Metal cutting can be accomplished in two distinct ways: fusion cutting and sublimation cutting. The vast majority of metals profiling, such as operates daily at hundreds of laser jobshops relies upon the former approach:
The laser head contains not only a lens to focus the beam onto the workpiece but also a nozzle which directs high pressure gas at the focus. After initial material piercing of the sheet or tube metal, the laser melts the material (up to 30mm of mild steel for example) all the way through the thickness and the gas blows the material away, leaving a ‘kerf’ or cut region.
The gas is selected to meet the needs of the process; for example an inert gas such as Nitrogen will produce a relatively clean, unoxidised finish on steels, whereas Titanium will require Argon. An extension of fusion cutting is exothermic cutting which uses oxygen as the process gas. This cut method generates additional heat via the oxidation process that results, thereby permitting faster cut rates.
Plastics, ceramics, wood and leather are normally cut by sublimating or vapourising material away from the kerf and do not rely upon a process gas other than to reduce oxidation and protect optics. Such cutting can be effected using galvo-steered optics (see Laser Marking section for schematic) as well as fixed optic heads.
In all but the thinnest or most easily cut materials sublimation cutting may achieve separation by repeated passes – erosion cutting. This method can also be used to cut metals and is particularly useful if grooves, rather than full body cuts are required. By extension, depth control of such surface profiling can be used to patter or texture material surfaces – see applications section on Laser Structuring.
Laser cutting commonly utilises CO2, Nd:YAG and fibre lasers, variously in continuous wave, pulsed or modulated outputs. All will leave evidence of the thermal process to some degree in the form of a heat affected zone.
Two other families of industrial lasers are able to cut or surface profile many materials with far less thermal impact: short wavelength and short pulse duration laser sources. Examples of these lasers could include frequency tripled (355nm) versions of Q-switched Nd:YAG (1064nm) lasers and pico or femtosecond (ps or fs) laser sources. These sources are far more expensive than ‘traditional’ cutting lasers and are likely to achieve much slower cutting, but there may be niches where these trade-offs with edge quality become worth it.
Lasers operated in the UV region of the spectrum are sometimes able to achieve ‘cold’ cutting with less thermal damage when cutting organic materials by virtue of the fact that some discrete polymer bond energies are low enough to be broken with a single, high energy UV photon. This event results in most of the heat applied by the photon being taken away from the kerf in the form of the heat of vapourisation of the material.
By contrast, lasers operating in the infrared region of the spectrum (10.6µm for CO2, 1.064µm for Nd:YAG) tend to rely on an accumulation of photons to be able to break such bonds, necessarily resulting in a period of time for heat conduction away from the cut area, resulting in a heat-affected zone.
Picosecond and to a greater extent, femtosecond lasers have such short pulse durations that they deliver sufficient numbers of photons, even at infrared energies, to vapourise most materials, within a timeframe which is shorter than typical structural vibration cycles within metal crystals or amorphous organics. This means that one can imagine the receiving structure (the material to be cut) as simply having insufficient time to transfer the heat (in the form of structural vibrations) to adjacent atoms or molecules before it is ejected from the structure by vapourisation.