Process Requirements for Laser Cutting Stainless Steel Thick Plates

With the increasing economic development, the application fields of stainless steel medium and thick plates have become more and more extensive. The products manufactured by it are now widely used in construction engineering, machinery manufacturing, container manufacturing, shipbuilding, bridge construction and other industries. Nowadays, laser cutting is the main cutting method of stainless steel thick plates, and certain process skills are required to achieve high-quality cutting results.


Generally, the medium plate refers to a steel plate with a thickness of 10.0-25.0mm, a plate with a thickness of 25.0-60.0mm is called a thick plate. While the plate with a thickness of more than 60.0mm is an extra-thick plate.


1.Standards for the pros and cons of laser cutting thick plates


1. Roughness

The laser cutting section will form vertical lines, the depth of the grain determines the roughness of the cut surface, the shallower the lines, the smoother the cut section, the deeper the lines, the rougher the section. The shallower the grain, the higher the cutting quality.


2. Verticality

For thick sheet metal, the verticality of the cutting edge is very important. When it’s away from the focus, the laser beam becomes divergent, which may cause inconsistencies in the upper and lower width of the slit; the cutting edge deviates from the vertical line too much, which will cause the workpiece to be substandard 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 contour. In actual cutting, the parameters need to be adjusted to compensate for the cut material to ensure that the workpiece is the required size.


4. Grain

When cutting thick plates at high speed, the molten metal is not ejected from the incision below the vertical laser beam, but is ejected at the rear of the laser beam. This will form a curved line on the cutting edge. To solve this problem, it is necessary to reduce the feed rate at the end of the cutting process, which can greatly eliminate the formation of lines.


5. Burrs

The presence and amount of burrs is a very important factor in determining the quality of laser cutting. The removal of burrs requires additional work, which will be calculated in time and labor costs. Therefore, the presence of burrs is the basic criterion for judging whether the laser cutting is qualified.


6. Heat Affected Area

The heat-affected zone refers to the depth of the area where the internal structure changes. In laser cutting, the metal area near the cut is heated, which may cause changes of the structure of the metal. For example, some metals will harden.


7. Deformation

If the cutting heats the part sharply, it will deform, which is especially important in fine processing. Controlling the laser power and using short laser pulses can reduce parts heating and avoid deformation.


2. Process requirements for laser cutting of stainless steel thick plates


1. Nozzle selection

The diameter of the nozzle determines the shape of the gas flow entering the incision, the gas diffusion area, and the gas flow rate, which affects the melt removal and the stability of cutting. The air flow into the incision is large, the speed is fast, and the position of the workpiece in the air flow is appropriate, the stronger the jetting ability to remove the molten material. The thicker the stainless steel, the larger the nozzle should be used, the larger the proportional valve setting, and the larger the flow rate, the pressure can be ensured and the normal section effect can be cut.


Specifications: The nozzle specifications here mainly refer to the end aperture. Take Precitec's cutting nozzle as an example, the aperture is from 1.5mm to 5.0mm. The choice of aperture is mainly related to the cutting power. The greater the power is, the more heat is generated and the greater the air volume is required. When we cut sheets below 3mm, we generally choose nozzles with 2.0mm aperture; when cutting sheets from 3mm to 10mm, we choose 3.0mm nozzles, when cutting sheets above 10mm, we need to use nozzles of 3.5 and above.


Single and double-layer nozzles: Generally, double-layer nozzles are used for oxidative cutting (the auxiliary gas is oxygen), and single-layer nozzles are used for fusion cutting (the auxiliary gas is nitrogen). However, some lasers have special instructions, in this case, to use a single layer or a double layer, please follow the laser instructions.


2.Auxiliary gas and its purity

Various auxiliary gases are often used in stainless steel laser cutting processing, such as oxygen, nitrogen, air, etc. Different gas types are used, and the effect of cutting sections is different. Oxygen is a black section, air is light yellow, and nitrogen can keep the original color of stainless steel being oxidized. Nitrogen is the preferred auxiliary gas for stainless steel cutting.


3. Focus position

The focus is different, and the thickness, material, and quality that can be cut are also different. Different materials and thicknesses need to be adjusted to different focus. Before cutting, the actual zero focus is measured, and the test and analysis of the cutting process parameters can be carried out with the zero focus as the benchmark. The negative defocusing is the main process selection direction for stainless steel cutting.


4.The influence of laser frequency adjustment and pulse duty cycle on cutting quality


The influence of frequency change on stainless steel thick plate cutting:


The frequency is reduced from 500-100Hz, the cutting section effect becomes finer, and the layering gradually disappear. When the frequency is set to 100Hz, it cannot be cut and the blue light is reversed. Through frequency change, find the best frequency range. In order to ensure the best cutting section, the number of pulses must be perfectly matched with the energy of a single pulse.


The influence of pulse duty cycle change on stainless steel thick plate cutting:


The pulse duty cycle 45% is the critical value. If you continue to reduce the duty cycle, there will be traces of incomplete cutting on the lower surface. The duty cycle increases to 60%, the section becomes rough, the hierarchy is obvious, and the cut surface becomes yellow.

The pulse duty cycle refers to the proportion of the beam irradiation time in each pulse. The frequency is the number of times the peak power appears in a pulse, and the duty cycle is the ratio of the peak power and trough power in a pulse.

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