# Investigations of the Absorption Front in High-Speed Laser Processing Up to 600 m/min
## 3 Results  
### Reference Process  
Since processing speeds from 1 to 10 m/s were considered and the energy, mass, and momentum transfer between the keyhole/absorbing front and molten material changed significantly in the addressed range, the loss of mass with the associated metallographic cross-sections was examined at the beginning. In order to clarify the difference in the resulting loss of mass for the applied processing speeds and reduce the effect of measurement errors due to the weighing, the loss of mass was scaled up by processing complete areas. For this purpose, 300 single lines with a spacing of 100 \(\upmu\)m were performed within a constant area of 20 \(\times\) 20 mm\({}^{2}\) in the middle of the sample. For statistical validation, three samples were processed for every applied processing speed. The samples were cleaned and weighed after the trials. The resulting loss of mass for the different processing speeds can be seen in Figure 2. In addition, characteristic metallographic cross-sections of the individual process tracks are shown for selected processing speeds, which represent the significant changes in the processing result. It became apparent that the loss of mass increased with the application of a higher processing speed. The lowest loss of mass was measured with about 1.2 mg for 1 m/s. At a processing speed of 10 m/s, a mass of 12.2 mg was lost, which equaled an increase by a factor of 10. However, a plateau between 1 and 6 m/s was visible. At speeds above 6 m/s, there was a massive increase in the loss of mass up to 9 m/s. From 9 to 10 m/s, there was a slight decrease in the mean value visible. This might have been due to the decreased amount of deposited energy, which came along with the higher scanning speed. However, a significant difference was not found due to the overlapping standard deviation. The cross-sections from Figure 2 gave a first explanation of the resulting loss of mass. In addition to very substantial reductions in the penetration depth with increased speeds, it became apparent that humps occurred for the low processing speeds, while the formation of kerfs became visible for processing speeds of 7 m/s and beyond. Small layers of solidified melt remained at the sidewalls and the bottom of the kerfs for the higher processing speeds, while there was even less remaining melt visible for a 10 m/s processing speed. The samples were electrolytically etched with oxalic acid, which made the melt lines visible. Regarding the microstructure, no changes were seen at the applied magnification using light microscopy. The microstructure surrounding the melt area seemed to be unaffected in the shown cross-sections. No changes compared to the base material were found next to the melt lines, which can be explained by the high processing speeds and the low amount of dissipated heat [16]. The cross-sectional areas of the weld seams were measured to study these geometric changes in more depth. This allowed for the melt volume to be calculated via multiplication with the total length of the seams that were created during the areal processing. Only the melted area below the surface was considered for this purpose to exclude the influence of humps or solidified melt on the sheet's surface. The results for different processing speeds can be taken from Figure 3a. It can be seen that the melt volume decreased significantly with increased processing speed. In the same diagram, the calculated loss volume is shown by means of the second axis. The volume of the loss material was determined using the measured loss of mass and the density of AISI 304 (7.9 g/mm\({}^{3}\)[17]). The ratio of loss volume to melt volume is given in Figure 3b. It can be shown that the proportion of loss volume became larger with increasing processing speed, rising to a value of 0.53 for 10 m/s. At lower speeds up to 5 m/s, the fraction of volume that became lost during the process seemed rather negligible, with values below 4% of the melt volume.