Advanced trim-cut technique to visualize melt flow dynamics inside laser cutting kerfs

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. To visualize the laser cutting process, the trim-cut technique was invented many years ago and is being continuously improved by different authors in the last years as well as in current studies. 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. Without additional measures, the cutting gas jet expands during trimming in the half space freed by the lack of a second cutting edge. To maintain a guided supersonic gas jet path along the melt film and simultaneously enable the in-situ-observation of the cutting kerf with high-speed imaging, the missing cut flank is simulated by a transparent substrate, e.g., made of synthetic fused silica. In this manner, the gas jet is guided between the transparent substrate and the opposite real cutting flank. In this paper, a comprehensive review about trim-cut studies from different authors is given. Furthermore, an automated trim-cut test bench with two fast, highly accurate x/y-tables and versatile control features, as well as recent research results on melt flow characteristics during laser cutting of 6 mm thick stainless steel with 1 μm wavelength and nitrogen assist gas, are presented.