# Investigations of the Absorption Front in High-Speed Laser Processing Up to 600 m/min
## 3 Results
### Further Process Observations  
Up to this point, the penetration depth and inclination angle of the absorption front could be determined via process observations. In order to get more information about the area trailing the absorption front, a further experimental setup was developed, which allowed for gathering high-speed records from above with a 45\({}^{\circ}\) inclination angle of the camera (see Figure 9c). This perspective allowed for the determination of the keyhole length or even the length of the melt-free zone, which was found for higher processing speeds. For the trials, the seams were aligned parallel to the edge of the sample with a distance of 5 mm. A comparison of single captured frames for different processing speeds is given in Figure 9a. Similar to the records done in the half-section setup, an up-flowing melt was visible right behind the absorption front at 1 m/s. Based on this, it can be assumed that at this processing speed, a keyhole was apparent, as described for deep penetration laser welding in several studies [19]. On the left-hand side of the described frame, huge melt pool swelling was visible. The formation of these melt pool swellings is described as the humping effect in the literature [20]. As the processing speed increased, this up-flowing melt appeared at a growing distance from the absorption front, as can be seen for 2 m/s, 3 m/s, 4 m/s, and 5 m/s. These images demonstrate how the process zone changed at higher processing speeds such that instead of describing a keyhole, it is now referred to as an absorption front [13]. Figure 8: (**a**) Resultant inclination angle of the absorption front for different processing speeds; (**b**) resultant loss of mass and corresponding inclination angle of the absorption front. In order to quantify these observations, the length of the melt-free zone behind the absorption front was determined. For this purpose, the distance between the interaction zone of the laser beam and the point where the up-flowing melt reached the level of the sample surface was measured. The results can be seen in Figure 9b. It can be observed that the measured length for 2 m/s was longer than the measured length for 1 m/s by a factor of four. This huge difference led to the assumption that already at 2 m/s, no keyhole was surrounded by a circular melt flow. Beyond processing speeds of 5 m/s, it was not possible to measure this melt-free area. The formation of kerfs was visible beyond processing speeds of 5 m/s instead. The melt must be accelerated out of the processing zone as it was also visible for the higher processing speeds in the half-section setup. In order to clarify the formation of the melt ejections more precisely, another experimental setup was developed, which can be seen in Figure 10. The camera position was quite similar to that found in the half-section setup. The difference here was the focal position and the illumination mode. In this setup, the light of the high-power LED was reflected directly into the optical path of the camera by means of a mirror. This allowed for producing records in a transmitted light method, whereby objects in the focal plane of the  
Figure 9: (**a**) Snapshots of high-speed records; (**b**) length of the melt-free zone for different processing speeds; (**c**) camera setup for records from above at an angle of 45\({}^{\circ}\). camera were displayed dark with a very lit-up background. The position of the laser focus on the sheet was made similar to the previously described camera setup. A comparison of some of the captured images is given for different processing speeds in Figure 10. The frames made at 1 and 2 m/s again show the formation of huge melt pool swellings behind the interaction zone. This humping effect did not immediately appear for processing speeds beyond 2 m/s. They became smaller with increased speeds and could not be captured in the section shown. At 5 m/s, the formation of spatters at the absorption front was observed.