% Encoding: UTF-8 @COMMENT{BibTeX export based on data in FAU CRIS: https://cris.fau.de/} @COMMENT{For any questions please write to cris-support@fau.de} @inproceedings{faucris.120510324, abstract = {With regard to mechanical and chemical resistivity metal surfaces can benefit from hydrophobic behavior. Against this background, results of laser-induced generation of hydrophobic surfaces on three different steel alloys without any additional coating are presented. Therefore, a dual-scale structure - consisting of micro cones and nano-ripples and droplets - is generated by picosecond laser structuring in ambient air.
Firstly, the dependencies of the formation of these micro cones, called cone-like protrusions (CLP), on different parameters of the laser ablation process - the peak fluence, the spot diameter, the effective pulse number per unit area and the material - are investigated. Secondly, the hydrophobicity and surface energy of the resulting surface topography of the irradiated substrate are evaluated by means of contact angle measurements. For that purpose identical topographies are generated on stainless steel, hot- and cold-working steel. The time-dependency of the contact angle is investigated depending on the material and the morphology.
For CLP formation a high number of scans is preferred to provide a high effective pulse number per unit area, which is advantageous for homogeneous generation of larger micro cones. By measuring the contact angle a change from an initially hydrophilic to a hydrophobic behavior of the surfaces can be observed due to a change of surface chemistry over time. Thus, contact angles higher than 90° can be measured on laser treated surfaces of different steel alloys.},
author = {Häfner, Tom and Heberle, Johannes and Holder, Daniel and Schmidt, Michael},
booktitle = {Proc. Laser in Manufacturing Conference 2015},
date = {2015-06-22/2015-06-25},
faupublication = {yes},
keywords = {Micro processing; ablation},
pages = {1-10},
peerreviewed = {No},
title = {{Adjustment} of surface energy on steel surfaces due to {CLP} generation by picosecond laser processing},
venue = {München},
year = {2015}
}
@inproceedings{faucris.241843168,
abstract = {Glass welding by ultra-short pulsed lasers is temporally a highly dynamic process. These dynamics have a high impact on the shape and the thermodynamic properties of the molten zone as well as the plasma. In this work we analyze the local distribution of laser absorption in the melt by shockwave observation and the evolution of the subsequent pressure wave generated during the process. The pressure waves were observed using a pump-probe setup with a high-speed video camera. Furthermore we show a dramatic decrease of pressure waves propagation velocity inside the molten zone as an explicit indicator of the fluid state of the molten zone.},
author = {Cvecek, Kristian and Miyamoto, Isamu and Heberle, Johannes and Bergler, Michael and de Ligny, Dominique and Schmidt, Michael},
booktitle = {Procedia CIRP},
date = {2018-09-03/2018-09-06},
doi = {10.1016/j.procir.2018.08.128},
editor = {Michael Schmidt},
faupublication = {yes},
keywords = {Glass processing; Glass welding; Image processing; Pump probe; Sensing; Ultra-fast lasers; Ultra-short pulses},
note = {Created from Fastlane, Scopus look-up},
pages = {339-343},
peerreviewed = {unknown},
publisher = {Elsevier B.V.},
title = {{Analysis} of shockwave formation in glass welding by ultra-short pulses},
venue = {Furth},
volume = {74},
year = {2018}
}
@inproceedings{faucris.241842912,
abstract = {The modification of selective laser sintering (SLS) powder materials by nanoadditives offers the possibility to adapt the powder properties to the laser sintering process or the resulting part properties. To avoid agglomeration of the nanofiller, a new approach in which surfactant-free laser-generated colloidal nanoparticles are adsorbed onto the polymer surface directly in an aqueous solution is demonstrated. Based on this novel approach, polyamide 12 (PA12) powders are decorated with metal and oxide nanoparticles and processed via SLS. Electron microscopy and confocal laser scanning imaging are utilized to analyze the dispersion of the filler.},
author = {Hupfeld, Tim and Laumer, T. and Stichel, T. and Schuffenhauer, T. and Heberle, Johannes and Schmidt, Michael and Barcikowski, S. and Goekce, B.},
booktitle = {Procedia CIRP},
date = {2018-09-03/2018-09-06},
doi = {10.1016/j.procir.2018.08.103},
editor = {Michael Schmidt},
faupublication = {yes},
keywords = {Laser additive manufacturing; Nanoparticles; Polymer powders; Pulsed laser ablation in liquids; Selective laser sintering},
note = {Created from Fastlane, Scopus look-up},
pages = {244-248},
peerreviewed = {unknown},
publisher = {Elsevier B.V.},
title = {{A} new approach to coat {PA12} powders with laser-generated nanoparticles for selective laser sintering},
venue = {Furth},
volume = {74},
year = {2018}
}
@article{faucris.265386220,
abstract = {Thermoplastic polymers such as polyamide 12 (PA12) are of great interest for functional coatings in industry due to their good material properties (e.g., chemical and wear resistance and biocompatibility). In order to provide a high local selectivity of polymer deposition and to shorten the process chain, a laser-based coating process represents a promising approach for generating functional coatings. In this work, a laser-based coating process for PA12 powder on stainless steel substrates is investigated experimentally by using a thulium-doped fiber laser with a wavelength of 1.94 μm. Due to the higher inherent absorption of the laser radiation in the powder material, a thulium-doped fiber laser is more appropriate for processing PA12 powder in comparison to more common near-infrared laser beam sources with wavelengths around 1 μm. The influence of the main process parameters (laser power, scanning speed, hatch distance, and substrate temperature) on the resulting coatings was evaluated. For this purpose, optical microscopic analyses were performed to characterize the coating surfaces and cross sections in terms of possible defects. In order to assess the adhesion between the coating and the substrate, cross-cutting values were determined. On the basis of the quality characteristics regarding the coating surface, a process window was determined to produce a closed melting film, which fully covers the metallic substrate. By an adjustment of the scanning speed (5 mm/s) and line overlap (50%/62.5%), a broadening of the process window could be achieved. An additional substrate heating (T ≥ 90°C) is an appropriate strategy, which enlarges the size of the process window significantly. Optical microscopy analyses and cross-cutting tests verified the suitability of a thulium-doped fiber laser for processing dense PA12 coatings with an adequate adhesion to the stainless steel substrate. The substrate temperature is a decisive process parameter to obtain a homogeneous morphology and to further improve the adhesion between the metallic substrate and PA12 layer.},
author = {Wittmann, Alexander and Heberle, Johannes and Huber, Florian and Schmidt, Michael},
doi = {10.2351/7.0000501},
faupublication = {yes},
journal = {Journal of Laser Applications},
peerreviewed = {Yes},
title = {{Consolidation} of thermoplastic coatings by means of a thulium-doped fiber laser},
volume = {33},
year = {2021}
}
@inproceedings{faucris.114103484,
abstract = {Piezokeramiken bieten aufgrund ihrer Eigenschaften vielseitige Anwendungsmöglichkeiten. Werden sie beispielsweise in Sturkturbauteile integriert, so kann über eine frequenzmodulierte Ansteuerung eine aktive Schwingungsdämpfung realisiert werden. Die Integration von Piezoaktoren in Strukturbauteile stellt jedoch in der Großserienfertigung aufgrund der Komplexität der Verarbeitung und Kontaktierung bisher ein ungelöstes Problem dar. Aufgrund der hohen Anforderungen an die Kontaktierung wird am Bayerischen Laserzentrum das Laser Droplet Brazing entwickelt, welches sich durch eine sehr definierte Energieeinbringung in die Fügestelle auszeichnet. Es wird hierbei eine Hartlotkugel in eine Kapillare eingebracht, dort mittels eines Laserpulses aufgeschmolzen und durch einen, in der Kapillare herrschenden, Überdruck ausgestoßen. Die schmelzflüssige Lotkugel fällt auf ein sich unter der Düse befindliches Kontaktpad und benetzt dort sowohl die Metallisierung des Substrates als auch den Kupferdraht. Im Anschluss erstarrt die Lotkugel, wodurch es zu einer stoffschlüssigen Kontaktierung von Substrat und Kupferdraht kommt. Mit dem Laser-Droplet-Brazing-Verfahren lassen sich thermisch stabile Hartlötverbindungen auf wenigen µm starken Metallisierungen realisieren, wobei eine thermische Schädigung des Substrates aufgrund der definierten Energiedeposition vermieden werden kann. Es eignet sich somit sowohl für die Kontaktierung von Piezokeramiken als auch für andere Aufgabenstellungen, bei denen hochtemperaturstabile Kontaktierungen auf dünnen Metallisierungen erzeugt werden müssen, ohne das Substrat thermisch zu schädigen. Das Verfahren sowie die System- und Prozesstechnik werden im vorliegenden Artikel vorgestellt. Daneben werden die Einflüsse von Prozessparametern wie Pulsdauer und Pulsleistung auf das Kontaktierungsergebnis aufgezeigt und Kontaktierungsergebnisse diskutiert, sowie deren Anbindung metallografisch und durch Schertests untersucht.},
address = {Bamberg},
author = {Stein, Stefan and Heberle, Johannes and Hugger, Florian and Roth, Stephan and Schmidt, Michael},
booktitle = {Tagungsband des 17. Seminars Laser in der Elektronikproduktion und Feinwerktechnik},
date = {2014-03-12/2014-03-13},
editor = {M. Schmidt, S. Roth},
faupublication = {yes},
isbn = {978-3-87525-359-7},
keywords = {Laserstrahllöten; Hartlöten; Piezokeramik; Kontaktierung; Laser Droplet Brazing;},
pages = {75–86},
peerreviewed = {No},
publisher = {Meisenbach},
title = {{Düsenbasiertes} {Laserstrahl}-{Löten} mit {Formteilen} zur hochtemperaturfesten {Kontaktierung} piezokeramischer {Substrate}},
url = {http://www.lef.info/lef2014.html},
venue = {Fürth},
year = {2014}
}
@article{faucris.106575084,
author = {Häfner, Tom and Heberle, Johannes and Hautmann, Hubert and Zhao, Rong and Tenner, Jennifer and Tremmel, Stephan and Merklein, Marion and Schmidt, Michael},
doi = {10.2961/jlmn.2017.02.0015},
faupublication = {yes},
journal = {Journal of Laser Micro Nanoengineering},
pages = {132-140},
peerreviewed = {Yes},
title = {{Effect} of picosecond laser based modifications of amorphous carbon coatings on lubricant-free tribological systems},
volume = {12},
year = {2017}
}
@article{faucris.204045222,
abstract = {Ophthalmic intraocular lenses are conventionally machined by diamond tools. A promising alternative approach is contour cutting by ultrashort pulsed laser micromachining. To improve process knowledge, a parametric study of picosecond and femtosecond laser machining of medical grade hydrophilic copolymers and PMMA is carried out. Material removal rates and machining quality with respect to main process parameters are determined. Reasons for chipping and formation of heat affected zones are identified and an optimized process strategy is derived. By choosing a defined pulse overlap, heat accumulation is kept minimal while increasing absorptivity through incubation avoids chipping.
Electro-optical deflectors (EOD) and acousto-optical deflectors (AOD) are based on deflection of laser light within a solid state medium. As they do not contain any moving parts, they yield advantages compared to mechanical scanners which are conventionally used for laser beam deflection. Even for arbitrary scan paths high feed rates can be achieved. In this work the principles of operation and characteristic properties of EOD and AOD are presented. Additionally, a comparison to mirror based mechanical deflectors regarding deflection angles, speed and accuracy is made in terms of resolvable spots and the rate of resolvable spots. Especially, the latter one is up to one order of magnitude higher for EOD and AOD systems compared to conventional systems. Further characteristic properties such as response time, damage threshold, efficiency and beam distortions are discussed. Solid state laser beam deflectors are usually characterized by small deflection angles but high angular deflection velocities. As mechanical deflectors exhibit opposite properties an arrangement of a mechanical scanner combined with a solid state deflector provides a solution with the benefits of both systems. As ultrashort pulsed lasers with average power above 100 W and repetition rates in the MHz range have been available for several years this approach can be applied to fully exploit their capabilities. Thereby, pulse overlap can be reduced and by this means heat affected zones are prevented to provide proper processing results. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.},
address = {Bellingham, WA},
author = {Heberle, Johannes and Bechtold, Peter and Strauß, Johannes and Schmidt, Michael},
booktitle = {Laser-based Micro- and Nanoprocessing X, Proc. SPIE 9736},
date = {2016-02-13/2016-02-18},
doi = {10.1117/12.2212208},
editor = {Klotzbach, Udo; Washio, Kunihiko; Arnold, Craig B.},
faupublication = {yes},
isbn = {9781628419719},
keywords = {laser beam scanner; electro-optical deflector; acousto-optical deflector;},
pages = {97360L-1 - 97360L-10},
peerreviewed = {No},
publisher = {SPIE},
title = {{Electro}-optic and acousto-optic laser beam scanners},
venue = {San Francisco},
year = {2016}
}
@article{faucris.123676784,
abstract = {Tailored intensity distribution enables efficient microstructuring as the maximum pulse energy of an ultrashort-pulsed laser can be applied. Dynamic adaptation of the intensity profile can be realized by holographic laser beam shaping using phase-only liquid crystal on silicon (LCoS) displays as a spatial light modulator (SLM). To achieve the desired intensity profile at the focal plane an accurate phase retardation of the SLM's liquid crystals has to be guaranteed. In this publication we present the phase calibration of three LCoS SLM from different manufacturers. Each system is evaluated for direct laser structuring applications. As one of the tested SLM show a spatially nonlinear behavior, a compensation method proposed by Engström et al. in 2013 is applied. Finally, the necessity of this phase calibration is presented and discussed by applying a squared flat-top profile.},
author = {Strauß, Johannes and Häfner, Tom and Dobler, Michael and Heberle, Johannes and Schmidt, Michael},
doi = {10.1016/j.phpro.2016.08.122},
faupublication = {yes},
journal = {Physics Procedia},
keywords = {laser; diffractive optics; holographic beam shaping; spatial light modulator},
pages = {1160-1169},
peerreviewed = {Yes},
title = {{Evaluation} and {Calibration} of {LCoS} {SLM} for {Direct} {Laser} {Structuring} with {Tailored} {Intensity} {Distributions}},
url = {http://www.sciencedirect.com/science/article/pii/S1875389216302292},
volume = {83},
year = {2016}
}
@article{faucris.114107444,
abstract = {The functional and geometrical requirements to electronic, optoelectronic and mechatronic devices have significantly increased during the last years. Within the scope of the development and the fabrication of such components a very important aspect is the fast and flexible generation of conductive circuits respectively structures on work-pieces of various dimensions and materials. The presented work gives an overview about three promising laser-based approaches to generate microscopic and macroscopic circuits as well as conductive layers with a thickness in the order of a few hundred nanometers. In addition to processes for the fabrication of conductive structures laser-based soldering of piezo-composites is investigated.
},
author = {Amend, Philipp and Hentschel, Oliver and Scheitler, Christian Josef and Baum, Marcus and Heberle, Johannes and Roth, Stephan and Schmidt, Michael},
doi = {10.2961/jlmn.2013.03.0015},
faupublication = {yes},
journal = {Journal of Laser Micro Nanoengineering},
keywords = {conductive circuits; ITO nanoparticles; stereolithography; ADDIMID technology; Molded Interconnect Device; Laser-Beam-Melting; laser-based soldering; piezo-composites;},
pages = {276-286},
peerreviewed = {Yes},
title = {{Fast} and {Flexible} {Generation} of {Conductive} {Circuits}},
url = {http://www.jlps.gr.jp/jlmn/upload/b9e3a7c5b186909a8dca1665dec97e38.pdf},
volume = {8},
year = {2013}
}
@article{faucris.111843644,
abstract = {The spectral dispersion of ultrashort pulses allows the simultaneous focusing of light in both space and time, which creates so-called spatiotemporal foci. Such space–time coupling may be combined with the existing holographic techniques to give a further dimension of control when generating focal light fields. In the present study, it is shown that a phase-only hologram placed in the pupil plane of an objective and illuminated by a spatially chirped ultrashort pulse can be used to generate three-dimensional arrays of spatio-temporally focused spots. By exploiting the pulse front tilt generated at focus when applying simultaneous spatial and temporal focusing (SSTF), it is possible to overlap neighboring foci in time to create a smooth intensity distribution. The resulting light field displays a high level of axial confinement, with experimental demonstrations given through two-photon microscopy and the non-linear laser fabrication of glass.},
author = {Sun, Bangshan and Salter, Patrick and Roider, Clemens and Jesacher, Alexander and Strauß, Johannes and Heberle, Johannes and Schmidt, Michael and Booth, Martin J.},
doi = {10.1038/lsa.2017.117},
faupublication = {yes},
journal = {Light: Science & Applications},
keywords = {Micro-optics; Nonlinear optics; Ultrafast photonics},
month = {Jan},
peerreviewed = {Yes},
title = {{Four}-dimensional light shaping: manipulating ultrafast spatiotemporal foci in space and time},
volume = {7},
year = {2018}
}
@article{faucris.121636944,
abstract = {In deep drawing processes sustainability can be increased and processing steps can be omitted by abolishing any lubricants. Tailored tools with locally textured surfaces offer a possibility to compensate higher strains caused by the changed tribological system. Ultrashort pulsed laser machining is an advantageous approach to generate surface features. Thus, very hard and brittle materials can be processed inducing negligible heat affected zones so that the surrounding tool material keeps its initial properties. The material-dependent process parameters for efficient picosecond laser structuring are applied. The effects of features with single feature sizes in the range of 100 µm to 500 µm on friction of the tribological pairing are presented. The dependencies of the friction coefficient on the properties of the micro features - the geometry, the shape, the density and the orientation - are investigated by using a ring-on-disc-tribometer. The ring representing the tool is made out of the cold work steel 1.2379. The zinc-coated deep-drawing steel DC04 is used as disc respectively workpiece material. During the ring-on-disc-tests a constant contact pressure of 2.1 MPa and a mean sliding velocity of 100 mm/s are applied. To obtain the significant influences of micro features on friction, screening tests by varying the parameters according to the Shainin method are carried out. Because of the stochastically occurring high wear observed in reference experiments a changed methodology of ring-on-disc-tests is proposed. Applying this method the effects of textured ring surfaces on friction coefficient of steel-zinc-sliding are evaluated and compared to untextured rings. The latter are tested non-lubricated as well as with lubrication. The screening tests show that the feature orientation is the significant parameter influencing the friction. Selecting this parameter together with the feature density the friction coefficient can be adjusted with regard to untextured surfaces.},
author = {Häfner, Tom and Heberle, Johannes and Dobler, Michael and Gränitz, Michael and Alexeev, Ilya and Schmidt, Michael},
faupublication = {yes},
journal = {Key Engineering Materials},
keywords = {Tribology, laser micro machining},
pages = {57-64},
peerreviewed = {Yes},
title = {{Friction} {Adjustment} within {Dry} {Deep} {Drawing} by {Locally} {Laser} {Textured} {Tool} {Surfaces}},
year = {2015}
}
@inproceedings{faucris.290232669,
abstract = {In deep drawing processes sustainability can be increased and processing steps can be omitted by abolishing any lubricants. Tailored tools with locally textured surfaces offer a possibility to compensate higher strains caused by the changed tribological system. Ultrashort pulsed laser machining is an advantageous approach to generate surface features. Thus, very hard and brittle materials can be processed inducing negligible heat affected zones so that the surrounding tool material keeps its initial properties. The material-dependent process parameters for efficient picosecond laser structuring are applied. The effects of features with single feature sizes in the range of 100 μm to 500 μm on friction of the tribological pairing are presented. The dependencies of the friction coefficient on the properties of the micro features-the geometry, the shape, the density and the orientation-Are investigated by using a ring-on-disc-tribometer. The ring representing the tool is made out of the cold work steel 1.2379. The zinc-coated deep-drawing steel DC04 is used as disc respectively workpiece material. During the ring-on-disc-tests a constant contact pressure of 2.1 MPa and a mean sliding velocity of 100 mm/s are applied. To obtain the significant influences of micro features on friction, screening tests by varying the parameters according to the Shainin method are carried out. Because of the stochastically occurring high wear observed in reference experiments a changed methodology of ring-on-disc-tests is proposed. Applying this method the effects of textured ring surfaces on friction coefficient of steel-zincsliding are evaluated and compared to untextured rings. The latter are tested non-lubricated as well as with lubrication. The screening tests show that the feature orientation is the significant parameter influencing the friction. Selecting this parameter together with the feature density the friction coefficient can be adjusted with regard to untextured surfaces.},
author = {Häfner, Tom and Heberle, Johannes and Dobler, Michael and Gränitz, Michael and Alexeev, Ilya and Schmidt, Michael},
booktitle = {Key Engineering Materials},
date = {2015-03-16/2015-03-18},
doi = {10.4028/www.scientific.net/KEM.639.57},
editor = {Marion Merklein},
faupublication = {yes},
isbn = {9783038354505},
keywords = {Laser micro machining; Shainin; Tribology},
note = {CRIS-Team Scopus Importer:2023-03-07},
pages = {57-64},
peerreviewed = {unknown},
publisher = {Trans Tech Publications Ltd},
title = {{Friction} adjustment within dry deep drawing by locally laser textured tool surfaces},
venue = {Erlangen-Nurnberg, DEU},
volume = {639},
year = {2015}
}
@inproceedings{faucris.221875161,
abstract = {An interferometry-based method for direct observation of phase modifications caused by transient temperature and pressure changes during irradiation of glass using ultra-short laser pulses is shown. The method provides a 3-dimensional time-resolved phase distribution and allows to distinguish between reversible and irreversible laser induced phase changes inside the glass.},
author = {Cvecek, Kristian and Heberle, Johannes and Bergler, Michael and Miyamoto, Isamu and de Ligny, Dominique and Schmidt, Michael},
booktitle = {Optics InfoBase Conference Papers},
date = {2019-05-05/2019-05-10},
doi = {10.1364/CLEO{\_}AT.2019.ATu3I.1},
faupublication = {yes},
isbn = {9781557528209},
note = {CRIS-Team Scopus Importer:2019-07-09},
peerreviewed = {unknown},
publisher = {OSA - The Optical Society},
title = {{Glass} machining and in-situ metrology: {Recovery} of spatio-temporal phase distribution from 2-dimensional interference fringe movement caused by irradiation of glass with ultra-short laser pulses at high pulse repetition rates},
venue = {San Jose, CA},
volume = {Part F127-CLEO{\_}AT 2019},
year = {2019}
}
@article{faucris.231279460,
abstract = {Powder Bed Fusion with Laser Beam of Metals (PBF-LB/M) is one of the fastest growing technology branches. More and more metallic alloys are being qualified, but processing of aluminum wrought alloys without cracks and defects is still challenging. It has already been shown that small parts with low residual porosity can be produced. However, suering from microscopic hot cracks, the fracture behavior has been rather brittle. In this paper dierent combinations of temperature gradients and solidification rates are used to achieve specific solidification conditions in order to influence the resulting microstructure, as well as internal stresses. By this approach it could be shown that EN AW-2024, an aluminum-copper wrought alloy, is processable via PBF-LB/M fully dense and crack-free with outstanding material properties, exceeding those reported for commonly manufactured EN AW-2024 after T4 heat treatment.
pulsed laser processing is an effective technology for high-precision cutting,
preforms of copper based braze (CuSn12) with a liquidus temperature of 990 °C [1]. In the process, a braze preform is induced into a ceramic capillary, molten by a laser pulse and expelled out of the machining head via nitrogen overpressure. After a flight phase the braze droplet wets the surface of the electrode structure and the copper resulting in a firm joint after solidification. The shear strengths of the joints were measured to be 31 MPa for joints on Al2O3 substrates and 37 MPa for joints generated on LTCC substrates with screen printed Ag electrode structures. The shear strengths after temperature shock tests according to DIN EN 60068-2-14 indicate good performance of the joints in harsh environments. The average resistivity of a joint was tested by four wire testing and found to be 1.56 ± 0.33 mΩ. The introduced setup is capable to join thin metallic layers of 30 - 15 μm thickness with cu conductors of 100 μm diameter without the need of flux.},
author = {Stein, Stefan and Heberle, Johannes and Suchy, Michael and Tenner, Felix and Hugger, Florian and Roth, Stephan and Schmidt, Michael},
doi = {10.2961/jlmn.2016.01.0021},
faupublication = {yes},
journal = {Journal of Laser Micro Nanoengineering},
keywords = {micro joining; laser droplet brazing; joining of foils; electronics; contacting; electrodes;},
pages = {111-116},
peerreviewed = {Yes},
title = {{High} {Temperature} {Laser} {Based} {Drop} on {Demand} {Micro} {Joining} of {Thin} {Metallic} {Layers} or {Foils} using {Bronze} {Braze} {Preforms}},
url = {http://www.jlps.gr.jp/jlmn/upload/2633dd44a74cc80a417d9c07831e60ca.pdf},
volume = {11},
year = {2016}
}
@inproceedings{faucris.114806824,
abstract = {Since high-power ultrashort pulsed (usp) laser sources are commercially available, customers are facing the issue of delivering high power appropriately onto their work piece. This results in an inefficient usage of the provided laser power. Current technologies allow either for fast deflection or spatial adaption of the laser beam. To access a fast temporal and spatial distribution of the laser power the use of the acousto optic (AO) effect was proposed and experimentally confirmed. While the usage of AO beam deflection is well known for decades, the application as a beam shaper is subject of current research. This technology is capable to operate at several MHz. Therefore, two orthogonal AO deflectors (AOD) are placed in the Fourier plane of a lens. If a proper acoustic signal is applied synchronized to the laser pulses, each pulse is affected by a quasi-static arbitrary grating. The desired intensity distribution can now be observed in the image plane of the lens. The AO deflectors are driven by a two-channel arbitrary waveform generator followed by an amplifier stage. In this paper we present an algorithm based on an Iterative Fourier Transform Algorithm (IFTA) to calculate the acoustic field from an input intensity distribution. The intensity distribution is projected in x- and y-direction to convert into two separated 1D-profiles. Via the IFTA the necessary phase delays for both directions are calculated. Then the corresponding acoustic signals are computed and hardware restrictions like the modulation bandwidths of the AOD and the signal generators are consider for the iterative optimization. Also the amplifiers amplitude response characteristics are taken into account. The resulting 2D intensity distribution is simulated by convolution of both signals. We present results of intensity distributions by AO beam shaping. The practical restrictions of achievable beam profiles of an AO beam shaping device are discussed. Finally we show micro structures generated by such a device.},
author = {Strauß, Johannes and Vorndran, Martin and Heberle, Johannes and Schmidt, Michael},
booktitle = {International Congress on Applications of Lasers & Electro–Optics},
date = {2016-10-16/2016-10-20},
faupublication = {yes},
peerreviewed = {unknown},
title = {{IFTA} {Calculation} of {Frequency} {Patterns} for {Acousto} {Optical} {Laser} {Beam} {Shaping}},
venue = {San Diego, CA, USA},
year = {2016}
}
@article{faucris.123040984,
abstract = {This paper presents latest results on the influences of nozzle material and geometry on the electromechanical contacting of sensitive piezoceramic actuator modules. Two nozzle types have been investigated,a standard WC/Co nozzle which is used for soldering applications and a novelceramic nozzle. Applications for active piezoceramic components integrated in structural parts are e.g. active damping, energy harvesting, or monitoring of vibrations and material failure. Anup to now unsolved problem is the electrical contacting of such components without damaging the conductor or the metallization of the ceramic substrate. Since piezoelectric components are to be integrated into structures made of casted aluminum, requirements are high mechanical strength and temperature resistance. Within this paper a method forcontacting piezoceramic modules is presented. A spherical braze preform of tin bronze Cu89Sn11 with a diameter of 600 μm is located in a ceramic nozzle and is subsequently melted by a laser pulse. The liquid solder is ejected from the nozzlevia nitrogen overpressure and wets the surface of the metallization pad and the Cu-wire, resulting in a brazing joint after solidification. The process is called laser droplet brazing (LDB). To asses the thermal evolution during one cycle WC/Co and ZTA have been simulated numerically for two different geometries enabling a proposition weather the geometry or the material properties have a significant influence on the thermal load during one cycle. To evaluate the influence of the nozzle on the joint the positioning accuracy, joint height and detachment times have been evaluated. Results obtained with the ZTA nozzle show comparable positioning accuracies to a WC/Co nozzle with a lower standard deviation of solder detachment time.},
author = {Stein, Stefan and Heberle, Johannes and Gürtler, Franz-Josef and Cvecek, Kristian and Roth, Stephan and Schmidt, Michael},
doi = {10.1016/j.phpro.2014.08.078},
faupublication = {yes},
journal = {Physics Procedia},
keywords = {laser soldering; brazing; piezoceramic; bonding; laser droplet brazing},
pages = {709-719},
peerreviewed = {Yes},
title = {{Influences} of {Nozzle} {Material} on {Laser} {Droplet} {Brazing} {Joints} with {Cu89Sn11} {Preforms}},
url = {http://www.sciencedirect.com/science/article/pii/S1875389214002235},
volume = {56},
year = {2014}
}
@article{faucris.108626584,
abstract = {Efficient and accurate generation of micro holes and feature geometries is one object of investigation in ultrashort pulse laser processing. Different analytical models exist to describe the relation between energy input and removed volume. These models relate to the ablation threshold and the energy penetration depth representing material-dependent parameters. Both parameters are influenced by incubation. Against this background, incubation effects depending on the number of pulses applied on two steel alloys are presented in the paper. The ablation threshold and the energy penetration depth are analyzed by the zero-damage method and an analytical model based on Beer’s law describing the crater depth. The pulse frequency is chosen to fP = 100 Hz to investigate incubation excluding the influence of temporal effects between subsequent pulses. The used ultrashort pulsed laser has a pulse duration of 10 ps and a wavelength of 1064 nm. Craters are generated with defined pulse numbers between N = 1 and N = 1000. Two commonly used steel alloys – the cold working steel 1.2379 and the austenitic stainless steel 1.4301 – are investigated and compared. Additionally, the first one is investigated at two levels of hardness. All sample surfaces have the same initial average roughness Sa ≈ 0.02 µm which was realized by manual polishing. The comparison of two incubation models shows higher coincidence of the experimental results and the model proposed by Ashkenasi than for the model published by Jee, especially for pulse numbers N > 100. The linear optical penetration depths which are determined by ellipsometry measurements are larger than the energy penetration depths which are analyzed for the ablation regimes of the gentle phase. The main reasons for the pulse-number dependent decrease of the ablation threshold are investigated. Absorption changes due to surface modifications such as oxidation and generation of different surface morphologies which are observed by SEM. The obtained results are the basis for further studies on incubation including temporal multi-pulse effects.},
author = {Häfner, Tom and Heberle, Johannes and Dobler, Michael and Schmidt, Michael},
doi = {10.2351/1.4944445},
faupublication = {yes},
journal = {Journal of Laser Applications},
keywords = {Micro-machining; Picosecond laser; Incubation},
pages = {0226051-0226056},
peerreviewed = {Yes},
title = {{Influences} on {Incubation} in ps {Laser} {Micro}-machining of {Steel} {Alloys}},
url = {http://lia.scitation.org/doi/abs/10.2351/1.4944445?journalCode=jla},
volume = {Volume 28},
year = {2016}
}
@article{faucris.206693353,
abstract = {
Laser Beam Melting (LBM) of metals is an innovative additive manufacturing technology for producing complexly shaped parts. However, the spectrum of available materials is yet limited and the qualification of further alloys is subject of ongoing research. Considering tooling applications e.g. for injection moulding low alloyed tool steels like 1.2343 (AISI H11) would be of particular interest. The feasibility of processing 1.2343 by LBM has already been shown. Besides the LBM-process itself, also a heat-treatment process has to be taken into account. Heat-treatment is necessary to reduce the process-inherent internal stress and to adjust the desired mechanical properties. Hence, an experimental study on the heat-treatment of laser beam molten specimens made from 1.2343 is conducted. The resulting microstructure is characterised by metallographic microsections and electron backscatter diffraction (EBSD). Additionally, hardness measurements and tensile tests give information about the mechanical properties in dependence of the build direction and the heat-treatment strategy. The ultimate tensile strength after annealing reached 2148 ± 16 MPa along with an elongation at break of 8.8 ± 1.1 %. The hardness of the LBM-generated material was determined to 737 ± 16 HV1 after hardening and to 585 ± 9 HV1 after annealing.
Large-area processing with high material removal rates by ultrashort pulsed (USP) lasers is coming into focus by the development of high-power USP laser systems. However, currently the bottleneck for high-rate production is given by slow and inefficient beam manipulation. On the one hand, slow beam deflection with regard to high pulse repetition rates leads to heat accumulation and shielding effects, on the other hand, a conventional focus cannot provide the optimum fluence due to the Gaussian intensity profile. In this paper, we emphasize on two approaches of dynamic laser beam shaping with liquid crystal on silicon spatial light modulation and acousto-optic beam shaping. Advantages and limitations of dynamic laser beam shaping with regard to USP laser material processing and methods for reducing the influence of speckle are discussed. Additionally, the influence of optics induced aberrations on speckle characteristics is evaluated. Laser material processing results are presented correlating the achieved structure quality with the simulated and measured beam quality. Experimental and analytical investigations show a certain fluence dependence of the necessary number of alternative holograms to realize homogeneous microstructures.