Which Assist Gas Should I Use for Fiber Laser Cutting?
During the laser cutting process, a laser is used as a heat source to melt or vaporize materials.
To cut the materials with a high-quality edge, an assist gas blows through a cutting nozzle on the laser cutting head. Then the molten material pushes out of the hot spot where the laser light is hitting the material. This area is known as the kerf.
The right assist gas can improve the edge quality and can increase the cutting speeds substantially for certain materials.
Additionally, the assist gas acts as a positive pressure within the head to protect the laser optics/lens from molten material spatter. This provides longer processing life with reduced maintenance.
There are several options for assist gas, and it is important to understand which to use for different applications and why.
Assist gases can be reactive or non-reactive. If the gas is reactive, it can change the properties of the metal it is cutting within the kerf. On the part edge, it will react with the components of the metal during cutting and could cause the metal to become oxidized, introducing rust after a short period of time.
Reactive gases are acceptable to use when edge quality is not important. For example, when mild steel material continues to another step of the process within 24 hours.
Non-reactive assist gases can be used for critical applications where edge quality is important, for example if the next process is laser welding.
We are going to focus on 4 types of assist gas: compressed air, nitrogen, oxygen, and argon.
Compressed air is the simplest of the assist gas options which can connect into an existing shop air setup.
Most shop compressors run between 75-175 psi, so depending on the application, a pressure booster may be needed.
For example, a 1.5 mm/0.060”/16 ga mild steel might cut best in the 150-200 psi range. The air will need to be cleaned to remove any oil or moisture prior to entering the head to avoid contaminating the optics, so a filter is required to ensure clean air.
Compressed air as an assist gas is a blend of the other gases including nitrogen and oxygen. Due to its oxygen content, compressed air is considered a reactive cutting gas for processing of metals. However, it is less reactive than oxygen assist gas.
Compressed air can have high-quality cuts through aluminum, and at somewhat higher speeds than for nitrogen or oxygen in thinner material thicknesses. Depending on application requirements, compressed air can be used with many other metals.
Nitrogen is considered the best edge quality producing assist gas in most scenarios.
For most purposes, with the exceptions of some exotic materials like titanium, nitrogen is considered a non-reactive or inert assist gas. Meaning the nitrogen does not react with any of the components of the metal during cutting.
This means no major chemical differences are present in the cut edge, and the mechanism of material removal is simply the pressure of the gas jet pushing the molten metal out of the cut. Because this is a colder process, nitrogen assist gas cutting results in a very high-quality edge for a wide range of materials with a very thin heat affected zone.
Nitrogen is best used with aluminum, mild steels, galvanized steels, and UHSS automotive steels.
This assist gas is often used on parts that need to be stored for a period of time before being used, as oxygen and air cutting can introduce oxides around the cut edge with prolonged storage.
Oxygen was one of the earliest assist gases used, due to its reactive nature when cutting.
When processing steel, the oxygen burns the carbon from the steel in the kerf. The kerf produces extra heat which allows for earlier, lower powered lasers to cut through thicker materials.
Although, this same reactivity can also have some negative side effects.
Often, a material will be limited in speed and gas pressure used if a good edge quality is desired. As a result, the chemical reaction increases with increased gas pressure and flow rate. This leads to added heat that increases the melt of an edge.
This is not always desirable for high quality edges.
Oxygen cutting tends to run at a lower flow rate and pressure than nitrogen cutting, resulting in a lower gas consumption and less operating cost at the expense of a slightly slower cutting speed in some scenarios.
Oxygen cut materials often will also have oxides formed along the edge of the cut. The oxides can interfere with attempts to paint the material if not immediately fed into a painting line.
Some materials like stainless steel can form blackened “icicles” of dross when cut with oxygen. Many metals may be cut with oxygen assist gas when a lower gas usage rate is desired. However, it is important to note that the edge may not be clean.
Argon is the rarest and most expensive gas encountered for use by most processors.
Materials that can be cut with nitrogen well can also be cut with argon with similar high-quality edge.
The main reason to use the more expensive argon is for metals that are still chemically reactive being cut in pure nitrogen.
Most commonly, argon is used in processing titanium. At the temperatures a laser cutter raises the metal to, titanium is chemically reactive even in a pure nitrogen atmosphere. This is the primary reason why argon would be chosen over nitrogen.
Even though argon is inert and cuts similarly to nitrogen, there are two differences that might limit more widespread use.
Argon has a higher specific heat than the other assist gases on this list. This means that, while nitrogen is said to cool a cut as it is made, and oxygen is said to add heat to a cut, argon removes heat from a cut even faster than nitrogen.
The heat affected zone is narrow, but for some materials like a Martensite steel, this can lead to the narrow HAZ being quite brittle as if heated and quenched, which may lead to premature cracking.
Conversely, this same property can be used on some steel formulations, where a slightly hardened edge is desirable.
There are various combinations when it comes to metals and the assist gas to be used. As shown in the table below, there are recommendations for which gases to use. In the end, it is the application that will have to drive what assist gas is best.