# Effect of Local Gas Flow in Full Penetration Laser Beam Welding with High Welding Speeds
## 2 Materials and Methods  
The experimental procedure was carried out for full penetration laser welding of stainless austenitic steel 1.4301 (X5 CrNi18-10, AISI 304). The essential chemical composition of the steel is given in Table 1 below. The bead-on-plate welding process was operated by using an orthogonally arranged disk laser (TruDisk 5000.75, Trumpf Laser und Systemtechnik GmbH, Ditzingen, Germany) which was equipped with a stationary welding optics (Figure 1a). The specifications of the laser system are shown in Table 2 below. The laser power was determined and set for a full penetration process regime, whereby the keyhole is fully penetrating the bottom side of the specimen. The specimens (\(150\times 40\times 2\) mm) were clamped (clamping force of 295 N) at the front of a six-axis robot (KR 60 HA, Kuka AG, Augsburg, Germany) which have been used for the handling process. The experiments were recorded by means of a Photron SA-X2 high-speed camera (10.000 frames per second) which was equipped with a Navitar zoom lens. For a more detailed sight on the effect of the gas flow, the camera was arranged in three different positions (top view (60\({}^{\circ}\)), plane-parallel (0\({}^{\circ}\)) and bottom view (\(-\)60\({}^{\circ}\))). To mask out process emissions, a narrow-band filter with a center wavelength of 808 nm was placed in front of the camera. During the top side and bottom side recording, the process was illuminated using a Cavitar Cavilux laser system (808 nm). In 0\({}^{\circ}\) orientation, the process was recorded based on thermal radiation in order to illustrate the spatter detachment more specific. Subsequently, the captured data were processed using a Matlab-based image analysis. Thereby, it was possible to determine the trajectories of spatter formation (superposition of 250 sequential frames) as well as the angle of detachment and the speed of each spatter. Besides the recordings based on thermal radiation, the Schlieren technique was used in the plane-parallel (0\({}^{\circ}\)) camera orientation. Based on the difference in density and in the refractive index of the supplied gas and the surrounding atmosphere, it was possible to visualize the forming flow pattern. Figure 1b shows a visualized flow pattern of helium, whereby the gas was used exemplary for illustration reasons due the high difference in density. Otherwise, the experiments were carried out using argon, to ensure comparable test conditions to preliminary investigations [29; 30]. Further details in schlieren imaging are given in [31]. To quantify the effect of spatter formation, the resulting loss of material was weighed by using a high-precision balance (Kern PLJ 2000-3A, Balingen-Frommern, Germany). Therefore, the mass of the specimens was determined before and after the welding process, based on a seam length of 150 mm. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Cr** & **Ni** & **C** & **Si** & **Mn** & **Fe** \\ \hline
17.00–19.00 & 8.00–10.00 & \(\leq\)0.07 & \(\leq\)1.00 & \(\leq\)2.00 & Bal. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of stainless steel 1.4301 in mass % according to DIN EN 10088-3. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Wavelength** & **Fiber** & **Focal Length** & **Focal Length** & **Spot** & **Focal** & **Rayleigh** \\  & **Diameter** & **Collimation** & **Focus** & **Diameter** & **Position** & **Length** \({}^{\star}\) \\ \hline
1030 nm & 200 \(\upmu\)m & 200 mm & 280 mm & 274 \(\upmu\)m & 0 mm & 2.38 mm \\ \hline \hline \end{tabular} \({}^{\star}\) Measured by using FocusMonitor (Primes GmbH, Pfungstadt, Germany).  
\end{table} 
Table 2: Laser properties and settings. For the local supply of the gas, the setup was equipped with a nozzle unit (Figure 1c). A three-dimensional positioning of the nozzle was realized by three linear guides and one rotary disk. In this way, the system could be positioned with an accuracy of 0.05 mm and a reproducibility of 0.02 mm.