## 3 Results and Discussion  
The temperature measurement inside the refractory probe was initially conducted with a single parameter set that resulted in a comparatively good quality of the weld seam when welding pure aluminum. The welding velocity was chosen to lie within the range of the Rosenthal regime so that the vapor capillary was expected to have circular cross-sections perpendicular to the laser beam axis with a homogeneous temperature distribution along its contour. The laser was operated at a power of \(P\) = 5000 W with a focus shifted below the workpiece surface, thereby enlarging the keyhole diameter. The defocus was chosen to result in a keyhole diameter on the workpiece surface \(d_{\rm key}\) between 600 \(\upmu\)m and 900 \(\upmu\)m, based on Skupin [5] and the previously measured beam profile. The welding velocity was set to \(v_{\rm weld}\) = 50 mm/s, resulting in an energy per unit length of \(E_{\rm dist}\) = 100 J/mm. An example temperature curve is depicted in Figure 10. The timespan during which temperature values exceeding the minimum temperature of the pyrometer were obtained is referred to as region of measurement (RoM). For better orientation, the origins of the time and length scales were chosen to coincide with the moment or position, respectively, where the center axis of the laser beam reached the tip of the probe. This convention is used for the presentation of all the following results. Although the curve depicted in Figure 10 shows temperature values from past the keyhole position containing different temperature maxima, as well as a pronounced cooling-down phase, according to the experimental design, only measurements taken from the keyhole prerunning zone can contain reliable information on the temperature. The region past the keyhole cannot be interpreted due to destruction of the probe. Nonetheless, information like the course of the cooling-down of the molten material obtained from the trailing region behind the keyhole can be used for validation of the temperature calculations by indicating whether the temperature curve matches significant points of the phase transitions. Synchronizing the high-speed recordings with the pyrometric measurements, a small region is identified between \(t=-25\) ms and \(t=-10\) ms where the position of the keyhole front coincides with the position of the tantalum foil, and, therefore, the temperature of the front keyhole should be approximated by the measurement. Inside this short region, the maximum temperatures recorded from different experiments range from \(T_{\rm high}\) = 1415 K to 2398 K. The fact that the initial slopes of all recorded temperature curves are steep and start at high values close to the melting temperature of the base material is probably due to imperfections in the drill hole geometries. A cross-section polish of the hole shows that, at the bottom, the shape shifts from cylindrical to conical. In the bottom region, the diameter decreases with the increasing depth; thus, the refractory probe cannot establish a laminar contact with the bottom of the drill hole. The small space between the tantalum foil sealing the probe and the bottom of the drill hole traps air, which acts as a thermal isolator. Not enough heat for the foil to emit radiation in the range detectable by the pyrometer is transferred from the weld material until the melt reaches the cavity and starts transferring heat to the foil directly by heat conduction. Therefore, the temperature curves exhibit almost no trace of the prerunning heat. A more problematic effect on the experimental procedure resulting from the entrapped air is that, depending on the volume of air and the stability of the melt pool, the air can expand rapidly, causing a massive blow-out and disturbing the measurements completely. In the measurements conducted during this study, such a blow-out at the point of interest occurred in almost 50% of cases.