Welding with a new generation of high brightness lasers

TWI Bulletin, May - June 2008

In March 2005, TWI reported the results of the first welding work using the then new high power fibre laser at TWI Technology Centre in South Yorkshire, in a Bulletin article. ^{[Verhaeghe & Hilton, 2005]} In January 2007, a further Bulletin article ^{[Verhaeghe & Hilton, 2007]} compared the performance of this laser to that of two new lasers, in the form of the thin disc laser, and a second, higher beam quality, fibre laser. As Lien Nguyen and Paul Hilton report it is a mark of how rapidly the fibre delivered laser technologies are progressing, that it is felt useful to update Members further on the welding capabilities of these lasers and this article is based onan evaluation made in preparation for experimental work in the current TWI Core Research Programme.

The advantages of optical fibre laser beam delivery have been well documented in the past. A critical issue for fibre delivered laser beams is the available beam quality, usually measured by beam parameter product (BPP). A laserwith a high beam quality has a numerically low BPP. One consequence of a laser with a high beam quality is that the beam can be focused into a small diameter beam delivery fibre. For step index fibres (those generally used to transporthigh power laser beams), the laser process head, mounted 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.

The beam 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 fibrediameter 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 betweenworkpiece 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, a distinct advantage of a high beam quality, fora laser process head of a given diameter, is that, for a given minimum spot diameter, a larger stand-off distance can be used, thus providing better protection for the optics. This also gives a greater depth of focus, here defined asthe 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 laser output power, a high beam quality can give a smaller minimum spotdiameter, which in turn produces a higher power density at the beam focus.

As a result of the above, the manufacturers of fibre lasers have directed their recent development efforts in two directions. The first was to increase the power available in a discrete Yb-fibre laser module. These modules producesingle mode beams, which effectively have the ultimate beam quality available of the order 0.3mm.mrad. Currently IPG Photonics offers single mode fibre lasers with output powers up to 3kW, capable of producing very high power densitieswhen focused. The second direction was to increase the power available in the multi-mode lasers ( ie the type of laser used in TWI South Yorkshire), with a corresponding increase in beam quality. The latter has been achieved by using the power developments in single mode modules mentioned above. This in turn means thatfewer modules and hence fewer fibres have to be coupled together to achieve the required output power.

As a result the beam quality is better. The power limit now appears to be simply a question of how deep is your pocket, in that effectively any power can be achieved, by coupling more of the individual modules together. The laser inSheffield has a power of 7kW with a beam quality of 18mm.mrad, a record only four years ago. The same manufacturer, IPG Photonics, now offers a 10kW laser with a beam quality of 6mm.mrad, and a 2kW laser with a beam quality of2.5mm.mrad. The IPG web site indicates a 50kW laser is available and a 17kW, 12mm.mrad laser is operational in a laboratory in Germany. In reference 2, an arbitrary figure was established to define a 'high brightness' fibre deliveredlaser, that being one with a beam quality less than 7mm.mrad. Using this definition, a 10kW fibre laser would now fit into this category.

In parallel to the development of high power fibre lasers, lower power fibre lasers (powers of 300W and below say), have also made recent advances. One of the most significant is that at the start of 2007, only two companies offeredsuch lasers (IPG and SPI Lasers) but by June 2007, over five manufacturers were apparently offering commercially available fibre lasers in this power range. At lower powers, the technical advances made recently involve the introductionof Q switched pulsed fibre lasers and fibre lasers operating at 1.5 and 2µm in wavelength.

The market demand for fibre lasers has also witnessed a continuous increase. For example, in 2006 the sale of fibre lasers in the global market grew by 61% compared to 2005. It is anticipated that by 2010, the sale of fibre laserswill reach US$680 million, which will account for at least a quarter of the US$2.8 billion market for industrial lasers. ^{[Peach, 2006]}

The thin disc Yb:YAG technology, invented several years ago at Stuttgart university, has also developed recently with the introduction of 2-8kW disc lasers meeting the above definition of 'high brightness' in that their beamqualities are of the order 7mm.mrad. Such high power lasers are manufactured by Trumpf and low power versions, with outputs of the order of 10 to a few hundred watts are available from Jenoptik and PrenovaTec. Some of these smalllasers have beam qualities very close to single mode and can produce very small focused spot diameters.

There is a reasonable amount of published information on the welding capabilities of the smaller lasers, but the majority of this comes from the laser manufacturers. On the other hand the majority of published work on the high powerlasers mentioned above has been made by research organisations. This article will review some currently available processing information with the objective of raising awareness of the welding capabilities, and advantages anddisadvantages, of these high brightness fibre delivered laser systems.

Available spot sizes and power densities

Using a typical low power single mode fibre laser, if the beam is expanded to about 12mm diameter before being focused by a doubler lens, a spot size as small as 7µm in diameter can be achieved. A 200W laser beam focused tothis small spot, produces a power density of the order 5x108W/cm ², or 500MW/cm ². If the single mode power is raised to 2kW, a larger spot size is usually chosen (to produce a greater working distance) of say 100µm in diameter. In this case the power density of the spot will be 25MW/cm ². The 17kW fibre laser mentioned earlier might be used with a spot size of about 500µm, in which case the available power density would be 8.6MW/cm ². These values should be compared to the typical power density of 1.5MW/cm ², available with the conventional 4kW Nd: YAG lamp pumped laser. It might be expected that the high power densities available from these new high beam quality lasers should have a significant effect on weldingperformance, both in terms of penetration and welding speed.

Welding with high power multi-mode high brightness lasers

Figure 1 shows the performance for two fibre lasers, operating at 17kW and 10kW, when welding aluminium alloy and compares this to the results from a 4kW Nd:YAG laser. Certainly in terms of depth of penetration and weldingspeed, the performance of both these lasers is significant, with a depth of penetration of 5mm available at a speed of 20m/min.

Fig.1. Performance for two fibre lasers, operating at 17kW and 10kW, when welding aluminium alloy ^{[Thomy et al, 2005]} compared to results from a 4kW Nd:YAG laser at TWI.

When the cross sections of welds made with high power, high brightness lasers are compared to those made with more conventional fibre delivered laser beams, other changes become evident, particularly the weld shape. Figure 2, for example, shows the cross section of a weld in 16mm thick CMn steel, made with 10.5kW of power, at a relatively slow speed of 1.2m/min. Note the narrowness of this weld, which has a profile much closer to that which might be expected from an electron beam, than from a laser and the small heat affected zone.

Fig.2. Cross section of a weld in 16mm thick steel at a welding speed of 1.2m/min using an IPG fibre laser operating at 10.5kW ^{[Thomy et al, 2005]}

For lower power multi-mode lasers, differences in weld profile become obvious as the beam quality of the laser gets better. For example, ^{[Brenner et al, 2005]} compared the profiles of welds made in steel with a disc laser, of BPP 7mm.mrad and a fibre laser, of BBP 2mm.mrad when welding steel at a power of 4kW. The results, shown in Fig.3, show a great variation in weld profile as a function of speed for both lasers sources and for speeds above 4m/min, the better beam quality of the fibre laser produces deeper penetration than available with the disclaser. Note however the extreme narrowness of the fibre laser welds at 4 and 8m/min. It is also worth noting that at the lower speed of 1m/min, the fibre laser penetration is not much better than that achieved at 4m/min (but the weld is much wider). In contrast, the poorer beam quality of the disc laser appears to produce an increase in penetration at the lowest speed of 1m/min, when compared to the same laser at 4m/min.

Fig.3. Cross sections of welds carried out with a disc laser HLD4002 (a) and a fibre laser YRL 4000 (b) Both lasers operated at 4kW ^{[Brenner et al, 2005]}

As a result of offering a greater working distance, high brightness fibre delivered laser beams are highly suitable for use with optical scanning heads of the type used in what has become known as 'remote' welding applications.Here, a long focal length lens is used and the beam is manipulated by two rapidly oscillating mirrors, to describe the welding path. High brightness lasers can be used for this and still retain a small focused spot size. Both fibre anddisc lasers (as well as Nd:YAG and CO ₂ lasers) have been used in this application. ^{[Klotzbach, 2005]} compared the cross sections of remotely welded joints made with Nd:YAG, disc and fibre lasers, see Fig.4. Fibre lasers produced narrower welds with significantly higher penetration.  

Fig.4. Cross sections of welds made using mirror scanning with

4a) Nd:YAG laser

4b) disc laser and

4c) fibre laser. The operating parameters were kept the same in three cases: power = 2.5kW, welding speed = 2.5m/min. ^{[Klotzbach, 2005]}

Welding with single mode high brightness lasers

As mentioned earlier, single mode fibre lasers have extremely high beam quality. Using such a laser, the spot size at the focus position can be lower than 10µm. Most published results to date with these lasers have used powersup to say 300W. Although higher power single mode fibre lasers are available, little work, particularly in the area of welding, has been reported above 1kW, as the high welding speeds available tend to cause humping. Single mode fibrelasers of up to 300W ^{[Vollertsen and Thomy, 2004]} have been used for welding of a variety of materials, such as stainless steel, steel, aluminium, titanium and copper (due to the high power densities available), with thickness less than 1mm. A welding speed as high as 200m/minhas been demonstrated in both 0.15 mm stainless steel and aluminium, using a 200W fibre laser, see Fig.5. Figure 6 shows the cross section of a weld from this series, in stainless steel, made at a speed of 20m/min. A penetration of 0.2mm in copper at a welding speed of 6m/min, in pulsed mode, with an average power of 190W has alsobeen reported.

Fig.5. Bead-on-plate welding of stainless steel and aluminium using a 200W single mode fibre laser ^{[Vollertsen and Thomy, 2004]}

Fig.6. Cross section of a bead-on-plate weld, only 40µ in width, made using a single mode fibre laser at 200W and at a speed of 20m/min, in stainless steel. The size of the focused spot was 14µm. ^{[Vollertsen and Thomy, 2004]}

^{[Kancharla, 2006]} has welded 0.5mm thick Ti at 10m/min using a 300W single mode fibre laser. A penetration of 0.76mm in 304 stainless steel, welded at 3m/min and using 143W, has also been reported. At a higher laser power of 542W, penetration can reach 2.25mm at 3m/min in the same material. ^{[Ream, 2005]}

Lap welds with single mode fibre lasers have also been successful in a variety of materials such as stainless steel, copper and titanium. Figure 7 shows a lap weld consisting of three sheets of 100 µm stainless steel produced using a 200W fibre laser, at 80m/min. ^{[Grupp and Klinker, 2007]}

Fig.7. Lap weld of three 100µm stainless steel sheets at 200W and 80m/min ^{[Grupp and Klinker, 2007]}

Discussion

This new generation of high beam quality fibre and disc lasers has already demonstrated potential in the field of both macro and micro welding. Despite being still a relatively new technology, fibre and disc lasers promise wide-spread use and are already enjoying an increasing market demand. The curves of weld penetration against welding speed and the weld cross sections discussed above, demonstrate the interest currently being shown in the process capability of high brightness laser sources, and some distinct potential advantages of using this technology in welding applications. There are, however, still some areas of concern when using these laser sources, particularly at powers above 1kW, which need to be addressed.

For example, in Fig.3. it is not clear why the performance of the fibre laser, in terms of penetration, is excellent at 4m/min, but no greater at 1m/min. Clearly the very high brightness of this laser produces different welding conditions to the lower beam quality disc laser, the latter showing increased penetration at 1m/min compared to that at 4m/min. In addition, many papers report high levels of weld spatter from the high brightness lasers. Maintaining the cleanliness of the focusing optics also seems critical for optimum welding performance and, in some cases, the power density on the quartz optics used to shape the laser beam, causes them to distort and induce a change in focus position of the laser beam. Also obvious from the above narrow weld profiles, in some cases almost indistinguishable from electron beam welds, is that in butt joints, component fit up must be very good. The effects of the fine 'needle' like weld roots also need to be taken into consideration. TWI has a current two year Core Research Project, started in 2007, which will address some of the above problems in the use of high brightness fibre delivered laser energy for welding, and the results of this work will be reported to TWI Members as the project progresses.

The story so far... References

1. Brenner, B., Gobel, G., Dittrich, D., Schedewy, R., Standfub, J., 'New effects in welding of light weight alloys and steel with fibre lasers', 1st International Fraunhofer Workshop on Fibre Laser, 2005.
2. Grupp M., Klinker K.: 'Micro joining with single-mode fibre lasers.' Proceedings of the 4th International Conference on Lasers in Manufacturing, 2007, 659 - 661.
3. Hilton, P., Verhaeghe, G., Chong, P., Allen, C., Shi, S., 'Two years' experience with high power fibre laser', 1st International Fraunhofer Workshop on Fibre Laser, 2005.
4. Kancharla V.: 'Application review: materials processing with fibre lasers under 1 kW'
5. Klotzbach A.: 'Fibre laser remote processing', 1st International Fraunhofer Workshop on Fibre Laser, 2005.
6. Peach M.: 'Fiber lasers grab industrial market.' Optics and Laser Europe, issue 139, May 2006, 5.
7. Ream S.: 'High-speed fibre laser welding for fuel cell components.' Proceedings of the 25th International Congress on Applications of Laser and Electro-Optics, 2005, 586 - 594.
8. Thomy. C., Seefeld, T., Vollertsen, F., 'Laser and Laser-GMA welding applications using high power fibre lasers', 1st International Fraunhofer Workshop on Fibre Laser, 2005.
9. Verhaeghe G., Hilton P: 'Welding - using the new generation of high-power fibre-delivered lasers', TWI Bulletin, January - February 2007.
10. Verhaeghe G., Hilton P: 'Deep penetration welding - the fibre laser way', TWI Bulletin, March - April 2005.
11. Vollertsen F., Thomy C.: 'Welding with fibre lasers from 200W to 17,000W. 'Proceedings of the 24th International Congress on Applications of Laser and Electro-Optics, 2004, 254 - 263.