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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The joining of metals is often associated to metallurgical problems. Specifically, metallurgical transformations in the HAZ (Heat Affected Zone) can occur, what lead to undesirable microestructural changes. The HAZ is a difficult region for studying due to its small dimensions and high thermal gradientes. Thus, in order to overcome this limitation of the HAZ study, there are in the literature different proposes of HAZ-simulator machines (physical simulation), which uses the Joule effect for heating specific coupons that cool down by conduction and convection. This approach intends to simulate the thermal cycle in a real HAZ obtained in a real welding. However, the use of traditional coupons with cylindrical geometry presents restrictions of portraying the real case, i.e., the welding. In the cylindrical geometry option, the obtained thermal cycles do not present temperature gradients closer to the ones in real weldments. Hence, to overcome this limitation, finite elements modeling was carried out and different coupon geometries were simulated. The objective is to reach thermal cycles as close as possible to the ones obtained in a real situation, for a subsequent physical simulation. This approach showed proper and the physical and numerical present coherent results. The next step is the physical simulation validation by comparing to real weldments. This would be the most intuitive way. However, it was proposed to conduct this validation by determining CCT (Continuous Cooling Transformation) diagrams. This approach has the advantage of reaching important technological results at the same time of validating the physical simulation, since CCD diagrams dedicated to welding are very difficult to find in literature. It is possible to concluded that the physical simulation does represent the HAZ and can be used to build up CCT diagrams / Freqüentemente a união de materiais metálicos está associada a problemas metalúrgicos. Especificamente, podem ocorrer modificações metalúrgicas na ZAC (zona afetada pelo calor), levando a alterações microestruturais não adequadas. Contudo esta é uma região de difícil estudo devido a sua pequena dimensão e grandes gradientes térmicos. Assim, a fim de superar esta limitação com relação ao estudo da ZAC, existem na literatura proposições de máquinas de simulação física, que fazem uso do Efeito Joule para o aquecimento de determinados corpos de prova, que se resfriam por condução e convecção, de forma a simular o ciclo térmico obtido por soldagem real. Entretanto, o uso de corpos de provas cilíndricos apresenta restrições em retratar o caso real. Nesta opção de geometria cilíndrica, os ciclos térmicos não apresentam gradientes de temperatura que sejam fiéis àqueles encontrados em uma soldagem. Desta forma, foram realizadas simulações numéricas em elementos finitos, variando-se a geometria dos corpos de prova, de forma a obter ciclos térmicos o mais próximos da realidade para uma posterior simulação física. Esta abordagem de variação da geometria do corpo de prova calculada via simulação numérica mostrou-se adequada, onde os resultados obtidos pela simulação física e numérica mostraram-se coerentes. Como forma de validar os resultados finais obtidos via simulação física, ao invés de se conduzir soldagens reais, o que seria mais intuitivo, optou-se pelo levantamento de Diagramas CCT (Transformação em Resfriamento Contínuo) dedicados à soldagem. Este direcionamento tem a vantagem de se, concomitantemente à validação da simulação física, atingir resultados tecnologicamente muito importantes que são as curvas CCT de difícil acesso na literatura. Conclui-se pela viabilidade da simulação física em retratar a ZAC e pela viabilidade em se conseguir Diagramas CCT para soldagem. / Mestre em Engenharia Mecânica

The Al-Zn-Mg-Cu (7XXX series) alloys are amongst the strongest aluminum available. However, they are considered unweldable with conventional fusion techniques due to the negative effects that arise with conventional welding, including hydrogen porosity, hot cracking, and stress corrosion cracking. For this reason, friction stir welding has emerged as the preferred technique to weld 7XXX series alloys. Aluminum 7449 is one of the highest strength 7XXX series aluminum alloy. This is due to its higher zinc content, which leads to a higher volume fraction of eta' precipitates. It is typically used in a slight overaged condition since it exhibits better corrosion resistance. In this work, the welds of friction stir welded aluminum 7449 were studied extensively. Specific focus was placed in the heat affected zone (HAZ) and nugget. Thermocouples were used in the heat affected zone for three different depths to obtain thermal profiles as well as cooling/heating profiles. Vicker microhardness testing, transmission electron microscope (TEM), and differential scanning calorimeter (DSC) were used to characterize the welds. Two different tempers of the alloy were used, a low overaged temper and a high overaged temper. A thorough comparison of the two different tempers was done. It was found that highly overaged aluminum 7449 tempers show better properties for friction stir welding. A heat gradient along with a high conducting plate (Cu) used at the bottom of the run, resulted in welds with two separate microstructures in the nugget. Due to the microstructure at the bottom of the nugget, higher strength than the base metal is observed. Furthermore, the effects of natural aging and artificial aging were studied to understand re-precipitation. Large improvements in strength are observed after natural aging throughout the welds, including improvements in the HAZ.

This project is in collaboration with the Hydro-Quebec Research Institute (IREQ) and the Canadian Centre for Electron Microscopy (CCEM) on the mechanical test performance of miniature-scale micro-tensile specimens. The objective of the thesis project is to create an efficient and reliable fabrication route for producing micro-tensile specimens and to validate the accuracy of a newly custom-built micro-tensile bench at IREQ. The fabrication techniques developed and outlined in this thesis use the underlying fundamental physical mechanisms of secondary electron microscopy (SEM), focused-ion beam (FIB), and the femtosecond (fs)-laser machining for producing optimal quality micro-tensile specimens. The mechanical testing of the specimens is geared towards studying the localized deformation occurring in the microstructure when the size of the specimen only limits a number of grains and grain boundaries in order to target the specific detailed measurement of the mechanical behaviour of individual grains and interfaces. The goal for creating an optimal fabrication route for micro-tensile specimens is to carry out micro-mechanical testing of the primary turbine steels of 415 martensitic stainless steel used in the manufacture of Francis turbine components at Hydro-Quebec. The mechanical testing of single phase and interphase interface 415 steel micro-tensile specimens are considered building blocks to developing digital twin models of the steel microstructure. The experimental data from the mechanical tests would be fed into the crystal plasticity finite element models (CPFEM) that are currently being developed by researchers at IREQ. With the development of digital twin models, engineers at IREQ would be able to predict crack initiation at the microstructure level (prior to crack propagation into macro-scale cracks) by observing the evolution of the grain’s crystallographic orientation and morphology, as well as deformation mechanisms such as martensite formation and twinning produced from localized induced strains in the microstructure. In addition, self-organized dislocation processes such as dislocation nucleation and dislocation escape through the free surface can also be studied using the CPFEM models for size-limited mechanical deformation behaviour of miniature-scale mechanical test specimens. The fabrication routes studied in this thesis project use the combination of the fs-laser and plasma focused ion beam (PFIB) to machine the micro-tensile specimens. (100) single crystal copper was the ideal material chosen to validate the accuracy of the micro-tensile bench and quality of the fs-laser-machined tensile specimens, due to its ductile nature and well-characterized properties studied in literature. A mechanical size effect was studied for single crystal copper specimens with different gauge thicknesses. It was observed from the micro-tension testing that the strength of the specimens increased with decreasing gauge thickness occurring in the size-limited tensile gauges. In addition, it was determined there was negligible differences in the size effect seen between the PFIB-machined copper micro-tensile specimens and the fs-laser-machined micro-tensile specimens, demonstrating that the fs-laser is a reliable machining route for the micro-tensile specimens. X-ray computed tomography was used to validate the correct geometry of the machined gauge section produced from an innovative gauge thinning method adopted from IREQ’s research collaborator, Dr. Robert Wheeler. As well, finite-element analysis (FEA) was performed to determine the deformation behaviour under both linear-elastic and non-linear elastoplastic conditions of (100) copper and 415 steel models simulated in pure tension, prior to the fabrication of the micro-tensile specimens, respectively. Furthermore, significant progress has been made towards targeting martensite grains in the 415-steel microstructure using electron backscattered diffraction (EBSD) analysis to produce single crystal and interphase interface micro-tensile specimens. A workflow towards grain targeting using EBSD analysis has been developed, as well as for the relocation of grains using reference fiducial marks for future fabrication of the single crystal and interphase interface 415 micro-tensile specimens. / Thesis / Master of Applied Science (MASc) / Hydro-Quebec is an energy utilities company that operates the design of Francis hydro-turbines to supply hydroelectric power across the province of Quebec. The hydro-turbines have an expected service life of 70 years. Unfortunately, the turbines can get replaced by new ones prior to reaching half of its service life, due to the development of severe fatigue crack growth in the primary components of the turbines. A solution proposed by the researchers at the Hydro-Quebec Research Institute (IREQ) is to determine a linkage between the turbine’s steel’s microstructure and the mechanical behaviour of the turbine steels. Deformation of the material starts at the microstructure level, where dislocations glide through the material lattice, causing both reversible (elastic) and irreversible (plastic) deformation. Therefore, a solution was proposed by the researchers at IREQ to create computational models of the steel microstructure to predict the deformation of the steel’s microstructure. Being able to predict the deformation mechanisms through the simulation models of the microstructures allows for engineers at Hydro-Quebec to schedule regular maintenance of the turbines more efficiently and provide metallurgists the knowledge on what is occurring at the microstructure level and what can be done to improve the chemical and physical composition of the steel. To develop the digital twin models, experimental data must be collected through mechanical testing of miniature mechanical test specimens of the turbine steels. The mechanical properties of the single phases and interphase interface specimens are fed into the models as building blocks to building a microstructure map of the turbine steels. Micro-tension testing of micro-tensile specimen provides direct information about the material’s mechanical properties. In this work, a reliable and efficient fabrication route for micro-tensile specimens was developed for the purpose of extracting mechanical properties of single phase and interphase interface turbine steel specimens using focused ion beam (FIB) and femtosecond laser machining.

The calculation of the loadbearing capacity of hollow section joints using the design formulas according to EN 1993-1-8 assumes full-strength welded connections if the non-linear stress distribution over the circumference is not covered by calculation. Sufficient ductility in the connections ensures a plastic redistribution capacity within each joint. In the case of hollow section structures made of high-strength steels, it becomes more difficult to meet the requirement for full-strength welds as the yield strength of the base material increases. This article begins with a summary of the current rules for the design and execution of welded hollow section joints. The softening behaviour of high-strength steels and the loadbearing capacity of welded connections with local softening in the HAZ are then discussed. That is followed by the presentation of the results from the AiF-FOSTA research project P1020 regarding the influence of manufacturing parameters on the properties of welded connections and the HAZ. The experimental test programme for single-sided welded T-connections and the parameter studies within the research project P1453 are also described. Finally, a design approach is presented to account for HAZ failure and hybrid failure modes of hollow section connections.

The present dissertation reports the findings of a study of cracking behavior of a newly developed superalloy, Allvac 718Plus during high power beam welding and post-weld heat treatment. Microstructures of the base alloy, heat affected zone (HAZ) and fusion zone (FZ) of welded and post-weld heat treated (PWHT) coupons were examined by the use of standard metallographic techniques involving optical microscopy, analytical scanning electron microscopy (SEM) and analytical transmission electron microscopy. Moreover, grain boundary segregation behavior of boron atoms during pre-weld heat treatments was evaluated using secondary ion mass spectroscopic system. In the first phase of the research, 718Plus was welded using a low and high heat input CO2 laser to assess its weld cracking response. Detailed examination of the welds by analytical electron microscopic technique revealed the occurrence of cracking in the HAZ of low heat input welds, while their FZ was crack free. However, both the FZ and HAZ of high heat input welds were crack-free. Resolidified constituents were observed along the cracked grain boundaries of the lower heat input welds, which indicated that HAZ cracking in this newly developed superalloy was associated with grain boundary liquation. However, despite a more extensive liquation of grain boundaries and grain interior in the HAZ of high heat input welds, no cracking occurred. This was attributed to the combination of lower welding stresses generated during cooling, and relaxation of these stresses by thick intergranular liquid. Although HAZ cracking was prevented by welding with a high heat input laser, it resulted in a significant damage to the parent microstructure through its extensive liquation. Thus, the use of low heat input welding is desirable. However, this resulted in HAZ cracking which needs to be minimized or eliminated. Therefore, during the second phase of this research, the effects of pre-weld thermal processing on the cracking response of 718Plus were investigated. Results from the quantification of the cracking of the alloy showed that HAZ cracking may be significantly reduced or eliminated through an adequate selection of pre-weld thermal cycle. In the third stage of this research, crack-free welds of 718Plus were post-weld heat treated using standard thermal schedules. A significant solid state cracking of the alloy occurred during the PWHT. The cracking was attributed to the presence of embrittling phases on HAZ grain boundaries, coupled with aging contraction stresses that are generated by a considerable precipitation of gamma prime phase during aging.

The present dissertation reports the findings of a study of cracking behavior of a newly developed superalloy, Allvac 718Plus during high power beam welding and post-weld heat treatment. Microstructures of the base alloy, heat affected zone (HAZ) and fusion zone (FZ) of welded and post-weld heat treated (PWHT) coupons were examined by the use of standard metallographic techniques involving optical microscopy, analytical scanning electron microscopy (SEM) and analytical transmission electron microscopy. Moreover, grain boundary segregation behavior of boron atoms during pre-weld heat treatments was evaluated using secondary ion mass spectroscopic system. In the first phase of the research, 718Plus was welded using a low and high heat input CO2 laser to assess its weld cracking response. Detailed examination of the welds by analytical electron microscopic technique revealed the occurrence of cracking in the HAZ of low heat input welds, while their FZ was crack free. However, both the FZ and HAZ of high heat input welds were crack-free. Resolidified constituents were observed along the cracked grain boundaries of the lower heat input welds, which indicated that HAZ cracking in this newly developed superalloy was associated with grain boundary liquation. However, despite a more extensive liquation of grain boundaries and grain interior in the HAZ of high heat input welds, no cracking occurred. This was attributed to the combination of lower welding stresses generated during cooling, and relaxation of these stresses by thick intergranular liquid. Although HAZ cracking was prevented by welding with a high heat input laser, it resulted in a significant damage to the parent microstructure through its extensive liquation. Thus, the use of low heat input welding is desirable. However, this resulted in HAZ cracking which needs to be minimized or eliminated. Therefore, during the second phase of this research, the effects of pre-weld thermal processing on the cracking response of 718Plus were investigated. Results from the quantification of the cracking of the alloy showed that HAZ cracking may be significantly reduced or eliminated through an adequate selection of pre-weld thermal cycle. In the third stage of this research, crack-free welds of 718Plus were post-weld heat treated using standard thermal schedules. A significant solid state cracking of the alloy occurred during the PWHT. The cracking was attributed to the presence of embrittling phases on HAZ grain boundaries, coupled with aging contraction stresses that are generated by a considerable precipitation of gamma prime phase during aging.

Ziel der Dissertation ist die ebenso valide wie effiziente Vorhersage der Schmelz- und der Wärmeeinflusszone der Schweißverbindung beim Thermit-Schweißen. Dazu werden reduzierte Modelle vorgestellt, darunter ein niederdimensionales Modell des Schienenstegs und dreidimensionale Modelle des Gießsystems für das Schweißverfahren. Mit dem niederdimensionalen Modell werden mittels Parameterschätzung unbekannte Randbedingungen der Vorwärmung des Gießsystems ermittelt sowie mittels Sensitivitätsanalyse systematisch Einflüsse auf die Schmelz- und die Wärmeeinflusszone untersucht. Durch den systematischen Vergleich der vorgestellten Modelle werden Gültigkeitsgrenzen der Modelle gezielt auf die Modellreduktionen zurückgeführt und über die Modelle hinausgehende Aspekte für die Beschreibung des Schmelz- und Erstarrungsverhaltens identifiziert. Dabei wird die Validität der Modelle anhand von experimentellen Daten der Schmelz- und der Wärmeeinflusszone im Schienenlängsschnitt untersucht. / This dissertation aims at providing a valid and efficient prediction of the melting zone and heat-affected zone of thermite welds. For this purpose, reduced models are presented, including a low-dimensional model of the rail web and three-dimensional models of the casting system for the welding process. With the low-dimensional model, unknown boundary conditions of the preheating of the casting system are determined by means of parameter estimation and influences on the melting zone and the heat-affected zone are systematically examined by means of a sensitivity analysis. By a systematic comparison of the models presented, the validity limits of the models are specifically traced back to the model reductions and aspects beyond these models for the description of the melting and solidification behavior are identified. The validity of the models is examined on the basis of experimental data from the melting zone and the heat-affected zone in the longitudinal section of the rails.

An effective procedure and parameter window was developed for underwater friction stir welding (UWFSW) 304L stainless steel with a PCBN tool. UWFSW produced statistically significant: increases in yield strengths, decreases in percent elongation. The ultimate tensile strength was found to be significantly higher at certain parameters. Although sigma was identified in the UWFSWs, a significant reduction of sigma was found in UWFSWs compared to ambient FSWs. The degree of sensitization in UWFSWs was evaluated using double loop EPR testing and oxalic acid electro-etched metallography. Results were compared to base metal, ambient FSW, and arc welds. Upper and lower sensitization localization bands were identified in the UWFSWs. Although higher sensitization levels were present in UWFSWs compared to the arc weld, ambient FSW, and heat treated base metals, the UWFSWs were found less susceptible to corrosion than arc welds due to the subsurface location of the sensitization bands. A SCC analysis of UWFSWs relative to base metal and arc weldments was performed. U-bends were exposed to two 3.5% NaCl cyclic immersion experiments at 21 °C and 63 °C for 1000 hours each. A tertiary test was conducted in a 25% NaCl boiling solution. The UWFSW u-bends were no more susceptible to SCC than base metal in the cyclic immersion tests. In the boiling NaCl test, the SCC of the UWFSWs showed significant improvement over the SCC of arc welds. Arc u-bends cracked entirely within the weld bead and HAZ, while SCC in the UWFSWs showed no cracking localization.