With the development of the economy, the application of stainless steel plate is more and more extensive.
Its products are now widely used in construction engineering, machinery manufacturing, container manufacturing, shipbuilding, bridge construction and other industries.
At present, the cutting method of stainless steel thick plate is mainly laser cutting.
The technology of laser cutting stainless steel thick plate is becoming more and more mature, which gradually replaces the traditional stainless steel cutting technology.
In order to achieve high-quality cutting effect, we need to master certain process skills.
Generally speaking, the medium plate refers to the steel plate with a thickness of 10.0-25.0mm, the plate with a thickness of 25.0-60.0mm is called thick plate, and the plate with a thickness of more than 60.0mm is extra thick plate.
In order to cut high-quality thick stainless steel, stainless steel product manufacturers need to fully understand the laser cutting processes.
Criteria for laser cutting thick plate
The laser cutting section will form vertical lines.
The depth of the grain determines the roughness of the cutting surface.
The shallower the grain, the smoother the cutting section.
The deeper the grain, the coarser the section.
In addition, the shallower the grain, the higher the cutting quality.
For thick sheet metal, the perpendicularity of the cutting edge is very important.
When it is far away from the focus, the laser beam becomes divergent, which may lead to inconsistency between the upper and lower widths of the slit;
If the cutting edge deviates too much from the vertical line, the workpiece will not be standard enough and difficult to use;
The more vertical the edge, the higher the cutting quality.
3. Cutting width
The cutting width determines the inner diameter of the profile.
During actual cutting, it is necessary to adjust parameters and compensate the cut materials to ensure that the workpiece is the required size.
When cutting thick plates at high speed, the molten metal is not ejected from the notch below the vertical laser beam, but ejected at the back of the laser beam.
This will form a curved pattern at the cutting edge.
To solve this problem, it is necessary to reduce the feed rate at the end of the cutting, which can greatly eliminate the formation of grain.
Burr is a very important factor to determine the quality of laser cutting.
Burr removal requires additional work, which will be calculated in time and labor cost.
Therefore, whether there is burr is the basic standard to judge whether the laser cutting is qualified.
6. Heat affected area
The heat-affected area refers to the depth of the area where the internal structure changes.
In laser cutting, the metal area near the notch is heated, which may lead to the change of metal structure.
For example, some metals harden.
If cutting causes a part to heat rapidly, it will deform, which is particularly important in fine machining.
Controlling laser power and using short laser pulses can reduce component heating and avoid deformation.
Technological requirements of laser cutting thick stainless steel plate
1. Nozzle selection
The nozzle diameter determines the gas flow shape, gas diffusion area and gas flow rate into the notch, which affects the stability of melt removal and cutting.
The larger the air flow into the notch, the faster the speed, and the proper position of the workpiece in the air flow, the stronger the ability of jet to remove melt.
The thicker the stainless steel, the larger the nozzle should be used.
The larger the setting of the proportional valve and increase the flow, the pressure can be ensured and the normal section effect can be cut.
The specification of the nozzle here mainly refers to the end aperture.
Taking Precitec’s cutting nozzle as an example, its aperture ranges from 1.5mm to 5.0mm.
The selection of aperture is mainly related to cutting power.
The greater the power, the more heat generated, and the greater the amount of gas required.
When we cut the plate below 3mm, the nozzle with 2.0mm aperture is generally selected;
When cutting 3mm to 10mm plates, select 3.0mm nozzle;
When cutting plates above 10mm, nozzles of 3.5mm and above are required.
△3.5 aperture single layer nozzle
Single-layer nozzle or double-layer nozzle?
Generally speaking, double-layer nozzles are used for oxidative cutting (auxiliary gas is oxygen) and single-layer nozzles are used for melt cutting (auxiliary gas is nitrogen).
However, some lasers have special instructions, whether to use single layer or double layer. In this case, operate according to the laser instructions.
2. Auxiliary gas selection and gas purity
Various auxiliary gases, such as oxygen, nitrogen and air, are often used in stainless steel laser cutting.
Different gas types have different cutting effects.
Oxygen is black section, the air is light yellow, and nitrogen can keep the primary color of stainless steel from being oxidized.
Nitrogen is the preferred auxiliary gas for stainless steel cutting.
Recommended oxygen and nitrogen purity:
|Advantages||High cutting speed, able to cut thick plates||Avoid oxidation of the cutting edge, so the workpiece does not need to be reworked|
The test data of nitrogen flow, nozzle diameter and gas pressure are shown in the figure below.
The ordinate is the gas flow test value, the abscissa is the adjustment setting, different gas pressure changes, and different colors represent the test conditions of different nozzle diameters.
It can be seen from the graph that the flow is determined by the set gas pressure and nozzle diameter, which is a linear positive correlation.
3. Focus position
In order to ensure the cutting effect and protect the nozzle from damage, a coaxial test needs to be conducted before cutting to ensure that the nozzle is coaxial with the laser output beam.
Focus position test method
Stick the transparent tape paper on the end face of the nozzle outlet, adjust the laser output power for drilling, observe whether there is a central hole on the transparent tape paper and the position of the central hole, and synchronously adjust the adjusting screw on the mirror cavity handle until the hole punched by the laser on the transparent tape paper coincides with the center of the nozzle.
The focus is different, and the thickness, material and quality can be cut are also different.
Cutting different materials and thicknesses need to be adjusted into different focus.
Before cutting, measure the actual zero focus, and test and analyze the cutting process parameters based on the zero focus.
Negative defocus is the main process selection direction for stainless steel cutting.
|Location of focus||Cutting material and section features|
Zero focal length: the laser focus is on the surface of the cutting workpiece
|Suitable for thin carbon steel below 1mm;|
The focus is on the workpiece surface, the upper surface is cut smoothly, and the lower surface is not smooth.
Negative focal length: the laser focus is below the surface of the cutting workpiece
|Cutting method of stainless steel;|
The focus is below the board surface, so the smooth surface range is large, the slit is wider than the zero focal length slit, the gas flow is large during cutting, and the perforation time is longer than the zero focal length.
Positive focal length: the laser focus is above the surface of the cutting workpiece
|Oxygen is used when cutting carbon steel;|
The surface is blackened and the section is rough.
4. Influence of laser frequency adjustment and pulse duty ratio on cutting quality
Influence of frequency change on the cutting of stainless steel thick plate:
The frequency decreases from 500-100hz, the cutting section effect becomes fine, and the stratification is slowly improved.
When the frequency is set to 100Hz, it cannot be cut and the blue light is reflected.
Find out the best frequency range through the change of frequency.
In order to ensure the best cutting section, the perfect matching of pulse times and single pulse energy must be ensured.
Influence of pulse duty cycle change on stainless steel thick plate cutting:
The pulse duty cycle of 45% is the critical value.
Continue to reduce the duty cycle, and the undercut mark appears on the lower surface
When the duty cycle increases to 60%, the section becomes rough, the stratification is obvious, and the cutting surface turns yellow.
The pulse duty cycle refers to the proportion of beam irradiation time in each pulse.
Frequency is the number of times peak power occurs in a pulse, and duty cycle is the ratio of peak power to low power in a pulse.