## 4 Results and Discussion
### Influence of the Shielding Gas Flow Rate on the Weld Bead Geometry and the Melt Pool Temperature  
For the build-up of defect-free and geometrically accurate components, precise knowledge about the dependence of the weld bead geometry on the input parameters of the process is essential. Therefore, the influence of the shielding gas flow rate on the geometry of the weld beads was examined. In these further investigations, only the optimized shielding gas nozzle was employed. In Figure 9, the weld bead width and height are plotted as a function of the flow rate. The weld bead width exhibited a descending trend with increasing flow rates. This can be attributed to the increased cooling of the melt pool due to convection. For a constant focal distance, a lower melt pool temperature is typically associated with a lower expansion of the melt pool and a higher surface tension ([PERSON] and [PERSON], 2018). In contrast, no distinct correlation between the flow rate and the weld bead height was evident. An influence of the increasing gas pressure above the surface, which is expected at higher flow rates and could result in a flattening and widening of the melt pool, was not observed for the parameter range investigated. Therefore, the cooling effect of the shielding gas flow is assumed to be the dominant influencing factor for the resulting weld bead geometry.  
Figure 8: Surfaces of individual weld beads deposited at shielding gas flow rates ranging from 10 l/min to 70 l/min using (**a**) the lateral nozzle and (**b**) the optimized nozzle.  
In order to confirm the cooling effect of the shielding gas flow, the melt pool temperature was monitored during the process using an in-axis pyrometer. The temperature data of the first 25 mm and the last 8 mm of the weld beads were omitted, so that only the stationary phase of the process was considered [64]. In Figure 10, the average melt pool temperatures during the stationary phase of the process are shown as a function of the flow rate. The absolute difference between the maximum and the minimum average temperature was approximately 15 K. It is worth noting that a closed-loop control system could be employed to compensate for the temperature deviations of the melt pool induced by the different shielding gas flow rates [65].  
To quantify the existing fluctuations in the temperature signals, the coefficient of variation \(c_{\mathrm{v}}\), i.e., the ratio of the standard deviation to the mean value, was calculated (see Figure 10). At higher flow rates, the fluctuations in the temperature signals tended to decrease, indicating a less dynamic melt pool. This was also reflected in the reduced appearance of ripples in the weld beads, which were described above. Therefore, with regard to an economical process, a large covered area and the prevention of ripples in the weld beads, a shielding gas flow rate of 40 l/min appears to be a suitable value for the given setup.  
Figure 10: Influence of the shielding gas flow rate when using the optimized nozzle on the average melt pool temperatures \(T_{\mathrm{m}}\) during the stationary phase of the process and the coefficients of variation \(c_{\mathrm{v}}\) of the melt pool temperature signals.  
Figure 9: Influence of the shielding gas flow rate when using the optimized nozzle on the weld bead width \(w\) and the weld bead height \(h\); the error bars show the respective standard deviation.