Laser cutting of metals offers many advantages over more traditional cutting techniques. The cutting speed is higher, the cutting width is smaller and a laser cutting machine is generally more accurate. Also, the heat affected area is much smaller which limits internal stresses or changes in the material.
In this article we are going to talk about the different aspects involved in machining metals with a laser cutting machine. (Cutting, welding, surface machining, etc…). We want to give you some insight into the practical aspects of cutting with a high power laser.
Table of Contents
- Laser sources for laser cutting
- Structure of a laser source
- Properties of laser radiation and laser beams
- Machine aspects
- Cutting Principles
- Parameter Influences
- Other components of a laser cutting machine
Laser cutting involves focusing a laser beam on the material to be cut. This causes the material to melt and/or vaporize locally. The process is often aided by a cutting gas that is blown into the cut. This drives the molten material out of the cut. In some applications, a reactive gas is used so that it also promotes the evaporation/melting of the material.
The laser beam should preferably fall perpendicular to the product surface. The laser and gas jet move opposite the product resulting in an almost straight (perpendicular) cut. The cutting edge shows a certain roughness with a striations pattern and will increase towards the bottom of the cut and progress at higher speeds and thicker materials. If the cutting process is carried out properly with the right settings corresponding to the material, the roughness is limited to a few tens of micrometers (Rz).
Laser cutting has compared to more conventional cutting techniques a high cutting speed (about 15m/min in some cases). In addition, the cut quality is generally better and there is no need for post-processing. This is because burr formation on the underside of the cut is very small or non-existent.
In addition, laser cutting is also more precise compared to other thermal cutting techniques such as electron beam, oxyfuel and plasma cutting. It has a smaller cutting width, sharp angles are possible in the shapes being cut, and the impact on the material of the heat produced is very small and localized.
Against mechanical separation techniques, such as cutting, punching and nibbling, it is much more flexible and can cut shapes that were otherwise impossible (both in 2D and 3D products). Only waterjet cutting comes close to the flexibility of a laser machine.
The biggest disadvantage, however, is the limited thicknesses that a laser can cut (steel up to approx. 20mm). Larger thicknesses are possible, but do not offer the same quality and are not economically advantageous. In addition, a laser cutting machine is a relatively high investment, although in most cases it can be earned back quickly due to the higher cutting speeds.
Laser sources for laser cutting
Structure of a laser source
LASER stands for Light Amplification by Stimulated Emission of Radiation. Specifically, it refers to amplification of light by stimulated emission of radiation. Any laser source in which laser radiation is generated consists of:
- A vibrating cavity (or resonator). This is a space made up of reflective materials (mirrors). In between is an active medium (CO2 gas with a CO2 laser and a crystal with a Nd:YAG and fiber laser)
- An energy source, which is responsible for the production of added energy. In the case of the CO2 laser, this is done by an electrical discharge into a CO2 gas. With the Nd:YAG laser, this is done by means of flash lamps or diodes. The fiber laser works exclusively with diodes.
This added energy is then changed into laser light in the vibrating cavity. The laser beam we use to process materials is the light that passes through a small part of the uncoupling mirror. This light has only one wavelength (or color). This wavelength determines the extent to which it will be absorbed by the material to be cut. Each material has its own optical properties. The choice of your type of laser source, or in other words, the choice of the delivered wavelength, is therefore closely related to its absorption in the chosen workpiece.
The supply of laser energy to the material to be processed can be pulsed or continuous. CW (continuous wave) lasers deliver continuous energy. Pulsed supply is referred to as looped lasers.
CO2 lasers have high available power and are used for cutting thicker metal products. The beam quality of a diode Nd:YAG laser is better than a flash Nd:YAG laser. This makes a diode Nd:YAG laser more suitable for laser cutting. In addition, diodes have a much longer life than lamps, but they are more expensive. (Diode life: 10 000 hours, lamp life: 1000 hours).
Properties of laser radiation and laser beams
A laser source is basically a high energy light source. Nothing special you might think, but a laser has a lot of properties that are different from a normal light source. A laser cutting machine makes use of these properties during the cutting process. Especially the low beam divergence and high power density are important for the cutting process.
The power density (I) can be calculated using the following formula:
I = P/O = 4P/π d² [W/m2 ]
It is the laser power (P) per surface area (O) of the beam cross section. The area is calculated by the diameter (d) of the laser beam.
Power densities up to 1020 W/m2 can be achieved with a laser cutting machine. This gives lasers a high power density, allowing very high cutting speeds to be achieved. The cutting width (+/- 0.2 mm) is another advantage that makes the accuracy very high. It also ensures that only a small area is affected by the heat produced.
Low beam divergence
No beam of light is perfectly parallel and it is no different for a laser beam. The diameter of the laser beam increases with distance.
Close to the source we speak of a small constriction called waist (do [m]). After that, the beam diameter increases steadily with a certain divergence angle (θ(z) [rad]). This divergence angle depends on the wavelength of the laser light and on the waist diameter. At a large distance from the waist, the divergence angle reaches an asymptotic value θ0 [rad].
The divergence of a laser source is extremely small compared to normal light sources. Laser energy can therefore be transported over large distances without a significant decrease in power density.This also ensures that the beam can be very well focused leading to a very small cut width.
A laser beam has a certain surface area. You would think that over that surface area the energy delivered would be the same everywhere. However, this is not the case.
The energy distribution or intensity distribution over the cross section of the laser beam depends mainly on the technique used, the construction of the resonator and which optical elements are chosen to transport the laser beam.
In CO2 lasers, we often see a gaussian intensity distribution. Thus, the peak intensity is in the center of the laser beam. However, if the energy is transported through a fiber (as with a Nd:YAG laser) we see a different pattern, called the top hat intensity distribution.
To determine the intensity distribution of a laser, you need to let the laser burn in on a perspex for a short are at low power (e.g. 0.1 seconds at 100W). This will not cut through the material but you will cut a small pit in it. Where the energy was higher you obviously hit deeper. The intensity distribution has a big influence on the cut quality.
Each laser beam has a quality number M².
M²= π/4λ d0 θ0
d0 = smallest insoering of the beam
θ0 = the asymptotic value of the angle of divergence
M² is usually greater than 1 for the various possible intensity distributions. For a Gaussian intensity distribution, M²=1. A beam quality of about 1 is very favorable for laser cutting. This means that the laser beam can be focused on a very small spot and therefore high cutting speeds and a small cutting width are possible. As a result, the supplied heat is also lower and therefore also its effects on the material. Also the distance between lens/mirror and the product can be large. This makes it safe for the optics of the laser cutting machine (splashing) and increases the accessibility of the product.
Absorption of laser radiation
Laser cutting is done by evaporating/melting material. So the more laser energy you can get into the material, the more heat is generated, the easier it is to cut. With metals, the conversion of energy to heat happens in a very thin surface layer (about 300 mm thick).
The percentage of the energy falling on the surface that is absorbed is called the absorption coefficient A [%]. The energy that is not absorbed is reflected. The absorption coefficient of a material depends on its optical properties.
The more energy that is absorbed by a metal the higher obviously the absorption coefficient and therefore the easier the material can be processed. For metals this coefficient increases significantly as the wavelength of the laser light increases. The absorption coefficient of iron, for example, is less than 20% with a CO2 laser, but with a Nd:YAG laser it is almost 40%.
In addition to absorption through the surface, energy is also absorbed through the walls of the cut. This can increase the absorption even more significantly.
The components and technology in your machine also largely determine the cutting result, of course.
Bundle transport by means of mirrors and glass fibers
Once the laser source has produced a laser beam, it must of course be transported to the processing site via the focusing optics. With a CO2 laser, this is done via mirrors. A sliding or partially transmissive mirror is used to guide the laser beam to the processing site or to divide it into several laser bundles so that it can be used in different places.
These zmirrors are usually made of copper and equipped with a water-based cooling system. They do cause a loss of power. This can be as much as 4% of the total power of the laser beam. Distance is also a factor. The greater the distance (e.g. when cutting large plates), the larger the beam diameter becomes. However, this can be solved by a bundle extender (or telescope)
In addition to the use of mirrors, Nd:YAG lasers also use fiber optics. Internal reflections keep the laser light trapped in the fiberglass. This transport does affect the diameter of the laser beam. The smaller the core diameter of the fiber, the smaller the focus. A smaller focus increases the cutting speed. However, with a smaller core diameter, a lot of laser light is lost during the coupling in process.
A high power density is required to cut the metal. Therefore, lenses and mirrors are used to focus the laser beam. Compare it to a magnifying glass and sunlight. The more you focus the energy on a small point, the greater the effects.
In a CO2 laser, the lenses are usually made of zinc selenide. There is a power loss of about 1% per lens. This loss increases with the lifetime of the lens (approximately 3000 hours). ZnSe cannot withstand higher laser powers, but with cooling they can be used up to a maximum of 5kW. For higher powers, mirrors are therefore used.
Nd:YAG lasers make greater use of quarter lenses with a power loss of 1% per lens.
A laser spot typically has a diameter of about 0.1 to 0.2mm. The smaller that focus diameter becomes the more power density our laser beam has. This increases the cutting speed and reduces the cutting width.
A small focus diameter can be obtained with a high quality laser source or with a strong lens with a small focal length. But a smaller focal length also limits the working distance. The optics must be protected from splashing during cutting. For a safe working distance you should still count on 5 to 20cm.
Depth of field
The depth of field is defined as twice that distance from the focus over which the beam diameter is larger than the diameter df of the focus by a factor of √2. So at the distance s/2 from the focus, the energy density is only half that at the focus.
The greater the depth of field the straighter (more angular) a cut will be. In addition, around the focus, there will not be a large increase in beam diameter so the intensity will not decrease much. As a result, the positioning of the workpiece opposite the focus is a lot less critical.
By going with the optics for a long focal length, you can achieve a large depth of field, but a longer focal length increases the diameter of the focus.
Optics and/or workpiece manipulation
Since laser cutting can be done at relatively high speeds and has a high accuracy, this means that the manipulators must be very accurate. To move a laser spot accurately over the material, it is best to use product manipulators with stationary options.
Depending on the products to be machined, it is therefore best to choose a 1D system (pipes, for a 2D or 2½D system (sheet metal) or a 3D system (three-dimensional products). The more axes the system occupies, the less accurate and more expensive it becomes.
To cut 3D products one uses relatively accurate CNC and gantry manipulators. In addition, robots are also available. With a glass fiber, a robot forms a very flexible production tool.
Contour cutting & offline programming
A constant speed over the product is important for delivering a constant cutting quality. Accelerations or decelerations of the laser spot cause deviations of the cutting contour. The result is also an excessive heat input. This problem occurs especially with sharp contours.
To increase the accuracy of the manipulator, it is important to reduce the cutting speed at the location of sharp contours. At the same time, the laser power is reduced or switched to pulsed cutting to maintain the same cutting quality.
A rapid traverse can also be programmed to avoid this type of positioning problem.
Excessive heat input can occur for products in which many small contours must be cut close together. This can be minimized by programming the sequence of the individual cutting contours so that there is sufficient time for cooling between cuts.
There are many programming software packages that automatically take this into account. A CAD file of the product is required, however. They are also capable of providing run-in contours and starting holes. The rapid traverses and thus the production time can be optimized.
Three principles are distinguished:
- sublimation cutting
- melting with a non-reactive gas
- melting with a reactive gas
At higher speeds, a gas is usually used.
In sublimation cutting, the cut material is vaporized directly from a solid phase to a vapor phase. A gas is then used to blow it out of the cut. Usually nitrogen is used to avoid oxidation.
The major advantages of sublimation cutting are a low roughness of the cutting edge (hardly any striations) and a small sin affected by the heat. The disadvantages are a lower cutting speed compared to melt cutting and a high required energy density.
Sublimation cutting is thus mainly for applications where the quality requirements of the cutting edge are very high or for cutting non-metals, such as wood, paper, ceramics, and plastics. CO2 lasers are mainly used for this purpose.
Melting and expelling with a non-reactive gas
In laser fusion cutting, we are going to melt the material and expel it with a non-reactive gas (usually nitrogen). By using a non-reactive gas we can guarantee an oxide free cut and if the gas speed is high enough, also a burr free cut.
Melt cutting is faster than sublimation cutting. The speed depends on the material, thickness and power. The roughness of the cutting edge is greater and also the area affected by the heat transfer is larger.
Melting and expelling with a reactive gas
When we start using a reactive gas, we speak of laser fire cutting. The laser heats the material to above its combustion temperature. Then an exothermic reaction occurs with the reactive gas (usually oxygen). This causes a combustion.
This generates a much higher energy than that put in by the laser beam itself. The cutting speed is therefore two to three times higher than when cutting with a non-reactive gas.
The vapors produced are carried along by the gas flow, but some of the oxidation products remain on the cutting edge.
There are several parameters that affect the laser relustate. Below we discuss the most important ones.
Laser power and cutting speed
The cutting speed and the power of the laser are the main parameters that determine the laser cut. There is a difference between cw cutting (a constant laser power) and pulsed cutting.
With cw cutting, there is a lot of heat development in the cutting edges at high power and this creates a wide zone that is affected by this heat increase. This is less the case with pulsed cutting. However, this zone is still much smaller than with other thermal cutting methods.
With pulsed laser cutting, one can also work with regular pulses or super-pulses. When super-pulses are used, power peaks occur that are higher than the maximum cw power that the laser machine can produce.
The advantage of pulsed laser cutting is that the material is melted, vaporized and extruded just as in cw laser cutting. However, during the out time there is room for cooling. This provides a lower thermal load. However, the cutting speeds are lower.
Super-pulses have their advantages when drilling starting holes and when cutting highly reflective materials. The high intensity created by pulses helps advance these processes.
Proper positioning of the laser beam is crucial to making a good cut. It has to do with the diameter and the focus opposite the surface to be cut.
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Poor focus position results in round cutting edges, a large cutting width, burr formation, and a low cutting speed. The cutting edges are also often not parallel. Especially when using lenses with short focal lengths, focus position is crucial because they have a smaller depth of field.
The optimum focus position depends on the material type and the thickness of the workpiece. It ensures the smallest possible cutting width. In addition, for cutting with nitrogen, the optimal position is at the bottom of the material.
The machine supplier usually provides tables that show the optimal focus position for different material thicknesses.
Laser cutting is usually done using a cutting gas that is blown into the cut. It drives the molten or vaporized metal out of the cut. Generates etra heat and protects the optics from splashing. It can additionally be used to cool the material next to the cut.
Which cutting gas is best has to do with the type of material you wish to use. Oxygen works best with steel, but nitrogen is generally used for stainless steel. With nitrogen the cutting speed is significantly lower, but can be increased by means of a higher gas pressure.
Unlike with steel, where oxygen is common, with stainless steel and aluminum, reaction products form and adhere to the underside of the cuts. This creates burrs that are sometimes difficult to remove.
Titanium alloy cutting, on the other hand, requires a different approach. Both oxygen and nitrogen cause a very strong reaction. Therefore, for titanium, one uses argon or helium as cutting gas. However, these are significantly more expensive.
Gas pressure and consumption
The best gas pressure is very much dependent on the gas type, the diameter of the nozzle orifice, and the thickness and type of the material.
The greater the thickness of the workpiece, the lower the cutting speed. Here the gas pressure can be reduced. On the other hand, with thin products it is necessary to increase the gas pressure. After all, the cutting speed is much higher.
When cutting with a non-reactive gas (argon, nitrogen, helium) a high pressure is needed to expel the molten metal.
The gas consumption depends on the gas pressure and the diameter of the nozzle orifice and can be considerable.
You might not think it right away, but the geometry of the nozzle opening, the distance from the product, and the gas pressure have a major impact on the cutting quality and speed. This is mainly due to the small diameter that is cut. This is a lot smaller than the nozzle opening. In any case, gas cannot be controlled as well as laser light. The gas will therefore only partially end up in the cut.
The nozzle opening diameter is often twice the cutting width. The smaller the diameter, the lower the gas consumption, but the more difficult and important the alignment is. A small nozzle is also more sensitive to changes in gas pressure.
A larger nozzle diameter is a lot fussier, but very easy to align and less sensitive to variations in gas pressure.
Alignment of gas nozzle
When the nozzle is about 0.5mm behind the laser beam, you can work with a lower gas pressure. And this naturally results in lower gas consumption. A lower gas pressure ensures better cutting quality and less chance of developing burrs.
Nozzle plate distance
The distance of the nozzle from the plate also have an impact. It is best to keep the distance smaller than the diameter of the nozzle itself. If the distances are greater, turbulent vortices are created within the cutting gas. This is an undesirable process.
Turbulence, certainly in combination with a small cutting width, causes pressure fluctuations. And you notice that on your cutting surface. The quality will not be the same everywhere.
A height sensor and control system is a must to keep the position of your nozzle constant.
To start a cut, one drills a star hole with the laser. The laser beam does not move relative to the material until a hole is made. This starting hole can be made either on the cutting contour or next to it.
You can drill a starting hole with cw laser power. This requires a relatively high continuous laser power in combination with a gas pressure of about 4 bar. Often the distance of the gas nozzle also increases during drilling.
One can also drill in pulsed mode with a non-reactive gas at low pressure (1 bar). This results in a starting hole whose diameter is not larger than the desired cutting width. This also allows you to start on the cutting contour.
The cw laser method is two to three times faster, but has a heavy thermal load on the product. Also, the diameter is much larger than the cutting width. So you can’t start on the cutting contour. It also involves a lot of spatter which contaminates your nozzle opening.
The properties of the material are important in any operation. Aluminum, steel, plastic, paper and wood can all be lasered. But there is a big difference between the way. Below are the material properties that are important:
- Condition of the product surface
- absoption coefficient
- thermal conductivity and thermal diffusion coefficient (the lower the better)
- Melting temperature and melting heat (the lower the better)
- Evaporation temperature and heat of evaporation (the lower the better)
- Viscosity of the liquid material. (the higher the more likely to burr)
The machine supplier usually provides tables with the optimal machine settings for different materials.
Other components of a laser cutting machine
The product to be cut must be accurately clamped. A laser cutting machine has a small shear depth and focus, so any inaccuracy in the clamping process has major consequences. Especially if a height sensor is used it is curcial.
Despite the usually small heat input, the product can also still be thermally deformed. A balanced cutting sequence and/or adequate product effort are therefore important.
Take into account the following aspects.
- Internal stresses in the material can lead to deformation
- Point clamping or high clamping forces can lead to local deformation around the clamp.
- When the workpiece moves, it can deform under the influence of inertia.
- High gas pressure can lead to deformation.
- The optics head and the laser beam need a clear path along the fixtures. A collision can cause damage.
- A point support minimizes the reflection of the laser beam. The gas beam also has a free path. This increases the cutting quality.
- Use heat-resistant effort materials with a splash-resistant coating.
- Good accessibility to the effort clamps is easy for removing slag and spatter.
To maintain a constant nozzle-plate distance, a height control is used. This also keeps the focus position constant, which is especially important when using lenses with a short focal length or when cutting 3D products.
The height control can be controlled mechanically or electronically.
The mechanical way consists of a wheel (or a ball). This is attached to the cutting head and rolls over the plate. This ensures a constant distance and focus position. It is suitable for sheet metal, but not usually for 3D workpieces. Rolling can also cause scratches and grooves.
With an electronic height control, the electrical capacity is measured between nozzle and product. This determines the distance that controls a manipulator to adjust the position. In contrast to mechanical control, it is a lot more flexible to use, but also more expensive. The electronic height control is also only suitable for electrically conductive materials.
We hope this article has given you some more insights into cutting metals with high-power lasers. If you are looking for more information, feel free to look around the rest of our knowledge base or get in touch. We are always ready to help you.
References and sources
 Römer, G.R.B.E.; Hoogvermogen lasers voor het bewerken van metalen, VM121, Vereniging FMECWM, Zoetermeer, 2002, aangepast in 2009.
 R.F. de Graaf; Laser cutting of hybrid laminates. Proefschrift Universiteit Twente, 2002, ISBN 90-365-1703-6.
 VDI 2906: Richtlinie Blatt 8: Schnittflächenqualität beim Schneiden, Beschneiden und Lochen von Werkstücken aus Metall; Laserstrahlschneiden.
 DIN 2310: Teil 1: Thermisches Schneiden; Allgemeine Begriffe und Benennungen.
Teil 5: Thermisches Schneiden; Laserstrahlschneiden von metallischen Werkstoffen; Verfahrensgrundlagen, Güte, Maßtoleranzen.
 NEN-EN 10825: Veiligheid van laserproducten – Apparatuurclassificatie, eisen en gebruikershandleiding.
 NEN-EN 12626: Veiligheid van machines – Machines die gebruikmaken van lasers – Veiligheidseisen.
 NEN-EN 12254: Afschermingen voor werkplekken met lasers – Veiligheidseisen en beproeving.
 NEN-EN 12584: Onvolkomenheden bij brandsnijvlakken, lasersnijvlakken en plasmasnijvlakken.
 NEN-EN-ISO 9013: Thermisch snijden; Classificatie van thermische doorsnijdingen; Geometrische productspecificatie en kwaliteitstoleranties.
 NEN-EN-ISO 15616 (ontw.): Acceptance tests for CO2-laser beam machines for welding and cutting.
 VM 114: Scheidingstechnieken voor metalen. Vereniging FME-CWM, Zoetermeer, 1998.
 Tech-Info-blad TI.99.12; IOP Metalen nr. 2.5: Laser- en waterstraalsnijden van gelamineerde en beklede plaat. Vereniging FME-CWM, Zoetermeer, 2000.
- Trumpf Laser Nederland
- Rofin-Baasel Benelux
- Demar Laser
- Hoek Loos
- Leerstoel Toegepaste Lasertechnologie van de Universiteit Twente te Enschede.