The past few years have been a period of transition for laser cutting assist gas. Conventional wisdom is being questioned, and because of that, in fact, the market now has more options than ever. Bulk and bottled nitrogen and oxygen are still popular, but other options are moving quickly into the fray. These include using a blend of oxygen, nitrogen generation, as well as dried-air systems.
According to Steve Albrecht, president of Hartland, Wis.-based Liberty Systems, the industry’s assist gas usage has evolved significantly over the past few years, a trend implied by the company’s sales record. Liberty, best known for its nitrogen-generation systems, is now selling more high-performance air systems.
“Of the systems we’ve sold this year, between 20% and 30% are nitrogen- generation systems,” he said, adding that another 10% consists of blended nitrogen-oxygen systems. “The rest is almost entirely high-performance air.”
The traditional assist gas options like bulk nitrogen aren’t going anywhere. Still, Albrecht said he wouldn’t be surprised that, with the rise of fiber lasers and, especially, laser power, high-performance air might eventually become, if not the most popular, at least a major form of laser cutting assist gas that the industry uses.
How exactly? First, it helps to define some terms. When you talk with Albrecht about cutting with high-performance air, don’t call it “shop air.”
“If you’re talking about regular shop air, you’re talking about relatively wet air that you might be able to use in material up to 1/8 in. thick,” he said. “But cut thicker than that and it’s not going to look pretty.”
As it stands today, lasers use one of about three types of assist gas that come from ambient air, all of them filtered for cleanliness (minimizing particulates) but each with a different level of dryness. One is refrigerant dry air that Albrecht considers “wet.” This can work fine on thin stock, depending on what a job’s edge requirements are, but might present challenges for thicker cutting. The midlevel comprises dry-air systems that use desiccant.
“Then you get to the extremely dry, high-performance air systems,” Albrecht said. “This is very dry air and, in many cases, gives the performance and speed you need.”
In the days when CO2 lasers dominated, cutting with nitrogen assist gas became the norm for precision work that required a shiny edge, while oxygen cutting held its place for thick carbon steel, which benefited from oxygen’s exothermic reaction that stimulated the cut.
By the early 2000s, many were espousing the benefits of dried shop air, especially for thin stock, but it still wasn’t recommended for thick, edge-critical work where nitrogen cutting continued to reign supreme. At the same time, nitrogen generators began emerging in the fab shop, yet they weren’t widespread—and for good reason.
“In the early 2000s, those who adopted the early nitrogen-generation systems were using systems with fundamental designs going back to the 1970s,” Albrecht said. “The compressors weren’t all that refined either.”
Between 2005 and 2008 the first higher-end nitrogen-generation systems, both the membrane and pressure swing absorption (PSA) varieties, began appearing in the market. Also in the late 2000s came, of course, the emergence of the fiber laser and with it the need for greater assist gas pressure. Early on the fiber laser’s nitrogen consumption was a rude awakening for some. This in turn gave a boost to nitrogen generation, a technology that has become increasingly accepted by more laser machine OEMs.
Still, as Albrecht explained, although assist gas usage has increased, it hasn’t gone through the roof as the fiber laser has taken over the industry. The story’s a bit more nuanced than that.
On most early fiber laser machines, operators used the same straight nozzles—with hole diameters of 2, 2.5, 3, or 4 mm—that they used on their CO2 laser cutting machines. “With these nozzles, the machines consumed so much more assist gas because the required pressure was much higher than what CO2 lasers required,” Albrecht explained. “It’s simple physics. If you have a certain size hole and you elevate pressure, you have more gas flow and more gas consumption.”
To combat this the industry has seen innovations in nozzle technology. Nozzle geometries change the gas dynamics so a smaller orifice can produce a wider kerf and clean cut. For instance, some nozzles emit a 2-mm stream of gas with a curtain of flow around it. “´Curtain’ isn’t the best term to describe it,” Albrecht said. “Regardless, the effect is that a 2-mm nozzle can create the same kerf as a 4-mm nozzle.” The nozzle produces the necessary pressure increase without a significant increase to gas flow, hence the savings on assist gas consumption. Other nozzles “kiss” the material surface to reduce the amount of assist gas that escapes before entering the kerf.
“Other laser machine OEMs have refined the laser parameters to reduce the required pressure,” he said. “And when you reduce the pressure, you can reduce the flow rate.”
Finally, the continual increase in fiber laser power is starting to have an effect on assist gas usage. “The assist gas pressures have been able to be reduced as lasers have become more powerful,” Albrecht said.
Over recent years Liberty has been developing and refining its nitrogen-oxygen blended-gas systems, which continued to find popularity in critical and challenging cutting applications, especially those involving aluminum. “In these systems, you typically have a bulk supply of nitrogen combined with a bank of oxygen cylinders,” Albrecht explained. “When cutting aluminum, the oxygen content helps. In fact, it’s very similar to high-performance air cutting.”
That’s not a coincidence. When developing their mixed-gas systems, Liberty Systems technicians kept reducing the amount of nitrogen in the mix, eventually to the point where it was very similar to the mix found in the air we breathe. If that air could be filtered, cleaned of particulate, and dried to the extreme, could air cutting work for edge-critical applications, even in thicker material?
As it turned out, yes. The company’s high-performance air system has a dehydration unit that, according to Albrecht, “is similar to a desiccant but a little different.”
The first high-performance air systems were installed in 6-kW fiber lasers. These systems cut with air for anything 0.25 in. and less. As power increased, though, so did the capabilities of cutting with air. “When the 8-kW systems came out, the rule of thumb was 5/16-in. material,” Albrecht said. “Now, for 10-kW systems, that rule has changed to between 3/8 in. and ½ in. So as fiber laser powers continue to increase, you’re going to see additional thicknesses being cut with high-performance air.”
He added that these thicknesses are only rules of thumb; air cutting’s efficacy depends on the mix of materials a shop processes, the lasers it has on the floor, the cutting parameters the shop uses, and the edge quality customers require.
Regardless, Albrecht said the assist gas market share might shift in the coming years. Oxygen cutting relies on that exothermic reaction and, hence, can occur only so quickly. A decade ago this wasn’t an issue, considering how the fiber laser crawled as it cut thick plate. But as fiber laser powers reach 12, 15, even 20 kW and beyond, the story is changing.
“Oxygen is fairly inexpensive, but high-performance air cutting is really taking over,” Albrecht said. “For many applications, air might displace oxygen cutting.”
He added that oxygen cutting won’t go the way of the dodo and neither will conventional nitrogen cutting. It’s hard to imagine an effective laser cutting operation with 1.25-in.-thick carbon steel being cut by a laser beam surrounded not by purified oxygen but by a jet of air.
That said, never say never. “I’ve seen 1-in.-thick stainless steel cut with high-performance air,” Albrecht said. “The edge didn’t look pretty but nonetheless it was done.”