Pyrometry diagnostic in laser cutting technology

Abstract

The measurement data of temporal temperature fluctuation on the cut front while laser cutting of sheet metal (3 mm, 6 mm and 10 mm thickness) are obtained using two-color multi-channel pyrometer. Measurements were carried out for several values of cutting speed and pressure of assisted gas (oxygen), using a 1500W CO₂ laser radiation and 1800W Ytterbium fiber laser. The relationship between the temperature fluctuations and the deformation of the melt flow surface on the cutting front in the frequency range above 3 kHz was proven. It is shown that in the case of CO₂ laser cutting the temperature fluctuations RMS is greater than 10 K in the range of sub-millimeter capillary waves, but in the case of Fiber Laser cutting the temperature fluctuations RMS is less than 3 K. The spectrum of capillary waves in case of Fiber- Laser cutting is formed under the effect of forced surface deformation at lower frequencies, in particular related to the assisted gas jet. It is shown, that thermo-capillary effect with capillary-wave turbulence generation can be observed in the locations, where exposition intensity of melt surface to CO₂ laser radiation exceeds 1 MW/cm². Thus, an additional mechanism of the anomalous absorption on the front of cutting can compensate the low absorption of the metal in case of 10.6 ìm laser in comparison with the absorption of the metal in the near infrared range.