Metal laser cutting machines can use oxygen or nitrogen as an assist gas. In this article, we discuss the tradeoffs. To summarize before we dive in,
- Oxygen effectively adds energy to the cut, increasing maximum material thickness.
- Oxygen leaves a glassy scaling that might need to be removed before finishing.
- Nitrogen prevents oxidation, leaving a cut that requires less post-processing.
- For some material thicknesses, nitrogen cuts are very expensive compared to oxygen.
Laser-cut 1/8″ thick mild steel. The top part, with its glassy edge, was cut with oxygen. The bottom part was cut with nitrogen.
As an inert gas, nitrogen prevents oxidation of the cut surface, helps clear the cut, and reduces or eliminates slag. For most people, that makes sense – as in welding, we use an inert gas to keep oxygen off the cut. In contrast, the use of oxygen might make less sense. Why use one gas that prevents oxidation, and then another that basically guarantees it?
Unlike nitrogen, oxygen assist is used as an oxidizer to assist in the vaporization of the material. The oxygen doesn’t burn (on it’s own, oxygen isn’t flammable!) But pure oxygen mixed with, say, metal dust, absolutely does burn. Injecting oxygen into the cut effectively adds energy to the process, allowing thicker materials to be processed.
Laser-cut 1/8″ thick mild steel, oxygen assist gas vs. nitrogen assist.
Thick steel, aluminum, and even relatively thin copper require oxygen assist. Thinner material can be cut with either oxygen or nitrogen, depending on the desired surface finish. Oxygen cuts leaves a glassy scale layer on steel. It actually looks good, but it might not hold paint. Powder coating processes might not work on it, either. In contrast, a nitrogen-assist cut comes off the machine ready to go.
For that reason, nitrogen is usually the preferable choice wherever it is an option. We use nitrogen whenever we can, and oxygen for thicker steel.