## 3 Results
### Effect of a Local Gas Flow on Loss of Mass
The behavior can be explained by a shift of the process regime due to a changed keyhole and melt pool behavior depending on welding speed for the laser spot size of 300 \(\upmu\)m. A greater keyhole front inclination due to a higher welding speed affected the interaction between metal vapor and the keyhole rear wall [56]. The upward melt flow velocity at the keyhole rear wall was increased. According to [10] the increase in flow velocity around the keyhole is nonlinear and therefore higher than the welding speed increase. Thus, higher welding speeds decrease the effect of the gas flow for the smaller laser spot size. In summary, the use of gas flow for 12 m/min and 16 m/min reduced the loss of mass for both laser spot sizes. The gas flow was particularly beneficial at a welding speed of 12 m/min. This welding speed was investigated further in the following. The reduced loss of mass was also evident in the form of less spatter adhesions on the sample top sides for the welding speed of 12 m/min (see Figure 9). Consequently, the possibility that spatter adhesions on the top of the specimen distorted the loss of mass results can be excluded. In general, all specimen top sides revealed few spatter adhesions for the welding speed of 12 m/min suggesting a high kinetic energy of the spatters due to a high velocity. Figure 9: Weld seam surfaces for a welding speed of 12 m/min for the reference experiment and using a gas flow: (**a**) Laser spot size of 300 \(\upmu\)m; (**b**) Laser spot size of 600 \(\upmu\)m.  
Figure 10 shows typical weld seam cross sections for samples welded at 12 m/min, but different laser spot sizes and different gas flows. The cross sections show a distinct underfill due to the loss of melt volume for the experiments without gas flow, a laser spot size of 300 \(\upmu\)m, and a welding speed of 12 m/min (see Figure 10a), which reflects the determined loss of mass. Using a gas flow rate of 1.5 L/min, scattered spatter adhesions were observed and no underfill was visible due to the reduced loss of mass (see Figures 9a and 10a). By increasing the gas flow rate to 5.0 L/min, the process behavior shifted into the \"pre-humping\" regime. Melt accumulations from displaced melt occurred at the weld seam surface due to an enhanced momentum transfer from the gas flow to the melt during the welding process. The penetration depth in the experiments using gas flow was comparable to the reference experiment. Increasing the laser spot size to 600 \(\upmu\)m resulted in a wider weld seam and scattered spatter adhesions on the sample surface for the reference experiment without gas (see Figures 9b and 10b). The gas flow with a flow rate of 1.5 L/min enlarged the seam width compared to the reference experiment due to a widening of the keyhole top side (see Section 3.4). An increase in the gas flow rate to 5.0 L/min resulted in an excess weld reinforcement (see Figure 9). This indicated a transition to the \"pre-humping\" regime. Due to a larger melt pool behind the keyhole in comparison to the experiments with a smaller laser spot size, the momentum transfer of the gas flow did not result in a switch to the \"pre-humping\" regime. In summary, the loss of mass determination is not distorted by adhering spatters using the gas flow and is considered suitable for assessing the welding process with gas flow. To what extent the loss of mass reflects the reduction of the spatter formation is investigated in the following by evaluation of the lateral high-speed images.