Industrial gases and engineering company the Linde Group launched its Lasgon series in central Europe in the 1990s as an alternative to using pure helium for laser welding that conserves the gas and has an increased adaptability to welding, says welding, cutting and gas products supplier Afrox national safety and gas equipment specialist Hennie van Rhyn.
Afrox is a subsidiary of the Linde Group of companies.
The laser gas mixtures are available for various welding tasks and materials, guaranteeing improved working conditions for laser welding processes, says Van Rhyn.
The Lasgon product range includes dedicated mixtures for all material categories, including the laser welding of non-alloyed and low-alloy materials, stainless chrome-nickel steels, aluminium and aluminium alloys, he adds.
Van Rhyn explains that while uisng pure helium is often essential to suppress a plasma plume during welding with high-powered carbon dioxide (CO2 ) lasers, Linde Group technologists have long recognised that welding process efficiency could be improved by substituting helium with a mix of lower-cost inert gases such as argon, and active ingredients such CO2 or oxygen to enhance the welding-process gas nozzle geometry.
Since fibre-delivered lasers do not generate the plasma plume associated with CO2 lasers and do not always require helium, there is a greater scope to vary the gas composition to increase weld productivity, as minimum plasma plume is still widely accepted, says Van Rhyn.
At the shorter wavelength of the fibre-delivered laser, steel has a much smaller Brewster angle (an angle of incidence at which light with a particular polarisation is perfectly transmitted through a transparent dielectric surface, with no reflection) than a CO2 laser. Therefore, 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, resulting in a fibre-delivered laser cutting thin sheets up to three times faster than a CO2 laser of the same power, he adds.
However, a higher nitrogen pressure is required to remove the molten metal effectively from the area being welded at this very high cutting speed. As a consequence of the smaller Brewster angle, the cut front is less inclined and, therefore, a slightly larger nozzle is used. As a result, nitrogen consumption for each hour is higher, but nitrogen consumption for each metre cut length is quite similar.
Fibre Optics and Laser Technology
Afrox has acknowledged the importance of fibre-delivered lasers for cutting, hence the introduction of several 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 the Linde Group over the last three years has resulted in the introduction of 300 bar nitrogen cylinder bundles in the European market, 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, says Van Rhyn. Conventionally, the bundle must be replaced by a full one once it has reached 40 bar, leaving 20% of the nitrogen unused.
In a 300 bar bundle, containing about 190 m3 of useable nitrogen gas, only 13% of the nitrogen remains unused.
Van Rhyn highlights that when customers’ nitrogen demand exceeds the capacity of cylinder bundles and when higher delivery pressures are required, Afrox offers the Trifecta system that operates with lower-pressure cryogenic bulk tanks. Trifecta units provide a constant supply of nitrogen, oxygen and argon with no downtime; minimal blow-off losses, compared with high-pressure bulk tanks; and no production interruptions for tank filling, making it an efficient option.
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.
A brief history of lasers
Continuous progress has been made in laser application technology since the invention of the laser in the 1960s. In the decade thereafter, industrial laser applications harnessed relatively low power, compared to current standards, where carbon dioxide (CO2) lasers of about 1 kW were used for cutting wooden die boards with air and cutting steel with oxygen.
The rationale for using oxygeninstead of air or nitrogen was that the exothermal reaction with the steel resulted in substantial amounts of energy being added to the cutting process, increasing the marking speed and even making it possible to achieve a cut. Nitrogen cutting was possible, achieving an unoxidised edge, but laser power was too low to make this a commercially attractive option.
Since then, using CO2 laser power of up to 6 kW for cutting has been steadily increasing. At this power level, it became realistically possible to cut stainless steel with nitrogen at a reasonable speed, with the crucial advantage of achieving oxide-free, brightly cut edges.
Further, it became more popular to cut thin mild steel plate with nitrogen, since the high-powered laser enabled a higher cutting speed than cutting 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 shops, delivering custom-cut components from one-offs to thousands of parts, within very short delivery times.
High-powered neodymium-doped yttrium aluminum garnet lasers were also used for cutting, primarily in the automotive industry, where the fibre-guided delivery made robot applications a reality.
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 bar to 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 with the introduction of the fibre optics and fibre-delivered laser. It became clear that the laser and fibre technology originally developed for telecom applications could 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-powered diode lasers necessary to "pump" fibre lasers. The gap in investment in new equipment during 2008/9 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.
Currently, most new laser welding machines are powered by a fibre-delivered laser, which has a much shorter wavelength than CO2 lasers and, in addition to the big advantage of this wavelength being transmittable 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.