@article{KeferZettlEsenetal.2022, author = {Kefer, Stefan and Zettl, Julian and Esen, Cemal and Hellmann, Ralf}, title = {Femtosecond Laser-Based Micromachining of Rotational-Symmetric Sapphire Workpieces}, series = {Materials}, volume = {15}, journal = {Materials}, number = {18}, doi = {10.3390/ ma15186233}, pages = {6233 -- 6233}, year = {2022}, abstract = {Sapphire is a robust and wear-resistant material. However, efficient and high-quality micromachining is still a challenge. This contribution demonstrates and discusses two novels, previously unreported approaches for femtosecond laser-based micromachining of rotational-symmetric sapphire workpieces, whereas both methods are in principal hybrids of laser scanning and laser turning or laser lathe. The first process, a combination of a sequential linear hatch pattern in parallel to the workpiece's main axis with a defined incremental workpiece rotation, enables the fabrication of sapphire fibers with diameters of 50 µm over a length of 4.5 mm. Furthermore, sapphire specimens with a diameter of 25 µm over a length of 2 mm can be fabricated whereas an arithmetical mean height, i.e., Sa parameter, of 281 nm is achieved. The second process combines a constant workpiece feed and orthogonal scanning with incremental workpiece rotation. With this approach, workpiece length limitations of the first process are overcome and sapphire fibers with an average diameter of 90 µm over a length of 20 cm are manufactured. Again, the sapphire specimen exhibits a comparable surface roughness with an average Sa value of 249 nm over 20 cm. Based on the obtained results, the proposed manufacturing method paves an innovative and flexible, all laser-based way towards the fabrication or microstructuring of sapphire optical devices, and thus, a promising alternative to chemical processes.}, subject = {Femtosekundenlaser}, language = {en} } @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{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{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} } @article{BischoffMueckeRothetal.2022, author = {Bischoff, Kay and M{\"u}cke, Dominik and Roth, Gian-Luca and Esen, Cemal and Hellmann, Ralf}, title = {UV-Femtosecond-Laser Structuring of Cyclic Olefin Copolymer}, series = {Polymers}, volume = {14}, journal = {Polymers}, number = {14}, doi = {https://doi.org/10.3390/polym14142962}, pages = {2962 -- 2962}, year = {2022}, abstract = {We report on the laser ablation of cyclic olefin copolymer using an amplified ultrashort pulsed laser in the ultraviolet spectral range. In addition to a high ablation depth per laser-structured layer up to 74 μm at a fluence of 22 J cm-2, an excellent mean roughness 𝑅𝑎 of laser-patterned surfaces down to 0.5 μm is demonstrated. Furthermore, with increasing fluence, increasing ablation efficiencies up to 2.5 mm3 W-1 min-1 are determined. Regarding the quality of the ablation, we observed steep ablation flanks and low debris formation, though for fluences above 10.5 J cm-2 the formation of troughs was observed, being attributed to multiple reflections on the ablation flanks. For comparison, laser ablation was performed under identical conditions with an infrared laser wavelength. The results highlight that UV ablation exhibits significant advantages in terms of ablation efficiency, surface roughness and quality. Moreover, our results show that a larger UV focus spot accelerates the ablation process with comparable quality, paving the way for high-power UV ultrashort pulsed lasers towards an efficient and qualitative tool for the laser machining of cyclic olefin copolymer. The production of complex microfluidics further underlines the suitability of this type of laser.}, subject = {Femtosekundenlaser}, language = {en} } @article{BischoffKeferWienkeetal.2023, author = {Bischoff, Kay and Kefer, Stefan and Wienke, Alexander and Overmeyer, Ludger and Kaierle, Stefan and Esen, Cemal and Hellmann, Ralf}, title = {Integration of Bragg gratings in aerosol-jetted polymer optical waveguides for strain monitoring capabilities}, series = {Optics Letters}, volume = {48}, journal = {Optics Letters}, number = {7}, doi = {https://doi.org/10.1364/OL.481801}, pages = {1778 -- 1781}, year = {2023}, abstract = {We demonstrate and discuss the integration of Bragg gratings in aerosol-jetted polymer optical waveguides, produced in the optical assembly and connection technology for component-integrated bus systems (OPTAVER) process. By using a femtosecond laser and adaptive beam shaping, an elliptical focal voxel generates different types of single pulse modification by nonlinear absorption in the waveguide material, which are arranged periodically to form Bragg gratings. Integration of a single grating structure or, alternatively, an array of Bragg grating structures in the multimode waveguide yields a pronounced reflection signal with typical multimodal properties, i.e., a number of reflection peaks with non-Gaussian shapes. However, the main wavelength of reflection, located around 1555 nm, is evaluable by means of an appropriate smoothing algorithm. When loaded by mechanical bending, a pronounced Bragg wavelength shift of this reflected peak up to 160 pm is detected. This demonstrates that the additively manufactured waveguides can be used not only for signal transmission but also as a sensor.}, subject = {Bragg-Reflektor}, language = {de} } @article{BischoffEsenHellmann2023, author = {Bischoff, Kay and Esen, Cemal and Hellmann, Ralf}, title = {Preparation of Dispersed Copper(II) Oxide Nanosuspensions as Precursor for Femtosecond Reductive Laser Sintering by High-Energy Ball Milling}, series = {Nanomaterials}, volume = {13}, journal = {Nanomaterials}, number = {19}, doi = {https://doi.org/10.3390/nano13192693}, pages = {2693 -- 2693}, year = {2023}, abstract = {This contribution demonstrates and discusses the preparation of finely dispersed copper(II) oxide nanosuspensions as precursors for reductive laser sintering (RLS). Since the presence of agglomerates interferes with the various RLS sub-processes, fine dispersion is required, and oversized particles must be identified by a measurement methodology. Aside from the established method of scanning electron microscopy for imaging individual dried particles, this work applies the holistic and statistically more significant laser diffraction in combination with dynamic image analysis in wet dispersion. In addition to direct ultrasonic homogenization, high-energy ball milling is introduced for RLS, to produce stable nanosuspensions with a high fine fraction, and, above all, the absence of oversize particles. Whereas ultrasonic dispersion stagnates at particle sizes between 500 nm and 20 μm, even after 8 h, milled suspension contains a high proportion of finest particles with diameters below 100 nm, no agglomerates larger than 1 μm and a trimodal particle size distribution with the median at 50 nm already, after 100 min of milling. The precursor layers produced by doctor blade coating are examined for their quality by laser scanning microscopy. The surface roughness of such a dry film can be reduced from 1.26 μm to 88 nm by milling. Finally, the novel precursor is used for femtosecond RLS, to produce homogeneous, high-quality copper layers with a sheet resistance of 0.28 Ω/sq and a copper mass concentration of 94.2\%.}, subject = {Laserstrahlsintern}, language = {en} } @inproceedings{BischoffEsenHellmann2023, author = {Bischoff, Kay and Esen, Cemal and Hellmann, Ralf}, title = {Acceleration of Femtosecond Reductive Laser Sintering of Copper(II) Oxide for Conductive Copper Patterns on Cyclic Olefin Copolymers}, series = {International Congress on Applications of Lasers \& Electro-Optics (ICALEO) 2023}, booktitle = {International Congress on Applications of Lasers \& Electro-Optics (ICALEO) 2023}, year = {2023}, abstract = {We report on the acceleration of femtosecond reductive laser sintering of copper (II) oxide on the surface of transparent cyclic olefin copolymers (COC) by varying the focus size to generate metallic copper (Cu) layers. By using different focusing conditions, different process speeds are achieved, and the structural accuracies are controlled. The formation of Cu lines is observed for different focus sizes at various scan speeds, which is attributed to laser exposure duration and chemical reaction time. The formed lines are categorized and measured by optical microscopy and provide the basis for two-dimensional Cu surfaces. By hatching, high quality conductive films are produced with an increased focus diameter of up to 400 µm at a scan speed of 400 mm/s and a hatch distance of 250 µm resulting in a multiple increased process speed by a factor of thirteen compared to previous research. Using 4-tip measurement , a low sheet resistance of 0.165 Ω/sq was validated on these Cu films, which have an atomic Cu content of 89.5\%.}, subject = {Femtosekundenlaser}, language = {en} } @inproceedings{BischoffMueckeEsenetal.2023, author = {Bischoff, Kay and M{\"u}cke, Dominik and Esen, Cemal and Hellmann, Ralf}, title = {UV-Femtosecond-Laser Structuring of Silicon Carbide}, series = {International Congress on Applications of Lasers \& Electro-Optics (ICALEO) 2023}, booktitle = {International Congress on Applications of Lasers \& Electro-Optics (ICALEO) 2023}, year = {2023}, abstract = {We report on ultraviolet ultrashort pulsed laser processing of silicon carbide. Laser ablated cavities are evaluated with respect to their ablation rates, surface roughness and overall quality by varying the laser pulse fluence. Using this type of laser, high edge quality and surface roughness down to 107 nm can be achieved with an ablation efficiency of up to 0.228 mm 3 W -1 min -1. Functional surfaces are produced by generating laser induced periodic surface structures. The comparison with infrared wavelength underlines the advantages of the ultraviolet wavelength for laser micro machining this material. Significant differences with respect to the measured ablation depths and roughness as well as generated micro-and nanostructures appear. While infrared ablation is dominated by a chipping mechanism above a critical fluence of 9.93 Jcm -2 , higher ablation rates are observed with strong quality losses at the same time. In comparison to the infrared emission wavelength, in general , a significantly higher processing quality is achieved with the ultraviolet emission wavelength. In addition, the influence of spot size and repetition rate in UV processing is investigated. By increasing these parameters, a process acceleration without quality losses is enabled. The generation of a sophisticated microstructure exemplifies the advantages of processing silicon carbide with the UV laser reported here.}, subject = {Femtosekundenlaser}, language = {en} } @article{GvozdovskyyBratovaKazantsevaetal.2023, author = {Gvozdovskyy, Igor and Bratova, Daria and Kazantseva, Z. and Malyuta, S. and Lytvyn, Petro and Schwarz, Simon and Hellmann, Ralf}, title = {Light-controllable hybrid aligning layer based on LIPSS on sapphire surface and PVCN-F film}, series = {Journal of Molecular Liquids}, volume = {387}, journal = {Journal of Molecular Liquids}, number = {keine}, doi = {10.1016/j.molliq.2023.122623}, pages = {122623 -- 122623}, year = {2023}, abstract = {The creation of aligning layers for the uniform orientation of liquid crystals is significant for both research and the application of liquid crystals. For all applications, the creation of aligning layers possessing controllable characteristics such as azimuthal and polar anchoring energies, easy-axis of director alignment and pretilt angle, in the same way as it is achieved by using photoaligning layers processed by light, is very important. Here, aligning properties of hybrid aligning layers created on the basis of sapphire surfaces additionally coated by photoaligning layer of PVCN-F are studied. These hybrid layers possess the properties of the nano-structured sapphire layer and the photosensitive PVCN-F layer, and complement each other. The irradiation time dependence of the azimuthal anchoring energy of the hybrid layers is studied. By using certain experimental conditions during irradiation of hybrid layers, e.g., polarization of light and irradiation time, a minimum value of the azimuthal anchoring energy, close to zero, was obtained. Atomic force microscope studies of the irradiated hybrid layers were also carried out. It was found that the behavior of the contact angle of nematic droplets placed on treated sapphire surfaces are in good agreement with properties of hybrid aligning layers and parameters of structuring surface obtained from AFM images.}, subject = {Fl{\"u}ssigkristall}, language = {de} } @article{BischoffMueckeSchubertetal.2024, author = {Bischoff, Kay and M{\"u}cke, Dominik and Schubert, Andreas and Esen, Cemal and Hellmann, Ralf}, title = {Rheological Investigation of Highly Filled Copper(II) Oxide Nanosuspensions to Optimize Precursor Particle Content in Reductive Laser-Sintering}, series = {Liquids}, volume = {4}, journal = {Liquids}, number = {2}, publisher = {MDPI AG}, issn = {2673-8015}, doi = {http://dx.doi.org/10.3390/liquids4020019}, pages = {382 -- 392}, year = {2024}, abstract = {In this article, the particle concentration of finely dispersed copper(II) oxide nanosuspensions as precursors for reductive laser sintering (RLS) is optimized on the basis of rheological investigations. For this metallization process, a smooth, homogeneous and defect-free precursor layer is a prerequisite for adherent and reproducible copper structures. The knowledge of the rheological properties of an ink is crucial for the selection of a suitable coating technology as well as for the adjustment of the ink formulation. Different dilutions of the nanosuspension were examined for their rheological behavior by recording flow curves. A strong shear thinning behavior was found and the viscosity decreases exponentially with increasing dilution. The viscoelastic behavior was investigated by a simulated doctor blade coating process using three-interval thixotropy tests. An overshoot in viscosity is observed, which decreases with increasing thinning of the precursor. As a comparison to these results, doctor blade coating of planar glass and polymer substrates was performed to prepare precursor layers for reductive laser sintering. Surface morphology measurements of the resulting coatings using laser scanning microscopy and rheological tests show that homogeneous precursor layers with constant thickness can be produced at a particle-solvent ratio of 1.33. A too-high particle content results in an irregular coating layer with deep grooves and a peak-to-valley height Sz of up to 7.8 μm. Precise dilution control allows the fabrication of smooth surfaces with a Sz down to 1.5 μm.}, subject = {Laserstrahlsintern}, language = {en} } @article{BischoffEsenHellmann2024, author = {Bischoff, Kay and Esen, Cemal and Hellmann, Ralf}, title = {Femtosecond reductive Laser Sintering under multiple focus conditions for rapid production of conductive copper layers}, series = {Procedia CIRP}, volume = {124}, journal = {Procedia CIRP}, publisher = {Elsevier BV}, issn = {2212-8271}, doi = {http://dx.doi.org/10.1016/j.procir.2024.08.189}, pages = {629 -- 633}, year = {2024}, subject = {Femtosekundenlaser}, language = {en} } @article{SommerStockfleetHellmann2024, author = {Sommer, David and Stockfleet, Henry and Hellmann, Ralf}, title = {Optimization of Mechanical Properties and Evaluation of Fatigue Behavior of Selective Laser Sintered Polyamide-12 Components}, series = {Polymers}, volume = {16}, journal = {Polymers}, number = {10}, publisher = {MDPI AG}, issn = {2073-4360}, doi = {http://dx.doi.org/10.3390/polym16101366}, year = {2024}, abstract = {In this paper, a comprehensive study of the mechanical properties of selective laser sintered polyamide components is presented, for various different process parameters as well as environmental testing conditions. For the optimization of the static and dynamic mechanical load behavior, different process parameters, e.g., laser power, scan speed, and build temperature, were varied, defining an optimal parameter combination. First, the influence of the different process parameters was tested, leading to a constant energy density for different combinations. Due to similarities in mechanical load behavior, the energy density was identified as a decisive factor, mostly independent of the input parameters. Thus, secondly, the energy density was varied by the different parameters, exhibiting large differences for all levels of fatigue behavior. An optimal parameter combination of 18 W for the laser power and a scan speed of 2666 mm/s was determined, as a higher energy density led to the best results in static and dynamic testing. According to this, the variation in build temperature was investigated, leading to improvements in tensile strength and fatigue strength at higher build temperatures. Furthermore, different ambient temperatures during testing were evaluated, as the temperature-dependent behavior of polymers is of high importance for industrial applications. An increased ambient temperature as well as active cooling during testing was examined, having a significant impact on the high cycle fatigue regime and on the endurance limit.}, subject = {Laserstrahlsintern}, language = {en} } @article{SommerHornungEsenetal.2024, author = {Sommer, David and Hornung, Simon and Esen, Cemal and Hellmann, Ralf}, title = {Surface roughness optimization of hybrid PBF-LB/M-built Inconel 718 using in situ high-speed milling}, series = {The International Journal of Advanced Manufacturing Technology}, volume = {132}, journal = {The International Journal of Advanced Manufacturing Technology}, number = {3-4}, publisher = {Springer Science and Business Media LLC}, issn = {0268-3768}, doi = {http://dx.doi.org/10.1007/s00170-024-13382-5}, pages = {1741 -- 1751}, year = {2024}, abstract = {AbstractWe report on the optimization of the surface roughness of hybrid additive manufactured Ni superalloys, combining a conventional laser powder bed fusion process with in situ high-speed milling. This remarkable hybrid approach has only recently been applied to different steel types and barely to Ni superalloys which opposite to steel appear to be challenging for milling processes, particularly within the powderbed of laser powder bed fusion. Different influencing factors on the surface roughness are varied in this study, following the Taguchi method. Their effect is evaluated with respect to the average surface roughness and the maximum surface roughness. The signal-to-noise ratio for the varied parameters infeed, z-pitch, feed rate, and spindle speed is calculated, determining their relevance on the surface roughness, and defining an optimal parameter combination. As the surface quality is optimized to \$\$\varvec{R_a=0.47\, \mu m}\$\$ R a = 0.47 μ m , the definition of the optimal parameter combination is of the highest relevance for the application of this novel manufacturing approach for Inconel. Using linear regression, the resulting surface roughness of these parameters is predicted, getting validated by the experimental evaluation. Due to a further analysis, including EDX analysis and a quantitative element analysis at different positions of the flank of the milling cutter, wear characteristics as well as the dissipation of the coating of the milling cutter are detected. The flank wear and the resulting breakage of the cutting edge are defined as the main reasons of a rising surface roughness.}, subject = {Hochgeschwindigkeitsfr{\"a}sen}, language = {en} } @inproceedings{SommerHornungEsenetal.2024, author = {Sommer, David and Hornung, Simon and Esen, Cemal and Hellmann, Ralf}, title = {Optimization of mechanical properties of additive manufactured IN 718 parts combining LPBF and in-situ high-speed milling}, series = {Laser 3D Manufacturing XI}, booktitle = {Laser 3D Manufacturing XI}, editor = {Gu, Bo and Chen, Hongqiang}, publisher = {SPIE}, doi = {http://dx.doi.org/10.1117/12.3000952}, year = {2024}, subject = {Hochgeschwindigkeitsfr{\"a}sen}, language = {en} } @inproceedings{SommerSafiEsenetal.2024, author = {Sommer, David and Safi, Abdulrahman and Esen, Cemal and Hellmann, Ralf}, title = {Additive manufacturing of Nickel-based superalloy: optimization of surface roughness using integrated high-speed milling}, series = {Laser 3D Manufacturing XI}, booktitle = {Laser 3D Manufacturing XI}, editor = {Gu, Bo and Chen, Hongqiang}, publisher = {SPIE}, doi = {http://dx.doi.org/10.1117/12.3000972}, year = {2024}, subject = {Hochgeschwindigkeitsfr{\"a}sen}, language = {en} } @article{SchleierAdelmannEsenetal.2022, author = {Schleier, Max and Adelmann, Benedikt and Esen, Cemal and Hellmann, Ralf}, title = {Image Processing Algorithm for In Situ Monitoring Fiber Laser Remote Cutting by a High-Speed Camera}, series = {Sensors}, volume = {22}, journal = {Sensors}, number = {8}, publisher = {MDPI AG}, issn = {1424-8220}, doi = {10.3390/s22082863}, year = {2022}, abstract = {We present an in situ process monitoring approach for remote fiber laser cutting, which is based on evaluating images from a high-speed camera. A specifically designed image processing algorithm allows the distinction between complete and incomplete cuts by analyzing spectral and geometric information of the melt pool from the captured images of the high-speed camera. The camera-based monitoring system itself is fit to a conventional laser deflection unit for use with high-power fiber lasers, with the optical detection path being coaxially aligned to the incident laser. Without external illumination, the radiation of the melt from the process zone is recorded in the visible spectral range from the top view and spatially and temporally resolved. The melt pool size and emitted sparks are evaluated in dependence of machining parameters such as feed rate, cycles, and focus position during cutting electrical sheets.}, subject = {Laserschneiden}, language = {en} } @article{SchleierEsenHellmann2022, author = {Schleier, Max and Esen, Cemal and Hellmann, Ralf}, title = {High speed melt flow monitoring and development of an image processing algorithm for laser fusion cutting}, series = {Journal of Laser Applications}, volume = {34}, journal = {Journal of Laser Applications}, number = {4}, publisher = {Laser Institute of America}, issn = {1042-346X}, doi = {10.2351/7.0000785}, year = {2022}, abstract = {This contribution presents high-speed camera monitoring of melt pool dynamics for steel during laser fusion cutting and compares the images with recordings in aluminum. The experiments are performed by a 4 kW multimode fiber laser with an emission wavelength of 1070 nm. To visualize the thermal radiation from the process zone during the cutting process, the kerf is captured at sample rates of up to 170 000 frames per second without external illumination with a spectral response between 400 and 700 nm, allowing measurements of the melt flow dynamics from geometric image features. The dependencies of the melt flow dynamics on laser processing parameters, such as feed rate, gas pressure, and laser power, can be evaluated. The monitoring system is placed both off-axis and mounted to a conventional cutting head, with the monitoring path aligned to the processing laser for a coaxial and lateral view of the cut kerf. The measured signal characteristics of the images captured from the melt pool are examined in the visible spectral range of the emitted thermal radiation from the process zone. Moreover, a specifically developed image processing algorithm is developed that process and analyze the captured images and extract geometric information for a measurement of the melt flow.}, subject = {Laserschneiden}, language = {en} } @article{SchleierEsenHellmann2023, author = {Schleier, Max and Esen, Cemal and Hellmann, Ralf}, title = {Evaluation of a Cut Interruption Algorithm for Laser Cutting Steel and Aluminum with a High-Speed Camera}, series = {Applied Sciences}, volume = {13}, journal = {Applied Sciences}, number = {7}, publisher = {MDPI AG}, issn = {2076-3417}, doi = {10.3390/app13074557}, year = {2023}, abstract = {We report on a monitoring system based on a high-speed camera for fiber laser fusion cutting. The monitoring system is used without an external illumination retrofit on a conventional cutting head, with the optical path aligned coaxially to the incident laser, permitting a direct, spatially, and temporally resolved detection of the melt pool area in the cut kerf from the top view. The dependence of the melt pool area on laser processing parameters such as laser power and feed rate are thus evaluated for stainless steel, zinc-coated steel, and aluminum, respectively. The signal characteristics of the images captured from the melt pool are examined in the visible spectral range of the emitted secondary thermal radiation from the process zone. An ad hoc developed image processing algorithm analyzes the spectral and geometric information of the melt pool from high-speed camera images and distinguishes between complete and incomplete cuts.}, subject = {Laserschneiden}, language = {en} } @article{SchleierEsenHellmann2025, author = {Schleier, Max and Esen, Cemal and Hellmann, Ralf}, title = {Vision transformer based cut interruption detection and prediction of laser fusion cutting from monitored melt pool images}, series = {Journal of Laser Applications}, volume = {37}, journal = {Journal of Laser Applications}, number = {1}, publisher = {Laser Institute of America}, issn = {1042-346X}, doi = {10.2351/7.0001611}, year = {2025}, abstract = {Incomplete cuts during laser fusion cutting result in a closed kerf, preventing the workpiece from detaching from the sheet and resulting in rework or rejection. We demonstrate the approach of a vision transformer, used for image classification, to detect cut interruption during laser fusion cutting in steel and aluminum. With events impending an incomplete cut in steel, we attempt to predict cut interruption before they even occur. To build a data set for training, cutting experiments are carried out with a 4 kW fiber laser, forcing incomplete cuts by varying the process parameters such as laser power and feed rate. The thermal radiation from the process zone during the cutting process is captured with a size of 256 × 256 px2 at sample rates of 20 × 103 fps. The kerf is recorded with a spectral sensitivity between 400 and 700 nm, without external illumination, which enables the melt to be observed in the range of the visual spectrum. The vision transformer model, which is used for image classification, splits the image into patches, linearly embedded with an added position embedding, and fed to a standard transformer encoder. For training the model, a set of images was labeled for the respective classes of a complete, incomplete, and impending incomplete cut. With the trained model, incomplete cuts in steel and aluminum can then be recognized and impending incomplete cuts in steel can be predicted in advance.}, subject = {Laserschneiden}, language = {en} } @inproceedings{LutzSommerhuberKettneretal.2024, author = {Lutz, Christian and Sommerhuber, Ryan and Kettner, Matthias and Esen, Cemal and Hellmann, Ralf}, title = {Towards process control by detecting acoustic emissions during ultrashort pulsed laser ablation of multilayer materials}, series = {Laser-based Micro- and Nanoprocessing XVIII}, booktitle = {Laser-based Micro- and Nanoprocessing XVIII}, editor = {Kling, Rainer and Pfleging, Wilhelm and Sugioka, Koji}, publisher = {SPIE}, doi = {10.1117/12.3000954}, year = {2024}, abstract = {We report on process sensing using a membrane-free optical microphone to monitor the acoustic emission during ultrashort pulsed laser ablation of multilayer materials. The acoustic emission during ablation is used to detect material transitions, with the specific signatures allowing to create a reliable process control for identifying individual layers. The outstanding properties of membrane-free optical microphones in terms of high bandwidth and high temporal resolution are ideally qualified for characterizing an ultrashort pulsed laser process, with its properties and capabilities being presented in this contribution. In particular, for layer- and material-selective ablation of multilayer printed circuit board components, copper and polyimide layers are ablated and the material transition is detected by analyzing the acoustic signal at different frequency levels, which is a novelty in the field of ultrashort pulsed laser process sensing. The investigations show, that the optical microphone can be used to resolve both the scanning paths and ablated layers by means of interruptions in a time-resolved acoustic spectrogram. Furthermore, as a result of a higher ablation rate of polyimide compared to copper and thus the increase of the emitted acoustic energy, the material transition between copper and polyimide layers can clearly be detected. The detection of this process event can be used for process control.}, subject = {Ultrakurzzeitlaser}, language = {en} }