second welding head. Due to the varying welding positions � horizontal, downhill, uphill and overhead � it makes sense to adjust the power sources in such way that the power supply and the wire speed are regulated according to the position. This can be done automatically, semi automatically, but also manually. The hot pass is welded downhill by two welding heads set with the same welding parameters. After finishing the second layer, the welding heads are removed from the guiding band and are transported to the next joint. The following pair of welding heads now welds the filling layers (downhill) without oscillation of the torch. Depending on the wall thickness of the pipe to be welded, up to five working stations have to be employed, requiring a total of 10 welding heads. The welding is done with a solid wire and the gas mixture depends on the layer, which is being welded. It is recommendable to install an automatic gas mixture unit on the welding tractor or to use gas bottles, which already contain the required gas mixture. 2.4.3 conclusion about MAG orbital technology All variations of MAG orbital welding are fully developed, but they demand a 100 % compliance with the required conditions, if perfect welding joints are to be obtained. Therefore, despite its maturity, the MAG orbital technology has reached its limits, due to high repair quotes, break down times, as well as short comings resulting from the operation personnel. The operators of the welding heads must be highly qualified, not only in the welding sector, but also in the field of electronics. Welding parameter control units, which influence the welding process fully automatically in any welding position, have the disadvantage that any external influences (e. g. welding spatters or influences from the atmosphere) require the interaction of the welding operator, who has to manipulate the welding process, in order to minimize the defect. Finally, the high investment costs are another factor to be taken into consideration. No matter if the welding is done with two or four welding wires on one welding head, the problems do not decrease, but increase. Therefore a completely new way of thinking was needed in order to achieve an increase of output and an improvement of quality. For this reason VIETZ Company had several laser welding systems tested, in order to

Pipeline Technology 2006 Conference become able to use the laser technology with its specific advantages for pipeline construction. 3. potential uses of laser beam sources in pipeline construction 3.1 basics The made-up word �laser� is an acronym for �Light Amplification by Stimulated Emission of Radiation� and strictly speaking it is only the description of a physical process, the basic principle of which was firstly postulated by Albert Einstein in 1917. Nevertheless, from its first practical realization on, it has been used as the term, which describes the device, in which this process happens. Since the 1970ies the laser has been successfully used for many tasks in the field of material processing (e. g. for welding, cutting, curing and so on) and nowadays it still shows high increments, as still not all potential applications have been found. Numerous atoms, molecules and ions can be animated to emit laser light. The laser types are grouped according to their laser active mediums and mainly differ in terms of wave length and radiation intensity. For industrial laser welding gas lasers (CO2- lasers), solid-state lasers (e. g. Nd:YAG-Laser) and diode lasers (HPDL) (e. g. GaAs) are applied. Moreover, the latest developments in laser physics have permitted the market-ready supply of high performance fiber lasers with beam powers up to 17 kW. This kind of laser will be shortly described in the following: 3.2 high power fiber laser The fiber laser belongs to the group of the solid-state lasers, just like the Nd:YAG-Laser. The laser active medium normally consists of an Ytterbium-doped glass fiber, which is optically pumped at its ends and at the shell surface by diodes. The wave length is 1.07 µm. With an effectiveness of approximately 30 %, devices with a beam power of 17 kW are presently available in the market. The investment costs for a high power fiber laser with 17 kW beam power amount to approximately 1,800,000 $ for the laser beam at present. Figure 4 shows the laser beam source, which is used by BIAS at the moment.

fig. 4 high power fiber laser YLR-17000

Pipeline Technology 2006 Conference 3.3 comparison of the beam sources Table 1 gives an overview of the main characteristics of high power beam sources, which are in the market at the moment and which are also relevant for a potential use for welding under field conditions. table 1: properties of industrially applied laser systems, with regard to their suitability for mobile pipeline welding CO2-Laser Nd:YAG-Laser HPDL high power fiber

laser wave length [µm] 10.6 1.064 0.78 .. 0.94 1.070 max. beam power [kW] 40 5 6 17

efficiency [%] 10 3 .. 10 20 .. 40 30 beam guidance mirror fiber direct/fiber fiber suitability for heavy gauge welding + 0 - +

mobility - - + + robustness - - 0 + price of laser beam source [�] 550.000

[10 kW] 410.000 [4 kW]

330.000 [6 kW]

700.000 [10kW]

price of laser beam source/kW

[� / kW] 55.000 102.500 55.000 70.000

+ 0 -

good neutral bad

With regard to orbital welding in pipeline construction under field conditions, the fiber laser is obviously most suitable, as the CO2-Laser as well as Nd:YAG-Laser can hardly be used as mobile systems, due to their energy and space requirements and their construction and weight. The diode laser, which could at least generally be described as a mobile system, does not under normal conditions permit deep penetration welding, because of its lower intensity. Thus, thick-walled pipes could only be welded with multi-pass technique. Solely the new beam tool �high power fiber laser� provides the necessary potential as well as the mobility, flexibility and robustness, which are required especially in onshore pipeline construction, so that a new, interesting application area for this tool is given here � with decreasing investment costs [2]. You can find information on the use of different high power fiber laser systems also for other application areas in [3], [4], and [5]. 3.4 aspects of the laser application in pipeline construction Laser welding technology has been successfully applied e. g. in the stationary production of pipes ([6], [7]). For orbital welding different laser methods can be proposed, none of which is however available for the practical use on a production site, yet. The demands the user puts into the laser, which is applied for orbital welding on a job site are the following:

• The entire welding process is to be done from outside only • the laser beam must be able to endure shocks and different climate conditions • the laser beam must bridge a distance of about 30 m to the laser head

Pipeline Technology 2006 Conference

• the required power supply must not exceed 160 kVA, because bigger welding sets are difficult to handle on a job site and are thus out of question, due to economic and technical reasons

• the quality of the welding joint must meet the requirements, especially with regard to hardness increase and notch toughness

• the root must be welded perfectly without weld pool backup • the welding speed must be higher than the speed of MAG orbital welding • the investment cost must be kept in a reasonable economic frame

Welding systems, which are presently available in the market do not � or not sufficiently � meet these requirements. Due to the restricted mobility of conventional laser systems, attempts of orbital welding with lasers were made almost only on stationary or offshore appliances, where the mobility aspect does not play any role. For the onshore appliance different concepts have been discussed, however none of these concepts has yet found its way into practical appliance or into an industrially applicable product, respectively. Due to this situation VIETZ Company has carried out some preliminary tests with the diode laser. Within the framework of these preliminary tests it was possible to butt weld a 3 mm root. All following layers would have had to be welded with a cored wire according to the MAG orbital method. Moreover, the first tries with the diode laser already showed that this laser is relatively sensitive to external interferences and cannot be used under certain climate conditions. Therefore the idea of applying a diode laser for pipeline welding was abandoned. The high power laser however has the potential to meet all above mentioned requirements. Therefore a concept for the appliance of this laser on the job site was developed by VIETZ together with BIAS and some preliminary welding tests were carried out with different high power fiber lasers. The main results will be described in the following. 4. the concept of VPL System The concept of VPL System (fig. 5) is partially based on the experiences made in the context of the development of MAG welding systems, however the specific features of the high power fiber laser, as well as those of the novel welding process are to be taken into account. fig. 5 The VPL System

The system itself, which has been patented within the framework of the PTC-procedure ([8], [9]), consists of an equipment carrier and an air-conditioned chamber. The carrier among other things comprises a generator, the cooled fiber laser (mounted in a vibration-insulated way), the cooler for the laser, the gas supplying device for the welding process and a compressor unit for the cross-jet of the welding optic. Finally it includes all necessary systems for the control and supervision of the process.

Pipeline Technology 2006 Conference A hose package, which includes among other things the beam delivery fiber, the cooling circuit for the optical system, the compressed air supply and the shielding gas supply, builds the connection to the orbital welding unit. The orbital welding unit is in a climatized chamber and is orbitally movable on a guide band, which is installed relatively to the joint assembly. The tubes are aligned and clamped by a pneumatic internal line-up clamp. The orbital welding unit itself mainly consists of a welding optic, which is orbitally movable on the guide band and a moving unit for the linear positioning of the welding optic, relatively to the joint. The positioning is done online by a positioning sensor, without time loss. The complete penetration is controlled by a plasma sensor system. No welder is needed inside the chamber during the welding process, as a camera is installed for the supervision of the welding process. 5. results of the preparatory series of experiments In the following some important results of the preparatory studies concerning the application of high power fiber laser welding for girth welding of pipe steel will be explained. 5.1 qualification of the fiber laser For qualifying the welding performance of the system, which consists of the high power fiber laser YLR 10000 (with a max. beam power of 10.5 kW and a beam parameter product of 11,6 mm*mrad) and the welding optic Precitec YW 50 (focal distance 200 or 250 mm), penetration depth / welding speed curves were taken at a focal position of 0. Moreover, variations of the focal position between z= -10 mm and z=+5 mm were tested in order to examine the influence of the focal position on the seam cross section (especially seam width and penetration depth). All tests were carried out with vertical beam incidence. Figure 6 shows a penetration curve for X70 for welding speeds between 2 m/min and 15 m/min, with a beam performance of 10 kW. For the welding speed of 7 m/min the influence of the focal position is indicated as well. Moreover, the influence of the welding speed on the seam width at the level of the sheet surface is indicated. fig. 6: penetration curve

Pipeline Technology 2006 Conference Figure 7 shows that the penetration depth decreases only slightly up to a defocusing in the steel of about �5 mm, so that a possibly necessary defocusing to increase process stability can be carried out without negative influences on the welding speed. Moreover it shows that the welding process is relatively insensitive with regard to wrong positioning along the optic axis. fig. 7: variation of focal positions at X70

The XLR-17000, which has been used since March 2005 instead of the YLR-10000 has a max. beam power of 16,7 kW at the work piece and a beam parameter product of. 11,7 mm*mrad. It supplies comparable results regarding the variations of the focal position, but it has a considerably higher penetration depth at a comparable welding speed. Thus, at a welding speed of 20 m/min, almost 4 mm penetration are achieved, a figure, which the YLR-10000 achieves at about 12 m/min. Focal distances of up to 500 mm can be applied here. 5.2 welding in flat position and out-of-position with the YLR-10000 fig 8: X70, thickness 11.2 mm, welding positions PA to PE, laser power 10.2 kW, welding speed 2.2 m/min

After qualifying the complete system, sheet materials (X70, 11.2 mm thickness) were welded in the welding positions PA to PE and the results were metallographically examined and x-rayed (DIN EN 1435). Figure 8 shows the according cross sections for the welding positions PA to PE.

As a result it was established that with 10.2 kW beam power and a welding speed of 2.2 m/min welding without fissures and pores is possible. This was also confirmed by the results of the x-ray examinations. The formation of the very thin seam is evenly fine-scaly, also at the root. There are no undercuts. The slight influence of gravitation, which is recognizable in the sections does not have any impact on the quality of the welding joint and the process stability. Moreover, additional investigations were carried out with different pre-heating temperatures in order to

PA PB PC

PD PE

Pipeline Technology 2006 Conference

fig. 9: X70, thickness 11.2 mm, welding position PA, laser power 15 kW, welding speed 2.2 m/min

establish which pre-heating is necessary (regarding very small t8/5 cooling times in the range of 1 s) to obtain the required toughness. It was proven that with preheating up to about 350°C the maximum hardness increase in the WEZ 260 does not exceed HV0,3. From experience this is uncritical. With the welding parameters determined here, it is potentially possible to weld a pipe DN 1000, X70, wall thickness 11.2 m in approximately 1.5 minutes. In order to prove that a high power fiber laser of the 10 kW category is suitable for economic welding of about 80 % of the wall thicknesses, which occur in practice, welding processes were additionally carried out with S 355 material with a wall thickness of 16 mm, which is comparable with regard to the penetration depth. Here welding speeds of up to 1.2 m/min could be achieved, always considering the necessary welding joint quality. This means that a pipe DN 1000 can be welded in less than 3 minutes. Even higher potentials with regard to a further increase of the performance of the method can be achieved, if even more powerful lasers are applied. 5.3 welding with the YLR-17000 In order to use these potentials it was decided to carry out all further examinations at the work piece with the high power fiber laser IPG YLR-17000 with a maximum beam power of 16.7 kW. Within the framework of the first examinations it could be proven that X70 (t=11.2 mm) can be welded with this laser with a power of 15 kW with a welding speed of 2.8 m/min. Figure 9 shows the traverse section of such a weld.

This welding joint also has a very thin appearance in the traverse section. No significant fissures or pores could be detected. Also in the area of the root a complete connection was achieved, the formation of the root is fine-scaly and relatively even. Also the sheet top edges are completely remelted. A minor edge misalignment of about 0.3 mm could be bridged also in the area of the roots without problems. Further examinations are being carried out concerning the question of the edge misalignment, which can be of more than 2 mm even with high quality pipes, due to diameter and ovality tolerances. At present, edge misalignments of up to 1.4 mm in the root area can be bridged without explicit parameter optimization and without a decrease in welding speed. Figure 10 shows the traverse sections of 2 welds with edge misalignments of 1 mm and 1.4 mm.

For compensating the weld sagging, which appears at an edge misalignment of 1 mm or more, it is recommendable to use laser-MSG-hybrid welding or a MAG top pass.

Pipeline Technology 2006 Conference

fig. 10: influence of the edge misalignment: edge misalignment 1.0 mm (left side) and 1.4 mm (right side), X 70, laser power 15 kW, welding speed 2.9 m/min

6. résumé and outlook Faced with steadily growing demands with regard to quality and economic efficiency in pipeline laying the conventional orbital welding technology is reaching its limits, so that completely new ways have to be forged. For this reason VIETZ GmbH has followed up the technological development of the high power fiber laser from the beginning, as it is the only commercialized beam source, which is suitable for mobile application on the job site. Together with BIAS tests were carried out for the qualification of the beam source and in order to prove the suitability of the system for economically efficient welding, which observes the high quality demands for the joining of pipe steels. Welding a DN 1000 pipe in a welding time of about 1.5 min per joint in one pass seems possible for the first time so that the welding of 12 joints in one hour (including setting-up times) under field conditions with only one welding station seems possible. Due to these enormous potentials with regard to the economic efficiency of the system, a concept for a practicable complete system was developed and patented within the PTC procedure. In order to allow soon field trials of the system and a qualification of the system according to the accordant rule types, BIAS is working hard in order to push the development of the system. Soon the first deliveries of the system may be expected. 7. literature [1] Vietz, E.: Schweißverfahren im weltweiten Pipeline-bau, abgestimmt auf die

Rohrstahlqualität � gestern, heute und morgen. 18. Oldenburger Rohrleitungsforum 2004: Rohrleitungen im Jahr der Technik, Vulkan-Verlag, Essen, 2004, p.14-29.

[2] H. Kohn, C. Thomy, M. Grupp, F. Vollertsen: Neue Entwicklungen beim Laserstrahlschweißen von Rohren. 18. Oldenburger Rohrleitungsforum 2004: Rohrleitungen im Jahr der Technik, Vulkan-Verlag, Essen, 2004, p. 50-74.

Pipeline Technology 2006 Conference [3] C. Thomy, M. Grupp, T. Seefeld, F. Vollertsen: Schweißen mit dem

Hochleistungs-Faserlaser. 6. Konferenz �Strahltechnik�, Halle, 26.-28. April 2004, p. 39-45.

[4] H. Kohn, C. Thomy, F. Vollertsen: Schweißtechnische Anwendungen mit einem 10 kW Faserlaser � Erste Ergebnisse. Tagungsband EALA European Automotive Laser Application 2005 (CD), 02.-03.02.2005, Bad Nauheim.

[5] C. Thomy, T. Seefeld, F. Vollertsen: High-Power Fibre Lasers � Application Potentials for Welding of Steel and Aluminium Sheet Material. Advanced Materials Research Vols. 6-8 (2005), p. 171-178.

[6] C. Thomy, M. Schilf, T. Seefeld, F. Vollertsen, G. Sepold, R. Hoffmann: CO2-Laser-MSG-Hybridschweißen in der Rohrfertigung. Schweißen und Schneiden 2003, DVS-Berichte Bd. 225, DVS-Verlag, Düsseldorf, p. 167-173.

[7] C. Thomy, M. Schilf, T. Seefeld, F. Vollertsen, G. Sepold, R. Hoffmann: CO2-Laser-MSG-Hybridschweißen in der Rohrfertigung. wt Werkstattstechnik online 93 (2003) 6, p. 462-466.

[8] USP 60/528,189: Girth Welding of Pipelines with Fiber laser, 10. 12. 2003. [9] PCT/EP2004/014089: Orbitalschweißvorrichtung für den Rohrleitungsbau, 10.

12. 2004.