In situ high speed diagnosis - A quantitative analysis of melt flow dynamics inside cutting kerfs during laser fusion cutting with 1 mm wavelength

Instabilities of the melt flow dynamics on the laser cutting front cause loss of quality due to the formation of striations on the cut flank. By using an advanced trim-cut test bench, in situ visualization of multiple reflections inside the cutting kerfs was possible for the first time. The existence of multiple reflections could be clearly demonstrated. The consequences regarding the striation pattern and its sectioning could be shown. Further evaluations of the melt flow dynamics inside laser cutting kerfs regarding characteristic temporal and spatial variables of the cutting process and their dependences are still necessary. Within this paper, the melt flow dynamics of the laser cutting process during laser cutting of 6mm thick stainless steel using a disc laser with a laser power of 5 kW and nitrogen assist gas with a pressure up to 20 bar is visualized and analyzed by means of in situ high-speed video diagnostics (>70 000 fps). An advanced algorithm is used to determine depending on the cutting depth the velocity distributions of melt accumulations which are sliding down on the cutting front apex. The influence of the variation of focus position, cutting velocity, and assist gas pressure is investigated. Furthermore, a link to the quality of the cut flank is described.