@article{MaiwaldHierl, author = {Maiwald, Frederik and Hierl, Stefan}, title = {Absorber-free laser transmission welding of transparent polymers}, series = {Laser Systems Europe}, volume = {50}, journal = {Laser Systems Europe}, number = {Spring 2021}, publisher = {Europa Science}, address = {Cambridge}, language = {en} } @misc{MaiwaldSchulzeSchmidtetal., author = {Maiwald, Frederik and Schulze, Julian and Schmidt, Michael and Hierl, Stefan}, title = {In-situ optical coherence tomography for spatio-temporal analysis of melt pool crystallization in absorber-free laser transmission welding of polymers}, series = {The 9th International Congress on Laser Advanced Materials Processing (LAMP2025), Ise-city, Mie-prefecture, Japan, June 10 to June 13, 2025}, journal = {The 9th International Congress on Laser Advanced Materials Processing (LAMP2025), Ise-city, Mie-prefecture, Japan, June 10 to June 13, 2025}, publisher = {Japan Laser Processing Society}, language = {en} } @article{ReindlMeisnerHierl, author = {Reindl, Thomas and Meisner, Dennis and Hierl, Stefan}, title = {Benchmarking of plastic-based Additive Manufacturing Processes}, series = {RTejournal - Forum f{\"u}r Rapid Technologie}, journal = {RTejournal - Forum f{\"u}r Rapid Technologie}, publisher = {Fachhochschule Aachen}, doi = {https://doi.org/10.58134/fh-aachen-rte_2023_002}, abstract = {Additive Manufacturing (AM) is a future-oriented manufacturing technology that is experiencing an enormous boom in the times of Industry 4.0. As a result, various AM technologies and printer models from different manufacturers are entering the market over a short time span. With the advancing establishment of this manufacturing technology for series applications, the expectations and requirements of the fabricated components are also increasing. However, a major challenge is the application-specific selection of the most suitable AM process due to a lack of comparable data. Furthermore, there needs to be more know-how regarding the geometrical and mechanical characteristics of AM parts. This paper addresses this problem by comparing the three most common plasticbased AM processes in the areas of surface quality, dimensional accuracy, and mechanical properties. Roughness measurements, evaluation of a benchmark artifact, tensile tests, and load increase tests are carried out. Based on the results, the individual possibilities and limitations of the compared AM processes can be detected.}, language = {en} } @inproceedings{MaiwaldKrothLaskinetal., author = {Maiwald, Frederik and Kroth, Lea and Laskin, Alexander and Hierl, Stefan and Schmidt, Michael}, title = {Enlarging the process window in absorber-free laser transmission welding of polymer foils using tailored laser intensity distribution}, series = {Procedia CIRP}, volume = {124}, booktitle = {Procedia CIRP}, publisher = {Elsevier}, doi = {10.1016/j.procir.2024.08.159}, pages = {489 -- 493}, abstract = {Absorber-free laser transmission welding enables precise and clean joining of polymer foils without absorbent additives or adhesives. It is well suited for applications in medical technology and food industry, which impose high demands on process reliability. To achieve a large process window and thus a reliable process, a homogeneous weld seam temperature is desirable. For this purpose, the intensity distribution of the laser beam is adapted locally by refractive beam shaping optics. Using a donut-shaped intensity distribution, the weld seam temperature is homogenized. Thus, the process window for welding polypropylene or polyethylene foils is enlarged up to a factor of 4 compared to a conventional, Gaussian-shaped distribution. This enables the reliable welding of even 85 µm thin foils, which could only be welded to a limited extent with a conventional laser intensity distribution.}, language = {en} } @article{KuettnerRathsFischeretal., author = {Kuettner, Andreas and Raths, Max and Fischer, Samuel and Laumer, Tobias}, title = {Heat staking of polymer parts generated by fused layer modeling}, series = {The International Journal of Advanced Manufacturing Technology}, volume = {128}, journal = {The International Journal of Advanced Manufacturing Technology}, publisher = {Springer Nature}, doi = {10.1007/s00170-023-11850-y}, pages = {547 -- 562}, abstract = {Heat staking is a joining technology by which thermoplastic pins are formed by force and temperature to create a form- and force-fitting connection between components. This paper examines the characteristics of 3D printed pins in comparison to conventionally turned pins for heat staking applications. The 3D printed pins are created using fused layer modeling, with variations in horizontal and vertical building directions, as well as different layer thicknesses. The study investigates the impact of significant factors on the heat staking process, including the forming force and temperature. Tensile tests, micrographs, and micro-CT measurements were conducted to determine the properties of the heat-staked joints. Additionally, a stage plan was developed to enhance the understanding of the forming process of both printed and conventionally turned materials. The findings suggest that, under specific process parameters, 3D printed pins exhibit comparable strength to conventionally manufactured pins. The research also demonstrates that the anisotropy resulting from the layer-by-layer construction of the pins significantly influences the strength of the connection. Furthermore, the study reveals that 3D printed pins exhibit good forming accuracy during the heat staking process, and the cavities formed during printing can be substantially reduced.}, language = {en} } @misc{MaiwaldHierl, author = {Maiwald, Frederik and Hierl, Stefan}, title = {Laser welding device for clamping and welding components and method for clamping and welding components}, organization = {Ostbayerische Technische Hochschule Regensburg}, language = {en} } @article{RathsBauerKuettneretal., author = {Raths, Max and Bauer, Lukas and Kuettner, Andreas and Fischer, Samuel and Laumer, Tobias}, title = {Gradual error detection technique for non-destructive assessment of density and tensile strength in fused filament fabrication processes}, series = {The International Journal of Advanced Manufacturing Technology}, journal = {The International Journal of Advanced Manufacturing Technology}, number = {131}, publisher = {Springer}, address = {London}, issn = {1433-3015}, doi = {10.1007/s00170-024-13280-w}, pages = {4149 -- 4163}, abstract = {Fused filament fabrication (FFF) is a widely used additive manufacturing process for producing functional components and prototypes. The FFF process involves depositing melted material layer-by-layer to build up 3D physical parts. The quality of the final product depends on several factors, including the component density and tensile strength, which are typically determined through destructive testing methods. X-ray microtomography (XCT) can be used to investigate the pore sizes and distribution. These approaches are time-consuming, costly, and wasteful, making it unsuitable for high-volume manufacturing. In this paper, a new method for non-destructive determination of component density and estimation of the tensile strength in FFF processes is proposed. This method involves the use of gradual error detection by sensors and convolutional neural networks. To validate this approach, a series of experiments has been conducted. Component density and tensile strength of the printed specimens with varying extrusion factor were measured using traditional destructive testing methods and XCT. The cumulative error detection method was used to predict the same properties without destroying the specimens. The predicted values were then compared with the measured values, and it was observed that the method accurately predicted the component density and tensile strength of the tested parts. This approach has several advantages over traditional destructive testing methods. The method is faster, cheaper, and more environmentally friendly since it does not require the destruction of the product. Moreover, it facilitates the testing of each individual part instead of assuming the same properties for components from one series. Additionally, it can provide real-time feedback on the quality of the product during the manufacturing process, allowing for adjustments to be made as needed. The advancement of this approach points toward a future trend in non-destructive testing methodologies, potentially revolutionizing quality assurance processes not only for consumer goods but various industries such as electronics or automotive industry. Moreover, its broader applications extend beyond FFF to encompass other additive manufacturing techniques such as selective laser sintering (SLS), or electron beam melting (EBM). A comparison between the old destructive testing methods and this innovative non-destructive approach underscores the possible fundamental change toward more efficient and sustainable manufacturing practices. This approach has the potential to significantly reduce the time and cost associated with traditional destructive testing methods while ensuring the quality of FFF-manufactured products.}, language = {en} } @article{BartschBurgerGradetal., author = {Bartsch, Alexander and Burger, Moritz and Grad, Marius and Esper, Lukas and Schultheiß, Ulrich and Noster, Ulf and Schratzenstaller, Thomas}, title = {Enhancement of laser cut edge quality of ultra-thin titanium grade 2 sheets by applying an in-process approach using modulated Yb:YAG continuous wave fiber laser}, series = {Discover Mechanical Engineering}, journal = {Discover Mechanical Engineering}, number = {10}, publisher = {Springer}, doi = {10.1007/s44245-023-00018-3}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:898-opus4-65647}, pages = {9}, abstract = {Titanium is used in many areas due to its excellent mechanical, biological and corrosion-resistant properties. Implants often have thin and filigree structures, providing an ideal application for fine cutting with laser. In the literature, the main focus is primarily on investigating and optimizing the parameters for titanium sheets with thicknesses greater than 1 mm. Hence, in this study, the basic manufacturing parameters of laser power, cutting speed and laser pulse of a 200 W modulated fiber laser are investigated for 0.15 mm thick grade 2 titanium sheets. A reproducible, continuous cut could be achieved using 90 W laser-power and 2 mm/s cutting-speed. Pulse pause variations between 85 and 335 μs in 50 μs steps and a fixed pulse width of 50 μs show that a minimum kerf width of 23.4 μm, as well as a minimum cut edge roughness Rz of 3.59 μm, is achieved at the lowest pulse pause duration. An increase in roughness towards the laser exit side, independent of the laser pulse pause duration, was found and discussed. The results provide initial process parameters for cutting thin titanium sheets and thus provide the basis for further investigations, such as the influence of cutting gas pressure and composition on the cut edge.}, language = {en} } @unpublished{GradHaagHahnetal., author = {Grad, Marius and Haag, Lydia and Hahn, Konstantin and Schultheiß, Ulrich and Esper, Lukas and Noster, Ulf}, title = {Influence of carbon content on the formation of TiC at diffusion bonded titanium-steel interface}, publisher = {Elsevier}, doi = {10.2139/ssrn.4261928}, abstract = {Hot pressing of pure Ti and various carbon steels in a temperature range of 950 - 1050 °C creates an up to 9 μm thick compound layer of TiC at the Ti/ steel interface. The calculation of the activation energy for layer formation is 126.5 - 136.7 kJ/mol, independent of the steels carbon content. As the carbon content of the steel increases, the layer thickness also increases, which provides enormous potential for the surface modification of Ti/ Ti-alloys.}, language = {en} } @unpublished{BurgerBartschGradetal., author = {Burger, Moritz and Bartsch, Alexander and Grad, Marius and Esper, Lukas and Schultheiß, Ulrich and Noster, Ulf and Schratzenstaller, Thomas}, title = {Enhancement of laser cut edge quality of ultra-thin titanium grade 2 sheets by applying in-process approach using modulated Yb:YAG continuous wave fibre laser}, doi = {10.21203/rs.3.rs-2520041/v1}, abstract = {Titanium is used in many areas due to its excellent mechanical, biological and corrosion-resistant properties. Implants often have thin and filigree structures, providing an ideal application for laser fine cutting. In literature, the main focus is primarily on investigating and optimizing the parameters for titanium sheet thicknesses greater than 1 mm. Hence, in this study, the basic manufacturing parameters of laser power, cutting speed and laser pulsing of a 200 W modulated fibre laser are investigated for 0.15 mm thick titanium grade 2 sheets. A reproducible, continuous cut could be achieved using 90 W laserpower and 2 cutting-speed. Pulse pause variations between 85-335 μs in 50 μs steps and fixed pulse duration of 50 μs show that a minimum kerf width of 23.4 μm, as well as a minimum cut edge roughness Rz of 3.59 μm, is achieved at the lowest pulse pause. An increase in roughness towards the laser exit side, independent of the laser pulse pause, was found and discussed. The results provide initial process parameters for cutting thin titanium sheets and thus provide the basis for further investigations, such as the influence of cutting gas pressure and composition on the cut edge.}, language = {en} } @misc{EsperNosterSchultheissetal., author = {Esper, Lukas and Noster, Ulf and Schultheiss, Ulrich and Bund, Andreas}, title = {Quasi-in-Situ Analysis of Electropolished Additively Manufactured Stainless Steel Surfaces}, series = {ECS Meeting Abstracts, F01: Advances in Industrial Electrochemistry and Electrochemical Engineering}, volume = {MA2023-02}, journal = {ECS Meeting Abstracts, F01: Advances in Industrial Electrochemistry and Electrochemical Engineering}, publisher = {The Electrochemical Society}, issn = {2151-2043}, doi = {10.1149/MA2023-02241342mtgabs}, pages = {1342 -- 1342}, abstract = {Progress in additive manufacturing is leading to the emergence of new areas of application. Laser Powder Bed Fusion (L-PBF) is increasingly used for the development of metallic medical implants, but for high-risk implants like vascular support structures (stents), surface quality is critical to ensure successful implantation without harming the surrounding tissue and ensure the patients' health. Therefore, enhancing the surface quality is crucial. Electropolishing is a method for removing surface roughness by smoothing out micro-peaks and valleys. However, L-PBF structures have a high surface roughness due to metal particles adhering on the surface. To achieve a smooth surface for additively manufactured implants like stents using electropolishing, the removal of these particles needs to be studied in more detail. The objective of this study is to examine the electropolishing mechanism of 316L stainless steel samples additively manufactured through Laser Powder Bed Fusion (L-PBF). The main objective is to investigate the removal properties and surface characteristics during electropolishing. To achieve this, various surfaces were characterized for morphology and roughness during Hull cell experiments. Markings are utilized on the Hull cell sample surfaces to identify points of interest during quasi-in-situ measurements. The surfaces are then analyzed after multiple time steps, applying different currents to investigate particle dissolution. The surface characteristics are analyzed through scanning electron microscopy, and surface roughness is analyzed using laser scanning microscopy. The results show that the electropolishing process preferentially removes the adhering particles present on the surface of the samples. Increasing the current density results in faster particle dissolution and a smoother surface (see Figure 1a and b). The mechanism of material removal of various surface features, as shown in Figure 1 (red circle, yellow arrow and red square), was assessed based on the experimental results of the surface structures seen on the SEM images. It was found that different surface features were removed during the experiment at different polishing times and current densities. The amount of charge flowed was found to correlate with surface morphology. Based on the obtained results, various surface features (such as large adherent particles, agglomerates of smaller particles, and valleys) and their changes with increasing test duration and current density were observed by quasi-in situ analyses. A reduction in the diameter of round particles adhering to the surface was observed at both low and higher current densities (see Figure 1a red circle a). Increasing the polishing time resulted in leveling of both large particles and valleys (see Figure 1b red square). Also, dissolution of agglomerates of smaller particles occurred at different polishing times as a function of current density and polishing time (see Figure 1a yellow arrow) are observed. Smoothed surface structures can be observed in regions with equivalent surface charge density (see Figure 2). As a result, comparable surface morphologies may appear at the same area charge density, irrespective of a specific current density. So, it may be adequate to only consider the amount of charge flowed to describe the electropolishing of additive materials. In conclusion, comprehending the dissolution characteristics of particles on L-PBF surfaces is essential for attaining satisfactory surface finish in electropolishing. The results of this study offer valuable perspectives into the electropolishing mechanism of additively manufactured 316L stainless steel and can guide future investigations on surface finishing and polishing of additive manufactured implants like stents. Figure 1}, language = {en} } @article{EsperSchultheissGradetal., author = {Esper, Lukas and Schultheiss, Ulrich and Grad, Marius and Noster, Ulf and Bund, Andreas}, title = {Application of the Hull Cell for Identifying Electropolishing Parameters to Adjust Surface Morphology in Additive Manufacturing}, series = {ECS Advances}, volume = {4}, journal = {ECS Advances}, number = {4}, publisher = {The Electrochemical Society}, issn = {2754-2734}, doi = {10.1149/2754-2734/ae184f}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:898-opus4-86295}, pages = {9}, abstract = {Additively processed materials are increasingly used to manufacture customized parts, e.g. medical implants. Implant surfaces often require a smooth finish, which can be achieved by post-processing and well-defined process parameters. In this study, the effects of electropolishing of metal parts produced by laser powder bed fusion are investigated using Hull cell experiments and a three-electrode setup. Current density voltage curves were measured with the three-electrode setup to identify the regimes for electropolishing. Subsequently different constant-currents were applied and Hull cell experiments were conducted. The surface roughness (Sz, Sa) and the mass removal were analysed. Surface morphologies were assessed using laser scanning and scanning electron microscopy. A reduction of the initial surface roughness of more than 90\% to Sa \< 0.3 μm has been achieved. Considering the passed electrical charge during electropolishing, results from Hull cell experiments are systematically correlated with current-controlled electropolishing. This approach enables the precise tailoring of polishing parameters to achieve surfaces with defined roughness. Furthermore, the study demonstrates the suitability of Hull cells in determining electropolishing parameters for additive materials and highlights their contribution to post-processing in additive manufacturing.}, language = {en} } @article{GerschNosterSchulzetal., author = {Gersch, Sebastian and Noster, Ulf and Schulz, Carsten and Bagdahn, J{\"o}rg}, title = {Influence of the Process-Related Surface Structure of L-PBF Manufactured Components on Residual Stress Measurement Using the Incremental Hole Drilling Method}, series = {Applied Sciences}, volume = {15}, journal = {Applied Sciences}, number = {18}, publisher = {MDPI}, issn = {2076-3417}, doi = {10.3390/app15189861}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:898-opus4-84929}, pages = {17}, abstract = {Laser Powder Bed Fusion (L-PBF) parts combine geometric freedom with process-induced rough surfaces that challenge residual-stress metrology. We evaluated the accuracy of the incremental hole-drilling (IHD) method with electronic speckle pattern interferometry (ESPI) by applying defined stresses via four-point bending to stress-relieved AlSi10Mg coupons, rather than measuring unknown process stresses. Flat specimens (2 mm, thin per ASTM E837) were analyzed on up-skin, side-skin, and CNC-milled surfaces; thin-specimen calibration coefficients were used. After a preliminary inter-specimen check (three specimens per surface; spread \< 8 MPa), one representative specimen per surface was tested with three drill sites to assess intra-specimen uniformity. Measured IHD-ESPI stresses agreed best at 70 MPa: deviations were ~4.1\% (up-skin), 6.0\% (side-skin), and 6.24\% (CNC-milled). At 10 MPa the relative errors increased (23.6\%, 18.4\%, and 1.40\%), consistent with reduced ESPI signal-to-noise and fixture compliance in the low-stress regime. At 140 MPa, deviations rose again (21.1\%, 14.3\%, and 13.1\%), reflecting operation near the ~60\% Rp0.2 elastic limit of hole-drilling and potential local plasticity. Surface-dependent artifacts also mattered as follows: the side-skin required no coating and performed comparably to CNC-milled, whereas the up-skin's roughness plus matting spray introduced fringe distortions and chip/coating debris near the hole. This controlled study indicates that IHD-ESPI can provide reliable results on L-PBF AlSi10Mg in the mid-stress range when surface preparation, coating, and rig compliance are carefully managed. Limitations include excluding down-skin surfaces and testing only one specimen per condition; thus, results should be generalized cautiously.}, language = {en} } @phdthesis{Grad, author = {Grad, Marius}, title = {Oberfl{\"a}chenmodifikation von Titanbasiswerkstoffen durch das Eindiffundieren von Kohlenstoff}, publisher = {Technische Universit{\"a}t Ilmenau}, address = {Ilmenau}, doi = {10.22032/dbt.66089}, abstract = {Titan und seine Legierungen finden aufgrund ihrer geringen Oberfl{\"a}chenh{\"a}rte vorwiegend in wenig verschleißbeanspruchten Bereichen Anwendung. Zur Erh{\"o}hung der H{\"a}rte werden oft Verschleißschutzschichten durch chemische oder physikalische Gasphasenabscheidung eingesetzt. Diese Schichten weisen jedoch mitunter eine schlechte Haftung auf, was zu vorzeitigem Versagen f{\"u}hren kann. Eine Alternative bietet das Diffusionsschweißen, bei dem durch den Konzentrationsgradienten verschiedener Elemente eine Diffusionsschicht zwischen Titan und einem F{\"u}gepartner entsteht. Bei hochkohlenstoffhaltigen St{\"a}hlen bildet sich an der Grenzfl{\"a}che eine TiC-Schicht. Durch chemisches Aufl{\"o}sen des Stahls kann diese Schicht freigelegt und genutzt werden. Jedoch ist dieses Verfahren noch nicht g{\"a}nzlich untersucht. Verschiedene Ver{\"o}ffentlichungen treffen widerspr{\"u}chliche Aussagen, ob neben TiC auch Eisen-Titan-Verbindungen (FeTi oder Fe2Ti) entstehen. Neben der Zusammensetzung ist auch die Mikrostruktur der entstehenden Schicht nicht abschließend untersucht. Ziel dieser Arbeit ist es, das Schichtwachstum anhand thermodynamischer Gesetzm{\"a}ßigkeiten zu untersuchen. Dabei werden der Einfluss von Temperatur, Zeit und Kohlenstoffkonzentration auf das Diffusionsverhalten und die Schichtbildungsgeschwindigkeit analysiert. Besonders relevant ist, dass der Kohlenstoff in interstitieller Form vorliegt. Die Schichtbildungskinetik wird bildgebend untersucht. Die chemische Zusammensetzung wird mittels Glimmentladungsspektroskopie (GDOES) und energiedispersiver R{\"o}ntgenspektroskopie (EDS) bestimmt, w{\"a}hrend die Phasenanalyse durch R{\"o}ntgendiffraktometrie (XRD) erfolgt. Zur Charakterisierung der Mikrostruktur werden EBSD-Messungen herangezogen. Nanoindentation dient der Ermittlung mechanischer Kennwerte wie E-Modul und H{\"a}rte. Die Ergebnisse zeigen, dass der Kohlenstoffgehalt des Stahls die Schichtbildung wesentlich beeinflusst. Ein hoher Kohlenstoffanteil kann die Bildung intermetallischer Fe-Ti-Verbindungen unterdr{\"u}cken. Die mikrostrukturellen Eigenschaften k{\"o}nnen hingegen nur durch die Prozessparameter (Temperatur und Zeit) beeinflusst werden. Die mechanischen Kennwerte sind nur durch die Phasenzusammensetzung der Schicht, welche in großen Teilen konstant ist, bestimmt. Durch das chemische Entfernen des Stahlsubstrats aus dem Verbund und dem damit einhergehenden Freilegen der Schicht, stellt das Verfahren des Diffusionsschweißens eine alternative M{\"o}glichkeit der Beschichtung von Titanbasiswerkstoffen dar. Die vorliegenden Untersuchungen belegen, dass der Prozess bestimmten Grenzen unterliegt, dieser jedoch innerhalb des erarbeiteten Prozessfensters beherrschbar ist. Dies ist der Grund, weshalb dem Diffusionsschweißen großes Potenzial im Bereich der Beschichtungstechnik zuzuordnen ist.}, language = {de} }