The fibre laser: A newcomer for material welding and cutting

Why this new type of laser is attracting so much interest from the materials processing industry

G Verhaeghe

Paper published in Welding Journal August 2005.

Introduction

The first reports on the use of lasers for material processing in commercial applications were published in the early seventies. For many years, the CO ₂ laser remained the only laser of choice when deep penetration keyhole welding was required. A significant step forward was made when continuous wave (CW) solid state lasers, in the form of the lamp-pumped Nd:YAGlaser, 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. More recently, the industry has seen both improvements of these existing technologies, with the introduction of the diffusion-cooled, or slab geometry, CO ₂ laser and the diode-pumped version of the Nd:YAG laser, as well as developments of completely new laser sources, such as the direct diode laser, the disc laser and the fibre laser. Particularly since the output powers of the fibre laser have exceeded one kilowatt, has the materials processing industry gained interest in this new technology as an addition to, or a possible replacement for, the more conventional CO ₂ and Nd:YAG lasers currently used. This article sets out to investigate this new technology, some of its claims and its suitability for welding, based on experience using the latest fibre laser technology at TWILtd (Cambridge, UK).

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 lasing medium to emit photons, an action known as pumping, at a wavelength specific to the rare earth element used as the doping element. Ytterbium is generally used for the high powerfibre lasers currently available for material processing and emits a wavelength approximately the same as Nd:YAG lasers, i.e. between 1.060 and 1.085 micron. The doped fibre is surrounded by a low refractive index material that acts as a waveguide for the pumping light and ensures optimum transfer of this energy to the lasing medium. 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 lasing 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 from unwanted back reflections from the workpiece surface.

To date, 700W 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] 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, with the resulting laser beam still having properties suitable for transmission through small diameter optical fibres and for welding.

Comparison with existing laser technology

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 CO ₂ lasers. For instance, a 17kW Yb-fibre laser with a beam parameter product (BPP) of around 12mm.mrad was recently installed in Europe and a 5kW system with a BPP of 2mm.mrad is now available.

But what is beam quality? At any point, the laser beam can be characterised by a divergence angle and a beam width, or diameter. 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 (for which the shape of propagation is a hyperbola) ^[2] . The beam quality of a solid state laser, often referred to as the beam parameter product (BPP), 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, but in this paper, beam quality is quoted in terms of half beam diameter and half divergence angle, in line with the ISO standard for laser beam propagation. ^[3] Table 1 provides an overview of the characteristics of some of the commercially available CW laser sources. In this table, beam quality is expressed in mm.milliradians with the CO ₂ laser value appropriately converted. Figures are given for the maximum output power commercially available for each at the moment, and, for comparison purposes, an estimate has also been made of the BPP available for each technology at an output power of 1kW.

Table 1: Laser source comparison

But what impact, in practical terms, does beam quality have on welding performance? A high beam quality means that the beam can be focussed into a small diameter optical delivery fibre, which translates, on the processing side, fora 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. 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, which is a measure for how divergent the beam remains over a given distance. ^[2] So, 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.

A reliability in the order of 100,000 hrs before maintenance/failure of the laser diode pumps is claimed for the fibre lasers. This is an order of magnitude higher than the maintenance interval of conventional solid-statelasers, which 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. The 100,000hrs claim is based on the fact that fibrelaser pumping technology uses less stressed laser diodes instead of diode stacks. ^[4] With only a few Yb-fibre lasers with an output power higher than 5kW in operation in Europe, and the first one of those 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% . ^[4] For the 7kW 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 a four-way optical switch, Figure 1. This is significantly better than the power conversion efficiency of around 8 and 3% for CO ₂ and lamp-pumped Nd-YAG lasers respectively. 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.

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 footprint of TWI's 7kW laser source, without chiller, is approximately 1m ² (10.76 sq ft). This is over four times smaller than that of a commercially available 4kW lamp-pumped Nd:YAG laser source, and many times smaller, for instance, than that of the 10kW cross-axial flow CO ₂ laser source used at TWI in the early 1980s.

Because of its inherently simple and compact design, the 7kW Yb-fibre laser 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.

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 of laser and chiller, the reduced floorspace 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. Because ofthe 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. A limiting factor in this price drop however, is the cost of the laser diode pumps.

Using fibre lasers for welding and cutting

Laser welding is not uncommon in today's manufacturing industry. Steel and aluminium, for instance, are welded successfully for a range of industrial applications. The automotive industry in particular has been instrumental in getting laser technology onto the shop floor. Whereas the high-speed capability is particularly attractive for automotive applications, other industry sectors also gain benefit from its single-pass, deep penetration welding capability.Output powers of up to 15kW are commercially available for CO ₂ lasers, but often the necessary use of mirrors for beam manipulation is considered restrictive in applying this technology for flexible manufacturing. Fibre-delivered solid-state lasers on the other hand, offer the required flexibility, but, so far, have been limited in output power, i.e. typically less than 4kW. The advent of this new solid-state laser with higher output powers than before (and a better beam quality than existing solid-statelasers), evidently offers new opportunities for thin and thick-section applications. Below are a few of welds produced to date, at TWI, using the latest fibre laser technology.

The diagrams shown in Figures 2 and 3, obtained from a series of fully penetrating bead-on-plate runs carried out on steel and aluminium, show the welding performance, i.e. depth of penetration versus speed, of the 7kW fibre laser for a measured power at the workpiece of 4 and 7kW. The process set-up was similar to that typically used for welding with a 4kW lamp-pumped Nd:YAG laser, comprising an optical glass cover slide, an airknife and helium shielding for the aluminium welds.

Figure 4 compares the cross-sections of zero-gap, square-edge butt joints completed in 8mm (5/16-in.) thickness C-Mn steel using 4kW Nd:YAG (4a) and 7kW Yb-fibre (4b) laser power at the workpiece. Both were completed using a600 micron laser spot size focused on the material surface, and at a welding speed of 0.5m/min (20 in./min) in case of the Nd:YAG laser and 1.6m/min (63 in./min) for the Yb-fibre laser. The difference in power density and heat input clearly affects the weld and HAZ region. A stringent weld quality in accordance with BS EN ISO 13919-1 was achievable for both the lasers used.

The cross-section in Figure 4a is one of a zero-gap, square-edge butt joint completed in a 6.35mm (1/4-in.) thickness 7000-series aluminium alloy using 3.5kW Nd:YAG laser power at the workpiece focused into a 600 micron laser spot size. The lowpower density, just enough to achieve complete joint penetration, resulted in a slow welding speed of 0.3m/min (12 in./min), i.e. high heat-input, and a weld with an aspect ratio of approximately 1:1. Doubling the power to 7kW, in case of the Yb-fibre laser, resulted in a much higher power density, allowing welding speeds of between 2.8 and 4.8m/min (110 and 190 in./min), i.e. a ten to sixteen-fold increase. The resulting welds, Figures 4b and 4c, have a much higher aspect ratio, typical for laser welding. A stringent weld quality in accordance with BS EN ISO 13919-1 was achieved for all welds.

When welding lap joints in 1.2mm to 1.2mm (3/64-in. to 3/64-in.) thickness ultra-high strength steel (UHSS) using 4kW Nd:YAG laser power, as measured at the workpiece, a welding speed of 2.5m/min (100 in./min) was achieved for alaser spot size of 600 micron, Figure 6a. Using a 4kW Yb-fibre laser with a BPP of 5mm.mrad, resulted in a welding speed of 4 and 17m/min (160 and 275 in./min) for a 450 micron and a 140 micron laser spot diameter respectively. Note the high aspect ratio ofthe weld, with a width of the weld (including HAZ) less than 1mm.

By choosing a hybrid laser-arc configuration, i.e. combining an arc in the same weld pool as that of the 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. Plenty of examples exist in the literature demonstrating this concept for CO ₂ or Nd:YAG lasers. The welds shown in Figures 7 and 8 demonstrate how the 7kW Yb-fibre laser was used in the hybrid configuration on 8mm (5/16-in.) thickness X60 pipesteel and 12.7mm (1/2-in.) thickness 7000-series aluminium respectively. Both these were carried out in the PF position at a welding speed of 1.8 and 0.9m/min (70 and 35 in./min) respectively.

Because the beam quality of these new lasers is now in the same order of magnitude as CO 2lasers, fibre lasers should also be considered for cutting applications, a market currently dominated by CO ₂ lasers. Some of the first cutting results using a Yb-fibre laser with a BPP of 17mm.mrad are shown in Figure 8. The 2 and 4mm (5/64 and 5/32-in.) thickness mild steel samples (2 ^nd and 4 ^th from the top) were cut using oxygen assist gas, whereas the 0.8mm (1/32-in.) Zn-coated steel (top), the 3mm and 6mm (1/8 and 1/4-in.) austenitic stainless steel (3 ^rd and 6 ^th from top) and the 5mm (3/16-in.) 5000-series aluminium alloy (5 ^th from top) were all cut using high pressure inert gas.

Concluding remarks

The manufacture of lasers has become a highly competitive market, with lasers being introduced in different industry sectors for a wide range of applications, including cutting, drilling, welding, marking and surface engineering. Welding trials at TWI using the latest fibre laser technology confirm that this new type of laser source should now be considered as an alternative to the CO ₂ or Nd:YAG laser for the welding of materials, such as steel and aluminium. As the fast pace of development continues to push up the power levels and improve the beam quality, the range of industrial applications for this new laser technology will expand undoubtedly, to possibly include, for instance, cutting and remote welding applications. Its compact design, easy set-up and minimal cooling requirement also makes it an ideal laser source foron-site welding, for pipeline welding or shipbuilding, or for remote repair applications, for instance. With the power conversion efficiency and claimed reliability also very attractive from an economic point of view, industrial confidence in this new laser technology is on the up. It remains to be seen however, if this competition between laser technologies will also result in more affordable lasers.

Acknowledgements

This article has benefited from the technical contributions of Paul Hilton, Steve Shi, Pak Chong and Anthony Elliott.

References

1. Woods S. 2003. Fibre lasers - the new high power, high quality, high efficiency source. AILU magazine. The Industrial Laser User, Issue 33, December 2003, PP. 32-33.
2. Hilton P.A. 1998. Fibre optic beam delivery for high-power CW Nd:YAG lasers. Confidential TWI Report 88277/49/98, December 1998.
3. BS EN ISO 11146:1999: Lasers and laser related equipment - Test methods for laser beam parameters - Beam Widths, Divergence Angles and Beam Propagation Factor.
4. Shiner B. 2004. kW fibre lasers for material processing markets. AILU magazine. The Industrial Laser User, Issue 35, June 2004, pp. 23.