## 3 Results
### Spatter Quantity, Area, and Velocity for Both Laser Spot Sizes  
The spatters were quantified by lateral high-speed images to investigate if the loss of mass correlated with the spatter quantity. A change in spatter size could result in differences between sample loss of mass and spatter quantity. The results were obtained for the seam parts with constant welding depth (see Section 2). The determined number of spatters was divided by the welding seam length to obtain the number of spatters per millimeter seam to provide a better comparability to the loss of mass. Figure 11 shows the spatter quantities per millimeter for the welding speed of 12 m/min and different local gas flow rates in combination with typical lateral high-speed images. Figure 10: Exemplary cross sections from the center of the sample for a welding speed of 12 m/min of the reference and gas flow experiment: (**a**) Laser spot size of 300 \(\upmu\)m; (**b**) Laser spot size of 600 \(\upmu\)m.  
A spatter reduction of 76% was reached for the laser spot size of 300 \(\upmu\)m and a gas flow rate of 1.5 L/min. A further increase in flow rate to 5.0 L/min reduced the number of spatters per millimeter up to 80% which correlated well with the measured loss of mass (see Figure 11a). Figure 11a also shows a superposition of the lateral high-speed images. A visible reduction in spatter formation and a reduced angle of spatter detachment occurred with increasing gas flow. Using the laser spot size of 600 \(\upmu\)m the gas flow even reduced the spatter quantity by 95% for 1.5 L/min and by 98% for 5.0 L/min. Again, these values correlated well with the measured loss of mass (see Figure 11b). The already observed low spatter quantity under the use of a gas flow is particularly evident in the superposition of the high-speed images. Also, the decrease of the spatter detachment angle with increasing gas flow rate can be seen nicely. In addition, the already described spatter reabsorption is discernible in the superposition images. In summary, the determination of the spatter quantities from the lateral high-speed images showed a good correlation with the loss of mass. Therefore, the use of a local gas flow results in a significant reduction in spatter formation during laser beam welding with different laser spot sizes. Figure 12 shows the measured cross-sectional spatter areas \(\mathrm{A_{sp}}\) for different gas flow rates and laser spot sizes. The spatter areas were related to their volume, however, due to the irregular shape of the spatters only their measured areas were regarded. In addition to the median and mean values in Figure 12, the gray boxes represent 50% of the measured values. Whiskers are limited to 1.5 times the interquartile range (IQR) in the figure. Furthermore, the outliers are displayed as individual points. For a laser spot size of 300 \(\upmu\)m, similar median values were measured for the reference experiment without gas flow and for a gas flow rate of 1.5 L/min. For the flow rate of 5.0 L/min the median value of the spatter area was slightly increased by 0.08 mm\({}^{2}\). However, the mean values were in the same range. Considering the measurement method and comparing the gray boxes with 50% of the measured values, the spatter area was assumed to be about constant for the experiments with a laser spot size of 300 \(\upmu\)m. The experiments with the larger laser spot size of 600 \(\upmu\)m showed similar median values to the experiments with the 300 \(\upmu\)m laser spot size. Additionally, both gas flow rates did not exhibit a change in the spatter area. In summary, the spatter area and thus the spatter volume and surface were neither influenced by the gas flow rate nor by the laser spot size. Figure 11: Spatter quantity and superposition of the lateral high-speed images for a welding speed of 12 m/min: (**a**) laser spot size of 300 \(\upmu\)m; (**b**) laser spot size of 600 \(\upmu\)m.  
In addition to the spatter size, the spatter velocity was of interest.