Flame cutting requires oxygen as a cutting gas. The oxygen is blown into the kerf at a pressure of up to 6 bar. In the kerf, the metal melt reacts with the oxygen: it burns and oxidizes. The chemical reaction releases a large amount of energy. This energy supports the laser beam. Flame cutting permits high cutting speeds in thin sheets, and the processing of thick sheets. For instance, structural steel of thicknesses up to more than 30 millimeters can be cut.

Of course, there are drawbacks as well: the cut edge is covered with an oxide layer. In the case of structural steel, the oxide layer must be removed before painting or powder coating, because the paint or the powder coat does not adhere well to the oxidized surface. If the protective layer breaks up, the metal is no longer protected against corrosion. The oxide layer also destroys the anticorrosive protection of stainless steel and interferes with welding. Therefore, when necessary, the edges have to be reworked.

 Conclusion: flame cutting is more appropriate for structural steel and quicker than fusion cutting. It is appropriate when the oxide layer at the edge is not detrimental or the costs for flame cutting and reworking are more advantageous than for other cutting methods.

Gases like nitrogen or argon are used for fusion cutting.

They are forced through the kerf at pressures between 2 and 20 bar. In contrast to flame cutting, the cutting gas does not react with the metal surface of the kerf. Therefore, it is called inert. The gas blows the molten metal out of the kerf and protects the cut edge from the air. Nitrogen is suitable for almost all metals. The exception is titanium. Titanium reacts strongly with oxygen and nitrogen and is therefore cut with argon. Fusion cutting has the advantage that the cut edges remain unoxidized. They no longer have to be reworked. However, only the energy of the laser is available for the cutting process. As a result, cutting speeds are only as high as for flame cutting in thin sheet. Piercing is also more difficult. With some cutting machines, piercing occurs with oxygen and further cutting is done with nitrogen.

Conclusion:fusion cutting produces edges that are free of burrs and oxides. The higher quality requires more time and money because of higher gas and energy costs.

If buying gas is undesirable, air can be used to cut thin sheet. Compressed air at 5 to 6 bar is sufficient to blow the molten metal out of the kerf. Since air consists of almost 80% nitrogen, compressed air cutting is a fusion cutting procedure. At first glance, compressed air cutting seems to be an attractive alternative to cutting with nitrogen. Air is free, but it must be compressed, dried and de-oiled.

This puts the cost advantage compared to nitrogen into perspective. The cut edges are rougher than after fusion cutting with nitrogen. The thickness of sheet that can be cut depends on the pressure available from the compressed air mains and on the laser power. With a 5 kW laser output and 6 bar of pressure, 2 mm sheet can be cut without burrs. The best cutting results with compressed air are obtained in aluminum.

Conclusion: cutting with compressed air is relatively fast but produces lower edge quality and is not always more cost effective.

In all the processes described above, the molten metal is rapidly and completely blown out of the kerf. The procedure is different with plasma-supported fusion cutting with the CO2 laser. Here, use is made of an effect that also occurs in laser welding. A plasma cloud forms in the kerf, consisting of ionized metal vapor and ionized gas.

Plasma can be generated when slag, molten metal and gas collect in the kerf. Plasma was long considered as only a nuisance and cause for alarm, because usually the cut broke off soon after the plasma cloud appeared. The laser beam no longer penetrated the material completely; and as a result of the processing error, the part became unusable. Then it turned out that cutting with plasma is quicker if the parameters are well chosen. The plasma brings more energy to the workpiece. In thin sheets, the process accommodates cutting speeds of 40 meters per minute and more. The maximal sheet thickness depends on the laser power. At 6 kW of power, for instance, 4 mm aluminum.

Conclusion: plasma-supported high-speed cutting is always used when the process is supposed to be particularly rapid and rough edges are not important.