Lasers were first used for cutting back in the 1970s. In modern industrial production, laser cutting is widely used in the processing of sheet metal, plastic, glass, ceramics, semiconductors, textiles, wood and paper.
In the next few years, the application of laser cutting in the field of precision machining and micromachining will also achieve substantial growth.
laser cutting
When a focused laser beam is shone onto a workpiece, the irradiated area heats up dramatically to melt or vaporize the material. As soon as the laser beam penetrates the workpiece, the cutting process begins: The laser beam moves along the contour while melting the material. A jet of air is usually used to blow the melt away from the kerf, leaving a narrow gap between the cut part and the plate holder, almost as wide as the focused laser beam.
Flame cutting
Oxygen cutting is a standard process for cutting mild steel using oxygen as the cutting gas. Oxygen pressurized up to 6 bar is blown into the incision. There, the heated metal reacts with oxygen: combustion and oxidation begin. The chemical reaction releases a large amount of energy (up to five times the power of the laser) to assist the laser beam in cutting.
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Figure 1 The laser beam melts the workpiece, and the cutting gas blows away the molten material and slag in the incision
Melt cutting
Fusion cutting is another standard process used when cutting metal. Can also be used to cut other fusible materials such as ceramics.
Nitrogen or argon is used as the cutting gas, and the gas with a pressure of 2-20 bar is blown through the incision. Argon and nitrogen are inert gases, which means they don't react with the molten metal in the incision, just blowing it away towards the bottom. At the same time, the inert gas can protect the cutting edge from being oxidized by air.
compressed air cutting
Compressed air can also be used to cut thin sheets. Air pressurized to 5-6 bar is sufficient to blow molten metal out of the cut. Since air is nearly 80 percent nitrogen, compressed air cutting is basically fusion cutting.
plasma assisted cutting
If the parameters are chosen properly, a plasma cloud will appear in the plasma-assisted melting cutting kerf. The plasma cloud consists of ionized metal vapor and ionized cutting gas. The plasma cloud absorbs the energy of the CO2 laser and transfers it into the workpiece, so that more energy is coupled to the workpiece, and the material will melt faster, resulting in faster cutting speed. Therefore, this cutting process is also called high-speed plasma cutting.
Plasma clouds are virtually transparent to solid-state lasers, so only CO2 lasers can be used for plasma-assisted melting cutting.
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gasification cutting
Gasification cutting evaporates the material, minimizing the thermal effect on surrounding materials. This can be achieved by evaporating low-heat, high-absorbing materials such as thin plastic films as well as non-melting materials such as wood, paper, foam, etc., using continuous CO2 laser processing.
Ultrashort-pulse lasers allow this technique to be applied to other materials. Free electrons in the metal absorb the laser light and heat up violently. The laser pulses do not react with the molten particles and plasma, and the material sublimates directly, giving no time to transfer energy in the form of heat to surrounding materials. Picosecond pulses ablate material without significant thermal effects, melting and burr formation.
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Figure 3 Gasification cutting: The laser vaporizes and burns the material. The pressure of the steam makes the slag discharge from the incision
Parameters: Adjusting the machining process
Many parameters affect the laser cutting process, some of which depend on the technical performance of the laser and machine tool, while others vary.
degree of polarization
The degree of polarization indicates what percentage of the laser light is converted. A typical degree of polarization is around 90%. This is more than enough for a high quality cut.
focal diameter
The focal diameter affects the kerf width, and the focal diameter can be changed by changing the focal length of the focusing mirror. A smaller focal diameter means a narrower incision.
focus position
The focus position determines the beam diameter and power density on the workpiece surface as well as the shape of the incision.
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Figure 4 Focus position: inside the workpiece, on the surface of the workpiece and above the workpiece
laser power
The laser power should match the type of processing, material type and thickness. The power must be high enough that the power density on the workpiece exceeds the machining threshold.
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Figure 5 Higher laser power can cut thicker materials
Operating mode
The continuous mode is mainly used for cutting standard profiles of metal and plastics in millimeter to centimeter sizes. To melt perforations or create precise contours, low-frequency pulsed lasers are used.
cutting speed
Laser power and cutting speed must match each other. Cutting speeds that are too fast or too slow will result in increased roughness and burr formation.
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Figure 6 Cutting speed decreases as the thickness of the sheet increases
Nozzle diameter
The diameter of the nozzle determines the flow rate and shape of the gas flow from the nozzle. The thicker the material, the larger the diameter of the gas jet and, accordingly, the diameter of the nozzle opening.
Gas Purity and Barometric Pressure
Oxygen and nitrogen are often used as cutting gases. The purity and pressure of the gas affect the cutting effect.
When cutting with oxy-fuel, a gas purity of 99.95 % is required. The thicker the steel plate, the lower the gas pressure used.
Fusion cutting with nitrogen requires a gas purity of 99.995 % (ideally 99.999 %), and higher gas pressures are required for fusion cutting thicker steel plates.
Technical Data Sheet
In the early days of laser cutting, users had to decide the setting of processing parameters by themselves through trial operation. Well-established processing parameters are now stored in the control unit of the cutting system. For each material type and thickness, there is corresponding data. The technical data sheet enables the smooth operation of laser cutting equipment even for those who are not familiar with this technology.
Laser cutting quality evaluation factors
There are many criteria for judging the quality of a laser cut edge. Standards such as burr form, depression, and texture can be judged with the naked eye; verticality, roughness, and incision width, etc., need to be measured with special instruments. Material deposition, corrosion, heat-affected zone and deformation are also important factors to measure the quality of laser cutting.
