@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{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{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} }