TY - JOUR A1 - Sommer, David A1 - Götzendorfer, Babette A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Design Rules for Hybrid Additive Manufacturing Combining Selective Laser Melting and Micromilling JF - Design and Post Processing for Metal Additive Manufacturing N2 - We report on a comprehensive study to evaluate fundamental properties of a hybrid manufacturing approach, combining selective laser melting and high speed milling, and to characterize typical geometrical features and conclude on a catalogue of design rules. As for any additive manufacturing approach, the understanding of the machine properties and the process behaviour as well as such a selection guide is of upmost importance to foster the implementation of new machining concepts and support design engineers. Geometrical accuracy between digitally designed and physically realized parts made of maraging steel and dimensional limits are analyzed by stripe line projection. In particular, we identify design rules for numerous basic geometric elements like walls, cylinders, angles, inclinations, overhangs, notches, inner and outer radii of spheres, chamfers in build direction, and holes of different shape, respectively, as being manufactured by the hybrid approach and compare them to sole selective laser melting. While the cutting tool defines the manufacturability of, e.g., edges and corners, the milling itself improves the surface roughness to Ra < 2 µm. Thus, the given advantages of this hybrid process, e.g., space-resolved and custom-designed roughness and the superior geometrical accuracy are evaluated. Finally, we exemplify the potential of this particular promising hybrid approach by demonstrating an injection mold with a conformal cooling for a charge socket for an electro mobile KW - hybrid additive manufacturing; high-speed milling; selective laser melting; construction rules KW - Selektives Laserschmelzen KW - Hochgeschwindigkeitsfräsen Y1 - 2021 UR - https://www.mdpi.com/1996-1944/14/19/5753 U6 - https://doi.org/https://doi.org/10.3390/ma14195753 VL - 14 IS - 19 SP - 1 EP - 23 ER - TY - JOUR A1 - Sommer, David A1 - Pape, Dominik A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Tool Wear and Milling Characteristics for Hybrid Additive Manufacturing Combining Laser Powder Bed Fusion and In Situ High-Speed Milling JF - Ultra-Precision Manufacturing Technology for Difficult-to-Machine Materials N2 - 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 KW - hybrid additive manufacturing; high-speed milling; laser powder bed fusion; tool wear KW - Hochgeschwindigkeitsfräsen KW - Werkzeugverschleiß Y1 - 2022 UR - https://www.mdpi.com/1996-1944/15/3/1236 U6 - https://doi.org/https://doi.org/10.3390/ma15031236 VL - 15 IS - 3 SP - 1 EP - 13 ER - TY - CHAP A1 - Sauer, Theresia A1 - Kefer, Stefan A1 - Ruppert, Wolfgang A1 - Hellmann, Ralf A1 - Kaloudis, Michael T1 - Integration of Bragg grating sensors in components made of carbon fiber reinforced polymers T2 - Tagungsband der 20. GMA/ITG-Fachtagung Sensoren und Messsysteme 2019, 25.06.2019 - 26.06.2019, Nürnberg, Deutschland N2 - This contribution discusses the integration of polymer planar Bragg grating sensors (PPBG) into carbon fiber reinforced polymer (CFRP) components. For the first time, it is shown that PPBGs based on cyclic olefin copolymers can be integrated into commercial-grade composites, thereby withstanding the demanding production processes. Pre-impregnated fibers are stacked and partially modified to form a sensor pocket. Afterwards, the CFRP specimen containing the optical sensor is cured in a heated mechanical press for 2 hours at a pressure of 7 bar and a temperature of 120 °C. A subsequent evalutaion of the sensor signal shows a Bragg wavelength shift of 1236 pm and a decline in signal amplitude of -2 dB. Three-point flexural tests of the cured sample reveal a linear behavior of the sensor signal towards external loads. The determined sensitivity in dependence of the CFRP specimen's maximum central deflection is -112 pm/mm, while correlation to the applied force results in a sensitivity of -5 pm/N. KW - Bragg grating sensors KW - carbon fiber reinforced polymers KW - cyclic olefin copolymers KW - structural health monitoring KW - Bragg-Reflektor Y1 - 2019 SN - 978-3-9819376-0-2 U6 - https://doi.org/10.5162/sensoren2019/P3.12 SP - 806 EP - 810 ER - TY - JOUR A1 - Rung, Stefan A1 - Bokan, Kevin A1 - Kleinwort, Frederick A1 - Schwarz, Simon A1 - Simon, Peter A1 - Klein-Wiele, Jan-Hendrik A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Possibilities of Dry and Lubricated Friction Modification Enabled by Different Ultrashort Laser-Based Surface Structuring Methods JF - Lubricants N2 - In this contribution we report on the possibilities of dry and lubricated friction modification introduced by different laser surface texturing methods. We compare the potential of Laser-Induced Periodic Surface Structures and Laser Beam Interference Ablation on 100Cr6 steel in a linear reciprocating ball-on-disc configuration using 100Cr6 steel and tungsten carbide balls with load forces between 50mN and 1000 mN. For dry friction, we find a possibility to reduce the coefficient of friction and we observe a pronounced direction dependency for surfaces fabricated by Laser Beam Interference Ablation. Furthermore, Laser-Induced Periodic Surface Structures result in a load-dependent friction reduction for lubricated linear reciprocating movements. This work helps to identify the modification behaviour of laser generated micro structures with feature sizes of approximately 1μm and reveals new possibilities for surface engineering KW - laser-induced periodic surface structures KW - laser beam interference ablation KW - dry friction KW - lubricated friction KW - laser surface texturing KW - Laser KW - Oberflächenstruktur Y1 - 2019 U6 - https://doi.org/10.3390/lubricants7050043 VL - 7 IS - 43 SP - 1 EP - 13 ER - TY - JOUR A1 - Rung, Stefan A1 - Bokan, Kevin A1 - Rutsch, Kevin A1 - Schwarz, Simon A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Laser Induced Periodic Surface Structures on 100Cr6 Steel for Modification of Friction Demonstrated with Stribeck Test JF - Journal of Laser Micro/Nanoengineering N2 - In this contribution, we report on surface functionalization by introducing laser induced periodic surface structures on 100Cr6 bearing steel to modify complex tribological properties. The advanced approach of this study is the measurement of the coefficient of friction by performing a ball-on-disk Stribeck test on laser structured surfaces with polytetrafluoroethylene balls in a lubricant environment. The Stribeck test reveals the modified friction behavior using translation speeds up to 106 mm/min and load forces between 100 mN and 5000 mN. Our results show increased stiction and coefficient of friction for laser structured surfaces in the regime of boundary lubrication which is attributed to the laser induced surface asperities. Decreased coefficient of friction is observed in the regime of mixed and hydrodynamic lubrication, i.e. for velocities higher than 1000 mm/min, for surfaces covered by laser induced periodic surface structures with low spatial frequency. KW - Laser KW - Oberflächenstruktur Y1 - 2018 U6 - https://doi.org/10.2961/jlmn.2018.03.0004 VL - 13 IS - 3 SP - 160 EP - 165 ER - TY - JOUR A1 - Kefer, Stefan A1 - Sauer, Theresia A1 - Hessler, Steffen A1 - Kaloudis, Michael A1 - Hellmann, Ralf T1 - Microstructure-Based Fiber-to-Chip Coupling of Polymer Planar Bragg Gratings for Harsh Environment Applications JF - Sensors (Basel) N2 - This article proposes and demonstrates a robust microstructure-based fiber-to-chip coupling scheme for planar Bragg grating devices. A polymer planar Bragg grating substrate is manufactured and microstructured by means of a micromilling process, while the respective photonic structures are generated by employing a sophisticated single-writing UV-exposure method. A stripped standard single mode fiber is inserted into the microstructure, which is filled with a UV-curable adhesive, and aligned with the integrated waveguide. After curing, final sensor assembly and thermal treatment, the proposed coupling scheme is capable of withstanding pressures up to 10 bar, at room temperature, and pressures up to 7.5 bar at an elevated temperature of 120 °C. Additionally, the coupling scheme is exceedingly robust towards tensile forces, limited only by the tensile strength of the employed single mode fiber. Due to its outstanding robustness, the coupling scheme enables the application of planar Bragg grating devices in harsh environments. This fact is underlined by integrating a microstructure-coupled photonic device into the center of a commercial-grade carbon fiber-reinforced polymer specimen. After its integration, the polymer-based Bragg grating sensor still exhibits a reflection peak with a dynamic range of 24 dB, and can thus be employed for sensing purposes. KW - Bragg grating sensors KW - cyclic olefin copolymers KW - fiber-to-chip coupling KW - microstructures KW - micromilling Y1 - 2020 UR - https://www.mdpi.com/1424-8220/20/19/5452/htm U6 - https://doi.org/10.3390/s20195452 VL - 20 IS - 19 SP - 5452 EP - 5452 ER - TY - JOUR A1 - Kefer, Stefan A1 - Zettl, Julian A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Femtosecond Laser-Based Micromachining of Rotational-Symmetric Sapphire Workpieces JF - Materials N2 - 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. KW - sapphire KW - femtosecond laser KW - micromachining KW - laser turning KW - ablation threshold KW - Femtosekundenlaser KW - Saphir Y1 - 2022 UR - https://www.mdpi.com/1996-1944/15/18/6233 U6 - https://doi.org/10.3390/ ma15186233 VL - 15 IS - 18 SP - 6233 EP - 6233 ER - TY - JOUR A1 - Sommer, David A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Static and Dynamic Mechanical Behaviour of Hybrid-PBF-LB/M-Built and Hot Isostatic Pressed Lattice Structures JF - Mechanical Properties of Polymeric, Metallic, and Composite Materials N2 - 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. KW - Hybrid additive manufacturing KW - Lattice structures KW - Hot isostatic pressing KW - Fatigue behaviour KW - Isostatisches Heißpressen KW - Selektives Laserschmelzen Y1 - 2023 UR - https://www.mdpi.com/1996-1944/16/9/3556 U6 - https://doi.org/https://doi.org/10.3390/ma16093556 VL - 16 IS - 9 SP - 1 EP - 14 ER - TY - JOUR A1 - Kefer, Stefan A1 - Sauer, Theresia A1 - Hessler, Steffen A1 - Kaloudis, Michael A1 - Schmauss, Bernhard A1 - Hellmann, Ralf T1 - Robust Polymer Planar Bragg Grating Sensors Embedded in Commercial-Grade Composites JF - Polymers N2 - This contribution demonstrates the functionality of polymer planar Bragg grating (PPBG) sensors integrated into commercial-grade carbon fiber reinforced polymer (CFRP) components. Multiple CFRP specimens are generated by curing a stack of pre-impregnated fibers inside of a heated mechanical press, exposing the polymer sensor to a pressure of 7 bar and a temperature of 120 °C for 2 h. After integration, the sensor still exhibits a strong and evaluable signal. Subsequent flexural experiments reveal a linear response of the integrated sensor’s Bragg wavelength to the CFRP specimen’s maximum deflection. Additional findings demonstrate that the embedded PPBG can be used to detect plastic deformations of a CFRP workpiece, whereas a linear correlation of plastic deformation to the resulting Bragg signal offset is determined. A plausibility check of the obtained results is delivered by a comparison of three-point flexural experiments on bulk CFRP workpieces, without integrated sensors and additional specimens featuring external optical sensors affixed to their surface. It is found that PPBGs based on cyclic olefin copolymers are able to overcome the temperature-related limitations of traditional polymer-based optical sensors and can thus be directly integrated into commercial-grade composites during production. KW - Bragg grating sensors KW - carbon fiber reinforced polymers KW - cyclic olefin copolymers KW - Bragg-Reflektor KW - Optischer Sensor Y1 - 2020 UR - https://www.mdpi.com/2073-4360/12/3/715/htm U6 - https://doi.org/10.3390/polym12030715 VL - 12 IS - 3 SP - 715 EP - 715 ER - TY - JOUR A1 - Kefer, Stefan A1 - Bischoff, Kay A1 - Roth, Gian-Luca A1 - Haubner, Julian A1 - Schmauss, Bernhard A1 - Hellmann, Ralf T1 - Tunable Bulk Polymer Planar Bragg Gratings Electrified via Femtosecond Laser Reductive Sintering of CuO Nanoparticles JF - Advanced Optical Materials N2 - This contribution demonstrates and discusses electrically tunable polymer planar Bragg gratings based on bulk cyclic olefin copolymers. A lithographic single-writing-step method and femtosecond laser reductive sintering of copper(II) oxide nanoparticles are subsequently employed in order to generate buried photonic structures and copper conducting paths on top of the polymer substrate. This way, the necessary number of process steps for fabricating a planar polymer-based electro-optical device is greatly reduced. The response of a fully electrified grating structure follows temperature changes, induced by the copper conducting path, with sensitivities up to −31 pm K−1. Dilatometric measurements show that the specimen's behavior is correlated to the situationally reduced thermal expansion of the bulk polymer substrate. In consequence, the tuning response of the photonic platform follows a second order polynomial, whereas a direct current of 30 mA, which correlates to a power consumption of 18.3 mW, leads to a local temperature increase and a residual Bragg wavelength shift of 19.6 K and −547 pm, respectively. Moreover, the outstanding flexibility of the proposed fabrication concept is underlined by demonstrating alternative conducting path geometries, whereas one of the additional designs is adapted to control the spectral width of the Bragg grating's reflection peak. KW - Bragg gratings KW - CuO nanoparticles KW - cyclic olefin copolymers KW - electro-optic tuning KW - integrated photonics KW - Femtosekundenlaser KW - Bragg-Reflektor Y1 - 2021 UR - https://onlinelibrary.wiley.com/doi/10.1002/adom.202002203 U6 - https://doi.org/https://doi.org/10.1002/adom.202002203 VL - 9 IS - 13 SP - 2002203 EP - 2002203 ER - TY - JOUR A1 - Kefer, Stefan A1 - Limbach, Tobias A1 - Pape, Natalie A1 - Klamt, Kathrin A1 - Schmauss, Bernhard A1 - Hellmann, Ralf T1 - Birefringence in Injection-Molded Cyclic Olefin Copolymer Substrates and Its Impact on Integrated Photonic Structures JF - Polymers N2 - This contribution quantifies the birefringence within injection-molded cyclic olefin copolymer plates and discusses its impact on the mechanical properties of the plates. It also focuses on the impact of birefringence on integrated waveguides and Bragg gratings and provides fabrication guidelines for such structures. The anisotropy in all three dimensions of the workpiece is examined by means of polarimetry and a prism coupler. It is found that the birefringence is inhomogenously distributed within the workpieces, whereas the maximum birefringence not only varies locally, but also depends on the observation direction. Overall, a maximum birefringence of 10 × 10−4 is found at the plate’s surface near the injection gate. The anisotropy then reduces exponentially towards the center of the workpiece and saturates at 1.8 × 10−4, in a depth of 0.4 mm. Thus, the birefringence strongly affects near-surface photonic structures. It is found that, depending on their orientation and the local birefringence of the substrate, waveguides and Bragg gratings fabricated with comparable parameters behave completely differently in terms of polarization-dependent optical attenuation, cross-sectional intensity distribution and Bragg reflection signal. For example, the support of the TM mode can vary between total loss and an optical attenuation of 0.9 dB × cm−1. In consequence, this study underlines the importance of quantifying the birefringent state of an injection-molded cyclic olefin copolymer workpiece if it is supposed to serve as a substrate for integrated photonic structures. The study furthermore demonstrates that birefringence effects can be omitted by burying the photonic structures deeper into the volume of the thermoplastic. KW - cyclic olefin copolymer KW - birefringence KW - integrated photonics KW - Bragg grating KW - injection molding KW - Bragg-Reflektor KW - Wellenleiter KW - Doppelbrechung Y1 - 2024 UR - https://www.mdpi.com/2073-4360/16/2/168 U6 - https://doi.org/10.3390/polym16020168 VL - 16 IS - 2 SP - 168 EP - 168 ER - TY - JOUR A1 - Kefer, Stefan A1 - Pape, Natalie A1 - Bischoff, Kay A1 - Schmauss, Bernhard A1 - Hellmann, Ralf T1 - Lattice-Like Waveguides With Integrated Bragg Gratings in Planar Cyclic Olefin Copolymers JF - Journal of Lightwave Technology N2 - This contribution demonstrates femtosecond laser direct writing of lattice-like waveguides in planar cyclic olefin copolymer substrates. Based on numerical simulation and experimental near-field analysis, stable single-mode waveguiding around wavelengths of 1550 nm is demonstrated. The waveguiding mechanism is based on a hexagonal array of laser-induced, positive refractive index modification lines. Thus, the lateral extension of the guided mode can be adapted by varying the fabrication parameters and, in consequence, the resulting cross-sectional arrangement of the refractive index perturbations. With an optical attenuationof 2.2 dB·cm-1 around 1550 nm, the fabricated waveguides are well-suited for on-chip integrated photonic devices. Moreover, the waveguides can also be equipped with Bragg gratings to enable the application of the photonic platform as a sensing device. Dependingon their length, the Bragg grating structures exhibit reflectivities ofup to 99% and spectral widths down to 0.3 nm. The flexibility of the fabrication process and the sensing capabilities of the lattice-like waveguides with integrated Bragg gratings are underlined by an exemplary application study demonstrating a relative pressure sensor. For that, a photonic platform is micromilled to generate a 300 µm thick diaphragm and a reference pressure chamber. The strain introduced to the diaphragm by external pressure changes can then be quantified by the integrated photonic structures.This way, absolute pressure sensitivities of up to 38 pm·kPa-1 can be achieved in a relative pressure range from −60 to 100 kPa. The newly-developed lattice-like waveguides with integrated Bragggratings are therefore well-suited for the realization of novel and adaptable photonic devices and sensors. KW - Bragg grating KW - cyclic olefin copolymers KW - femtosecond laser KW - integrated photonics KW - lattice-like waveguide KW - pressure sensor KW - Wellenleiter KW - Bragg-Reflektor Y1 - 2024 U6 - https://doi.org/10.1109/JLT.2023.3328323 SN - 0733-8724 VL - 42 IS - 18 SP - 6302 EP - 6311 PB - Institute of Electrical and Electronics Engineers (IEEE) ER - TY - JOUR A1 - Häfner, Tom A1 - Bischoff, Kay A1 - Helm, Jonas A1 - Kunz, Tim A1 - Rung, Stefan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Picosecond laser microvia drilling of ABF material using MHz burst mode JF - Materials Research Express N2 - 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. KW - Ultrakurzzeitlaser KW - Pikosekundenbereich Y1 - 2023 VL - 10 IS - 9 SP - 096301 EP - 096311 ER -