Laser Cutting Process
The laser cutting process uses a focused laser beam and assist gas to sever metallic plate with high accuracy and exceptional process reliability. The laser beam is generated by a resonator, and delivered through the cutting nozzle via a system of mirrors.
Advantages of laser technology
Laser technology has the following advantages:
- High accuracy
- Excellent cut quality
- High processing speed
- Small kerf
- Very small heat-affected zone compared to other thermal cutting processes
- Very low application of heat, therefore minimum shrinkage of the cut material
- It is possible to cut complex geometrical shapes, small holes, and beveled parts
- Cutting and marking with the same tool
- Cuts many types of materials
- No contact between the material and machining tool (focusing head) and therefore no force is applied to the work-piece
- Easy and fast control of the laser power over a wide range (1-100%) enables a power reduction on tight or narrow curves
- The oxide layer is very thin and easily removed with laser torch cutting
- High-pressure laser cutting with nitrogen enables oxide-free cutting
Principles of Laser Cutting
A laser beam delivery system is shown below, consisting of:
- CO2 laser resonator
- rear mirror
- gas excitation generates single wavelength light
- output mirror
- polarizing mirror
- telescope mirror
- beam bender
- machine gantry
- constant beam length carriage
- beam bender
- beam bender
- cutting carriage
- beam bender
- adaptive mirror
- focusing mirror
- cutting head
- cutting nozzle
The focusing device consists of either a zinc-selenide lens or a parabolic mirror which brings the laser beam to a focus at a single point. Depending on the laser beam power, a power density of more than 107 W/cm2 is achieved at the focus point. The focal length gives the distance of the focal point from the focusing optics.
The focal point is positioned above, on or below the material surface according to the requirements of the material. The high power density results in rapid heating, melting and partial or complete vaporization of the material. The gas flowing from the cutting nozzle removes the molten mass from the kerf.
The machine moves the cutting head over the metal sheet according to the programmed contour, cutting the work-piece from the sheet.
Laser Cutting Methods
Depending on the material to be cut the cutting methods used differ :
Fusion Cutting ( high pressure cutting):
- The material is fused by the energy of the laser beam.
- The gas, in this case nitrogen at high pressure (10 to 20 bar), is used to drive out the molten material from the kerf.
- The gas also protects the focusing optics from splashes
This cutting method protects the cut edges from oxidation and is mainly used with stainless steels, aluminum and their alloys.
Oxidation Cutting (laser torch cutting):
- The material is heated by the laser beam to combustion temperature.
- The gas, in this case oxygen at a medium pressure (0.4 to 5 bar) is used to oxidize the material and to drive the slag out of the kerf.
- The gas also protects the focusing optics from splashes.
- The exothermic reaction of the oxygen with the material supplies a large part of the energy for the cutting process.
This cutting method is the quickest and is used for the economical cutting of carbon steels.
Parameters Affecting Laser Cutting
The following points are especially important for achieving good cutting results: Laser power
- Pulse frequency
- Type and pressure of cutting gas
- Diameter and type of nozzle
- Distance between the cutting nozzle and the work-piece
- Focal length of the focusing optics
- Focal position
- Cutting speed
- Work-piece surface
- Work-piece shape
- Material thickness
- Work-piece support
The laser power must be adjusted to suit the type and thickness of the work-piece. A reduction in the laser power may be necessary to achieve high accuracy on complex shaped work-pieces or very small parts. In contrast a laser power of at least 1000 W is needed for cutting carbon steel thicker than 5/16”.
As with the laser power, the pulse frequency can be matched to the relevant machining task. For example, it is recommended small contours are cut with reduced pulse frequency. The pulse frequency is also reduced when piercing in the ramp mode.
Type of gas
The type of material and the requirements of the cutting results determine the cutting gas to be used. A combustible material such as wood must not, for example, be cut with oxygen, as the work-piece would catch fire. Oxygen should only be used for metallic work-pieces with oxide-free edges. Oxygen forms a thin oxide layer during exothermic combustion.
With the laser torch cutting of metallic materials the quality of the applied oxygen is particularly important for the cutting results. Traces of water or nitrogen lead to the formation of burrs. This type of cutting gas contamination may be caused by bottle replacement and the connection of contaminated bottles. Therefore we recommend that the gas is supplied from gas tanks.
Recommended oxygen purity: 99.95 % (3.5)
With the use of oxygen with a purity of 99.5% (2.5) the possible cutting speed is reduced by approximately 10%.
The quality of the cutting gas (N2) is also very important for the high pressure cutting of stainless steel. Even slight traces of oxygen lead to the formation of a fine oxide layer.
The material thickness of the work-piece must be matched to the gas pressure. When torch cutting, thin metallic materials are cut with a higher gas pressure than thicker materials. The gas pressure must be set very carefully, because the cutting quality is affected by even slight changes in the oxygen pressure.
If the pressure is too low, the fluid slag remains adhered to the base material, forming a permanent burr or closing the kerf again.
If the pressure is too high, the lower edges of the cut are burnt out and often make the cut unusable. In contrast, with high pressure cutting thicker work-pieces are cut at higher gas pressure.
Cutting nozzles and nozzle size
The selection of the correct nozzle for the process is very important. For example, with high pressure cutting, nozzles with a larger hole are used than for standard cutting. A deformed nozzle hole, e.g. oval-shaped after a collision, can as with an eccentric laser beam, lead to directionally dependent cutting errors.
In general the nozzles specified in the data sets can be used. If the nozzle is slightly too large, cutting gas consumption is increased, but the cutting quality is not significantly affected.
If the nozzle is too small, the cut edge is not cleanly cut and slag clings to the lower edge of the kerf. In the extreme case the material is not parted.
The nozzle distance is held at the programmed value with a capacitive height control without touching the work-piece. The nozzle distance between the work-piece and the material surface has a great effect on the cutting quality with laser cutting. The smaller the nozzle distance, the better the cutting quality. But there is the following restriction: To ensure safe cutting, a minimum distance should not be maintained. This minimum distance is approx. 0.025”. For hole piercing the nozzle distance is selected to be the same or larger depending on the material thickness and type of hole-piercing.
The focusing optic causes the laser beam to be focused into a single spot through the nozzle. The focusing optic may either be a zinc-selenide lens or a parabolic mirror.
- Laser beam
- Cutting gas
- Focusing lens
- Cutting head / nozzle
- Blow-out molten mass
The focusing lens must be installation correctly. The surface which curves outwards must always point to the top.
A dirty focusing lens heats up due to more intense absorption of the laser radiation and deforms. This leads to the focal position drifting towards the top.
Important: Heavy contamination can lead to the focusing lens being damaged.
Effects: With increasing cutting length burrs start to form and build up; the kerf and surface roughness increase.
- in carbon steel there is a tendency to cratering
- in the extreme case the work-piece is not fully severed
Optical systems with 5" and 7.5" focal lengths are typically used for cutting. 5" optics are only suitable for thin materials. For thicker materials the 7.5" optics are used. With the 5" optics the kerf is narrower compared to the 7.5" optics, giving a higher energy density for the same laser power. The possible cutting speeds for the 5" optics are therefore slightly higher for the same material thickness and laser power. If mainly thin materials are cut, the 5" optics are therefore to be recommended for reasons of economy.
The 7.5" optics has the advantage of a greater depth of focus, i.e. the maximum cutting thickness is greater. The 7.5" optics can be used universally for a large range of thickness, but they are mainly used for thicker materials.
Exact positioning of the focal point is an important requirement for good cutting results.
Basically the following applies for the laser-beam torch cutting of carbon steel:
For sheet thicknesses up to about 6 mm the optimum focal position is on the sheet surface.
With sheet thicknesses of 8 mm and over, the focal point must be positioned above the sheet surface.
High pressure cutting of stainless steel or aluminum:
The focus is positioned in the sheet.
As a rule of thumb, the focus position can be positioned at about 2/3 the sheet thickness in the sheet.
Therefore, each change of plate thickness normally means a change of focus position.
Centering the nozzle
The focusing lens must be set such that the focused laser beam is placed in the centre of the nozzle hole. The focused laser beam may at most be +0.002” off centre with respect to the nozzle.
With otherwise good cutting quality, a non-centered laser beam can lead to the cutting quality being dependent on direction. In the extreme case the cut is very good in one direction and in the other directions the material is not cleanly cut or even not parted.
With the torch cutting of carbon steel sparks can form on the surface of the sheet when cutting takes place in a direction opposed to the eccentricity.
1 = Nozzle Orifice
2 = Laser Beam
a is centered, b and c are not centered.
The cutting speed must be matched to the type and thickness of the work-piece. A speed which is too fast or too slow leads to increased roughness, burr formation and to large drag lines.
The acceleration is linked to the machine constants and generally it does not need any attention since it is a setting specific to the machine. With high pressure cutting the acceleration should be limited from about 1/8” sheet thickness, because the cutting process can easily be interrupted if the acceleration is too high.
The characteristics of metallic materials have a decisive effect on how easily they are cut with a laser beam. Thermal conductivity, absorption, reflectance are just a few to mention. These are influenced by the composition of the materials and their production methods.
Iron and steel sheets:
Ideal laser steel is free from internal stresses and has very low silicon, phosphorous and carbon contents. This makes very high cutting speeds possible with clean and burr-free cut edges. With types of sheet with a high carbon content hardening of the material along the cut edge should be expected. Steel with a high alloying content is more difficult to cut than sheet with a low content.
Due to the high reflectance and thermal conductivity, aluminum alloys can only be cut in thicknesses up to approx. 30% of the cutting thickness achieved in carbon steel. The cutting quality is dependent on the type of alloy. Normally, improvements are found with higher alloying proportions.
Brass and copper can only be cut to a limited extent due to the very high reflectance and thermal conductivity. The maximum sheet thickness for brass is approx. 2 mm and for copper 1 mm. Caution: Personal Protective Equipment is essential for the operator due to reflected radiation.
In practice silver and gold cannot be cut with a laser beam.
With non-metallic materials the cutting behavior using a laser beam must be checked in each individual case. In general it can be said:
Organic natural materials such as leather, wood, cardboard and paper can be cut with good results, but a slight discoloration due to carbonization at the cut edges must be expected.
Organic synthetics such as acryl glass, PVC and polyurethane can be parted with good results, but measures must be taken to capture and treat hazardous fumes which may be generated.
Important: Dust collection and filtration systems designed for metals are not adequate for cutting of organic synthetic materials!
Inorganic materials such as quartz or ceramics can be cut with good results.
With glass there is the risk of cracks forming due to the thermal cutting processes.
Shiny material surfaces, such as for example pure aluminum, produce strong reflection of the laser beam and therefore also poor cutting results. With laser cutting, rolling marks and grooves, stamps and mechanical damage to cavities deflect laser beams and gas flows in undefined directions. This is more noticeable with thick sheets and causes unclean cut edges and it reduces cutting speed and performance.
Spray finishes and paint and plastic coatings affect the cutting result.
Mill scaleon the on the surface of the sheet impairs the cutting result. A loose mill scale of varying thickness does not permit the focused laser beam to impinge directly on the surface. The cutting gas enveloping the laser beam is also deflected.
Residues of sand grains in sand-blasted surfaces reflect the laser beam and the roughened (pyramid-shaped) surface structure deflects the cutting gas flow on the surface. In addition sand contains silicon which also causes problems with laser beam cutting. The rough texture of surfaces treated by abrasive grain techniques (cup-shaped) can also deflect the cutting gas flow on the surface. Small solidified balls of pierce slag also tend to stick to a blasted surface, which can cause cutting defects when the laser beam cuts over them or, if they are large enough, when the nozzle contacts them causing a crash.
In contrast, a thin layer of oil as often present on sheet material does not impair the cutting result. The oil layer has a positive effect during piercing with 100 % laser power, since the slag accumulation on the sheet surface is significantly reduced.
Plastic coated stainless steel can be cut without burrs in thicknesses up to 1/8” using high pressure cutting. The requirements are:
- Film is on top of the sheet.
- Polyethylene film 100 μm thick, self-adhesive
Important: When cutting plastic coated sheet, hazardous fumes may be produced and adequate extraction must be ensured.
Galvanized steel sheet can be cut without burrs in thicknesses up to 1/8” using high pressure cutting. The requirements are:
- galvanized layer applied electrolytically
- 60 g/m2 = approx. 8 μm layer thickness
Certain shapes of the work-piece, such as fine bridges, acute angles, or small holes (edge length / diameter small than 2x sheet thickness) sometimes give problems with laser cutting. These geometrical elements are therefore cut with reduced parameters:
- reduced laser power
- reduced cutting speed
- lower pulse frequency
Otherwise there is the risk that too much heat will be applied to the work-piece and parts of the shape will be burnt away.
With increasing material thickness the roughness of the cut edges increases for metallic materials and the laser power required for cutting is greater. With greater material thicknesses noticeably lower cutting speeds are achieved for the same laser power.
Work support table
With high pressure cutting interruption of the cutting process may occur when cutting over the work-piece support bars. When crossing the bars small grooves may be produced on the lower edge of the sheet. Splashes produced by cutting into the work-piece support may adhere to the bottom of the work-piece.
ESAB’s Alpharex CNC Laser Cutting Machine automates virtually all process parameters, can be equipped with up to 6KW resonators, and handles large multiple large sheets. It is truly the most versatile and flexible laser cutting machine available.