@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} } @inproceedings{LutzHelmEsenetal.2024, author = {Lutz, Christian and Helm, Jonas and Esen, Cemal and Hellmann, Ralf}, title = {Computational optimization of borehole sequences for the reduction of heat accumulation in drilling processes using ultrashort pulse lasers}, 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 = {10.1117/12.3000930}, year = {2024}, abstract = {We report on laser drilling of borehole arrays using a high-power ultrashort pulse laser with particular focus on reducing heat accumulation in the workpiece by optimizing the drilling sequence, particularly for highly efficient multi-spot drilling. Different optimization approaches are chosen to improve the drilling sequence, also comparing a simplex algorithm and an evolutionary algorithm. From a laser application point of view, we also compare drilling sequences using a single spot and up to 16-fold multi-spots generated by a spatial light modulator, as to accelerate the drilling process in terms of the number of drilled holes per second. To evaluate the temperatures generated during drilling of up to 40,000 holes in less than 76 seconds in stainless steel foil, temperatures are measured by a thermal imaging camera and subsequently compared to a COMSOL-based simulation for all optimized drilling sequences. With respect to an average temperature of 706 °C without optimization, a reduction by 252 °C, i.e., a reduction by nearly 36 \% based on the Celsius scale, is achieved using a 4 × 4 beam splitter and an optimized drilling sequence with a drilling rate of 526 holes per second. In addition, using a 2 × 2 beam splitter, a temperature reduction of up to 40.5 \% is achieved for a drilling process with a rate of 129 holes per second using an optimized drilling sequence.}, subject = {Ultrakurzzeitlaser}, language = {en} } @article{MarxSeefeldtHaskeetal.2025, author = {Marx, Jan and Seefeldt, Malte and Haske, Damian and Lutz, Christian and Hellmann, Ralf and Esen, Cemal and Ostendorf, Andreas}, title = {Holographic freeform micro hole processing using Bessel beams}, series = {Optics \& Laser Technology}, volume = {183}, journal = {Optics \& Laser Technology}, publisher = {Elsevier BV}, issn = {0030-3992}, doi = {10.1016/j.optlastec.2024.112287}, year = {2025}, abstract = {Bessel beams are advantageous tools for micro drilling applications due to their diffraction-free properties, their long focal length, and their small focal spot diameter. However, their application in industry is limited because they are challenging to integrate into flexible beam shaping systems. This work is intended to overcome limitations in flexibility by presenting a new approach paving the way to high-aspect ratio freeform holes. Therefore, a spatial light modulator was integrated into an axicon-based setup to merge diffraction-free ablation with the flexibility of holographic spatial beam shaping. A dynamic change of the phase hologram displayed on the spatial light modulator during the process allows for the generation of arbitrary ablation geometries. In contrast to other holographic approaches, the diffraction-free nature of the Bessel beam allows ablation over an extended focal depth without any focus control needed. The applicability of the approach was demonstrated by processing arbitrary hole shapes, such as ellipses, squares, or star-shapes. The hole sizes of the freeform holes range from 4 μm to 80 μm. Furthermore, up-scaling of the process is demonstrated by using the setup for generation of multi-Bessel spots.}, subject = {Mikrobearbeitung}, 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} } @inproceedings{LutzSchwarzRungetal.2021, author = {Lutz, Christian and Schwarz, Simon and Rung, Stefan and Marx, Jan and Esen, Cemal and Hellmann, Ralf}, title = {Optical system for multi Bessel beam high power ultrashort pulsed laser processing using a spatial light modulator}, series = {Lasers in Manufacturing - LiM 2021}, booktitle = {Lasers in Manufacturing - LiM 2021}, year = {2021}, abstract = {We report on an optical setup for multi Bessel beam processing combining a refractive axicon and a spatial light modulator. Based on their particular beam profile, Bessel beams exhibit various advantages over conventional Gaussian beams for ultrashort pulsed laser processing. Especially for micromachining of transparent materials, applications such as micro-hole drilling or the generation of voids benefit from the increased focal length of the applied Bessel beam. In addition, on account of the significantly increased average output power of industrial ultrashort pulsed lasers over the last years, there is a high demand on multi spot applications for using the available laser power in efficient production processes. Our optical concept combines the dynamic possibilities of beam splitting using spatial light modulator with the benefits of Bessel beams facilitating multi Bessel beam processing.}, subject = {Ultrakurzzeitlaser}, language = {en} } @inproceedings{KeferZettlSchmaussetal.2023, author = {Kefer, Stefan and Zettl, Julian and Schmauss, Bernhard and Esen, Cemal and Hellmann, Ralf}, title = {High-Precision Micromachining of Sapphire Towards Optical Waveguiding Structures using Femtosecond Lasers}, series = {Laser-based Micro- and Nanoprocessing XVII}, booktitle = {Laser-based Micro- and Nanoprocessing XVII}, editor = {Kling, Rainer and Pfleging, Wilhelm and Watanabe, Akira}, isbn = {9781510659230}, doi = {10.1117/12.2648758}, pages = {3 -- 11}, year = {2023}, abstract = {While sapphire is one of the most durable materials, its properties entail that high-precision machining, especially in the sub-millimeter regime, is still challenging. This contribution demonstrates and discusses novel femtosecond laser-based micromachining approaches for the fabrication of rotational-symmetric sapphire workpieces, specifically the generation of optical fibers by means of laser lathe of sapphire rods and the practical realization of windmill fibers. In addition, volume refractive index modification in planar sapphire substrates is presented to induce photonic crystal waveguides. The micromachined structures are comprehensively examined with respect to geometric fidelity, surface roughness, refractive index modification, and potential optical waveguiding properties. All micromachining approaches are done by means of frequency-doubled or frequency-tripled femtosecond laser radiation. Different laser optical setups including laser scanning head, spatial beam profilers including a spatial light modulator and axial rotatory movement of the specimen are employed for micro structuring and in-depth refractive index modifications. In particular for laser lathe, a sophisticated scanning pattern, in combination with an incremental axial rotatory movement of the specimen, allows for the precise diameter reduction of sapphire rods with 250 µm diameter to fibers with outer diameters of 25 µm. By supporting the workpiece with a V-groove fixture, multi-mode fibers with lengths up to 20 cm can be processed with an average surface roughness of 250 nm. Additionally, an adapted ablation scanning sequence enables the first practical demonstration of sapphire windmill fibers. Furthermore, using a spatial light modulator allows for the adaption of the laser propagation properties as to enable volume refractive index modifications with free-form arrangement. Hexagonal patterns of refractive index modifications surrounding a pristine waveguide core are fabricated and single-mode waveguiding at 1550 nm is verified. Finally, the possibility of integrating Bragg gratings into this photonic waveguide type is demonstrated}, subject = {Femtosekundenlaser}, language = {en} } @article{YangBischoffMueckeetal.2024, author = {Yang, Yongting and Bischoff, Kay and M{\"u}cke, Dominik and Esen, Cemal and Hellmann, Ralf}, title = {UV-ultrashort pulsed laser ablation of fused silica}, series = {Journal of Laser Applications}, volume = {36}, journal = {Journal of Laser Applications}, number = {1}, publisher = {Laser Institute of America}, issn = {1042-346X}, doi = {http://dx.doi.org/10.2351/7.0001197}, year = {2024}, abstract = {The authors report on ultraviolet ultrashort pulsed laser ablation of fused silica and compare the achievable micromachining results to those obtained by using the fundamental emission wavelength in infrared. Ablation in ultraviolet reveals a stable efficiency for increasing fluences, whereas using an infrared beam exhibits a decreasing trend of the ablation efficiency at higher and increasing fluences. In addition, a significant improvement in the surface quality is found by using an ultraviolet wavelength in a fluence range up to 20 J/cm2 compared to infrared, e.g., revealing an Ra of down to 0.45 μm on using the ultraviolet wavelength compared to Ra = 0.56 μm on using infrared at fluences up 15 J/cm2. Moreover, taking advantage of the high available pulse energy, the authors compare the achievable ablation efficiency and surface roughness using a conventionally focused ultraviolet beam and a defocused ultraviolet beam, finding that the defocused ultraviolet beam possesses a processing quality comparable to that of the focused beam. Finally, the authors exemplify the potential of ultraviolet ultrashort pulsed laser ablation by using a Tesla mixer for microfluidic integration of fused silica.}, subject = {Laserablation}, 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} } @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{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} } @inproceedings{LutzSchwarzMarxetal.2024, author = {Lutz, Christian and Schwarz, Simon and Marx, Jan and Esen, Cemal and Hellmann, Ralf}, title = {Combination of an axicon and a spatial light modulator for the generation of multiBessel beams for microdrilling using ultrashort pulsed laser}, 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 = {10.1117/12.3000946}, year = {2024}, abstract = {Due to their special beam profile, Bessel beams offer advantageous properties for micro-drilling and internal volume processing, especially for transparent materials. In addition, the laser power of industrial ultrashort pulsed lasers hasnincreased significantly in recent years, offering the possibility of highly efficient processes using multi-spot profiles. We report on optical simulations and an experimental optical setup for generating multi-Bessel beams by combining a refractive axicon and a spatial light modulator. This setup offers a new possibility of multi-Bessel beam generation by functional division of the Bessel beam generation by the axicon and the possibility of separation flexible beam splitting by a spatial light modulator. The beam profiles generated with this setup are analyzed and compared in this study using optical simulations and experiments. In addition to the analysis of the Bessel beams in the machining plane, they are also analyzed in the propagation direction by means of intensity distributions measured by a camera in small-step z positions. Furthermore, the experimental applicability of the module-based beam shaping system is demonstrated by integrating into an existing laser system.}, subject = {Ultrakurzzeitlaser}, language = {en} } @article{SommerHornungEsenetal.2025, author = {Sommer, David and Hornung, Simon and Esen, Cemal and Hellmann, Ralf}, title = {Design Guide for Hybrid-Additive Manufacturing of Inconel 718 Combining PBF-LB/M and In Situ High-Speed Milling}, series = {Journal of Manufacturing and Materials Processing}, volume = {9}, journal = {Journal of Manufacturing and Materials Processing}, number = {3}, publisher = {MDPI AG}, issn = {2504-4494}, doi = {http://dx.doi.org/10.3390/jmmp9030088}, year = {2025}, abstract = {As the correlation between design rules and process limitations is of the upmost importance for the full exploitation of any manufacturing technology, we report a design guide for hybrid-additive manufacturing of Inconel 718. Basic limitations need to be evaluated for this particular hybrid approach that combines laser powder bed fusion (PBF-LB/M) and in situ high-speed milling. Fundamental geometric limitations are examined with regard to the minimum feasible wall thickness, cylinders, overhanging structures, and chamfers. Furthermore, geometrical restrictions due to the integrated three-axis milling process with respect to inclinations, inner angles, notches, and boreholes are investigated. From these findings, we derive design guidelines for a reliable build process using this hybrid manufacturing. Additionally, a design guideline for the hybrid-additive manufacturing approach is presented, depicting a step-to-step guide for the adjustment of constructions. To demonstrate this, a powder nozzle for a direct energy deposition (DED-LB/M) process is redesigned following the previously defined guidelines. This redesign encompasses analysis of the existing component and identification of problematic areas such as flat angles, leading to a new construction that is suitable for a hybrid-additive manufacturing approach.}, subject = {Hochgeschwindigkeitsfr{\"a}sen}, language = {en} } @article{SommerTruetschEsenetal.2025, author = {Sommer, David and Truetsch, Ben and Esen, Cemal and Hellmann, Ralf}, title = {Fractographic Analysis and Fatigue Behavior of Additively Manufactured Ni-Superalloy Components with Post Processing Heat Treatment and Hot Isostatic Pressing}, series = {Advanced Engineering Materials}, volume = {27}, journal = {Advanced Engineering Materials}, number = {12}, publisher = {Wiley}, issn = {1438-1656}, doi = {http://dx.doi.org/10.1002/adem.202500078}, year = {2025}, abstract = {A report is made on a study of mechanical properties and fractographic characteristics of laser powder bed fusion (PBF-LB/M)-built Inconel 718, performing heat treatments and hot-isostatic pressing. For this, tensile components are heat-treated by different processes, as namely stress relief (SR), SR and double aging (SR + DA), and hot-isostatic pressing are conducted. For the mechanical testing, the ultimate tensile strength (UTS) as well as the fatigue behavior are evaluated, examining differences in maximum load behavior, elongation, and the different regimes of fatigue. As changes in material structure can be observed, the sole SR leads to a diminished UTS, while the combination of SR + DA develops an UTS of Rm = 1277 MPa. Within the fatigue behavior, the HIP shows a very balanced material structure with an increased high cycle and very high cycle regime, as the texture gets homogenized during the heat treatment. The metallographic analysis can quantify the material changes, as the density and the hardness are improved by virtue of the heat treatments. Furthermore, the fractographic analysis shows the differences in fracture behavior, arising due to the microstructural changes, as crack initiation points, crack propagation, and forced fractures can be categorized by scanning electron microscopy.}, subject = {Rapid Prototyping, Fertigung}, language = {en} } @article{HaefnerBischoffHelmetal.2023, author = {H{\"a}fner, Tom and Bischoff, Kay and Helm, Jonas and Kunz, Tim and Rung, Stefan and Esen, Cemal and Hellmann, Ralf}, title = {Picosecond laser microvia drilling of ABF material using MHz burst mode}, series = {Materials Research Express}, volume = {10}, journal = {Materials Research Express}, number = {9}, pages = {096301 -- 096311}, year = {2023}, abstract = {We report on a comprehensive study of laser percussion microvia drilling of Ajinomoto build-up film (ABF) material using an ultrashort pulsed laser in MHz burst mode. After laser processing, microvia drilling quality is being evaluated by the fabricated diameter and taper using laser scanning microscopy and metallography. The influences of the incubation effect, heat accumulation and shielding effects as a result of pulse to pulse interactions are being discussed on the ablation threshold, penetration depth and laser microvia drilling quality. We find that an increasing heat accumulation in MHz burst mode processing is responsible for the void formation and delamination of the insulating ABF layer. Therefore, the parameter clearance is introduced to evaluate these effects on the microvia sidewalls. For a comparable clearance, applying 2 intra-burst pulses achieves an average reduced taper of down to 19.5\% compared to single pulse mode. At the same time, a reduced laser drilling time of 16.7\% per microvia highlights the enormous potential of the MHz burst mode for laser drilling of ABF material in printed circuit board fabrication.}, subject = {Ultrakurzzeitlaser}, language = {en} } @inproceedings{FranzMueckeKunzetal.2023, author = {Franz, Daniel and M{\"u}cke, Dominik and Kunz, Tim and Rung, Stefan and Esen, Cemal and Hellmann, Ralf}, title = {UV picosecond laser drilling of ABF film material for printed circuit boards using laser burst mode and beam shaping}, 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 an experimental study of laser microvia percussion drilling of Ajinomoto build-up film (ABF) material using an ultrashort pulsed laser in the ultraviolet region. In order to optimize the drilling quality in terms of the fabricated taper and microvia diameter, the MHz burst mode is used in combination with both a Gaussian and a top-hat laser intensity distribution. To evaluate the influence of the top-hat beam shape on defocusing, microvias were drilled at different z-positions in the range of 200 μm evenly distributed around the focal plane. The drilling quality was evaluated by laser scanning microscopy and metallography. When using a top-hat beam shape, a reduced taper of down to 26.8 \% on average is achieved compared to a Gaussian beam. For the application of 3 intra-burst pulses, we find an improvement of the taper by 11.6\% compared to the single pulse mode due to the increasing heat accumulation. Using a top-hat beam shape in combination with the MHz burst mode, an 18.2 \% reduction in laser drilling time for a microvia with a similar taper is remarkable, demonstrating the high potential for scaling throughput in electronics manufacturing.}, subject = {Pikosekundenbereich}, language = {en} } @article{FranzYangMicheletal.2023, author = {Franz, Daniel and Yang, Yongting and Michel, Luis and Esen, Cemal and Hellmann, Ralf}, title = {Evaluation of an ultrashort pulsed laser robot system for flexible and large-area micromachining}, series = {Journal of Laser Applications}, volume = {35}, journal = {Journal of Laser Applications}, number = {4}, pages = {042057 -- 042065}, year = {2023}, abstract = {We report for the first time on the realization, characterization and application of an ultrashort pulsed laser robot system for flexible and large-area 2D and 3D laser micromachining with 6 articulated axes. To characterize the dynamic positioning of the laser beam during and after axes movement, CMOS image sensors were integrated into the beam path. A method introduced for the alignment of the optical axes allows a reduction of the deviations in laser beam positioning to less than 141.8 ± 92.9 μm within a 110° rotation range of axis 4. In addition, a high laser beam positioning repeatability of less than 102.2 μm is demonstrated over a total period of 14 h for a movement of axis 5 within a range of 0° to 90°. Initial laser cutting, laser structuring and laser marking applications on automotive dashboards and glass substrates are presented for flexible and large area 2D and 3D manufacturing. By applying a special laser cutting strategy for processing AF 32 eco thin glass, high cutting quality is achieved with a taper of up to 96.3\% without the generation of cracks, demonstrating the innovative potential of the high-precision laser robot system. Nonetheless, different identified inherent influences of each axis 1-5 during robot axis movement demand for an innovative beam stabilization concept to achieve high precision in laser beam positioning.}, subject = {Ultrakurzzeitlaser}, language = {en} } @inproceedings{FranzRothRungetal.2021, author = {Franz, Daniel and Roth, Gian-Luca and Rung, Stefan and Esen, Cemal and Hellmann, Ralf}, title = {Superimposed beam deflection using acousto-optical deflectors in combination with a galvanometer scanner}, series = {Lasers in Manufacturing - LiM 2021}, booktitle = {Lasers in Manufacturing - LiM 2021}, year = {2021}, subject = {Galvanometer}, language = {en} } @article{FranzSchusterSchwarzetal.2025, author = {Franz, Daniel and Schuster, David and Schwarz, Simon and Rung, Stefan and Esen, Cemal and Hellmann, Ralf}, title = {Fabrication and analysis of through-glass vias for glass-based electronic packaging using an ultrashort pulsed laser}, series = {Optics and Lasers in Engineering}, volume = {193}, journal = {Optics and Lasers in Engineering}, publisher = {Elsevier BV}, issn = {0143-8166}, doi = {10.1016/j.optlaseng.2025.109106}, year = {2025}, subject = {Ultrakurzzeitlaser}, language = {en} } @article{FranzYangEsenetal.2026, author = {Franz, Daniel and Yang, Yongting and Esen, Cemal and Hellmann, Ralf}, title = {Flexible 2D Laser Cutting of Ultra-Thin Glass Using an Ultrashort Pulsed Laser Robotic System}, series = {Journal of Laser Micro/Nanoengineering}, volume = {21}, journal = {Journal of Laser Micro/Nanoengineering}, number = {1}, editor = {Esen, Cemal}, publisher = {Japan Laser Processing Society}, issn = {1880-0688}, doi = {10.2961/jlmn.2026.01.2004}, year = {2026}, subject = {Laserschneiden}, language = {en} } @inproceedings{FranzSchusterSchwarzetal.2025, author = {Franz, Daniel and Schuster, David and Schwarz, Simon and Rung, Stefan and Esen, Cemal and Hellmann, Ralf}, title = {Ultrashort pulsed laser drilling of through-glass vias for glass core packaging}, series = {Proceedings of LAMP2025 - the 9th International Congress on Laser Advanced Materials Processing, Ise-City, Japan, June 2025}, booktitle = {Proceedings of LAMP2025 - the 9th International Congress on Laser Advanced Materials Processing, Ise-City, Japan, June 2025}, year = {2025}, subject = {Ultrakurzzeitlaser}, language = {en} } @inproceedings{SommerTruetschEsenetal.2025, author = {Sommer, David and Truetsch, Ben and Esen, Cemal and Hellmann, Ralf}, title = {Microstructural Fracture Behaviour of PBF-LB/M Inconel 718 Components within different HIP Processes}, series = {Lasers in Manufacturing, M{\"u}nchen, Juni 2025}, booktitle = {Lasers in Manufacturing, M{\"u}nchen, Juni 2025}, year = {2025}, abstract = {We report on a study of different hot isostatic pressing (HIP) cycles, improving the mechanical properties of additively manufactured Inconel 718 components. For this, PBF-LB/M built components are post-processed by different HIP sequences, as gas pressure and processing time are varied, leading to differences in microstructure and material characteristics. Static and dynamic mechanical testing are performed, evaluating the changes in mechanical properties with particular focus on the ultimate tensile strength and endurance limit. Furthermore, metallographic analysis is employed to investigate the achieved density and microhardness. Microstructural analysis, showing the grain boundaries, is used to identify generated phases and precipitations of the material matrix. Moreover, the fracture behaviour is classified by grain deformation during mechanical testing. As the HIP leads to microstructural changes of Inconel 718 components, mechanical properties can be improved significantly, enhancing the ultimate tensile strength and simultaneously the endurance limit.}, subject = {Isostatisches Heißpressen}, language = {de} } @inproceedings{SommerPetersEsenetal.2025, author = {Sommer, David and Peters, Maximilian and Esen, Cemal and Hellmann, Ralf}, title = {Influence of in-situ high-speed milling within a hybrid additive manufacturing approach on the fatigue behaviour of Inconel 718 lattice structures}, series = {Lasers in Manufacturing, M{\"u}nchen, Juni 2025}, booktitle = {Lasers in Manufacturing, M{\"u}nchen, Juni 2025}, year = {2025}, abstract = {As lattice structures in various designs are used in additive manufacturing for lightweight components, the mechanical characterisation and fracture behaviour is of upmost importance for their industrial application. In this study, the fatigue behaviour of Inconel 718 lattice structures is evaluated, comparing sole PBF-LB/M to a hybrid additive manufacturing process combining PBF-LB/M with in-situ high-speed milling. At first, the static and dynamic mechanical load behaviour of different packing densities is analysed, determining the compressive strength and the endurance limit. Secondly, hybrid additive manufactured components are compared to PBF-LB/M built parts with respect to these mechanical properties, revealing improved compressive properties and modified regimes of fatigue. In addition, differences in fracture behaviour are qualified by fractographic and surface analysis. Overall, it can be summarized that the mechanical load characteristics, especially the fatigue behaviour, are improved for hybrid additively manufactured components with a superior surface quality of Ra < 1 µm.}, subject = {Hochgeschwindigkeitsfr{\"a}sen}, language = {de} } @inproceedings{BischoffKeferEsenetal.2024, author = {Bischoff, Kay and Kefer, Stefan and Esen, Cemal and Hellmann, Ralf}, title = {Towards entirely ultrashort pulsed laser based process chain to manufacture polymer lab-on-chip systems with electrical, photonic, and microfluidic functionalities}, series = {The 5th Smart Laser Processing Conference (SLPC)}, booktitle = {The 5th Smart Laser Processing Conference (SLPC)}, year = {2024}, subject = {Ultrakurzzeitlaser}, language = {de} } @article{SommerHornungEsenetal.2025, author = {Sommer, David and Hornung, Simon and Esen, Cemal and Hellmann, Ralf}, title = {In situ high-speed milling and thermal post-processing for the optimization of fatigue behaviour of additive-manufactured IN718 components}, series = {Journal of Materials Research}, volume = {40}, journal = {Journal of Materials Research}, number = {16}, publisher = {Springer Science and Business Media LLC}, issn = {0884-2914}, doi = {https://doi.org/10.1557/s43578-025-01656-x}, pages = {2388 -- 2401}, year = {2025}, abstract = {We report on the optimization of tensile properties and fatigue behaviour of Laser Powder Bed Fusion (PBF-LB/M)-built IN718 components employing an innovative hybrid additive-subtractive technology and various heat treatments. The hybrid approach comprises PBF-LB/M with in situ high-speed milling. Double Ageing, solution and ageing treatment, as well as Hot Isostatic Pressing are used to improve ultimate tensile strength and endurance limit. As a result, the advantages of the hybrid-machined components are clearly demonstrated by their improved mechanical properties. These enhanced properties are associated to changes in microstructure, which are identified as precipitations of IN718. Furthermore, crack initiation and crack growth for high cycle numbers as well as sudden fractures are determined.}, subject = {Hochgeschwindigkeitsfr{\"a}sen}, language = {en} } @article{SommerTruetschEsenetal.2025, author = {Sommer, David and Truetsch, Ben and Esen, Cemal and Hellmann, Ralf}, title = {Effect of Hot Isostatic Pressing and Sequenced Heat Treatment on the Mechanical Properties of Hybrid Additive Manufactured Inconel 718 Components}, series = {Journal of Manufacturing and Materials Processing}, volume = {9}, journal = {Journal of Manufacturing and Materials Processing}, number = {11}, publisher = {MDPI AG}, issn = {2504-4494}, doi = {https://doi.org/10.3390/jmmp9110378}, year = {2025}, abstract = {We report on the effect of hot isostatic pressing combined with solution and ageing treatment in different sequences on the mechanical properties of Inconel 718 specimens, which in turn have been fabricated by a hybrid additive manufacturing approach. The latter combines conventional laser powder bed fusion and in-situ high speed milling, yielding superior surface quality as being quantified by Ra about 1 μm. In a comparative study between hybrid additively manufactured parts and those built without milling, we find that, in general, any combination of heat treatment leads to a higher ultimate tensile strength and an improved endurance limit, while, however, hot isostatic pressing affects these figures of merit most. In addition, metallographic analysis reveals increased density and hardness for hot isostatic pressed parts due to precipitation hardening. These improvements of the mechanical properties are found to be even more pronounced when the printed parts are manufactured by the hybrid additive approach, i.e., for parts with improved surface conditions.}, subject = {Isostatisches Heißpressen}, language = {en} } @article{SommerPetersEsenetal.2026, author = {Sommer, David and Peters, Maximilian and Esen, Cemal and Hellmann, Ralf}, title = {Enhancing mechanical properties of double-periodic lattice structures using hybrid-additive manufacturing}, series = {The International Journal of Advanced Manufacturing Technology}, volume = {142}, journal = {The International Journal of Advanced Manufacturing Technology}, number = {7-8}, publisher = {Springer Science and Business Media LLC}, issn = {0268-3768}, doi = {https://doi.org/10.1007/s00170-025-17373-y}, pages = {4065 -- 4071}, year = {2026}, abstract = {We report on a study of the mechanical properties of hybrid-additive manufactured Inconel 718 double-periodic lattice structures. For this, a hybrid approach, combining Laser Powder Bed Fusion (PBF-LB/M) and in-situ high-speed milling, is employed. Within this technique, the PBF-LB/M process is interrupted after several layers, as the milling process starts, enabling a machining of inlaying structures.}, subject = {Hochgeschwindigkeitsfr{\"a}sen}, language = {en} } @article{SommerHornungEsenetal.2025, author = {Sommer, David and Hornung, Simon and Esen, Cemal and Hellmann, Ralf}, title = {Design Guide for Hybrid-Additive Manufacturing of Inconel 718 Combining PBF-LB/M and In Situ High-Speed Milling}, series = {Journal of Manufacturing and Materials Processing}, volume = {9}, journal = {Journal of Manufacturing and Materials Processing}, number = {3}, publisher = {MDPI AG}, issn = {2504-4494}, doi = {https://doi.org/10.3390/jmmp9030088}, year = {2025}, abstract = {As the correlation between design rules and process limitations is of the upmost importance for the full exploitation of any manufacturing technology, we report a design guide for hybrid-additive manufacturing of Inconel 718. Basic limitations need to be evaluated for this particular hybrid approach that combines laser powder bed fusion (PBF-LB/M) and in situ high-speed milling. Fundamental geometric limitations are examined with regard to the minimum feasible wall thickness, cylinders, overhanging structures, and chamfers. Furthermore, geometrical restrictions due to the integrated three-axis milling process with respect to inclinations, inner angles, notches, and boreholes are investigated. From these findings, we derive design guidelines for a reliable build process using this hybrid manufacturing. Additionally, a design guideline for the hybrid-additive manufacturing approach is presented, depicting a step-to-step guide for the adjustment of constructions. To demonstrate this, a powder nozzle for a direct energy deposition (DED-LB/M) process is redesigned following the previously defined guidelines. This redesign encompasses analysis of the existing component and identification of problematic areas such as flat angles, leading to a new construction that is suitable for a hybrid-additive manufacturing approach.}, subject = {Hochgeschwindigkeitsfr{\"a}sen}, 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}, number = {124}, publisher = {Elsevier BV}, issn = {2212-8271}, doi = {10.1016/j.procir.2024.08.189}, pages = {629 -- 633}, year = {2024}, abstract = {Reductive laser sintering is an advantageous alternative to conventional precious metal- or mask-based processes for the electrification of polymers and has been demonstrated in a variety of sensor applications. Ultrashort pulsed lasers promise precise control of the energy applied during sintering and ablation. Besides fine tracks, microelectronic circuits include wide contact pads, which leads to time-consuming processing with small laser spot sizes. The use of larger focal diameters, which results in wider lines and thus a higher metallization rate, is accompanied by a loss of structure resolution. We show the influence of focusing on the femtosecond reductive laser sintering of copper(II) oxide on cyclic olefin copolymers. While small focus diameters down to 23 µm provide high accuracy, increasing the laser power and the focus diameter up to 400 µm significantly accelerates the process, first by enlarging the line width and second by increasing the scanning speed due to the exposure time. Two-dimensional copper electrodes for 23 µm and 200 µm focal diameters were generated by hatching individual lines and compared with respect to their morphology. The enlarged spot can be used for high-speed generation of electrodes with a surface conductivity of 117 mΩ/sq. Hybrid processes with production under combined focusing conditions are presented for the first time.}, subject = {Laserstrahlsintern}, language = {en} } @article{SommerTruetschEsenetal.2025, author = {Sommer, David and Truetsch, Ben and Esen, Cemal and Hellmann, Ralf}, title = {Fractographic Analysis and Fatigue Behavior of Additively Manufactured Ni-Superalloy Components with Post Processing Heat Treatment and Hot Isostatic Pressing}, series = {Advanced Engineering Materials}, volume = {27}, journal = {Advanced Engineering Materials}, number = {12}, publisher = {Wiley}, issn = {1438-1656}, doi = {https://doi.org/10.1002/adem.202500078}, year = {2025}, abstract = {A report is made on a study of mechanical properties and fractographic characteristics of laser powder bed fusion (PBF-LB/M)-built Inconel 718, performing heat treatments and hot-isostatic pressing. For this, tensile components are heat-treated by different processes, as namely stress relief (SR), SR and double aging (SR + DA), and hot-isostatic pressing are conducted. For the mechanical testing, the ultimate tensile strength (UTS) as well as the fatigue behavior are evaluated, examining differences in maximum load behavior, elongation, and the different regimes of fatigue. As changes in material structure can be observed, the sole SR leads to a diminished UTS, while the combination of SR + DA develops an UTS of R m  = 1277 MPa. Within the fatigue behavior, the HIP shows a very balanced material structure with an increased high cycle and very high cycle regime, as the texture gets homogenized during the heat treatment. The metallographic analysis can quantify the material changes, as the density and the hardness are improved by virtue of the heat treatments. Furthermore, the fractographic analysis shows the differences in fracture behavior, arising due to the microstructural changes, as crack initiation points, crack propagation, and forced fractures can be categorized by scanning electron microscopy.}, subject = {Nickellegierung}, language = {en} } @article{SommerHornungEsenetal.2025, author = {Sommer, David and Hornung, Simon and Esen, Cemal and Hellmann, Ralf}, title = {Evaluation of the Impact of Heat Treatment Processes and Surface Conditions on the Mechanical Properties of PBF-LB/M-Built Inconel 718 Components}, series = {Materials Science Forum}, volume = {1155}, journal = {Materials Science Forum}, publisher = {Trans Tech Publications, Ltd.}, issn = {1662-9752}, doi = {https://doi.org/10.4028/p-9gR9cI}, pages = {11 -- 17}, year = {2025}, abstract = {A study of the mechanical properties of hybrid additive manufactured IN718 components is presented, optimising mechanical properties due to an in-situ high-speed milling and different heat treatment processes. At first, the impact of different heat treatment processes is investigated, as the changes in microstructure during the process lead to different mechanical properties. Static and dynamic mechanical load behaviour is tested, quantifying microstructural changes by means of the Ultimate Tensile Strength (UTS) and the endurance limit. Furthermore, sole PBF-LB/M- and hybrid built components are compared, investigating the effect of a surface finish on the static and dynamic load behaviour, as superficial cracks and melting errors diminish the UTS and the endurance limit of PBF-LB/M-built components. Within these experiments, a change of fatigue behaviour for the heat-treatedstates can be observed, compared to the as-built state of the PBF-LB/M, as the development of different phases during heat treatment leads to an improvement of the endurance limit for, e.g., solution and ageing treated components. Additionally, the improvement of the surface quality to Ra = 2 µm leads to a significant increase of the dynamic mechanical load behaviour of hybrid-built components, as superficial cracks and surface defects are reduced.}, subject = {Hochgeschwindigkeitsfr{\"a}sen}, language = {en} }