Deep penetration welding - the fibre laser way

TWI Bulletin, March/April 2005

A relative newcomer in the field of laser material processing is making its presence known.

Geert Verhaeghe joined TWI in 1996 where, as a Senior Project Leader in the Arc Welding Group first, and since 1999 in the Laser Welding Group, he has managed a variety of projects on a range of arc and laser welding processes and applications. He has gained specific experience in the welding of aluminium, the use of robotics and automation for welding, the prediction and prevention of weld distortion and hybrid laser-arc welding. More recently, he has been responsible for the set-up and initial performance testing of the 7kW Yb-fibre laser at TWI. Geert has particular involvement with projects for the aerospace and road transport industry sectors.

Paul Hilton After graduating with a PhD in solid state physics, Paul worked for five years at an international R&D organisation in France, before returning to the UK in 1979 to work for Oxford Instruments. In 1983 he became Research & Development Manager at Control Laser Ltd, which was taken over by Integrated Laser Systems in 1987, at which point he became Technical Director. Paul joined TWI in 1990 as Head of Department of the Laser Centre, where he was responsible for the day to day operation of the department and for the alliance between the laser centres of AEA Culham Laboratory and TWI. In 1994 he became 'Technology Manager - Lasers', with a strategic responsibility for use of laser technology throughout TWI. Paul is currently President of the UK's Association of Industrial Laser Users.

Over the past 35 years, an increasing number of laser sources have been introduced into a range of industry sectors, including automotive, shipbuilding and aerospace, for welding applications. As Geert Verhaeghe and Paul Hilton report, for many years the CO ₂ laser remained the only laser of choice where deep penetration or keyhole welding was required.

A significant step forward was made when continuous wave solid state lasers, in the form of the lamp-pumped Nd:YAG laser, became commercially available, covering at least part of the power range of CO ₂ lasers, and introducing the benefits and advantages of optical fibre delivery of the laser beam to the workpiece. The pace of laser technology development however, has increased considerably. Recently, improvements to existing technology, such as the introduction of the diffusion-cooled, or slab geometry, CO ₂ laser and the diode-pumped version of the Nd:YAG laser, plus developments of completely new laser sources, such as the direct diode laser, the disc laser and the fibre laser, are beginning to find their place in manufacturing applications. It is the fibre laser source, which is the subject of this article.

Fibre laser technology

Fibre lasers, not to be confused with fibre-delivered lasers, where the fibre is merely an optical delivery mechanism, are solid state lasers in which an optical fibre doped with low levels of a rare earth element is the lasing medium. Laser diodes are used to stimulate the doping atoms, an action known as pumping, forcing them to emit photons at a specific wavelength. The wavelength of the emitted laser light depends on the rare earth element used as the doping element, with wavelengths between 1540 and 1550nm, between 1800 and 2100nm and between 1060 and 1085nm, emitted when using erbium, thulium and ytterbium respectively.

Ytterbium is generally the doping element used for the high power fibre lasers currently available for material processing. The use of pumping power available from the laser diodes is maximised by using a low refractive index, internal cladding surrounding the inner core, or doped fibre, as a waveguide for the pump light. An even lower refractive index external cladding contains the pump energy and ensures maximum absorption of this energy by the rare earth elements in the doped fibre.

Diffraction gratings are used as rear mirror and output coupler, to form the laser resonator, creating a long thin laser, which due to the flexibility of the optical fibre, (it is simply coiled up) can be very compact. Although it is possible to use the doped fibre as the beam delivery fibre, using appropriate beam shaping and focusing optics at its end, de-coupling of the beam delivery fibre from the lasing fibre is preferred for lasers used for material processing, in case of fibre damage.

Low power single-mode fibre lasers of a few watts have been used for many years as signal amplifiers in the telecommunications industry. The slump in the telecommunications industry however, led manufacturers to seek new applications for fibre laser technology, and in 2001, a single fibre laser unit with an output power of 100W was produced, which opened new market opportunities. To date, 200 and 370W single-mode fibre laser modules are commercially available, with prototype single-mode Yb-fibre lasers of up to 1000W of output power already being assessed in laboratory conditions.

The manufacturing route currently preferred for achieving output powers suitable for deep penetration welding of metals is by combining the outputs from a series of these commercially low power eg 200W single-mode units into a single fibre output. Although this beam combining technique, proprietary to the laser manufacturer, reduces the beam quality, the reduction is relatively small and the resulting laser beam has properties suitable for keyhole laser welding.

Most propagating laser beams diverge naturally and, when focused into a spot, the shape of propagation of a perfect beam is a hyperbola. At any point, the laser beam can be characterised by a divergence angle and a beam width, or diameter, derived from the power density distribution in the direction perpendicular to the beam propagation. The beam quality is defined as the ratio of the beam width and divergence angle product of the actual beam to that expected for a perfect beam. The beam quality of a solid state laser, usually given the term BQ but often also known as the beam parameter product BPP, is generally quoted in mm.milliradians, with a low value indicating a high beam quality.

Confusion sometimes arises as beam quality can be expressed using either full or half beam diameter and divergence angles. In this article however, beam quality is quoted in terms of half beam diameter and half divergence angle, in line with the ISO standard for laser beam propagation. But what does beam quality actually mean for practical laser welding applications?

One consequence of a laser with a high beam quality is that the beam can be focused into a small diameter optical beam delivery fibre. The laser process head on the other end of the beam delivery fibre is simply imaging the end of that fibre, with the diverging laser beam exiting from the fibre first made parallel (or collimated) using one or more lenses. It is then focused, into a minimum beam waist diameter, often also referred to as the laser spot. The relationship between the ratio of the collimating and focusing lens focal lengths and the ratio of the beam delivery fibre diameter and the spot size, determines the maximum power density available at the workpiece, an important parameter when deep-penetration keyhole welding.

Also important for welding applications is the stand-off distance between workpiece and focusing lens, which must be large enough to provide a degree of confidence that spatter from the welding process will not detrimentally affect the processing optics. Thus, the main advantage of a high beam quality for a laser process head of a given diameter is that, for a given minimum spot diameter, a larger stand-off distance can be used, thus improving protection of the optics.

This also gives a greater depth of focus, here defined as the length along the laser central axis over which the focused laser beam diameter increases in size by 5%. Alternatively, for a given stand-off distance and output power, a high beam quality can give a smaller minimum spot diameter, which in turn produces a higher power density at the beam focus.

Comparison with existing technology

Since laser output powers of one kilowatt and more have become available, the material processing industry has shown an increased interest in fibre laser technology as an addition to, or possible replacement for, the more conventional CO ₂ and Nd:YAG lasers currently used. The fibre laser technology seems to allow, for the very first time, the manufacture of easily scaleable lasers, in a compact form, with no obvious limit to the power available, other than money.

Today, the output power of a fibre laser far exceeds that available using Nd:YAG laser technology, whilst also offering a better beam quality. In fact, fibre laser power and beam quality are fast approaching, and in certain cases already even exceeding, those of CO ₂ lasers. In addition to the power and beam qualities now available, the fibre laser manufacturers claim high reliability and high power conversion efficiency. This, coupled with the additional benefits of a small footprint, compact design and no moving parts, merits a closer investigation of this new laser technology for material processing.

To investigate some of these claims and to access the performance of this new laser technology for welding of metals, a 7kW Yb-fibre laser was recently installed at TWI, Fig.1. The YLR-7000 laser, manufactured by IPG Photonics GmbH, comprises a series of 200W single-mode fibre units, the outputs of which are combined, using proprietary IPG technology, into a 10m long, 200µm diameter, single optical fibre. The output laser power is transmitted into a four-way optical switch maximising the system's flexibility to process industrial components of different sizes and shapes, Fig.2. Twenty metre long, 300µm diameter, single optical fibres transmit the laser power from the optical switch to the laser process head. Based on existing experience at TWI using up to 9kW of Nd:YAG laser power, by combining laser sources of very different beam qualities, the optical diameter of the laser process heads for the fibre laser, ie the beam diameter at the focusing lens, was set at 43mm, providing a fairly compact head, useful for instance when access is restricted.

The beam quality of the laser beam through the four beam paths was measured to be between 17.7 and 18.7mm.mrad. With this beam quality, focusing lenses with a 250mm and a 160mm focal length were chosen to produce a nominal minimum spot diameter of 0.625mm and 0.4mm respectively, similar to those obtained with the 4kW lamp-pumped Nd:YAG laser also available at TWI. During testing an average minimum spot diameter of 604µm and 386µm was measured for the 250mm and the 160mm focusing lenses respectively. A Kawasaki ZX130L 6-axis articulated robot arm, equipped with Interbus-S to ensure high-speed communication between robot and laser, was used for beam manipulation.

The measured beam quality of between 17.7 and 18.7mm.mrad for the fibre laser is better than that of a 4kW lamp-pumped Nd:YAG laser. But to make a like-for-like comparison between laser sources in terms of beam quality, it should be noted that beam quality should be considered at the same output power. Table 1 gives an overview of beam quality values for a number of lasers commercially available today.

Table 1. Laser source comparison

In case of the Yb-fibre laser for instance, a beam quality of less than 1mm.mrad has been claimed for the latest development of a 1kW single-mode Yb-fibre laser source, whereas 5mm.mrad is claimed for a 5kW multi-mode source.

A claimed reliability in the order of 100,000hrs before maintenance/failure of the diode pumps is much higher than those quoted for other laser sources, see Table 1, and is based on the fact that fibre laser technology uses less stressed laser diodes instead of diode stacks as used in diode-pumped Nd:YAG lasers for instance. The maintenance interval of conventional solid-state lasers refers to the time between two consecutive changes of the pumping source, ie flashlamps in case of lamp-pumped, and diode bars in case of diode-pumped Nd:YAG lasers, and is an order of magnitude less than that claimed for the fibre lasers.

There are currently only three Yb-fibre lasers with output powers higher than 5kW installed in Europe, ie the earlier mentioned 7kW system at TWI, an 8kW system at Cranfield University and a 10kW system shared between BIAS, a Bremen-based laser research institute, and a Rostok shipyard. As the first operational system has only been running for less than a year, it is, at this stage, impossible to either confirm or deny this 100,000hrs claim.

The long, thin fibre geometry allows effective cooling and is thus ideal to minimise thermal effects due to the pump energy. That, and the inherently high gain of the fibre laser source, translates in a high power conversion efficiency, which is the ratio of optical power available at the workpiece to the electrical power consumed, claimed to be between 20% and 30%.

For the YLR-7000 system installed at TWI, a power conversion efficiency of 21% was calculated for an output power of 4 and 7kW, based on the optical power measured after the optical switch and a 250mm focal length process head. Compared with a power conversion efficiency of around 8 and 3% for CO ₂ and lamp-pumped Nd-YAG lasers respectively, this is significantly higher. The immediate economic impact is two-fold, in that less power is required to run the laser at full power, and less power is also required to dissipate the heat generated by the laser. Air cooling for instance, is now available for Yb-fibre lasers up to 2kW, whereas higher output powers require water-cooling. These units however, are much smaller than their CO ₂ and Nd:YAG laser equivalents.

As the lasing fibres can be coiled up, and no bulky moving parts are required, the footprint of the fibre laser is significantly reduced when compared with conventional laser technology. The approximately 1m ² footprint of the YLR-7000, without the chiller, for instance, is over four times smaller than a commercially available 4kW lamp-pumped Nd:YAG laser source, and many times smaller than the 10kW cross-axial flow CO ₂ laser source used at TWI in the early 1980s, ( Fig.3).

Note that the chiller, placed outside the building because of its size, ie approximately the size of a waste container, and the noise it produced, is not shown in this picture. Because of its inherently simple and compact design, the YLR-7000 was, in contrast with the requirements for CO ₂ and Nd:YAG lasers, installed in only a few hours. This included connecting the laser to the chiller, the four-way optical switch and laser process head, and preparing the system for a beam and output power analysis.

Nevertheless, as lasers are still considered by many as a manufacturing tool too expensive to use, the above benefits should be weighed up against the ultimate question of capital investment cost and how this compares with other available lasers. Notwithstanding large variations in quoted prices, the author estimates that in the current competitive market, at the time of writing, the cost per kW of Yb-fibre laser technology is about the same as that of a diode-pumped Nd:YAG laser and between 15 and 20% more expensive when compared with a lamp-pumped Nd:YAG laser source. The manufacturer of the other new laser technology currently being promoted, ie the Yb-YAG disc laser, claims the same cost per kW as a lamp-pumped Nd:YAG laser, Table 1.

Because of the fast pace of technological development and increasing competition between existing and new laser technologies, it is expected that overall prices per kW of laser output power will come down much faster than before. As all the new solid state laser technology relies on diode pumping, the driving force in selling price will be the cost of the pumps. What also should be born in mind, is that not only capital investment, but also the cost of running the laser should be considered in any economical evaluation of laser technology. That is where the fibre laser, with its high power conversion efficiency, has a clear advantage over most conventional lasers capable of deep-penetration keyhole welding.

Welding performance

A series of welding trials were completed using up to 7kW of Yb-fibre laser power, measured at the workpiece, and a 0.6mm minimum spot diameter, to demonstrate the welding performance of the YLR-7000 laser on thick section C-Mn steel. Using a set-up similar to the one used for welding with a 3 or 4kW lamp-pumped Nd:YAG laser, comprising an optical glass cover slide and an airknife, operating at a pressure of between 5 and 6 bar, just below the processing optic to provide a degree of confidence that spatter from the welding process would not detrimentally affect the processing optics, fully penetrating bead-on-plate runs were produced at various speeds and laser output powers to produce power-penetration-speed diagrams, such as the one shown in Fig.4 for C-Mn steel.

Based on the earlier comments related to beam quality, power density, stand-off distance, and their importance for welding applications, and the choice of the 250mm focusing lens to produce the 0.6mm minimum spot diameter, no major differences in processing conditions were expected between the Yb-fibre laser and the lamp-pumped Nd:YAG laser for equivalent output powers.

The curves in Fig.4 give the values for depth of penetration in C-Mn steel when welding with 3 and 4kW of Yb-fibre laser and Nd:YAG laser power. The Nd:YAG values in Fig.4 were obtained from earlier TWI work. Although the curves do not exactly overlap, a better performance for either the Yb-fibre or the Nd:YAG laser can not be concluded, as the Yb-fibre laser would appear to have a slight performance advantage over Nd:YAG when evaluating the 4kW values, with the opposite true when considering the 3kW values. Small differences in the way the trials were performed or the depth of penetration recorded for instance, have probably contributed to the differences shown between the two laser sources, rather than these differences resulting from a change of laser source. For a more comprehensive welding performance comparison between laser sources, execution of the trials has to exclude all such non-process factors, and such a study is currently underway at TWI.

The cross-section of a zero-gap, square-edge butt joint in 8mm thickness C-Mn steel welded using 7kW of Yb-fibre laser power at the workpiece, focused into a 0.6mm spot diameter and at a travel speed of 1.6m/min, is shown in Fig.5. Nd:YAG and CO ₂ comparisons, also shown in this figure, were welded with 4kW of laser power at the workpiece, a minimum nominal spot diameter of 0.6 and 0.3mm respectively and a welding speed of 0.5m/min and 1.2m/min respectively. Standard plume and plasma suppression methods were applied for the Nd:YAG and the CO ₂ laser weld respectively. The sizes and shapes of the welds are determined by the difference in power density, ie 25, 14 and 32kW/mm ² respectively, and the difference in laser energy input, ie 263, 480 and 200J/mm respectively, which also affects the size of the heat-affected zone. A stringent weld quality in accordance with BS EN ISO 13919-1 was achievable for all three laser sources.

The cross-sections in Fig.6 show zero-gap, square-edge butt joints completed in 12.7mm and 12mm thickness C-Mn steel welded with the Yb-fibre and TWI's decommissioned CL10 CO ₂ laser respectively. The welds were completed using 7kW and 7.4kW of laser power at the workpiece, a minimum spot of 0.6 and 0.3mm in diameter and a travel speed of 0.325 and 0.75m/min respectively.  

The geometries of the weld and heat-affected zone were determined by the power density and laser energy input used for welding, ie 25 and 105kW/mm ² and 840 and 592J/mm respectively. Because of the large weld pool size when welding with the Yb-fibre laser, welding was carried out in the PC, or horizontal-vertical, position, as welding in the PA, or flat, position resulted in occasional drop-through of the weld at that welding speed. Whereas plasma suppression was used for the CO ₂ laser welds, no plume suppression was used for the fibre laser weld giving it its typical nail-head profile. An effective plume suppression is expected to increase penetration by around 15% for welding speeds equal to or smaller than 1.5m/min and give the weld a more parallel cross-sectional aspect.

The sections in Fig.7 were taken from welds in close-fitting C-Mn steel T-joints comprising an 8mm thickness web and a 12.7mm thickness flange produced using 7kW of Yb-fibre laser and 4kW of Nd:YAG laser power at the workpiece. A travel speed of 0.8 and 0.3m/min respectively, with a minimum spot of 0.6mm in diameter used for both, resulted in a power density of 24.8 and 14.2kW/mm2 and a laser energy input of 525 and 800J/mm respectively. Notwithstanding a good initial weld profile, the weld bead geometry, in particular at the weld root, will be improved upon further, in future trials, by optimising the focal position, beam-to-joint alignment and process work angle.

By combining an electric arc in the same weld pool as that of a laser beam, faster welding speeds, larger depth of penetration, improved quality and/or improved tolerance to joint fit-up can be achieved, compared with the individual processes. In other words, the technical benefits of laser welding are retained or enhanced whilst the economy of the process is improved. The cross-section in Fig.8 illustrates the initial results of combining 7kW of Yb-fibre laser power (at the workpiece) with approximately 5.4kW of MAG arc power.

Conclusions

The laser manufacturing business has become a highly competitive market with lasers being introduced in a wide variety of industry sectors for a range of applications, including cutting, drilling, welding, marking and surface engineering. Initial welding trials using a 7kW Yb-fibre laser at TWI confirm that this new type of laser source should now be considered as an alternative to CO ₂ or Nd:YAG for welding. Over the next few years, it is expected that various industry sectors will further investigate the viability of the fibre laser as a production tool, because of its power conversion efficiency and claimed reliability. Its compact design, easy set-up and minimal cooling requirement also makes it an ideal laser source for on-site welding, for pipeline welding or shipbuilding, or for remote repair applications, for instance. The high beam quality at high powers is also attractive from a cutting point of view in a market currently dominated by CO ₂ lasers. Current confidence in fibre laser technology, as well as advances made with other laser technologies, such as the Yb-disc laser, is increasingly pushing the boundaries of optical fibre delivered beam quality and output power. Besides a technological impact, this also contributes to much more affordable laser technology.