## 4 Conclusions  
The present study describes the effect of a local gas flow of inert gas on spatter detachment during laser partial penetration welding of high-alloy steel AISI 304 (1.4301, X5 CrNi18-10) for the welding speeds 8 m/min, 12 m/min, and 16 m/min. A gas nozzle provided an argon flow with flow rates of 1.5 L/min and 5.0 L/min. The investigations were carried out for laser spot sizes of 300 \(\upmu\)m and 600 \(\upmu\)m. The gas flow reduced the sample loss of mass by up to 92% for both laser spot sizes and a welding speed of 12 m/min. In addition to the determination of the loss of mass due to spatter detachment, high-speed imaging from the top and lateral views was used to analyze the spatter formation and keyhole behavior. Therefore, a similar reduction of spatter formation was also found in the analysis of the spatter quantities. The cross-sectional spatter areas and velocities were investigated to determine the cause of the spatter-reducing effect of the gas flow. In particular, the gas flow rate of 1.5 L/min caused a velocity reduction for both laser spot sizes with similar spatter sizes. A measurement of the keyhole geometry was carried out for further causal research. The gas flow resulted in a keyhole length reduction for the laser spot size of 300 \(\upmu\)m and reduced the oscillation frequencies of the keyhole for both laser spot sizes. However, the reduction of the oscillation frequency was not considered to be the main cause of the reduced spatter detachment due to the unchanged high oscillation frequencies. The velocity of the occurring melt pool swellings was determined in a further investigation. The gas flow had only an insignificant influence on the upward melt flow velocity for the smaller laser spot size of 300 \(\upmu\)m. A significant velocity reduction of up to 56% was evident using the gas flow for the laser spot size of 600 \(\upmu\)m.  
The effect of the gas flow Is attributed to an increase in surface tension of the melt pool due to oxygen shielding for the smaller laser spot size. The kinetic energy of the melt required for spatter detachment is increased and fewer spatters detach from the melt pool swellings. Increasing the laser spot size to 600 \(\upmu\)m affects the keyhole geometry in such a way that the melt flow at the keyhole rear wall slows down and the fluid energy decreases. Combined with the increased surface tension of the melt pool, fewer spatters are detached from the melt pool swellings. If the local gas flow is too high, the increased momentum transfer from the gas flow to the melt pool results in the formation of melt accumulations especially for the smaller laser spot size. In summary, the study extends the understanding of the effect of a local gas flow on laser beam welding and spatter formation using different laser spot sizes. A reduced spatter formation allows higher welding speeds in industrial production with the same weld seam quality as at lower welding speeds. This increases the profitability of manufacturing companies by using a local gas flow during laser beam welding. Further investigations will focus on the effect of the gas flow on keyhole geometry. High-speed synchrotron X-ray imaging will be used to measure the keyhole depth, inclination angle, and keyhole length at different keyhole positions. Furthermore, the melt flow at the keyhole rear wall can be determined directly. **Author Contributions:** Conceptualization, C.D., L.S., T.M., K.S., P.W. and J.P.B. ; methodology, C.D., L.S., T.M., K.S., T.S., P.W. and J.P.B. ; software, C.D. ; formal analysis, C.D. and T.M. ; investigation, C.D., L.S. and T.M. ; data curation, C.D. ; writing--original draft preparation, C.D. and K.S. ; writing--review and editing, J.P.B. and P.W. ; visualization, C.D., L.S., K.S. and T.M. ; funding acquisition, J.P.B., K.S. and L.S. All authors have read and agreed to the published version of the manuscript.