A friction stir weld (FSW) is a solid state joining operation whose processing parameters are currently determined by lengthy trial and error methods. To implement FSWing rapidly in various applications will require an approach for predicting process parameters based on the physics of the process. Based on hot working conditions for metals, a kinematic model has been proposed for calculating the shear strain and shear strain rates during the FSW process, validation of the proposed model with direct measuring is difficult however. Since the shear strain and shear strain rates predicted for the FSW process, are similar to those predicted in metal cutting, validation of the FSW algorithms with microstructural studies of metal chips may be possible leading to the ability to predict FSW processing parameters.

Friction stir welding (FSWing) is a solid state, thermo-mechanical process that utilizes a non-consumable rotating weld tool to consolidate a weld joint. In the FSW process, the weld tool is responsible for generating both the heat required to soften the material and the forces necessary to deform and consolidate the former weld seam. Thus, weld tool geometry, material selection, and process parameters are important to the quality of the weld. To study the effects of the weld tool geometry on the resulting welds, a previous study was conducted using varying degree taper, microwave-sintered tungsten carbide (WC) weld tools to FSW Ti-6Al-4V. Fully consolidated welds were down selected for this study to evaluate the resulting mechanical properties and to document the microstructure. X-ray diffraction (XRD) was used to compare the parent material texture with that in the weld nugget. The purpose of this study is to quantify the temperatures obtained during FSWing by interpreting the resulting microstructure. This information is useful in process optimization as well as weld tool material selection.

The Swedish model for final disposal of nuclear fuel waste is based on copper canisters as a corrosion barrier with an inner pressure holding insert of cast iron. One of the methods to seal the copper canister is to use the Friction Stir Welding (FSW), a method invented by The Welding Institute (TWI). This work has been focused on characterisation of the FSW joints, and modelling of the process, both analytically and numerically. The first simulations were based on Rosenthal’s analytical medium plate model. The model is simple to use, but has limitations. Finite element models were developed, initially with a two-dimensional geometry. Due to the requirements of describing both the heat flow and the tool movement, three-dimensional models were developed. These models take into account heat transfer, material flow, and continuum mechanics. The geometries of the models are based on the simulation experiments carried out at TWI and at Swedish Nuclear Fuel Waste and Management Co (SKB). Temperature distribution, material flow and their effects on the thermal expansion were predicted for a full-scale canister and lid. The steady state solutions have been compared with temperature measurements, showing good agreement. Microstructure and hardness profiles have been investigated by optical microscope, Scanning Electron Microscope (SEM), Electron Back Scatter Diffraction (EBSD) and Rockwell hardness measurements. EBSD visualisation has been used to determine the grain size distribution and the appearance of twins and misorientation within grains. The orientation maps show a fine uniform equiaxed grain structure. The root of the weld exhibits the smallest grains and many annealing twins. This may be due to deformation after recrystallisation. The appearance of the nugget and the grain size depends on the position of the weld. A large difference can be seen both in hardness and grain size between the start of the weld and when the steady state is reached. / QC 20101207

The historical success of friction stir welding (FSW) on materials such as aluminum and magnesium alloys is associated with the absence of melting and solidification during the solid-state process. However, commercial adoption of FSW on steels and other non-ferrous high-strength, high-temperature materials such as nickel-base and titanium-base alloys is limited due to the high costs associated with the process. In this dissertation, the feasibility of using an FSW approach to fabricate certain structural components made of nitrogen containing austenitic stainless steels that go into the vacuum vessel and magnetic systems of tokamak devices was demonstrated. The FSW weldments possessed superior application-specific mechanical and functional properties when compared to fusion weldments reported in the technical literature. However, as stated earlier, the industrial adoption of FSW on high temperature materials such as the ferrous alloys used in the present study is greatly limited due to the high costs associated with the process. The cost is mainly dictated by the high temperature FSW tools used to accomplish the weldments. Commercially available high temperature FSW tools are exorbitantly priced and often have short lifetimes. To overcome the high-cost barrier, we have explored the use of integrated computational materials engineering (ICME) combined with experimental prototyping validation to design next-generation tool materials with high performance and relatively low cost. Cermet compositions with either tungsten carbide or niobium carbide as the hard phase bonded by high entropy alloy binders were processed via mechanical alloying and spark plasma sintering. The feasibility and effectiveness of the newly developed cermet tool materials as potential next generation high temperature FSW tool materials was evaluated.

Os objetivos deste trabalho foram processar e caracterizar microestrutural e mecanicamente superligas à base de cobalto do sistema Co-Al-W-Ni-Cr-(Nb,Ta)- C-B com microestrutura ?/?\'. Visando possível aplicação em ferramenta para Soldagem por Atrito com Pino Não Consumível (SAPNC), em inglês Friction Stir Welding (FSW) nas composições: (Co-10Al-7,5W-30Ni-10Cr-3,0Nb-0,6C-0,06B %at. ) - 30Ni-3Nb, (Co-10Al-7,5W-40Ni-10Cr-3,0Nb-0,6C-0,06B %at.) - 40Ni-3Nb, (Co-10Al-7,5W-50Ni-10Cr-3,0Nb-0,6C-0,06B %at.) - 50Ni-3Nb além da liga patenteada (Co-10Al-7,5W-40Ni-10Cr-3,0Ta-0,6C-0,06B %at.) - 40Ni-3Ta considerada padrão para este projeto. A caraterização microestrutural foi feita por microscopia eletrônica de varredura (MEV), microscopia eletrônica de varredura por emissão de campo (MEV-FEG), medidas de microanálise semi-quantitativa e mapeamento químico por EDS, caracterização por difração de raios X, além do ensaio mecânico de dureza Vickers em todas as amostras. Os resultados da caracterização microestrutural e mecânica para a liga 40Ni-3Ta mostraram-se semelhantes aos encontrados na literatura enquanto que a composição 40Ni-3Nb foi a que apresentou resultados mais próximos em relação à liga padrão. / The objectives of this study were to process and characterize microstructural and mechanical cobalt-based superalloys from Co-Al-W-Ni-Cr-(Nb,Ta)-C-B system with ?/? \' microstructure. Aiming possible application for Friction Stir Welding (FSW) tool in the compositions: (Co-10Al-7.5W-30Ni-10Cr-3.0Nb-0.6C-0.06B %at.) - 30Ni-3Nb, (Co-10Al-7.5W-40Ni-10Cr-3.0Nb-0.6C-0.06B %at. ) - 40Ni-3Nb, (Co-10Al-7.5W-50Ni-10Cr-3.0Nb-0.6C-0.06B %at.) - 50Ni-3Nb, including the patented alloy (Co-10Al-7.5W -40Ni-10Cr-3.0Ta-0.6C-0.06B %at.) - 40Ni-3Ta considered the standard alloy for this project. The microstructural characterization was made by Scanning Electron Microscopy (SEM), Scanning Electron Microscopy with Field Emission Gun (SEM-FEG), semi-quantitative microanalysis measures and chemical mapping by EDS, characterization by X - ray diffraction and mechanical test of hardness Vickers in all the samples. The results of the microstructural and mechanical characterization for 40Ni-3Ta alloy were similar to those found in the literature. The 40Ni-3Nb alloy showed the closest results, among the other alloys studied in comparison with the standard alloy.

Os objetivos deste trabalho foram processar e caracterizar microestrutural e mecanicamente superligas à base de cobalto do sistema Co-Al-W-Ni-Cr-(Nb,Ta)- C-B com microestrutura ?/?\'. Visando possível aplicação em ferramenta para Soldagem por Atrito com Pino Não Consumível (SAPNC), em inglês Friction Stir Welding (FSW) nas composições: (Co-10Al-7,5W-30Ni-10Cr-3,0Nb-0,6C-0,06B %at. ) - 30Ni-3Nb, (Co-10Al-7,5W-40Ni-10Cr-3,0Nb-0,6C-0,06B %at.) - 40Ni-3Nb, (Co-10Al-7,5W-50Ni-10Cr-3,0Nb-0,6C-0,06B %at.) - 50Ni-3Nb além da liga patenteada (Co-10Al-7,5W-40Ni-10Cr-3,0Ta-0,6C-0,06B %at.) - 40Ni-3Ta considerada padrão para este projeto. A caraterização microestrutural foi feita por microscopia eletrônica de varredura (MEV), microscopia eletrônica de varredura por emissão de campo (MEV-FEG), medidas de microanálise semi-quantitativa e mapeamento químico por EDS, caracterização por difração de raios X, além do ensaio mecânico de dureza Vickers em todas as amostras. Os resultados da caracterização microestrutural e mecânica para a liga 40Ni-3Ta mostraram-se semelhantes aos encontrados na literatura enquanto que a composição 40Ni-3Nb foi a que apresentou resultados mais próximos em relação à liga padrão. / The objectives of this study were to process and characterize microstructural and mechanical cobalt-based superalloys from Co-Al-W-Ni-Cr-(Nb,Ta)-C-B system with ?/? \' microstructure. Aiming possible application for Friction Stir Welding (FSW) tool in the compositions: (Co-10Al-7.5W-30Ni-10Cr-3.0Nb-0.6C-0.06B %at.) - 30Ni-3Nb, (Co-10Al-7.5W-40Ni-10Cr-3.0Nb-0.6C-0.06B %at. ) - 40Ni-3Nb, (Co-10Al-7.5W-50Ni-10Cr-3.0Nb-0.6C-0.06B %at.) - 50Ni-3Nb, including the patented alloy (Co-10Al-7.5W -40Ni-10Cr-3.0Ta-0.6C-0.06B %at.) - 40Ni-3Ta considered the standard alloy for this project. The microstructural characterization was made by Scanning Electron Microscopy (SEM), Scanning Electron Microscopy with Field Emission Gun (SEM-FEG), semi-quantitative microanalysis measures and chemical mapping by EDS, characterization by X - ray diffraction and mechanical test of hardness Vickers in all the samples. The results of the microstructural and mechanical characterization for 40Ni-3Ta alloy were similar to those found in the literature. The 40Ni-3Nb alloy showed the closest results, among the other alloys studied in comparison with the standard alloy.

Este trabalho apresenta um modelo inovador de coletor solar plano, com placa absorvedora fabricada por Estampagem Incremental e Soldagem FSW (Friction Stir Welding). Esta placa absorvedora é constituída de duas chapas de alumínio AA1200-H14 com espessura de 1 mm, estampadas e soldadas em simetria, a fim de que na união das chapas sejam deixados canais para passagem de água. Neste estudo foi realizada a caracterização do material por Ensaio de Tração e Ensaio Nakajima. Para determinação dos parâmetros de Estampagem Incremental foram realizados 16 experimentos com ferramenta de diâmetro df = 9,5 mm, variando a rotação de N = 50 rpm a 800 rpm e o incremento vertical de Δz = 2 mm a 0,2 mm, mantendo o avanço em = 250 mm/min. E também foram realizados 3 experimentos com ferramenta df = 22 mm, variando o incremento vertical de Δz = 2 mm a 0,5 mm, mantendo a rotação em N = 50 rpm e o avanço em = 250 mm/min. Para determinação dos parâmetros de Soldagem FSW foram realizados 4 experimentos com ferramenta de ombro de diâmetro 8 mm e pino roscado M3x0,5, mantendo a rotação em N = 1500 rpm e variando o avanço entre = 100 mm/min a 400 mm/min. Em seguida foi fabricado um protótipo de placa absorvedora de coletor solar com área de 0,12 m². Nos experimentos realizados foi constatado que é possível obter maiores deformações na Estampagem Incremental do que na Estampagem Convencional e que as deformações são mais elevadas quando se utiliza menores diâmetros, maiores rotações e menores incrementos verticais da ferramenta. Na Soldagem FSW não foi constatada influência na qualidade do cordão de solda em relação à variação do avanço da ferramenta. Neste estudo também se verificou que é possível fabricar protótipos de placas absorvedoras de coletores solares pelos processos de Estampagem Incremental e Soldagem FSW. No entanto, para coletores em tamanho comercial, novos estudos são necessários para melhorar a forma de fixação das chapas durante a Soldagem FSW. / This work presents an innovative model of flat plate solar collector, with absorber plate manufactured using Incremental Sheet Forming (ISF) and Friction Stir Welding (FSW). This absorber plate consists of two AA1200-H14 aluminum sheets with a thickness of 1 mm, stamped and welded in symmetry, in order to leave channels for the passage of water. In this study the characterization of the material by Nakajima Test and Traction Test was performed. In order to determine the parameters of ISF, 16 experiments were performed with a tool of diameter df = 9.5 mm, varying the rotation speed of N = 50 rpm at 800 rpm and the step down of Δz = 2 mm to 0.2 mm, maintaining the feed rate at = 250 mm/min. Also, 3 experiments with tool df = 22 mm were performed, varying the step down of Δz = 2 mm to 0.5 mm, maintaining the rotation speed at N = 50 rpm and the feed rate at = 250 mm/min. For determination of FSW parameters, 4 experiments with 8 mm diameter shoulder tool and M3x0.5 pin were performed, maintaining the rotation speed at N = 1500 rpm and varying the feed rate from = 100 mm/min to 400 mm/min. A prototype solar collector absorber plate with a 0.12 m² area was then manufactured. In the experiments carried out, it was found that it is possible to obtain greater deformations in the ISF than in the Conventional Stamping and that the deformations are higher when using smaller diameters, higher rotations and smaller step downs of the tool. In FSW, no influence was observed in the quality of the weld bead in relation to the variation of the tool feed rate. In this study it was also verified that it is possible to manufacture prototypes of solar collector absorber plates by the processes of ISF and FSW. However, for commercial size collectors, further studies are needed to improve the way the plates are fixed during FSW.

This work has been focused on characterisation of FSW joints, and modelling of the process, both analytically and numerically. The Swedish model for final deposit of nuclear fuel waste is based on copper canisters as a corrosion barrier with an inner pressure holding insert of cast iron. Friction Stir Welding (FSW) is the method to seal the copper canister, a technique invented by The Welding Institute (TWI). The first simulations were based on Rosenthal’s analytical medium plate model. The model is simple to use, but has limitations. Finite element models (FEM) were developed, initially with a two-dimensional geometry. Due to the requirements of describing both the heat flow and the tool movement, three-dimensional models were developed. These models take into account heat transfer, material flow, and continuum mechanics. The geometries of the models are based on the simulation experiments carried out at TWI and at Swedish Nuclear Fuel Waste and Management Co (SKB). Temperature distribution, material flow and their effects on the thermal expansion were predicted for a full-scale canister and lid. The steady state solutions have been compared with temperature measurements, showing good agreement. In order to understand the material flow during welding a marker technique is used, which involves inserting dissimilar material into the weld zone before joining. Different materials are tested showing that brass rods are the most suitable material in these welds. After welding, the weld line is sliced, etched and examined by optical microscope. To understand the material flow further, and in the future predict the flow, a FEM is developed. This model and the etched samples are compared showing similar features. Furthermore, by using this model the area that is recrystallised can be predicted. The predicted area and the grain size and hardness profile agree well. Microstructure and hardness profiles have been investigated by optical microscopy, Scanning Electron Microscopy (SEM), Electron Back Scatter Diffraction (EBSD) and Rockwell hardness measurements. EBSD visualisation has been used to determine the grain size distribution and the appearance of twins and misorientation within grains. The orientation maps show a fine uniform equiaxed grain structure. The root of the weld exhibits the smallest grains and many annealing twins. The appearance of the nugget and the grain size depends on the position of the weld. A large difference can be seen both in hardness and grain size between the start of the weld and when the steady state is reached. / QC20100719

Friction Stir Welding (FSW) is an emerging solid state welding process. It has been a proven method for welding high strength aluminium alloys which were previously not recommended for conventional fusion welding. Since the invention of the process by The Welding Institute, United Kingdom, in 1991, a number of studies have been conducted on the material flow, microstructural evolution and mechanical properties of friction stir welds. However, there is not enough conceptual background available on FSW process for physical understanding of the mechanism of weld formation. In addition to that, FSW welds of high strength precipitation hardenable aluminium alloys suffer from reduced joint efficiency due to overaging in the heat affected zone. In the present investigation, experimental analysis has been carried out to understand the mechanism of weld formation and parameter optimization for aluminium alloys 7020-T6 and 6061-T6. For this purpose the investigations have been made on both the process aspects and the material aspects. The process aspects are analyzed with the objective of learning the mechanism to produce defect free welds. For this purpose experiments have been carried out to analyze the effect of FSW parameters, material flow and the frictional characteristics between the tool and base metal. Preliminary experiments are conducted on aluminium alloy 7020-T6 with different tool geometries to analyze the interaction of the tool with the base metal using a knee-type vertical milling machine. Then, the tool geometry which produced defect-free weld is used for further experimentation. The role of tool pin, shoulder and axial load on the formation of defect free weld is analyzed in an innovative experiment, where the tool and base metal interaction is continuously increased by continuously increasing the interference between the tool and base metal. In another experiment the initial abutting interface position with respect to the tool is continuously varied to study the interaction of the tool with the initial interface and to find the positional information where the initial interface is completely eliminated. Further, the tool metal interface condition is studied using a specially designed tribological experiment which simulates the FSW condition. From the base metal point of view, due to the strain, strain rate and temperature imposed on the base metal during the process, the microstructure is altered. In precipitation hardenable aluminium alloys the strengthening precipitates are dissolved or overaged in the weld region depending on the peak temperature in the region, which reduces the joint efficiency. However, the dissolution and overaging are kinetic process. In order to analyze this time dependant softening behavior of the base metal 7020-T6 and 6061-T6, isothermal annealing and differential scanning calorimetric studies are performed. In order to obtain FSW welds with maximum joint efficiency, the welding temperature should not exceed the “softening temperature” of the base metal. But, to produce defect free welds favorable material flow in the weld nugget is necessary. The material flow and consolidation depend on the process temperature. Hence, for a given tool to produce defect free weld there is a need for minimum temperature. If the weld formation temperature is less than the base metal softening temperature, the weld can be made with 100% joint efficiency. In order to optimize the FSW parameter which gives defect free weld with lowest possible temperature, an instrumented programmable FSW machine is designed and developed. The machine is designed in such a way that welding parameters – rotation speed, traverse speed and plunging depth – can be continuously varied from the start to end of the weld between given two values. This reduces the number of experimental trials, material and time. Based on the experimental results the following conclusions are derived. 1.The minimum diameter of the pin required for FSW depends on the base metal and tool material property for a given set of parameters. If the pin diameter is insufficient for a given set of welding parameters, it fails during plunging operation itself. 2.There is a minimum diameter of the shoulder for a given diameter of the pin which produces defect free weld. The ratio of pin to shoulder to produce a defect free weld is not a constant value. It changes with tool geometry and process parameters. 3.Increasing the area of contact between the tool and shoulder for a given set of parameters increases the heat input and results in increased weld nugget grain size. 4.Initial abutting interface of the base metal is eliminated at the leading edge of the tool. However, new surfaces are generated due to interaction with the tool and the newer surfaces are consolidated at the trailing edge of the tool. Importantly, the weld strength is controlled by the defects generated due the improper elimination of newly generated surfaces. 5.Optimal axial load is required to generate the required pressure to consolidate the transferred material at the trailing edge of the tool and should be equal to the flow stress of the material at the processing temperature. The optimal axial load is 8.1kN for a tool having 20mm diameter shoulder with 6mm diameter frustum shaped pin. 6.Only the material that approaches the tool at the leading edge on the advancing side is stirred and the remaining material is simply extruded around the tool. Further, the initial abutting interface is completely removed only when it is located in the stirring zone, otherwise the initial abutting interface is not eliminated. In the present study the interface is completely stirred when it is located on the advancing side of the tool between 0.5mm away from the centerline and edge of the tool. 7.The temperature and pressure at the tool–base metal interface is above the temperature and pressure required for seizure to occur for given tool material (H13) and base metal (7020-T6). Hence, it is clear that during FSW the base metal transfers on to the tool and interaction occurs between transferred layer on tool and base metal. The coefficient of friction between the given tool material and base metal in FSW condition is in the range of 1.2 – 1.4. 8.The minimum temperature requirement for FSW of 7020-T6 is 400oC and 6061-T6 is 430oC. However, 7020-T6 and 6061-T6 softens at lower temperatures than that of the minimum FSW temperature. 7020-T6 softens 30% in 7min at 250oC, 4min at 300oC, 2min at 350oC and 1min at 400oC. After softening 30%, there is 10% recovery in hardness and the hardness remains constant thereafter. Whereas 6061-T6 softens gradually up to 47% in 7min at 350oC and 400oC, below the temperature of 250oC for 7020-T6 and 350oC for 6061-T6 there is no softening observed in 7min. 9.The maximum joint efficiency of the 7020-T6 weld is 82% and 6061-T6 weld is 60%. 10. The reduction in joint efficiency is attributed to overaging of the material in the heat affected zone.

<p>The Swedish model for final disposal of nuclear fuel waste is based on copper canisters as a corrosion barrier with an inner pressure holding insert of cast iron. One of the methods to seal the copper canister is to use the Friction Stir Welding (FSW), a method invented by The Welding Institute (TWI).</p><p>This work has been focused on characterisation of the FSW joints, and modelling of the process, both analytically and numerically. The first simulations were based on Rosenthal’s analytical medium plate model. The model is simple to use, but has limitations. Finite element models were developed, initially with a two-dimensional geometry. Due to the requirements of describing both the heat flow and the tool movement, three-dimensional models were developed. These models take into account heat transfer, material flow, and continuum mechanics. The geometries of the models are based on the simulation experiments carried out at TWI and at Swedish Nuclear Fuel Waste and Management Co (SKB). Temperature distribution, material flow and their effects on the thermal expansion were predicted for a full-scale canister and lid. The steady state solutions have been compared with temperature measurements, showing good agreement.</p><p>Microstructure and hardness profiles have been investigated by optical microscope, Scanning Electron Microscope (SEM), Electron Back Scatter Diffraction (EBSD) and Rockwell hardness measurements. EBSD visualisation has been used to determine the grain size distribution and the appearance of twins and misorientation within grains. The orientation maps show a fine uniform equiaxed grain structure. The root of the weld exhibits the smallest grains and many annealing twins. This may be due to deformation after recrystallisation. The appearance of the nugget and the grain size depends on the position of the weld. A large difference can be seen both in hardness and grain size between the start of the weld and when the steady state is reached.</p>