# 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_
#### 3.3.5 Effect on Keyhole Fluctuations  
Based on the investigations of the static keyhole dimensions, the keyhole length was analyzed depending on time by quantifying the frequency and the normalized amplitude of keyhole fluctuation, in order to specify the effect of the gas flow on keyhole dynamics (Figure 9). For this purpose, the length of the keyhole was measured in 250 sequential frames by an increment of 0.01 ms. Subsequently, the determined raw data was processed using a median filter to limit measurement irregularities. Following, using a fast Fourier transformation algorithm (FFT), it was possible to calculate the normalized amplitude and the frequency of keyhole fluctuations. In order to approve the found solution and to get an information about the harmonicity of the fluctuation, a sine was fitted into the filtered data based on the determined oscillating period.  
By analyzing the raw data of the keyhole length of the reference process at a welding speed of 12 m/min as function of time (Figure 9a), it was possible to observe a high noise in data. Using a median filter method, the curve was characterized by a change in oscillating amplitude and oscillating period. These characteristics could be also seen by all tested welding speeds of 8 m/min, 10 m/min and  
Figure 8: Keyhole dimensions as function of welding speed and flow rate: (**a**) Keyhole width; (**b**) Keyhole length.  
16 m/min, respectively, the local gas supply (Figure 9b). In contrast to the reference process, the local gas supply seems to increase the oscillating amplitude.  
Based on this data set, the frequency spectrum has been determined using a fast Fourier transformation algorithm (FFT). The spectrum of the reference process indicates a high inharmonicity of the keyhole oscillation by showing multiple peaks of the normalized amplitude (Figure 9c). By analyzing the spectrum of the gas-assisted process, these inharmonicity could be also seen (Figure 9d). By comparing the determined frequencies (Table 4), no significant effect on gas flow neither on welding speed could be observed. Following, a mean frequency of keyhole fluctuation of \(\approx\)720-760 Hz was found. In contrast, the normalized amplitude was rising by using the local gas flow. This can be attributed to an additional impulse transfer from the gas to the melt, which can result in an amplification of the keyhole oscillation. However, based on the determined reduction in the loss of mass due to the local gas supply, this is contrary to the state of the art regarding the condition of keyhole stability (see Section 1). Besides this relation, the normalized amplitude increased for an enhancement in welding speed. Thereby, these characteristics can be traced back to the rising melt flow velocities at high welding speeds, which results in an increase in melt pool dynamics [3; 4].  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Process Parameters**} & \multicolumn{2}{c}{**8 m/min \({}^{*}\)**} & \multicolumn{2}{c}{**10 m/min**} & \multicolumn{2}{c}{**12 m/min**} & \multicolumn{2}{c}{**16 m/min**} \\  & **2.700 W** & \multicolumn{2}{c}{**3.100 W**} & \multicolumn{2}{c}{**3.700 W**} & \multicolumn{2}{c}{**4.800 W**} \\ \hline
**Mode** & Ref & Gas & Ref & Gas & Ref & Gas & Ref & Gas \\
**Flow rate in L/min** & / & 8 & / & 6.4 & / & 4.8 & / & 4.8 \\
**Frequency in Hz** & 720 \({}^{*}\) & 440 \({}^{*}\) & 760 & 760 & 720 & 760 & 640 & 760 \\
**Normalized amplitude** & 0.012 \({}^{*}\) & 0.016 \({}^{*}\) & 0.016 & 0.048 & 0.041 & 0.048 & 0.038 & 0.058 \\ \hline \hline \end{tabular} \({}^{*}\)High noise in measurement data  
\end{table}
Table 4: Frequency and normalized amplitude of keyhole fluctuation determined by FFT.  
Figure 9: Comparison of reference process and gas-assisted process at a welding speed of 12 m/min regarding: (**a,b**) Keyhole length as function of time; (**c,d**) Frequency spectrum of keyhole oscillation.