@article{GradNadammalSchultheissetal., author = {Grad, Marius and Nadammal, Naresh and Schultheiss, Ulrich and Lulla, Philipp and Noster, Ulf}, title = {An Integrative Experimental Approach to Design Optimization and Removal Strategies of Supporting Structures Used during L-PBF of SS316L Aortic Stents}, series = {Applied Sciences}, volume = {11}, journal = {Applied Sciences}, number = {19}, publisher = {MPDL}, doi = {10.3390/app11199176}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:898-opus4-25612}, pages = {1 -- 22}, abstract = {One of the fundamental challenges in L-PBF of filigree geometries, such as aortic stents used in biomedical applications, is the requirement for a robust yet easily removable support structure that allows each component to be successfully fabricated without distortion. To solve this challenge, an integrative experimental approach was attempted in the present study by identifying an optimal support structure design and an optimized support removal strategy for this design. The specimens were manufactured using four different support structure designs based on the geometry exposed to the laser beam during the L-PBF. Support removal procedures included sand blasting (SB), glass bead blasting (GB), and electrochemical polishing (ECP). The two best-performing designs (line and cross) were chosen due to shorter lead times and lower material consumption. As an additional factor that indicates a stable design, the breaking load requirement to remove the support structures was determined. A modified line support with a 145° included angle was shown to be the best support structure design in terms of breaking load, material consumption, and manufacturing time. All three procedures were used to ensure residue-free support removal for this modified line support design, with ECP proving to be the most effective.}, language = {en} } @inproceedings{MaiwaldTroegerHierl, author = {Maiwald, Frederik and Tr{\"o}ger, Johannes and Hierl, Stefan}, title = {Automated weld seam evaluation and 2D simulation parameter calibration for absorber-free laser transmission welding}, series = {Lasers in Manufacturing Conference (LIM 2023), 26. Juni bis 29. Juni 2023, M{\"u}nchen}, booktitle = {Lasers in Manufacturing Conference (LIM 2023), 26. Juni bis 29. Juni 2023, M{\"u}nchen}, publisher = {Wissenschaftliche Gesellschaft Lasertechnik und Photonik e.V. (WLT)}, pages = {10}, abstract = {Absorber-free laser transmission welding enables clean and precise joining of plastics without additives or adhesives. It is therefore well suited to produce optical and medical devices, which place high demands on cleanliness and accuracy. However, the weld usually has an undesirably large vertical expansion, causing bulges and distortion. To improve this, the intensity distribution of the laser beam as well as the processing strategy must be adapted. Due to the complexity, this is aided by process simulation. However, simulation parameter calibration and verification are usually done considering the seam width and height, which is of limited significance. To overcome this, we propose a new method for image processing of microtome sections, determining the spatially resolved geometry of the weld. Thus, the deviation between experiment and simulation can be calculated pixel by pixel. This spatially resolved value is predestined for the calibration of the simulation parameters: For a parameter field with 18 different settings, the total deviation between experiment and simulation is less than 11 \% after calibration.}, language = {en} } @article{GradNadammalTytkoetal., author = {Grad, Marius and Nadammal, Naresh and Tytko, Darius and Noster, Ulf}, title = {On the nanoindentation behavior of a TiC layer formed through thermo-reactive diffusion during hot pressing of Ti and cast iron}, series = {Materials Letters: X}, volume = {15}, journal = {Materials Letters: X}, number = {September}, publisher = {Elsevier}, address = {Amsterdam, Niederlande}, issn = {2590-1508}, doi = {10.1016/j.mlblux.2022.100161}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:898-opus4-50066}, pages = {1 -- 4}, abstract = {Hot pressing of pure Ti and cast iron under vacuum formed a ~15 μm wide TiC-layer through the thermoreactive diffusion process. Nanoindentation testing of the TiC-layer revealed average hardness and elastic modulus values of ~35 GPa and ~400 GPa, respectively. High-resolution indentation mapping indicated hardness change across the boundary with peak values observed within the TiC-layer. Solid state high temperature diffusion holds significant potential for developing uniform hard coatings on pure Ti/Ti-alloys.}, language = {en} } @misc{GradHahnNosteretal., author = {Grad, Marius and Hahn, Konstantin and Noster, Ulf and Spieß, Lothar}, title = {Increasing the lifetime of titanium implants through a diffusion-controlled surface treatment}, series = {Th{\"u}ringer Werkstofftag 2023, 14. M{\"a}rz 2023, ILmenau}, journal = {Th{\"u}ringer Werkstofftag 2023, 14. M{\"a}rz 2023, ILmenau}, doi = {10.13140/RG.2.2.35933.59360}, language = {en} } @misc{MeisnerKaftiranisMaiwaldetal., author = {Meisner, Dennis and Kaftiranis, Nikitas and Maiwald, Frederik and Rapp, Petra and Hierl, Stefan}, title = {Enhancing Layer Adhesion in Material Extrusion AM with Adaptive Laser Preheating Integrated in Printhead}, series = {LiM Conference 2025, Lasers in Manufacturing, June 23 - 26, 2025, Munich, Germany}, journal = {LiM Conference 2025, Lasers in Manufacturing, June 23 - 26, 2025, Munich, Germany}, publisher = {Wissenschaftliche Gesellschaft Lasertechnik und Photonik e.V.}, abstract = {Material extrusion is a widely used AM process and is gaining more acceptance in industry applications due to its material variety, flexibility, and low cost. However, its usage is limited by a process-related anisotropy caused by insufficient interlayer bonding due to reduced temperature in the process zone. To enhance layer adhesion, an adaptive laser preheating system is integrated in a conventional printhead around the extrusion nozzle. The setup with eight fiber coupled diode laser allows preheating in feed direction and investigation of various intensity profiles. The article describes the experimental setup and significant improvements achieved. Laser preheating with different intensity distributions (spot, sickle or ring shaped) show an incasement of up to 85 \% of the mechanical properties in the build-up direction. However, due to the additional energy input by the laser, controlled cooling of the workpiece becomes a crucial factor and is investigated as well.}, language = {en} } @article{WiesentSchultheissLullaetal., author = {Wiesent, Lisa and Schultheiß, Ulrich and Lulla, Philipp and Nonn, Aida and Noster, Ulf}, title = {Mechanical properties of small structures built by selective laser melting 316 L stainless steel - a phenomenological approach to improve component design}, series = {Materials Science \& Engineering Technology}, volume = {51}, journal = {Materials Science \& Engineering Technology}, number = {12}, publisher = {Wiley}, doi = {10.1002/mawe.202000038}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:898-opus4-14718}, pages = {1615 -- 1629}, abstract = {Experimental investigations are conducted to quantify the influence of specimen thickness and orientation on the mechanical properties of selective laser melted stainless steel 316 L. The results indicate that the mechanical strength and ductility increase with increasing specimen thickness until a saturation value is reached from a specimen thickness of about 2 mm. Specimen orientation dependency is pronounced for thin specimens (<1.5 mm), whereas only small deviations in strength are observed for thicker specimens with orientations of 30°, 45° and 90° to build direction. The mechanical properties of the specimen orientation of 0° to build direction shows great deviation to the other orientations and the smallest overall strength. A reliable design of selective laser melted components should account for specimen thickness and orientation, e. g. by a correction factor. Furthermore, it is recommended to avoid loads vertical (90°) and parallel (0°) to build direction to guarantee higher ductility and strength.}, language = {en} } @article{WiesentSchultheissLullaetal., author = {Wiesent, Lisa and Schultheiß, Ulrich and Lulla, Philipp and Noster, Ulf and Schratzenstaller, Thomas and Schmid, Christof and Nonn, Aida and Spear, Ashley}, title = {Computational analysis of the effects of geometric irregularities and post-processing steps on the mechanical behavior of additively manufactured 316L stainless steel stents}, series = {PLoS ONE}, volume = {15}, journal = {PLoS ONE}, number = {12}, publisher = {PLOS}, doi = {10.1371/journal.pone.0244463}, pages = {1 -- 30}, abstract = {Advances in additive manufacturing enable the production of tailored lattice structures and thus, in principle, coronary stents. This study investigates the effects of process-related irregularities, heat and surface treatment on the morphology, mechanical response, and expansion behavior of 316L stainless steel stents produced by laser powder bed fusion and provides a methodological approach for their numerical evaluation. A combined experimental and computational framework is used, based on both actual and computationally reconstructed laser powder bed fused stents. Process-related morphological deviations between the as-designed and actual laser powder bed fused stents were observed, resulting in a diameter increase by a factor of 2-2.6 for the stents without surface treatment and 1.3-2 for the electropolished stent compared to the as-designed stent. Thus, due to the increased geometrically induced stiffness, the laser powder bed fused stents in the as-built (7.11 ± 0.63 N) or the heat treated condition (5.87 ± 0.49 N) showed increased radial forces when compressed between two plates. After electropolishing, the heat treated stents exhibited radial forces (2.38 ± 0.23 N) comparable to conventional metallic stents. The laser powder bed fused stents were further affected by the size effect, resulting in a reduced yield strength by 41\% in the as-built and by 59\% in the heat treated condition compared to the bulk material obtained from tensile tests. The presented numerical approach was successful in predicting the macroscopic mechanical response of the stents under compression. During deformation, increased stiffness and local stress concentration were observed within the laser powder bed fused stents. Subsequent numerical expansion analysis of the derived stent models within a previously verified numerical model of stent expansion showed that electropolished and heat treated laser powder bed fused stents can exhibit comparable expansion behavior to conventional stents. The findings from this work motivate future experimental/numerical studies to quantify threshold values of critical geometric irregularities, which could be used to establish design guidelines for laser powder bed fused stents/lattice structures.}, subject = {Koronarendoprothese}, language = {en} } @article{MaiwaldEnglmaierHierl, author = {Maiwald, Frederik and Englmaier, Stephan and Hierl, Stefan}, title = {Online pyrometry for weld seam localization in absorber-free laser transmission welding of transparent polymers}, series = {Journal of Laser Micro/Nanoengineering}, volume = {Vol. 16}, journal = {Journal of Laser Micro/Nanoengineering}, number = {1}, publisher = {Japan Laser Processing Society}, issn = {1880-0688}, doi = {10.2961/jlmn.2021.01.2002}, pages = {8 -- 13}, abstract = {Production of optical and medical devices makes high demands on cleanliness, precision, visual appearance and reliability. Advantages of laser transmission welding - contactless input of energy, high precision, no adhesives and no particle formation - can be fully exploited for this use. For absorber-free welding, lasers emitting in the polymers' intrinsic absorption spectrum between 1.6 μm and 2 μm are used. Focusing the laser beam with high NA provides large intensity gradients inside the specimen and enables selective fusing of the joining zone without affecting the surface. Since a molten upper surface leads to visible and palpable irregularities and causes warpage, the verticalexpansion of the weld seam is crucial. To meet the high quality and reliability required in the medical and optical industry, online monitoring of the seam is necessary. The aim of this work is the localization and evaluation of the weld seam using pyrometry. First, the emitted thermal radiation is computed, enabling signal analysis regarding origin and spectrum. Thus, assessment of measurement spectrum and forecast of the signal in dependence on seam shape and position is possible. Finally, welding tests with Topas 8007-04 are performed at 200 mm/s feed rate and monitored with 100 kHz. The comparison of tracked data with thin-cuts demonstrates that weld seam localization by pyrometry is possible, enabling online assessment of the process.}, subject = {Laserdurchstrahlschweissen}, language = {en} } @inproceedings{MaiwaldEnglmaierHierl, author = {Maiwald, Frederik and Englmaier, Stephan and Hierl, Stefan}, title = {Online pyrometry for weld seam localization in absorber-free laser transmission welding of transparent polymers}, series = {21st International Symposium on Laser Precision Microfabrication: Dresden, 26.06.2020}, booktitle = {21st International Symposium on Laser Precision Microfabrication: Dresden, 26.06.2020}, publisher = {Japan Laser Processing Society}, abstract = {Optical and medical devices made of transparent polymers are gaining popularity because of its cost advantages. The manufacturing technology must be able to meet the high demands of these applications. Advantages of laser transmission welding - contactless input of energy, precision, no adhesives and no particle formation - can be exploited here. For absorber-free welding, lasers emitting in the polymers' intrinsic absorption spectrum are used. Focusing the laser beam with high NA enables selective fusing of the joining zone. Since a molten upper surface leads to visible and palpable irregularities, the lateral expansion of the weld seam is monitored in this work. Welding tests with 1 mm thick plates of COC are performed and monitored by pyrometry. Distances between seam and surface measured in thin cuts and are compared with the pyrometer signal. This shows that the localization of the weld seam is possible and proper and faulty parts can be distinguished by pyrometry.}, subject = {Laserdurchstrahlschweissen}, language = {en} } @article{MaiwaldHierl, author = {Maiwald, Frederik and Hierl, Stefan}, title = {Pyrometerbasierte Prozess{\"u}berwachung beim Laserkunststoffschweißen}, series = {Photonik}, journal = {Photonik}, number = {52}, publisher = {AT-Fachverl.}, address = {Fellbach}, pages = {22 -- 25}, subject = {Laserschweissen}, language = {de} } @inproceedings{KaesbauerSchmailzlPrehmetal., author = {K{\"a}sbauer, Johannes and Schmailzl, Anton and Prehm, Jens and Loose, Tobias and Hierl, Stefan}, title = {Simulation of Quasi-Simultaneous Laser Transmission Welding of Plastics}, series = {Procedia CIRP}, volume = {94}, booktitle = {Procedia CIRP}, publisher = {Elsevier}, doi = {10.1016/j.procir.2020.09.136}, pages = {737 -- 741}, abstract = {Thermo-mechanical simulation offers great opportunities to optimize welding processes of plastics. For realistic simulation, the temperature dependent mechanical properties need to be implemented from ambient temperature to temperatures above the flow temperature. Standard test methods are insufficient for characterization in the entire temperature range because close to the flow temperature the material is too soft for tensile tests and too stiff for rheometry. Therefore, an optimization strategy is developed, that determines unknown material parameters by testing in welding simulations. The unknown parameters are iteratively adjusted to minimize the mismatch between computed and measured set-paths. Thus, important process characteristics are calculated realistically, enabling the computer aided assessment of the weld quality.}, subject = {Kunststoffschweißen}, language = {en} } @inproceedings{MaiwaldEnglmaierHierl, author = {Maiwald, Frederik and Englmaier, Stephan and Hierl, Stefan}, title = {Absorber-free Laser Transmission Welding of Transparent Polymers using Fixed Focus Optics and 3D Laser Scanner}, series = {Procedia CIRP}, volume = {94}, booktitle = {Procedia CIRP}, publisher = {Elsevier}, doi = {10.1016/j.procir.2020.09.117}, pages = {686 -- 690}, abstract = {Laser transmission welding is a well-known joining technology for thermoplastics, providing reliable and hermetical sealing without adhesives or particle formation. The main advantages of laser-based energy input - high precision, no additional adhesive and no particle emission - are essential for medical and optical applications. To obtain sufficient absorption in visually transparent polymers, thulium fibre lasers emitting in the polymers' intrinsic absorption spectrum are used. Optics with high numerical aperture provide large intensity gradients inside the specimen, enabling selective fusing in the joining zone. Although the basic feasibility has already been demonstrated, the welding process lacks stability and productivity. Aim of this work is the determination of optimized settings for a fast and reliable welding process. Thus, the interplay of process parameters as well as their impact on the seam are analysed by thermal simulations. Calculated settings are verified by welding tests with COC, PA6 and PETG, using a fixed-focus optics and a 3D laser scanner with up to 200 mm/s feed rate.}, language = {en} } @article{SchmailzlKaesbauerMartanetal., author = {Schmailzl, Anton and K{\"a}sbauer, Johannes and Martan, Jiř{\´i} and Honnerov{\´a}, Petra and Sch{\"a}fer, Felix and Fichtl, Maximilian and Lehrer, Tobias and Pruš{\´a}kov{\´a}, L. and Tesař, Jiř{\´i} and Sk{\´a}la, J. and Honner, Milan and Hierl, Stefan}, title = {Measurement of Core Temperature through Semi-Transparent Polyamide 6 using Scanner-Integrated Pyrometer in Laser Welding}, series = {International Journal of Heat and Mass Transfer}, volume = {146}, journal = {International Journal of Heat and Mass Transfer}, number = {January}, publisher = {Elsevier}, doi = {10.1016/j.ijheatmasstransfer.2019.118814}, abstract = {Predicting the core temperature during welding is an ambitious aim in many research works. In this work, a 3D-scanner with integrated pyrometer is characterized and used to measure the temperature during quasi-simultaneous laser transmission welding of polyamide 6. However, due to welding in an overlap configuration, the heat radiation emitted from the joining zone of a laser transmission weld has to pass through the upper polymer, which is itself a semi-transparent emitter. Therefore, the spectral filtering of the heat radiation in the upper polymer is taken into account by calibrating the pyrometer for the measurement task. Thermal process simulations are performed to compare the temperature field with the measured temperature signal. The absorption coefficients of the polymers are measured, in order to get precise results from the computation. The temperature signals during welding are in good agreement with the computed mean temperature inside the detection spot, located in the joining area. This is also true for varying laser power, laser beam diameter and the carbon black content in the lower polymer. Both, the computed mean temperature and the temperature signal are representing the core temperature. In order to evaluate the spatial sensitivity of the measurement system, the emitted heat radiation from both polymers is calculated on basis of the computed temperature field. Hereby it is found, that more than 90 percent of the detected heat radiation comes from the joining area, which is a crucial information for contact-free temperature measurement tasks on semi-transparent polymers.}, language = {en} } @inproceedings{SchmailzlQuandtHierletal., author = {Schmailzl, Anton and Quandt, Benjamin and Hierl, Stefan and Schmidt, Michael}, title = {Correlation between Joint Strength and Process Temperature in Quasi-Simultaneous Laser Transmission Welding of Polyamide 6}, series = {Proceedings of LiM2019 - Lasers in Manufacturing (23 June 2019, Munich)}, booktitle = {Proceedings of LiM2019 - Lasers in Manufacturing (23 June 2019, Munich)}, publisher = {Wissenschaftliche Gesellschaft Lasertechnik e.V.}, pages = {1 -- 10}, abstract = {The joint strength is gaining importance in quasi -simultaneous laser transmission welds, especially in structural components. A correlation between the joint strength and a process characteristic is a basic requirement for selecting the best process parameter setting. In this work, the temperature is measured during welding polyamide 6 by using a scanner-integrated pyrometer with an InGaAs-detector. The filtering of the heat radiation in the upper joining partner is taken into account by calibrating the mea surement system. By this, the measured temperature signal represents the temperature in the joining zone. A correlation is found between the measured temperature and the resulting joint strength. Moreover, high joint strengths are also seen for welds with short welding times, as far as the temperature is sufficiently high. With this knowledge, a process window can be derived easily in order to produce welds with high strength and short welding times.}, language = {en} } @article{KaesbauerSchmailzlWeberetal., author = {K{\"a}sbauer, Johannes and Schmailzl, Anton and Weber, Ulrich and Hierl, Stefan and Jaus, Tobias and Schwalme, Michael}, title = {Simulationsgest{\"u}tzte Evaluierung von Strahloszillationsmustern beim quasi-simultanen Laser-Durchstrahlschweißen}, series = {JOINING PLASTICS - F{\"u}gen von Kunststoffen}, volume = {13}, journal = {JOINING PLASTICS - F{\"u}gen von Kunststoffen}, number = {2}, publisher = {DVS-Verl., Verl. f{\"u}r Schweißen und Verwandte Verfahren}, address = {D{\"u}sseldorf}, pages = {102 -- 109}, language = {de} } @inproceedings{QuandtSchroeckerHierl, author = {Quandt, Benjamin and Schr{\"o}cker, Korbinian and Hierl, Stefan}, title = {Prozess{\"u}berwachung beim quasi-simultanen Laser-Durchstrahlschweißen glasfaserverst{\"a}rkter Thermoplaste}, series = {9. Landshuter Leichtbau-Colloquium: Leichtbau in Forschung und industrieller Anwendung von der Nano- bis zur Makroebene, 27. / 28. Februar 2019 Hochschule Landshut, Tagungsband zum Colloquium}, booktitle = {9. Landshuter Leichtbau-Colloquium: Leichtbau in Forschung und industrieller Anwendung von der Nano- bis zur Makroebene, 27. / 28. Februar 2019 Hochschule Landshut, Tagungsband zum Colloquium}, editor = {Huber, Otto and Bicker, Marc and Patzelt, Peter}, publisher = {LC-Verlag}, address = {Landshut}, pages = {56 -- 64}, language = {de} } @misc{KaesbauerSchmailzlLooseetal., author = {K{\"a}sbauer, Johannes and Schmailzl, Anton and Loose, Tobias and Hierl, Stefan}, title = {Thermo-Mechanical Modeling of Quasi-Simultaneous Laser Transmission Welding using LS-DYNA with Focus on Accuracy of Heat Input Calculation}, series = {Technology Day 2020 - Plastics on the test rig, Traboch, 04.03.2020}, journal = {Technology Day 2020 - Plastics on the test rig, Traboch, 04.03.2020}, language = {en} } @article{HuentelmannHierl, author = {H{\"u}ntelmann, Sven and Hierl, Stefan}, title = {AM-gerechte Bauteilgestaltung}, series = {additive - Das Magazin f{\"u}r generative Fertigung}, journal = {additive - Das Magazin f{\"u}r generative Fertigung}, number = {1}, publisher = {Konradin-Verlag Robert Kohlhammer GmbH}, address = {Leinfelden-Echterdingen}, pages = {57}, language = {de} } @misc{KaesbauerSchmailzlLooseetal., author = {K{\"a}sbauer, Johannes and Schmailzl, Anton and Loose, Tobias and Hierl, Stefan}, title = {Potentials of the EFG-Method for Modeling Quasi-Simultaneous Laser Transmission Welding Considering the Melt Flow}, series = {Simulationsforum 2019 - Schweißen und W{\"a}rmebehandlung, Weimar}, journal = {Simulationsforum 2019 - Schweißen und W{\"a}rmebehandlung, Weimar}, language = {en} } @misc{MaiwaldHierl, author = {Maiwald, Frederik and Hierl, Stefan}, title = {Absorberfreies Laser-Durchstrahlschweißen transparenter Kunststoffe}, series = {InnovationForum Medizintechnik, Tuttlingen, 24.10.2019}, journal = {InnovationForum Medizintechnik, Tuttlingen, 24.10.2019}, language = {de} } @inproceedings{MeisnerForstnerKaftiranisetal., author = {Meisner, Dennis and Forstner, Lukas and Kaftiranis, Nikitas and Hierl, Stefan}, title = {Investigation of process improvements through laser preheating in extrusion-based additive manufacturing process}, series = {Lasers in Manufacturing Conference (LIM 2023), 26. Juni bis 29. Juni 2023, M{\"u}nchen}, booktitle = {Lasers in Manufacturing Conference (LIM 2023), 26. Juni bis 29. Juni 2023, M{\"u}nchen}, publisher = {Wissenschaftliche Gesellschaft Lasertechnik und Photonik e.V. (WLT)}, pages = {10}, abstract = {Fused layer modeling (FLM) is widely used and is gaining more acceptance in the industry mainly due to its material variety and low costs. However, the usage is limited by a process-related anisotropy of the produced parts. The strength and ductility of the printed parts are significantly lower in the build-up direction than perpendicular to it. This is caused by insufficient interlayer bonding resulting from a reduced surface temperature in the process zone. To overcome this problem, a diode laser is integrated into the conventional FLM process to increase the surface temperature between the already printed surface and the newly applied substrate directly at the deposition zone. The investigations carried out show a significant improvement in the mesostructure, as well as a clear reduction in the anisotropy of the printed test specimens.}, language = {en} }