## 4 Results and Discussion  
### Simulative Evaluation of the Shielding Gas Coverage  
First, the oxygen distribution when using the optimized nozzle was analyzed based on simulations. Figure 6a shows the contour plot of the oxygen distribution in the process zone at a shielding gas flow rate of 50 l/min. The oxygen concentrations in parts per million (ppm) were calculated from the mass fractions obtained in the simulation. Directly below the nozzle outlet, a cylindrical free jet region is formed. In this area, no turbulent flow and, thus, no unacceptable mixing with the surrounding atmosphere is apparent. Once the free jet reaches the substrate plane, it propagates along the surface and forms a laminar zone. Towards the outer edges of the substrate, gradual mixing with the atmosphere occurs, resulting in a decreasing thickness of the shielding gas layer. The flow above the hot process zone also serves as a convection cooling, which could have a positive effect with regard to a higher cooling rate. In addition, a map of the oxygen concentration on the substrate plane is displayed in Figure 6b.  
In order to reliably avoid unacceptable oxidation during the process, the dimensions of the area sufficiently covered with shielding gas are relevant. This area is referred to as the \"effective protective region\" [25]. For the processing of titanium in DED processes, the effective protection region was defined by [PERSON] et al. [28] as well as [PERSON] et al. [25] as the area in which the oxygen concentration was below 2000 ppm. Although stainless steel is less critical than titanium in terms of the maximum acceptable oxygen concentration, this value was used as a conservative limit in the simulations. The effective protection region is indicated in Figure 6b. To evaluate the shielding effect based on CFD simulations, the flow rate was varied between 5 l/min and 100 l/min in steps of 51/min. To quantify the  
Figure 6: Contour plots of the oxygen distribution at a shielding gas flow of 50 l/min: (**a**) outlet of the optimized shielding gas nozzle as a cross-sectional view; (**b**) oxygen map on the substrate plane.  
shielding effect, the average diameter of the effective protection region on the substrate plane was determined for each flow rate. This diameter is referred to as the \"effective protection length\" [25]. Figure 7 shows the effective protection length as a function of the flow rate. A distinct trend is visible, where a higher flow rate results in a larger protected area. Between 51/min and 401/min, the effective protection length increases rapidly from 0 mm to 65.9 mm. In contrast, a further increase of the flow rate from 401/min to 1001/min results in only a 2.7% increase of the effective protection length.  
In this study, only the fundamental case when depositing on a flat substrate was considered. It is noteworthy that the protected area is smaller when larger components with arbitrary geometries are built. The volumetric area with a sufficiently low oxygen concentration strongly depends on the respective component geometry below the nozzle outlet. In the case of a thin wall, it can be assumed that at least the cylindrical free jet region with a diameter of approximately 28 mm is sufficiently covered with shielding gas [19]. In order to minimize oxidation, it is therefore crucial that the hot process zone cools down rapidly enough while still covered by shielding gas. The main influencing factors for this are the heat dissipation in the component and the traverse speed.