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Sökning: WFRF:(Kaplan Alexander) > Doktorsavhandling

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1.
  • Da Silva, Adrien (författare)
  • Aspects of material and heat transfer in drop- and powder-based laser additive manufacturing
  • 2023
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Additive Manufacturing became a major research topic and part of industrial production in the past years. Numerous techniques now allow to build 3D structures with a wide choice of materials. When it comes to processing of metals, a laser beam is often used as a heat source to melt either a wire or powder. Novel approaches of material deposition are also developed, such as Laser Droplet Generation, which could potentially be applied to Additive Manufacturing. During the process, the laser beam light is partly absorbed by the material, and is then converted to heat, which can induce melting and even vaporization. Additive Manufacturing presents several processing challenges, such as the recoil pressure acting on the drops and powder particles that affects their trajectory. Storage and recycling of the powders is also an important aspect since the powder properties are changed through aging. Another challenge is the adjustment of process parameters according to varying deposition conditions, where the use of process monitoring techniques is crucial.Therefore, this thesis aims at better understanding (i) the effects of recoil pressureon metal drops and powder particles, (ii) powder aging and its effects on the process, and (iii) process optimisation and stability via monitoring. In the six adjoined papers, high-speed imaging and thermal imaging were used to observe laser Additive Manufacturing processes involving both metal drops and powders. The videos enabled to observe drop detachments, measure trajectories, plot powder density maps, quantify powder catchment in the melt pool, measure themelt pool geometry, detect oxides, and extract cooling rates. The experimental results were supplemented with material analysis and theoretical calculations of thermodynamics, recoil pressure and surface tension.These studies allowed to conclude that the recoil pressure induced by laser irradiation on a drop or a powder particle can have some significant effect such as acceleration, change of trajectory, or disintegration. However, these effects seem to be considerably lower than what theoretical models predict. It was also found that the recoil pressure can be used to accurately detach drops from a wire, which was utilised as a new material deposition method for Additive Manufacturing. In Directed Energy deposition, it was showed that aging of the aluminium powder feedstock should be avoided since it induces high porosity, high dilution and decreased mechanical properties. Finally, to guarantee a defect-free deposition during the whole process, it was demonstrated that a thermal camera can be used to monitor the melt pool size, which allows to apply appropriate laser power adjustments to compensate for changing building conditions.  
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2.
  • Dewi, Handika Sandra (författare)
  • Laser-induced phase transformations in microalloyed steels
  • 2022
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • The application of thermal cycles below melting temperature can induce solid-to-solid phase transformation in steels, which is the transition between different crystalline structures of the same compound. There are many types of crystalline structures in steels produced, depending on the characteristics of the applied thermal cycle. For instance, rapid cooling can generate martensite structure that tends to increase the hardness of the steels, while slow cooling will more likely produce ferrite structure, which is less hard than the martensite structure. Laser heat treatment is one example where the laser becomes a thermal energy source, inducing thermal cycles below melting point and an extremely rapid cooling rate, which results in unexpected microstructures upon cooling. The mechanism of such phase transformations is still widely unknown, although the knowledge can be beneficial for many laser processes. Accordingly, studies on laser induced phase transformation are necessary.The purpose of my work is explaining underlying mechanisms of solid-to-solid phase transformation in microalloyed steels due to short thermal cycles of the laser heat treatment. My work aims to (1) find the correlation between energy input distributions from the laser beam and temperature history during the laser heat treatment process and (2) describe how changes in the thermal cycle induced by laser illumination influence the phase transformation dynamics. This work focuses on martensitic transformation and infrared laser (1070 nm).To explain martensitic transformation during laser heat treatment, this work involved ex-situ observations of the laser heat treated specimens. The study consists of varying the laser parameters, measuring the surface temperature of the specimens and simulating the in-depth temperature. Consecutively, characteristics (i.e., holding time, peak temperature, and cooling rate) of the measured and/or calculated thermal cycles were extracted, and the microstructures of the specimens were observed using microscopes. Finally, the thermal cycle characteristics and the microstructure of the specimens were related.The results show that the energy input distributions from the laser beam (e.g., laser beam profile) determine the geometry of the treated area, while processing speed and laser power influence the cooling rate and peak temperature of the thermal cycle respectively. The short thermal cycles induced by the laser beam are able to induce martensitic structure in the specimen. However, ferrite structure unexpectedly remains in the treated area. The holding time, which is the duration of temperature staying above austenisation temperature, has an inverse correlation to the appearance of ferrite structure in the treated area. This relates to the carbon diffusion occurring during the process, in which the carbon atoms have to diffuse from rich-carbon-austenite into low-carbon-austenite before cooling. Accordingly, the amount of martensite structure in the treated area depends on the holding time value of the process. There are indications that the rapid cooling induced by the laser beam can abruptly stop the diffusion process. It is clear that the laser provides an opportunity to control martensite structure.
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3.
  • Fedina, Tatiana (författare)
  • Towards sustainability in additive manufacturing: material and process aspects
  • 2023
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • The acceptance of additive manufacturing (AM) depends on the quality of final parts and process repeatability. Recently, many studies have been dedicated to the establishment of the relationship between the process behavior and material performance. Phenomena such as laser-material interaction, melt pool dynamics, ejecta formation and particle movement behavior on a powder bed are of a particular interest for the AM community as these events directly influence the outcome of the process. Another aspect, which hinders the adoption of AM, is the need for cost-efficient powder materials, their sustainable processing and recycling. The research work presented in this thesis explores scientific aspects related to the above-mentioned topics, with a particular focus on the material and process behavior phenomena in powder bed fusion-laser melting (PBF-LM) and directed energy deposition (DED) processes. Paper A shows a comparative study of dissimilarly shaped gas and water atomized low alloy steel powders regarding their processability, packing capacities, particle movement behavior and powder performance in PBF-LM. The impact of chemical composition and morphology of the powders on the process behavior was revealed. Powder spattering and melt pool instabilities were discussed in detail. Paper B contains research on the particle movement and denudation behavior on a powder bed when using near-spherical and non-spherical steel powders. The influence of particle morphology on the dynamics of arbitrary-shaped powder particles was studied by applying an analytical correlation formula to calculate the drag force exerted on powder particles of various shape. Particle entrainment of gas and water atomized powders in front of the laser beam was measured, revealing a significant difference in the powder transfer towards the melt pool.Paper C explains the role of ejecta in the recycled powder and the changing behavior of the material due to ejecta pick-up. The impact of multiple powder recycling steps on the degradation of low alloy steel powder in laser powder bed fusion was studied. Oxygen content, particle size and ejecta occurrence gradually increased after each recycling step and were identified as the main contributors to the property alterations observed in the powder during recycling. In addition, a direct correlation between the increase in oxygen and more frequent spatter ejection with repeated recycling was established. Paper D focuses on the impact of powder aging on the degradation of AlSi10Mg powder during processing in PBF-LM. The analysis of the powder properties, affected by laser exposure and the aging procedure, showed a change of chemical and morphological characteristics of the powders in virgin and aged conditions. The oxygen content in the powders appeared to have a significant effect on the powders' surface appearance and light absorbance, gradually deteriorating the processability of the powders with the increase of oxygen level. Porosity occurrence and its influence on the mechanical properties of the powders was also studied, demonstrating a rapid decrease of ultimate tensile strength and elongation from virgin condition to aged.Papers E and F investigate the possibilities of iron ore waste reduction using Al powder as a reducing agent and a laser beam as a heat source. Paper E focuses on the Fe2O3-Al interaction behavior and extent of the iron ore reduction, whereas Paper F reports on the high-speed imaging investigation possibilities of laser beam-material surface interaction when processing Fe2O3-Al powders and an Fe2O3 powder-AlSI5 wire combination in DED. In-situ observation of various melt pool phenomena and exothermic reaction behavior of the material combinations using high-speed imaging was carried out. In addition to that, the influence of feed materials and laser power on the thermite reaction time was discussed in detail, showing their dissimilar behavior.All six papers include research on laser additive manufacturing using powder feedstocks. The papers discuss various phenomena regarding powder processability, recycling and laser beam-material interaction behavior in both PBF-LM and DED. High-speed imaging was used as the main tool to observe and study the above listed topics.  
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4.
  • Hauser, Tobias (författare)
  • In-situ analysis of process characteristics in Directed Energy Deposition
  • 2022
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • In recent years, the interest in Additive Manufacturing on an industrial scale has risen due to the various new processes and the increase in potential use cases. In this thesis, the two Additive Manufacturing processes, Wire Arc Additive Manufacturing and Laser Metal Deposition were investigated. Both processes belong to the Directed Energy Deposition processes in Additive Manufacturing and are already used in different industrial areas such as automotive, aviation, railway, or medical engineering. A major challenge for the industrialization of Additive Manufacturing is the insufficient quality and repeatability of the manufactured parts due to the complexity of the processes and the lack of process knowledge. Therefore, this work focused on a deeper understanding of the process characteristics and their correlations with different sensors used for in-situ analysis.  One of the sensors used for in-situ analysis was high-speed imaging. High-speed imaging during Wire Arc Additive Manufacturing revealed that different lead angles of the welding torch have an influence on fluctuation effects in the manufactured structures. To avoid such fluctuation effects, which mainly origin from too low or too high interlayer temperatures, it was found that a pushing Wire Arc Additive Manufacturing process with a slightly tilted lead angle is working best. Apart from fluctuation effects, oxidation of aluminium can be also critical for the process stability of Wire Arc Additive Manufacturing. It was found that the surface oxidation on aluminium parts changed from an amorphous oxide layer into a white duplex oxide layer during the process. It was also found that oxidation anomalies, which can occur during processing due to process instabilities or lack of shielding gas, can be detected by light emission spectroscopy during manufacturing as peaks in the light spectrum arise when they occur. Another challenge in Wire Arc Additive Manufacturing of aluminium is the porosity in parts as it can have a significant impact on the resulting mechanical properties. It has been observed that as the shielding gas flow rate increases, the porosity in aluminium parts also increases due to the rapid solidification of the melt pool by the forced convection of gas flow. In addition, it has been shown that gas inclusions escaping from the melt pool leave cavities on the surface that can be observed by process imaging, which reveals information about the porosity of the part. Another promising sensor for in-situ analysis of the process characteristics are microphones that capture acoustic emissions. For Wire Arc Additive Manufacturing, the investigations showed that the main acoustic emissions origin from the plasma expansion of the arc. The acoustic emissions and the process anomalies that occur correlate mainly with the size of the arc because that is essentially the ionized volume that leads to the air pressure and causes the acoustic emissions. For Laser Metal Deposition, it was found that the main acoustic emissions are created by the interaction between the powder particles and the laser beam, because they create an air pressure when the particles expand from the solid state to the liquid state while melting. Another major area investigated in this thesis was multi-material Additive Manufacturing, as process and material characteristics can change significantly. For processing of multi-material parts in Wire Arc Additive Manufacturing, it was found that the strength was limited by the properties of the individual aluminium alloys and not by those of the material transition zones. Process monitoring algorithms have been investigated to determine the chemical composition of the processed material. It was shown that the voltage, current, acoustic, and spectral emission data can be used for in-situ analysis of the chemical differences between two aluminium alloys. For Laser Metal Deposition, the design freedom of Additive Manufacturing with multiple materials was also demonstrated. It was shown that material transitions can be implemented discrete and graded, but the gradual material transition showed advantages in avoiding cracks in the material transition zones. In summary, all scientific papers contributed to a deeper process understanding of the process, the processed materials, and the resulting mechanical properties. In addition, the contributions provided crucial insights into the interrelationships of the process characteristics and the physical principles of various sensors used for in-situ analysis of the processes. 
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5.
  • Heralic, Almir, 1981- (författare)
  • Monitoring and Control of Robotized Laser Metal-Wire Deposition
  • 2012
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • The thesis gives a number of solutions towards full automation of the promising manufacturing technology Robotized Laser Metal-wire Deposition (RLMwD). RLMwD offers great cost and weight saving potentials in the manufacturing industry. By metal deposition is here meant a layered manufacturing technique that builds fully-dense structures by melting metal wire into solidifying beads, which are deposited side by side and layer upon layer. A major challenge for this technique to be industrially implemented is to ensure process stability and repeatability. The deposition process has shown to be extremely sensitive to the wire position and orientation relative to the melt pool and the deposition direction. Careful tuning of the deposition tool and process parameters are therefore important in order to obtain a stable process and defect-free deposits. Due to its recent development, the technique is still manually controlled in industry, and hence the quality of the produced parts relies mainly on the skills of the operator. The scientific challenge is therefore to develop the wire based deposition process to a level where material integrity and good geometrical fit can be guaranteed in an automated and repeatable fashion. This thesis presents the development of a system for on-line monitoring and control of the deposition process. A complete deposition cell consisting of an industrial robot arm, a novel deposition tool, a data acquisition system, and an operator interface has been developed within the scope of this work. A system for visual feedback from the melt pool allows an operator to control the process from outside the welding room. A novel approach for automatic deposition of the process based on Iterative Learning Control is implemented. The controller has been evaluated through deposition experiments, resembling real industrial applications. The results show that the automatic controller increases the stability of the deposition process and also outperforms a manual operator. The results obtained in this work give novel solutions to the important puzzle towards full automation of the RLMwD process, and full exploitation of its potentials.
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6.
  • Naesstroem, Himani (författare)
  • Phenomena in laser based material deposition
  • 2021
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • This thesis is regarding the use of a laser beam to deposit material. Phenomena in two processes, laser beam welding with filler wire and blown powder directed energy deposition (DED) also known as laser metal deposition (LMD)1, are discussed. High-speed imaging is used as a central tool, supported by cross-sectional macrographs, surface images, X-ray images, computed tomography scans and quantitative analysis of the acquired results to observe many phenomena. Several results generated could be used in the manufacturing industry.A novel concept of feeding the filler wire off-axis to the joint in laser beam welding is presented. The formation of defects called undercuts depended mainly on the stability of the wire feed and irregular melting of its tip. Process parameters played a key role in the robustness of the process, with higher welding speeds and laser powers increasing the chance for formation of defects.Powder catchment in DED, and the various influencing factors are discussed. The position of initial interaction between powder grains and the melt pool plays an important role in defining incorporation behaviour. Powder grains can float on the surface of melt pool and travel along the direction of surface tension driven melt flows before fully incorporating. In high-deposition rate DED, an island of unmelted powder can form in the melt pool, depending on the laser beam shape and powder feeding configuration used. This island could lead to formation of spatter from the melt pool and porosity in resulting clads. Solid oxide skins present on the melt pool in low temperature areas can act like a barrier preventing complete incorporation of powder grains or possibly causing localised boiling, forming spatter.For the first time, near-unprocessed material was used as feedstock in the DED process. A single large melt pool is formed in the relatively calm process, and phenomena like cloud formation while feeding of material and spatter were observed. Single and multi-layered deposition resulted in porous tracks and delamination from the substrate. While the process is not industrially useable in its current state, it is a step towards processing cheap unprocessed material with a laser beam to manufacture low cost parts or for in-situ reduction. The roles of material composition and surface conditions of the substrate in DED are also presented. Both, the composition and surface condition affect the absorption of the laser radiation. Material composition influences the time taken for incorporation of powder grains. The size of the melt pool and dilution depends on the thermal conductivity of the substrate material. Surfaces that are rough or coated with (several sorts of) paint produce wider tracks, with better wetting angles as compared to milled or ground surfaces. Coatings like paints or cold-galvanising primers do not negatively affect the process. Deposition directly on rough or painted surfaces could significantly reduce processing time and the resources needed for cleaning before cladding or repair processes. 
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7.
  • Näsström, Jonas (författare)
  • Phenomena in wire based multi-layer laser welding and hybrid deposition
  • 2019
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Several laser materials processing technologies using metal wire addition have been researched during the last decades. Especially in the field of joining, as well as in the field of Additive Manufacturing (AM), multiple major benefits have been reached, e.g. higher welding speeds and lower heat input. With laser and arc hybrid welding techniques, additional prospects become accessible. These can combine and improve both deep penetration of autogenous laser welding and gap bridging capabilities of traditional arc welding. In the field of AM, wire feed has been a much-appreciated way of supplying additional material. Reasons include clean and easy handling, high utilisation and availability. A high intensity heat source, e.g. a laser beam or an electrical arc, continuously melts a metal wire; the melt being deposited onto a substrate in one or multiple layers to generate a new surface or three dimensional structure. An alternative joining process is Narrow Gap Multi-Layer Welding (NGMLW). This technique utilises the former mentioned AM processes to fill a gap to join sheets together, instead of depositing on an open surface. NGMLW is a capable competitor to the above-mentioned joining processes due to its prospects of being able to join essentially any thickness of sheets, as long as the beam and wire can accurately reach the gap floor and a sufficient number of layers are used.In this thesis, multiple types of NGMLW, Papers A – D, and hybrid material deposition, Papers E and F, using laser and hybrid heat sources with metal wire addition have been studied. Techniques such as High-Speed Imaging (HSI), 3D and Computed Tomography (CT) scanning have been used to gain greater insight into the workings of these modern manufacturing processes. The multi-layered way of material deposition within a gap to form a welded joint and onto a surface for AM have many similarities, e.g. wire melting behaviour and melt flow.Paper A introduces the workings of NGMLW, highlighting possible welding imperfections and welded joint morphology. HSI of the process is analysed both qualitatively and quantitatively: qualitative analysis identifying possible causes for said imperfections; quantitative analysis highlighting the potential for using similar and lower frame rate camera footage for closed loop control to suppress the formation of such imperfections.In Paper B, an alternative near-vertical building strategy for NGMLW is presented and compared to its more common horizontal counterpart. This upright strategy is found to be fully capable of producing sound welded joints, sporting less than 0.3% cavities. The near-vertical welded joints also have potential for unique material properties due to their much different thermal history.Papers C and D return to the topic of horizontal NGMLW, but with resistance heating of the metal wire for easier processing, also referred to as Laser Hot-Wire Welding (LHWW). Process behaviour and the resulting morphology of welded joints are the main topics of Paper C. Theoretical reasoning for the formation of occasional centre-line cracks, relating to the shape of the melt pool during solidification, are presented. Arcing is observed in some of the experiments, although prior theory indicates that the applied wire voltage was too low for arcing to occur. This arcing phenomenon is further covered in Paper D, where HSI observations are used to correlate process parameters to arcing probability and a theoretical explanation of why arcing can occur is suggested.Papers E and F take the step out of the gap, studying the impact of laser beam augmentation in different orientations on Wire-Arc Additive Manufacturing (WAAM). Paper E focuses on a method of quantifying melt pool movement. Fluctuations of the melt pool surface decreased by more than 35% with the introduction of a laser beam to the process. Paper F analyses the generated structures, evaluating the usable portion of the “as deposited” shapes and material composition. Surface irregularities decreased by more than 50% on application of a trailing laser beam. Additional aspects relating to the resulting morphology are also presented, including observations and reasoning for surface irregularities and sloping.The knowledge gained and methods used in the presented work intertwine to form a strong insight into both laser and laser-hybrid materials processing with wire addition. They also introduce approaches for processing and quantifying HSI footage for process evaluation and improvement.
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8.
  • Olsson, Rickard, 1959- (författare)
  • Analysis and Monitoring of Laser Welding and Surface Texturing
  • 2019
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • This thesis can be conveniently divided into three sections as follows;Part I. Monitoring of laser welding In laser materials processing there has always been a need for suitable methods to supervise and monitor the processes on-line, to ensure correct production quality or to trigger alarms when failures are detected. Numerous investigations have been made in this field, including experimental and theoretical work. It is common practice in this field to monitor surface temperature, plasma radiation and back-reflected laser light, coaxially with the laser beam.  Traditionally, the monitoring systems involved carry out no statistical analysis of the signals received – they merely involve thresholds.The first two papers in this thesis look at the feedback collected during laser welding using a co-axial setup from a Digital Signal Processing point of view and also uses high speed video photography to correlate signal perturbations with process anomalies. Digital signal processing techniques such as Kalman filtering, Principal Component Analysis and Cluster Analysis have been applied to on-line measurement data and have generated new ways to describe laser welding behaviour using parameters such as reflected pulse shape. The limitations of commercially available welding supervision systems have been studied and design suggestions for the next generation of on-line weld monitoring equipment have been formulated.Having progressed from thin section welding with continuous wave lasers to pulsed laser welding, the thesis then moves on to pulsed laser surface melting.Part II Analysis of surface texturing of titanium. The second part of the thesis concentrates on laser structuring of titanium surfaces for medical implants. The two papers in this section present an analysis of the laser-material interactions which create surfaces suitable for osseointegration (bone attachment). The work concentrates on a commercially available surface used for screw implants in dentistry; BioHelix™. This surface is generated by an intense bombardment of laser pulses and the surface is thus disrupted during solidification. The formation of various levels and types of roughness are analysed and it is noted that laser generated rough surfaces are fundamentally different from those with a mechanically produced roughness. One key point is that laser generated rough surfaces can include overhanging features. This finding lead to the research carried out in part III of the thesis.Part III. Analysis and classification of laser generated surfaces.The final section of the thesis presents research which uses statistical techniques to identify whether or not a roughened surface includes overhanging features. The presence or otherwise of such features is important because they can affect the wettability of surfaces and thus their suitability for implant surfaces, adhesive bonding and lubrication etc. Micro Computer Tomography was used to generate a typical cross section of the surface under investigation. At equally spaced positions the profile of this cross section is then allocated vectors which are normal to the profile at each point. The angles of the vectors can then be analyzed to reveal the presence or otherwise of overhanging features. The presence of overhangs on the material surface is indicated by the existence of normal vectors with angles that exceed 180°. The papers in this section also investigate possibilities and limitations of using statistical methods in conjunction with Micro Computer Tomography.The papers have in common processing of data for laser materials processing, by advanced methods to identify and extract essential information from the processes and the resulting material properties.
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9.
  • Pocorni, Jetro, 1988- (författare)
  • Laser cutting and piercing: Experimental and theoretical investigation
  • 2017
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • This thesis concerns experimental investigations of laser cutting and piercing, with theoretical and practical discussions of the results. The thesis is made up of an introduction to laser cutting and six scientific Papers. These Papers are linked in such a way that each of them studies a different aspect of laser cutting: process efficiency in Paper I, morphology and melt flow on the laser cut front in Papers II, III and IV and laser piercing in Papers V and VI.Paper I investigates the effect of material type, material thickness, laser wavelength, and laser power on the efficiency of the cutting process for industrial state-of-the-art CO2 and fibre laser cutting machines. Here the cutting efficiency is defined in its most fundamental terms: as the area of cut edge created per Joule of laser energy.In Paper II a new experimental technique is presented which has been developed to enable high speed imaging of laser cut fronts produced using standard, commercial parameters. The results presented here suggest that the cut front produced when cutting 10 mm thick medium section stainless steel with a fibre laser and a nitrogen assist gas is covered in humps which themselves are covered in a thin layer of liquid. Paper III presents numerical simulations of the melt flow on a fibre laser ablation-driven processing front during remote fusion cutting, RFC. The simulations were validated with high speed imaging observations of the processing front. The simulation results provide explanations of the main liquid transport mechanisms on the processing front, based on information on the temperature, velocity and pressure fields involved. The results are of fundamental relevance for any process governed by a laser ablation induced front. In Paper IV cutting fronts created by CO2 and fibre lasers in stainless steel at thicknesses between 2 mm and 10 mm have been ‘frozen’ and their geometry has been measured. The resulting three-dimensional shapes have been curve fitted as ninth order polynomials. Various features of the cutting front geometry are discussed, including the lack of correlation of the cut front inclination with either the relevant Brewster angle or the inclination of the striations on the cut edge. In this paper, mathematical descriptions of the cutting fronts are obtained, which can be used as input parameters by any researcher in the field of laser cutting simulations.Paper V investigates the subject of laser piercing. Before any cut is started the laser needs to pierce the material. In this paper the laser piercing process is investigated using a wide range of laser pulse parameters, for stainless steel using a fibre laser. The results reveal the influence of pulse parameters on pierce time and pierced hole diameter. A high speed imaging camera was used to time the penetration event and to study the laser-material interactions involved in drilling the pierced holes. In Paper VI a ‘dynamic’ or ‘moving beam’, laser piercing technique is introduced for processing 15 mm thick stainless steel. One important aspect of laser piercing is the reliability of the process because industrial laser cutting machines are programmed for the minimum reliable pierce time. In this work a comparison was made between a stationary laser and a laser which moves along a circular trajectory with varying processing speeds. High speed imaging was employed during the piercing process to understand melt behavior inside the pierce hole.Throughout this work experimental techniques, including advanced high speed imaging, have been used in conjunction with simulations and theoretical analysis, to provide new knowledge for understanding and improving laser beam cutting and its associated piercing process.
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10.
  • Robertson, Stephanie (författare)
  • Laser welding and laser heat treatment of high strength steels
  • 2021
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Laser materials processing, including thermal treatment and laser welding has been undergoing continuous growth in the manufacturing industry for decades. A laser beam offers high precision and energy transfer, capable of various processing. For many cases a Gaussian beam is applied, but lately development of more complex beam shapes has been developed, where e.g. multiple beams (beam splitting) can be used for increased tolerances during welding.This thesis presents six papers (Papers A-F) on welding of high strength steels, laser pulse shaping, thermal treatments, and microstructural investigations. Different methods for obtaining a desirable weld were investigated through tailoring of the laser beam process. This affected the resulting temperature fields and thermal histories of the specimens. Experimental analysis was supported through various in-situ observation techniques and metallurgical studies.Papers A-C present thermal processing and chemical manipulation to obtain the desired microstructure, by introducing and applying the here introduced Snapshot method. Paper A explores tailoring a laser pulse to mimic a hybrid welding process, Paper B elaborates the simulation to a multi-cycle process, and Paper C explores dilution. The manuscripts utilize a specialized experimental setup, optical analysis methods, and standard thermal measuring techniques. Metallographic analysis showed that thermal process optimization and/or dilution rate control during welding improved weld zone characteristics.Improvements also include joint macrostructure characteristics, which are impacted by process stability, the theme of Papers D-F. Melt pool phenomena are studied in depth in Papers D and E. Paper D explores material ejections in a single beam welding scenario. Paper E investigates six beam shapes, from a single beam to a quad-beam arrangement. Paper F studies hybrid welding, a process that was simulated in Papers A-C but focused on the stability of the process instead of thermally guiding the microstructure. The studies complement each other in knowledge and methods. Welding of high strength steel is joining method-dependent, which imposes a unique thermal profile that affects the microstructures. The microstructure is also influenced by the chemical composition, an important point when multiple materials are used. The studies contribute an analysis of certain aspects of thermal and chemical effects of different laser-based processes to further optimize processing of specifically high strength steels, though the aspects can be generalized to other metals
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