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Electron beams. --- High pressure (Science) --- Pressure --- Beams, Electron --- Electron optics --- Electronics --- Particle beams

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621.9.048 --- Electron beams --- -Industrial electronics --- -Electronics in industry --- Factories --- Electronic apparatus and appliances --- Electronics --- Beams, Electron --- Electron optics --- Particle beams --- Erosion machining, blasting. Spark erosion. Ultrasonic, vibration machining. Machining with particle beams. Electron-beam machining etc. --- Industrial applications --- -Congresses --- Congresses --- Electronic equipment --- Electric equipment --- -Erosion machining, blasting. Spark erosion. Ultrasonic, vibration machining. Machining with particle beams. Electron-beam machining etc. --- 621.9.048 Erosion machining, blasting. Spark erosion. Ultrasonic, vibration machining. Machining with particle beams. Electron-beam machining etc. --- -621.9.048 Erosion machining, blasting. Spark erosion. Ultrasonic, vibration machining. Machining with particle beams. Electron-beam machining etc. --- Electronics in industry --- Industrial electronics --- Industrial applications&delete& --- Erosion machining, blasting. Spark erosion. Ultrasonic, vibration machining. Machining with particle beams. Electron-beam machining etc

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"These proceedings comprise cutting-edge contributions by researchers at the frontiers of beam physics, free-electron-based light sources, and advanced accelerators. It represents a snap-shot of activity in these fields at a critical historical juncture, where rapid experimental progress is being reported, and new facilities such as X-ray free-electron lasers are under construction. The volume features invited contributions from leading researchers from the international beam physics community that summarize the state-of-the-art research in individual topics, as well as timely contributions from participants that arose during the workshop itself."

High-brightness accelerators --- Electron beams --- Beams, Electron --- Electron optics --- Electronics --- Particle beams --- Accelerators, High-brightness --- High-brightness particle accelerators --- Particle accelerators --- Industrial applications

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Proefschriften --- Thèses --- Academic collection --- 621.9.048 <043> --- #BIBC:T1998 --- #TWER:DISS --- Erosion machining, blasting. Spark erosion. Ultrasonic, vibration machining. Machining with particle beams. Electron-beam machining etc.--Dissertaties --- Theses --- 621.9.048 <043> Erosion machining, blasting. Spark erosion. Ultrasonic, vibration machining. Machining with particle beams. Electron-beam machining etc.--Dissertaties

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Selectief Laser Smelten is een zeer interessante technologie voor het vervaardigen van complexe metalen onderdelen. Er zijn echter een aantal belangrijke problemen, met name de optredende residuele spanningen en de smeltbad-instabiliteiten, die het gebruik van de techniek als werkelijke Rapid Manufacturing techniek in de weg staan. Restspanningen ontstaan ten gevolge van optredende temperatuursgradiënten en ten gevolge van de afkoeling van de opeenvolgend toegevoegde materiaallaagjes. De grootte en het verloop van deze spanningen over de hoogte van de geproduceerde stukken werd voorspeld door middel van een theoretisch model en opgemeten via verschillende experimentele methodes. Hieruit bleek duidelijk dat een opdeling van een 2D doorsnede in kleinere sectoren die achtereenvolgens worden gescand, resulteert in een significante vermindering van de thermische spanningen. Ook de voorverwarming van het bouwplatform en het nagloeien van het materiaal d.m.v. de laserstraal resulteren in lagere residuele spanningen. Door middel van een coaxiaal optisch systeem werd het gedrag van het materiaal in de smeltbadzone geanalyseerd. Hiervoor werd zowel een hogesnelheids-CMOS camera gebruikt als een fotodiode. Door middel van de cameraobservaties werd zowel het transient als het (quasi) steady-state gedrag van het SLM proces geobserveerd. Het effect van de belangrijkste procesparameters, met name het laservermogen en de scansnelheid, op de smeltbadgeometrie bleek complexer dan voorspeld door een eenvoudig theoretisch model. Eén van de redenen voor de afwijking tussen het theoretische model en de experimentele observaties, zijn de stralings-warmteverliezen, welke bij lage scansnelheden niet mogen worden verwaarloosd. Ook de smeltbadinstabiliteiten die zich voordoen ten gevolge van de aanwezige oppervlaktespanningen (Rayleigh-instabiliteit of ’balling’), werden onderzocht. Naast de scanparameters (laservermogen en scansnelheid) werd ook het effect van het zuurstofgehalte in de atmosfeer alsook de aanwezigheid van bepaalde legeringselementen onderzocht. Het is duidelijk dat een hoog zuurstofgehalte alsook de aanwezigheid van legeringselementen met hoge zuurstofaffiniteit (bv. silicium) de stabiliteit van het smeltbad verslechteren. Met behulp van het coaxiaal optisch observatiesysteem werden tenslotte de smeltbadinstabiliteiten die zich voordoen ten gevolge van de drastische wijzigingen in de conductieve warmteafvoer bij veranderende – tweedimensionele of driedimensionele - geometrie, onderzocht. Een eenvoudige proportioneel-integratieve feedback controle werd gebruikt om wijzigingen in de smeltbadgrootte te minimaliseren om zo een verbetering van de kwaliteit van de geproduceerde stukken te verkrijgen. Selective Laser Melting is a very interesting technology for the production of complex metallic parts. However, some important issues, namely residual stresses and melt pool instability may form an obstacle for the successful application of the technology in the field of Rapid Manufacturing. Residual stresses arise in SLM parts due to the Temperature Gradient Mechanism and due to the obstructed shrinkage of the added molten layers. The basic shape of the evolution of the normal stresses along the build height could be predicted by a simple theoretical model that uses the conservation of force and momentum to calculate the residual stresses. These predictions were validated experimentally using the Crack Compliance Method and using XRD stress measurement experiments. In general, parts that stay connected to the base plate contain very high tensile residual stress levels, in the range of the materials yield strength. Parts that have been removed from their base plate, contain much lower stress levels, but they have suffered from deformation during part removal.The basic stress profile along the build direction of a part removed from its base plate consists of a zone of large tensile stresses just below the upper surface, followed by a large zone of compressive stress, to end again with a tensile stress zone at the bottom. The magnitude of the stresses depends amongst others on the part height and the stiffness and height of the base plate. The exposure strategy has a large influence on the residual stress levels. In general, the largest normal stresses occur in the direction perpendicular to the scan direction, while the lowest normal stress values are found along the scan direction. A subdivision of the surface in smaller sectors results in a lower maximum stress value and in equally large stresses in X and Y direction. Heating of the substrate plate results in a reduction of the residual stress levels, since temperature gradients are reduced. Preheating temperatures up to 200 degrees were used in the experiments, leading to a stress reduction of about 10 % for stainless steel parts. Higher preheating temperatures can yield a larger stress reduction. When annealing cycles are used to reduce the residual stress content of the SLM parts, it is recommended to perform the heat treatment before removal of the part from its base plate, to avoid large part deformations. Furthermore, it was shown that it is possible to reduce the residual stress levels by applying a heat treatment using the laser source. However, no drastic reductions were obtained and extra research is necessary to determine the optimal post scanning parameters. In order to investigate the SLM process’ steady state and transient behavior, an in-situ optical process observation system has been developed and has been shown to be a useful tool in investigating the melt pool instabilities. The optical process observation system can be done with a high-speed CMOS camera to obtain images of the actual melt pool, or with an integrating photodiode reflecting the total melt pool area allowing easy SISO feedback control. Steady state melt pool observations revealed a discrepancy between analytical moving point source solutions and the real SLM process at low scanning velocities. Radiant heat losses are significant at low scanning velocities and may thus not be neglected in simulations. In general, a raising scanning velocity results in a raise of the melt pool area (and length) for a fixed laser power up to a certain velocity. If the velocity is increased beyond this value, this trend is inverted and the melt pool area (and length) will become smaller again, improving again the melt pool stability. This threshold velocity raises with raising laser power. The occurrence of a liquid melt pool leads to a number of instability problems, resulting from surface tension, capillary and gravity forces acting on the liquid melt pool. Rayleigh instabilities were investigated theoretically and experimentally. Loose powder scanning experiments on ferrous powders revealed that the length-to-width ratio is the critical factor determining the melt pool stability. This ratio is influenced by the scanning velocity and the laser power. In general, raising the scanning velocity will raise the length-to-width ratio and will lead to balling, in case of loose-powder scanning. Apart from the scanning parameters, also the oxygen content of the atmosphere and the alloying elements were found to have an important influence in case of ferrous powders. If the laser scans over previously solidified layers, the length-to-width ratio will be much closer to 1 at the same scanning parameters due to a much smaller melt pool length, compared with the powder supported situation, and balling will usually not occur. The transient response of the melt pool to laser power variations, can be approximated as a first order response. The rise time depends on the scanning velocity being used, but is typically about 10 ms at typical SLM scanning speeds. Using the coaxial optical system, it was shown that raises in the melt pool dimensions occur in various situations, like the u-turn in zig-zag scanning patterns, successions of scanning vectors becoming shorter, overhang planes, etc. By simultaneous sampling of a high speed CMOS camera and a planar photodiode, it was demonstrated that the photodiode output signal correlates to the melt pool area and that it can thus be used in feedback control applications stabilizing the melt pool dimensions. Using simple proportional and proportional-integrative control of the laser power implemented using Labview, the stability of the melt pool was improved in case of a narrowing geometry with successive scanning vectors becoming shorter. Also in case of scanning of overhang planes,  feedback control drastically improved the melt pool stability and resulted in a significant improvement of the resulting part quality. Dit doctoraat behandelt de controle van Selectief Laser Sinteren- (SLS) en Smelten (SLM). Dit zijn twee recent ontwikkelde productietechnieken die complexe drie-dimensionele metalen componenten opbouwen in dunne laagjes metaalpoeder, uitgaande van een computermodel. Door middel van een gefocusseerde laserstraal worden deze poederlaagjes op elkaar gemolten, wat een metalen product oplevert. Deze nieuwe techniek biedt interessante mogelijkheden voor de vervaardiging van complexe metalen producten, vermits geen dure gereedschappen zijn vereist. Deze technieken bieden daarom zeer interessante voordelen voor lucht- en ruimtevaart componenten (gewichtsbesparing) en voor medische prothesen (complexe, patient-specifieke vormgeving). Twee probleemaspecten die met het SLS/SLM proces worden geassocieerd, worden behandeld, namelijk de thermische spanning die optreedt tijdens het proces en de instabiliteit van de vloeibare zone rond de laserspot. Hiervoor werd o.a. een optisch feedbacksysteem ontwikkeld dat de laserparameters in real-time bijstuurt, wat leidt tot een stabieler proces en een hogere productkwaliteit. This PhD concerns the control of Selective Laser Sintering (SLS) and Selective Laser Melting (SLM) processes. These are two recently developed technologies used to produce complex shaped metallic parts by adding successive metal powder layers, according to a computer model of the part being built. By means of a focused laser beam, these powder layers are molten and connected, resulting in a solid metal product. This new technique offers interesting possibilities for manufacturing complex metal products, since no expensive tooling is required. These techniques therefore offer interesting possibilities for aerospace applications (weight reduction) and for medical prostheses (complex, patient-specific designs). Two problem aspects associated with the SLS/SLM process, namely the thermal stresses and the instability of the liquid zone around the laser spot, are investigated. An optical feedback system was developed that controls the laser parameters in real-time, leading to a more stable melt pool and a higher product quality.

Academic collection --- 621.9.048 <043> --- Erosion machining, blasting. Spark erosion. Ultrasonic, vibration machining. Machining with particle beams. Electron-beam machining etc.--Dissertaties --- Theses --- 621.9.048 <043> Erosion machining, blasting. Spark erosion. Ultrasonic, vibration machining. Machining with particle beams. Electron-beam machining etc.--Dissertaties