The laser cutting process is closely related to the following factors:
- Laser mode
- Laser power
- Focus location
- Nozzle height
- Nozzle diameter
- Auxiliary gases
- Auxiliary gas purity
- Auxiliary gas flow
- Auxiliary gas pressure
- Cutting speed
- Sheet material
- Sheet surface quality (e.g., rust, foreign objects, etc.)
The process parameters associated with laser cutting are shown in the figure below.
Fig. 1 Cutting process parameters
The mode of the laser has a great influence on the cutting, and the mode required to reach the surface of the steel plate is better when cutting.
This is directly related to the mode of the laser and the quality of the external optical path lens.
The distribution of light intensity on the cross-section of the laser beam is called laser transverse mode.
The transverse mode is generally regarded as the laser mode.
The symbol TEMmn is used to represent various transverse modes.
TEM stands for transverse electromagnetic wave.
M and N are positive integers.
They represent the ordinal numbers of points with zero light intensity in the x-axis and Y-axis directions respectively, which is called mode ordinal.
The following figure shows the facula of several different transverse modes of the laser beam.
TEM00 mode is also called basic mode, and the intensity of any spot in the spot is not zero.
If there is a spot in the X direction with zero intensity, it is called TEM10 mode;
There is a point in the Y direction with zero light intensity, which is called TEM01 mode.
By analogy, the larger the mode ordinals m and N are, the more points with zero light intensity in the spot are.
Laser beams with different transverse modes are called multimode.
Fig. 2 Mode Spot
In the above figure, the TEM00 module is called the fundamental mode.
TEM * 01 module is a single ring mode, also called quasi fundamental mode. To distinguish it from TEM01, an asterisk * is added.
In fact, TEM01 mode and TEM10 mode can be regarded as the same mode, because X and Y axes are originally artificially divided. The stereograms of several modes are shown below.
Fig. 3 Stereogram of TEM00 mode
Fig. 4 Stereogram of TEM20 mode
Fig. 5. Stereogram of TEM23 mode
Figure 6 Multimode
II. Focal position
The focus position is a key parameter, which should be adjusted correctly.
1. The relationship between focus position and cutting surface
|Focal position||Schematic diagram||Features|
|Zero focal length:
The focus is on workpiece surface.
|It is suitable for thin carbon steel below 5mm.
The focus is on the upper surface of the workpiece, so the cutting is smooth and the lower surface is not smooth.
|Negative focal length:
The focus is under the surface of the workpiece.
|Aluminum, stainless steel and other workpieces adopt this method.
The focus is in the center and lower part, so the range of smooth surface is large.
The results show that the cutting width is wider than zero focal length, the cutting gas flow is larger, and the perforation time is longer than zero focal length.
|Positive focal length:
The focus is on the workpiece surface
|When cutting thick steel plate, oxygen is used.
When cutting thick steel plate, the oxidation of oxygen for cutting must be from top to bottom.
Because of the thick plate, the cutting width should be wide, the setting can get a wider cutting width.
The cutting section is similar to gas cutting, which can be said to be blown by oxygen, so the section is rough.
2. The influence of focus position on cutting section
|1.5mm above the surface||0.5mm above the surface||2.5mm above the surface|
3. Focus seek
It needs to use masking glue to light the nozzle by adjusting the focus distance, and then check the size of the holes that are punctured.
The position with the smallest hole is the focus.
The best focus for cutting is found according to the cutting process after finding out the focus.
Nozzle shape, nozzle diameter, nozzle height (distance between nozzle outlet and workpiece surface) will affect the cutting effect.
Fig. 7 Nozzle
1. Function of the nozzle
1) It can prevent the impurities such as melting stains from rebounding upward, passing through the nozzle and polluting the focusing lens.
2) It can control the gas diffusion area and size to control the cutting quality.
Fig. 8 Gas ejection without nozzle
Fig. 9 Gas ejection with nozzle
2. The relationship between nozzle and cutting quality
The coaxiality between the center of the nozzle outlet hole and the laser beam is one of the important factors affecting the cutting quality.
The thicker the workpiece, the greater the influence.
The coaxiality will be affected directly when the nozzle deforms or there is melt stain.
Therefore, the nozzle should be carefully stored to avoid damage and deformation.
The shape and size of the nozzle are of high manufacturing accuracy, and the correct installation method should be paid attention to.
Because of the poor condition of the nozzle, the cutting conditions need to be changed, so it is better to replace a new nozzle.
If the nozzle is different from the laser axis, the cutting quality will be affected as follows.
1) Influence on cutting section
As shown in the figure, when the auxiliary gas is blown out from the nozzle, the gas volume is uneven, and there is molten stain on one side and not on the other side.
It has little influence on the cutting of thin plate less than 3 mm, but more serious when cutting more than 3 mm, sometimes it can not be cut through.
Fig. 10 Influence of coaxiality on cutting section
2) Impact on sharp corners
If the workpiece has sharp angle or small angle, it is easy to produce over melting phenomenon, and thick plate may not be cut.
3) Impact on perforation
The perforation is unstable, the time is not easy to control, the thick plate will be over melted, and the penetration conditions are not easy to master.
It has little effect on the thin plate.
3. Adjustment of coaxiality between nozzle hole and laser beam
The adjustment steps of coaxiality between nozzle hole and laser beam are as follows:
1) It is necessary to apply ink pad on the outlet end face of the nozzle (generally in red), and stick the adhesive tape on the nozzle outlet end face. As shown in the figure.
Fig. 11 Step 1 of adjusting coaxial
2) Use 10 ~ 20 watts power, manual drilling.
3) Remove the self-adhesive paper, pay attention to maintain its direction to compare with the nozzle.
Under normal circumstances, the self-adhesive paper will leave a black spot, is burned by the laser.
However, if the center of the nozzle deviates too much from the center of the laser beam, the black spot will not be visible (the laser beam hits the wall of the nozzle).
Fig. 12 Nozzle deviation too large
4) If the center point is large or small, please pay attention to whether the conditions are consistent and whether the focusing lens is loose.
Fig. 13 Loose focusing mirror
5) Pay attention to observe the direction of the black spot away from the center of the nozzle and adjust the position of the nozzle.
Fig. 14 Adjusting the position of the coaxial laser beam
4. Nozzle diameter
The aperture size has a key influence on cutting quality and perforation quality.
If the nozzle aperture is too large, the molten material splashed around during cutting may pass through the nozzle hole and splash the lens.
The larger the aperture, the higher the probability, the worse the protection of the focusing lens and the worse the lens life.
Comparison of the nozzle aperture
|Nozzle aperture||Gas flow rate||Melt removal capacity|
The difference between φ1 and φ1.5 nozzle
|Nozzle diameter||Thin plate (less than 3mm)||Thick plate (over 3mm)
High cutting power, longer cooling time and longer cutting time
|φ1||The cutting surface is fine.||The gas diffusion area is small and unstable, but it is basically available.|
|φ1.5||The cutting surface will be thicker, and the corner is easy to have solution stains||The gas diffusion area is large, the gas velocity is slow, and the cutting is stable.|
5. Adjustment of nozzle height
The nozzle height is the distance between the nozzle outlet and the workpiece surface.
The setting range of this height is between 0.5mm and 4.0mm, and we usually set it at 0.7mm-1.2mm when cutting.
If it is too low, the nozzle will easily collide with the workpiece surface.
If it is too high, the concentration and pressure of auxiliary gas will be reduced, resulting in the decline of cutting quality.
When drilling, the height should be slightly higher than the cutting height, and the height should be set at 3.5mm-4mm.
In this way, the focus lens can be effectively prevented from being polluted by splashes produced during drilling.
Fig. 15 Nozzle height
Capacitive sensor control box
As shown in the figure , the nozzle height adjustment item is shown.
IV. Cutting speed
Cutting speed directly affects the width and surface roughness of the incision.
There is an optimum value of cutting speed for different material thickness and cutting gas pressure, which is about 80% of the maximum cutting speed.
1. Too fast speed
If the cutting speed is too fast, the following consequences may be caused.
1) It may not be cut through and sparks will be sprayed randomly.
2) Some areas can be cut through, but some areas cannot.
3) The whole section is thick, but no solution is found.
The cutting section is inclined striation, and the lower part of the cut section has solution stains.
Fig. 18 Too fast speed
2. Too slow speed
1) It causes over melting and rough cutting surface.
2) The slit widened and the sharp corner melted.
3) Affect the cutting efficiency.
3. Determine the appropriate cutting speed
Judging whether the feed speed can be increased or decreased from the cutting spark
1) Sparks spread from top to bottom
Fig. 19 Normal cutting speed
2) If the spark is inclined, the cutting speed is too fast.
Fig. 20 Too fast cutting speed
3) If the sparks are not diffused and few, and they gather together, the speed is too slow.
Fig. 21 Too slow cutting speed
The feed speed is appropriate.
As shown in the figure, the cutting surface presents a relatively smooth line, and there is no melting on the lower half.
Fig. 22 Normal cutting speed
V. Cutting auxiliary gas
When selecting the type and pressure of cutting auxiliary gas, the following aspects should be considered:
Oxygen is generally used in cutting ordinary carbon steel with low-pressure drilling and low-pressure cutting.
Air cutting is generally used for nonmetal cutting.
Generally, nitrogen is used to cut stainless steel.
The higher the gas purity, the better the cutting quality.
The purity of cutting low carbon steel plate should be at least 99.6%, and the oxygen purity should be above 99.9% when cutting carbon steel plate more than 12mm.
The nitrogen purity of cutting stainless steel plate should be above 99.6%.
The higher the purity of nitrogen, the better the quality of the cutting section.
If the purity of cutting gas is not good, it will not only affect the quality of cutting, but also cause lens pollution.
1. The influence of auxiliary gas on cutting quality
1) The gas can help heat dissipation and combustion, blow off the solution and improve the quality of the cutting surface.
2) The influence of insufficient gas pressure on cutting
a. The cutting surface is melted.
b. Cutting speed can not be increased, affecting efficiency.
3) The influence of high gas pressure on cutting quality
a. When the airflow is too large, the cutting surface is thicker and the seam is wider.
b. When the air flow is too large, the cut-off part will melt and the good cutting quality cannot be formed.
2. The effect of auxiliary gas on perforation
1) When the gas pressure is too low, it is not easy to penetrate and the time increases.
2) When the gas pressure is too high, the penetration point will melt and a large melting point will be formed.
Therefore, the perforation pressure of thin plate is higher than that of thick plate.
3. Auxiliary gas for cutting plexiglass
Plexiglass is flammable, in order to get a transparent and bright cutting surface, nitrogen or air is chosen for flame retardant.
If oxygen is selected, the cutting quality is not good enough.
It is necessary to select the appropriate pressure according to the actual situation when cutting.
The smaller the gas pressure is, the higher the brightness of the cutting light is, and the narrower the hair section is.
But the gas pressure is too low, resulting in slow cutting speed, flame under the plate surface, which affects the quality of the lower surface.
VI. Laser power
The laser power required by laser cutting mainly depends on the cutting type and the properties of the material to be cut.
The laser power required by vaporization cutting is the largest, followed by melting cutting and oxygen cutting.
Laser power has great influence on cutting thickness, cutting speed and incision width.
With the increase of laser power, the thickness of material can be cut increases, the cutting speed is accelerated, and the width of incision is also increased.
Laser power has a decisive influence on the cutting process and quality.
1. The power is too small to cut
Fig. 26 Too low power
2. If the power is too high, the whole cutting surface will melt.
Fig. 27 Excessive power
3. The power is insufficient, resulting in molten stains after cutting.
Fig. 28 Power deficiency
4. The power is appropriate, the cutting surface is good without molten stains.
Fig. 29 Proper power
Summary of cutting parameters
The main parameters of laser cutting
Given the laser power density and material, the cutting speed conforms to an empirical formula.
As long as it is above the threshold, the cutting speed of the material is proportional to the laser power, that is, increasing the power density can increase the cutting speed.
The cutting speed is also inversely proportional to the density and thickness of the material being cut.
Factors to improve cutting speed:
(1) Increase power (500-3000w);
(2) Change the beam mode;
(3) Reduce the focus spot size (such as using a short focal length lens)
For metal materials, other process variables are kept unchanged, and the laser cutting speed can have a relative adjustment range while still maintaining a satisfactory cutting quality. This adjustment range appears to be relatively wide when cutting thin metals.
After the laser beam is focused, the spot size is proportional to the focal length of the lens.
After the beam is focused by the short focal length lens, the spot size is small, and the power density at the focal point is very high, which is very beneficial for material cutting.
But its disadvantages are that the depth of focus is very short and the adjustment margin is small. It is generally suitable for high-speed cutting of thin materials.
For thick workpieces, since the telephoto lens has a wider focal depth, as long as it has sufficient power density, it is more suitable for cutting it.
Due to the highest power density at the focal point, in most cases, the focal position is just at the surface of the workpiece or slightly below the surface of the workpiece during cutting. To ensure a constant relative position between the focal point and the workpiece is an important condition for obtaining stable cutting quality.
Sometimes the lens is heated due to poor cooling during operation, which causes the focal length to change, which requires timely adjustment of the focal position.
The auxiliary gas is sprayed coaxially with the laser beam to protect the lens from pollution and blow away the molten slag at the bottom of the cutting area.
For non-metallic and some metallic materials, compressed air or inert gas is used to remove molten and evaporated materials, and at the same time prevent excessive combustion in the cutting area.
Auxiliary gas pressure
Most metal laser cutting uses reactive gas (oxygen) to form an oxidative exothermic reaction with hot metal.
This additional heat can increase the cutting speed by 1/3-1/2.
When cutting thin plates at high speeds, higher gas pressure is required to prevent the back of the cut from sticking to slag.
When the material thickness or cutting speed is slow, the gas pressure can be appropriately reduced.
Laser output power
The laser power and the quality of the mode will have an important impact on the cutting.
In actual operation, the maximum power is often set to obtain a high cutting speed or to cut thick materials.
Wrap it up
The above paragraph shows most of the factors that may affect the parameters of the cutting effect.
The following table lists typical values of cutting parameters.
They are not applicable to specific cases, but they can be used as a reference to find out the correct startup parameters.
Typical values of DC030 stainless steel cutting parameters
|Distance from nozzle to plate
Typical values of DC030 low carbon steel cutting parameters
|Distance from nozzle to plate
Typical values of DC025A1Mg3N2 steel cutting parameters
|Distance from nozzle to plate
The following pictures show the cutting of 15mm carbon steel sheet and 8mm stainless steel sheet at different focal lengths: