## 4 Results and Discussion
### Deposition of Single Weld Beads  
In order to compare the effectiveness of the optimized shielding gas nozzle and the lateral nozzle, individual weld beads were deposited using both configurations. In each case, the shielding gas flow rate was systematically varied in steps of 10 1/min from 101/min to 701/min, corresponding to the maximum possible output of the system used. Figure 8 shows sections of the various weld bead surfaces. Using the lateral nozzle, a strong oxidation in the form of a dark oxide layer on the weld beads is evident at flow rates ranging from 10 l/min to 60 l/min (see Figure 8a). At a flow rate of 70 l/min, no stable melt pool could be formed and the melt was blown to the side in droplets, preventing the deposition of a continuous weld bead. With this lateral configuration, despite the high cooling rate when depositing a single bead directly on the substrate plate, the covered area was not sufficiently large to avoid strong oxidation. Apparently, for the continuous weld beads, the degree of oxidation even increased with an increasing flow rate. This could be confirmed through measurements in cross-sections of the weld beads, in which the mean thickness of the oxide layer increased from approximately 4.9 um at 10 l/min to 8.8 um at 60 l/min (see Appendix A). This observation can be explained by a more turbulent flow at higher velocities. Due to the small nozzle diameter, a high flow velocity occurs at comparatively low volume rates, which causes high Reynolds numbers. The critical Reynolds number for the outflow of a free jet is in the range of 1500-2300, above which the flow transitions into turbulence [19]. This turbulence causes increased mixing with the atmosphere, which was also demonstrated by [PERSON] et al. [61] and [PERSON] et al. [62].  
Figure 7: Effect of the shielding gas flow rate on the effective protection length (indicated by the blue markers) on the substrate plane at a stand-off distance of 10 mm obtained from the simulations.  
Thus, at higher flow rates, the amount of atmospheric oxygen within the process zone was significantly larger.  
In contrast, the weld beads produced using the optimized shielding gas nozzle exhibited a completely bright surface (see Figure 8b). For a shielding gas flow of 10 l/min, slight discolorations could be observed, while for the other experiments no discolorations were visible. Since the formation of discolorations is an indicator for the presence of residual oxygen in the welding atmosphere, a complete protection of the hot process zone can be assumed for shielding gas flow rates ranging from 20 l/min to 70 l/min regarding the given setup. It is noteworthy that higher flow rates apparently did not result in significant turbulence and, thus, mixing with the atmosphere due to the nozzle being designed for a laminar flow. Remarkably, for smaller flow rates, ripples in the weld beads could be observed. According to [PERSON] et al. (2018), such ripples occur at high heat inputs, where an incipient dripping of the wire leads to a discontinuous deposition. In the present case, this is related to the decreased cooling effect at low shielding gas flows. The correlation between the shielding gas flow rate and the melt pool temperature is analyzed in the following section.