# Effect of Local Gas Flow in Full Penetration Laser Beam Welding with High Welding Speeds
## 3 Results
### 3.3. _Spatter Reduction by Use of Local Gas Flow_
#### 3.3.3 Characterization of Particle Speed  
Based on the plane-parallel high-speed recordings (see Section 2), the detached spatter was analyzed in more detail. Therefore, 250 sequential frames of high-speed recordings were merged into one picture by superposition. The particle speed as well as the angle of spatter detachment could be determined by image processing. This method allows for the analysis of spatter formation on the top and bottom sides separately. The following information about the particle speed (Figure 7) only refer to the top-sided analysis, as no influence of the particle speed could be determined on the bottom side. For statistical purposes, the particle speed was calculated using the arithmetic mean of three or more detected spatter per each tested parameter. Nevertheless, in the case of a sequence with less spatter (n \(<\) 3), the particle speeds are also shown for a better illustration. These values are highlighted by a grey color.  
At a welding speed of 8 m/min, the curve of particle speed was not significantly affected by varying flow rates (Figure 7a), except measuring points with a low detection rate of n \(<\) 3. By increasing the welding speed to 10 m/min (Figure 7b), the particle speed of the reference process was not affected. Using the local gas flow, the particle speed raised by an increase in flow rate. Based on the starting values of the reference process of approximately 2.0 \(\pm\) 0.98 m/s, the particle speed increased to a maximum of approximately 5.6 \(\pm\) 2.7 m/s at a flow rate of 9.6 L/min. This relation can be attributed to an enhanced impulse transfer, while the applied dynamic pressure is rising by an increase in flow rate (see Equation (4)). Furthermore, the change in spatter formation mechanism from 8 m/min (Rosenthal regime) to 10 m/min (single wave regime) contributes to the detachment of larger spatters (see Section 3.1). Due to an increase in windage, the transmitted impulse from the gas to the spatter can be affected, which can result in a change of particle speed. At welding speeds of 12 m/min (Figure 7c) and 16 m/min (Figure 7d), the correlation of particle speed and flow rate shows a similar characteristic as described before at a welding speed of 10 m/min  
By comparing the superimposed images of the reference and the gas-assisted process (Figure 7b, bottom), it is possible to observe a significantly reduction in spatter trajectories at the top side of the specimen due to the local gas supply. This correlates to the determined decrease in loss of mass (see discussion to Figure 4). However, the gas flow did not affect the amount of spatter trajectories at the bottom side. By analyzing the effect of the gas by smaller increments in flow rate (Figure 7c, bottom), it is possible to observe a higher amplification of gas/metal vapor intensity at increasing flow rates. Therefore, it can be assumed that the applied dynamic pressure of the gas flow leads to a change in the outgassing conditions of the keyhole, which was also discussed in the context of schlieren imaging (see Figure 6). In contrast, a change in flow rate showed no influence on the determined angle of spatter detachment. To sum up, it can be stated that an increase in flow rate results in a rise of particle speed, while the formation of spatter is significantly reduced.