## 3 Results and Discussion
### Metallographic Analysis of Mild Steel to Reveal the Thermal Energy Deposition  
Whereas the first part of this investigation was dedicated to proof the process improvement using beam oscillation, the second part focused on understanding the mechanisms that lead to these improvements. Therefore, the influence of beam oscillation on the energy deposition during LBFC has been analyzed. Laser beam energy converts mainly into thermal energy during LBFC in order to melt the material. This energy absorbs not only on the cut front, but also on the lateral cut edge and creates the HAZ or recast layer. A method for visualizing the HAZ and recast layer inside the material consists on performing polished cross-sections of the cut materials as depicted in Figure 5. The metallographic analysis has been performed for selected mild steel samples. The reason is due to the three times higher heat conduction coefficient of mild steel compared to stainless steel, which tends to form a pronounced HAZ and recast layer. The cut kerr area corresponds to the black area of the microscopic images in Figure 5a,b and represents the boundary of the recast layer in one direction. Microstructural transformations between HAZ and recast layer indicate the second border. The recast layer represents the portion of the melt film that was not completely ejected and in general attaches at the lower part of the cut edge. This material heats up into the molten phase and solidifies subsequently, which is evident through the cast structure (Figure 5b). A ferritic-pearilic structure is distinctive for the basic material of mild steel. The pearlite grains transforming through heat input and indicate the beginning HAZ (Figure 5b,c). Figure 5: Microscopically images of polished and etched cross-sections of 12 mm laser beam fusion cut mild steel using static beam shaping indicates the cut kerr, recast layer, heat affected zone (HAZ), and basic material at different magnifications: (**a**) 20 fold; (**b**) 100 fold; (**c**) 250 fold. It is known that high-quality cuts require a small HAZ with preferably no recast layer [3, 11, 19, 42]. Moreover, parallel and ample cut kerfs simplify the removal of the cut parts from the remaining plate. Figure 5a depicts a polish of cross-section cut with static beam shaping. Contrary to ideal desires, the cut kerf varies greatly and has no straight-line boundaries. The recast layer attaches from approximately half the plate thickness and opens into strong burr adhesion on the bottom side of the plate. The HAZ is evident over the entire plate thickness. At the top of the plate is the HAZ layer uniformly thin and broadens significantly when the recast layer adhesion initiates. Figure 6 shows exemplary polish of cross-sections of mild steel cut with very different beam oscillation parameters. The images have been selected since they represent different behaviors. For example, for 60 \(\upmu\)m amplitude and 250 Hz frequency (Figure 6a) are the cut kerf walls still wavy, but there is just one constriction. The beginning of the recast layer adhesion shifts slightly towards the top side of the plate and narrows compared to the static beam shaping from Figure 5a. The burr height and HAZ area reduces. Increasing the frequency to 4000 Hz (Figure 6b) results in parallel, straight cut kerf walls. The recast layer and HAZ are not significantly different from the results obtained in Figure 6a, but the burr height grows. The increased amplitude (Figure 6c) minimizes the recast layer, HAZ, and burr significantly; hence, no heat input is visible on the top side of the plate. The influence of the oscillation parameters on the HAZ and recast layer width is shown in Figure 7. A rising frequency increases the HAZ area with a maximum at 2 kHz (Figure 7a). Additionally, the average recast layer width slightly increases with rising frequency. Increasing amplitudes minimizes the HAZ area, reaching its minimum at 400 \(\upmu\)m amplitude. The average recast layer width shows a similar behavior, however, the minimum is located at 300 \(\upmu\)m amplitude.