Article by Hennie van Rhyn, Afrox, Member of the Linde Group
Much progress has been made in laser application technology since the invention of the laser in the 1960s. Over the next decade industrial laser applications harnessed relatively low power, compared to today’s standards, where carbon dioxide (CO2) lasers of around 1 kW were used for cutting wooden die boards with air and for cutting steel with oxygen. There was a simple rationale behind the use of oxygen, as opposed to air or nitrogen: the exothermal reaction with the steel contributed substantial amounts of energy to the cutting process, increasing the speed and even making it possible to achieve a cut. Nitrogen cutting was possible, achieving an unoxidised edge, but laser power was simply too low to make this a commercially attractive option.
Since then there was a steady increase in CO2 laser power up to 6 kW for cutting. At this power level it became realistically possible to cut stainless steel with nitrogen at a reasonable speed, with the big advantage of achieving oxide-free, bright cut edges. It even became attractive to cut thin mild steel plate with nitrogen, since the high laser power enabled a higher cutting speed than with oxygen, with the added benefit of a clean edge that could immediately be painted. The advent of high powered CO2 lasers and the development of reliable laser cutting machines created an entirely new market segment of laser cutting job shops, delivering custom-cut components from one-offs to thousands of parts, at very short delivery times. High power Nd:YAG lasers were also used for cutting, primarily in automotive industry where the fibre-guided delivery made robot applications a reality.
Afrox, a company in The Linde Group, operates a state-of-the-art laboratory where highly skilled operators manufacture custom LASERMIX® gas mixtures to the tolerances required for any OEM supplied laser machine. This includes four-, five- and six-component laser resonator mixtures for the latest generation high powered CO2 lasers available in South Africa.
Pressure boosting systems have also been developed, using piston pump compressors and other pressure raising methods in combination with conventional lower pressure cryogenic nitrogen or oxygen vessels. For example, Trifecta units are widely used in South Africa to produce nitrogen gas at 25 – 30 bar pressure, using conventional low pressure nitrogen bulk tanks.
The recovery period after the global economic downturn in 2008 witnessed a step-change in laser technology for laser cutting and welding: the fibre- and fibre-delivered laser. It had become clear that the laser and fibre technology originally developed for telecom applications was able to handle the extremely high power needed for cutting and welding operations. The advance of these lasers for metal fabrication was driven by the development of cost-effective, reliable high power diode lasers necessary to "pump" fibre lasers. The gap in investment in new equipment during 2008/2009 created a window for fibre-delivered laser manufacturers to enter the market with new, attractive lasers. Uptake was rapid, even replacing old CO2 laser machines for cutting and especially welding.
Today most new laser welding machines are powered by a fibre-delivered lasers, which have a much shorter wavelength than CO2 lasers and, in addition to the big advantage that this wavelength can be transmitted by optical fibre, there is also a significant difference, based on wavelength, in how these two types of laser interact with metal, for cutting and welding.
At the shorter wavelength of the fibre-delivered laser, steel has a much smaller so-called Brewster angle than the CO2 laser. In essence, when cutting thin steel sheet with nitrogen, the radiation of the fibre-delivered laser is used much more efficiently than the radiation of a CO2 laser. Therefore a fibre-delivered laser cuts thin sheet up to three times faster than a CO2 laser of the same power. However, a higher nitrogen pressure is required to remove the molten metal effectively at this very high cutting speed. Moreover, as a consequence of the smaller Brewster angle, the cut front is less steeply inclined and therefore a slightly larger nozzle is used. As a result nitrogen consumption per hour is higher, but nitrogen consumption per metre cut length is very similar.
Afrox has recognised the importance of fibre-delivered lasers for cutting and has introduced a number of improved or alternative supply options in addition to nitrogen cylinders and bulk tanks to accommodate the needs of every customer. For example, a global investment programme by Linde over the last three years has seen the introduction of 300 bar nitrogen cylinder bundles which will be available in South Africa in due course. Compared to a 200 bar bundle, this cylinder holds 50% more nitrogen, but for the laser operator, the benefit is even better. Conventionally, when the bundle has reached a pressure of 40 bar it must be replaced by a full one, leaving 20% of the nitrogen unused. In a 300 bar bundle containing approximately 190 m3 of useable nitrogen gas, the remaining unused nitrogen is only 13%.
Where customers’ nitrogen demand exceeds the capacity of cylinder bundles and where higher delivery pressures are required, Afrox offers the Trifecta system that operates with lower pressure cryogenic bulk tanks. Trifecta units provide a constant supply with no downtime, minimal blow-off losses compared to high pressure bulk tanks and no production interruptions for tank filling, making it a very efficient option.
The availability of high power lasers was also an enabler for another important laser application — welding. As the beam quality requirement for welding is not as stringent as for cutting, even higher power lasers, up to 15 kW, were produced. For the first time in decades, a completely new technology was able to produce the weld geometries impossible with arc- and plasma techniques, such as deep-penetration butt-welds in automotive gears, which require a narrow, low heat input welding process. To avoid oxidation and plasma plume formation, most welding applications required pure helium as a welding process gas. Since helium is difficult to liquefy, it is always supplied as a compressed gas in cylinders or tube tanks.
Linde's LASGON® series is an attractive alternative to pure helium for laser welding. Not only do these innovative laser gas mixtures conserve valuable helium reserves, they have the added advantage of adaptability. Bespoke mixtures are available for the most varied of welding tasks and materials, guaranteeing optimum working conditions for laser welding processes. The proven LASGON® family includes dedicated mixtures for all material categories, including laser welding of non-alloyed and low-alloy materials, stainless CrNi steels, aluminium and aluminium alloys. Linde complements its LASGON® gases with a range of high-performance LASERLINE® supply solutions for security of supply.
While the use of pure helium is often essential to suppress a plasma plume during welding with very high power CO2 lasers, Linde technologists have long recognised that welding process efficiency could be improved by the substitution of a mix of helium with lower cost inert gases such as argon, together with the addition of active ingredients such CO2 or oxygen and an optimisation of the welding process gas nozzle geometry.
Since fibre delivered lasers do not generate the plasma plume associated with CO2 lasers and therefore don’t always require helium, there is a greater scope to vary the gas composition to optimise weld productivity.
For further information contact Hennie van Rhyn at Afrox on 011 255 5717 or 079 883 5716.