Outside of metallic engineering circles, the use (and need) for a laser cutting assist gas is mostly unknown. However, it has a big impact on the final aesthetic finish of the cut. Make the right choices and the finish will be smooth and long lasting. Conversely, wrong choices will result in a jagged finish which will wear quickly.
A laser cutter works by focusing a beam of coherent light onto the material. The concentrated energy heats the metal above its melting point. This cuts through the metal, but the molten material doesn’t simply vanish into thin air. Instead, it partially attaches itself to the surrounding hot material, creating a very rough surface around the cut.
Not only would this look bad, but it would also be a safety hazard as the shards of metal would be very sharp. To avoid this, a gas is blown onto the steel at high pressure alongside the laser beam nozzle to remove the molten or vaporised material away from the cut. The most commonly used gases for this purpose are nitrogen and oxygen.
When oxygen is used as a laser assist gas, it reacts with the metal in an exothermic reaction. An exothermic reaction creates heat in the surrounding material, which has the benefit of speeding up the cutting process. Without going into any more complex chemistry, oxygen essentially magnifies the laser power.
Therefore, oxygen is a more cost-effective method as less energy is required.
However, the heat caused in the reaction also means that the gas cannot properly do its job of removing the molten material. Some material will remain on the underside of the cut, creating a residue. Not only does this look untidy, but a powder coat will not be able to properly adhere to it. That means the powder coat will start to break down long before it would under normal circumstances.
Using nitrogen as a laser assist gas is a modern innovation; traditionally, oxygen was mainly used. However, nitrogen has the distinct advantage of being an inert gas, which means it does not react with the molten metal like oxygen does.
Therefore, using nitrogen when laser cutting will result in a clean edge because all the molten material can be removed.
When cutting thin gauge material there’s not much need for additional speed, so nitrogen is more suitable. It’s only on thicker material that the additional heat of oxygen will help to cut the material noticeably faster.
Its also not recommended to use oxygen to assist cutting through a galvanised material because it will give off harmful gases.
Also, nitrogen is more suited for stainless steel as it does not cause much, if any, discolouration around the cut.
Oxygen is still widely used because of its cost benefits. However, because of the clean cut it achieves and the good protection it offers for the powder coat finish, we have always used nitrogen for our planter edging. We argue that for the reasons listed above, oxygen is not suitable for architectural non-industrial purposes.