## 3 Results
### Effects of the Local Gas Flow on the Keyhole Length  
The following results describe the effect of the gas flow on the keyhole length. Figure 14 shows the measured keyhole length values for each parameter and the red line represents their median value. The gas flow of 1.5 L/min resulted in a decrease in keyhole length of 26%, from 0.50 mm to 0.37 mm at a laser spot size of 300 \(\upmu\)m. Increasing the gas flow rate to 5.0 L/min led to a small length increase to 0.39 mm. Figure 14 also shows a decrease in the deviation of the measurement values for the flow rate of 1.5 L/min compared to the other two flow rates. It seems the deviation of keyhole lengths around the median was reduced by a comparably low gas flow. The median absolute deviation was reduced by 40% in comparison to the experiment without gas flow. Using the larger laser spot size of 600 \(\upmu\)m, the average keyhole length was 0.59 mm for the reference experiment. That means the keyhole length was smaller than the laser spot size due to the changed beam caustic caused by defocusing. Applying a gas flow expanded the keyhole slightly to 0.62 mm for 1.5 L/min and to 0.64 mm for 5.0 L/min. Consequently, the gas flow had only a minor influence on keyhole length. Especially for 5.0 L/min the deviation in the measurement values increased. The subsequent frequency analysis will consequently show higher amplitudes. Figure 14: Measured keyhole length for different laser spot sizes and gas flow rates at a welding speed of 12 m/min. The gas flow also changed the shape of the keyhole. Figure 15 shows the ratio of keyhole width to length of each measurement point. A ratio of one corresponds to a circular keyhole shape. If the ratio is less than one, the keyhole is elongated in the welding direction. It should be noted that the inclination angle of the welding optics resulted in an elliptical intensity distribution on the sample surface which favors an elliptical keyhole opening. Further information regarding the measured keyhole width is given in Appendix A. In general, the use of a local gas flow changed the keyhole shape for both laser spot sizes. The keyhole shifted from an elongated to a more circular shape using the flow rate of 1.5 L/min. This shift of the keyhole geometry was also described in [33]. The change in keyhole geometry was explained by the pressures affecting the keyhole during the welding process. Therefore, the keyhole closing Laplace pressure is calculated on the basis of surface tension and keyhole geometry [36]. The surface tension and the pressure of the melt flow mainly determine the keyhole pressure at high welding speeds [57]. Consequently, the increase in surface tension resulted in an equalization of the pressures and the keyhole geometry changed to a more circular shape. The higher flow rate of 5.0 L/min resulted in a similar ratio as in the experiment without gas flow for both laser spot sizes. The momentum transfer of the gas flow to the keyhole rear wall outweighed the effect of the increased surface tension. Therefore, the keyhole was elongated in the direction of the gas flow. The amplitudes and frequencies of the keyhole length are depicted in Figure 16 for an analysis of the keyhole oscillation at a welding speed of 12 m/min. Therefore, the frequencies and amplitudes were determined from the measured values shown above, and the maximum amplitudes are marked in the diagrams. The figure depicts the relative oscillation amplitudes over the frequencies. Relative values were calculated for comparability of the oscillation amplitudes in relation to keyhole size. The respective amplitude was divided by the median value of the 1000 measured values resulting in a dimensionless value that enabled comparability between the different laser spot sizes. The frequency of 1140 Hz had the maximum amplitude for a laser spot size of 300 \(\upmu\)m and the reference experiment. A flow rate of 1.5 L/min decreased the frequency of the maximum amplitude by 40% to 680 Hz. The amplitudes were similar in both cases.