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Oct 22, 2024

Laser cutting assist gas technology evolves

The fiber laser cutting machine never cuts with the laser alone. It instead relies on several technologies working in concert. There’s the beam itself, of course, including its raw power and how it’s

The fiber laser cutting machine never cuts with the laser alone. It instead relies on several technologies working in concert. There’s the beam itself, of course, including its raw power and how it’s distributed across the beam diameter. This works together with the assist gas that envelops the beam as it cuts through the sheet metal or plate below.

When it comes to assist gases, many fabricators have shifted from oxygen to nitrogen, even for cutting thicker mild steel. Dialed in correctly, nitrogen assist gas leaves no oxide layer and, thanks to the incredible energy density of the high-powered fiber laser, cleanly evacuates molten material out of the kerf. A few catches remain, though: Nitrogen consumption can be extraordinarily high. Moreover, in thick plate especially, the kerf can be extremely narrow and leave the denesting team struggling to lift cut pieces out of the nest.

Here, a confluence of gas mixing and nozzle technologies are helping shops perfect the cut and, ultimately, help fabricators get the most out of their fiber laser investment.

Gas mixing technology usually takes place in a separate unit. In some lasers today, however, both the gas mixing and assist gas flow optimization occur within the nozzle itself. This describes the approach taken by Mitsubishi Laser. The company’s nozzles flow, and sometimes mix, air and nitrogen in such a way that aims to reduce consumption and increase overall cutting efficiency.

The company calls the technology Assist Gas Reduction, or AGR, comprising of proprietary nozzles coupled with a compressor that delivers low-pressure air. The technology applies to various assist gas setups: low-pressure shop-air cutting (AGR-AIR), oxygen cutting (AGR-O2 Boost), nitrogen cutting (AGR-N2), and mixed gas cutting (AGR-MIX).

To delve deeper, The Fabricator spoke with Ryan Conroy, laser product manager at MC Machinery. His message: No one assist gas technology fits all situations, and today, fabricators have more assist gas options to consider. Couple the right laser with the right assist gas and cutting application, and the opportunities for efficiency abound.

In one sense, air was laser cutting’s first “mixed gas,” being about 78% nitrogen and 21% oxygen. Most modern setups use what Conroy called “high-pressure air cutting,” which incorporates dried and filtered shop air delivered to the machine at about 400 PSI. Here, the primary cutting action comes from the laser beam heating the metal and the assist gas’s nitrogen content evacuating the molten material, though the process does get a boost from the slight oxygen content. Cutting mild steel, the method can achieve similar speeds as nitrogen cutting as well as similar levels of burr, especially for 7-ga. mild steel and thinner, though the presence of burr will vary depending on the laser wattage.

“For aluminum, however, high-pressure air cutting actually can achieve better edge quality than straight nitrogen,” Conroy said. “And in the middle-thickness range, the laser can achieve even faster speeds.”

Depending on wattage, the process does produce more pronounced burrs in mild steel material thicker than 7 ga. And because air is about 21% oxygen, the edge does have some oxidation, but it doesn’t have the flaky oxide edge like traditional oxygen cutting. The oxide layer from air cutting can be “baked into” the edge, which allows many operations to send cut parts directly to painting or welding, with no secondary process in between.

Of course, all this depends on the part and customer requirements. The paint will stick, but the coating might be more sensitive to impact. “For this reason, some operations choose to remove the oxide layer before the parts move on to secondary operations,” Conroy said. Then there’s the floor space consideration. The cutting method does require a large compressor that can reach 400 PSI.

Today, nitrogen is being mixed with air (which is about 21% oxygen) to produce a mixed assist gas. Here, Mitsubishi’s AGR-MIX technology mixes nitrogen and air within a mixing chamber in the nozzle. Air also shrouds the core mixed gas column. Images: MC Machinery Inc.

Low operational costs are a big reason why many choose to laser-cut with air, and to reduce those costs even further, Mitsubishi introduced its AGR-AIR technology. Air is delivered from a low-pressure compressor at about 200 PSI through a nozzle that helps drive the column of assist gas through the cut in an effective way. “Compared with high-pressure air, the compressor is smaller and has lower electrical and maintenance costs,” Conroy said, adding that the cuts give the same burr level as pure nitrogen on gauge material and similar cut speeds and burr levels as high-pressure air. “The method gives the lowest operational costs of all cutting methods, even high-pressure air.”

He added, however, that AGR-AIR still leaves oxides on the cut surface. And depending on the laser wattage, using low-pressure air can have some limitations when processing thick plate. A 10-kW laser can use low-pressure air to cut up to about 0.375-in.-thick mild steel and 0.25-in.-thick aluminum. For a 20-kW machine, low-pressure air can usually cut up to 1-in. mild steel, aluminum, and stainless while still maintaining the same or similar cut quality and speeds as high-pressure air.

In conventional oxygen cutting, oxygen assist gas exothermically reacts with the laser beam to create a higher-energy cutting process, especially useful when using a lower-wattage laser. Most oxygen cutting with the fiber laser occurs within thicker mild steel.

The process produces burr-free parts in thin to thick material. Because it uses low-pressure oxygen at very low consumption rates, the operational costs are very low. The kerf is also very wide, which makes for easy denesting, whether performed manually or automatically with a part sorting system.

Because the process uses oxygen, it does leave oxides on the cut surface, which tend to flake. “Parts that require welding or painting likely will require secondary finishing,” Conroy said. “Also, slow cutting speeds can cause heat to build up, especially in thick plate.”

To mitigate these issues, programmers might place parts farther away from each other on the nest, leading to lower material utilization. Also, the cutting method can work well if you have high-quality, laser-grade material, but it can be sensitive to certain material types.

Oxygen cutting’s notoriously slow speed is why many have shifted to nitrogen or nitrogen-oxygen mixes, especially after upgrading to a high-powered fiber laser. But as Conroy explained, these aren’t the only options.

He described a method called “high-speed oxygen cutting” using AGR-O2 Boost. “This setup also uses low-pressure, low-flow oxygen,” Conroy explained, “but is combined with a nozzle cooled by low-pressure air.”

He added that this combination has allowed some mild steel laser cutting applications—especially those involving thick plate—to reach the same or higher level of productivity as pure nitrogen cutting, but with lower operational costs.

Just like conventional oxygen cutting, Conroy said, the method still produces a wide kerf. And because the laser cuts faster, the material distorts less. Both factors, he added, ease the denesting process.

AGR-N2 incorporates a low-pressure compressor that sends a shroud of air around the core of nitrogen assist gas.

Nitrogen has become the standard assist gas for the age of the fiber laser. “An inert gas, nitrogen in most cases doesn’t add any energy to the cutting process,” Conroy said, adding that it occasionally achieves what’s known as a “laser-induced plasma,” which, if controlled in the right way, can boost the cutting action in a positive way. “But for the most part, nitrogen cutting uses assist gas simply to eject molten material through the kerf.”

The method produces an oxide-free cut surface, which means oxides don’t need to be removed by a secondary finishing process. It produces a narrow heat-affected zone and minimal burr on various materials, including mild steel, stainless, copper, brass, and galvanized stock.

Traditional nitrogen cutting does have its drawbacks, however. Denesting can be a challenge in certain applications, especially when nitrogen cutting with high-powered fiber lasers on thicker materials. The machines might cut 1-in. mild steel extraordinarily quickly, but the denesting team might struggle to lift those pieces out of the nest, and automated part sorting becomes more challenging. Then there’s the significant nitrogen consumption, which increases the cost of operation.

Conroy described one technology, AGR-N2, that aims to change this. “It incorporates a low-pressure compressor that sends a shroud of air around the core of nitrogen assist gas. This arrangement allows the nozzle diameter to be reduced, leading to large reductions in nitrogen assist gas consumption. It also helps guide the nitrogen through the kerf and prevents some nitrogen from escaping. Most of the gas that does escape comes from that outer shroud of low-pressure, low-cost air.”

For years, the industry has known about the benefits of mixing nitrogen and oxygen. Again, shop air is, in essence, a “mixed gas.”

This spurred the development of mixed gas cutting. Today’s gas mixing technology, applied to a wide variety of material grades, effectively blends several gases—usually nitrogen from a bulk tank and oxygen from cylinders or dewars—in a controlled way, allowing fabricators to monitor the mixture levels and optimize them for specific material grades and thicknesses. When dialed in, the blended gas works in concert with beam and nozzle technologies to improve productivity and deliver a clean cut.

Having an external mixing unit and oxygen tanks, however, isn’t the only way to make mixed gas. “Again, air is a mixed gas, with about 21% oxygen, and today it can be mixed with nitrogen to achieve similar results as external blending systems,” Conroy said.

He was referring to AGR-MIX, which uses low-pressure compressed air, some of which flows out of a nozzle in a shroud surrounding the core column of assist gas. Here, though, a portion of the air gets funneled into a mixing chamber in the nozzle. There, the air combines with pure nitrogen to create a mix of, say, 95% nitrogen and 5% oxygen (or other percentages).

“This method achieves higher cut quality over straight nitrogen,” Conroy explained, “eliminating the ‘fiber fuzz’ dross we see on many mild steel materials. It also provides an even greater reduction in nitrogen assist gas consumption.” The mixing characteristics also can be changed with the material grade and thickness. And it produces a wider kerf than nitrogen for easier denesting.

Conroy reiterated that no one assist gas strategy fits all situations. “Nitrogen cutting still dominates in the fiber laser cutting arena, but there’s still a place for, and plenty of technological advancement in, mixed gas, oxygen, and air cutting.”

He added that an application’s assist gas strategy depends on the types and grades of material being processed, the cut quality requirements, and the fiber laser power available. “A 20-kW fiber laser cutting mild steel would likely have a very different assist gas strategy from a 10-kW fiber laser cutting stainless steel all day.”

Modern fiber laser cutting is about building that optimal “soft cutting tool” for the situation at hand: the power density and energy profile of the laser beam, the assist gas, the nozzle, the assist gas supply (plumbing, tank setup, etc.), cutting speeds, optimal toolpaths, and more. It’s about balancing operational cost at the laser with the overall costs of the entire metal fabrication value stream.

This includes not just the cutting speed and cut quality but everything that happens after the cut, including denesting and secondary operations. Put another way, a laser cutting strategy should optimize flow and minimize overall costs—not just costs at the laser. Today’s assist gas mixing and delivery technologies are becoming an important piece of the puzzle.