@article{SommerPapeEsenetal.2022, author = {Sommer, David and Pape, Dominik and Esen, Cemal and Hellmann, Ralf}, title = {Tool Wear and Milling Characteristics for Hybrid Additive Manufacturing Combining Laser Powder Bed Fusion and In Situ High-Speed Milling}, series = {Ultra-Precision Manufacturing Technology for Difficult-to-Machine Materials}, volume = {15}, journal = {Ultra-Precision Manufacturing Technology for Difficult-to-Machine Materials}, number = {3}, doi = {https://doi.org/10.3390/ma15031236}, pages = {1 -- 13}, year = {2022}, abstract = {We report on milling and tool wear characteristics of hybrid additive manufacturing comprising laser powder bed fusion and in situ high-speed milling, a particular process in which the cutter mills inside the powder bed without any cooling lubricant being applicable. Flank wear is found to be the dominant wear characteristic with its temporal evolution over utilization period revealing the typical s-shaped dependence. The flank wear land width is measured by microscopy and correlated to the achievable surface roughness of milled 3D-printed parts, showing that for flank wear levels up to 100 μm a superior surface roughness below 3 μm is accessible for hybrid additive manufacturing. Further, based on this correlation recommended tool, life scenarios can be deduced. In addition, by optimizing the finishing tool start position and the number of afore-built layers, the milling process is improved with respect to the maximum millable angle for undercut surfaces of 3D-printed parts to 30° for the roughing process and to 40° for the entire machining process including finishing}, subject = {Hochgeschwindigkeitsfr{\"a}sen}, language = {de} } @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{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{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{LutzJungTschirpkeetal.2022, author = {Lutz, Christian and Jung, Marcel and Tschirpke, Katrin and Esen, Cemal and Hellmann, Ralf}, title = {Optimization of Heat Accumulation during Femtosecond Laser Drilling Borehole Matrices by Using a Simplex Algorithm}, series = {Materials}, volume = {15}, journal = {Materials}, number = {14}, publisher = {MDPI AG}, issn = {1996-1944}, doi = {10.3390/ma15144829}, year = {2022}, abstract = {We report on an optimization study of percussion drilling thin metal sheets employing a high repetition rate, high power femtosecond laser with respect to the resulting heat accumulation. A specified simplex algorithm was employed to optimize the spatial drilling sequence, whereas a simplified thermal simulation using COMSOL was validated by comparing its results to the temperature measurements using an infrared camera. Optimization for drilling borehole matrices was aspired with respect to the generated temperature across the processed specimen, while the drilling strategy was altered in its spatial drilling sequence and by using multi-spot approaches generated by a spatial light modulator. As a result, we found that an optimization strategy based on limited consecutive holes in a Moore neighborhood led to reduced temperatures and the shortest process times.}, subject = {Femtosekundenlaser}, language = {en} } @article{VoigtBockHilpertetal.2022, author = {Voigt, Jorrit and Bock, Thomas and Hilpert, Uwe and Hellmann, Ralf and M{\"o}ckel, Michael}, title = {Increased relative density and characteristic melt pool Signals at the edge in PBF-LB/M}, series = {Additive Manufacturing}, volume = {2022}, journal = {Additive Manufacturing}, number = {57/102798}, doi = {https://doi.org/10.1016/j.addma.2022.102798}, pages = {* -- *}, year = {2022}, abstract = {Limited process control can cause metallurgical defect formation and inhomogeneous relative density in laser powder bed fusion manufactured parts. This study shows that process monitoring, based on optical melt-pool signal analysis is capable of tracing relative density variations: Unsupervised machine learning, applied to cluster multiple-slice monitoring data, reveals characteristic patterns in this noisy time-series signal, which can be co-registered with geometrical positions in the build part. For cylindrical 15-5 PH stainless steel specimens, manufactured under constant process parameters and post-analyzed by µ-computer tomography, correlations between such patterns and an increased local relative density at the edge have been observed. Finite element method (FEM) modeling of thermal histories at exemplary positions close to the edge suggest pre-heating effects caused by neighboring laser scan trajectories as possible reasons for the increased melt pool intensity at the edge.}, subject = {Maschinelles Lernen}, language = {en} } @article{MarxLutzHellmannetal.2022, author = {Marx, Jan and Lutz, Christian and Hellmann, Ralf and Esen, Cemal}, title = {Holographic multi-spot generation for ultra-short pulse Bessel beam processing of stainless steel}, series = {Procedia CIRP}, volume = {111}, journal = {Procedia CIRP}, publisher = {Elsevier BV}, issn = {2212-8271}, doi = {10.1016/j.procir.2022.08.003}, pages = {648 -- 652}, year = {2022}, subject = {Ultrakurzzeitlaser}, language = {en} }