Definition: cutting of material using intense laser beams
More general terms: laser material processing, laser machining
Category: laser material processing
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Author: Dr. Rüdiger Paschotta
Various methods of laser cutting (or laser beam cutting) – separating solid parts by forming a small gap (kerf, see Fig. 2) between them with an intense laser beam – are an important discipline of industrial laser material processing, or more specifically laser beam machining. Laser cutting is applied to a wide range of materials, including many metals (e.g. different types of steel, titanium alloys, brass, copper, aluminum), ceramics, glasses, semiconductors and other crystals. For some of those, hardly any alternative cutting methods would be available, while others compete with traditional techniques like punching or plasma cutting. Even in those areas, laser cutting becomes tentatively more important, for example because some hard steels which allow for lower-weight solutions in automobile fabrication cause problems for punching machinery.
Applications and Limitations of Laser Cutting
In industrial manufacturing, there are numerous situations where some kind of solid materials need to be cut:
- Many metallic parts, for example for machine tools, ships, automobiles and household items (e.g. kitchen sinks) are originally made from larger metal plates from which they need to be cut out.
- Frequently, one needs to remove some material, e.g. to reduce the weight, make feedthroughs for cables, insert other components, etc. For such purposes, one or several areas of rectangular or circular shape, for example, need to be cut out of workpieces.
- Similar operations are required for many parts consisting of other materials, such as semiconductor chips, ceramics, glasses for displays, solid plastics and polymer foils, wood for furniture, etc.
- A special area is laser micromachining, where extremely fine structures (e.g. for stents to be implanted in heart arteries, or for micro-electromechanical systems) are produced.
Therefore, certain technological developments depend on special laser cutting processes, and even every household now contains a huge number of items which have been cut with lasers.
In most cases, laser cutting is applied in the context of large-scale industrial manufacturing; it is less common in small manufacturing sites with a larger part of manual operations, where the cost of laser cutting equipment is often prohibitive. Typical advantages in industry result from the non-contact processing, particularly avoiding the degradation of mechanical tools, and from the convenient integration into complex computer-controlled machinery, which results in high speed and great flexibility. A disadvantage is the often high electricity consumption, comparing with mechanical tools e.g. for punching; however, the consumption is still significantly lower than for plasma cutting. The quality of processing results depends very much on various details such as processed materials, employed cutting method etc., and can range from just satisfactory to excellent.
In many cases, laser cutting is followed by further laser-based processes, for example by methods for improving the edge quality or by laser welding.
Piercing and Contour Cutting
Laser cutting is in some respects similar to drilling, but it is aimed at separating parts over some length. It often begins with drilling (here: called piercing) to get some initial hole, from where the contour cutting process can continue by the smooth movement of the laser processing head and/or the workpiece. In the simplest cases, cutting occurs along straight lines, but it is also possible to cut sophisticated contours, even with steep edges. Rectangular and circular contours are most common, but rather sophisticated shapes are required for some applications.
Although the piercing usually concerns only a tiny fraction of the total processed volume, it can substantially contribute to the processing time and introduce quality issues. Special methods have been developed e.g. for doing the piercing slightly outside the required contour and appropriately leading the beam into the cutting contour, sometimes with a ramping of speed and laser power up to their final values.
Melting-based Cutting Processes (Melt-and-blow Cutting, Fusion Cutting)
For cutting of thick steel plates, as required for heavy machinery, e.g. in ship building, one mostly uses cutting processes which are based only on melting the metal. This is because melting requires substantially less heat than vaporization, so that reasonable processing speeds can be realized without excessive laser power. The material is expelled with a high-pressure gas jet, directed from the laser processing head (cutting head) to the workpiece over a small working distance of the order of 1 mm to 2 mm. The gas jet also protects the laser processing head against debris.
Multiple kilowatts of laser power are usually required (sometimes more than 10 kW), particularly for thicker plates, but not necessarily with very high beam quality, since the required kerf width (typically around 2 mm) is anyway relatively large – one must prevent that the parts are recombined by re-solidification of molten material. Therefore, one can work with a relatively large beam diameter, and the effective Rayleigh length is then easily much longer than the sheet thickness.
Both continuous-wave and pulsed lasers can be used; high-power CO2 lasers have been used for many years, particularly for cutting relatively thick sheets, but are more and more replaced with diode-pumped high-power solid-state lasers including fiber lasers, particulary for thinner sheets. For beam delivery, one often uses high-power fiber cables; here, it is quite acceptable that a very high beam quality at the output is not possible.
Several meters of cutting per minute can be achieved with a few kilowatts of laser power if the metal sheet thickness does not exceed a few millimeters; for larger thickness, the process becomes substantially slower. The quality of the obtained cutting results is often not very high; one typically observes substantial ripples and other imperfections, also substantial heat-affected zones with oxidization.
Reactive Cutting (Oxygen-assisted Cutting, Flame Cutting)
Cutting speeds can be several times higher when a jet of purified oxygen is injected together with the laser beam. This causes oxidation (burning) of the expelled metal (e.g. steel) and thus supplies substantial additional heat to the process – that can be substantially more than the heat introduced with the laser. The cutting efficiency of such reactive cutting (also flame cutting or burning stabilized laser gas cutting) with respect to the required laser power is correspondingly improved. On the other hand, the quality of the results is tentatively lower both in terms of surface roughness (ripples) and because of the tendency to form oxide layers on the cut surfaces. Such oxidized layers can cause problems in subsequent processing steps such as laser welding and coating. If additional processes are needed to fix such problems, the advantage of higher processing speed may fade away.
Vaporization Cutting (Sublimation Cutting)
For fine cutting (precision cutting) of thinner metal sheets with substantially higher processing quality, adapted cutting methods are employed. Here, one mostly uses pulsed lasers, ideally with nanosecond or even shorter pulse durations; solid-state lasers are dominating in this area. A higher beam quality (sometimes even diffraction-limited) is also required. The applied optical intensity is made so high (e.g. 108 W/cm2, by stronger focusing and by pulsing) that the expulsion of material occurs dominantly in the form of vapor. Oxidization is prevented by using an inert process gas like nitrogen (N2) or argon. Unfortunately, the processing efficiency is correspondingly lower, i.e., for the same sheet thickness one requires substantially more laser energy per unit length.
The term sublimation cutting is also used, although sublimation in a strict sense usually does not occur. In the context of laser cutting, it is essentially meant that the vaporization occurs so fast that no appreciable amount of melt is present at the workpiece at any time. That largely eliminates the problem of ripples generated by the melt dynamics.
Water-beam Guided Laser Cutting
A refined technology has the laser beam guided in a small-diameter water jet (water-beam guided laser drilling, water microjet laser drilling). Here, a water jet is formed by a nozzle with a diameter of the order of 50 μm and falls downwards towards the work piece over a distance of a few millimeters or a few centimeters. The laser beam (e.g. from a nanosecond YAG laser) is injected into the water jet, which acts as an optical waveguide, guiding the light by total internal reflection at its outer surface up to the workpiece. Due to the high refractive index contrast (water to air), the beam guidance works well even for a quite low beam quality of the laser. Accurate control of the working distance is not required, since the diameter of the water jet does not vary a lot. The water also serves to remove debris.
At the workpiece, the light can escape and interact with the workpiece, while the water effectively cools it, thus avoiding any significant heat affected zone. Note that the laser pulses can ablate material despite the presence of cold water, since the pulsed heating of the hit surface is so rapid that during that short time there is minimal energy loss either through the water or by heat conduction into the processed material. The cooling by the water occurs mainly in the much longer time intervals between the pulses.
There are also other techniques of laser cutting in liquids, not involving a water beam, but also with the purpose of removing debris.
For micro-machining, one uses more strongly focused beams and even shorter pulse durations in the picosecond or even femtosecond domain. The required ultrafast lasers usually have a high beam quality, making it easy to tightly focus their radiation. Unfortunately, they are substantially more expensive and more limited than longer-pulse lasers in terms of average power. The processing speed is then lower due to the lower average power, but it is practically less important for the relatively small dimensions of the required cuts.
An often presented application example is the cutting of stents for implantation into heart arteries and other blood vessels, e.g. in the brain for treating strokes.
See also the article on laser micromachining.
In most cases, laser cutting is done with a rather small working distance of the order of 1 mm. However, there is also the possibility for remote cutting, i.e., with much larger working distances. That can result in substantial practical advantages, such as high productivity at least for cutting of relatively thin materials, using a suitable laser scanner.
While the application of a laser beam over a larger distance is no problem as long as its beam quality is high enough, it is not possible to supply a high-pressure jet of process gas over large distances. Therefore, remote cutting usually has to work without such gas, which implies serious limitations. On the other hand, the avoided gas consumption is of course an advantage, and the construction of the cutting head is then also substantially simpler.
2D and 3D Cutting
Many laser cutting processes essentially work only in two dimensions, e.g. on flat metal sheets. It is then sufficient to move either the workpiece or the laser head in two dimensions, keeping the directional orientation of the laser head fixed (mostly for normal beam incidence on the workpiece).
More flexible laser cutting machines allow for 3D cutting, i.e., also including changes of the laser beam direction or the orientation of the workpiece. That is required, for example, for cutting operations on workpieces which are already bent or have more complicated geometrical shapes. Such machines need to have correspondingly more sophisticated motion equipment (with actuators, position sensors, electronics and software).
In any case, some machines are optimized for high-speed cutting. They need both fast motion equipment and a high laser power, also potentially equipment for fast loading and unloading of parts. Economically, it can be advantageous to minimize processing times for reaching maximum daily throughput in order to best utilize an expensive laser and optical system.
Influences of Polarization
The polarization of the laser light can have a substantial impact on the absorption and thus on the processing results. A linear polarization direction perpendicular to the cutting direction may be ideal, but that requires the adjustment of polarization direction when the cutting does not occur long straight lines only. For simplicity, one sometimes uses circular polarization to avoid polarization-related difference between different cutting directions.
Cutting of Glasses
The cutting of glasses is required in various industrial application areas, such as the fabrication of flat-panel displays, automotive and others windows including optical windows and architectural windows, and electronics. Because glasses are quite brittle and also are transparent to visible and near-infrared light, special glass cutting methods have been developed. In many cases, the used mechanism is substantially different from that of cutting metals. For example, one can exploit thermally induced mechanical stress, which can lead to controlled cracking (cleaving) of a glass (laser scribe and break, thermal stress cutting).
In other cases, one locally melts the glass and blows the melt away with a gas jet, similar to some metal cutting techniques. Such methods tend to induce substantial stress, which may lead to cracks, but that tendency depends strongly on the expansion coefficient of the glass, and different types of glasses vary substantially in that respect. Also, one can mitigate such problems by cutting the glass in a rather hot state, where it already gets slightly soft.
Similar methods are applied to other hard and brittle materials, e.g. to silicon wafers.
Such processes can be quite fast, also considering that additional processes for improving the surface quality are often not required.
The radiation of near-infrared lasers would normally not be absorbed by glasses. Two different approaches can be used to obtain the required absorption:
- One can use a CO2 laser a meeting around 10.6 μm, where most glasses are strongly absorbing. Such lasers provide substantial output powers at moderate cost. However, the pulse durations are longer than ideal.
- Ultrafast lasers can be used (usually with femtosecond pulse durations), when the local intensity gets high enough to obtain laser-induced breakdown. This allows most accurate glass cutting, even with materials having a large thermal expansion coefficient and without heating all the glass, but unfortunately the cost of the laser system is then substantially higher.
Key Parameters of Laser Cutting
The following parameters are often of high importance in the context of laser cutting:
- First of all, the given parameters of the workpieces, such as the material, its thickness and the required cutting contours, define the basic conditions. Note that different variants of a material – for example, different types of steel – can exhibit substantial differences in their behavior in laser cutting. The cutting process often needs to be tuned to the specific material variant.
- Concerning the cutting results, the obtained kerf width is often important. It needs to be large enough to obtain a safe separation, but should not be larger than necessary in order to minimize the loss of material and the amount of debris to be removed.
- Various parameters can be used to assess the quality of cutting results, e.g. the shape of the kerf, the width of the heat affected zone and some parameters concerning the smoothness of the obtained cut surfaces and the amount of material deposited on the backside. It can depend on the application what kind of parameters are most appropriate to quantify the quality of results.
- The cutting speed in meters per minute is often of high practical importance and depends substantially on the used laser power, the material type and thickness, apart from various process details such as beam focusing and the flow and pressure of the applied process gas.
- The electricity consumption and the gas consumption (specified per meter of cut material) determine a substantial part of the operation cost, apart from maintenance requirements. The electricity consumption can strongly depend on the wall-plug efficiency of the used laser; modern devices like diode-pumped thin-disk lasers are far more efficient than various older laser types. The improved efficiency also reduces the cooling requirements.
Various qualitative features can also be of high importance. For example, some processes allow for high quality piercing, avoiding the need to pierce outside the final contour. Another aspect of potentially higher relevance is the processing of sharp edges in contours concerning achieved quality and required time.
Lasers Used for Cutting
CO2 lasers have partly been replaced with solid-state lasers, but are still quite common particularly for cutting thick metal sheets, because they are quite powerful, robust and reasonably power-efficient. They also often used for cutting other materials with lower thermal conductivity and good absorption for the long-wavelength radiation.
Particularly for precision machining, diode-pumped solid-state lasers are nowadays very often used. They are often pulsed with nanosecond durations, or sometimes with shorter pulse durations in the picosecond or even femtosecond domain.
Very different laser types are used for special cases such as the cutting of glasses and polymer materials. Even excimer lasers are sometimes used, where high absorption of intense ultraviolet pulses is essential.
Laser Cutting Machines
A typical industrial laser cutting machine contains the following:
- The core part is the laser source, e.g. a Q-switched solid-state laser system with an average output power of several hundred watts or even several kilowatts. It usually requires some auxiliary units, particularly for cooling the laser.
- A beam delivery system transports the laser light to the application area. For bridging large distances, one often uses high-power fiber cables.
- The laser processing head needs to be moved relative to the workpiece. That may be done by moving the workpieces only, by moving the laser head only (flying optics), or with hybrid solutions, assigning certain degrees of freedom to one or the other side.
- The processing head is often equipped with a supply of processing gas. In some cases, the gas nozzles occasionally need to be exchanged in order to use optimized gas jets for different processing conditions.
- There may be additional facilities for automatically loading and removing workpieces, removing debris and fumes, etc.
- Due to the substantial laser powers, laser safety is an important issue. It may be provided with very different means, e.g. by doing the cutting only within a closed housing with an interlock, or by not having any personnel in the whole area during automated production.
Advanced laser cutting machines can often control movements with five or even six axes, allowing not only to cut pieces in a plane, but also 3D parts, e.g. accessed by a laser head on a robot arm from different directions.
While some cutting machines are entirely optimized for one specific application, there are also multi-purpose machines with substantial flexibility to adapt to different scenarios.
Flexible use of cutting machines with sophisticated computer control is essential for maximum productivity, i.e., also for optimally utilizing an expensive laser system. Therefore, particularly larger machines do not only contain fast high-precision motion control, but also have interfaces e.g. to CAD/CAM software and are well integrated into the fabrication environment. They also contain a large number of sensors for monitoring the process as well as various internal parameters for quickly identifying possible faults. The health condition of a machine may be automatically communicated to the manufacturer so that necessary maintenance operations can be efficiently planned and arranged.
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See also: laser material processing, laser machining, lasers for material processing
and other articles in the category laser material processing
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