## 3 Results
### Effect of a Local Gas Flow on Loss of Mass  
Figure 8 shows the loss of mass over welding speed and the resulting keyhole and melt pool behavior for both laser spot sizes depending on different flow rates of argon. The initial loss of mass of 0.04 \(\pm\) 0.02 mg/mm for the reference process with a laser spot size of 300 \(\upmu\)m at a welding speed of 8 m/min was not significantly reduced at flow rates of 1.5 L/min and 5.0 L/min due to the already negligible number of spatters in the reference measurement (see Figure 8a). Furthermore, the spatters detached from inside the keyhole. There was no spatter detachment from the melt pool swellings. A flow rate of 1.5 L/min enabled a 77% reduction of the loss of mass from 1.10 \(\pm\) 0.05 mg/mm down to 0.25 \(\pm\) 0.02 mg/mm for the welding regime with the highest loss of mass of all reference experiments for the welding speed 12 m/min. The further increase of the gas flow rate up to 5.0 L/min showed also a reduction down to 0.34 \(\pm\) 0.05 mg/mm which, however, is a slightly decreased reduction of loss of mass compared to a flow rate of 1.5 L/min. The high-speed images (see Figure 8a) show a change in melt pool behavior under the influence of the gas flow. Similarly to the reference process without gas flow, melt pool swellings behind the keyhole occurred. However, significantly fewer spatters detached from the melt pool swellings. If a spatter detached, it often fell back into the melt pool and was reabsorbed. Reabsorption was favored by an elongation of the melt pool due to the gas flow. Figure 8: Loss of mass for various welding speeds, keyhole and melt pool behavior under the use of local gas flow at a welding speed of 12 m/min: (**a**) laser spot size of 300 \(\mu\)m; (**b**) laser spot size of 600 \(\mu\)m.  
Furthermore, the spatter trajectories were influenced by the gas flow and the changed flow of the escaping metal vapor. The spatters also tended to be reabsorbed into the melt pool if they had low kinetic energy. The influence on the melt pool swellings and spatter formation was similar for both flow rates and will be explained in the following investigations. The subsequent rise of the welding speed to 16 m/min showed a decrease in loss of mass from 1.06 \(\pm\) 0.02 mg/mm to 0.60 \(\pm\) 0.02 mg/mm for 1.5 L/min while an increased gas flow rate of 5.0 L/min resulted in a loss of mass of 0.53 \(\pm\) 0.02 mg/mm. Thus, the use of a local gas flow resulted in a positive effect on loss of mass for welding speeds of 12 m/min and 16 m/min. A laser spot size of 600 \(\upmu\)m revealed a similar loss of mass for the reference process at 8 m/min. An increase in welding speed up to 12 m/min resulted in a reduced loss of mass to 0.10 \(\pm\) 0.04 mg/mm by applying a gas flow of 1.5 L/min. A similar reduction was achieved for a gas flow rate of 5.0 L/min with a reduction of 92% to 0.07 \(\pm\) 0.02 mg/mm. The percentage of loss of mass was higher for this laser spot size because the value of the reference experiment without gas flow was already lower than for the smaller laser spot size. The effect of the gas flow on the melt pool behavior was similar to experiments with the smaller laser spot size. Spatters were only detached occasionally. A welding speed of 16 m/min caused an initial loss of mass of 0.55 \(\pm\) 0.05 mg/mm for the reference process while the local gas flow reduced it down to 0.15 \(\pm\) 0.02 mg/mm for both gas flow rates. At 8 m/min welding speed, the gas flow had a negligible effect on the loss of mass for both laser spot sizes. The highest reduction of loss of mass was obtained for a welding speed of 12 m/min for both laser spot sizes. However, a similar but slightly lower effect was noticed for a welding speed of 16 m/min and both laser spot sizes.