# Investigations of the Absorption Front in High-Speed Laser Processing Up to 600 m/min
## 2 Materials and Methods  
The experiments were carried out as bead-on-plate welds with stainless austenitic steel sheets (AISI 304, X5 CrNi18-10). The essential chemical composition of the steel is given below in Table 1.  
In order to implement the relative motion between the workpiece and the laser beam, a 400 W single-mode fiber laser (TruFiber 400, Trumpf Laser GmbH, Ditzingen, Germany) was equipped with a 2D galvanometric scanning system with an f-Tetha lens (PFO 20, Trumpf Laser GmbH, Ditzingen, Germany). The technical data of the laser and scanning system are listed in Table 2. The focal position was chosen based on the maximum resulting penetration depth in cross-sections of bead-on-plate welds. The laser power was set to the maximum of 380 W for all experiments, while the laser was operated in cw-mode.  
A 5 mm thick borosilicate glass plate was attached to the steel sheets for the trials, as depicted schematically in the experimental setup in Figure 1. Borosilicate glass was used in this setup because of its high thermal shock resistance and low thermal expansion. The samples were cut to size using wire-cut EDM to achieve high-quality cut edges. This realized a minimum gap between the glass plate and the sheets along the edge during clamping. Due to the small focal diameter of 16.5 microns, the challenge here was to place the laser focus on the edge of the sample with its complete diameter. Even the slightest lateral displacements in both \(y\) directions lead to unusable results. The authors henceforth refer to this setup as the half-section setup.  
For a side view, the high-speed camera (Fastcam SA-X2, Photron Deutschland GmbH, Reutlingen, Germany) was positioned perpendicular to the \(x\)-\(z\) plane with a distance of  
\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 [15]).  
Figure 1: Schematic depiction of the experimental setup for high-speed recordings in the half-section setup.  
about 100 mm. A Navitar 12\(\times\) zoom lens was used to achieve sufficient magnification for the measurements of several dimensions, even in small processing zones. A high-power LED system with a wavelength of 630 nm in combination with a narrow band-pass filter in the optical path of the camera was used to illuminate the processing zone. This setup allowed for the inclination angle of the absorption front to be determined, in addition to the penetration depth. The high-speed records in this work were captured at a framerate of 150,000 fps. Another two camera setups were applied in this study to capture further information about the processing zone. Their setups are described in detail in the respective results sections.  
The loss of mass of the samples was determined via weighings before and after the laser process with a precision balance (ME 2355, Sartorius AG, Goettingen, Germany). Because of the high resolution of the balance (0.01 mg), the samples received a two-step cleaning procedure with acetone and isopropyl alcohol in an ultrasonic bath prior to each weighing.  
The sample number \(n\) is given in each diagram. As such, the error bars show the corresponding standard deviation of the sample. Furthermore, for a concise presentation of the results, the processing speeds in this work are henceforth given in meters per second.