In situ visualization of multiple reflections on the cut flank during laser cutting with 1 μm wavelength

Instabilities of the laser cutting front cause loss of quality due to the formation of striations on the cut flank. The mechanisms of striation generation during laser cutting are still not yet fully understood. In this paper, the frequently disputed and not completely clarified effect of multiple reflections in the cut kerf during laser cutting with 1 μm wavelength is investigated during cutting of 6 mm thick stainless steel using a laser power of 5 kW and nitrogen assist gas. To visualize the laser cutting process, the trim-cut technique was invented many years ago and has been and still is continuously improved by different authors. During a trim-cut, the laser beam is moved in parallel along an existing sheet flank and cuts off a stripe of less than a standard kerf width. The cutting front and the transition to the resultant cut flank are directly visible. In order to maintain a kerf-guided supersonic gas jet along the melt film and simultaneously enable the in situ observation of the cutting kerf with high-speed imaging, the missing cut flank is substituted by a transparent substrate made of quartz glass. The existence and the effect of multiple reflections and their manipulation are demonstrated during laser cutting by means of an automated in situ trim-cut test bench with versatile control features. Depending on the focus position, the behavior of the melt flow and of laterally extended cutting fronts due to multiple reflections is visualized. Moreover, measurable results such as the profile of the cut flanks of trim-cuts and the corresponding real reference cuts are presented and discussed. The theoretical intensity distribution on the flank of trim-cuts is calculated taking into account multiple reflections during the beam propagation in the interaction zone and compared with corresponding high-speed videos.