## 2 Materials and Methods
The analysis of the melt pool velocity and the spatter detachment took place within the static area of the welding process that is described below. The keyhole behavior was described for varying parameters by means of high-speed imaging of the sample top side (see Figure 4a). The evaluation of the image data was done by a third image processing algorithm for this purpose. The first step was to binarize the image and select the ROI (see Figure 4b). Afterward, the morphological operation \"closing\" closed defects in the detected keyhole geometry. Finally, the algorithm measured the length and width of the keyhole opening. The centroid of the area was considered the center of the keyhole geometry through which the measurement lines had to pass. One-thousand frames were measured for each sample to evaluate the keyhole geometry on the specimen's top side. The keyhole behavior was studied based on the generated data of keyhole length by analyzing frequencies and amplitudes of the keyhole oscillation. The  
Figure 3: Image processing of spatter formation: (**a**) single frame from the high-speed recording; (**b**) determination of the upward melt flow velocity at the keyhole rear wall; (**c**) superposition of the evaluated frames with spatter trajectories. data of the 1000 frames was pre-processed by a median filter (3 \(\times\) 1) to limit measurement irregularities. A FFT algorithm determined the normalized amplitudes and frequencies. In order to exclude temporal changes in the keyhole oscillation over the observation time, time-dependent changes in the keyhole oscillation were investigated. For this purpose, several calculations were performed for the 1000 frames with an area of 100 frames in each case. The first calculation was thus performed for the frame range one to 100. Subsequently, the range was shifted by one frame to the area two to 101. This was carried out 901 times to regard all 1000 frames. All calculated results were compiled in one three-dimensional diagram. Two exemplary time-wise windowed FFTs of the keyhole length without gas flow and welding speed of 12 m/min are depicted in Figure 5.  
No significant variations of amplitudes or frequencies occurred for both laser spot sizes during the investigation period. Consequently, FFTs of the entire range of 1000 frames were sufficient to evaluate the keyhole dynamic. Figure 4: Image processing of the keyhole geometry to determine its length and width: (**a**) high-speed imaging raw image; (**b**) identified geometry of the keyhole opening. Figure 5: Windowed FFT of the keyhole length for 12 m/min. The previously described evaluations of the high-speed imaging were carried out within the area of a consistent penetration depth. This ensured the comparability of the experimental results with different process parameters. Reasons for a deviation of penetration depth were the acceleration and deceleration of the axis and also a necessary time to reach the steady state after starting the weld. For this purpose, the measurement values of an OCT were used. The beam paths of the measuring OCT beam and the laser beam were similar. The applied system (PRECITEC IDM) was mounted at the welding optics by means of a beam splitter module. The wavelength was 1.55 um with a sampling rate of 70 kHz. To reduce the measurement noise, values with a difference greater than 0.1 mm to the previous value were filtered. Figure 6 shows three exemplary measurements of the OCT system for the two laser spot sizes and welding speed of 12 m/min with intervals for keyhole measurement and spatter evaluation. The OCT signal for the welding speed of 12 m/min and both laser spot sizes showed fluctuations at the beginning since the process was not yet in a steady state. For this reason, the start of the evaluation was set to 0.15 s. At the end of the welding process, an increase in welding depth was observed due to a deceleration of the axis.