The Role of Assist Gas in Metal Laser Cutting

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How the assist gas works

The laser beam is focused on the cut line, penetrating the metal — it starts with a pierce hole and then proceeds to contour cutting through thermal action. At the same time, an assist gas (also called process gas or working gas) is delivered coaxially to the cut zone. It blows molten material out of the kerf. Depending on the gas selected, it may also react with the metal or cool it. A portion of the metal is vaporized.

During cutting, under the action of the laser beam, the material in the cut zone is melted, ignited, vaporized, or blown out by the gas jet.

Gas purity

Clean (high-quality) gas affects edge quality and cutting speed at a given laser power. Low-quality gas means the metal will not cut cleanly at the standard speed — dross, scaling, and discoloration may appear on high-alloy steels.

Gas quality is normally expressed as the percentage content of the main molecule. For nitrogen, oxygen, and argon the purity is typically 99 % plus several decimal digits — the more "9"s after the decimal point, the higher the purity. The thicker the metal, the higher the purity required.

Industrial laser cutters usually use technical-grade oxygen at 99.5 % or 99.7 %. There is no need to chase ultra-pure gas with many "9"s — 99.99 % is sufficient.

Gas selection depends on material type, sheet thickness, and downstream operations. With nitrogen, purity is not always the top priority; with oxygen, results depend directly on its purity.

Factors driving gas consumption

Fiber-laser metal cutting gas consumption depends on:

gas type;
pressure;
material thickness;
nozzle.

Gas types for laser cutting

Compressed air (gas mixture);
Oxygen (active gas);
Nitrogen (conditionally inert gas);
Argon and Helium (truly inert gases);
Liquefied carbon dioxide (active gas).

Cylinder color coding

Carbon dioxide — black cylinder, yellow lettering;
Oxygen — blue cylinder;
Nitrogen — black cylinder with a brown stripe;
Argon — black cylinder with a white stripe;
Helium — brown;
Compressed air — black cylinder, white lettering.

Gas storage formats

40-L or 70-L cylinders — not optimal; they need frequent replacement.
Cylinder pack (manifold) — cylinders connected with hoses. Lasts longer, but refilling and transport are inconvenient and they take up a lot of space.
Cryogenic tank (gasifier) — a large vessel storing the gas as a liquid. The preferred option: cleaner gas, lower consumption.
On-site gas station — generates gas directly; used at large facilities.

Active and inert gases

Active gases. Carbon dioxide shields the metal from contact with oxygen, whereas oxygen is used as an oxidizing component: it reacts with the metal and forms metal oxides. Since the surface of nearly all metals and alloys is covered with a thin oxide film, the gas enters an exothermic reaction — well suited for carbon steel cutting.

Inert gases have nearly zero reactivity and do not enter the exothermic reaction. This prevents oxidation and protects the edge and microstructure of high-alloy steels from corrosion and structural damage.

Functions of the assist gas

Melt removal from the cut zone — the primary function.
Edge cooling. A key advantage of laser cutting over other cutting methods is the absence of thermal distortion. Only the cut zone is strongly heated; surrounding areas do not heat enough to deform. The gas jet cooling is largely responsible for this.
Plasma suppression. The gas jet prevents the formation of a plasma plume that would unpredictably alter the process.
Optics protection. Properly directed gas shields the laser optics from molten and vaporized metal that could damage them.
Reaction function — depends on the gas:
- an active gas enters the exothermic reaction and makes cutting faster and more efficient;
- an inert gas instead shuts out the reactive components of air and prevents the edges from reacting with them.

Compressed air (Air)

Constant components of air: oxygen — 21 %, nitrogen — 78 %, inert gases (mainly argon) — 0.94 %; variable components: carbon dioxide — 0.03 %, plus impurities (water vapor, dust, sulfur and nitrogen oxides, and other gases).

Since air contains roughly 78 % nitrogen, it is in principle sufficient as an assist gas for cutting thin sheet (typically up to 3 mm). It works best on high-carbon alloys. Low-carbon steel should not react with the impurities and CO₂ in air, so nitrogen is preferable for it. Air must be properly dried and filtered.

Compressed air is typically used to cut black steel, galvanized steel, brass, and aluminum.

Cuts made with compressed air are medium-quality at best. Air contains around 75 % nitrogen and 25 % oxygen (by volume), and the oxygen can oxidize the material, degrading edge quality. On the other hand, air also gives a slight oxygen-burn effect combined with nitrogen's higher speed — best of both worlds in a sense. However, you will notice a slight yellowish tint on the cut edge from the oxygen. If significant post-processing is planned, compressed air is not recommended.

Although compressed air is the most cost-effective option, it has serious drawbacks.

Atmospheric air is delivered by a compressor that brings the line up to the required pressure. Effective air treatment is essential.

Oil and water particles settle on the laser head's protective window, reducing its transmittance. Light transmission drops quickly and the window must be replaced. Air carries moisture and oil droplets from the compressors, contaminating the air, damaging the protective lens, and in the worst case the cutting head. The whole air line gets contaminated. Strict, uncompromising filtration is required.

What happens to stainless steel when cut with air

Stainless steel is strong and resistant to aggressive environments. Unlike black steel it does not readily corrode when exposed to air and moisture — at least within the rating of the specific grade. This is achieved with alloying additions, the main one being chromium.

The problem with chromium is that it readily reacts with the CO₂ always present in atmospheric air. Theoretically the reaction also occurs under normal conditions, but the rate is so low that no observable changes happen in a human lifetime. In the molten state during laser cutting, however, chromium reacts with CO₂ instantly.

Consequences:

The resulting chromium-carbide grains reduce part strength — the homogeneous metallic-bonded matrix becomes peppered with inclusions that do not support those bonds. Lots of weak links appear.
Chromium bound this way stops performing its alloying duty. Otherwise good stainless behaves as if half the chromium had been stolen — it suddenly rusts in conditions where it should not.

If you start cutting titanium in atmospheric air, the material is ruined — air must be replaced with pure argon.

Oxygen (O₂)

Oxygen is a chemically active gas that enters exothermic iron-oxidation reactions. It acts as a catalyst, adding nearly 40 % more energy in the cut zone. Low-viscosity oxides are produced. Oxygen accelerates the oxidation reaction: first it oxidizes the molten material, then it blows away the residue.

Oxygen cutting is based on the metal's ability to burn in a stream of technically pure oxygen once heated near steel's melting temperature. It involves:

heating the metal to the ignition temperature in the oxygen jet;
burning of the metal;
blowing the oxides and molten particles out with the oxygen jet.

The required preheat temperature at the start of the cut depends on the mass (thickness) and primarily on the composition of the metal: more mass and more alloying elements mean higher preheat. Oxidation intensity grows with oxygen purity and temperature. Oxide blowing begins simultaneously with metal oxidation.

So oxygen does more than just blow out the melt — together with the laser radiation it participates in melting and flowing the metal out of the laser-heated zone. With oxygen cutting, gas pressure setting, plus laser power, cutting speed, and focus position must all be set carefully.

For thin steel (up to 3 mm) both oxygen and nitrogen can be used. With oxygen on thin material the laser power must be lower than with nitrogen to avoid edge burn, but cutting speed drops accordingly.

Thin-metal cutting with 99.5 % oxygen has speed and quality similar to compressed air, while for thick steel (above 10 mm) this grade is not suitable.

Because oxygen is an active gas, it is used at lower pressure than nitrogen or air, reducing consumption. Precise pressure control is needed: after piercing, pressure must be raised, otherwise spatter occurs during cutting. Oxygen cutting pressure typically stays below 1 bar, whereas nitrogen runs up to about 20 bar. Oxygen is more economical than nitrogen.

Oxygen is normally used for low- and medium-alloy steel grades, except for parts whose cut edges will be painted afterwards. The focal distance is shorter with oxygen, and the beam focus should normally sit on the top surface of the steel.

Interestingly, with oxygen — opposite to nitrogen — as thickness increases the pressure should be decreased, not increased, to prevent runaway exothermic reactions that could ruin the cut and the whole workpiece. For steel thicker than 12 mm, oxygen pressure of no more than 1 bar is usually enough. The flip side of such low pressure: even small pressure variations can noticeably affect cut uniformity — reliable pressure regulators are essential.

Oxygen cutting has downsides: the oxidizer also acts on the edges, which is very undesirable. With accurate parameters this side effect can be managed on carbon and low-alloy steel. On stainless steel it is much harder to control. Under high temperature and excess oxygen the edge metal oxidizes and burns — burn-through is quite likely. The metal then loses its main property: corrosion resistance. So laser cutting stainless steel in an oxygen environment cannot deliver quality and turns the edge from "stainless" into "rusting" steel.

Oxygen cutting on aluminum gives a rough cut edge. If rough edges and an oxide layer on the cut are acceptable to you (these will be a problem for powder coating of edges), oxygen is fine.

For copper piercing and cutting, high-pressure oxygen is the typical assist gas.

What does oxygen do when cutting aluminum? It ruins the laser's signature feature — clean, straight cut edges. Aluminum cut with oxygen comes out rough and with burrs. Aluminum used to be cut with oxygen anyway because laser power was not enough — then edges were machined off afterwards. That made production longer and costlier. Today, with powerful sources, nitrogen yields clean cuts in one pass.

Nitrogen (N₂)

Nitrogen is considered conditionally inert. For laser cutting, nitrogen with purity from 99.5 % can be used. Nitrogen and other inert gases do not cause exothermic reactions. It is only "conditionally" inert because the triple bond in the N₂ molecule is hard to break — nitrogen is reluctant to react with anything. But in principle its molecules can react with other substances. With titanium this is critical — titanium-nitride molecules form, which damage the material's strength just as chromium carbides damage stainless.

Therefore titanium is cut in argon — a truly inert gas whose atoms are "self-sufficient" and do not react.

Nitrogen makes up more than 70 % of air, so it is easy to source. Unlike argon it does combine with other substances and take part in chemical reactions. It is treated as inert because it is not an oxidizer: there is no combustion and no plasma formation in a nitrogen atmosphere.

Nitrogen is mainly used to cut: stainless steel, high-alloy steel, nickel, aluminum. Nitrogen can also process black metals but is far less productive than oxygen on them. With stainless properties absent to begin with, the oxidation caused by oxygen can be ignored.

Some materials cannot be cut cleanly with nitrogen, let alone oxygen — one is titanium (common in certain industries). Nitrogen lacks the inertness, reacts with titanium, and the material begins to crumble, changing properties and structure. A more strongly neutral gas is required — argon, which does not interact with any materials.

Advantages of nitrogen cutting on stainless steel:

no change in the structure or chemistry of the stainless steel in the cut area;
high-quality contour cutting and hole perforation in thin sheet metal;
excellent precision plus no thermal or mechanical distortion — parts can go to the next assembly step without additional machining.

Why nitrogen is used:

Stainless steel. Even a small amount of oxygen during high-temperature processing oxidizes the edge and destroys the corrosion-resistant properties. In a nitrogen atmosphere the chromium-bearing passive oxide film remains intact and the stainless properties are preserved.
Galvanized surfaces, including zinc-coated. Nitrogen cutting preserves the protective layer.
Painted surfaces. Nitrogen as assist gas avoids scaling and minimizes secondary finishing.
Edges destined for painting. Nitrogen prevents oxide formation on the edge, which would impair powder-coating adhesion.
Aluminum and non-ferrous metals. Oxygen does not increase cutting speed on these materials, and cut quality is higher with nitrogen.

For brass, nitrogen is the right assist gas.

If you are cutting thin steels and care about speed and quality, and are not bothered by the higher gas-related cost — choose nitrogen.

With nitrogen, the laser focus should sit closer to the bottom surface of the sheet. The resulting kerf is wider and more compressed nitrogen flows through it. Nozzles of 1.5 mm diameter or larger are typically used.

Argon (Ar) and Helium (He)

Truly inert gases — argon and helium — neither oxidize nor react with the molten material at all. They also displace from the cut zone any gases that could have reacted with it.

For most metals in laser cutting this is not needed. Nothing bad happens if aluminum can react with nitrogen. But for titanium, for example, it matters.

Titanium is rightly considered one of the harder-to-process materials. It is slightly tougher than stainless steel, so the same thickness of titanium cannot be cut. Interestingly, titanium is nearly 40 % lighter than stainless.

Cutting titanium in other gases produces not only unwanted oxides that ruin edge quality, but also titanium-nitrogen compounds — these are extremely brittle and should not be present. So titanium cannot be cut in nitrogen, but it can be cut in truly inert gases. Inert gases are also often used on stainless steel and aluminum — they prevent oxidation and ensure a clean, smooth cut surface. However, these gases cost significantly more than nitrogen, so they are used only when truly necessary.

Argon:

air content: 0.9 % by volume, 1.3 % by mass;
density at standard conditions: 1.78 kg/m³;
boiling point: −186 °C.

Argon is an inert gas that does not react with most metals and does not enter chemical reactions. One advantage of argon cutting is the absence of oxidation on the resulting surface. Argon delivers high purity and precision of the cut, smooth and even edges — especially important for thin sheet.

Argon and helium are seldom used in fiber-laser cutting. Argon has a higher specific heat capacity — meaning it can absorb heat from the cut zone. This can cause local hardening and rapid cooling at the cut, potentially damaging the material. So it is used only as an alternative for metals that may react with nitrogen.

Argon cutting handles metals of various thicknesses and compositions — stainless steel, aluminum, alloyed metals, and others. But its high cost, consumption, and risk of damaging the material make argon a rare, specialty gas for laser cutting.

Helium. It has the lowest boiling point of any known substance. Helium is non-combustible, non-toxic, and non-explosive. Helium has higher thermal conductivity than argon, allowing higher speed and reduced heat input to the metal. Helium is often used to cut thin metals where high precision is needed.

Liquefied carbon dioxide (active gas)

In laser cutting — specifically on fiber machines — CO₂ as an assist gas is rare, although it reacts weakly with the metal without oxidizing it. CO₂ is heavier than air, so it reliably isolates the molten metal from atmospheric contact. It serves as an alternative when there is no compressor: cheaper than nitrogen, but more expensive than oxygen. Suitable for stainless steel, with quality clearly inferior to nitrogen. CO₂ compresses poorly — less gas fits into a cylinder.

This gas is used only as a last resort and is not recommended by any laser equipment manufacturer.

Why laser cutting brass and copper is so difficult

Low absorption of infrared laser light makes these metals hard to cut.
Copper and brass (copper-zinc alloy) are good reflectors — and therefore poor absorbers — of infrared (IR) laser light, especially when solid.
Pure solid copper reflects > 95 % of near-IR light (~1 µm wavelength).
The reflectivity of copper and other reflective metals decreases on heating and drops sharply on melting (e.g., to < 70 % for molten copper). In the molten state these metals absorb significantly more laser energy.

Because metals such as bronze, brass, and aluminum oxidize readily, cutting them on laser equipment requires inert assist gas. Inert gases create an oxygen-free atmosphere at the cut zone, preventing rapid oxidation. Cutting aluminum, copper, or bronze in an oxygen atmosphere produces uneven, burr-laden edges that need secondary mechanical finishing.

In practice, nitrogen is most often used in production. It is the most economical choice and generally delivers the required cut quality.

Factors to consider when planning gas supply

Type and thickness of materials (dielectrics, metals, which metals exactly);
presence and number of consumption peaks, plus peak-load demand;
average monthly consumption;
working pressure planned at the point of use (cutting head);
nozzle diameter;
peak-load pressure drop between the gas source (cryogenic tank or manifold) and the point of use;
the minimum residual liquid in the cryogenic tank required for safe, uninterrupted operation;
delivery frequency per month;
expected production growth and future gas demand.