Universität Stuttgart
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Item Open Access Determination of the thermally induced focal shift of processing optics for ultrafast lasers with average powers of up to 525 W(2018) Faas, Sebastian; Förster, Daniel J.; Weber, Rudolf; Graf, ThomasThe continuous increase of the average laser power of ultrafast lasers is a challenge with respect to the thermal load of the processing optics. The power which is absorbed in an optical element leads to a temperature increase, temperature gradients, changing refractive index and shape, and finally causes distortions of the transmitted beam. In a first-order approximation this results in a change of the focal position, which may lead to an uncontrolled change of the laser machining process. The present study reports on investigations on the focal shift induced in thin plano-convex lenses by a high-power ultra-short pulsed laser with an average laser power of up to 525 W. The focal shift was determined for lenses made of different materials (N-BK7, fused silica) and with different coatings (un-coated, broadband coating, specific wavelength coating).Item Open Access Estimation of the depth limit for percussion drilling with picosecond laser pulses(2018) Förster, Daniel J.; Weber, Rudolf; Holder, Daniel; Graf, ThomasItem Open Access Process limits for percussion drilling of stainless steel with ultrashort laser pulses at high average powers(2022) Brinkmeier, David; Holder, Daniel; Loescher, André; Röcker, Christoph; Förster, Daniel J.; Onuseit, Volkher; Weber, Rudolf; Abdou Ahmed, Marwan; Graf, ThomasThe availability of commercial ultrafast lasers reaching into the kW power level offers promising potential for high-volume manufacturing applications. Exploiting the available average power is challenging due to process limits imposed by particle shielding, ambient atmosphere breakdown, and heat accumulation effects. We experimentally confirm the validity of a simple thermal model, which can be used for the estimation of a critical heat accumulation threshold for percussion drilling of AISI 304 steel. The limits are summarized in a processing map, which provides selection criteria for process parameters and suitable lasers. The results emphasize the need for process parallelization.Item Open Access Tuning the hardness of produced parts by adjusting the cooling rate during laser-based powder bed fusion of AlSi10Mg by adapting the process parameters(2022) Leis, Artur; Traunecker, David; Weber, Rudolf; Graf, ThomasThe mechanical properties of parts produced by laser-based powder bed fusion (LPBF) are mainly determined by the grain structure in the material, which is governed by the cooling rate during solidification. This cooling rate strongly depends on the scan velocity and the absorbed laser power. Experiments with varying process parameters were performed to develop and validate an analytical model that predicts the hardness of printed AlSi10Mg parts. It was found that it is possible to tune the hardness of additively manufactured parts of AlSi10Mg in a range between 60 ± 9 HV0.5 and 100 ± 10 HV0.5 by adjusting the cooling rate during solidification with adapted process parameters.Item Open Access Heat accumulation during pulsed laser materials processing(2014) Weber, Rudolf; Graf, Thomas; Berger, Peter; Onuseit, Volkher; Wiedenmann, Margit; Freitag, Chistian; Feuer, AnneLaser materials processing with ultra-short pulses allows very precise and high quality results with a minimum extent of the thermally affected zone. However, with increasing average laser power and repetition rates the so-called heat accumulation effect becomes a considerable issue.Item Open Access Process window for highly efficient laser-based powder bed fusion of AlSi10Mg with reduced pore formation(2021) Leis, Artur; Weber, Rudolf; Graf, ThomasItem Open Access Analytical model for the depth progress of percussion drilling with ultrashort laser pulses(2021) Holder, Daniel; Weber, Rudolf; Graf, Thomas; Onuseit, Volkher; Brinkmeier, David; Förster, Daniel J.; Feuer, AnneA simplified analytical model is presented that predicts the depth progress during and the final hole depth obtained by laser percussion drilling in metals with ultrashort laser pulses. The model is based on the assumption that drilled microholes exhibit a conical shape and that the absorbed fluence linearly increases with the depth of the hole. The depth progress is calculated recursively based on the depth changes induced by the successive pulses. The experimental validation confirms the model and its assumptions for percussion drilling in stainless steel with picosecond pulses and different pulse energies.Item Open Access High-quality percussion drilling with ultrashort laser pulses(2021) Feuer, Anne; Weber, Rudolf; Feuer, R.; Brinkmeier, David; Graf, ThomasThe influence of the laser fluence on the quality of percussion-drilled holes was investigated both experimentally and by an analytical model. The study reveals that the edge quality of the drilled microholes depends on the laser fluence reaching the rear exit of the hole and changes with the number of pulses applied after breakthrough. The minimum fluence that must reach the hole’s exit in order to obtain high-quality microholes in stainless steel was experimentally found to be 2.8 times the ablation threshold.Item Open Access Dry metal forming using volatile lubricants injected into the forming tool through flow-optimized, laser-drilled microholes(2020) Henn, Manuel; Reichardt, Gerd; Weber, Rudolf; Graf, Thomas; Liewald, MathiasA novel tribologic system was developed in which volatile lubricants (carbon dioxide-CO2 or nitrogen-N2) were used as a substitute for mineral oil-based lubricants in deep drawing processes. This process allows an intermediate medium to be introduced into the tool contact surfaces under high pressure by flow-optimized, laser-drilled microholes. This eliminates the need for subsequent cost-intensive cleaning processes as volatile lubricants evaporate while expanding to ambient pressure without leaving any residue. This article gives an overview of the current findings to enable and characterize the novel tribologic system. The areas of microhole laser drilling by ultrashort pulsed laser radiation, characterization of the novel tribologic system and realization of the system using a prototype tool will be described.Item Open Access Influence of pulse duration on X-ray emission during industrial ultrafast laser processing(2022) Holland, Julian; Weber, Rudolf; Sailer, Marc; Graf, ThomasSoft X-ray emissions during the processing of industrial materials with ultrafast lasers are of major interest, especially against the background of legal regulations. Potentially hazardous soft X-rays, with photon energies of >5 keV, originate from the fraction of hot electrons in plasma, the temperature of which depends on laser irradiance. The interaction of a laser with the plasma intensifies with growing plasma expansion during the laser pulse, and the fraction of hot electrons is therefore enhanced with increasing pulse duration. Hence, pulse duration is one of the dominant laser parameters that determines the soft X-ray emission. An existing analytical model, in which the fraction of hot electrons was treated as a constant, was therefore extended to include the influence of the duration of laser pulses on the fraction of hot electrons in the generated plasma. This extended model was validated with measurements of H (0.07) dose rates as a function of the pulse duration for a constant irradiance of about 3.5 × 1014 W/cm2, a laser wavelength of 800 nm, and a pulse repetition rate of 1 kHz, as well as for varying irradiance at the laser wavelength of 1030 nm and pulse repetition rates of 50 kHz and 200 kHz. The experimental data clearly verified the predictions of the model and confirmed that significantly decreased dose rates are generated with a decreasing pulse duration when the irradiance is kept constant.