Das Laserstrahlhartlöten ermöglicht die Herstellung von Fügeverbindungen mit exzellenten Nahtqualitäten. Daher hat es sich bei anspruchsvollen Anwendungen, wie zweiteiligen Heckklappen und der Verbindung von Dach und Seitenwandrahmen (Dachnullfuge) etabliert. Um die hohen Qualitätsanforderungen durch das Laserstrahlhartlöten realisieren zu können, sind allerdings anspruchsvolle konstruktive Randbedingungen zu erfüllen, die über die Fertigungskette nicht immer vollständig sicherzustellen sind. Das Ergebnis solcher Fertigungs- und Materialschwankungen äußert sich dann oft als Nahtimperfektionen, die während des Laserlötprozesses entstehen. Diese verursachen vor allem mit steigenden Prozessgeschwindigkeiten einen erhöhten und kostenintensiven Nacharbeitsaufwand und sollten daher vermindert bzw. gänzlich vermieden werden. Das Ziel ist somit den Laserlötprozess so robust wie möglich auszulegen, um auf diese Fertigungsschwankungen ohne Einschränkungen in der Nahtqualität reagieren zu können. Im ersten Teil dieser Arbeit werden wesentliche Einflussfaktoren auf die Ausbildung der Nahtqualität am schrägen Bördelstoß untersucht und die systemtechnischen Grenzen mit einem statischen und runden Laserspot aufgezeigt. Weiterhin werden die resultierenden Nahtqualitäten durch das Laserstrahllöten mit gescanntem Laserstrahl in Vorschubrichtung untersucht. Die Ergebnisse zeigen, dass sich die Nahtqualität mit diesem innovativen Ansatz bezüglich des Anbindungsquerschnittes und der Oberflächenqualität weiter steigern lassen. Dies lässt sich ebenfalls für höhere Prozessgeschwindigkeiten realisieren. Abschließend werden neuartige Möglichkeiten der Prozessüberwachung, sowie erste Ansätze zur Prozessregelung des Laserstrahlhartlötens am schrägen Bördelstoß vorgestellt. Die erzielten Resultate zeigen, dass sich der Laserstrahlhartlötprozess durch die Regelung der Laserleistung in Verbindung mit evaluierten Temperaturfeldern im Bereich der Prozesszone online kontrollieren und sich dadurch die Prozessstabilität merklich steigern lässt.

Das Laserstrahlhartlöten ermöglicht die Herstellung von Fügeverbindungen mit exzellenten Nahtqualitäten. Daher hat es sich bei anspruchsvollen Anwendungen, wie zweiteiligen Heckklappen und der Verbindung von Dach und Seitenwandrahmen (Dachnullfuge) etabliert. Um die hohen Qualitätsanforderungen durch das Laserstrahlhartlöten realisieren zu können, sind allerdings anspruchsvolle konstruktive Randbedingungen zu erfüllen, die über die Fertigungskette nicht immer vollständig sicherzustellen sind. Das Ergebnis solcher Fertigungs- und Materialschwankungen äußert sich dann oft als Nahtimperfektionen, die während des Laserlötprozesses entstehen. Diese verursachen vor allem mit steigenden Prozessgeschwindigkeiten einen erhöhten und kostenintensiven Nacharbeitsaufwand und sollten daher vermindert bzw. gänzlich vermieden werden. Das Ziel ist somit den Laserlötprozess so robust wie möglich auszulegen, um auf diese Fertigungsschwankungen ohne Einschränkungen in der Nahtqualität reagieren zu können. Im ersten Teil dieser Arbeit werden wesentliche Einflussfaktoren auf die Ausbildung der Nahtqualität am schrägen Bördelstoß untersucht und die systemtechnischen Grenzen mit einem statischen und runden Laserspot aufgezeigt. Weiterhin werden die resultierenden Nahtqualitäten durch das Laserstrahllöten mit gescanntem Laserstrahl in Vorschubrichtung untersucht. Die Ergebnisse zeigen, dass sich die Nahtqualität mit diesem innovativen Ansatz bezüglich des Anbindungsquerschnittes und der Oberflächenqualität weiter steigern lassen. Dies lässt sich ebenfalls für höhere Prozessgeschwindigkeiten realisieren. Abschließend werden neuartige Möglichkeiten der Prozessüberwachung, sowie erste Ansätze zur Prozessregelung des Laserstrahlhartlötens am schrägen Bördelstoß vorgestellt. Die erzielten Resultate zeigen, dass sich der Laserstrahlhartlötprozess durch die Regelung der Laserleistung in Verbindung mit evaluierten Temperaturfeldern im Bereich der Prozesszone online kontrollieren und sich dadurch die Prozessstabilität merklich steigern lässt.

Yes / Diffusion brazing of SiC ceramic to IN738 using an Ag-Cu-Ti powder-mixture as an interlayer was carried out for the first time. The impact of the bonding time (30 and 45 min) on metallurgical features and shear strength of the joints was assessed. The results revealed that raising the bonding time resulted in expanding of the brazing layer from 46.98 µm to 55.31 µm. Besides, increasing the bonding time also enhanced the shear strength of the SiC/Ag-Cu-Ti/IN738 joints.

The need for industrial gas turbines to operate at higher temperatures and/or speeds has resulted in the continual modification of nickel-based superalloys to provide better high-temperature strength and corrosion resistance for components such as hot-section turbine blades. Thermal-Mechanical Fatigue (TMF) cracking, accelerated by the oxidation that forms as a result of the exposure of bare metal during the crack-opening stages, is one of the most common forms of damage experienced by service-run turbine blades. Due to the high costs associated with manufacturing nickel-based superalloy components, damaged turbine blades must be repaired to restore their original mechanical properties. One such method, Activated Diffusion Brazing (ADB), is under development for this purpose, and involves melting a two-part powder mixture into a damaged region. However, the tenacious oxides formed on nickel-based superalloy components provide an obstacle for the repair process, and must be removed. Fluoride-Ion Cleaning (FIC) uses flowing hydrogen and HF gas to remove tenacious oxide scales through a set of chemical reactions, leaving cleaned components free of oxide compounds and depleted of the strong oxide-formers of Al and Ti. GTD111 is a nickel-based superalloy containing the strong oxidizing elements of Al, Ti, and Cr, and is similar in composition to other nickel-based superalloys such as DD8 and Rene95. Literature concerning the oxidation, cleaning, and brazing of this particular alloy is limited, and as such this thesis serves as a comprehensive overview of the chemical effects of each above process on GTD111. The objectives of this project are to determine, through SEM-EDX and element mapping analysis, the oxidation behavior of nickel-based superalloy GTD111, the effects of oxidation and FIC on the chemistry near the surface of this particular alloy, and the effects of mixing ratio and paste viscosity on the quality of repairs made by ADB. Notches of 8 mm depth and 0.25 mm width were made in coupons of GTD111 via wire-EDM and samples were oxidized between 1 and 452 hours at 900°C. Samples oxidized between 96 and 452 hours were sectioned in half and one half of each sample was cleaned via the standard FIC process at Ti-Coating Inc. Notches of 8 mm depth and 1 mm width, also made via wire-EDM, were repaired by the ADB process with a bonding temperature of 1220°C and a holding time of 65 minutes. Time-dependent multi-layer oxide growth was observed on all samples, consisting of an innermost discontinuous aluminum oxide region, followed by a thin continuous band of Ni-W-Ta oxide and a thicker, very dense chromium oxide layer. Some oxidation times exhibited the presence of weak, inconsistent oxide regions rich in Ni and/or Ti. Since the GTD111 alloy does not contain sufficient amount of Al to form a continuous layer – as 5-7% Al is required – oxidation resistance was provided mainly by the formation of the dense chromium oxide layer. A region heavily depleted of Al and Ti and therefore the strengthening gamma prime phase was observed below and surrounding the Al-rich oxide regions. Chemical analysis of cleaned samples showed that the standard FIC process at Ti-Coating Inc. was able to remove all oxide compounds formed during oxidation at 900°C, and that the prior oxidation time had no effect on the chemistry within the surface of the cleaned samples; however, the depths of elemental and gamma prime phase depletion were affected. The elemental depletions of Al and Ti have been observed in past studies, but depletions of Ni and concentrations of Cr near the surfaces of cleaned components have not been previously observed. Preliminary brazing trials made with varying paste viscosities demonstrated the importance of paste pre-placement and maintaining the molten filler metal within the notch, as better pre-placement resulted in higher densities in the braze-repaired region of the brazing trial samples. Although porosity was observed on all samples, the paste pre-placement was found to be more important in reducing porosity than the mixing ratio and paste viscosity, although using an appropriate paste viscosity allowed for better pre-placement.

The need for industrial gas turbines to operate at higher temperatures and/or speeds has resulted in the continual modification of nickel-based superalloys to provide better high-temperature strength and corrosion resistance for components such as hot-section turbine blades. Thermal-Mechanical Fatigue (TMF) cracking, accelerated by the oxidation that forms as a result of the exposure of bare metal during the crack-opening stages, is one of the most common forms of damage experienced by service-run turbine blades. Due to the high costs associated with manufacturing nickel-based superalloy components, damaged turbine blades must be repaired to restore their original mechanical properties. One such method, Activated Diffusion Brazing (ADB), is under development for this purpose, and involves melting a two-part powder mixture into a damaged region. However, the tenacious oxides formed on nickel-based superalloy components provide an obstacle for the repair process, and must be removed. Fluoride-Ion Cleaning (FIC) uses flowing hydrogen and HF gas to remove tenacious oxide scales through a set of chemical reactions, leaving cleaned components free of oxide compounds and depleted of the strong oxide-formers of Al and Ti. GTD111 is a nickel-based superalloy containing the strong oxidizing elements of Al, Ti, and Cr, and is similar in composition to other nickel-based superalloys such as DD8 and Rene95. Literature concerning the oxidation, cleaning, and brazing of this particular alloy is limited, and as such this thesis serves as a comprehensive overview of the chemical effects of each above process on GTD111. The objectives of this project are to determine, through SEM-EDX and element mapping analysis, the oxidation behavior of nickel-based superalloy GTD111, the effects of oxidation and FIC on the chemistry near the surface of this particular alloy, and the effects of mixing ratio and paste viscosity on the quality of repairs made by ADB. Notches of 8 mm depth and 0.25 mm width were made in coupons of GTD111 via wire-EDM and samples were oxidized between 1 and 452 hours at 900°C. Samples oxidized between 96 and 452 hours were sectioned in half and one half of each sample was cleaned via the standard FIC process at Ti-Coating Inc. Notches of 8 mm depth and 1 mm width, also made via wire-EDM, were repaired by the ADB process with a bonding temperature of 1220°C and a holding time of 65 minutes. Time-dependent multi-layer oxide growth was observed on all samples, consisting of an innermost discontinuous aluminum oxide region, followed by a thin continuous band of Ni-W-Ta oxide and a thicker, very dense chromium oxide layer. Some oxidation times exhibited the presence of weak, inconsistent oxide regions rich in Ni and/or Ti. Since the GTD111 alloy does not contain sufficient amount of Al to form a continuous layer – as 5-7% Al is required – oxidation resistance was provided mainly by the formation of the dense chromium oxide layer. A region heavily depleted of Al and Ti and therefore the strengthening gamma prime phase was observed below and surrounding the Al-rich oxide regions. Chemical analysis of cleaned samples showed that the standard FIC process at Ti-Coating Inc. was able to remove all oxide compounds formed during oxidation at 900°C, and that the prior oxidation time had no effect on the chemistry within the surface of the cleaned samples; however, the depths of elemental and gamma prime phase depletion were affected. The elemental depletions of Al and Ti have been observed in past studies, but depletions of Ni and concentrations of Cr near the surfaces of cleaned components have not been previously observed. Preliminary brazing trials made with varying paste viscosities demonstrated the importance of paste pre-placement and maintaining the molten filler metal within the notch, as better pre-placement resulted in higher densities in the braze-repaired region of the brazing trial samples. Although porosity was observed on all samples, the paste pre-placement was found to be more important in reducing porosity than the mixing ratio and paste viscosity, although using an appropriate paste viscosity allowed for better pre-placement.

Microchannel process technology (MPT) offers several advantages to the field of nanomanufacturing: 1) improved process control over very short time intervals owing to shorter diffusional distances; and 2) reduced reactor size due to high surface area to volume ratios and enhanced heat and mass transfer. The objective of this thesis was to consider how nanomaterials, produced in part using MPT, could be used to solve problems associated with the fabrication of MPT devices. Specifically, many MPT devices are produced using transient liquid-phase brazing involving an electroplated interlayer consisting of a brazing alloy designed for melting temperature suppression. Unfortunately, these alloys can form brittle secondary phases which significantly reduce bond strength. In contrast, prior efforts have shown that it is possible to leverage the size-dependent properties of nanomaterials to suppress brazing temperatures. In this prior work, thin films of off-the-shelf elemental nanoparticles were used as interlayers yielding joints with improved mechanical properties. In the present investigation, efforts have been made to characterize the synthesis and deposition of various elemental nanoparticle suspensions for use in the transient liquid-phase brazing of aluminum and stainless steel. Advances were used to demonstrate the nanoparticle-assisted diffusion brazing of a microchannel array. In the first section, a silver nanoparticle (AgNP) interlayer was produced for the diffusion brazing of heat exchanger aluminum. Efforts are made to examine the effect of braze filler particle size (~5 nm and ~50 nm) and processing parameters (heating rate: 5ºC/min and 25ºC/min; brazing temperature: 550ºC and 570ºC) on thin coupons of diffusion-brazed 3003 Al. A tensile strength of 69.7 MPa was achieved for a sample brazed at 570°C for 30 min under 1 MPa with an interlayer thickness of approximately 7 μm. Further suppression of the brazing temperature to 500ºC was achieved by sputtering a 1 µm thick layer of Cu before depositing a 5 nm thick film of AgNPs resulting in a lap shear strength of 45.3±0.2 MPa. In the middle section of this thesis, several techniques are investigated for the synthesis of sub 10 nm diameter nickel nanoparticles (NiNPs) to be used in the diffusion brazing of 316L stainless steel. The average NiNP size was varied from 9.2 nm to 3.9 nm based on the synthesis technique, solvent and reducing agent used. Conventional wet-chemical synthesis using NiCl₂.6H₂O in ethylene glycol (solvent) and N₂H₄.H₂O (reducing agent) resulted in the formation of 5.4 ± 0.9 nm NiNPs. Continuous flow synthesis using a microchannel T-mixer (barrel diameter of 521µm) and a 10 second residence time of reactants in a bath temperature of 130ºC resulted in a particle size of with 5.3 ± 1 nm. To make the synthesis safer and less energy intense, microwave heating was used along with less toxic Ni(CH₃CO₂)₂·4H₂O (nickel salt), propylene glycol (solvent) and NaPH₂O₂ (reducing agent) yielding 3.9 ± 0.8 nm diameter NiNPs. For the final section, nickel nanoparticles were synthesized using NiCl₂.6H₂O (nickel salt), de-ionized water (solvent), NaBH₄ (co-reducing agent), N₂H₄.H₂O (reducing agent) and polyvinylpyrolidone (capping agent) yielding 4.2 ± 0.6 nm NiNP. Several deposition techniques were investigated for controlling film thickness and uniformity in the diffusion brazing of 316L stainless steel (SS). Using in-house prepared NiNP and automated dispensing, a hermetic joint up to 70 psi (tested pressure) was obtained in 316L SS substrates under brazing conditions of 800ºC, 2 MPa and 30 min. Throughout the course of this thesis, techniques used for characterizing nanoparticles, films and joints included FT-IR, XRD, SEM, TEM, HRTEM, EDS, EPMA, DSC, mass spectrometry, and lap-shear testing. / Graduation date: 2012

Os aços inoxidáveis duplex (AID) caracterizam-se pela sua microestrutura composta por austenita numa matriz ferrítica, com fração volumétrica média de 50% para cada fase. A combinação destas características confere-lhes excelente resistência mecânica e à corrosão. A soldagem/junção destes aços é frequentemente uma operação crítica, já que, ao sofrer ciclos térmicos, estes aços têm suas propriedades alteradas. Portanto, processos de junção com ausência de gradientes de temperatura, como a brasagem, mostram-se uma solução prática para a junção destes aços. No entanto, o adequado desenvolvimento do processo de brasagem em AID envolve considerações importantes a respeito da escolha dos parâmetros de processamento e metal de adição em conjunto com os ciclos térmicos de aquecimento e resfriamento. O presente estudo pretende avaliar a brasabilidade dos AID UNS S32101(baixa liga), UNS S32304 (baixa liga), UNS S32507 (superduplex) e UNS S32707(hiperduplex), mediante a caracterização da junta brasada. Estes aços foram brasados em forno continuo com metal de adição BNi-7 (Ni-Cr-P), na temperatura de 1100oC, por tempos de 32 min e 12 min, seguidos de resfriamento em forno, utilizando-se folgas de 0,5, 0,3 e 0,0 mm. A junta brasada foi caracterizada utilizando-se microscopia ótica e microscopia eletrônica de varredura (MEV). A identificação microestrutural foi realizada via Microanálise Química de Energia Dispersiva de Raios-X (EDS) e difração de Raios-X. Foi identificada na junta brasada dos diferentes AID a fase rica em níquel, assim como fases prejudiciais compostas por fosfetos de níquel e cromo. A fase rica em níquel, que usualmente é denominada de -Ni, apresenta-se preferencialmente em toda a região da junta, sem a presença de fase intermetálica contínua no centro da junta. No caso dos aços inoxidáveis super e hiperduplex esses apresentaram a formação de fase sigma na região da interface da junta, do lado do metal de base, devido aos ciclos térmicos de brasagem / The duplex stainless steels (DSS) are characterized by its microstructure consisting of austenite in a ferritic matrix with mean volumetric fraction of 50% for each phase. The combination of these features gives them excellent mechanical strength and corrosion resistance. The welding / joining of these steels are often considered as a critical operation, since, subjected to thermal cycles, they have their microstructures changed and, consequently, their properties. Therefore, joining processes without temperature gradients, like brazing, are shown as a practical solution for joining these steels. However, the proper development of brazing process of DSS involves important issues concerning the choice of processing parameters and the brazing filler metal together with heating and cooling brazing thermal cycles. This study aims at evaluating the brazeability of DSS UNS S32101 (lean duplex), UNS S32304 (lean duplex), UNS S32507 (superduplex) and UNS S32707 (hyperduplex) by characterizing the brazed joint. These steels were brazed in a furnace with filler metal BNi-7 (Ni-Cr-P) at 1100°C for times of 32 min and 12 min, followed by cooling in a continuous brazing furnace, with joint gaps of 0.5 , 0.3 and 0.0 mm. The brazed joint was characterized using optical microscopy and scanning electron microscopy (SEM). The phase identifications were performed by microanalysis using energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction. Results showed, for non-ideal joint gaps, in all duplex used in this work, the Ni rich solid solution, as well as deleterious phases composed of nickel and chromium phosphides. The ideal gap presented Ni rich solid solution, usually called -Ni, was found continuously without a continuous intermetallic region in the joint center. In the case of super duplex and hyperduplex brazing, the brazing thermal cycles produced the formation of sigma phase in the region of the joint interface, in the base metal.

Advanced aluminum graphite composites have unique thermal properties due to opposing coefficients of thermal expansion of aluminum and graphite. The thermal and mechanical properties of such composites are anisotropic due to directional properties of graphite fibers and their designed orientation. A joint with different fiber orientations would theoretically produce an isotropic material for thermal management. This paper presents results for welding and brazing of the composite using different joining techniques. Laser welding of Al-Gr composite showed that a power density above 30kW/mm2 gives a weld with microstructure defects. Also the laser beam melts the matrix and delaminates the graphite fibers. The molten aluminum reacts with graphite to form aluminum carbide (Al4C3). The joint strength is compromised when laser welding at optimal conditions to minimize the carbide formation. Also porosity and redistribution of graphite fibers is seen during laser welding. These defects prompt us to consider a low temperature joining. Brazing is considered since the low melting temperature of a filler material suppresses the formation of Al4C3 while minimizing pores and microstructural defects in the joint. Microstructural study and shear test are performed to analyze the joints. Shear strengths of brazed joints are determined to be 20-21MPa which is comparable to the composite shear strength (46.5MPa in x-y plane and 19MPa in z plane). The fracture surface is found to be mostly on the composite rather than in brazed material or along the interface. Also, the microstructural study showed no Al4C3 formation and minimal porosity in the brazed region. These results show a successful joining of the composite using laser brazing and resistance brazing methods.

Due to the high performance of electronic components, the heat generation is increasing dramatically. Heat dissipation becomes a significant issue in efficiency promotion and stable operation. Microchannels are of current interest for use in heat exchangers where very high heat transfer performance is desired. Microchannels provide high heat transfer coefficients because of their small hydraulic diameters. In this study, the design and experimental investigation of fluid flow and heat transfer in a microchannel heat exchanger is conducted. Water and air are used as the working fluids and flowed through microchannels. The heat exchanger has been designed with 6 rows of microchannels for water flow and 7 rows of microchannels for forced flow of air. The heights of the microchannels are 4 mm and 10 mm respectively for water and air flows. Microchannels are brazed to form the heat exchanger. For forced convection cooling with air, a military fan is used. A constant heat source has been specifically designed for experiments. Water flow and heat transfer experiments are conducted on the aluminum microchannel heat exchanger. An experimental method of imposing a constant heat flux to water prior to the entrance to the microchannel heat exchanger, to adjust the inlet temperatures is used. v From the data obtained, the rate of heat transfer, effectiveness and various other parameters have been computed and the results have been compared with those from an available commercial heat exchanger. The results indicate that the heat exchanger performs well and provides 681 W of cooling in a volume 677.6 cm3 while the commercial heat exchanger provides 702.5 W of cooling in a volume 2507.5 cm3. In addition, air-side Colburn modulus has been obtained with respect to Reynolds number.

Les composites à matrice céramique (CMC) SiCf/SiC sont des matériaux envisagés pour le gainage du combustible des futurs réacteurs à neutrons rapides. Une des problématiques de mise en oeuvre concerne leur assemblage et notamment la fermeture de ces gaines contenant le combustible. A l'heure actuelle, aucune composition de joint d'assemblage suffisamment réfractaire n'est mentionnée dans la littérature. Les objectifs de cette étude sont : (1) la recherche d'un matériau d'assemblage et du procédé associé répondant aux contraintes de mise en oeuvre et de fonctionnement, (2) la validation d'un procédé "nucléarisable" de chauffage localisé et rapide afin de ne pas dégrader le composite lors de l'assemblage et (3) la définition de tests mécaniques pour caractériser ces assemblages. Après une étude bibliographique approfondie des systèmes d'assemblage potentiels et la sélection d'une composition à travers des travaux expérimentaux exploratoires par frittage flash, la solution proposée est de réaliser un brasage à partir d'une composition mixte de disiliciure métallique (MSi2) et de carbure de silicium (SiC). Le choix du brasage a nécessité une étude de mouillabilité et de cinétique d'étalement de la brasure en prenant en compte les aspects réactifs de celle-ci sur le carbure de silicium. Différents procédés de chauffage localisé de la brasure tels que le laser CO2, les micro-ondes ou le chauffage inductif haute fréquence ont été testés. La faisabilité de différents procédés d'élaboration du joint a été montrée, notamment par dépôt physique en phase vapeur (PVD), par projection plasma et par enduction de suspensions de poudres. Un essai de flexion 4-points a également été défini afin d'évaluer la résistance mécanique des assemblage. / The present work is part of the Fourth Generation Fast Reactors program. One of the key issues is the joining of the SiCf/SiC ceramic matrix composites (CMC) to seal the combustible cladding. At the present time, no chemical composition as a joint is refractive enough to face the expected operating temperatures. The aimes of this study are the following : (1) the identification of a joining composition and its associated elaboration process answering the specifications, (2) the validation of a local and fast heating process and (3) the definition of mechanical tests to caracterise these joinings. We describe the methodology and the results for joining SiC and SiCf/SiC substrates at solid and liquid state using metallic silicides. Joint integrity and joint strength can be improved by adding small SiC particles to the silicides powders. Chemical reactivity, wettability tests and thermo-mechanical properties analysis have been carried out on the joints. Cross sections of the assembly were prepared to study the joint/substrate chemical bonding, the cracking and the crack deflection in the vicinity of the interface. Also, one of the challenge consists in using a local heating at a high temperature (around 1800°C) for a short time to avoid the degradation of the composite structure. The assemblies have been then performed in an inductive furnace but in order to prepare the joining technology, trials of local heating have also been investigated with a CO2 laser beam and a microwave generator. Finally, descriptions of the 4-points bending mechanical test used and the associated results are presented.