Pak Chong has worked for TWI since October 2004. She completed her PhD at the Corrosion and Protection Centre within the School of Materials at the University of Manchester (previously UMIST). Based at TWI Yorkshire in Sheffield, she is a Senior Project Leader in the Laser and Sheet Processes Group. Her field of interest includes laser welding, laser hybrid welding and laser cutting and she is presently responsible for a number of fibre laser projects. She was involved in the first Yb-fibre laser cutting work in TWI.
According to the January 2006 issue of 'Industrial Laser Solutions', in 2005, just over 35,000 production systems using laser beams for materials processing were installed world wide. The sales value of these systems was estimated at $US 4.3bn. It is significant that 24% of these systems were for laser cutting and that this figure corresponds to 40% of the total revenue for all laser systems. As Paul Hilton and Pak Chong report it is perhaps also surprising that the vast majority of these laser cutting systems still uses a carbon dioxide gas laser - the same type of laser used by Peter Houldcroft, in the first ever experiments on gas assisted laser cutting performed, here at TWI, in 1967.
Of course the CO 2 gas lasers used in today's laser cutting systems have evolved quite significantly over the years and are now fully integrated into the highly accurate and efficient machine tools currently used in production. The CO 2 gas laser maintains its prominent position in laser cutting for several reasons. The three most important are probably first, the fact that the laser powers necessary for cutting are readily available with this technology....second, the fact that the beams from these lasers can be focused to small spots, with the very circular and symmetric energy distributions required for laser cutting....and third, the fact that in terms of cost per kW of delivered power, at the powers needed for laser cutting, CO 2 laser technology provides one of the most cost effective solutions.
It is quite clear from the above that the advantages of any laser technology trying to usurp the current position of CO 2 lasers in cutting, will need to be very significant to succeed. In fact, it is only very recently that an alternative laser source, with real potential to replace the CO 2 laser, has become available - and this is the fibre laser. 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. The pumping mechanism used with fibre lasers is the laser diode, another solid state device. Ytterbium is generally the doping element used for the high power fibre lasers currently available for materials processing and this produces laser light at about 1µm wavelength, very close to that of the Nd:YAG solid state lasers, but ten times shorter than the wavelength of the light produced by the CO 2 laser.
So what are the potential advantages for this type of laser for cutting metal? Firstly, the fibre lasers are completely solid state, and are very compact in size, kW for kW, significantly smaller than the equivalent CO 2 gas laser. Secondly, these lasers are very efficient in terms of energy usage, being of the order of four times better than current CO 2 lasers, and thirdly, they have no recognised consumables (because the laser diodes have projected lifetimes of the order of 100,000 hours) and require essentially no maintenance. These advantages are particularly significant today, in terms of incorporation of a laser source into a machine tool.
From the process point of view, the fibre laser also offers certain potential advantages over the CO 2 laser. The beams from fibre lasers are delivered via optical fibres, which adds additional flexibility and ease of use when compared to the 10µm wavelength beams from CO 2 lasers, which can only be manipulated by mirrors.
This advantage is probably most significant for very high speed, 3D or remote cutting applications. In addition, the quality of the beams generated by fibre lasers can be very high, particularly at the powers currently found useful for laser cutting. This means the beams can be easily focused to small, circular symmetric spots, very similar to those produced by CO 2 lasers and significantly better than those produced by other solid state lasers, such as the Nd:YAG laser. Finally, for the cutting of metallic material, it would be generally expected that the 1µm fibre laser beam would have advantages over the 10µm CO 2 laser beam, because the absorption of the shorter wavelength light from the fibre laser will be higher.
TWI has recently conducted two series of laser cutting trials using fibre laser sources and some of the results of this work are reported here. The first series of trials was undertaken using the YLR-7000, high power fibre laser, at the TWI Technology Centre in Yorkshire. This laser, with its maximum output power of 7kW, was manufactured by IPG Laser GmbH and, since its installation, has been mainly used for welding. For the cutting work, a standard industrial laser cutting head (with optics modified for the 1µm fibre laser beam), manufactured by Precitec, was used.
In a later series of trials, conducted in conjunction with Cambridge University, a second IPG fibre laser was used, a YLR-2000, which could produce 2.2kW of output power, but at a significantly better beam quality (ie it could be focused to a smaller spot) than TWI's Yorkshire based laser. As a good example of the 'plug and play' capability offered using optical fibre delivery, the cutting head used in Yorkshire was used in Cambridge, without any modifications.
The beam from the YLR-7000 was delivered to the cutting head via a 300µm diameter fibre. Using a 150mm focusing lens, the minimum spot size that could be produced was 0.36mm in diameter. Using a lens with a focal length of 80mm produced a minimum focused spot size of 0.2mm. The beam from the higher beam quality laser, the YLR-2000, was delivered to the cutting head using a 50µm diameter fibre. As a result, using the same lens of 150mm focal length, the minimum spot size that could be produced dropped to 0.08mm in diameter, a very significant change, particularly in terms of power density at the focus, twenty times higher for the YLR-2000, at any given power.
The depth of focus for the spot generated with the YLR-2000 would also be expected to be much greater than that available using the YLR-7000. For comparison purposes, on today's production cutting machines, the CO 2 lasers available might typically produce focused spot sizes in the range from 0.2 to 0.4mm in diameter, with focal lengths between 125 and 190mm commonly in use.
It is generally accepted that a large number of variables come into play in any laser cutting experiments. In this work laser power, cutting speed, cutting assist gas type, cutting assist gas pressure, cutting tip nozzle diameter, cutting tip stand off distance (the distance between the nozzle tip and the workpiece surface) and the vertical focus position of the laser beam, were investigated during the trials.
The nozzles used were a standard 'conical design' with a flat tip. Oxygen, in the pressure range from zero to 10 bar and nitrogen, in the pressure range from zero to 20 bar were used as assist gases. When using the YLR-7000 laser, the cutting head was held in the arm of an articulated robot and when using the YLR-2000, the cutting head was fixed and the samples manipulated on an x-y table. Figure 1 shows the cutting head in action using the YLR-2000 fibre laser. During the work, a range of C-Mn and microalloyed steels from 0.8mm to 4mm in thickness, stainless steels from 3mm to 13mm in thickness and aluminium alloy of 5mm and 8mm thickness were cut. Figure 2 shows a selection of cuts on different materials and thicknesses made using the YLR-7000 fibre laser.
Fig.2. A section of the materials cut using the YLR-7000 fibre laser, including C-Mn steel, stainless steel and aluminium alloy
The majority of laser cutting undertaken on C-Mn steels uses oxygen assist gas and the work using the YLR-7000 began on this material (4mm thick), with a 0.36mm diameter focused spot. The combinations of parameters chosen for the initial trials were based on experience drawn from CO 2 laser cutting. However, it took several changes to the parameters to obtain a cutting quality comparable to that using a CO 2 laser. The best cut, in terms of a combination of edge quality and speed, in this material/thickness combination, was found using a power of 2kW, at a speed of 3.5m/min, with the 0.36mm diameter spot focused at the material surface. These conditions gave a sample with an almost mirror like finish, with a surface roughness, Ra, of between 1 and 2µm. This sample is the fourth from the top in Figure 2. For the C-Mn steels and using oxygen as the assist gas, it was generally felt that the tolerance box for achieving a high quality cut with the fibre laser, was tighter than for the CO 2 laser.
One of the areas where it is felt the fibre laser should perform well is in the cutting of thin (<1mm) material. Here the very high power densities available with the fibre laser, combined with its short wavelength, might be expected to provide advantages. The 0.8mm thick steel was cut using both the YLR-7000 and YLR-2000 lasers, using both oxygen and nitrogen assist gases, with laser powers between a kilowatt and 2.2kW. For each of the focal spot sizes investigated (0.08mm, 0.2mm and 0.36mm diameter) very linear relationships between the laser power and the maximum available cutting speed were found, and the optimum gas pressure of between 10 and 12 bar, was independent of the actual gas used.
A maximum cutting speed of 59m/min was found at 2.2kW using the 0.08mm diameter spot and oxygen assist gas. The edge quality of this very high speed cut can be seen in Figure 3. At this material/thickness combination the effect of beam quality/spot size is very large. When cutting the 0.8mm thick material using nitrogen assist gas, the maximum cutting speed was lower, falling for example to about 19m/min using the YLR-7000, at 2.2kW, with a spot size of 0.2mm and to about 13m/min (at the same power) using a spot size of 0.36mm. The highest cutting speed recorded at 59m/min is very comparable to the best published data obtained using CO 2 lasers.
Fig.3. Edge quality on 0.8mm thick steel, cut using the YLR-2000 fibre laser, at a speed of 59m/min and high pressure oxygen assist gas
Most commercial laser cutting systems have to work on a range of materials and thicknesses and it might be expected that one of the advantages of the combination of small spot size and large depth of focus offered by the fibre laser might assist in the cutting of thicker material. However, for CO 2 laser cutting of thicker materials, in practice, it is often found necessary to operate away from the focal position to obtain good quality cuts.
Moving away from focus position on the surface of the material has the effect of widening the cutting kerf. To investigate the cutting of thicker materials the two fibre laser sources have been used to cut 3, 6, 8, 10 and 13mm stainless steel, using high pressure nitrogen as the cutting gas. The three, eight and 10mm thick stainless steel was cut using the YL-7000 laser, and the six, eight and 10mm stainless steel was cut using the YL-2000 laser. In this work, it was consistently found that when using either the 0.36mm diameter spot, or the 0.08mm diameter spot, better cuts were made with the position of the minimum spot diameter pushed down below the surface of the material. In fact this was exactly as would be expected for CO 2 laser cutting.
Figure 4 compares the surface quality for the 6mm thick stainless steel, cut using both laser sources. For the top, middle and bottom sections of the cut, the surface roughnesses, Ra, were 4.6, 5.3 and 5.5µm, respectively for the 0.08mm spot, and 4.8, 6.0 and 9.4µm, respectively for the 0.36mm spot. Unfortunately, these measurements were made at two different laser powers, 2.2kW and 3.0kW (corresponding to speeds of 0.9 and 1.6m/min respectively), so a direct comparison is not possible. However, these roughness values, although good, are no better than can be obtained using CO 2 laser cutting.
and the YLR-7000 fibre laser
Figure 5 shows the cut surface for 8mm and 10mm thick stainless steel, produced using the YLR-2000 laser. A quite difficult material for laser cutting is aluminium. It has a very high reflectivity to CO 2 laser light. It has a high thermal conductivity and it also forms a highly tenacious oxide layer. Nevertheless, good quality cuts in aluminium alloys can be made using CO 2 lasers and high pressure inert gas techniques. Figure 6 shows the edge quality obtained on 5mm thick Al alloy, using a high beam quality fibre laser at a speed of 3m/min, using 4kW of laser power.
Fig.6. Fibre laser cut in 5mm thick aluminium alloy. Cutting speed 3m/min
The work reported above is some of the first to be published regarding the use of fibre lasers for cutting. Further details of the work using the ILR-2000 laser on stainless steels was published at the ICALEO conference in the USA in November 2006. These results have shown that, for the first time, there is probably an optical fibre delivered laser beam, which might become a serious contender to replace some CO 2 laser cutting applications (and possibly introduce new applications due to the 'remote' cutting possibilities offered by fibre optic delivery).
The cutting speeds available on thin materials using a high beam quality fibre laser are high. It has also been shown that the other materials most commonly cut using CO 2 lasers, such as stainless steel and aluminium, can also be cut effectively with the fibre laser. It is very possible, that with more attention to the interaction between the assist gas flow from the cutting nozzle and the molten material ejected from the kerf, it may be possible to improve the quality and speed of fibre laser cutting over that presented here. In this context it should be remembered that by 2007, we will have been cutting with CO 2 lasers for 40 years!
Acknowledgments
The authors are grateful to the TWI Member companies who supported the Group Sponsored Project 'Evaluation of high power Yb fibre lasers', for permission to publish some of the above results on cutting. Thanks also go to colleagues at the Centre for Industrial Photonics at the Institute for Manufacturing at Cambridge University, where the work using the ILR-2000 laser was completed.
