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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Optimization for high speed surface processing of metallic surfaces utilizing direct laser interference patterning

Lang, Valentin, Hoffmann, Tim, Lasagni, Andrés Fabián 12 August 2020 (has links)
Direct Laser Interference Structuring (DLIP) is a manufacturing technology capable to functionalize large areas with high-precision periodic patterns. However, for industrial use of this emerging technology, solutions must be developed for specific requirements. With the objective of optimizing Direct Laser Interference Patterning in terms of process speed, an advanced optical module was developed that permits to superimpose two laser beams obtaining the interference pattern within an elongated area (linear spot) to meet the requirements of high-speed processing. After that, the influence of the process parameters on the quality of the surface patterns produced with the developed optical assembly was determined. It could be shown that the pulse overlap, in contrast to the applied average fluence, has a significant influence on the resulting structure heights of the produced patterns. Furthermore, it became apparent that during the course of the process, the underlying physical process dynamics seem to change, which was indicated by the resulting structure heights variations over the process. The gained findings will make a contribution to improving the quality of surface patterns produced with DLIP and to enabling reliable manufacturing qualities in the future.
2

Fabrication of multifunctional aluminum surfaces using laser-based texturing methods

Milles, Stephan 18 August 2021 (has links)
Nature-inspired surfaces provide an endless potential for innovations and exploitations in material science and engineering for a broad range of applications. Particularly, significant progress has been achieved in the fields of ice formation and wetting phenomena on metallic surfaces. One of the most relevant wetting states is superhydrophobicity, which is characterized by the complete repellency of water droplets upon impinging on a surface. A superhydrophobic surface can be accompanied by additional functions such as anti- icing, corrosion-resistance or self-cleaning. A particularly attractive material to implement functional surfaces is aluminum, due to its outstanding mechanical properties such as lightweight and high strength combined with an excellent electrical conductivity and affordable price. Functionalized aluminum surfaces can further increase the added value of technical aluminum products which are used in the automotive, aerospace and life science industry among others. A promising strategy to achieve multifunctionalities is by fabricating micrometer and submicrometer features on the material’s surface. Thus, surface texturing of aluminum components is an extremely relevant topic in science and engineering which affects all facets of our lives. Until now, micropatterned aluminum surfaces, that combine water- repellent, self-cleaning and icephobic properties, have not yet been completely explored. The present doctoral thesis focuses on structuring aluminum substrates to fabricate multifunctional surfaces with superhydrophobic, self-cleaning and anti-icing properties. To accomplish this goal, scanner-based direct laser writing (DLW) and two- and four-beam direct laser interference patterning (DLIP) are applied to pattern micrometer and sub- micrometer features on aluminum. They are employed separately to fabricate single-scale textures, as well as in combination in order to obtain multi-scale geometries and complex patterns. The laser texturing parameters are optimized to maximize the addressed functionalities and their influence on the microstructure are studied. In order to explain the wetting and freezing behavior of the functional surfaces, numerical heat transfer simulation models are applied. The most promising textures are then selected and tested under realistic icing conditions simulating the freezing behavior of water droplets on aircraft parts during flight. Moreover, a new method to characterize the self-cleaning efficiency of laser-patterned aluminum is developed. The textured aluminum surfaces attained a water-repellent functionality with a static water contact angle of up to 163° and a sliding angle of 12° without chemical post-processing. This functionality permitted a self-cleaning property where the DLIP and DLW+DLIP structures provided a maximum self-cleaning efficiency with remaining contamination as low as 1 %. The ice-repellent characterization at a temperature of -20°C revealed that in all investigated laser-structured surfaces the freezing time of 8 μl droplets increased up to three times compared to an unstructured reference. Moreover, it was demonstrated, that optimized surface textures led to a reduction of the ice adhesion strength by up to 90 %.:Selbstständigkeitserklärung Kurzfassung Abstract Acknowledgements Table of content List of abbreviations and symbols 1 Motivation 2 Theoretical principles and definitions 3 State of the art 4 Materials and methods 5 Results and discussion 6 Conclusions 7 Outlook Literature Curriculum vitae of the author List of publications / Von der Natur inspirierte Oberflächen bergen ein endloses Potential für Innovationen auf den Gebieten der Materialwissenschaft und demonstrieren ein breites Anwendungsfeld. Insbesondere in den Bereichen der Eisbildung und der Benetzungsphänomene auf Metalloberflächen wurde ein bedeutender Fortschritt erzielt. Einer der relevantesten Benetzungszustände ist der der Superhydrophobizität, welcher sich durch die vollständige Abweisung von Wassertropfen auszeichnet, sobald diese auf eine Oberfläche auftreffen. Eine superhydrophobe Oberfläche kann von zusätzlichen Funktionen wie Vereisungsschutz, Korrosionsbeständigkeit oder Selbstreinigung begleitet werden. Dabei ist besonders der Werkstoff Aluminium zur Realisierung funktionaler Oberflächen attraktiv, aufgrund seiner mechanischen Eigenschaften wie etwa ein geringes Gewicht und eine hohe Festigkeit bei gleichzeitig hervorragender elektrischer Leitfähigkeit ergänzt durch einen günstigen Preis. Funktionalisierte Aluminiumoberflächen können die Wertschöpfung von technischen Aluminiumprodukten deutlich erhöhen. Diese werden u.a. im Automobilsektor, in der Luft- und Raumfahrtindustrie oder im Life-Science-Bereich eingesetzt. Ein vielversprechender Ansatz zur Realisierung multifunktionaler Eigenschaften basiert auf der Herstellung von Mikrometer- und Submikrometer-Strukturen auf der Oberfläche. Daher stellt die Texturierung von Aluminiumkomponenten ein äußerst relevantes Thema in der Wissenschaft und Technik dar, da sie sämtliche Facetten unseres täglichen Lebens tangiert. Bis heute sind laser-strukturierte Aluminiumoberflächen, die wasserabweisende, selbstreinigende und eisabweisende Eigenschaften vereinen, noch nicht vollständig erforscht. Die zugrunde liegende Dissertation thematisiert die Strukturierung von Aluminiumsubstraten zur Herstellung multifunktionaler Oberflächen mit superhydrophoben, selbstreinigenden und vereisungsmindernden Eigenschaften. Dafür, werden direktes Laserschreiben (engl. Direct laser writing, DLW) sowie die direkte Laserinterferenzstrukturierung (engl. Direct laser interference patternin, DLIP) auf Aluminium angewendet. Die Verfahren werden sowohl separat zur Herstellung von einskaligen Texturen als auch in Kombination eingesetzt, um mehrskalige komplexe Muster zu fertigen. Die Strukturierungsparameter werden zur Maximierung der erwähnten Eigenschaften hin optimiert, und ihr Einfluß auf die Mikrostruktur wird untersucht. Um das Benetzungs- und Vereisungsverhalten der funktionalisierten Oberflächen zu erklären, werden numerische Simulationsmodelle eingesetzt. Die vielversprechendsten Texturen werden unter realistischen Vereisungsbedingungen getestet, welche das Gefrierverhalten von Wassertropfen auf Flugzeugbauteilen während des Fluges simulieren. Darüber hinaus wird eine neue Methode zur Charakterisierung der Selbstreinigungseffizienz von laserstrukturiertem Aluminium entwickelt und angewendet. Die texturierten Aluminiumoberflächen erhielten ohne chemische Nachbearbeitung eine wasserabweisende Funktionalität mit einem statischen Wasserkontaktwinkel von bis zu 163° und einem Gleitwinkel von 12°. Diese Funktionalität ermöglichte eine Selbstreinigungseigenschaft, bei der die DLIP- und DLW+DLIP-Strukturen die höchste Effizienz mit einer Restverunreinigung von bis zu 1 % erzielten. Die eisabweisende Charakterisierung bei einer Temperatur von -20°C offenbarte, dass bei allen untersuchten laserstrukturierten Oberflächen die Vereisungszeit von 8 μl Wassertropfen bis um das Dreifache anstieg, im Vergleich zur unstrukturierten Referenz. Darüber hinaus konnte demonstriert werden, dass optimierte Oberflächentexturen zu einer Reduzierung der Eis- Adhäsionskraft um bis zu 90 % führten.:Selbstständigkeitserklärung Kurzfassung Abstract Acknowledgements Table of content List of abbreviations and symbols 1 Motivation 2 Theoretical principles and definitions 3 State of the art 4 Materials and methods 5 Results and discussion 6 Conclusions 7 Outlook Literature Curriculum vitae of the author List of publications
3

Application of the mesh-free smoothed particle hydrodynamics method in the modelling of direct laser interference patterning

Demuth, Cornelius 23 March 2022 (has links)
In this work, the mesh-free smoothed particle hydrodynamics (SPH) method is applied in the modelling of the direct laser interference patterning (DLIP) of metal surfaces. The DLIP technique allows the fabrication of periodic microstructures on technical surfaces using nanosecond laser pulses. Here, the interference of two coherent partial beams with a sinusoidal energy density distribution of the interference pattern is concerned, which is employed to generate line-like surface structures. However, the mechanisms effective during nanosecond pulsed DLIP of metals are not yet fully understood. The physical phenomena occurring due to the interaction of laser radiation with metallic materials are first considered and the governing differential equations are stated. The fundamentals of the SPH method and the approaches to the numerical treatment of the conservation equations are presented. Physical processes relevant to the modelling of laser material processing are solved by suitable SPH techniques, i.e. the approximations are verified with respect to test problems with analytical or known numerical solutions. Consequently, the SPH method is used to devise a thermal model of the DLIP process, considering the absorption of the laser radiation, the heat conduction into the workpiece and the latent heat of involved phase changes. This model is extended to compute the melt pool convection during DLIP, which is driven by surface tension gradients due to temperature gradients. For this purpose, an incompressible SPH (ISPH) method is used, representing a novel approach to the modelling of the laser-induced melt pool flow. The numerical model is employed to perform simulations of DLIP on metal substrates. Firstly, the thermal simulation of the single pulse patterning of stainless steel is in good agreement with experimental results. The application of DLIP to stainless steel and aluminium is then simulated by the comprehensive model including the melt pool flow. Moreover, this model is further extended to consider the non-linear temperature dependence of surface tension, as in liquid steel in the presence of a surface active element. The simulation results reveal a distinct behaviour of stainless steel and aluminium substrates. A markedly deeper melt pool and considerable velocity magnitudes of the thermocapillary convection at the melt surface are computed for DLIP of aluminium. In contrast, the melt pool flow is less pronounced during DLIP of stainless steel, whereas higher surface temperatures are predicted. Hence the Marangoni convection is a conceivable effective mechanism during the structuring of aluminium at moderate energy density. The different character of the melt pool convection during DLIP of stainless steel and aluminium is corroborated by experimental observations. Furthermore, the simulations for stainless steel with different sulphur content indicate distinct melt pool flow patterns and support the explanation of the microstructures found after DLIP experiments. The role of vapourisation and the induced recoil pressure in the microstructure evolution due to DLIP on metal substrates at elevated fluences could be prospectively investigated. In this regard, the consideration of the melt pool surface deformation in the ISPH algorithm, and particularly a suitable pressure boundary condition, is required.:I The research problem 1 Motivation 2 Modelling of laser material processing 2.1 Interaction of laser radiation with materials 2.1.1 Absorption of laser radiation 2.1.2 Heat conduction and phase change 2.1.3 Molten pool convection 2.1.4 Vapourisation regime 2.2 Mathematical modelling of laser material interaction 2.2.1 Conservation equations in Lagrangian formulation 2.2.2 Influence of surface tension 3 State of the art in laser microprocessing and the SPH method 3.1 Laser microprocessing 3.2 Simulation of direct laser interference patterning 3.3 The mesh-free smoothed particle hydrodynamics method 3.3.1 Fundamental approximations and kernel function 3.3.2 Particle distribution and interaction length 3.3.3 Approximation of derivatives 3.3.4 Treatment of boundaries 3.3.5 Neighbourhood search 3.4 Numerical modelling of laser material processing by SPH II SPH model development for direct laser interference patterning 4 SPH modelling of heat transfer and fluid flow 4.1 Solution of the heat diffusion equation 4.2 Formulation of equations governing fluid flow 4.2.1 Equation of continuity 4.2.2 Approximation of pressure gradient term 4.2.3 Treatment of viscosity 4.3 Weakly compressible SPH method for solving fluid flow 4.3.1 Particle motion 4.3.2 Time integration 4.3.3 Time step criteria 4.4 Incompressible SPH method for solving fluid flow 4.4.1 Time integration 4.4.2 Discrete incompressible SPH algorithm 4.4.3 Time step criteria 4.5 Simulation of thermal fluid flow using ISPH 4.5.1 Semi-implicit time integration 4.5.2 Solution of the pressure Poisson equation 5 Verification of the SPH implementation 5.1 Transient heat conduction in laser-irradiated plate 5.1.1 Problem description 5.1.2 Dimensionless formulation 5.1.3 Numerical solution and results 5.2 Viscous flow 5.2.1 Couette flow 5.2.2 Poiseuille flow 5.3 Thermal convection 5.3.1 Natural convection in a square cavity 5.3.2 Rayleigh--Marangoni--Bénard convection in liquid aluminium 6 SPH model of direct laser interference patterning 6.1 Characteristics of the process 6.2 Thermal model 6.2.1 Non-dimensionalisation 6.2.2 Numerical solution of governing equation 6.2.3 Verification of the computation 6.2.4 Numerical test 6.3 Thermofluiddynamic model 6.3.1 Non-dimensionalisation 6.3.2 Numerical solution of governing equations 6.3.3 Discretisation 6.3.4 Resolution independence study 7 SPH simulation of direct laser interference patterning 7.1 Thermal model 7.1.1 DLIP experiments on stainless steel substrates 7.1.2 Thermal simulation of DLIP on steel substrate 7.2 Thermofluiddynamic model 7.2.1 Material properties and simulation parameters 7.2.2 Numerical results for steel substrate 7.2.3 Numerical results for aluminium substrate 7.2.4 Discussion and comparison with experiments 7.3 Extended thermofluiddynamic model 7.3.1 Model parameters 7.3.2 Influence of sulphur content on DLIP of stainless steel 8 Conclusions and outlook Bibliography / In dieser Arbeit wird die direkte Laserinterferenzstrukturierung (Direct Laser Interference Patterning, DLIP) von Metallen mit der netzfreien Smoothed Particle Hydrodynamics (SPH) Methode modelliert. Das DLIP-Verfahren ermöglicht die Fertigung periodischer Mikrostrukturen auf technischen Oberflächen mit Nanosekunden-Laserpulsen. Hier wird die Zweistrahlinterferenz mit einer sinusförmigen Energiedichteverteilung des Interferenzmusters behandelt, die linienförmige Oberflächenstrukturen erzeugt. Die bei der direkten Interferenzstrukturierung von Metallen mit Nanosekunden-Laserpuls wirksamen Mechanismen sind jedoch noch nicht verstanden. Die aufgrund der Wechselwirkung von Laserstrahlung mit metallischen Werkstoffen auftretenden physikalischen Phänomene werden zuerst betrachtet und die sie bestimmenden Differentialgleichungen angegeben. Die Grundlagen der SPH-Methode sowie deren Herangehensweisen an die numerische Behandlung der Erhaltungsgleichungen werden vorgestellt. Für die Modellierung der Lasermaterialbearbeitung relevante physikalische Vorgänge werden mittels geeigneter SPH-Ansätze gelöst, d. h. anhand von Testproblemen mit bekannter Lösung verifiziert. Das mit SPH zunächst erstellte thermische Modell des DLIP-Prozesses berücksichtigt die Absorption der Laserstrahlung, die Wärmeleitung im Werkstück und die Enthalpien der Phasenübergänge. Das Modell wird zur Berechnung der Schmelzbadströmung bei der DLIP-Anwendung, angetrieben von Oberflächenspannungsgradienten verursacht durch Temperaturgradienten, erweitert. Hierbei wird eine inkompressible SPH (ISPH) Methode eingesetzt, in der Simulation laserinduzierter Schmelzbäder ein neuartiger Ansatz. Mit dem numerischen Modell werden Simulationen des DLIP-Verfahrens für metallische Substrate durchgeführt. Die thermische Simulation der Strukturierung von Edelstahl stimmt gut mit einem Experiment überein. Weiterhin wird die Anwendung von DLIP auf Edelstahl und Aluminium mit dem thermofluiddynamischen Modell simuliert. Außerdem wird das Modell um eine nichtlinear temperaturabhängige Oberflächenspannung, wie sie für Stahlschmelze in Anwesenheit eines oberflächenaktiven Elements vorliegt, ergänzt. Die Simulationen zeigen ein verschiedenes Verhalten von Edelstahl und Aluminium. Bei der Strukturierung von Aluminium treten ein deutlich tieferes Schmelzbad und erhebliche Geschwindigkeitsbeträge der thermokapillaren Konvektion an der Schmelzeoberfläche auf. Hingegen ist die Strömung bei der DLIP-Anwendung auf Edelstahl schwächer ausgeprägt und höhere Oberflächentemperaturen werden erreicht. Die Marangoni-Konvektion ist daher ein wirksamer Schmelzeverdrängungsmechanismus bei der Strukturierung von Aluminium mit moderater Energiedichte. Die unterschiedliche Schmelzbadströmung für die beiden Werkstoffe wird durch experimentelle Beobachtungen bestätigt. In Abhängigkeit des Schwefelgehalts von Edelstahl zeigen Simulationen verschiedene Strömungsmuster im Schmelzbad und unterstützen die Erklärung experimentell festgestellter Mikrostrukturen. Die Untersuchung der Wirkung der Verdampfung und des induzierten Rückstoßdruckes auf die Strukturausbildung bei höheren Fluenzen erfordert die Berücksichtigung der Oberflächendeformation sowie eine geeignete Druckrandbedingung im ISPH-Algorithmus.:I The research problem 1 Motivation 2 Modelling of laser material processing 2.1 Interaction of laser radiation with materials 2.1.1 Absorption of laser radiation 2.1.2 Heat conduction and phase change 2.1.3 Molten pool convection 2.1.4 Vapourisation regime 2.2 Mathematical modelling of laser material interaction 2.2.1 Conservation equations in Lagrangian formulation 2.2.2 Influence of surface tension 3 State of the art in laser microprocessing and the SPH method 3.1 Laser microprocessing 3.2 Simulation of direct laser interference patterning 3.3 The mesh-free smoothed particle hydrodynamics method 3.3.1 Fundamental approximations and kernel function 3.3.2 Particle distribution and interaction length 3.3.3 Approximation of derivatives 3.3.4 Treatment of boundaries 3.3.5 Neighbourhood search 3.4 Numerical modelling of laser material processing by SPH II SPH model development for direct laser interference patterning 4 SPH modelling of heat transfer and fluid flow 4.1 Solution of the heat diffusion equation 4.2 Formulation of equations governing fluid flow 4.2.1 Equation of continuity 4.2.2 Approximation of pressure gradient term 4.2.3 Treatment of viscosity 4.3 Weakly compressible SPH method for solving fluid flow 4.3.1 Particle motion 4.3.2 Time integration 4.3.3 Time step criteria 4.4 Incompressible SPH method for solving fluid flow 4.4.1 Time integration 4.4.2 Discrete incompressible SPH algorithm 4.4.3 Time step criteria 4.5 Simulation of thermal fluid flow using ISPH 4.5.1 Semi-implicit time integration 4.5.2 Solution of the pressure Poisson equation 5 Verification of the SPH implementation 5.1 Transient heat conduction in laser-irradiated plate 5.1.1 Problem description 5.1.2 Dimensionless formulation 5.1.3 Numerical solution and results 5.2 Viscous flow 5.2.1 Couette flow 5.2.2 Poiseuille flow 5.3 Thermal convection 5.3.1 Natural convection in a square cavity 5.3.2 Rayleigh--Marangoni--Bénard convection in liquid aluminium 6 SPH model of direct laser interference patterning 6.1 Characteristics of the process 6.2 Thermal model 6.2.1 Non-dimensionalisation 6.2.2 Numerical solution of governing equation 6.2.3 Verification of the computation 6.2.4 Numerical test 6.3 Thermofluiddynamic model 6.3.1 Non-dimensionalisation 6.3.2 Numerical solution of governing equations 6.3.3 Discretisation 6.3.4 Resolution independence study 7 SPH simulation of direct laser interference patterning 7.1 Thermal model 7.1.1 DLIP experiments on stainless steel substrates 7.1.2 Thermal simulation of DLIP on steel substrate 7.2 Thermofluiddynamic model 7.2.1 Material properties and simulation parameters 7.2.2 Numerical results for steel substrate 7.2.3 Numerical results for aluminium substrate 7.2.4 Discussion and comparison with experiments 7.3 Extended thermofluiddynamic model 7.3.1 Model parameters 7.3.2 Influence of sulphur content on DLIP of stainless steel 8 Conclusions and outlook Bibliography
4

Fabrication of Water- and Ice-Repellent Surfaces on Additive-Manufactured Components Using Laser-Based Microstructuring Methods

Kuisat, Florian, Ränke, Fabian, Baumann, Robert, Lasagni, Fernando, Lasagni, Andrés Fabián 30 May 2024 (has links)
Laser patterning techniques have shown in the last decades to be capable of producing functional surfaces on a large variety of materials. A particular challenge for these techniques is the treatment of additively manufactured parts with high roughness levels. The presented study reports on the surface modification of additive-manufactured components of Ti64 and Al–Mg–Sc (Scalmalloy), with the aim of implementing water- and ice-repellent properties. Different laser-based microstructuring techniques, using nanosecond and picosecond pulses, are combined to create multiscale textures with feature sizes between ≈800 nm and 21 μm. The wettability could be set to static water contact angles between 141° and 153° for Ti64 and Al–Mg–Sc, respectively. In addition, surface free energy is analyzed for different surface conditions.
5

Understanding the Relation between Pulse Duration and Topography Evolution of Polyether Ether Ketones Textures by Ultrashort Infrared Laser Interference Patterning

Mulko, Lucinda, Wang, Wei, Baumann, Robert, Kress, Joshua, Voisiat, Bogdan, Jaeger, Erwin, Leupolt, Beate, Vaynzof, Yana, Soldera, Marcos, Lasagni, Andrés Fabián 04 June 2024 (has links)
Advanced polymeric materials, such as polyether ether ketones (PEEK), have been placed as direct substitutes for metals and ceramics in diverse applications, such as the machinery industry and biomedical engineering. Moreover, surface treatments allow the emergence of brand-new properties or the improvement of preexisting ones, such as friction, lubrication, wettability, cellular infiltration, or osseointegration. A paramount approach to achieving topographical modifications is by using laser micro/nanoprocessing techniques such as direct laser interference patterning (DLIP). Herein, PEEK foils are structured with DLIP method using ultrashort pulses. The influence of the pulse duration between 266 fs and 15 ps and the pulse-to-pulse overlap on the resulting surface topography and chemistry is assessed. As a result, well-defined line-like textures with a period of 5.8 μm and aspect ratios up to 0.88 are achieved. Furthermore, it is possible to explore and understand the behavior of surface phenomena such as swelling, increase/decrease of laser–material interaction onset, and laser-induced periodic surface structures formation. A comprehensive topographical and chemical characterization study demonstrates that these distinctive topographical features occur because of multiphoton absorption, incubation effects, and heat accumulation. These phenomena allow structuring polymeric substrates that are low-absorbing and challenging to pattern with conventional nanosecond infrared (IR) laser sources.

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