11th Nordic Conference in Laser Processing of Materials, Lappeenranta, Finland, August 20-22, 2007
Abstract
The first experiment in laser materials processing which was subsequently to evolve into a significant industrial process was conducted in May 1967 when Peter Houldcroft used an oxygen assist gas to cut 1mm thick steel sheet with a focused CO 2 laser beam. Using archive photographs and early film, this presentation will describe the pioneering work on laser cutting using a 300W slow flow CO 2 laser. This laser was operational in the UK only two years after Patel had demonstrated lasing action from the CO 2 molecule. Early examples of laser cutting will be shown and reference made to the predictions of the early experimentalists. The paper will also discuss the evolution of the fast axial flow CO 2 laser and its subsequent use to produce the first keyhole laser welds.
The first gas assisted laser cutting experiments
Although laser materials processing is still often regarded as a 'new' technology, it might come as a surprise to some, to learn that the first gas assisted laser cutting was performed as long ago as May 1967. These first experiments probably mark the start of laser materials processing as we know it today. This is particularly important in that cutting is now the most significant application (in terms of market share) of the use of lasers in materials processing.
These first experiments were the idea of Peter Houldcroft, who was then Deputy Scientific Director at TWI (The Welding Institute) in Cambridge. He realised that the combination of a focused laser beam and an oxygen assist gas had the potential to improve the precision and speed offered by thermal cutting processes. In 1965, TWI became aware that a 300W CO 2 gas laser was operational at the Services Electronic Research Laboratory in Harlow, just down the road from TWI. The laser had been developed for military applications but potential industrial applications were also being considered. It is interesting to note that this laser was operational at 300W, only two years after Patel demonstrated lasing action from the CO 2 molecule.
The SERL laser was of the slow flow type, consisting of 5 discharge sections, making a total length of 10m. It was powered by a series of mains frequency AC supplies, each providing 45mA at 9kW. The device was unfolded and used a polished stainless steel rear mirror and a germanium output coupler. A maximum output power of 300W at 100Hz was available. Although no information remains on the beam quality of this laser, it was probably quite good, even by today's standards. Unfortunately no photographs of the SERL laser can be found. However, Ferranti subsequently supplied about six commercial versions of this laser, before developing the famous MF (multifold) laser series. One of these was delivered to BOC (The British Oxygen Company) and Figure 1 is an (old) photograph of part of this laser. It is not difficult to see that much progress has been made since 1967!
In early 1967 Houldcroft designed a 'laser cutting nozzle', its important feature being the oxygen pressure chamber which would provide the co-axial reactive assist gas stream in the region of the laser beam focus. This nozzle arrangement can be seen in the photograph in Figure 2, which was taken during the first series of cutting trials made during May 1967. The oxygen gas was kept in the chamber by a flat 'pressure window', and the beam focusing lens was positioned above the pressure chamber. The design of the nozzle tip itself, which had a circular orifice 2.5mm diameter and used a stand off distance of 1.5mm, is remarkably close to commercial nozzles in use today. The complete set up can be seen in Figure 3. The horizontal laser beam was diverted onto the focusing lens by an aluminised steel mirror. The lens (of focal length about 300mm) and pressure window were made from sodium chloride. It was sometime later that potassium chloride was used. The rectangular block at the top right of the photograph was attached to a rotating handle. This in fact formed the laser shutter and beam stop. Clearly, in those days, laser safety had still to be invented!
The results of the first experiments were published in August 1967 - 'Gas-jet laser cutting' by A B J Sullivan and P T Houldcroft, in the British Welding Journal. Arthur Sullivan worked at SERL and operated the laser. Cuts were made using oxygen assist gas in high carbon tool steel and stainless steel up to 2.5mm thick at speeds up to 1m/min. For these materials an optimum focal position was found to be on the workpiece surface and a focused spot size of 0.4mm was stated, although the authors do not say how this was measured. The kerf width was generally about one third wider than the laser beam focus diameter. Figure 2 shows a sample being cut. Small coupons were attached to a rotating disk and as a result the first cuts were curved. It is perhaps amazing that both halves of the sample shown being produced in Figure 2 have survived to this day and the edge quality of this cut can be seen in the photograph in Figure 4.
The paper goes on to describe the observations made during the first experiments. The kerf width, which was small (0.5mm), was seen to be dependent on focused spot size and not the gas jet size. In addition, the cut edges were free from microcracking and hot tears. Negligible distortion was reported and it was also pointed out that the process induces no mechanical forces on the cut part. The significance of these early observations becomes clear when it is realised that these same features are now widely cited by today's system manufacturers, as the benefits of laser cutting. It is also worth looking at the conclusions to the original paper which state:
'With the development of higher power lasers it should be possible to cut thicker and different materials including non metals'.
'The narrowness of the cut promises a precision not previously obtained with thermal cutting techniques'.
With 40 years hindsight the importance of these conclusions is also very clear. A vast range of materials can now be cut using lasers to commercial advantage, even though the main job-shopping materials are still mild and stainless steels. Current CO 2 lasers with high beam quality and high powers (up to and above 6kW) have extended the thickness range for quality cuts in mild steel using oxygen assist gas to 20mm thickness, and work is continuing to extend this range even further. The narrow kerf and precision offered by fast CO 2 laser cutting, has revolutionised ship construction, significantly reducing re-work caused by assembly of inaccurate and distorted components, and, in the automotive industry has provided a flexible method for the trimming of pre-production 3D pressed body panels. At the other end of the scale, the Nd:YAG laser, which is generally associated with the cutting of small high precision components for many industry sectors, has also become a very important industrial tool. The recent developments in high brightness of fibre and disc lasers, will continue to provide new applications for laser cutting.
On 3 August 1967, a short column on the first laser cutting work by Peter Houldcroft appeared in the Times newspaper. This is significant only because of the typographical error it contained. Instead of stating that the 'oxygen-jetlaser' had been cutting 1/10 inch stainless steel plate, this was unfortunately reported as 'cutting 1-10 inch stainless steel plate! Perhaps the industry has yet to fully recover from this original error.
Machine tool evolution
The evolution of the laser machine tool following these early cutting experiments is also very interesting. In August 1969, three gentlemen from the Boeing Company produced a paper on the CO 2 laser cutting of 'hard' materials, such as titanium, Hastelloy and ceramic, using the assist gas technique. They concluded that the laser 'could be an effective and economical cutting tool, but a great deal of research and development may be required before such a machine could be put on the production line'. Notwithstanding this latter statement, the authors present an 'artists conception' of what such a laser cutting tool might look like. This drawing can be seen in Figure 5, which has been scanned from the original paper. It is interesting to compare the configuration of this machine with that used by the successful 'Byflex' cutting machine made by Bystronic, whose principle is very similar to that shown in Figure 5. The industry, in fact, progressed very rapidly, particularly in the early 1970s. The first true laser cutting machine tool was probably that supplied by BOC in 1970 to William Thynes, a dieboard manufacturer, located in Scotland. This system used a laser not unlike the one shown in Figure 1, and was introduced in production to an industry completely dominated by trade unions! The first 2 axis sheet metal cutting system (certainly in the UK) is probably attributable to Andrew Greenslade who positioned the first Ferranti MF series laser on a BOC Falcon flame cutting machine in 1971. The first commercially available moving optics CO 2 laser cutting system, with a recognisable configuration comparable to a wide range of equipment available today, was probably supplied by Laser Work AG of Switzerland (now part of the Prima Industrie group), in1975. This machine can be seen in Figure 6 and it is still in operation today cutting holes in cow bells!
It is possible that approximately 48,000 commercial laser sheet metal cutting systems have been installed worldwide since 1980, when sales figures became generally available. The value of these systems, in 2006$ is probably about$28 billion. The majority of these machines are installed in Asia, with Europe accounting for 34% and North America accounting for 24%.
Perhaps the ultimate application of laser cutting is via 5 axis manipulation for 3-dimensional work. Such gantry beam delivery systems did not appear on the market until the 80s and the first of these systems was produced by the Italian company Prima Industrie. In the UK, the first such system was installed at the Swindon plant of Austin Rover in 1983 and was used very successfully for the trimming of pre-production car body panels during press tool development. This application is still the major use of multiaxis laser cutting throughout the world, with all the major automotive suppliers utilising the technique.
Thoughts on laser cutting
In the course of preparation of this article the author had the opportunity to speak to Peter Houldcroft (now retired) and one of the questions asked was: what had given him the original idea for gas assisted laser cutting? The answer was very surprising. In 1965 Peter Houldcroft had visited BMC (British Motor Company), where he was told some preliminary cutting trials had been undertaken using a plasma torch and an articulated arm robot, for the application of body panel trimming during press tool development. The problem was that the system was not accurate enough and produced burning. Peter was asked if he could think of any other suitable cutting process. On the drive back to Cambridge, the idea of combining an oxygen-jet with a focused laser beam began to form. The necessary catalyst for this idea was provided by the availability of 300W of CO 2 laser power at SERL.
In the introduction to his book 'Laser Materials Processing', Professor Bill Steen presents the argument that since the invention of the laser in 1960, we have entered into a new industrial revolution, based on the use of coherent optical energy. If you are prepared to subscribe to this idea, it is difficult to think of a better example of how this industrial revolution has progressed, than that of gas assisted laser cutting.
Higher power CO 2 lasers
Early attempts to weld metals with the slow flow lasers described earlier, found that although thin steels could be melted on the surface, the fusion zones were intermittent. These trials suggested that a laser beam with a low order transverse mode structure and a power of about 2kW was needed for practical welding of thin materials, say up to 3mm and at speeds above 0.5m/min. In principle, it would be possible to simply increase the length of a slow gas flow laser until the required power was reached. The power output is about 60 watts per metre of discharge from such a laser, and on this basis a 2kW slow flow laser would need more than 30m of discharge tube and would have to be optically folded many times. Cumulative power losses and optical distortion from so many reflections became a serious consideration, and the search was on for a more promising design for a 2kW CO 2 laser for welding, one which produced a relatively high power output per unit length of discharge, in a low order mode.
Lasers in general are inefficient. In a slow gas flow CO 2 laser, about 90% of the power from the discharge is not converted into laser power and goes into heating the gas, and as its temperature rises, the laser process becomes less efficient. The gas is cooled by conduction through the gas into the glass walls of the water-cooled discharge tube. Increasing the tube diameter in an attempt to increase the laser power doesn't work because it causes the temperature in the centre of the tube to increase. Hence the expression of power output in watts per unit length of discharge, rather than in watts per unit volume. The literature at the time suggested that the discharge could be effectively cooled by forced convection, by flowing the gas through the discharge at much higher speeds. It appeared that in this regime, power would be proportional to the mass flow of gas through discharges and would not be limited by the discharge length.
High gas flow could be achieved by pumping gas across a shallow channel with mirrors at each side. In principle electrodes could also be placed at the sides of the channel, but in practice it was more effective to have them at the top and bottom of the channel so that the gas flow, laser beam axis and discharge were all mutually orthogonal. Gas could be circulated using a high volume flow fan. Serious discharge problems with this approach were anticipated, however, and it would have been very difficult to produce a large volume homogeneous discharge which effectively filled the mode volume of the laser. This type of laser could well be very inefficient and could have a very asymmetrical transverse mode, making the beam less effective for welding. Given the other difficulties of producing as much as 2kW of laser power, it seemed prudent to ensure that whatever power was produced was in a low order, symmetrical mode, which could be focused to a small, high intensity spot. The cross-flow approach was, therefore, not pursued at TWI.
The requirements for a high quality output led the researchers at TWI to attempt to develop a fast flow CO 2 laser with discharges running axially in cylindrical tubes i.e. a fast axial flow laser.
The first fast axial flow test bed
First attempts to produce a fast axial flow CO 2 laser used the simplest possible optical arrangement with a mirror at one end and a gallium arsenide output window at the other. Two discharge tubes were arranged in line with their cathodes earthed. The high voltage anodes were insulated from earth by two lengths of glass tube through which the laser gas flowed before entering the discharge tubes. Gas was pumped through the discharge tubes by a large Rootes blower, backed by a rotary vacuum pump. The Rootes pump, similar to an automobile supercharger, was considerably more expensive and much bigger and heavier than a fan, but unlike a fan it was capable of developing the relatively high pressure differences which were necessary to force the gas through the relatively narrow fast axial flow laser tubes. The gas flow geometry was straight through, for simplicity, without gas recycling. As a result, a large cylinder of helium would be consumed in about 5 minutes, which led to the speedy development of rapid manual tuning techniques. First attempts to produce laser power were very disappointing. The problem seemed to be in the difficulty of producing a homogeneous glow discharge. Instead, it tended to cling to the sides of the tube in line axially with the electrodes. As the current was increased above a few milliamperes, the discharge became increasingly constricted into a number of unstable, long, thin and bright filaments, which moved about rapidly in the turbulent gas flow and consequently came to be called streamers. Getting rid of the streamers quickly became the researchers' one aim in life. Many arrangements of electrode, combined with different electrode materials, produced very little improvement and each time the current was increased the streamers appeared.
Since the discharge always clung to the wall of the discharge tube, the gas was finally given a radial component of velocity (i.e. towards the axis of the discharge tube) by inserting tubes of the same diameter, inside the larger diameter insulating tubes upstream of each discharge section. The gap between the inner tube and the discharge tube was set at a few mm to force the gas to enter the discharge with a radial component of velocity. From the first moment the laser was turned on with this new arrangement in place, the discharge behaved in a completely different manner. The plasma was seen as a soft homogeneous glow, with no sign of the streamers which formerly had always accompanied the raising of the discharge current. The laser power, which had always been negligible, quickly rose to several hundred watts, as the mirrors were rapidly turned before the helium bottle was emptied. It was found only later, that the introduction of the restriction into the gas path at the entry into the discharge tube, was causing the gas to reach supersonic velocities and it was the shock wave produced by this that was forcing the discharge to run homogeneously. In addition, the is entropic expansion also cooled the gas locally and allowed more power to be fed into the discharges.
Development of a 2kW prototype fast axial flow laser
A considerable amount of development using the test bed was still required, especially in the areas of the tube and electrode systems. In addition, a gas re-circulation system with the aim of providing a run of 8 to 10 hours on a single large bottle of helium was needed, if a usable laser was to be realised. The number of plasma tubes was doubled to four to reach the required 2kW power output. When optimised, each tube was 38mm diameter and 600mm long. The overall length of the laser was kept within about 4 meters by introducing a single optical fold containing two 90 degree mirrors. The maximum power was approximately 2.5kW, and 2kW could be maintained for extended periods. The first prototype 2kW laser is shown in Fig.7.
This laser was sold by TWI to the British Oxygen Company's Industrial Laser Division, along with manufacturing rights. Later, the fast axial flow design was taken up by many companies world wide, including Electrox in the UK, RofinSinar in Germany, Daihen in Japan, and PRC in the USA. In fact, fast axial flow CO 2 lasers, based directly or indirectly on the original TWI 2kW laser, became the industrial workhorse of the laser world for many years. Well over 50,000 of such lasers have been manufactured throughout the world for welding, cutting and other materials processing applications.
Acknowledgements
The author would like to thank all his (old) colleagues in the industry who have provided much of the background information needed to produce this paper in particular Peter Houldcroft.
References
- A B J Sullivan and P T Houldcroft: 'Gas-jet laser cutting', British Welding Journal, August 1967, pp.443.
- D Brod, R E Brasier and J Parks: 'A powerful CO 2 cutting tool', Laser Focus, August 1969, pp.36
Meet the Author
Paul Hilton is Technology Manager - Lasers, at TWI in the UK, where he has specific responsibility for TWI's strategic development of laser materials processing. He is a past president of the UK Association of Industrial Laser Users and a Vice President of the European Laser Institute.