@inproceedings{LaumerStichelBocketal., author = {Laumer, Tobias and Stichel, Thomas and Bock, Thomas and Amend, Philipp and Schmidt, Michael}, title = {Characterization of temperature-dependent optical material properties of polymer powders}, series = {AIP Conference Proceedings}, booktitle = {AIP Conference Proceedings}, number = {1}, publisher = {AIP Publishing}, doi = {10.1063/1.4918508}, abstract = {In former works, the optical material properties of different polymer powders used for Laser Beam Melting (LBM) at room temperature have been analyzed. With a measurement setup using two integration spheres, it was shown that the optical material properties of polymer powders differ significantly due to multiple reflections within the powder compared to solid bodies of the same material. Additionally, the absorption behavior of the single particles shows an important influence on the overall optical material properties, especially the reflectance of the powder bed. Now the setup is modified to allow measurements at higher temperatures. Because crystalline areas of semi-crystalline thermoplastics are mainly responsible for the absorption of the laser radiation, the influence of the temperature increase on the overall optical material properties is analyzed. As material, conventional polyamide 12 and polypropylene as new polymer powder material, is used. By comparing results at room temperature and at higher temperatures towards the melting point, the temperature-dependent optical material properties and their influence on the beam-matter interaction during the process are discussed. It is shown that the phase transition during melting leads to significant changes of the optical material properties of the analyzed powders.}, language = {en} } @article{StichelFrickLaumeretal., author = {Stichel, Thomas and Frick, Thomas and Laumer, Tobias and Tenner, Felix and Hausotte, Tino and Merklein, Marion and Schmidt, Michael}, title = {A Round Robin study for selective laser sintering of polymers: Back tracing of the pore morphology to the process parameters}, series = {Journal of Materials Processing Technology}, volume = {252}, journal = {Journal of Materials Processing Technology}, number = {February}, publisher = {Elsevier}, doi = {10.1016/j.jmatprotec.2017.10.013}, pages = {537 -- 545}, abstract = {The mechanical properties of polymer parts built by Selective Laser Sintering are strongly related to the internal microstructure which differs with the applied production parameters. The paper focuses on the back tracing of the pore morphology of laser sintered polyamide-12 samples to the process parameters. Therefore, a data base is used which is supplied by a Round Robin initiative and includes mechanical tensile tests and the microstructural analysis of the pore morphology of several different sample charges built with different machines. The pore morphologies (porosity, pore density, pore shape and pore arrangement) measured by X-ray computed tomography are compared and discussed regarding the employed parameters and the resulting mechanical properties. The investigations point out that pore density is a superior indicator than porosity for mechanical issues. This is especially valid along the build direction since pore morphology has shown to be strongly anisotropic. Moreover, the analysis revealed that pore density is strongly affected by the process temperature, which is proved to be essential for the fabrication of mechanical robust parts using Selective Laser Sintering.}, language = {en} } @article{StichelFrickLaumeretal., author = {Stichel, Thomas and Frick, Thomas and Laumer, Tobias and Tenner, Felix and Hausotte, Tino and Merklein, Marion and Schmidt, Michael}, title = {A Round Robin study for Selective Laser Sintering of polyamide 12: Microstructural origin of the mechanical properties}, series = {Optics \& Laser Technology}, volume = {89}, journal = {Optics \& Laser Technology}, publisher = {Elsevier}, doi = {10.1016/j.optlastec.2016.09.042}, pages = {31 -- 40}, abstract = {The mechanical and microstructural investigation of polymer parts (polyamide 12) fabricated by Selective Laser Sintering as part of a Round Robin initiative is presented. The paper focuses on the microstructural analysis of the Round Robin samples and their evaluation regarding their effect on mechanical properties with respect to each other. Therefore optical microscopy on microtomed samples, X-ray computed tomography and Differential Scanning Calorimetry is used to determine the morphology of residual particle cores and of internal pores.}, language = {en} } @inproceedings{LaumerSchmidtStichel, author = {Laumer, Tobias and Schmidt, Michael and Stichel, Thomas}, title = {Influence of temperature gradients on the part properties for the simultaneous laser beam melting of polymers}, series = {Proceedings of Laser in Manfacturing Conference 2015, June 22 - June 25, 2015 Munich, Germany}, booktitle = {Proceedings of Laser in Manfacturing Conference 2015, June 22 - June 25, 2015 Munich, Germany}, organization = {German Scientific Laser Society (WLT e.V.)}, abstract = {By Laser Beam Melting of polymers (LBM), parts with almost any geometry can be built directly out of CAD files without the need for additional tools. Thus, prototypes or parts in small series production can be generated within short times. Up to now, no multi-material parts have been built by LBM, which is a major limitation of the technology. To realize multi-material parts, new mechanisms for depositing different polymer powders as well as a new irradiation strategy are needed, by which polymers with different melting temperatures can be warmed to their specific preheating temperatures and be molten simultaneously. This is achieved by simultaneous laser beam melting (SLBM). In the process, two different materials are deposited next to each other and preheated a few degrees below their melting temperatures by infrared emitters and laser radiation (λ = 10.60 µm), before in the last step the two preheated powders are molten simultaneously by an additional laser (λ = 1.94 µm). So far, multi-material tensile bars have been realized and analyzed regarding their boundary zone between both materials. The experiments showed that the temperature gradients in the boundary zone and along the building direction seem to be of great importance for the process stability and the resulting part properties. Therefore, a detailed analysis of the occurring temperature gradients during the process is needed to identify adequate process adjustments regarding the temperature controlling. To analyze the temperature gradients, thermocouples positioned inside the powder bed are used. By varying the temperature of the building platform, the influence of different temperature gradients on the resulting part properties is shown.}, language = {en} } @article{LaumerStichelRiedlbaueretal., author = {Laumer, Tobias and Stichel, Thomas and Riedlbauer, Daniel and Amend, Philipp and Mergheim, Julia and Schmidt, Michael}, title = {Realization of multi-material polymer parts by simultaneous laser beam melting}, series = {Journal of Laser Micro / Nanoengineering}, volume = {10}, journal = {Journal of Laser Micro / Nanoengineering}, number = {2}, publisher = {Japan Laser Processing Society}, abstract = {In this paper, first results regarding the realization of multi-material parts by Simultaneous Laser Beam Melting (SLBM) of polymers are presented. This new approach allows the layerwise generation of parts consisting of different polymer materials within one building process. Besides the typical advantages of additive manufacturing technologies, such parts can fulfill different product requirements concomitant and therefore could enlarge the overall field of application. The powder materials used for this paper are polyethylene (PE) and a polyamide based thermoplastic elastomer (TPE). After depositing the powder materials next to each other, infrared-emitters heat the lower melting polymer and a CO2 laser provides the preheating temperature of the higher melting polymer. In the last step, a thulium fibre laser melts the two preheated powders simultaneously. The realized specimens are characterized by cross sections and their tensile strengths are determined. Additionally, the new approach of the simultaneous energy irradiation is investigated using a Finite Element Analysis in order to gain a more profound process understanding. In that sense, the influence of the size of the exposure area on the reachable maximum temperatures inside that area was analyzed by the simulation and compared to experimental studies.}, language = {en} } @article{LaumerStichelAmendetal., author = {Laumer, Tobias and Stichel, Thomas and Amend, Philipp and Schmidt, Michael}, title = {Simultaneous laser beam melting of multimaterial polymer parts}, series = {Journal of Laser Applications}, volume = {27}, journal = {Journal of Laser Applications}, number = {S2}, publisher = {Laser Institute of America}, issn = {1938-1387}, doi = {10.2351/1.4906303}, abstract = {By simultaneous laser beam melting (SLBM), parts consisting of different polymer powders can be additively manufactured within one building process. Besides the advantages of conventional LBM, e.g., not needing additional tools and being able to realize parts with almost any geometry, different product requirements can be achieved within a single part. Product requirements may be different chemical resistances or haptic material properties. Therefore, SLBM enlarges the application field for additive manufacturing in general. In the process, two different materials are deposited on the building platform and preheated a few degrees below the melting temperature of the lower melting polymer by infrared emitters. Afterward, a CO2 laser (λ = 10.6 μm) provides the energy for the temperature difference between the preheating temperatures of both materials. Finally, a digital light processing chip is used to achieve simultaneous and flexible energy deposition for melting both preheated polymers. By illuminating the chip with a laser, parts of the beam can be flexibly guided onto the powder bed or into a beam trap. As laser, a single mode thulium laser (λ = 1.94 μm) is used. After melting the layer, a new layer is deposited and the process starts anew. In this paper, polypropylene and polyamide 12 are used as materials. After analyzing the material and melting behavior during the process by a high-resolution thermal imaging system, the parts are qualified regarding their material compatibility at the boundary zone and porosity by cross sections.}, language = {en} } @inproceedings{LaumerStichelAmendetal., author = {Laumer, Tobias and Stichel, Thomas and Amend, Philipp and Roth, Stephan and Schmidt, Michael}, title = {Analysis of Temperature Gradients during Simultaneous Laser Beam Melting of Polymers}, series = {Physics Procedia}, volume = {56}, booktitle = {Physics Procedia}, publisher = {Elsevier}, doi = {10.1016/j.phpro.2014.08.159}, pages = {167 -- 175}, abstract = {By simultaneous laser beam melting (SLBM), different polymer powders can be processed to multi-material parts, which offers the potential to enlarge the field of application for conventional LBM. In a SLBM process, a powder bed consisting of different polymers and therefore with different melting and crystallization temperatures is deposited. Besides the use of infrared emitters for preheating the lower melting polymer, a CO2 laser distributes the necessary preheating temperature of the higher melting polymer. In the last step, a thulium fibre laser distributes the energy necessary for melting the two preheated powders simultaneously. In order to analyze the temperature gradients of the process on the powder surface and in deeper layers, a high-resolution thermal imaging system and thermocouples are used.}, language = {en} } @inproceedings{LaumerRothSticheletal., author = {Laumer, Tobias and Roth, Stephan and Stichel, Thomas and Schmidt, Michael}, title = {Strategien zur Erzeugung von dreidimensionalen Multi-Material-Bauteilen}, series = {4. Industriekolloquium des Sonderforschungsbereichs 814 - Additive Fertigung 814, 2015, N{\"u}rnberg}, booktitle = {4. Industriekolloquium des Sonderforschungsbereichs 814 - Additive Fertigung 814, 2015, N{\"u}rnberg}, organization = {Friedrich-Alexander-Universit{\"a}t Erlangen-N{\"u}rnberg, Sonderforschungsbereich 814 Additive Fertigung}, isbn = {978-3931864651}, language = {de} } @article{StichelLaumerSchmidt, author = {Stichel, Thomas and Laumer, Tobias and Schmidt, Michael}, title = {Simulation des (quasi-)simultanen Laserstrahlschmelzens zur Herstellung von Multi-Material-Bauteilen aus Polymeren}, series = {Rapid.Tech + FabCon 3.D - International Trade Show + Conference for Additive Manufacturing}, journal = {Rapid.Tech + FabCon 3.D - International Trade Show + Conference for Additive Manufacturing}, publisher = {Hanser}, doi = {10.3139/9783446458123.020}, pages = {312 -- 329}, language = {en} } @inproceedings{LaumerSchmidtStichel, author = {Laumer, Tobias and Schmidt, Michael and Stichel, Thomas}, title = {Correlation Analysis of Different Building Parameters on the Part Properties of Parts Built by Simultaneous Laser Beam Melting of Polymers}, series = {Fraunhofer Direct Digital Manufacturing Conference DDMC 2016 : Conference Proceedings, March 2016, Berlin}, booktitle = {Fraunhofer Direct Digital Manufacturing Conference DDMC 2016 : Conference Proceedings, March 2016, Berlin}, publisher = {Fraunhofer Verlag}, abstract = {Simultaneous Laser Beam Melting of polymers (SLBM) allows the generation of multi-material components,consisting of different thermoplastic polymers, within one additive building process. Besides the common advantagesof conventional Laser Beam Melting (LBM), multi-material components built by SLBM can fulfill different productrequirements like different chemical resistances or haptic material properties within a single part. To achieve suchparts, different powder materials are deposited next to each other and preheated a few degrees below their meltingtemperatures by infrared emitters and laser radiation (λ = 10.60 μm), before in the last step the preheated powdersare molten simultaneously by an additional laser source (λ = 1.94 μm). In this paper, different polymer powders likepolypropylene (PP) and polyamide 12 (PA12) are used for the generation of multi-material specimens. By varyingdifferent building parameters according to a specified design of experiments, their influence on the part properties isanalyzed. Important building parameters are the intensity and the irradiation time of the laser beam used for meltingthe preheated powders. Besides using tensile tests to determine the tensile strength and the elongation at break, theaverage part height in dependence of the energy input is analyzed. The overall aim is to specify the correlationbetween different building parameters regarding the energy deposition on the resulting part properties.}, language = {en} } @article{LaumerStichelRathsetal., author = {Laumer, Tobias and Stichel, Thomas and Raths, Max and Schmidt, Michael}, title = {Analysis of the Influence of Different Flowability on Part Characteristics Regarding the Simultaneous Laser Beam Melting of Polymers}, series = {Physics Procedia}, volume = {83}, journal = {Physics Procedia}, publisher = {Elsevier}, issn = {1875-3892}, doi = {10.1016/j.phpro.2016.08.098}, pages = {937 -- 946}, abstract = {Powder based Additive Manufacturing technologies offer huge potential for building parts with almost no geometrical restrictions, but both the process controlling as well as the part properties are strongly dependent on different material characteristics of the material, like the flowability. In this work, different weight percentages of nano-scaled silica dioxide particles (Aerosil®) are admixed to pure polyethylene and polypropylene powder and the resulting flowability is determined. Besides using the Hausner ratio as standardized value, the degree of coverage is introduced as a new characteristic to quantify the powder flowability. The degrees of coverage are compared to the Hausner ratios to allow a discussion and evaluation about the different characteristic values. Additionally, tensile bars consisting of polypropylene are generated to determine the porosity by cross sections and the mechanical part properties by tensile testing. As mechanical part properties, the tensile strength and elongation at break are determined and the effects of different powder flowability on these properties are analyzed.}, language = {en} } @article{GeisslerLaumerWuebbekeetal., author = {Geißler, Bastian and Laumer, Tobias and W{\"u}bbeke, Andrea and Lakemeyer, Patrick and Frick, Thomas and Sch{\"o}ppner, Volker and Schmidt, Michael}, title = {Analysis of the interaction between the temperature field and the weld seam morphology in laser transmission welding by using two different discrete laser wavelengths}, series = {Journal of Laser Applications}, volume = {30}, journal = {Journal of Laser Applications}, number = {3}, publisher = {AIP Publishing}, issn = {1938-1387}, doi = {10.2351/1.5040617}, abstract = {Laser transmission welding is a non-contact and efficient process technology for joining thermoplastic polymers. In the conventional process, laser sources in the wavelength range of 1 μm are usually used. Therefore, most of the laser radiation is transmitted through the upper joining partner and absorbed only in the lower joining partner. As a result, the possibilities to influence the temperature field especially in the upper joining partner are limited. To overcome these limitations, an additional thulium fiber-laser with a wavelength of 1.94 μm is used in this study and coaxially aligned with a diode laser. The use of an additional thulium fiber-laser leads to a significant absorption in the upper joining partner. Through this approach, it is shown that the temperature field and the weld seam geometry can be influenced by using these two different discrete laser wavelengths. Depending on the intensity distribution of both lasers, an increase of the size of the heat affected zone in the upper joining partner can be observed. In order to develop a better process understanding, a thermal finite element model is built up and verified by comparing the calculated size of the heat affected zone for different process parameters with the experimental data. The model is able to represent the influence of both laser sources on the temperature field and is used to calculate characteristics of the temperature field, such as maximum temperatures or cooling rates. The characteristics are then used to explain the weld seam morphology, such as occurrence and size of spherulitic structures in the weld seam.}, language = {en} } @article{GeisslerLaumerWuebbekeetal., author = {Geißler, Bastian and Laumer, Tobias and W{\"u}bbeke, Andrea and Frick, Thomas and Sch{\"o}ppner, Volker and Schmidt, Michael}, title = {Analysis of the Weld Seam Morphology of Polypropylene in Laser Transmission Welding}, series = {Journal of Manufacturing Science and Engineering}, volume = {140}, journal = {Journal of Manufacturing Science and Engineering}, number = {11}, publisher = {ASME}, doi = {10.1115/1.4040876}, pages = {7}, abstract = {Laser transmission welding is a well-known joining technology for welding thermoplastics. Although the process is already used industrially, fundamental process-structure-property relationships are not fully understood and are therefore the subject of current research. One aspect of these mentioned process-structure-property relationships is the interaction between the temperature field during the welding process, the weld seam morphology of semi-crystalline thermoplastics, and the weld seam strength. In this study, the influence of the line energy on the weld seam morphology of polypropylenes is analyzed. For this purpose, the size of spherulites in the weld seam is investigated, as well as different occurring phases of polypropylene (a- and b-phase). It is shown that both the spherulite size of the a-phase and the amount of b-phase increase with increasing line energy. For the explanation and discussion of the results, a temperature-dependent thermal simulation model is used to derive characteristic attributes of the temperature field (maximum temperatures, cooling rates, temperature gradients).}, language = {en} } @inproceedings{LaumerStichelAmendetal., author = {Laumer, Tobias and Stichel, Thomas and Amend, Philipp and Schmidt, Michael and Gachot, A.}, title = {Simultaneous Energy Deposition for Laser Beam Melting of Polymers}, series = {Proceedings of the Polymer Processing Society 29th Annual Meeting, N{\"u}rnberg, 2013}, booktitle = {Proceedings of the Polymer Processing Society 29th Annual Meeting, N{\"u}rnberg, 2013}, publisher = {American Institute of Physics}, address = {New York}, language = {en} } @article{SchmidtKroegerFlegeetal., author = {Schmidt, Thomas and Kr{\"o}ger, Roland and Flege, Jan Ingo and Horn-von Hoegen, Michael and Clausen, T. and Falta, J. and Janzen, Andreas and Zahl, P. and Kury, P. and Kammler, Martin}, title = {Less strain energy despite fewer misfit dislocations: the impact of ordering}, series = {Physical review letters}, volume = {96}, journal = {Physical review letters}, number = {6}, publisher = {APS}, doi = {10.1103/PhysRevLett.96.066101}, abstract = {The average strain state of Ge films grown on Si(111) by surfactant mediated epitaxy has been compared to the ordering of the interfacial misfit dislocation network. Surprisingly, a smaller degree of average lattice relaxation was found in films grown at higher temperature. On the other hand, these films exhibit a better ordered dislocation network. This effect energetically compensates the higher strain at higher growth temperature, leading to the conclusion that, apart from the formation of misfit dislocations, their ordering represents an important channel for lattice-strain energy relaxation.}, language = {en} } @article{KleinNabbefeldHattabetal., author = {Klein, C. and Nabbefeld, T. and Hattab, H. and Meyer, D. and Jnawali, G. and Kammler, Martin and Meyer zu Heringdorf, Frank-Joachim and Golla-Franz, A. and M{\"u}ller, B. H. and Schmidt, Thomas and Henzler, M. and Horn-von Hoegen, Michael}, title = {Lost in reciprocal space? Determination of the scattering condition in spot profile analysis low-energy electron diffraction}, series = {Review of scientific instruments}, volume = {82}, journal = {Review of scientific instruments}, number = {3}, publisher = {American Institute of Physics}, doi = {10.1063/1.3554305}, abstract = {The precise knowledge of the diffraction condition, i.e., the angle of incidence and electron energy, is crucial for the study of surface morphology through spot profile analysis low-energy electron diffraction (LEED). We demonstrate four different procedures to determine the diffraction condition: employing the distortion of the LEED pattern under large angles of incidence, the layer-by-layer growth oscillations during homoepitaxial growth, a G(S) analysis of a rough surface, and the intersection of facet rods with 3D Bragg conditions.}, language = {en} } @article{SchmidtKroegerClausenetal., author = {Schmidt, Thomas and Kr{\"o}ger, Roland and Clausen, T. and Falta, J. and Janzen, Andreas and Kammler, Martin and Kury, P. and Zahl, P. and Horn-von Hoegen, Michael}, title = {Surfactant-mediated epitaxy of Ge on Si(111): Beyond the surface}, series = {Applied Physics Letters}, volume = {86}, journal = {Applied Physics Letters}, number = {11}, publisher = {AIP}, doi = {10.1063/1.1882760}, abstract = {For a characterization of interface and "bulk" properties of Ge films grown on Si(111) by Sb surfactant-mediated epitaxy, grazing incidence x-ray diffraction and transmission electron microscopy have been used. The interface roughness, defect structure, and strain state have been investigated in dependence of film thickness and growth temperature. For all growth parameters, atomically smooth interfaces are observed. For thin Ge layers, about 75\% of the strain induced by the lattice mismatch is relaxed by misfit dislocations at the Ge/Si interface. Only a slight increase of the degree of relaxation is found for thicker films. At growth temperatures below about 600 °C, the formation of twins is observed, which can be avoided at higher temperatures.}, language = {en} }