Understanding the Role of Laser Cutting Gases
Laser cutting entails focusing a beam of coherent light onto the workpiece. The concentrated energy heats the metal above it’s melting point, which is how it cuts, but that alone doesn’t produce a clean edge. Without assist gas the heat flows into the sheet, and the molten metal doesn’t drop away as cleanly as we’d like. There’s also the problem of the hot metal reacting with all the various elements in the air.
The purpose of assist gas is to exclude regular atmospheric air and blow away the molten material. To do that, the laser nozzle is designed so the gas surrounds the laser beam. It’s like a protective tube, shielding the beam from the atmosphere.
These functions are what nitrogen and oxygen have in common. They also have substantial differences.
Blasting the gas at high velocity blows away the metal that’s been melted by the laser. It also takes away heat from the metal sheet, minimizing the heat affected zone.
The Special Properties of Oxygen and Nitrogen
Using oxygen around molten metal might seem an odd thing to do. After all, metal likes to grab oxygen atoms, (that’s the rusting process,) and does so all the more at high temperatures. However, that chemical reaction also adds power to the cutting process.
Without getting bogged down in chemistry, the laser heat promotes an exothermic reaction in the oxygen, meaning more heat is produced. In effect, oxygen magnifies the laser power, so it cuts faster. However, the cut edge does suffer oxidation in the process. That means some clean-up is needed, especially of pieces being welded or painted.
Nitrogen assist gas does the opposite. By excluding oxygen from the melt zone it prevents oxidation or discoloration and leaves a clean edge that’s ready for welding or painting.
The Importance of Purity
For both oxygen and nitrogen assist gases to do their job it’s essential they be pure. In the case of oxygen that means better than 99.7% oxygen. For nitrogen the standard is even higher: 99.999% purity is what’s needed. Both gases are available in those conditions, but there is of course a cost.
Making the Choice
Here’s the conundrum: Laser-cut sheet with a nitrogen assist gas and you get a clean edge. Cut the same sheet with oxygen assist gas and you get a rougher edge that needs clean-up, but thanks to the additional heat produced, you can cut much faster.
When cutting thin gauge material there’s not much need for additional speed. The beam slices through about as quickly as the machine can move the sheet. But on thicker material – say greater than 3/16” – the additional heat of oxygen assist helps cut faster.
It’s also worth noting that not all metals behave the same. Oxygen works well with carbon steels for example, although we wouldn’t use it with galvanized material because of the gases given off. Conversely, we might prefer nitrogen with a stainless steel because it leaves virtually no edge discoloration.
Like Robin or Ron Weasley, assist gases don’t get a lot of attention yet they play an important role. Gases aren’t all alike though, so it’s important to chose the right one for each job. Cutting speed and edge quality are the primary considerations, and that’s why to make clear about the fabrication’s purpose.