## 2 Materials and Methods
### Experimental Setup  
In the welding process, a TruDisk 12002 disk laser (Trumpf GmbH & Co. KG, Ditzingen, Germany) operating at a wavelength of \(\lambda\) = 1030 nm with a minimum beam parameter product of \(BPP_{\text{min}}\) = 8 mm\(\cdot\)mrad is used as the laser source. With a core diameter of the optical fiber of \(d_{\text{fiber}}\) = 200 \(\upmu\)m and an image ratio of 3:2, a focal diameter of the laser spot of \(d_{\text{focus}}\) = 300 \(\upmu\)m is achieved theoretically and is verified by a beam profile measurement. The Rayleigh length is given by \(z_{\text{R}}\) = 2.52 mm. The welding process is observed by high-speed imaging from the side, as well as by high-speed pyrometry. A schematic view of the experimental setup is presented in Figure 4. The high-speed imaging is performed using a Phantom VEO 410L (Vision Research, Wayne, NJ, USA) in combination with a Cavilux HF illumination laser (Cavitar Ltd, Tampere, Finland) operating at a wavelength of \(\lambda_{\text{ill}}\) = 810 nm and a narrow band-pass filter with a central wavelength of \(\lambda_{\text{BP}}\) = 810 nm with a full width at the half-maximum of \(FWHM\) = 12 nm. This way, the camera can effectively be protected from the scattered radiation from the welding process. The high-speed video is recorded at a framerate of \(f_{\text{HS}}\) = 17 kHz and is used for the evaluation of the process stability and keyhole fluctuations, as well as for the localization of the keyhole position relative to the probe. The temperature  
Figure 3: Schematic of the weld sample design with \(l\) = 60 mm, \(w\) = 40 mm, and \(h\) = 10.5 mm. The length of the probe was set to \(l_{\text{probe}}\) = 8 mm, whereas the \(z\)-position of the probe beneath the sample surface was meant to be varied in order to enable the measurement of a depth-dependent temperature profile. In this setup, the \(z\)-position of the probe was fixed to \(h_{\text{meas}}\) = 4 mm. Figure 2: Preparation of the refractory tantalum probes: (**a**) deep-drawing process of the tantalum foil, and (**b**) a finished, ready-to-use probe. measurement is executed using a KGA 740 (Kleiber Infrared GmbH, Unterwellenborn, Germany), a small-band pyrometer operating at wavelengths in the infrared spectrum at \(\lambda_{\text{pyro}}\) = 2000 nm to 2200 nm and capable of recording at sampling rates of up to \(f_{\text{pyro}}\) = 166 kS/s. The measuring spot has a minimum diameter of \(d_{\text{spot}}\) = 0.4 mm and is focused inside the tantalum tube and onto the tantalum foil at the end of the probe. Thus, the pyrometer records the temperature at a fixed point throughout the process from the beginning until the moment the laser reaches the probe. Due to the laser movement, and consequently, the movement of the melt pool, the relative position between the keyhole and the tantalum foil changes over time at the welding speed. After the destruction of the tantalum foil, the aluminum melt can penetrate the measuring channel so that the measuring spot of the pyrometer moves along with the melt inside the tube. At the same time, the tantalum temperature window is destroyed, and a material transition for the emissivity coefficient changing from the probe material of tantalum to the sheet material of aluminum must be considered when calculating the absolute temperature from the measured voltage signal of the pyrometer. For the validation of the measured temperature signals, additional experiments based on the same set of process parameters are conducted using the H322+ two-color pyrometer (Sensortherm GmbH, Sulzbach, Germany) operating at two discrete wavelengths, namely \(\lambda_{1}\) = 1640 nm and \(\lambda_{2}\) = 1400 nm, with a maximum sampling rate of \(f_{\text{pyro}}\) = 20 kS/s. The two-color pyrometry is almost independent of the measuring area and always gives values very close to the maximum temperature in the focused region.