Multiple strain path changes during forming lead to complex geometrically necessary dislocation (GND) development in strain gradient fields, inducing internal stresses that contribute to the Bauschinger effect, residual stresses, and springback which alters the final geometrical shape of the part. In order to analyze and design improved processing routes, models must capture the evolution of these internal stresses. However, most models capture the effects of these stresses via phenomenological approaches that require calibration to each new material and strain path. The development of models that capture the underlying physics at the sub-grain level is underway but requires in-depth studies of dislocation behavior (at the relevant meso length scale) in order to guide and validate them. The novel experimental campaign central to this thesis aims to tackle this problem by capturing unprecedented data of dislocation activity for several sheet metals during multiple strain path deformation. The resultant insights provide a new window into multi-path forming of metals, while also aiding the development and validation of two crystal plasticity (CP) models by collaborators at the University of New Hampshire (UNH). The models incorporate internal stresses at the grain and sub-grain levels, respectively. The hardening response due to strain path change during forming of AA6016-T4 was studied at the macro- and micro-level via combined experiments and an elasto-plastic self-consistent (EPSC) model. The experiments demonstrated that possible recombination and/or redirection of dislocations onto different slip systems under strain path change allowed for a gradual elasto-plastic transition, in comparison to a much sharper response upon continued deformation under the same strain path due to buildup and immediate activation of backstresses. The phenomenological backstress law of the EPSC model underpredicted the yield stress response for the strain path change deformations, possibly due to missing sub-grain GND development and an accurate description of associated backstresses. A more detailed experimental study of multi-path deformation for the AA6016-T4 was required in order to guide development of a strain gradient elasto-visco plasticity self-consistent model (SG-EVPSC); the model includes sub-grain strain gradient fields, and related internal stress fields. Total dislocation and GND density were tracked at various points of the deformation, and a complete 3D statistical volume element was characterized, to enable accurate modeling of the microstructure. The tests revealed a relatively lower yield stress response following strain path change, presumably aided by lower latent hardening than self hardening; the tests then showed a rapid accumulation of dislocations on the newly activated slip systems resulting in much higher final dislocation density without affecting the ductility of the pre-strained material. Interestingly, GND development was dominated by the precipitates instead of grain boundaries. These observations are vital for an accurate forming prediction from CPFEA models. Finally, optimized forming conditions of continuous bending under tension produced a ratcheting strain path resulting in a gradual GND development and a more complete retained austenite transformation in quenched-&-partitioned- and TRIP-assisted bainitic ferritic-1180 steels increasing their ductility by at least 360%.

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<p><strong>Abstract</strong></p> <p>The objective of this research is to develop an understanding of the mechanical behavior, failure and microstructure evolution of aluminum tubes under internal pressure loading, and to delineate the physical and mechanical origins of spatially-localized plastic deformation. Traditional approaches to the study of plastic instabilities, necking and failure have either been based on kinematic considerations, such as finite strain effects and geometric softening, or physics-based concepts. In this study, we develop a framework that combines both approaches to investigate the tube deformation and failure behavior at various loading conditions.</p> <p>A rate-dependent dislocation-based MTS model has been developed to study the tube hydro-forming process at high temperatures and at various strain rates. The development and application of the MTS model led to an advanced industrial application of PRF bottle forming, which has been fully investigated. This simulation shows a good agreement between experimental results and prediction. The model has been used extensively throughout the PRF bottle development, with several patent applications.</p> <p>The crystal plasticity based finite element model is selected to simulate surface roughening and localized necking in aluminum alloy tubes under internal pressure. The measured electron backscatter diffraction (EBSD) data are directly incorporated into the finite element model and the constitutive response at an integration point is described by the single crystal plasticity theory. The effects of the spatial grain orientation distribution, strain rate sensitivity, work hardening, and initial surface topography on surface roughening and necking are discussed. It is demonstrated that while localized necking is very sensitive to both the initial texture and its spatial orientation distribution, the initial surface topography has only a small influence on necking, but a large influence on surface roughness of the formed product.</p> <p>An elastic-viscoplastic based finite element model has been developed to study the necking behavior of tube expansion for rate dependent monolithic materials and laminated materials during dynamic loading. Numerical study shows that a high strain rate sensitivity can significantly delay the onset of necking for both monolithic and laminated sheets, and affect the multiple-neck formation in high speed dynamic loading. The model also shows that higher volume fractions of a clad layer with positive rate sensitivity material in laminated sheet could improve the sheet ductility as well.</p> <p>A commercial FE package, ABAQUS, is employed as a finite element method solver in this research work, and several user subroutines were developed to model various hydro-forming processes. Interfaces between the ABAQUS user subroutine UMAT and the ABAQUS main code are developed to allow further extension of the current method.</p> / Doctor of Philosophy (PhD)

High pressure die casting (HPDC) is both a cost-efficient and high throughput method for making near-net shape castings. 7xxx series aluminum alloys are excellent candidates for manufacturing structural components for significant light-weighting opportunities in the automotive industry. This project explores the development of a new 7xxx series aluminum alloy with iron additions to improve castability. The main objective of this project is to develop an optimized heat treatment process for the new Al-Zn-Mg-Fe alloy to achieve solute homogenization in the primary Al grains. The rationale behind adding iron as an alloying element was presented, as well as an analysis of the Al-Fe intermetallic phases to show their mitigating effects on hot tearing. A detailed analysis of the casting quality was carried out, including detailed microstructural analyses of defects and defect-free castings, correlating process parameters, shot profiles, uniaxial tensile properties, and fractography. Improvements on casting conditions and parameters were suggested. Solution heat treatments were carried out between 0.25 and 24 hours and quenched with forced air. Bulk hardness measurements were obtained following solution heat treatment to determine the arrest times for the precipitation reactions during natural aging. The uniaxial tensile properties of the alloy in the F- and T4-tempers were presented. Microstructural analyses of the alloy were carried out by optical and electron microscopy (SEM), including phase identification, phase fraction, average grain size, and distribution. A predictive model for the homogenization of the solutes in the Al matrix was developed using a one-dimensional diffusion model with spherical geometry, and a MATLAB code was developed to time for complete homogenization. Electron-dispersive X-ray spectroscopy (EDX) line scans were carried out on the F and T4 samples (0.25-2h) and the concentration profiles of Zn and Mg (the diffusing solutes) were extracted and analyzed. The models were verified and validated with experiment data. / Thesis / Master of Applied Science (MASc)

Corrosion of all aluminum microchannel heat exchangers present a challenge in automotive and heating, ventilation, and air conditioning (HVAC) industries. Reproducibility of Salt Water Acetic Acid Test (SWAAT) has been questioned and a need to new corrosion tests with better reproducibility has risen. Cyclic polarization, that is an electrochemical test, was explored for its suitability for the assessment of AA 3102 tube material that is currently a popular aluminum alloy used in manufacturing of heat exchanger. Corrosive electrolytes containing 3.5 % sodium chloride with 0.5 % ammonium sulfate (high chloride) or 0.5 % sodium chloride with 3.5 % ammonium sulfate (high sulfate) at their pH or acidic (pH=4) were used to measure corrosion potential (Ecorr), protection potential (Epp), pitting potential (Epit), Tafel constants (βa and βc), corrosion rate (mpy). Corrosive electrolyte used in SWAAT test (4.2% Sea Salt at pH 2.9) was also used to compare corrosion resistance of AA 3102 in SWAAT electrolyte compared to the other electrolytes used in this research. Scanning electron microscopy (SEM) was used to observe and document sample surface corrosion damage after each electrochemical test on all samples. Results of the cyclic polarization tests indicated that SWAAT electrolytes was the most aggressive electrolyte resulting in highest corrosion rates compared to all other electrolytes used in this investigation. SEM results indicated AA 3102 alloy exhibited higher pitting tendency in electrolytes with high sodium chloride whereas high sulfate electrolytes cause appearance of uniform corrosion surface damage on this alloy. Both high sulfate and SWAAT electrolytes showed intergranular corrosion but high chloride electrolyte showed severe pitting of AA 3102. Mohammad Navid Dorreyatim- Cyclic Polarization of AA 3102 in Corrosive Electrolytes Containing Sodium Chloride and Ammonium Sulfate. Master of Science (Mechanical and Energy Engineering), December 2016, 98 pp., references, 31 titles.