With the growth of the economy, the use of stainless steel sheets has become increasingly widespread. They are now commonly used in the construction industry, machinery manufacturing, container production, shipbuilding, bridge construction and other fields.
Currently, laser cutting is the main method for cutting thick stainless steel sheets. Stainless steel thick plate laser cutting technology is becoming more and more advanced, gradually replacing traditional cutting techniques.
To obtain a high-quality cutting result, it is crucial to master the process techniques.
In terms of thickness, medium plates refer to steel plates with a thickness of 10.0-25.0 mm, plates with a thickness of 25.0-60.0 mm are called thick plates, and plates with a thickness of more than 60 .0mm are called extra thick plates.
To cut high-quality thick stainless steel, it is essential for stainless steel product manufacturers to have a thorough understanding of the laser cutting process.
Criterion for laser cutting of thick sheet metal
1. Roughness
The laser cutting process results in vertical lines in the cut section. The roughness of the cutting surface depends on the grain depth. A shallower grit results in a smoother cut section, while a deeper grit leads to a coarser cutting surface. Additionally, a shallower grain results in a higher quality cut.
2. Perpendicularity
For thick sheets, the perpendicularity of the cutting edge is extremely important. If the laser beam is too far from focus, it will become divergent, which can result in uneven widths at the top and bottom of the slit.
If the cutting edge deviates significantly from a vertical line, the workpiece will not meet standards and may be difficult to use. The closer the cutting edge is to a vertical line, the higher the quality of the cut.
3. Cutting width
The width of the cut determines the internal diameter of the profile. During the actual cutting process, it is necessary to adjust parameters and compensate for the material being cut to ensure the part meets the desired size specifications.
4. Grain
When cutting thick plates at high speed, molten metal is expelled at the back of the laser beam instead of being ejected from the notch below the vertical laser beam. This results in a curved pattern on the cutting edge.
To solve this problem, it is necessary to decrease the feed rate at the end of the cutting process, which can effectively eliminate the formation of this curved pattern.
5. Burrs
Burr is a crucial factor in determining laser cutting quality. Burr removal requires additional effort, which results in increased time and labor costs. As a result, the presence or absence of burrs is the main criterion for determining whether a laser cut is considered to be of acceptable quality.
6. Heat Affected Area
The heat-affected area refers to the depth of the region where the internal structure has undergone changes. During laser cutting, the metal near the notch is heated, which can result in changes to the metal's structure. For example, some metals can become harder.
7. Deformation
Rapid heating during the cutting process can cause deformation, which is particularly critical in precision machining. To avoid this, it is necessary to control the laser power and use short laser pulses, which reduces the heating of the components and prevents deformation.
Technological requirements for laser cutting of thick stainless steel sheets
1. Nozzle selection
Nozzle diameter
The nozzle diameter determines the shape of the gas flow, the size of the gas diffusion area and the gas flow rate in the notch, which influences the stability of casting removal and cutting. Greater airflow into the notch results in faster speeds and greater ability to remove casting, as long as the part is positioned correctly within the airflow.
For thicker stainless steel, it is recommended to use a larger nozzle. To increase the pressure and ensure a normal cutting result, the proportional valve setting can be increased, resulting in a greater flow.
Nozzle Specifications
The nozzle specification mainly refers to the end opening. For example, the cutting nozzle manufactured by Precitec has an opening that varies from 1.5 mm to 5.0 mm. Aperture selection is primarily based on cutting power. The higher the power, the more heat generated and the greater the amount of gas required.
When cutting boards below 3 mm, a nozzle with an opening of 2.0 mm is generally used. To cut boards between 3mm and 10mm, a 3.0mm nozzle is selected. When cutting boards thicker than 10 mm, nozzles with openings of 3.5 mm or larger are required.
Single layer nozzle with △3.5 opening
Single layer nozzle or double layer nozzle?
Typically, double-layer nozzles are used for oxidative cutting (where oxygen is used as an auxiliary gas) and single-layer nozzles are used for fusion cutting (where nitrogen is used as an auxiliary gas). However, some lasers have specific instructions regarding the use of single or double layer nozzles. In these cases, it is important to follow the instructions provided by the laser.
two . Auxiliary gas selection and gas purity
A variety of auxiliary gases, including oxygen, nitrogen and air, are often used in laser cutting of stainless steel. Different gases have varying effects on the cutting process. Oxygen results in black sections, air results in light yellow sections, while nitrogen helps prevent oxidation and maintain the stainless steel's original color. As a result, nitrogen is the preferred auxiliary gas for cutting stainless steel.
Recommended purity of oxygen and nitrogen:
| Oxygen | Nitrogen | |
| Benefits | High cutting speed, capable of cutting thick sheets | Prevent oxidation of the cutting edge, so the part does not need to be reworked |
| Recommended purity | ≥99.999% | ≥99.995% |
The test data of nitrogen flow, nozzle diameter and gas pressure are shown in the figure below.
The vertical axis represents the gas flow test values, while the horizontal axis represents the adjustment setting. Different gas pressure changes are shown, and different colors represent the test conditions of different nozzle diameters.
The graph shows that the flow rate is determined by the adjusted gas pressure and the nozzle diameter, with a positive linear correlation.
3 . Focus position
To ensure the cutting effect and protect the nozzle from damage, it is necessary to perform a coaxial test before cutting. This test ensures that the nozzle is aligned with the laser output beam.
Focus Position Test Method
To perform the coaxial test, adhere a piece of clear tape to the end face of the nozzle outlet. Adjust the laser output power for punching and observe whether there is a central hole in the tape paper and its position. Adjust the adjustment screw on the mirror cavity handle until the laser-drilled hole in the paper tape aligns with the center of the nozzle.
Focus affects the thickness, material and quality that can be cut. Different materials and thicknesses require different focus adjustments. Before cutting, measure the actual zero focus and use it as a basis for testing and analyzing the parameters of the cutting process.
In stainless steel cutting, negative defocus is the preferred direction for process selection.
| Focus location | Cutting material and section features |
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Zero focal length: the laser focus is on the surface of the cutting part
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This method is suitable for cutting thin carbon steel with a thickness of less than 1mm. The focus is placed on the surface of the workpiece, resulting in a smooth top surface and a rough bottom surface. |
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Negative focal length : laser focus is below the surface of the cutting part
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This is the stainless steel cutting method. The focus is placed below the surface of the plate, which results in a wider smooth surface area, a wider slit compared to a zero focal length slit, a greater gas flow during cutting, and a longer drilling time. longer than with a zero focal length. |
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Positive focal length : the laser focus is above the surface of the cutting part
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When cutting carbon steel, oxygen is used as an auxiliary gas. This results in a blackened surface and an uneven cut section. |
4 . Influence of adjusting laser frequency and pulse ratio on cutting quality
Influence of frequency change on cutting thick stainless steel sheets:
As the frequency decreases from 500 to 100 Hz, the quality of the cut section improves and the layering becomes finer. However, if the frequency is set to 100 Hz, clipping will not be possible and blue light will be reflected. To determine the best frequency range, it is necessary to experiment with frequency changes.
To achieve optimal cutting results, it is important to ensure a perfect match between pulse time and single pulse energy.
Influence of changing pulse work cycle on cutting thick stainless steel sheets:
The critical value for the pulse duty cycle is 45%. If the duty cycle is further reduced, an undercut mark will appear on the bottom surface. On the other hand, if the duty cycle increases to 60%, the cut section becomes rough, with light layers and a yellow cutting surface.
The pulse duty cycle represents the proportion of time that the beam is radiated in each pulse. Frequency is the number of times peak power occurs in a pulse, while duty cycle refers to the relationship between peak power and low power in a pulse.
