Models (both physical and numerical) of the friction stir (FS) welding process are used to develop a greater understanding of the influence of independent process parameters on dependent process output variables, such as torque, power, specific weld energy, peak temperature, cooling rates and various metallurgical factors (e.g., grain size and precipitates). An understanding of how the independent process parameters influence output variables and ultimately their effect on resultant properties (e.g., strength, hardness, etc..) is desirable. Most models developed have been validated primarily for aluminum alloys with relatively small amounts of experimental data. Fewer models have been validated for steels or stainless steels, particularly since steels and stainless steels have proven more challenging to friction stir than aluminum alloys. The Gleeble system is also a powerful tool with the capability to perform thermomechanical simulations in a known and controlled environment and provide physical representation of resultant microstructure and hardness values. The coupling of experimental data and physical simulated data can be extremely useful in assessing the capabilities of friction stir numerical process models. The overall approach is to evaluate Isaiah an existing three-dimensional finite element code developed at Cornell University by comparing against experimental and physically-simulated data to determine how well the code output relates to real FS data over a range of nine processing conditions. Physical simulations replicating select thermomechanical streamline histories were conducted to provide a physical representation of resultant metallurgy and hardness. Isaiah shows promise in predicting qualitative trends over a limited range of parameters and is not recommended for use as a predictive tool but rather a complimentary tool, Once properly calibrated, the Isaiah code can be a powerful tool to gain insight into the process, strength evolution during the process and coupled with a texture evolution model may also provide insight into microstructural and texture evolution over a range for which it is calibrated.

Friction and wear have been concerns to industrialists, scientists and environmentalist for several decades. Many robust models have been developed to address friction and wear and their dependency on various parameters. Numerous experiments have been conducted in the past to model friction and wear. In order to get the experimental results as close as to the real world, the construction of the scientific instrument has to be precise as well. There are several factors that influence the experimental results such as setup of the experiment, dynamics of the instruments etc. These need to be analysed, characterised and suppressed their influence on the experiments. The dynamics of the tribometer when two surfaces are in sliding contact consist of friction induced vibrations and vibrations that influence friction. It is highly complex to device a model which defines interplay of friction and vibrations together. To understand the effect of dynamics of tribometer on sliding contact, vibrations signals were examined. The data was collected by mounting vibrations sensors on the tribometer and the stiffness of the tribometer was modified by changing the spring fixed in the tribometer. The vibrations signals and coefficient of friction data were analysed. The result showed that changing the stiffness of the tribometer did not influence vibrations and coefficient of friction to observable extent, in fact, the vibrations signals were the interaction between the sliding surfaces. The coefficients of friction and vibrations signals for several experiments were similar. The interplay between the vibrations and friction was observed and the vibrations were more like induced vibrations rather than vibrations in the tribometer affecting the sliding contact. The results of experiments comply with by the previously established theories. As the sliding speed decreases, vibrations also decrease and coefficient of friction increases because of increment in the area of contact. This also shows that as vibrations reduces, coefficient of friction increases. The roughness of the pins after the experiments with 40 N/mm spring constant were higher than the ones with 20 N/mm and 120 N/mm, even though the experiment with 20 N/mm were run for a fewer hour. The roughness of the disc for experiment with 120 N/mm were higher than that of the 40 N/mm and 20 N/mm. The experiment with 20 N/mm is higher than 40 N/mm because experiment ran for fewer number of hours than 40 N/mm and 120 N/mm. The roughness results did not have observable patterns and therefore it was concluded that changing one of the components of the tribometer do not influence the experiment at microscale. / Friktion och slitage har varit bekymmer för industrimän, vetenskapsmän och miljöpartister i flera decennier. Många robusta modeller har utvecklats för att hantera friktion och slitage och deras beroende av olika parametrar. Många experiment har utförts tidigare för att modellera friktion och slitage. För att få de experimentiella resultaten så nära den verkliga världen, måste konstruktionen av det vetenskapliga instrumentet också vara exakt. Det finns flera faktorer som påverkar de experimentella resultaten såsom experimentets uppställning, instrumentens dynamik etc. Dessa behöver analyseras, karaktäriseras och undertrycka deras inverkan på experimenten. Tribometerns dynamik när två ytor är i glidkontakt består av friktionsinducerade vibrationer och vibrationer som påverkar friktionen. Det är mycket komplucerat att skapa en modell som definierar samspelet mellan friktion och vibrationer. För att förstå effekten av tribometerns dynamik på glidkontakten undersöktes vibrationssignaler. Data samlades in genom att montera vibrationssensorer på tribometern och tribometerns styvhet modifierades genom att byta fjäder som var fixerad i tribometern. Vibrationssignalerna och friktionskoefficientdata analyserades. Resultatet visade att ändring av tribometerns styvhet inte påverkade vibrationer och friktionskoefficient i en observerbar utsträckning, i själva verket var vibrationssignalerna interaktionen mellan glidytorna. Friktionskoefficienterna och vibrationssignalerna för flera experiment var likartade. Samspelet mellan vibrationerna och friktionen observerades och vibrationerna var mer som inducerade vibrationer snarare än vibrationer i tribometern som påverkade glidkontakten. Resultaten av experiment överensstämmer med de tidigare etablerade teorierna. När glidhastigheten minskar minskar också vibrationerna och friktionskoefficienten ökar på grund av ökningen i kontaktområdet. Detta visar också att när vibrationerna minskar så ökar friktionskoefficienten. Stiftarnas grovhet efter försöken med 40 N/mm fjäderkonstant var högre än de med 20 N/mm 120 N/mm, även om försöket med 20 N/mm kördes i färre timmar. Råheten hos skivan för experiment med 120 N/mm var högre än den flr 40 N/mm och 20 N/mm. Experimentet med 20 N/mm är högre än 40 N/mm eftersom experimentet pågick under färre antal timmar än 40 N/mm och 120 N/mm. Ojämnhetsresultate§n hade inga observerbara mönster och därför drogs slutsatsen att förändring av en av komponenterna i tribometern inte påverkar experimentet i mikroskala.

Additive friction stir deposition (AFSD) is a newly developed solid-state metal additive manufacturing (AM) technology that adds a material feeding mechanism to the friction stir principle (Yu et al.., 2018). As a newly developed process, the development of a sound understanding of the process mechanics is necessary and may shed light on both limiting factors and new opportunities. This work explores the fundamental modes of deformation through an analytical decomposition of three flow components: 1) radial spreading, 2) rotating, and 3) traversing shear flow. The analytical models provide 'back-of-the-envelope' estimates of mechanical requirements (machine torque, for example), and straightforward algebraic equations for estimating the peak strain rate associated with deformation and the expected residence time of material underneath the AFSD tool head. A more complex, but preliminary, numerical modeling approach is then presented to models the steady state material flow as a fully coupled non-Newtonian fluid with rate and temperature dependent properties. Additionally, a transient thermal model is presented which captures the thermal history of the material along a dynamic printing trajectory. The numerical models provide insight into the pressure distribution underneath the AFSD tool, which impacts deformation bonding conditions at the interface, and suggest that temperature differences under the tool may be as high as 70℃. Several interface fracture experiments reveal a well-bonded center region, with high ductility and energy dissipation, and a poorly bonded outer edge region. Novel characterization work has been presented showing evidence of a nearly uniform 50μm thick shear layer on the top surface of a deposit. Analysis of the Prandtl number suggests that this shear layer is a consequence of a thin thermal boundary layer, which in the presence of frictional shear stress, becomes a thermo-mechanical boundary layer with a distinct flow regime from the bulk. Further characterization shows viscous mixing patterns in the wake of tool pins, and incomplete bonding at the edges of the deposition track. An additional application is presented for AFSD – selective area cladding of thin sheet metal. Substrates as thin as 1.4mm were clad without localized deformation, which is dependent on the clamping configuration of the substrate. Cladding quality, interface integrity, and certain failure modes are identified for thin cladding operations. In-situ monitoring and ex-situ laser scanning shows the slow evolution of thermal distortion during cooling of the cladding-on-sheet system. Finally, residual stress and strain estimates are made using curvature methods for bi-layer specimens extracted from the cladding. / Doctor of Philosophy / Additive manufacturing of metal components (colloquially called "3D printing") has generated significant interest and excitement as the manufacturing method of the future, where new materials with complex shapes and functionalities may unlock new possibilities for commerce and industry. Metal 3D printing also gives us new methods to repair aging and damaged structures, providing opportunities to extend the life of existing infrastructure. This work is centrally focused on understanding the most important factors and physical principles at play during a particular metal additive manufacturing process, additive friction stir deposition (ASFD). AFSD uses deformation to heat and bond materials together, building on the principles of friction welding and forge welding. A fundamental understanding of the process mechanics will allow for a better understanding of the current limits and potential opportunities this new technology can provide. Using a combination of analytical analysis, numerical modeling, and experiments, this work aims to provide a deeper understanding of the material flow, thermal fields, and mechanical forces associated with depositing material by AFSD, which may be insightful for new materials, tunable material properties, and may lead to new machine design opportunities.

No description available.

This work investigates the velocity-dependent coefficient of friction between concrete and 300 Maraging steel over short displacements. A modified torsional Hopkinson bar is utilized for rotating thin-walled steel rings in contact with a concrete disk under static precompression. Rotational velocity is varied between tests to determine the velocity dependence of the friction coefficient. Normal force is varied between certain tests to determine the pressure dependence of the friction coefficient between the concrete and steel. Three different types of concrete are tested to deduce any composition effect on the friction coefficient. Dry and greased conditions’ impact on the friction coefficient are also evaluated. Lastly, the displacement dependence (fade) is considered for the concrete with regards to the steel. Discussion of the usefulness of this data in modeling and experimentation of impact between concrete and steel is disclosed.

Tire parameters play a very important role in tire performance. Depending on the driving conditions for which a given tire is designed, its parameters must be chosen appropriately (e.g., the radius of the tire, the width of the tire, material properties of different sections). Among tire characteristics, the material properties of the rubber compounds have a vital role in tire behavior. Previous studies show that the material properties of the rubber are highly dependent on temperature. Thus, a comprehensive study on the effect of the material properties of the rubber on tire performance for different temperatures as well as different road conditions is required. In this study, a theoretical model has been developed for tire-ice interaction. The temperature changes obtained from the model are used to calculate the height of the water film created by the heat generated due to the friction force. Next, the viscous friction coefficient at the contact patch is obtained. By using the thermal balance equation at the contact patch, dry friction is obtained. Knowing the friction coefficients for the dry and wet regions, the equivalent friction coefficient is calculated. The model has been validated using experimental results for three similar tires with different rubber compounds properties. For the experimental part of this study, four tires have been selected for testing. Three of them have identical tire geometry and structure but different rubber tread compounds. Several tests were conducted for the chosen tires in three modes: free-rolling, braking, and traction. The tests were performed for two different normal loads (4 kN and 5.6 kN), two different inflation pressures (21 psi (144.8 kPa) and 28 psi (193 kPa)), and three tire temperatures levels (-10°C, -5°C, and -1 °C). The Terramechanics Rig at TMVS at Virginia Tech has been used for conducting the tests. The results from this study show the sensitivity of the magnitude of the tractive force with respect to parameters such as tire temperature, normal load, etc. The results also indicate that the tire with the lowest value of the Young modulus has the highest traction among all four tires used in this study. The model developed can be used to predict the temperature changes at the contact patch, the tire friction force, the areas of wet and dry regions, the height of the water film for different ice temperatures, different normal loads, etc. The results from this study coincide with the obtained results from the experiments. According to the data available, tire B with the smallest value of Young modulus and the smallest value of the specific heat parameter was shown to have the highest friction coefficient in both simulation and experiment. After validating the results using experimentally collected data, the model was used to perform a sensitivity analysis on the tire performance with respect to six material properties of the tread rubber: thermal conductivity, rubber density, Young's modulus, specific heat, roughness parameter of the rubber, and radii of spherical asperities of the rubber. The results from this study show the sensitivity of the magnitude of the friction coefficient to the rubber material properties. The friction coefficient has a direct relationship with the density of the rubber and has an inverse relationship with Young's modulus, specific heat, and roughness parameter. / Doctor of Philosophy / In order to decrease the number of deaths and injuries caused by driving on icy roads and increase the safety of the vehicle, it is important to improve the tire performance on ice. To this, understanding the effects of different tire and road parameters such as material properties of the rubber, loading condition, and temperature on the tire-ice performance is required. Tire parameters play a very important role in tire performance. Depending on the driving conditions for which a given tire is designed, its parameters must be chosen appropriately In this project, the effects of different tire and terrain parameters such as rubber material properties on tire performance on ice using an experimental and modeling approach have been studied. For the experimental part of this study, several tests were conducted for more than 30 tires with different material properties. The results of this study show what are the most important material properties of the rubber for designing a tire with the best performance on ice. For the modeling part of this study, a semi-analytical model was developed. The model was validated using collected experimental data and was used to predict the performance of the tire by having information about its material and physical properties. The developed model called ATIIM2.0 has several advantages. First, it is a unique model for a complete tire (not a rubber block) that can be used to predict the performance of the tire by using its material properties. In addition, this model can be connected to vehicle models to improve the performance of the vehicle in general. The model developed can be used to predict the temperature changes at the contact patch, the tire friction force, the areas of wet and dry regions, the height of the water film for different ice temperatures, different normal loads, etc. The results from this study coincide with the obtained results from the experiments.

Through a series of carefully designed experiments, characterization and some modeling tools, this work is aimed at studying the role of thermal profiles on different microstructural zones and associated properties like strength and corrosion through a variation of weld parameters, thermal boundary conditions and material temper. Two different alloys belonging to the Al-Cu and Al-Cu-Li system in different temper conditions- peak aged (T8) and annealed (O) were used. A 3D-thermal pseudo mechanical (TPM) model is developed for the FSW process using heat transfer module in COMSOL Multiphysics and is based on a heat source wherein the temperature dependent yield shear stress is used for the heat generation. The precipitation and coarsening model is based on the Kampmann and Wagner theoretical framework and accounts for the competition between the various nucleation sites for both metastable and equilibrium precipitates. The model predicts different precipitate mean radius and volume fraction for the various zones in the friction stir welded material. A model for the yield strength is developed which considers contributions from different strengthening mechanisms. The predictions of the each models have been verified against experimental data and literature. At constant advance per rotation, the peak temperature decreases with a decrease in traverse speed and increases with an increase in tool rotation. Weld properties were significantly affected by choice of thermal boundary conditions in terms of backing plate diffusivity. Weld conditions with a higher peak temperature and high strain rate results in more dissolution of precipitates and fragmentation of constituent particles resulting in a better corrosion behavior for the weld nugget. For a peak aged temper of 2XXX alloys, the weld nugget experiences dissolution of strengthening precipitates resulting in a lower strength and the Heat affected zone (HAZ) experiences coarsening of precipitates. For an annealed material, both the weld nugget and HAZ experiences dissolution of precipitates with an increase in strength in the weld nugget.

Lubricated friction issues are central to all hydraulic actuation systems undergoing motion and any in-depth understanding of the nature of lubricated friction will advance future component design. The classic friction models of hydraulic actuation systems under steady state conditions and their dependency on velocity and temperature have been studied extensively over the past years. A model which is commonly employed to represent the characteristics of friction is that of Stribeck in which the dependency of the friction force is based on velocity alone. However, experimentally, it has been found that lubricated friction is dependent on acceleration. Thus, the Stribeck model can be considered as a subset of a dynamic friction model in which acceleration is zero. Thus, it can be concluded that the Stribeck model is best applied to cases when the change rate of the velocities is very small. This thesis considers the dependency of lubricated friction on acceleration when pressure and temperature changes are relatively constant. As such, the basic hypothesis for this study was proposed as follows: “Lubricated friction in hydraulic actuation systems is not only a function of velocity, but is also a function of both velocity and acceleration”. In this thesis several terms are defined which facilitate the description under which friction models are developed. For example, the term non-steady state friction is used to account for the effect of acceleration on lubricated friction force while in motion. Further, the lubricated friction models are divided into two groups: steady state friction models and non-steady state friction models. Nonlinear friction modeling and measuring methods are reviewed in this dissertation. This review also includes nonlinear lubricated friction modeling in hydraulic actuation systems. A conclusion from this review was that limited research has been done in documenting and explicitly demonstrating the role of acceleration on lubricated friction. The research first introduced a methodology to experimentally measure friction as a function of acceleration and to demonstrate this dependency in the form of a three dimensional graph. A novel technique to experimentally obtain data for the lubricated friction model was introduced. This allowed the lubricated friction forces to be measured as a function of velocity in a continuous manner, but with acceleration being held constant as a family parameter. Two different valve controlled hydraulic actuation systems (VCHAS) were studied under a wide variety of accelerations at constant temperature and pressure. To enable repeatable data collection for the different friction conditions and to accommodate for the effect of hysteresis, a periodic parabolic displacement waveform was chosen which enabled the acceleration to be a family parameter. The second phase of the research introduced a method of representing the data (lubricated friction model) in a lookup table form. The relationship of lubricated friction (in this work, pressure differential, ΔP across the actuator) as a function of velocity and acceleration was presented in a unique semi-empirical 2D lookup table (2D LUT). Limitations of this experimental approach were identified, but the dependency on acceleration was clearly established. The last phase of the study implemented this 2D LUT model into a practical software model of an actuator and demonstrated its accuracy when compared to its experimental counterpart. The semi-empirical model (2D LUT) was experimentally verified by implementing the semi-empirical and Stribeck models into a real time simulation of an actuator and by comparing the experimental outputs against simulated outputs for a common sinusoidal input. A sinusoidal actuator displacement input was chosen to test the simulations as it was not used in the collection of the original data. The output of the simulation was compared to the experimental results and it was evident that for the range in which data could be collected in developing the model, the proposed 2D LUT model predicted an output that was superior to a model which used a standard Stribeck model. It was concluded that the semi-empirical model could be integrated into a simulation environment and predict outputs in a superior fashion when compared to the Stribeck friction model. Thus it was concluded that the stated hypothesis is consistent with the experimental evidence shown by all hydraulic actuators considered. Further, it was also observed that the traditional Stribeck form (steady state dynamic friction) does change with increasing acceleration to the point that the standard breakaway friction almost disappears. It is evident that the 2D LUT is a viable tool for modeling the non-steady state friction of hydraulic actuation systems. The semi-empirical 2D LUT model so developed is a more global representation of hydraulic actuator lubricated friction. In this research, only linear hydraulic actuators were considered; however, the novel nonlinear semi-empirical 2D LUT lubricated friction model can be applied to any actuator (linear and rotary) and provides a new way in which the dynamic friction can be viewed and modeled.

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.