# Investigations of the Absorption Front in High-Speed Laser Processing Up to 600 m/min
## 4 Process Models and Discussion  
When all the preceding aspects, findings, and measurements of this study are taken into account, a schematic process model can be developed. For this purpose, Figure 11 illustrates two-dimensional empirical process models for selected processing speeds in the  
Figure 10: Snapshots of high-speed records using the transmitted light method (**top**) and a schematic depiction of camera setup (**bottom**). range from 1 to 10 m/s. Theoretical considerations regarding the reflection of the incident laser beam were demonstrated by means of drawn-in red lines. Figure 11a shows that an almost vertically orientated keyhole was generated, which was surrounded by molten material. Considering the penetration depth from the cross-sections of about 420 \(\upmu\)m and the length of the 0 melt-free zone of about 131 \(\upmu\)m, the aspect ratio of the depicted keyhole was about 3.2. When the inclination angle from Section 3.2 is taken into account, the reflection of the incident laser beam can be assumed, as shown in Figure 10. In the case of the process model shown for 1 m/s, the incident laser beam was reflected toward the rear wall of the keyhole, which led to further multiple reflections and increased absorption of the incident laser energy. This effect resulted in significantly higher penetration depths compared to higher processing speeds. At processing speeds of 4 m/s (see Figure 11b), the absorption front was inclined by about 40\({}^{\circ}\) such that the incident laser beam was reflected nearly horizontally toward the up-flowing melt. If a second reflection would be considered, it might guide the laser beam upward, out of the processing zone, without further reflections and absorptions of the laser  
Figure 11: Empirical process models for (**a**) 1 m/s, (**b**) 4 m/s, (**c**) 7 m/s, and (**d**) 10 m/s. energy. The resultant melt flow at this speed still led to the formation of several melt pool swellings, as already observed for the lower processing speeds. At a processing speed of 4 m/s, the formation of spatters can also be seen at the absorption front. The spatters seemed to leave from the thin layer of melt running in front of the absorption front. When it came to processing speeds of about 7 m/s (see Figure 11c), the absorption front was even more inclined such that the incident laser beam was reflected directly upward, out of the processing zone. At the same time, it can be seen that the amount of spatter grew. For this reason, it can be assumed that a transfer of momentum on the trailing melt pool through the reflected laser beam did not necessarily increase the loss of mass. The resulting velocity of the melt flow at these speeds could explain this effect. It provided the kinetic energy of the melt such that it was sufficiently high to fulfill the detachment condition described in Equation (1). As a result, the flow velocity of the melt was much greater than the processing speed itself. It can be assumed, that the melt volume and, consequently, the melt layer leading the absorption front decreased with increased scanning speed, which was why the velocity of the melt flow increased due to the law of continuity. This approach was also followed in an inverted manner in [21] by means of an enlargement of the upper melt pool in order to reduce spatter formation. The preferred direction of the spatter detachment here was orientated against the scanning direction. Only a small amount of spatter left the absorption front laterally or frontward. Furthermore, it can be seen that no melt pool swelling resulted from the backward-oriented melt flow. Instead, it led to the formation of a solidified melt layer on the bottom of the resulting kerf, as already shown in the cross-sections in Figure 2. The generation of kerfs instead of seams was already shown in [1] for scanning speeds beyond 5 m/s for a focal diameter of 25 m and a laser power of 1 kW. In [22], the kerf generation was already determined at scanning speeds of 4.2 m/s for the same focal diameter and laser power of 3 kW.