## 3 Results
### Point of Origin and Growth Rate of the Pores  
Figure 3 shows single frames of an X-ray video of the process of pore formation, which is added in the Supplementary Materials, in steps of 5 ms. The process of pore formation is particularly well visible in the single X-ray images of welds processed with a high laser power and a high feed rate. For the sake of visibility, the investigation of single X-ray images is presented through the results of experiments at a high laser power of P = 8.5 kW and a high feed rate of 12 m/min. The dotted white line represents the position of the gap between the two sheets. The feed resulted from the movement of the sample from right to left, as described above. The red dashed line represents this movement by the displacement of a fixed local position on the sample, which is highlighted by red triangles. The feed rate of 12 m/min and the time step of 5 ms lead to the displacement of the triangle by 1 mm per 5 ms. Three individual bubbles can be identified in each frame of Figure 3 for 450 ms \(\leq\) t \(\leq\) 465 ms and are highlighted with white dashed circles. In order to indicate the growth and movement of these bubbles, white dashed lines interpolate their positions in the single frames.  
Figure 2: Single frame of an X-ray video of the pore formation process during welding in overlap configuration of uncleaned AA6016 sheets with a feed rate of 12 m/min, a beam diameter of 500 μm, and a laser power of 8.5 kW. Green-, yellow-, and white-colored areas indicate zones of low density.  
The comparison between the inclination of the white dashed lines and the inclination of the red dashed line showed that the velocity of the movement of the bubbles in x-direction was lower than the welding feed rate. No significant movement in the z-direction was observed. The bubbles appeared at t = 450 ms in the weld pool a few millimeters behind the capillary (on the left-hand side of the capillary in Figure 3) at the gap level between the two sheets (the dotted line at t = 445 ms). From this, it can be concluded that the bubbles originate at the gap's level (the dotted line at t = 445 ms) but form in the weld pool at the locations where the dashed white lines cross the dotted line, which is a few millimeters behind the capillary, as highlighted by the dashed ellipse.  
During their movement in the weld pool, the bubbles grew until bubble \"a\" reached the surface of the weld pool at t = 470 ms, while bubbles \"b\" and \"c\" coalesced at the same time. Figure 4 shows the growth of the three bubbles with their average diameter and their growth velocity as a function of time. The data points represent the average diameter and average growth velocity of bubbles \"a\", \"b\", and \"c\". The length of the error bars indicates the range between the minimum and maximum measured values within three measurements.  
Starting from a bubble diameter of 0 mm at t = 445 ms, Figure 4a shows that the bubbles grew continuously after they appeared. As shown in Figure 4b, the highest growth velocity of all observed bubbles was present at the beginning of their growing process. This indicated a sudden growth of the bubbles and confirmed the validity of the determined origin of the pores in Figure 3.  
Figure 3: Determination of the point of origin of the pores by extrapolation across single frames of an X-ray video showing the formation of pores. Two uncleaned AA6016 sheets were welded in overlap configuration with a feed rate of 12 m/min, a beam diameter of 500 μm, and a laser power of 8.5 kW.