## 3 Results
### Discussion of the Experimental Results  
The conclusive consideration of the effect mechanisms was done with regard to the energy balance (Equation (1)). Figure 18a shows the energy balance of spatter detachment and how the defining parameters were affected by the local gas flow and laser spot size. The momentum transfer based on the escaping metal vapor \(\mathrm{E_{vap}}\) was supposed to be equal for the reference and gas flow experiments during the investigation. The energy of the fluid \(\mathrm{E_{kin,fl}}\), which is composed of the melt density \(\rho\), the spatter volume \(\mathrm{V_{sp}}\), and the upward melt flow velocity \(\mathrm{v_{fl}}\), is mainly determined by the quadratic influence of the upward melt flow velocity. In particular, the gas flow reduced the upward melt flow velocity for the larger laser spot size of 600 \(\mathrm{\SIUnitSymbolMicro m}\). It was assumed, that the density of the melt was not affected by the shielding gas. Furthermore, the effect of the gas flow on the cross-sectional area of the spatters (see Figure 12) and thus on the spatter volume \(\mathrm{V_{sp}}\) were minor. The kinetic energy of the spatters is mainly determined by the quadratic influence of the spatter velocity. The velocity decreased using gas flow (especially 1.5 L/min) for both laser spot sizes and was the result of the influence of the keyhole and melt pool behavior. The inert gas also changed the surface energy \(\mathrm{E_{sur}}\) required for spatter detachment, which is determined by surface tension \(\sigma\) and spatter surface \(\mathrm{S_{sp}}\). The spatter surface is related to the cross-sectional area of the spatters and was therefore supposed to be constant, as described before. The surface tension is increased by the shielding of oxygen as already explained. Thus, the use of an inert shielding gas reduces the oxygen content in the atmosphere surrounding the melt pool and increases the energy required for spatter detachment. There were significantly fewer spatters which detached from the melt pool swellings, and the kinetic energy of the few spatters decreased. In summary, the gas flow diminishes the spatter formation and spatter velocity \(\mathrm{v_{sp}}\) by reducing the upward melt flow velocity and increasing the surface tension of the melt. A cooling of the melt pool by the gas flow and an additional heat input to the melt pool by the distraction of the metal vapor flow is considered negligible. Due to the minor influence on the upward melt flow velocity for the laser spot size of 300 \(\mathrm{\SIUnitSymbolMicro m}\), the primary reason for the spatter reduction is assumed to be the oxygen shielding. Consequently, the surface tension of the melt pool swellings increases and they are more pronounced if using the gas flow for both laser spot sizes (see Figure 18b). In addition to the increased surface  
Figure 18: Effect mechanisms: (**a**) energy balance with the effect of local gas flow and laser spot size; (**b**) schematic illustration using gas flow for various laser spot sizes. tension, the upward melt flow velocity is decreased by the gas flow as a result of a keyhole widening for the laser spot size of 600 \(\upmu\)m. The spatter reduction is therefore not only influenced by the surface tension but also by a changed upward melt flow velocity due to a manipulated keyhole geometry. The following two equations of the energy balance for spatter detachment summarize the investigation results considering the energy of the escaping metal vapor E\({}_{\text{vap}}\), the fluid flow E\({}_{\text{kinf,fl}}\), the spatters E\({}_{\text{kin,sp}}\), and the surface E\({}_{\text{sur}}\) (see Equations (2) and (3)).