Just a decade ago, fiber laser cutters were considered thin sheet specialists. Stores quickly found that they had to invest in them to compete, or at least cut their gauge material. For high-quality plate cutting, CO2 lasers are still the way to go. Sure, fiber lasers could cut thicker blanks, but the quality wasn't great, and their speed advantage almost disappeared when cutting very thick plates. Today, the world has changed.
Auxiliary gas technology has come a long way in just a few years and it is one of the key contributors to the rapidly changing field of laser cutting. Lens materials and their design have been improved, as have cutting heads and nozzles. Modern fiber laser beam delivery systems can be seen to be coping comfortably with huge photon powers. 20, 30, and even 50 kW ultra-high-power lasers can now slice thick plates quickly and cleanly.
"Clean" is the operative word here. Whether a laser makes economic sense comes down to cost per part. Today, high-power lasers are booming in the field of precision plate cutting. If a part used to be plasma cut and then deburred or finished on a milling machine, it may now be able to be done on a fiber laser.
Auxiliary gas mixing helps make it all possible. Even the thickest plates today are processed not with oxygen, but with a nitrogen-oxygen mixture. The auxiliary gas stream still consists primarily of nitrogen, an inert gas that expels the molten metal from the kerf, but a small portion of oxygen provides the chemical reaction that helps bring the kerf to the bottom for a dross-free edge.
The stand between the surface and the nozzle has been made so small as to be almost non-existent, all to allow a laminar flow of auxiliary gases through the kerf so that the nitrogen-oxygen mixture can work as intended. In precision plate cutting, excessive auxiliary gas turbulence is the enemy of clean laser cutting.
Early gas-mixing applications appeared more than a decade ago, not for thick steel, but for dross-free cutting of aluminum. Steve Albrecht, president of Pewaukee, Wisconsin-based Liberty Systems, a supplier of nitrogen generation and gas mixing, recalls using nitrogen-oxygen mixtures in the early 2010s, not for fiber lasers, but for a 4 kW CO2 system to cut 0.125-inch-thick aluminum.
"Aluminum has an oxide layer on top," Albrecht says, "and you need to burn it off to prevent any dross or burrs. As the application engineers discovered, a nitrogen-assisted air stream with a dose of oxygen helps eliminate the hard-to-remove scum on the edges of laser-cut aluminum.
As Albrecht recalls, when engineers started getting good results with oxygen levels approaching 20 percent, it opened the door to using ultra-dry air for cutting. This saved the manufacturer a lot of money, especially considering the amount of auxiliary gas consumed by the early fiber lasers.
"When the first 6 kW and 8 kW fibers came out," says Albrecht, "that's when ultra-dry air cutting really started to take off.
However, as fiber laser power continued to increase, the auxiliary gas strategy changed. Cutting conditions for the highest-power fiber lasers were built around precise nitrogen-oxygen mixtures with low oxygen content.
Laser cutter OEMs began experimenting with different nozzles and different approaches to achieve a smooth laminar flow of auxiliary gases around a more powerful beam. Nozzle designs were optimized. Some nozzle geometries trap the gas at the top of the metal. Other techniques use air "curtains" around the column of auxiliary gas. As Albrecht explains, these methods depend on the machine builder, but everyone is working toward the same goal: achieving the best-cut quality at the lowest cost per piece. This includes the utilization of auxiliary gases and, in particular, finding the optimal mixture to improve cut quality and speed.
