## 2 Materials and Methods
Due to the spatter glow, no additional illumination was necessary. Both cameras recorded the experiments with a framerate of 10 kHz. A gas nozzle specially designed for the application supplied the shielding gas (argon) locally to the keyhole. The nozzle tip had a length of 15 mm and an inner diameter of 1.4 mm. Three linear guides and a rotary axis enabled a flexible positioning of the gas nozzle with an accuracy of 50 \(\upmu\)m. Before the experiments, the gas nozzle was positioned to a spot weld of the laser beam on the sample surface using a coaxial pilot laser. Based on better welding results under a trailing orientation of the gas nozzle in comparison to a leading orientation, the gas nozzle unit was mounted on the welding optics in the direction of the keyhole rear wall [39]. The angle of incidence \(\alpha\) between the nozzle tip and sample surface was set to 48\({}^{\circ}\) and the distance to the welding spot h to 5 mm, similar to previous investigations [34]. Based also on the previous investigations, gas flow rates of 1.5 L/min and 5.0 L/min were used. The gas flow rate of 1.5 L/min was applied to significantly reduce the spatter formation. The second flow rate of 5.0 L/min was set at the transition area between spatter reduction and the formation of humps. The loss of mass due to the welding process was determined to quantify the influence of the gas flow on spatter formation. A weighing of each sample before and after the  
Figure 2: Schematic illustration of the experimental setup for high-speed imaging for the top side (1) and side view (2) under the use of a local gas flow. welding process was carried out by a high-precision balance (PLJ 2000-3A, Kern & Sohn, Balingen, Germany). To obtain a parameter that was independent of the weld seam length, the measured loss was divided by the weld seam length. All experiments were performed three times. The data points depicted in the following graphs are mean values for the loss of mass and the error bars represent the standard deviation. The weld seam cross sections were inspected using microscope images to investigate the effect of the gas flow on the weld seam. The samples were therefore hot mounted, ground, and polished after cutting. The microscope camera (Axiocam 208 color, Zeiss, Oberkochen, Germany) of an incident light microscope (Axio Scope.A1, Zeiss, Oberkochen, Germany) captured the cross sections. The evaluation of the lateral high-speed images was carried out by means of two different algorithms for image processing. The first algorithm evaluated the upward velocity of the melt pool swellings from the lateral high-speed recordings. Therefore, the algorithm extracted a region of interest (ROI) from the original image (see Figure 3a) and located the melt pool swelling based on its maximum (see Figure 3b). Subsequently, it tracked the growth of the melt pool swelling directly behind the keyhole and determined the velocity based on the first two frames that showed the swelling. Thus, the velocity was assumed to be representative of the upward melt flow velocity at the keyhole rear wall in combination with the melt flow around the keyhole. The second algorithm tracked spatters over multiple frames to determine their number and direction (see Figure 3c, more details are provided in [21]). The average distance for a detected spatter trajectory divided by the time defined by the spatter velocity. In addition, the program measured the visible area of each spatter. The multiple spatter areas of one trajectory were averaged to calculate the cross-sectional area of each spatter. The following evaluation of the spatter volumes and surfaces also considered a different deformation of the spatters depending on the welding parameters. Due to the small distance of the spatters between two frames at a frame rate of 10 kHz, only every fourth frame was considered in the investigation. The following depictions of the image superposition utilized a maximum intensity projection.