1. Vaporization Cutting.
In laser vaporization cutting, the material surface temperature rises to the boiling point so rapidly that melting caused by heat conduction is avoided; consequently, part of the material vaporizes and disappears, while the rest is blown away from the bottom of the kerf by the assist gas flow. This process requires very high laser power.
To prevent material vapor from condensing on the kerf walls, the material thickness must not significantly exceed the laser beam diameter. This process is therefore suitable only for applications where the expulsion of molten material must be avoided. In practice, it is used only in a very limited range of applications involving iron-based alloys.
This process is not applicable to materials such as wood and certain ceramics, which do not possess a liquid phase and thus are unlikely to cause vapor re-condensation. Furthermore, these materials typically require thicker cuts. In laser vaporization cutting, optimal beam focusing depends on material thickness and beam quality. Laser power and the heat of vaporization have only a limited influence on the optimal focal position. For a given sheet thickness, the maximum cutting speed is inversely proportional to the material's vaporization temperature. The required laser power density exceeds 10⁸ W/cm² and depends on the material, cutting depth, and beam focal position. Given sufficient laser power and a fixed sheet thickness, the maximum cutting speed is limited by the gas jet velocity.
2. Melt Cutting.
In laser melt cutting, the workpiece is locally melted, and the molten material is ejected by a gas stream. Since material transfer occurs only in the liquid state, the process is termed laser melt cutting.
A laser beam combined with a high-purity inert assist gas facilitates the removal of molten material from the kerf, while the gas itself does not participate in the cutting reaction. Laser melt cutting achieves higher cutting speeds than vaporization cutting. The energy required for vaporization is generally higher than that required for melting. In laser melt cutting, the laser beam is only partially absorbed. Maximum cutting speed increases with laser power and decreases-almost inversely-with increasing sheet thickness and material melting temperature. For a given laser power, the limiting factors are the gas pressure at the kerf and the material's thermal conductivity. Laser fusion cutting can produce oxidation-free cut edges on ferrous materials and titanium. For steel, the laser power density required to melt the material without vaporizing it ranges from $10^4$ W/cm² to $10^5$ W/cm².
3. Oxidative fusion cutting (laser flame cutting).
Fusion cutting typically employs an inert gas; however, if oxygen or another reactive gas is used instead, the material ignites under the laser beam. A vigorous chemical reaction with the oxygen generates an additional heat source that further heats the material; this process is known as oxidative fusion cutting.
Due to this effect, this method achieves higher cutting speeds than standard fusion cutting for structural steel of the same thickness. On the other hand, the cut quality may be inferior compared to fusion cutting. In practice, it results in wider kerfs, increased surface roughness, a larger heat-affected zone (HAZ), and poorer edge quality. Laser flame cutting is unsuitable for processing precision models or sharp corners (due to the risk of burning off the corners). Pulsed laser modes can be used to limit the heat-affected zone, with laser power determining the cutting speed. Given a constant laser power, the limiting factors are oxygen supply and the material's thermal conductivity.
4. Controlled fracture cutting.
For brittle materials susceptible to thermal damage, high-speed, controllable separation via laser beam heating is known as controlled fracture cutting. The process primarily involves heating a small area of the brittle material with the laser beam, inducing a steep thermal gradient and significant mechanical deformation that causes a crack to form. By maintaining a balanced heating gradient, the laser beam can guide the crack to propagate in any desired direction.
