Perforation is the first step in laser cutting, and the quality of perforation largely determines the cutting effect. The molten metal generated during the laser perforation process will splash on the surface of the processed plate and accumulate around the hole until the plate is penetrated. Excellent perforation quality is the basis to ensure the stability of beam and airflow, so the quality of perforation also directly affects the quality of cutting contour.
Laser-cut plates come in different materials and thicknesses. The thicker the plate, the lower the laser absorption rate of the material and the more difficult it is to perforate. In order to achieve the purpose of perforation, different processes such as pulse perforation and frequency conversion perforation may be used in actual processing scenarios. According to the different auxiliary gases, the perforation methods can also be divided into oxygen perforation, nitrogen perforation, air perforation and so on. The selection of the perforation method and the adjustment of the process parameters are very complicated knowledge. It is very important for the laser cutting operators to master the laser perforation process.
1) Perforation using pulsed conditions
In pulsed perforation, the laser repeatedly melts (evaporates) the material, discharges the molten material, and cools the material during the irradiation and stop of the irradiation, and the perforation gradually deepens. Any difference in the process of melting and discharging will cause the molten metal to spray upwards or perforate for a longer time. When the plate thickness of the carbon steel material is more than 9 mm, if the pulse condition is used to perforate, the processing time will increase sharply, but the diameter of the small hole pierced is only about 0.4mm, narrower than the slot, and the heat-affected zone is also small .
Piercing using pulse conditions can also be divided into fixed-frequency piercing and variable-frequency piercing. The pulse condition is determined by the two parameters of frequency and duty cycle. If the frequency and duty cycle are kept constant during the piercing process, this piercing method is called fixed-frequency piercing. In the perforation process using pulse conditions, as the perforation depth increases, the laser power density required for perforation increases, and the time for slag removal also increases. Therefore, the perforation form of frequency conversion perforation is currently produced in the fixed frequency perforation method. During the perforation process, a curve of frequency and duty cycle can be set to change linearly. With the increase of perforation time and perforation depth, the frequency of the laser will be lower and lower, and the duty cycle will be higher and higher, which can improve the perforation ability. , thereby shortening the perforation time and reducing the perforation range.
When using constant frequency piercing and the frequency is in the range of 100~200 Hz, the higher the pulse peak power is set, the better the piercing quality will be. If a higher frequency is used, only the melting capacity will become higher, and the discharge and cooling effect of molten metal will be reduced. When the frequency change value of frequency conversion piercing is 265~1000 Hz, the piercing effect is the best, and the duty cycle needs to be determined according to the piercing effect: if the duty cycle is too low, the piercing time will be increased or even impermeability will occur. Too high will cause excessive energy accumulation and cause blast holes.
2) Pierce using continuous condition
The disadvantage of continuous piercing is that a large amount of molten metal will be sprayed onto the surface of the workpiece, and when the molten metal cannot be discharged from the extremely small aperture above, it will cause overburning, but continuous piercing can greatly shorten the processing time. Figure 4.2 is a photo of the surface and back of the material after piercing 12 mm thick Q235 steel using nozzles of different diameters under continuous conditions (2000 W). The diameter of the nozzle is equivalent to the scope of spraying oxygen to the perforation, and the larger the diameter of the nozzle, the larger the diameter of the pierced hole.
3) other
Under normal conditions, the perforation conditions are adjusted by observing the progress of perforation under pulse conditions or continuous conditions. The most ideal perforation effect is that the aperture is small and the required time is short.
