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During laser penetration, the irradiated samples form a melt pool before perforation. Knowledge of the dynamics of this melt pool is of interest for the correct physical description of the process and leads to improved simulations. However, a direct investigation, especially at the location of high-power laser interaction with large spot diameters in the centimeter range is missing until now. Here, the applicability of 2D triangulation for surface topology observations is demonstrated. With the designed bidirectional 2D triangulation setup, the material cross-section is measured by profile detection at the front and back side. This allows a comprehensive description of the penetration process to be established, which is important for a detailed explanation of the process. Specific steps such as surface melting, indentations, protrusions during melt pool development and their dynamics, and the perforation are visualized, which were unknown until now. Furthermore, a scanning 3D triangulation setup is developed to obtain more information about the entire melt pool at the front side, and not just a single intersection line. The measurements exhibit a mirror-symmetric melt pool and the possibility to extrapolate from the central profile to the outer regions in most cases.

In this work, the energy transfer from intense continuous-wave laser beams with a wavelength of 1070 nm, a power in the kilowatt range, and with diameters in the millimeter and centimeter range to metal samples is investigated. While the absorptivity of iron and steel samples is almost constant for laser intensities below 3.4 kW/cm2, a decrease in the absorptivity is observed for higher intensities which is attributed to the formation of a vapor plume in the interaction zone. The dynamics of the formation and expansion as well as the emission of light in the visible spectral range from the vapor plume are further characterized with a fixed beam diameter of 2.6 mm at a laser power of 10 kW in detail for iron and aluminum samples. The analysis of high speed video sequences yields expansion velocities of the vapor plume of 5.0 m/s for the iron and 0.29 m/s for the aluminum samples. In the spectra from the aluminum samples, emission lines from atomic aluminum as well as emission bands from molecular aluminum monoxide are identified and allow for the estimation of the basic thermodynamic parameters. A special focus is on the investigation of the effect of vapor and plasma formation on the energy transfer from the laser to the sample and on the analysis of the role of inverse bremsstrahlung in this process. The measurements indicate that the metal vapor is partially ionized and that there is a significant contribution of inverse bremsstrahlung to the absorption of laser energy in the partially ionized vapor plume.

The phenomena occurring during laser-metal interaction depend on a variety of parameters such as laser wavelength, intensity, and thermophysical and optical properties of the irradiated material. This work comprises a detailed study of the perforation process of thin iron and steel plates under CW laser radiation at a wavelength of 1.07 mm. Experiments were carried out over a wide range of intensities between 0.1 and 100 kW/cm2, and with beam radii in the millimeter and centimeter range. Additionally, we describe a method applying contact-free radiative temperature measurements, which allows the measurement of melt-through times without an exact knowledge of the plate's spectral surface emissivity. A detailed numerical model is presented and employed to analyze, interpret, and predict data gathered from the experiments. The model is shown to produce accurate predictions and is in good agreement with experimental data. Furthermore, our results indicate that in the investigated regime perforation is governed by gravitational and surface forces. Based on this hypothesis, a criterion for the onset of the perforation explaining the observed local minimum in the perforation time versus beam radius curve is presented.

One phenomenon observed at hypervelocity impacts (HVI) is the generation of plasma with a very short lifetime of a few microseconds. Due to this short lifetime, characteristic plasma parameters such as the electron density and the electron temperature of the expanding plasma were not investigated thoroughly in the past. This paper will present a method to measure these parameters with a time-resolution of 500 ns for the full period of impact plasma expansion and discuss results gained in impact experiments. At the Fraunhofer EMI, impact experiments on solar panels were performed using a two-stage light-gas gun to accelerate aluminum spheres with a diameter of a few millimeters up to a speed of 8 km/s. A measurement system consisting of a spectrograph and a streak camera was applied for time-resolved spectroscopy of the impact plasma. To derive plasma properties, the recorded streak image was evaluated using different methods for different expansion states of the plasma cloud. The spectra show strong self-absorption lines in the first microseconds of expansion. In the present work, these features are explained by the electron density and temperature gradient in the plasma. For the determination of electron temperature and density, a one-dimensional radiative transfer model was adapted to the measured spectra. After 2 μs of expansion, the plasma is optically thin and emission lines can be observed. For this expansion state, the electron temperature was determined by the ratio of line to continuum radiation, whereas the electron density was determined through the line broadening due to the Stark effect. Using these methods, it was found that the electron temperature decreases in the first 3 μs of propagation from 45,000 K to 2,000 K in the experiments performed. The electron density decreases from 10 19 cm -3 to 10 17 cm -3 .