## 1 Introduction
During the build-up of multi-layer parts, these oxide layers are remelted and become impurities within the deposited material, which deteriorate the mechanical properties [26]. In a previous study, this effect was observed for LMD with coaxial wire feeding of stainless steel due to an insufficient shielding gas coverage [27]. Nevertheless, in contrast to an inert gas chamber, local shielding allows for efficient and economical processing independent of the part size. The method used to generate a local inert gas atmosphere is highly dependent on the employed process as well as the specific application. In the design of local shielding gas devices, one main objective is to generate a stable laminar flow to avoid mixing of the inert gas flow with the  
Figure 1: Schematic illustration of the process zone in laser metal deposition with coaxial wire feeding. atmosphere [19]. For the purpose of efficiently assessing the flow conditions, computational fluid dynamic (CFD) simulations are a commonly used tool. Various local shielding gas concepts were established for wire and arc additive manufacturing (WAAM) [28; 29; 30; 31; 32]. The common approach is to use a trailing shielding device with a laminar flow in order to protect not only the melt pool, but also the deposited bead while at higher temperatures [26]. Similarly, [PERSON] pursued an approach for powder-based LMD in which the shielding gas device was adapted to the deposition geometry and additionally sealed with a flexible aluminum foil [26]. However, although these approaches are advantageous with respect to gas consumption, they are not suitable for arbitrary 3D applications with frequent changes in the processing direction. For this reason, an axisymmetric nozzle is required. In powder-based LMD, the inert carrier gas used to feed the material to the melt pool also serves to generate a shielding gas atmosphere. However, the commercially available powder nozzles show limitations in terms of shielding the entire high-temperature zone. Therefore, some studies investigated a possible enlargement of the protected area by means of additional shielding gas nozzles for powder-based LMD, as described in the following. [PERSON] et al. [33] used a cone-shaped shielding gas nozzle in addition to an existing powder nozzle [34] to increase the area covered by shielding gas. CFD simulations were performed to investigate the argon concentration in the process zone and the effects of the additional shielding gas flow on the powder flow. The additional nozzle led to a significant increase in the shielded area, enabling oxide-free weld beads to be deposited, whereas significant oxidation occurred with the original nozzle. More complex nozzle geometries were not considered and the deposition experiments were limited to single weld beads. [PERSON] et al. [35] investigated the shielding gas coverage achieved with an additional local shielding gas nozzle for powder-based LMD of Ti-6 Al-4V. They used a 3D CFD model to evaluate the influence of the number of inlets to the nozzle, the inlet angle, and the flow rate on the argon concentration in the process zone. In a further study, [PERSON] et al. [25] extended the area covered with inert gas by adding an additional external shielding nozzle coaxial to the one described in [35]. The shielding gas coverage on a flat substrate plate was investigated at different flow rates using CFD simulations. Based on the simulations, the process parameters were adjusted to achieve an oxide-free deposition of individual weld beads as well as small multi-layer geometries. A similar concept was used by [PERSON] et al. [36], who added an additional coaxial inert gas nozzle to a commercial powder deposition nozzle. Internal lattice structures were included to improve the flow distribution inside the nozzle as well as the flow uniformity at the nozzle exit. To better understand the flow behavior of the gases, CFD simulations were employed.