# Investigations of the Absorption Front in High-Speed Laser Processing Up to 600 m/min
## 3 Results
### Characterization by Means of the Half-Section Setup
Furthermore, the standard deviation of the entire data series for the seam width in the half-section setup was significantly higher, which can be explained by small inaccuracies in the specimen-to-glass positioning that occurred during the process because of thermal expansion of the sample. This could lead to the formation of a small gap between the glass and the sample, which was filled with melt during the process. Considering the differences shown between the resultant penetration depth and seam width, it can be stated that there was a slight influence from the half-section setup. However, the underlying characteristics of the curves are comparable; therefore, the half-section setup can be considered for valid investigations. The lateral process observation also enabled the determination of the inclination angle of the absorption front, as is already shown in Figure 4. A qualitative comparison of the resultant inclination of the absorption front is given for selected processing speeds in Figure 7. White dotted lines mark the absorption front and the penetration depth in order to support better recognition. Figure 7a shows the smallest measured inclination angle of about 14\({}^{\circ}\), which was found with a processing speed of 1 m/s. Right behind the interaction area of the laser beam, a huge melt pool swelling formed by the up-flowing melt occurred. This aspect showed the presence of a keyhole at this processing speed. When the processing speed was increased, this melt pool swelling became smaller and the distance to the interaction zone with the incident laser beam simultaneously increased. Already at 4 m/s (see Figure 7b), no melt pool swelling was visible in the captured section. There was also a massive decrease in penetration depth, which came along with a significant increase in the inclination angle. A further increase in the processing speed to, for example, 7 m/s (see Figure 7c) led to the formation of spatter. It became apparent that major parts of the melt pool were ejected from the processing zone away from the scanning direction of the laser beam. Only a few spatters remained in the front. A further increase in the processing speed to 10 m/s (see Figure 7d) led to even bigger inclination angles of about 70\({}^{\circ}\), a lower penetration depth, and the formation of smaller spatters. Because of the extended formation of trajectories, the smaller spatters seemed to have a higher velocity and left the processing zone in more directions compared to those shown in Figure 7c. Figure 6: Resultant seam widths in the cross-section and half-section setups. For statistically validated values, three measurements of the inclination angle were made using each of the three videos. The results are depicted in Figure 8a, where a clear, approximately linear relationship between the resulting inclination angle and the processing speed can be seen. It is also noticeable that the inclination angle of the absorption front varied for every processing speed within a relatively small range. A linear relationship between the resulting inclination angle and processing speed was already determined by [18], where focal diameters of 200 to 600 \(\upmu\)m and processing speeds in the range of 0.016 to 0.16 m/s were applied. The resulting inclination angles were correlated with the resulting loss of mass from the removal trials afterward. The results are shown in Figure 8b. It became obvious that a significant rise in the loss of mass was found with an inclination angle of 60\({}^{\circ}\). Further increases in the inclination angle also led to a significant increase in the loss of mass. However, between the resultant inclination angles of 20\({}^{\circ}\) and 50\({}^{\circ}\), the loss of mass was almost the same. At this point, it became evident that there was a causal relationship between the resultant loss of mass and the inclination angle of the absorption front. Figure 4.0 Comparison of inclination angle of the absorption front for different processing speeds: (a) 1 m/s, (b) 4 m/s, (c) 7 m/s, and (d) 10 m/s.