CO2 Laser Cutting 101: The Basic Guide

What is CO2 laser cutting process?

CO2 laser cutting is to focus the CO2 laser beam on the material surface with a focusing lens to melt the material, blow away the melted material with the cutting gas coaxial with the laser beam, and make the laser beam move relative to the material along a certain track, so as to form a certain shape of notch.

CO2 laser cutting machine is most suitable for cutting low carbon steel with thickness less than 12mm, stainless steel with thickness less than 6mm and aluminum plate with thickness less than 4mm.

When processing stainless steel and aluminum alloy, the notch is smooth, the surface is free of oxidation and the sharp corner is good.

3000 ~ 7000W axial fast flow CO2 laser is widely used in metal material laser cutting machines at home and abroad.

The reason is that the operation cost is low, there are many kinds of processing and the cutting speed is fast.

CO2 Laser Cutting 101

(1) Cutting and perforation technology

In shape cutting, it is usually necessary to open a hole (perforation processing) when entering the cutting from the cutting starting point.

Previously, in the laser stamping compound machine, a hole was punched with a punch, and then cut from the small hole with a laser.

There are two basic perforation methods for laser cutting machines without stamping devices.

1) Blasting perforation, also known as continuous vibration mode.

After the material is irradiated by continuous laser, a pit is formed in the center, and then the molten material is quickly removed by the oxygen flow coaxial with the laser beam to form a hole.

Generally, the size of the hole is related to the plate thickness. The average diameter of the blasting hole is half of the plate thickness.

Therefore, the blasting hole diameter of the thicker plate is large and not round, so it is not suitable to be used on the parts with higher requirements.

In addition, because the oxygen pressure used for perforation is the same as that during cutting, the splash is large. When the continuous vibration mode is selected for perforation, the reason why the hole diameter becomes larger is that high-temperature splashes accumulate around the hole on the upper part of the cutting material, resulting in excess combustion with the auxiliary gas (oxygen).

In order to prevent excess combustion, inert gas can be mixed into the auxiliary gas (oxygen). In order to discharge the molten metal smoothly, the irradiation position of the laser can be moved during perforation.

In order to make the continuous vibration perforation mode with short time and high-quality hole shape, air can be mixed into the auxiliary gas (oxygen) and the laser moves in a circle.

2) Pulse perforation, also known as pulse vibration mode.

The pulse laser with peak power is used to melt or vaporize a small amount of materials, and air or nitrogen is used as auxiliary gas to reduce the hole expansion due to exothermic oxidation.

The gas pressure is lower than the oxygen pressure during cutting.

Each pulse laser produces only a small particle jet and gradually deepens, so the perforation time of the thick plate takes a few seconds.

Once the perforation is completed, immediately replace the auxiliary gas with oxygen for cutting.

In this way, the perforation diameter is smaller and its perforation quality is better than blasting perforation.

Therefore, the laser used should not only have high output power;

More importantly, the temporal and spatial characteristics of the beam, so the general cross flow CO2 laser can not meet the requirements of laser cutting.

In addition, pulse perforation also needs a more reliable gas path control system to realize the switching of gas type and gas pressure and the control of perforation time.

In the case of pulse perforation, in order to obtain high-quality incision, the transition technology from pulse perforation when the workpiece is stationary to constant speed continuous cutting of the workpiece should be paid attention to.

Theoretically, the cutting conditions of the acceleration section can usually be changed, such as focal length, nozzle position, gas pressure, etc., but in fact, it is unlikely to change the above conditions due to the short time.

It is more realistic to change the average laser power in plate processing.

There are three specific methods: changing the pulse width; Change the pulse frequency; Change the pulse width and frequency at the same time.

The actual results show that the best effect is to change the pulse width and frequency at the same time.

In the cutting of medium and thick plates, pulse vibration is generally used.

However, with the increase of plate thickness, the perforation time also increases.

(2) Process parameters of CO2 laser cutting

Laser cutting is a very complex thermophysical process, which is affected by many factors.

Some of the influencing factors are predetermined in the actual laser cutting production, such as the performance of the whole laser, beam quality, material and thickness of the processed object, etc;

While for other basic process parameters, such as laser power, cutting speed, auxiliary gas pressure, gas flow, the distance between cutting nozzle and workpiece, focal length and defocus, there is an adjustment range of cutting parameters, and different parameter combinations will significantly affect the cutting quality.

Among these parameters, laser power and cutting speed are a pair of interrelated and constrained parameters, which are not only easy to adjust, but also the main factors affecting the cutting quality.

Under normal cutting conditions, the spot diameter after focusing affects the notch width.

The minimum spot diameter can be obtained by adjusting the defocus amount, and once the defocus amount is set, it does not need to be adjusted frequently.

When the thickness of the workpiece is thin, the focus is on the surface of the workpiece, that is, the defocus amount is 0;

When the thickness is greater than 2mm and the focus is located at the downward 1 / 3 of the upper surface of the workpiece, it can ensure that the notch width is close to the spot diameter.

The laser power determines the energy irradiated on the processed part, which has a direct impact on the cutting.

The power is large, the corresponding cutting speed is large, and the maximum thickness that can be cut is also large.

Under the same power, the laser mode is low, the cutting speed is large, and the cutting quality is good. This is because the laser energy of the lower mode is relatively concentrated.

In laser cutting, the main function of auxiliary gas oxygen is to produce oxidation reaction where light acts, which can not only improve the absorption of laser energy by the workpiece, but also produce reaction heat and improve the cutting speed.

In addition, oxygen can blow away the metal vapor, reduce the absorption of light energy by the vapor, and protect the lens from pollution.

Properly increasing the flow of oxygen can improve the cutting speed and cutting quality.

The design of the nozzle must ensure that the auxiliary gas has both a certain flow rate and a certain flow rate.

Choosing proper polarized light and illumination time (cutting speed) can improve the quality of the incision.

Selecting the appropriate laser, setting the appropriate parameters according to different material characteristics and optimizing the cutting process can obtain satisfactory cutting results.

For different materials, due to their different thermophysical properties and the absorption rate of laser beam energy, they show different laser cutting adaptability, and the requirements for auxiliary gas are also different.

Generally, the material thickness is directly proportional to the required laser power and inversely proportional to the processing speed.

The choice of laser power, auxiliary gas, working pressure and focus point position are also different for different materials and thicknesses.

Only by comprehensively considering the properties of the processed materials and reasonably selecting the relevant cutting parameters can we achieve the optimal cutting processing.

The main parameters affecting the cutting quality are as follows:

1) Flare mode.

Laser cutting requires that the narrower the notch, the better, which requires that the laser beam has good focusing performance and can gather into a very small spot, which is closely related to the laser mode and divergence angle.

High power transverse flow CO2 laser generator usually adopts a stable cavity structure.

When the Fresnel number of the cavity is greater than 1, the output laser beam can be approximately described by Laguerre Gaussian beam.

Table 1 shows the spot radius on the lenses of various laser modes.

Table 1 Spot radius on lenses of various laser modes

It can be seen that the spot focused by the fundamental mode TEM00 is the smallest.

With the increase of the transverse mode order, the spot increases accordingly, that is, the higher the laser mode, the larger the spot radius.

If an aperture is set in the laser cavity to suppress the high-order mode and limit the oscillation of the high-order mode, the low-order mode or even single-mode output can be obtained, and the quality of the laser beam will be significantly improved.

Therefore, the laser spot mode determines the quality of the laser beam, which has a great impact on the laser cutting ability, notch size and notch roughness.

In two-dimensional cutting, the base mode is the best spot mode, and its spot radius and divergence angle are small, which is conducive to improve the cutting accuracy and cutting quality.

When cutting with multi-mode TEM21 beam, the cutting amplitude is more than twice that when cutting with basic mode beam.

And the cutting amplitude increases with the increase of power and the decrease of cutting speed.

2) Lens size and focus position.

Under the same other conditions, the smaller the focus spot diameter is, the greater the power density is.

One of the advantages of laser cutting is the high energy density of the beam and the small spot diameter, so as to produce a narrow notch.

The smaller the heat affected zone, the narrower the incision and the better the cutting quality.

At the same time, the spot diameter is also directly proportional to the focal depth of the lens. The smaller the focal depth, the smaller the spot diameter.

However, there is splash in cutting, and the lens is too close to the workpiece and easy to be damaged.

Therefore, the focal length of 127 ~ 190mm is widely used in high-power CO2 laser cutting industry, and the actual focal spot diameter is between 0.1 ~ 0.4mm.

For high-quality cutting, the effective focal depth is also related to the lens diameter and the material to be cut.

For example, cut carbon steel with a 127mm lens, and the focal depth is ± 2% of the focal length, i.e. about 5mm.

Therefore, it is very important to control the position of the focus relative to the surface of the cut material.

Considering the cutting quality, cutting speed and other factors, in principle, for metal materials with thickness less than 6mm, the focus is on the surface and the defocus amount is 0;

For carbon steel with thickness greater than 6mm, the focus above the surface is positive defocus; For stainless steel with thickness greater than 6mm, the focus is negative defocus below the surface.

Generally, when cutting materials with a thickness of less than 4mm, a lens with a focal length of about 127mm is selected.

The power density of the laser focus is directly proportional to the output power of the laser and inversely proportional to the square of the lens focal length.

Therefore, in the design of the focusing system, the design of focal length is very important.

When cutting metal, the focal length of the lens should be small.

Generally, the focal length of F is 50 ~ 100mm.

At this time, the spot size is small and the power density is large.

For cutting non-metallic materials, these materials are generally thick, so long focal length and long focal depth should be adopted.

Generally, the focal length with F of 200 ~ 300mm is selected to ensure the cutting quality.

Because the laser power density has a great influence on the cutting speed, the spot size of the focused beam is very small, and the power temperature at the focus is very high, which is beneficial to material cutting.

When cutting, the focus position is just on the surface of the workpiece, or slightly below the surface, the cutting effect is the best.

When cutting low-carbon steel, the focused light spot is generally set on the surface of the workpiece, which can improve the temperature at the front of the notch, so as to obtain a higher cutting speed.

When cutting thick plates, the beam with large focus depth should be used, and when cutting thin plates, the beam with small focus depth should be used.

In the cutting process, ensuring the constant relative position between the focus and the workpiece is an important condition to obtain stable cutting quality.

Sometimes, when the lens is heated due to poor cooling, the focal length will change, which requires timely adjustment of the focus position.

When the focus is in the best position, the incision is the smallest, the efficiency is the highest, and the best cutting speed and cutting effect can be obtained.

In most cases, the focus position is adjusted to just under the nozzle, and the distance between the nozzle and the workpiece surface is generally about 1.5mm, so the cutting effect is the best.

There are three simple methods to determine the focus position in plate cutting:

① Printing method:

Make the cutting head move from top to bottom, and carry out laser beam printing on the plastic plate, with the minimum printing diameter as the focus.

② Inclined plate method:

Use a plastic plate placed obliquely with the vertical axis to pull it horizontally to find the smallest point of the laser beam as the focus.

③ Blue spark method:

Remove the nozzle, blow air, hit the pulse laser on the stainless steel plate, and make the cutting head move from top to bottom until the maximum blue spark is the focus.

3) Laser cutting power.

Laser cutting power has a great influence on cutting thickness, cutting speed, cutting width and quality.

Generally speaking, the greater the laser cutting power is, the greater the plate thickness can be cut, and the cutting speed can also be increased.

However, with the increase of laser cutting power, the notch width increases slightly.

Table 6-18 shows the practical maximum cutting thickness of different materials that can be cut by CO2 laser with given power.

Generally, the laser cutting power is determined according to the thickness of the processed plate and the required cutting speed.

4) Cutting speed.

For the continuous output laser, the laser cutting power and mode will have an important impact on the cutting quality.

In practice, the maximum power is often set to obtain high cutting speed or to cut thicker materials.

The greater the laser cutting power is, the thicker the material is cut, and the faster the cutting speed is.

The cutting speed of materials is directly proportional to the laser cutting power density, that is, increasing the power can improve the cutting speed.

Under the same laser cutting power, the cutting speed is inversely proportional to the thickness of the material.

If the cutting speed is too fast, slag will stick to the lower edge of the incision and even the cutting surface, and even the workpiece cannot be cut through;

If the speed is too slow, the efficiency is low, the cutting surface is not smooth, and the lower edge of the incision is sticky.

In actual use, cutting should be carried out at a relatively high speed as far as possible without affecting the cutting quality.

When the laser cutting power and auxiliary gas pressure are constant, the cutting speed maintains a nonlinear inverse relationship with the notch width.

When the cutting speed increases, the notch width decreases;

If the cutting speed decreases, the notch width will increase.

There is a parabola relationship between cutting speed and surface roughness of notch. With the decrease of cutting speed, the value of surface roughness increases rapidly;

With the increase of cutting speed, the surface roughness decreases and the surface quality is improved. When the cutting speed exceeds a certain optimal value, the surface quality is improved slowly with the increase of cutting speed;

When the laser cutting speed increases to a certain value, the impervious material will be cut.

The surface roughness Ra of the notch is related to the workpiece thicknessδ, focus position, cutting speed and other factors.

Generally, it can be estimated by the following formula, Ra = 0.01 δ mm。

5) Cutting gas.

Usually, auxiliary gas is required for material cutting.

Because the laser reflectivity of the metal surface is as high as 95%, the laser energy cannot be effectively injected into the metal surface.

The absorption rate of the material to the laser can be improved by injecting auxiliary gas.

The auxiliary gas is ejected coaxially with the laser beam to protect the lens from pollution and blow away the cutting slag.

Common auxiliary gases include compressed air, nitrogen, oxygen, argon, etc.

For non-metallic materials and some metallic materials, use compressed air or inert gas.

The metal material is melted by laser heating, and then the non oxidizing gas (argon, helium, nitrogen, etc.) is injected through the nozzle coaxial with the beam, and the liquid metal is discharged by relying on the strong pressure of the gas to form a notch.

In a certain pressure range, increasing the pressure of auxiliary gas can increase the cutting thickness and improve the cutting efficiency.

It is mainly used for the cutting of some materials or active metals that are not easy to oxidize. The faster the cutting speed is, the easier the incision is to form ripple shape and the worse the cutting quality is.

For most metal laser cutting, oxygen is used to support combustion.

The cutting method uses the laser as the preheating heat source and oxygen and other active gases as the cutting gas.

On the one hand, the injected gas acts with the cutting metal to produce oxidation reaction and release a large amount of oxidation heat, which can increase the cutting speed by more than 30%.

On the other hand, the molten oxide and melt are blown out of the reaction zone to remove the slag, form a notch in the metal and inhibit the excessive combustion in the cutting area.

The faster the oxygen combustion supporting cutting speed is, the smaller the heat penetration is, and the better the cutting quality is.

When the laser power and cutting speed are constant, the oxygen pressure and oxygen flow are large, the oxidation reaction speed is accelerated, the oxidation calorific value is large, the notch is widened, the notch stripe is deep and thick, and the roughness of the cutting section is deteriorated;

When the oxygen pressure decreases and the oxygen flow decreases, the oxidation rate decreases, the incision narrows and the roughness of the incision section improve.

When the oxygen pressure is reduced to a certain value, the notch material will not be completely oxidized, and more melt adheres to the lower surface of the notch, and the cut material can not even be cut through.

Air can be supplied directly by the air compressor, so its price is very cheap.

Because the air contains oxygen, a small amount of oxide film will appear on the cutting surface.

However, since the air contains water and oil particles, which will pollute the lens, an oil-water separator shall be equipped.

Argon is an inert gas, which is used to prevent oxidation and nitriding in cutting. It is generally used to cut titanium and titanium alloys.

Compared with other gases, the price is higher, which increases the processing cost.

Table 2 practical maximum cutting thickness of different materials that can be cut by CO2 laser with given power

CO2 laser power / W

Practical maximum cutting thickness / mm

Carbon steel

Stainless steel

Aluminum alloy a5052

Copper

Brass

1000

12

9

3

1

2

1500

14

6

3

4

2000

22

12

5

5

3000

25

14

10

5

8

15000

80

55

 

Generally, oxygen is used as auxiliary gas for cutting low carbon steel, and nitrogen is used as auxiliary gas for cutting stainless steel, aluminum and aluminum alloy, which can obtain high cutting speed and good cutting quality.

Determining the appropriate auxiliary gas pressure is also a very important factor.

When cutting thin materials at high speed, higher gas pressure is required to prevent slag sticking on the back of the cut.

When the material thickness increases or the cutting speed is slow, the gas pressure must be appropriately reduced.

It should be noted that sometimes the pressure is too high, and the cutting surface will become thicker.

6) Distance from the nozzle to the workpiece surface.

The height between the nozzle and the workpiece surface is also very important.

If the nozzle is too close to the plate, it will produce strong rebound pressure on the lens, and it is easy to be polluted, which will have an adverse impact on the cutting quality;

However, if the distance is too long, the ejected auxiliary air flow is easy to fluctuates, the high-pressure center of the air flow is difficult to reach the workpiece surface, the blowing capacity is too poor, and unnecessary energy loss is caused, affecting the cutting quality and speed.

Therefore, in laser cutting, the nozzle height is generally reduced as much as possible, usually 0.5 ~ 2mm.

High end laser cutting machines generally use the imported capacitive cutting heads, and its principle is shown in figure 1.

The measuring circuit of frequency modulation capacitance sensor is shown in figure 2.

It can automatically adjust the nozzle height according to the set value within a certain range, so as to ensure the best mutual position relationship between the laser focus and the processed plate surface in the processing process, and has a certain anti-collision function.

working principle of capacitive sensor

Figure 1 Working principle of capacitive sensor

1 – nozzle sensor 2 – lead 3 – detection circuit 4 – transmission cable

Fig. 2 Measurement circuit diagram of frequency modulation capacitance sensor

7) Other parameters.

In addition to the above factors affecting the quality of laser cutting, there are many factors affecting the quality of laser cutting plates, which are briefly described as follows:

① NC programming.

When a metal plate needs to be laser cut, first use the process programming software for numerical control programming, and complete the blanking size calculation, layout and process parameter setting of the material at the same time.

② Plate surface condition.

For different materials, their thermophysical properties and laser absorptivity are different. The laser wavelength of CO2 laser is 10.6 μm.

Non metallic materials have better absorption and higher absorption rate, while metal materials have a higher absorption rate of 10.6 μ M beam absorption is poor.

The higher the absorption rate, the more conducive to cutting.

For the cutting of copper and aluminum plates, special anti-reflection devices need to be installed, and the laser power is large, generally more than 2500W.

The surface state of the material directly affects the absorption of the light beam, especially the surface roughness and surface oxide layer will cause obvious changes in the surface absorptivity.

In the practice of laser cutting, sometimes the surface of the aluminum plate is coated with an absorbing material layer, which can significantly improve the cutting speed.

(3) Influence of external light path system

The optical elements of the external optical path system shall be inspected regularly and adjusted in time to ensure the stability of the optical path and prevent bias, so as to obtain good cutting quality.

When the laser is used for a long time, sometimes the lens will be polluted and the power will drop.

Drop a few drops of analytical pure acetone on the lens cleaning paper, gently wipe the lens surface, and repeat several times until the lens surface is clean and free of dirt and residual traces. Pay attention not to press the lens with your fingers to avoid damaging the lens.

If there are spots on the lens that cannot be wiped off, it indicates that the lens has been damaged and must be replaced again.

(4) Optical path

Optical path refers to the distance traveled by the laser beam from the laser to the plate processing surface.

The increase of optical path will change the wavefront curvature of laser beam, and then affect the position of focus.

Especially when cutting stainless steel without oxide, the focus of laser beam must be accurately adjusted to obtain more stable cutting quality.

The adjustment of laser beam focus can be completed with the help of a set of servo auxiliary lens.

However, in high-pressure cutting, the lens will produce huge energy, so it is best to use adaptive optics system to complete this work.

In the adaptive optics system, the radius of curvature of the mirror can be corrected by means of piezoelectric power meter or cooling water pressure meter.

A tiny mirror deformation of one tens of a micron can shift the focus position by several millimeters.

The adaptive optics system can accurately locate the focus at the required position, and the distortion of the reflecting surface can be less than 0.1mm.

When the deformable spherical mirror is used, the vertical incidence angle must be maintained as far as possible in order to prevent astigmatism from affecting the focusing.

The laser cutting machine equipped with adaptive optics system can handle long-distance mobile processing and ensure stable high-quality laser cutting in the whole processing area.

Table 2 shows the cutting parameters of various metal materials by low-power CO2 laser.

Table 3 shows the cutting parameters of various metal materials by 2 ~ 20kW high-power CO2 laser.

Table 4 shows the cutting parameters of various metal materials by domestic 200 ~ 500w CO2 laser.

Table 5 shows the data of cutting some materials by laser cutting method for reference.

Table 2 Laser gas cutting parameters of various metal materials

Material SciencePlate thickness / mmCutting speed / (M / min)Slit width / mmLaser power / WJet gas
Ti-6A1-4V102.51.626002
Ti-6A1-4V6.52.81.025002
Ti-6A1-4V2.23.80.7621002
Ti-6A1-4V1.37.60.7621002
Pure titanium0.5150.3813502
Carbon steel C10103.20.551.019002
06Crl8Nil1Ti1.30.760.516502
Aluminum alloy0.45150.523002

Table 3 cutting parameters of various metal materials by high power CO2 laser

Material SciencePlate thickness / mmCutting speed / (M / min)Cutting width / mmLaser power / kwJet gas
Aluminum132.31.015 
carbon steel6.52.31.015 
Stainless steel 06cr19ni1051.32.020 
Stainless steel 06cr19ni10131.3 10N2
Heat treated steel 5061450.4 10N2
Heat treated steel 5061251.1 10N2
Aluminum 6061132.5 10N2
Titanium254 10N2
Stainless steel 06cr19ni1033 202
Nickel chromium alloy 751.012 202
Titanium alloy1.212 2air

Table 4 Cutting parameters of various metal materials by domestic 200 ~ 500w CO2 laser

Material ScienceThickness / mmCutting speed / (mm / s)Lens focal length / mmOxygen blowing pressure/( × 104N/m2)Laser power / W
High speed steel1.758.25015200
As steel211.15015200
Stainless steel2105015200
Alloy steel48.25015200
45 steel3105015200
Q345(16Mn)2.511.65015240
As steel735015060500
45 steel1228015040500
Stainless steel44309060500
Manganese steel853515040350
High strength steel (CrWMn)52809060500
Cobalt base alloy2.53509060500
Tinplate0.5>37509060500

Table 5 Data of cutting some materials by laser cutting method

Type

Material name

Thickness / mm

Laser power / W

Auxiliary gas

Cutting speed / (M / min)

Metal

Mild steel

2.3

850

02

1.80

Alloy steel

3.29.8

30006000

0202

3.301.50

Stainless steel

0.80.8

350850

0202

0.230.36

Aluminium alloy

612.5

380016000

CO2CO2

0.032.64

Titanium alloy

2.245.0

210850

0202

3.813.30

Nonmetal

Matte paper

0.33

60

air

28.8

Glazing paper

0.33

60

air

40.0

Propylene

3.1

300

air

1.83

Polyvinyl chloride

3.2

300

air

3.60

Vinylon

36th floor

330

air

0.026

Leatherwear

3

225

air

3.05

Oak

16

300

air

0.28

Pine

50

200

air

0.13

Hard fibre board

3.8

300

air

0.19

Plywood

4.8

350

air

5.30

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