Introduction
Laser technology was first used for cutting as early as the 1970s. In modern industrial production, laser cutting has been widely applied in processing sheet metal, plastics, glass, ceramics, semiconductors, as well as textiles, wood, and paper materials.
Laser Cutting
When a focused laser beam hits the workpiece, the irradiated area heats up rapidly, causing the material to melt or vaporize. Once the laser beam penetrates the workpiece, the cutting process begins: the laser beam moves along the contour line, melting the material. A jet of gas typically blows the molten material away from the cut, leaving a narrow slit, almost as wide as the focused laser beam.
Flame Cutting
Flame cutting is a standard process used for cutting low-carbon steel, using oxygen as the cutting gas. Oxygen is pressurized up to 6 bar and blown into the cut. There, the heated metal reacts with the oxygen: it starts burning and oxidizing. The chemical reaction releases a significant amount of energy (up to five times that of the laser energy) to assist the laser beam in cutting.
Melt Cutting
Melt cutting is another standard process used for cutting metals. It can also be used to cut other fusible materials, such as ceramics. Nitrogen or argon is used as the cutting gas, blown through the cut at a pressure of 2 to 20 bar. Argon and nitrogen are inert gases, meaning they do not react with the molten metal in the cut, merely blowing it towards the bottom. Inert gases also protect the cut edges from oxidation.
Compressed Air Cutting
Compressed air can also be used to cut thin sheets. Air pressurized to 5 to 6 bar is sufficient to blow away the molten metal in the cut. Since nearly 80% of air is nitrogen, compressed air cutting is essentially a form of melt cutting.
Plasma-Assisted Cutting
If the parameters are selected correctly, plasma clouds will form in the cut during plasma-assisted melt cutting. 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 to the workpiece, coupling more energy into the workpiece, causing the material to melt faster and increasing the cutting speed. Therefore, this cutting process is also called high-speed plasma cutting.
The plasma cloud is essentially transparent to solid-state lasers, so plasma-assisted melt cutting can only be used with CO2 lasers.
Vaporization Cutting
Vaporization cutting involves evaporating the material, minimizing the thermal effect on the surrounding material. Continuous CO2 lasers can achieve this by evaporating low-thermal-conductivity, high-absorption materials, such as thin plastic films, wood, paper, and foam, which do not melt.
Ultrashort pulse lasers make this technique applicable to other materials. Free electrons in the metal absorb the laser and heat up rapidly. The laser pulses do not interact with the molten particles and plasma; the material sublimates directly without transferring energy as heat to the surrounding material. When ablating material with picosecond pulses, there is no significant thermal effect, melting, or burr formation.
Many parameters influence the laser cutting process; some depend on the technical performance of the laser and the machine, while others are variable.
Polarization
Polarization indicates the percentage of the laser that is converted. A typical polarization degree is around 90%, which is sufficient for high-quality cutting.
Focal Diameter
The focal diameter affects the width of the cut and can be changed by altering the focal length of the focusing lens. A smaller focal diameter means a narrower cut.
Focal Position
The focal position determines the beam diameter and power density on the workpiece surface, as well as the shape of the cut.
Laser Power
The laser power should match the type of processing, material type, and thickness. The power must be high enough so that the power density on the workpiece exceeds the processing threshold.
Operating Mode
Continuous mode is primarily used for cutting standard contours in metals and plastics from millimeter to centimeter sizes. For melt drilling or creating precise contours, low-frequency pulsed lasers are used.
Cutting Speed
Laser power and cutting speed must match each other. Cutting too fast or too slow will lead to increased roughness and burr formation.
Nozzle Diameter
The nozzle diameter determines the flow and shape of the gas jet expelled from the nozzle. The thicker the material, the larger the diameter of the gas jet should be, and accordingly, the nozzle diameter should also be increased.
Gas Purity and Pressure
Oxygen and nitrogen are commonly used as cutting gases. The purity and pressure of the gas affect the cutting results.
When using oxygen for flame cutting, the gas purity must reach 99.95%. The thicker the steel plate, the lower the gas pressure used.
When using nitrogen for melt cutting, the gas purity needs to be 99.995% (ideally 99.999%). Higher gas pressure is required when melt cutting thick steel plates.
Technical Parameter Table
In the early stages of laser cutting, users had to determine processing parameters by trial and error. Now, mature processing parameters are stored in the control units of cutting systems. There are corresponding data for each material type and thickness. Technical parameter tables allow even those unfamiliar with the technology to operate laser cutting equipment smoothly.
There are many standards for judging the quality of laser-cut edges. Standards such as burr formation, indentations, and striations can be judged by the naked eye; verticality, roughness, and cut width require specialized instruments for measurement. Material deposition, corrosion, heat-affected zones, and deformation are also important factors in evaluating laser cutting quality.
