Fiber laser assist gas: oxygen, nitrogen or compressed air?

Use oxygen for thick mild steel, nitrogen when the edge must be clean and oxide-free on stainless, aluminium or mild steel, and dry, filtered compressed air for thin to mid-range sheet where a slightly darker edge is acceptable. Nitrogen costs the most to run because it needs high pressure and high flow through the nozzle.

What the assist gas does

The laser beam melts the metal. The assist gas, blown through the nozzle around the beam, pushes the molten material out of the kerf and protects the lens. Depending on the gas, it also takes part in the cut: oxygen burns with the steel, while nitrogen does not react. That difference sets the pressure, the speed, the edge finish and the running cost.

Oxygen, nitrogen and air compared

Gas	Typical use	Pressure and flow	Edge
Oxygen	Mild steel, especially thick plate	Low pressure, around 1 bar (15 psi) at the head, with modest flow	Oxide layer on the cut face
Nitrogen	Stainless steel, aluminium, and mild steel where a clean edge is needed	High pressure, typically 8 to 20 bar, and high flow	Bright, oxide-free
Compressed air	Thin and mid-range mild steel, stainless and aluminium	Similar pressure and flow to nitrogen	Darker or greyish; small oxide content

Oxygen

With oxygen, the burning reaction adds heat to the cut, which is why it cuts thick mild steel with relatively low laser power. A fabrication equipment maker's guide gives about 15 psi (1 bar) at the cutting head and flows from roughly 40 to 500 cubic feet per hour (about 1 to 14 m3/h), depending on nozzle size, and calls oxygen the only option for maximum-thickness cuts. The downside is the oxide layer on the edge. Parts that will be painted, powder coated or welded may need it removed.

Nitrogen

Nitrogen shields the cut from oxygen, so the edge stays clean and bright. Because it adds no heat, it relies on pressure and laser power to clear the melt. Atlas Copco, which makes nitrogen generators, puts fiber laser nitrogen demand at 8 to 20 bar depending on material and thickness. The same fabrication guide quotes around 200 psi (about 14 bar) and flows of roughly 650 standard cubic feet per hour (about 18 m3/h) with a 1.5 mm nozzle, rising to about 2,650 (about 75 m3/h) with a 3.0 mm nozzle. Flow grows quickly with nozzle diameter, which is why thick nitrogen cutting uses so much gas.

Compressed air

Air is about four-fifths nitrogen and one-fifth oxygen. Cutting with it gives an edge between the two pure gases: faster and cleaner than you might expect on thin sheet, but darker than nitrogen. It needs a dedicated high-pressure compressor with drying and a series of filters, because oil, water or particles in ordinary shop air can damage the cutting head. Machine capability varies: Bystronic lists compressed-air cutting from 3 to 30 mm for the ByStar Fiber, a high-power machine. Ask each maker for the air-cutting chart of the source you are quoted.

Purity

Gas purity affects edge colour and dross. Published figures vary by source and application:

• Atlas Copco says mild steel and aluminium cut cleanly with nitrogen at 99.9 to 99.99 percent, while thick stainless can need up to 99.999 percent. Its PSA generators produce 99.95 to 99.999 percent.
• The fabrication guide cited above says on-site nitrogen should reach 99.99 percent.
• For oxygen, the same guide notes that liquid oxygen is generally pure enough for cutting, while compressed oxygen needs a specified cutting grade.

Follow the laser maker's specification. Lower purity can be acceptable for some parts and saves money on a generator, but it shows on stainless edges.

Supply options

Supply	Suits	Consider
Cylinders or cylinder packs	Low use, trials, oxygen for occasional thick work	Highest cost per unit of gas; pressure falls as the cylinder empties; frequent changes and deliveries
Bulk liquid tank	Steady, high nitrogen or oxygen use	Tank location, foundation and permits; supply contract and rental terms; boil-off when idle
On-site nitrogen generator	High, predictable nitrogen use	Needs a compressor, storage and floor space; purity versus output trade-off; possible booster for high pressure; electricity and maintenance
Dedicated air compressor	Air cutting	Drying and filtration; oil-free or well-filtered air; noise and heat; maintenance

Generator makers argue that on-site nitrogen gives the lowest cost per cubic metre and removes delivery logistics. That depends on your volume. A generator sized for peak demand but running a few hours a week may never pay back, while a two-shift shop cutting stainless daily may find cylinders unworkable. Get consumption figures from the laser maker, estimate your arc-on hours, and ask gas suppliers and generator makers to quote against the same numbers. The site work for tanks and generators is covered in our floor preparation guide.

Edge quality and downstream work

• Welding. Oxide on oxygen-cut edges can affect weld quality; many shops cut weld preparations with nitrogen.
• Painting and powder coating. Oxide scale can flake and take coating with it. Nitrogen or air edges usually need less preparation.
• Visible stainless and aluminium parts. Nitrogen gives the cleanest result.
• Structural and hidden parts. Oxygen or air are often good enough and cost less.

What drives gas cost

• Nozzle diameter and pressure, which set the flow.
• Material thickness, since thicker plate needs larger nozzles and slower cutting.
• Arc-on hours and the number of pierces.
• Leaks in piping and fittings, which waste gas around the clock with high-pressure nitrogen.
• Supply method: delivery charges, tank or cylinder rental, and for generators and compressors, electricity and service.
• Purity: higher purity costs more from every supply route.

Gas handling basics

High-pressure nitrogen lines and cylinders store a lot of energy, so use hoses, regulators and fittings rated for the pressure, and secure every cylinder. Oxygen makes materials burn faster, so keep oil and grease away from oxygen fittings and repair leaks promptly. Bulk liquid tanks add cold-burn risks, and nitrogen released into a small, closed room can displace the air people breathe. The safety data sheets and site survey from your gas supplier should cover the details for your installation.

Choosing the right mix

Most production shops use more than one gas. A common pattern is nitrogen or air for thin and mid-range work, and oxygen for thick mild steel. Laser power also shifts the balance: higher-power sources can cut thicker mild steel with nitrogen or air at speeds where a smaller source would have to use oxygen. Our fiber laser power guide explains that trade-off, and fiber laser vs plasma covers thick plate where plasma may be the better tool.

When comparing fiber laser cutting machines, ask every seller for the cutting chart with gas type, pressure and nozzle for your top materials and thicknesses. Gas options and supply arrangements for machines such as the Trumpf TruLaser 1030 fiber or the Gweike LF3015GA can then be confirmed in the quote. Fire and fume controls for the cutting area are covered in our guide to laser cutting safety.