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Battle of the Sources - Using a high-power Yb-fibre laser for welding steel and aluminium (June 2005)

   
G. Verhaeghe and P Hilton

Paper published in WLT Conference - Lasers in Manufacturing 2005, 13-16 June 2005, Munich, Germany.

Abstract

The pace of development in laser technology has increased considerably in the last few years, with both improvements in existing technology, such as diffusion-cooled CO2 and diode-pumped Nd:YAG lasers, and developments of completely new laser sources, such as the direct diode, the disc and the fibre laser. Since the output powers for the fibre laser have exceeded 1kW, interest in this technology for materials processing has rocketed, because of the advantages it offers over existing solid-state lasers. The available output power, now well above the standard 4kW available for other solid-state sources, the high power conversion efficiency and the claimed reliability are particularly attractive. In this paper, fibre lasers and their claimed advantages over conventional laser welding technology are discussed, and their performance for the welding of both steel and aluminium assessed based on a 7kW Yb-fibre laser source installed at TWI.

1. Introduction

Since output powers of one kilowatt and more have become available for fibre lasers, the material processing industry has shown particular interest in this new technology as an addition to, or a possible replacement for, the more conventional CO2 and Nd:YAG lasers currently used. Fibre laser technology seems to allow, for the 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 commercially available 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 CO2 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. This paper sets out to investigate these claims, as well as suitability of fibre laser technology for welding, based on trials performed on steel and aluminium using a 7kW fibre laser.

2. 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. [1,2] 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 dopant, 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 laser diodes is maximised by using a low refractive index, internal cladding surrounding the inner doped core of the 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 (which 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 focussing optics at its end, de-coupling of the beam delivery fibre from the lasing fibre is preferred for lasers used for material processing as a means of protection.

To date, 200 to 400W 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. [1,2] The manufacturing route currently preferred for achieving output powers suitable for deep penetration keyhole welding of metals is by combining the outputs from a series of these commercially available 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 welding.

At the beginning of 2004, a 7kW Yb-fibre laser was installed at TWI Ltd, 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. Twenty metre long, 300µm diameter,single optical fibres transmit the laser power from the optical switch to the laser process heads. 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, i.e. the beam diameter at the focussing 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, focussing lenses with a 250mm and a 160mm focal length were chosen (with a 120mm focal length collimating lens) to produce a calculated 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 600µm and 390µmwas measured for the 250mm and the 160mm focussing 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.

Fig.1. The 7kW Yb-fibre laser welding trials set-up
Fig.1. The 7kW Yb-fibre laser welding trials set-up

 

3. Comparison with existing technology

The measured laser beam quality of between 17.7 and 18.7mm.mrad for the 7kW fibre laser installed at TWI is better than that of a 4kW lamp-pumped Nd:YAG laser, typically around 25mm.mrad. But what impact, in practical terms, does the beam quality have on welding performance? 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. [4] The beam quality of a solid state laser, usually given the term BQ but often also known as the beam parameter product, is generally quoted in mm.milliradians, with a low value meaning 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 paper 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. [5]

A consequence of a high laser beam quality is that the beam can be focussed into a small diameter optical delivery fibre. This translates, on the processing side for a given lens diameter, in a smaller minimum beam waist diameter or a larger stand-off distance. The laser process head images the end of the fibre onto the workpiece, by first collimating (i.e. making parallel) the diverging laser beam exiting from the fibre, before focusing to a minimum beam waist diameter, often also referred to as the laser spot. The relationship between the ratio of the collimating and focussing lens focal lengths and the ratio of the beam delivery fibre diameter and the spot size,determines, for a given output power, the maximum power density available at the workpiece, an important parameter when deep-penetration keyhole welding. The stand-off distance, the distance measured between the focusing lens and the surface of the material, should be large enough to provide a degree of confidence that spatter from the welding process will not damage the processing optics. The larger the stand-off, the greater also the depth of focus. [4] In summary, a higher beam quality can provide a higher power density at the beam focus, or, a larger stand-off distance/greater depth of focus, both of which influence the welding performance. To enable a like-for-like comparison between laser sources in terms of beam quality and its effect on welding performance, 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, as well as the beam quality values for each of these types of laser optimised for a nominal output power of 1kW.

 

Table 1: Laser source comparison

 CO2Lamp-pumped Nd:YAGDiode-pumped Nd:YAGYb-fibre (multi-mode)Thin disc Yb-YAG
Lasing medium Gas mixture Crystalline rod Crystalline rod Doped fibre Crystalline disk
Wavelength, nm 10,600 1060 1060 1070 1030
Beam transmission Mirror, lens Fibre, lens Fibre, lens Fibre, lens Fibre, lens
Typical delivery fibre Ø, mm - 0.6 0.4 0.1-0.2 0.15-0.2
Output powers a , kW Up to 15kW Up to 4kW Up to 6kW Up to 20kW Up to 4kW
Typical beam quality b , mm.mrad 3.7 25 12 20 7
3.7 12 <12 1.8 4
Maintenance interval, khrs 2 0.8-1 2-5 100 c 2-5
Power efficiency, % 5-8 3-5 10-20 20-30 10-20
Approximate cost per kW, k$ 60 130-150 150-180 130-150 130-150
Footprint of laser source large medium medium small medium
Laser mobility low low low high low
Notes:
a) Commercially available.
b) The top figures are for the max. available output powers, the bottom figures for the same type of laser but configured for optimum operation at 1kW.
c) Manufacturer's claim.

 

A claimed reliability in the order of 100,000hrs before maintenance/failure of the diode pumps is much higher than that quoted for other laser sources ( Table 1). The claim is based on the fact that fibre laser pumping technology uses less stressed lasers diodes instead of diode stacks, as used in diode-pumped Nd:YAG lasers for instance. [3] The maintenance interval of conventional solid-state lasers refers to the time between two consecutive changes of the pumping source, i.e. 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 a few Yb-lasers with an output power higher than 5kW in operation in Europe. With the first system only operational for just over 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%. [3] 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 CO2 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 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 CO2 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 1m2 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 CO2 laser source used at TWI in the early 1980s. Because of its inherently simple and compact design, the YLR-7000 was, in contrast with the requirements for CO2 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.

The initial investment cost of any laser is high, but this should be viewed together with the productivity advantages the technology offers, i.e. calculating the running costs of the technology per weld or per component. For the fibre laser for instance, this should take into account the use of a smaller chiller, the lower energy consumption, the reduced floor space and the minimal maintenance and service requirements. Notwithstanding large variations in quoted prices, the authors estimate 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 lamp-pumped Nd:YAG laser source. The manufacturer of the other new laser technology currently promoted, i.e. the Yb-YAG disc laser, has claimed the same prices ( 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 could drop further. The limiting factor in this price drop however, is the cost of the pumps, which is, as with all solid state laser technology, a major factor in the selling price.

Fig.2. Process set-up
Fig.2. Process set-up

4. Using fibre lasers for welding

The performance of the 7kW fibre laser system detailed above, was assessed for the welding of thick-section steel and aluminium, with thicknesses of 6mm and above considered thick-section for the purpose of this study. The process set-up was similar to that typically used for welding with a 4kW lamp-pumped Nd:YAG laser, and comprised an optical glass cover slide, an airknife and helium shielding for the aluminium welds ( Fig.2).

The airknife operated at a pressure of between 5 and 6 bar and was positioned just below the processing optic to provide a degree of confidence that spatter from the welding process would not damage the processing optics. Welding performance diagrams were created from a series of fully penetrating bead-on-plate runs carried out on steel and aluminium, Fig.3 and Fig.4. Welding conditions were then further refined to produce fully penetrating square-edge butt joints in various thicknesses of steel and aluminium, and T-joints in steel.

Fig.3. Performance curves for the welding of C-Mn steel using a Yb-fibre laser
Fig.3. Performance curves for the welding of C-Mn steel using a Yb-fibre laser
Fig.4. Performance curves for the welding of Al using a Yb-fibre laser.
Fig.4. Performance curves for the welding of Al using a Yb-fibre laser.

 

For a spot size of 0.6mm, achieved for the fibre laser with the 250mm focal length focusing lens, 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.5 show the thickness of the C-Mn steel sample that can be penetrated using 3 and 4kW of Yb-fibre laser and Nd:YAG laser power, with the Nd:YAG values in Fig.5 obtained from earlier TWI work. [6] Although the curves do not exactly overlap, a better performance for either the Yb-fibre or the Nd:YAG laser cannot 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 material composition or the way the trials were performed or depth of penetration recorded, for instance, have probably contributed to the differences shown, rather than these differences resulting from a change of laser source. A more detailed study, excluding all such non-process factors, is currently underway at TWI to determine how laser sources with different beam qualities may or may not affect welding performance.

Fig.5. Performance curves for the welding of C-Mn steel using a lamp-pumped Nd:YAG and a Yb-fibre laser.
Fig.5. Performance curves for the welding of C-Mn steel using a lamp-pumped Nd:YAG and a Yb-fibre laser.

 

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, and a travel speed of 1.6m/min, is shown in Fig.6. Nd:YAG and CO2 comparisons, also shown in this figure, were welded with only 4kW of laser power at the workpiece, a spot size of 0.6 and 0.3mm respectively and a welding speed of 0.5m/min and 1.2m/min respectively. [6,7] Standard plume and plasma suppression methods were applied for the Nd:YAG and the CO2 laser weld respectively, but not for the fibre laser weld. The sizes and shapes of the welds are determined by the difference in power density, i.e. 25, 14 and 57kW/mm2 respectively, and the difference in laser energy input, i.e. 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 ISO13919-1 was achieved for all three laser sources.

Fig.6. Fully penetrating, square-edge butt joints in 8mm thickness C-Mn steel
Fig.6. Fully penetrating, square-edge butt joints in 8mm thickness C-Mn steel

 

The cross-sections in Fig.7 show zero-gap, square-edge butt joints completed in 12.7mm and 12mm thickness C-Mn steel welded with the Yb-fibre and a 10kW CO2 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, 0.75 and 0.5m/min respectively. The geometry of the weld and heat-affected zone is determined by the power density, i.e. 25, 105 and 105kW/mm2 respectively, and laser energy input, i.e. 1292, 888 and 592J/mm respectively, used for welding. Whereas plasma suppression was used for the CO2 laser welds, [7] no plume suppression was used for the fibre laser weld, resulting in its typical nail-head profile. The use of effective plume suppression, normally recommended for welding speeds equal to or slower than 1.5m/min, would have given the weld a more parallel transverse aspect, and full penetration at a higher welding speed.

Fig.7. Fully penetrating, square-edge butt joints in 12.7 and 12mm thick C-Mn steel
Fig.7. Fully penetrating, square-edge butt joints in 12.7 and 12mm thick C-Mn steel

 

The sections in Fig.8 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 0.6 in diameter, 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.

Fig.8. Fully penetrating, close-fitting C-Mn steel T-joints joining an 8mm thickness web to a 12.7mm thickness flange, welded in the horizontal-vertical welding position
Fig.8. Fully penetrating, close-fitting C-Mn steel T-joints joining an 8mm thickness web to a 12.7mm thickness flange, welded in the horizontal-vertical welding position

 

By choosing a hybrid laser-arc combination, i.e. 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.9 illustrates the initial results of combining 7kW of Yb-fibre laser power (at the workpiece) with a MAG arc. Using a single set of welding conditions, a joint penetration of 8mm was achieved in the flat, vertical up and overhead welding position at a welding speed of around 1.8m/min. Notwithstanding a good initial weld profile, the weld bead geometry should be improved upon further, in future trials, by refining the wire feed speed, current and/or arc voltage setting.

Fig.9. Hybrid Yb-fibre laser - MAG weld completed in 8mm thickness X60 pipeline steel welded in the vertical-up welding position
Fig.9. Hybrid Yb-fibre laser - MAG weld completed in 8mm thickness X60 pipeline steel welded in the vertical-up welding position

 

The cross-sections shown in Fig.10 are of welds completed in 6.35mm aerospace grade aluminium alloy. The welds were completed using a laser power of 7kW at the workpiece focused into a 0.6mm and a 0.4mm diameter spot, resulting in a travel speed of 2.8and 4.8m/min respectively. In comparison, a typical welding speed for a 4kW Nd:YAG laser focused into a 0.4mm diameter spot would be between 0.5 and 1.0m/min. The cross-sections in Fig.11 show welds completed in a 12.7mm thickness aerospace grade aluminium alloy, produced using 7kW of laser power and a welding speed of 0.65 and 0.85m/min for a 0.6mm and a 0.4mm diameter spot respectively. Isolated pores were observed in some of the welds, not unusual for laser welded aluminium, believed to be originating from hydrogen or the helium shielding gas used. Notwithstanding a good initial weld profile, the weld bead geometry will be improved upon further, in future trials, by further refining the shielding gas support, laser beam focus position and welding speed.

Fig.10. Fully penetrating square edge butt joints in 6.35mm thick aluminium
Fig.10. Fully penetrating square edge butt joints in 6.35mm thick aluminium
Fig.11. Fully penetrating, square-edge butt joints in 12.7mm thick aluminium
Fig.11. Fully penetrating, square-edge butt joints in 12.7mm thick aluminium

5. Conclusions

Laser manufacture 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 CO2 or Nd:YAG for the welding of steel and aluminium. 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 available at high powers is also attractive from a cutting point of view, a market currently dominated by CO2 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 beam delivery and output power. Besides a technological impact, this should contribute to much more affordable lasers.

Bibliography

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  4. Hilton, P. A.: Fibre optic beam delivery for high-power CW Nd:YAG lasers. Confidential TWI report 88277/49/98, December 1998.
  5. BS EN ISO 11146:1999 'Lasers and laser related equipment - Test methods for laser beam parameters - Beam Widths, Divergence Angles and Beam Propagation Factor'.
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