# 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  
In order to gain more information about the dimensions and several effects of the process zone, the half-section setup was developed in accordance with the work of [11], which allowed for a lateral view into the process. Snapshots from the footage at different times of high-speed records at 1 m/s can be seen in Figure 4. The approximate position of the incident laser beam is marked with a red arrow in each frame. An exact indication of the laser beam position was difficult due to the small focal diameter of 16.5 \(\upmu\)m. The snapshot shows the appearance of an almost vertically orientated keyhole. The absorption front here seemed to be inclined slightly against the scanning direction. This effect can be attributed to a slight deviation in the positioning of the laser focus in the \(y\)-direction. The area right next to the focal position still appeared in the solid phase for this record. For this reason, the absorption front was outlined (white dotted line) by connecting the assumed focal position on the sheet surface with the visible ground of the resulting keyhole in Figure 4. As such, the focal position could be determined in the videos from a light reflection on the sheet's surface. Furthermore, the melt flow, as well as the melt pool, could be observed by means of the applied setup. In addition, the area of solidification became visible, which was marked out at 13.2 \(\upmu\)s by means of the white dotted solidification line. In order to verify the transferability of the results from the half-section setup, in the next step, the resultant penetration depth and seam width were compared to the measurements from the metallographic cross-sections. This enabled the direct depiction of the differences that were caused by the half-section setup due to changed thermal conditions. The results and an example for the determination of both penetration depths for a processing speed of 2 m/s are given in Figure 5. The penetration depths were measured at three different times in each of the three records and correspondingly in the cross-sections for nine single seams to calculate the mean values and standard deviations. Figure 3: (**a**) Melt volume and calculated loss volume; (**b**) ratio of loss volume to melt volume. It can be seen that the penetration depths in the cross-section were always lower than the values for the depth in the half-section, while the difference between the results from the 1 m/s and 2 m/s processing speeds was significantly higher than for the higher speeds. This was due to the larger heat accumulation, which occurred at the edge of the sample. The glass plate had a much lower thermal conductivity compared to AISI 304, which led to a larger amount of deposited heat. The determination of the values for the resulting seam width was carried out correspondingly to the ones for the penetration depth. For the measurement of the seam width in the half-section setup, the resulting seams were pictured in a top view by means of a microscope. The results for the measured widths can be seen in Figure 6. Regarding values for the cross-sections, it became obvious that the processing speed had no influence on the resulting seam width. Instead of a clear trend, there was a plateau for the complete range of applied processing speeds. For the measured seam widths created using the half-section setup, there was only a significant increase at 1 m/s and 2 m/s compared to the ones visible from the cross-section. This might have been due to the already mentioned amount of deposited heat, which was accordingly higher for the lower processing speeds. Figure 4: Snapshots from the footage of the high-speed records in half-section setup at a processing speed of 1 m/s. Figure 5: Resultant penetration depths in the cross-sections and half-section setup.