Gases For Laser Cutting And Welding Processes

Laser application technology has seen substantial progress and development since the invention of the laser in the 1960s. At the time it was recognized that nitrogen cutting was possible, giving an unoxidised edge, but laser power was simply too low to make this commercially attractive. The 1980s, 1990s and into the new millennium saw 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 oxide-free, bright cut edges. It became viable to even cut thin mild steel plate with nitrogen, as 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 the automotive industry where fibre-guided delivery made robot applications a reality.

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Laser gases play a significant role in the operation of a laser machine. Afrox has more than a decade of experience in developing laser gases for different applications in the market. “Our specialist expertise enables Afrox to advise on every aspect of laser usage, whether it’s assist gases, supply mode or laser mixed gases and supply,” says Hennie van Rhyn – Application Development Manager – Cutting, Heating and Safety for Afrox. The term LASER stands for Light Amplification by Stimulated Emission of Radiation. A laser emits a special type of light. Ordinary light such as from a light bulb or the sun is made of different wavelengths, colours and phase relation. The light emitted by a laser is monochromatic and coherent. This means the light contains waves of a single, precisely defined wavelength and frequency. Coherent means the electromagnetic waves are all in in-phase, in other words all in step with one another. These properties make it possible to bundle the light into a beam and focus it to a tiny point.

Afrox, a member of the Linde Group, started penetrating the laser cutting market around 2005 and the company’s growth in this field and market share of laser cutting has dramatically increased since then. Afrox has the latest state-of-the-art laboratory with highly skilled operators to mix any Lasermix requirement to meet required tolerances for any OEM supplied laser machine to the industry. This includes four part mixes for new 6 kW high-powered CO2 laser in South Africa. Afrox is licenced, as a member of the Linde Group, to sell five part mixed laser gases in South Africa and is very competitively priced, says van Rhyn.

Afrox’s Trifecta technology is used as a pressure boosting system, capable of boosting the liquid pressures up to 34 bar, vaporizing downstream, then regulating pressures from the operating point with special low pressure high flow regulators. Some gas suppliers still offer skid pump systems and other pressure raising methods in combination with conventional lower pressure cryogenic nitrogen or oxygen vessels. To cater for customers whose demand for nitrogen or oxygen exceeds the capacity of cylinder bundles and where higher delivery pressures are required, Afrox has included the Trifecta system offer, operating with lower pressure cryogenic bulk tanks. Features include constant supply with no down time, less blow off rates compared to high pressure units, and no pauses in production while tank filling takes place, making the Trifecta system a very efficient option.

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. This development was no coincidence – it became clear that laser and fibre technology originally developed for telecom applications was able to handle the very high power needed for cutting and welding. The advance of these lasers for metal fabrication was driven by the development of cost-effective, reliable high power diode lasers needed to pump fibre lasers. The gap in investment in new equipment by industry during 2008- 2009 provided the window for the fibredelivered laser manufacturers to enter the market with new, attractive lasers showing rapid growth in uptake and even replacing old CO2 laser machines for cutting, and especially welding. Today, more and newer laser welding machines are powered by a fibre-delivered laser.

Fibre-delivered lasers have a much shorter wavelength than CO2 lasers, and apart from the major advantage that this wavelength can be transmitted by optical fibre, there is also another significant difference, caused by the wavelength, in how these two types of lasers interact with metal, both 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. The detailed explanation goes beyond the scope of this article, but the consequence is that 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, in order to remove the molten metal effectively at this very high cutting speed, a higher nitrogen pressure is required. Moreover, as a consequence of the smaller Brewster angle, the cut front is less steeply inclined and therefore a slightly large nozzle must be used. The result of these two factors is that nitrogen consumption per hour is higher, but nitrogen consumption per metre cut length is very similar.

Although not available yet in South Africa, an investment programme over the last three years has seen the introduction of 300 bar nitrogen cylinder bundles in the industrial hubs of Pune and Bangalore in India. Compared to a 200 bar bundle, the 300 bar bundle holds 50% more nitrogen. Furthermore, when the 200 bundle has reached a pressure of 40 bar, it needs to be replaced by a full one, leaving 20% of the nitrogen unused. In a 300 bar bundle, the remaining unused nitrogen is only 13%. A 300 bar bundle holds approximately 190 m3 of useable nitrogen gas.

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