@inproceedings{SchwarzRungEsenetal.2020, author = {Schwarz, Simon and Rung, Stefan and Esen, Cemal and Hellmann, Ralf}, title = {All laser-based fabrication of optical elements}, series = {Proceedings of SPIE 11270, Frontiers in Ultrafast Optics: Biomedical, Scientific, and Industrial Applications XX}, booktitle = {Proceedings of SPIE 11270, Frontiers in Ultrafast Optics: Biomedical, Scientific, and Industrial Applications XX}, year = {2020}, subject = {Ultrakurzzeitlaser}, language = {de} } @article{ZettlEsenHellmann2023, author = {Zettl, Julian and Esen, Cemal and Hellmann, Ralf}, title = {Fundamental Considerations and Analysis of the Energy Distribution in Laser Turning with Ultrashort Laser Pulses}, series = {mdpi Micromachines}, volume = {14}, journal = {mdpi Micromachines}, number = {10}, doi = {10.3390/mi14101838}, pages = {1 -- 15}, year = {2023}, abstract = {This article discusses the process of the laser turning of rotational symmetric, cylindrical components using ultrashort laser pulses with respect to the geometrical conditions and the resulting energy distribution during the laser turning process. As a result, process predictions and potential process optimizations are feasible. Particular attention is drawn to the laser spot formation on the cylindrical surface of the work piece in conjunction with the positioning of the laser beam relative to the rotation axis of the specimen. Based on fundamental calculations and experimental results, an optimum processing strategy is discussed, whereat the use of a trepanning optic in the laser turning process and the forming of a particular surface structure is additionally being issued.}, subject = {Ultrakurzzeitlaser}, language = {en} } @article{SommerEsenHellmann2023, author = {Sommer, David and Esen, Cemal and Hellmann, Ralf}, title = {Static and Dynamic Mechanical Behaviour of Hybrid-PBF-LB/M-Built and Hot Isostatic Pressed Lattice Structures}, series = {Mechanical Properties of Polymeric, Metallic, and Composite Materials}, volume = {16}, journal = {Mechanical Properties of Polymeric, Metallic, and Composite Materials}, number = {9}, doi = {https://doi.org/10.3390/ma16093556}, pages = {1 -- 14}, year = {2023}, abstract = {We report on a comprehensive study of the mechanical properties of maraging steel body-centred cubic lattice structures fabricated by a hybrid additive manufacturing technology that combines laser powder bed fusion with in situ high-speed milling. As the mechanical properties of additive manufactured components are inferior to, e.g., cast components, surface modifications can improve the mechanical behaviour. Different hybrid additive manufacturing technologies have been designed using additive and subtractive processes, improving process quality. Following this, mechanical testing is performed with respect to static tensile properties and dynamic stress, hardness, and porosity, comparing specimens manufactured by laser powder bed fusion only to those manufactured by the hybrid approach. In addition, the influence of different heat-treatment techniques on the mechanical behaviour of the lattice structures is investigated, namely solution and aging treatment as well as hot isostatic pressing. Thus, the influence of the superior surface quality due to the hybrid approach is evaluated, leading to, e.g., an offset of about 14-16\% for the static testing of HIP lattice structures. Furthermore, the dynamic load behaviour can be improved with a finished surface, heading to a shift of the different zones of fatigue behaviour in the testing of hybrid-built specimens.}, subject = {Isostatisches Heißpressen}, language = {de} } @inproceedings{MarxEsenLutzetal.2023, author = {Marx, Jan and Esen, Cemal and Lutz, Christian and Hellmann, Ralf and Ostendorf, Andreas}, title = {Holographic tuning of physical axicons}, volume = {2023}, number = {154}, publisher = {LiM 2023 Proceedings}, pages = {1 -- 9}, year = {2023}, abstract = {Axicon generated Bessel beams are a popular tool for high aspect ratio precision laser drilling. Spot diameter and working distance are given by the geometric parameters of the axicon and the wavelength used. Thus, it is difficult to manipulate the beam shape of a Bessel beam for a given setup. Spatial light modulators (SLMs) overcome limitations in flexibility. However, due to the limited phase shift of SLMs, only Bessel beams with flat cone angles and large focal length can be generated. In this contribution, an approach for generating Bessel beams with a shorter, but tunable focal length is presented. A physical axicon was combined with an SLM. A holographic image of a negative axicon is put on the SLM to generate a ring beam, which is focused by a subsequent physical axicon to get a small focal diameter. Thus, different sized high aspect ratio micro holes can be drilled without using any moving components.}, subject = {Bessel-B{\"u}ndel}, language = {en} } @article{LutzHelmTschirpkeetal.2023, author = {Lutz, Christian and Helm, Jonas and Tschirpke, Katrin and Esen, Cemal and Hellmann, Ralf}, title = {Drilling Sequence Optimization Using Evolutionary Algorithms to Reduce Heat Accumulation for Femtosecond Laser Drilling with Multi-Spot Beam Profiles}, series = {MDPI Materials}, volume = {2023}, journal = {MDPI Materials}, number = {16/5775}, doi = {10.3390/ma16175775}, pages = {1 -- 12}, year = {2023}, abstract = {We report on laser drilling borehole arrays using ultrashort pulsed lasers with a particular focus on reducing the inadvertent heat accumulation across the workpiece by optimizing the drilling sequence. For the optimization, evolutionary algorithms are used and their results are verified by thermal simulation using Comsol and experimentally evaluated using a thermal imaging camera. To enhance process efficiency in terms of boreholes drilled per second, multi-spot approaches are employed using a spatial light modulator. However, as higher temperatures occur across the workpiece when using simultaneous multi-spot drilling as compared to a single-spot process, a subtle spatial distribution and sequence of the multi-spot approach has to be selected in order to limit the resulting local heat input over the processing time. Different optimization approaches based on evolutionary algorithms aid to select those drilling sequences which allow for the combination of a high efficiency of multi-spot profiles, a low-generated process temperature and a high-component quality. In particular, using a 4 x 4 laser spot array allows for the drilling of 40,000 boreholes in less than 76 s (526 boreholes/s) with a reduced temperature increase by about 35\%, as compared to a single spot process when employing an optimized drilling sequence.}, subject = {Laserbohren}, language = {en} } @article{LutzSchwarzMarxetal.2023, author = {Lutz, Christian and Schwarz, Simon and Marx, Jan and Esen, Cemal and Hellmann, Ralf}, title = {Multi-Bessel Beams Generated by an Axicon and a Spatial Light Modulator for Drilling Applications}, series = {MDPI Photonics}, volume = {2023}, journal = {MDPI Photonics}, number = {10/413}, doi = {10.3390/photonics10040413}, pages = {1 -- 9}, year = {2023}, abstract = {We report on an optical setup to generate multi-Bessel beam profiles combining a refractive axicon and a spatial light modulator. Based on their particular beam profile, Bessel beams offer advantageous properties for micro drilling processes and internal volume processing, especially for transparent materials. In addition, the laser power of industrial, ultrashort pulsed lasers has increased significantly over the last few years, offering the possibility for highly efficient processes using multi-spot profiles. Our optical concept combines the dynamic possibilities of beam splitting using a spatial light modulator with the benefits of Bessel beams, which facilitates multi-Bessel beam processing. Beside the simulation and experimental evaluation of the generated multi-Bessel beams, we exemplify the applicability of the developed module for the perforation of thin metal foils by micro drilling.}, subject = {Bessel-B{\"u}ndel}, language = {en} }