Assist gas mixing drives fiber laser technology forward

Laser assist gas mixing has been a key enabling technology, and today’s ultrahigh-powered fiber lasers couldn’t cut the way they do without it. zilber42/iStock/Getty Images Plus

Just a decade ago, fiber laser cutting machines were looked upon as thin sheet specialists. Shops soon found they had to invest in them to compete, at least to cut their gauge material. For high-quality plate cutting, the CO2 laser was still the way to go. Sure, fiber lasers could cut thicker stock, but the quality wasn’t great, and when cutting very thick plate, their speed advantage pretty much evaporated. Today, that world has changed. Perfect Stainless Welds

Assist gas mixing drives fiber laser technology forward

Assist gas technology has come a long way in just a few years, and it’s one big contributor to a fast-changing laser cutting landscape. Lens material and their designs have improved, as have the cutting heads and nozzles. A modern fiber laser beam delivery system can be seen taking immense amounts of photonic power in stride. Ultrahigh-powered lasers—20, 30, even 50 kW—are now slicing quickly and cleanly through thick plate.

“Cleanly” is the operative word here. Whether the laser makes economic sense boils down to the cost per part. High-powered lasers today are thriving in the precision plate cutting arena. If a part used to be plasma cut and then deburred or finish-machined on a mill, it now might be able to be done-in-one on the fiber laser.

Assist gas mixing has helped make all this a reality. Even the thickest plates today are being processed, not with oxygen, but with a nitrogen-oxygen mix. The assist gas stream still consists mainly of nitrogen, an inert gas that evacuates molten metal out of the kerf, but a small percentage of oxygen (usually between 1.5% and 5%) provides the chemical reaction that helps carry the cut through to the bottom for a dross-free edge.

Standoffs between the surface and the nozzle have gotten smaller to nearly nonexistent, all in an effort to get that laminar flow of assist gas flowing through the kerf, so that nitrogen-oxygen mix can work as intended. In the precision plate cutting arena, excessive assist gas turbulence is the enemy of a clean laser cut.

Early gas mixing applications emerged more than a decade ago—not for thick steel, but for dross-free cutting of aluminum. Steve Albrecht, president of Pewaukee, Wis.-based Liberty Systems, a nitrogen generation and gas mixing supplier, recalled working with nitrogen-oxygen mixes in the early 2010s, not for a fiber laser but for a 4-kW CO2 system cutting 0.125-in.-thick aluminum.

“Aluminum has an oxide layer on top of it,” Albrecht said, “and you need to burn through that to prevent any dross or burr.” As applications engineers discovered, a nitrogen assist gas stream with a certain dose of oxygen helped eliminate that hard-to-remove dross on the laser-cut edges of aluminum.

“Being a softer material, aluminum has some unique features for laser cutting,” said David Bell, president of Witt Gas Controls, Alpharetta, Ga. “And gas mixing helps. If you cut aluminum with oxygen, you burn it. If you cut it with nitrogen, you get edge striations. Cut it with a mix of the two, and you get a much cleaner cut.”

As fiber lasers began to take over the market, and available powers continued to grow, assist gas strategies evolved. Application engineers began experimenting with different combinations of nitrogen and oxygen.

As Albrecht recalled, when engineers began achieving good results as the oxygen content neared 20%, that opened the door for cutting with ultradry (and well-filtered) air. This offered fabricators significant savings, especially considering the amount of assist gas those early fiber lasers consumed.

“When the first 6- and 8-kW fibers came out,” Albrecht said, “that’s when ultradry air cutting really began to take off.”

As fiber laser power continued to rise, though, assist gas strategies changed. Cutting conditions for the highest power fiber lasers have been built around precise nitrogen-oxygen mixes, with lower amounts of oxygen (again, somewhere between 1.5% and 5%, depending on the application and machine).

Laser cutting machine OEMs began experimenting with different nozzles and different ways to achieve smooth, laminar flow of assist gas around an ever-more-powerful beam. Nozzle designs were optimized. Some nozzle geometries trap the gas on top of the metal. Other technologies use air “curtains” around the column of assist gas. As Albrecht explained, the approaches depend on the machine manufacturer, but everyone is aiming toward the same goal: to achieve the best cut quality at the lowest cost per part. This includes assist gas utilization and, not least, finding the best mix to boost both cut quality and speed.

Although machine OEMs have been working to optimize assist gas usage, high-powered fiber lasers cutting thick stock at high speed can still be assist-gas-hungry. A beam slicing through thick plate does so amid a large column of assist gas flowing through a nozzle and orifice designed to optimize it all. “If you cut faster, you use more gas,” Bell said. “It used to be that a laser used 2,000 cu. ft. an hour. Now they’re at 4,500 to 5,000 cu. ft. an hour.”

As Bell explained, laser cutting puts several unique demands on gas mixing. First is higher pressure. A fabricator that, say, uses an argon-CO2 mix for a welding shielding gas, the pressure likely won’t exceed 50 or 70 PSI. As long as the plumbing feeds 125 PSI into the work cell, the pressure demands can be met.

“For fiber lasers, the pressure can be four or five times that much,” Bell said. A mixer for laser might require 580 PSI into the system, simply because the pressure at the actual cutting process can be as high as 350 PSI. “It’s a whole different animal.”

Today, a fabricator might supply nitrogen and oxygen to a mixer in various ways. It might rely on bulk liquid nitrogen with individual dewars or cylinders of oxygen at each laser. Or it might have a generation system for the nitrogen, cylinders for the oxygen.

“In some applications, you can use high-pressure air systems to supply a mixer together with a supply of pure nitrogen, which in effect raises the purity levels,” Albrecht explained. “That way, fabricators don’t need to have cylinders of oxygen laying around.”

He added that this option is particularly popular for lasers between 8 and 15 kW. “That air system might cut 80% of your material, and the mixer helps improve quality when you cut thicker mild steel, for instance. Then they might use liquid nitrogen for stainless.”

He added that relying on an air supply for mixed-gas cutting currently isn’t being used for 20 kW and beyond. At present, most machine OEM application engineers have been working to dial in cutting conditions with traditional oxygen cylinders or dewars. Again, what method works best depends on the application and a laser OEM’s cutting conditions.

The need for a consistent gas mix under highly variable conditions has pushed mixing technology toward digital control interfaces—no more manually adjusting valves. As Bell described, some of the latest mixing systems are actually programmable, giving the ability to change the gas mix depending on the material being cut and other application variables. The mixer tanks are purged of the old gas mix before they are filled up with the new gas mix that’s dialed in for the job at hand. They can also communicate directly with the laser. When something’s awry with the gas mix supply, the laser knows and can shut down before creating a bad cut.

“The industry has come a long way in just a few years,” Bell said. “We’re no longer at gas mixing 101.”

When it comes to the nitrogen supply, fabricators have both liquid nitrogen and nitrogen generation options. Regardless of the setup, no one denies the importance of consistency—in gas quality as well as sufficient pressure and flow. A mixer needs to be able to meet the demands of carefully crafted cutting conditions, designed around a certain gas mix, pressure, and flow.

To do this, the oxygen must be evenly dispersed through the column of nitrogen throughout the cut program. If oxygen separates from the nitrogen, cut quality and performance can suffer. Various flow and diffusing technologies work to ensure mixed gas remains evenly mixed.

A mixer’s design also needs to account for changes in gas pressure and flow demand for each laser. When cutting parameters change—be it cutting speed, material thickness, or anything else—it demands different amounts of assist gas (that is, different pressures and flow volumes). That change in demand, Bell said, can alter the gas mix. What was once, say, a 95% nitrogen/5% oxygen mix suddenly changes to 90% nitrogen and 10% oxygen, which can be an excessive amount of oxygen for the cutting parameters designed for the job at hand.

Any change in the gas mix needs to happen in a controlled, intentional way, not just because a machine is drawing a different amount from the gas-mix system. As Bell explained, this is why many systems come with a buffer tank. These receiver-type gas mixers help maintain specific gas mixes under the varying flow conditions—from cutting a new thickness or using a new nozzle, for example.

As fabricators delve into higher laser powers, assist-gas plumbing becomes more critical in general. This in turn requires a good relationship with a gas supplier that knows the assist gas purity, flow, and pressure a high-powered fiber laser needs. A bad plumbing job that introduces contamination (flux from a brazed connection that wasn’t purged correctly, for instance) can throw a wrench into cut quality. A good gas supplier should know what best suits a fabricator’s application, including the purity and flow requirements.

If a fabricator uses liquid nitrogen, the quest for consistent flow starts at the tank and the plumbing that extends through the shop. Shops with multiple lasers plumb assist gas nitrogen flow from a bulk tank outside into the shop—not in a straight line to the lasers, but in a loop. The increased amount of piping between the nitrogen tank and lasers effectively makes a kind of “surge” tank. This gives the system the nitrogen reserve it needs when several high-powered fiber lasers start cutting at once.

“If you have multiple lasers and a straight pipe, usually the laser at the end will starve,” Albrecht said. “That’s why pipe looping is important if you have multiple lasers.”

Brazed pipes can present issues, since without proper purging there’s a chance that flux can contaminate the ID. For this reason, some mixing systems use hoses comprising heavy-braided Teflon and proper fittings to avoid any contamination. Some gas-mix tank systems are actually shipped full of pure nitrogen. Once the tanks are installed, technicians bleed the pure nitrogen and refill it with a gas mix. From there, the gas flows from the supply lines, through a filter at the mixer, into the system. Some setups even use a secondary filtration system downstream from the mixer but before the gas enters the laser work envelope.

“When you try to maintain a Grade 5 [99.999%] gas purity,” Bell said, “contamination can show up anywhere. Entire systems need to be designed to be gas-purity safe.”

Over the past 10 years, gas mixing has grown from being a niche process, mainly used for cutting aluminum, to becoming a defacto cutting method for thick carbon steel. Some might still use straight nitrogen for stainless, to achieve a shiny edge, but mixed gas has become a viable alternative here too.

“Over the past two or three years, we’ve seen that nearly any material can now be cut with mixed gas,” Bell said. “It depends on what laser and what power you have, and which mixed gas you use to achieve the best results.”

Mixed gas isn’t a magic bullet, but it has opened the door for some eye-popping cutting possibilities. As fiber laser powers climb, assist gas mixes are clearing the way, figuratively and literally, for some fast, high-speed cutting in thick plate, often with no secondary deburring required.

See More by Tim Heston

Tim Heston, The Fabricator's senior editor, has covered the metal fabrication industry since 1998, starting his career at the American Welding Society's Welding Journal. Since then he has covered the full range of metal fabrication processes, from stamping, bending, and cutting to grinding and polishing. He joined The Fabricator's staff in October 2007.