## 3 Results & Discussion
### Dependence on Laser Power and Feed Rate  
Figure 7 summarizes the variation of the melt pool area, determined from the high-speed camera top views, versus laser power and feed rate for 1.5 mm stainless steel, 1.0 mm, and 3.0 mm zinc-coated steel, respectively. In general, the melt pool area increases with increasing laser power and feed rate for all sheet thicknesses. Figure 7 shows that the recorded melt pool area is proportional to the laser power at a constant feed rate for a fixed sheet thickness with the investigated process window. During cutting, the absorbed laser power depends not only on the wavelength, polarization, and specific material properties but also on the angle of the kerf, i.e., the effective contact area of the impinging laser radiation. At low feed rates, a small melt pool area is observed, which can be attributed to the fact that in this regime, only a small portion of the laser radiation irradiates the nearly vertical cut front. At higher feed rates, the angle of the kerf becomes flatter, which leads to an enlarged contact area in the cutting direction. The raised flank of the kerf increases the  
Figure 6: Example images of the kerf in 1.0 mm, 3.0 mm, and 5.0 mm aluminum, recorded with a high-speed camera. Highlighted are the actual positions of the melt pool front in the feed direction by yellow arrows.  
absorption of the laser radiation, which leads to a larger measured intensity of the process radiation by the high-speed camera.  
Figure 8 summarizes the variation of the melt pool area in the cut kerf, detected from the high-speed camera top view, as a function of laser power and feed rate for 1.0, 3.0, and 5.0 mm aluminum. In addition, for aluminum, it can generally be seen that the melt pool area increases with increasing laser power and feed rate for all sheet thicknesses. The feed rate, however, does not seem to be directly related to the melt pool area.  
If the cutting parameters are unsuitable, e.g., if the feed rate is too high or the laser power too low, incomplete cuts occur, resulting in a significant increase in the melt pool area. Figure 9 compares the distribution of all melt pool areas for complete and incomplete cuts in stainless steel and zinc-coated steel. From this comparison, it can be seen that the melt pool area is not sufficient to distinguish incomplete cuts from complete cuts. Although for most cuts, the size of the melt pool area during an incomplete cut is significantly larger than during a complete cut. It can be observed that the same melt pool area is detected for incomplete and complete cuts in stainless steel and zinc-coated steel. Thus, further characteristics must be used for unambiguous cut interruption detection. The comparison of the melt pool area for complete and incomplete cuts in aluminum is depicted in Figure 10. For cutting aluminum, due to the lower melting temperature and resulting higher sampling rate, incomplete cuts often lead to overexposure of the camera image. This is particularly evident for cuts in a 5 mm aluminum sheet.  
Figure 7: Average size of the melt pool area depends on the laser power and feed rate for 1.5 mm thick stainless steel, 1.0 mm, and 3.0 mm zinc-coated steel.  
Figure 8: Average size of the melt pool area depends on the laser power and feed rate for 1.0, 3.0, and 5.0 mm thick aluminum.  
Figure 9: Comparison of the distribution of the melt pool area for complete and incomplete cuts in stainless steel and zinc-coated steel.