Aluminum alloy 6061-T6 is a common engineering material used in aerospace, automotive, structural applications. Despite its wide use, little has been published about the effects of damage from surface corrosion on its fatigue life. An investigation was performed where 6061-T6 extrusions were exposed to a 3.5% NaCl solution at pH 2 for 2 days and 24 days. The length of time and pH were chosen in order to create distinct surface flaws. The effect of these flaws on the fatigue life was then investigated and analyzed using scanning electron microscopy (SEM) and Weibull statistics. It was determined that samples corroded for both 2-days and 24-days exhibit fatigue lives that can be described using a 3-parameter Weibull distribution. The result of which was the determination of a threshold value for fatigue as well a general understanding of flaw geometry.

<div>Aluminum 6061 is extensively used in industry and welding and additive manufacturing (AM) of Al6061 offer flexibility in manufacturing. Solid-state welding and AM processes can overcome the shortcomings of fusion-based processes, such as porosity and hot cracking. In this thesis, friction stir welding and friction stir additive manufacturing, which are both based on the concepts of friction stir processing (solid-state), were studied. The welding parameters for a sound weld during friction stir welding of Al6061-T6 alloy were determined based on the experimental and numerical analysis. Formation of tunnel defects and cavity defects was also studied. A Coupled Eulerian-Lagrangian (CEL) finite element model was established to analyze the process, where the workpiece was modeled as an Eulerian body, and the tool as Lagrangian. The model was validated by conducting experiments and correlating the force measured by a three-axis dynamometer. The experimentally validated simulation model was used to find an optimum parameter set for the sound weld case.</div><div>To demonstrate the friction stir additive manufacturing process, a 40 mm × 8 mm × 8 mm (L×B×H) material was fabricated by adding five 1.6 mm thick plates. A similar coupled Eulerian-Lagrangian based finite element model was used to predict the effects of sound process parameters, such as the tool’s rotational speed and the translational speed. The temperature predicted by the model was used to predict the microhardness distribution in the sample and to further elucidate the hardness change in the weld zone, which showed a good agreement with the experimental results. The microstructure of the samples was analyzed, and the mechanical properties of the additive manufactured samples were characterized and compared with those of other AM techniques via tensile tests and tensile shear tests.</div>

This thesis is titled "A Framework for Enhancing the Accuracy of Ultra Precision Machining." In this thesis unwanted relative tool / workpiece vibration is identified as a major contributor to workpiece inaccuracy. The phenomenon is studied via in situ vibrational measurements during cutting and also by the analysis of the workpiece surface metrology of ultra precision diamond face turned aluminum 6061-T6. The manifestation of vibrations in the feed and in-feed directions of the workpiece was studied over a broadband of disturbance frequencies. It is found that the waviness error measured on the cut workpiece surface was significantly larger than that caused by the feed marks during cutting. Thus it was established that unwanted relative tool / workpiece vibrations are the dominant source of surface finish error in ultra precision machining. By deriving representative equations in the polar coordinate system, it was found that the vibrational pattern repeats itself, leading to what are referred to in this thesis as surface finish lobes. The surface finish lobes describe the waviness or form error associated with a particular frequency of unwanted relative tool / workpiece vibration, given a particular feed rate and spindle speed. With the surface finish lobes, the study of vibrations is both simplified and made more systematic. Knowing a priori the wavelength range caused by relative tool / workpiece vibration also allows one to extract considerable vibration content information from a small white light interferometry field of view. It was demonstrated analytically that the error caused by relative tool / workpiece vibration is always distinct from the surface roughness caused by the feed rate. It was also shown that the relative tool / workpiece vibration-induced wavelength in the feed direction has a limited and repeating range. Additionally, multiple disturbance frequencies can produce the same error wavelength on the workpiece surface. Since the meaningful error wavelength range is finite given the size of the part and repeating, study then focussed on this small and manageable range of wavelengths. This range of wavelengths in turn encompasses a broadband range of possible disturbance frequencies, due to the repetition described by the surface finish lobes. Over this finite range of wavelengths, for different machining conditions, the magnitude of the waviness error resulting on the cut workpiece surface was compared with the actual relative tool / workpiece vibrational magnitude itself. It was found that several opportunities occur in ultra precision machining to mitigate the vibrational effect on the workpiece surface. The first opportunity depends only on the feed rate and spindle speed. Essentially, it is possible to force the wavelength resulting from an unwanted relative tool / workpiece vibration to a near infinite length, thus eliminating its effect in the workpiece feed direction. Further, for a given disturbance frequency, various speed and feed rate combinations are capable of producing this effect. However, this possibility exists only when a single, dominant and fixed disturbance frequency is present in the process. By considering the tool nose geometry, depth of cut, and vibrational amplitude over the surface finish lobe finite range, it was found that the cutting parameters exhibit an attenuating or filtering effect on vibrations. Thus, cutting parameters serve to mitigate the vibrational effect on the finished workpiece over certain wavelengths. The filter curves associated with various feed rates were compared. These filter curves describe the magnitude of error on the ultra precision face turned workpiece surface compared with the original unwanted tool / workpiece vibrational magnitude. It was demonstrated with experimental data that these filter curves are physically evident on the ultra precision diamond face turned workpiece surface. It was further shown that the surface roughness on the workpiece surface caused by the feed rate was reduced with relative tool / workpiece vibrations, and in some cases the feed mark wavelength was changed altogether. Mean arithmetic surface roughness curves were also constructed, and the filtering phenomenon was demonstrated over a broadband of disturbance frequencies. It is well established that a decrease in the feed rate reduces the surface roughness in machining. However, it was demonstrated that the improved surface finish observed with a slower feed rate in ultra precision diamond face turning was actually because it more effectively mitigated the vibrational effect on the workpiece surface over a broadband of disturbance frequencies. Experimental findings validated this observation. By only considering the effect of vibrations on the surface finish waviness error, it was shown that the workpiece diamond face turned with a feed rate of 2 {tm / rev has a mean arithmetic surface roughness, Ra , that was 43 per cent smaller than when a feed rate of 10 μm / rev was used. / Thesis / Doctor of Philosophy (PhD)

The purpose of this project was to show the updated procedures and to make additions to the computer program called Tmin designed by E. I. DuPont De Nemours and Company. This program is used as a screening tool for determining the largest of the minimum pipe-wall thicknesses in a piping system. This project involved several additions that will be released in the next version of the Tmin computer program. The first major additions to be implemented are four alternating Stress-to-Number of cycles curves: Aluminum 1100, Aluminum 3003-0, Aluminum 6061-T6, and Nickel 200. In addition, procedures of the ASME for fatigue curve analysis and implementation of fatigue data were investigated. These four stress-to-number of cycles (S-N) fatigue curves were added to Tmin's internal Microsoft Access® database. Next, a 2-D vertical piping span configuration was incorporated. Finally, DuPont required a Microsoft Word® document output of the pipe-wall thickness data including the piping span model information. Other user-friendly additions were included. Since this computer program was to be American Society of Mechanical Engineers (ASME) compliant, a study of the ASME Pressure Vessel and Piping standards and codes was made to determine how pipe-wall thickness calculations were to be processed. The 2-D vertical piping span calculation procedures were investigated. Once the 2-D vertical piping span analysis was complete, the largest pipe-wall thickness value calculated were passed to a Microsoft Word® document. The last implementation is the inclusion of help files. Help file button additions in all input boxes allowed for the user to know exactly what was needed before a data entry was made. / Master of Science

The Cold-Gas Dynamic-Spray process also known as Cold Spray (CS) has been researched for three decades. The CS process is a solid-state deposition technique via supersonic velocity of powder particles at a temperature significantly below the melting point of the spray material. This thesis presents background on the overall CS process parameters, and additional information on the microstructural and mechanical properties of typical Cold Sprayed materials.This Thesis primarily presents a study on the microstructural annealing response of CS Al 6061. It should be noted that for this study, the term “annealing” is used in the sense of the classical metallurgical definition of annealing, and not a specific temper designation for the 6061 alloy. Cross sections of CS Al 6061 were imaged with a scanning electron microscope (SEM) in secondary electron (SE), backscatter electron (BSE), and electron backscatter diffraction (EBSD) imaging mode for quantitative and qualitative information on the grain size and orientation of the CS microstructure. The detailed SE, BSE and EBSD mode images present the grain size and grain orientation of the original powder, as received (AR) state and after heat treating at 200°C for 1 hour, 10 hours, and 100 hours. Three different regions, characterized with distinctly differing microstructures, are labeled as low, medium, and high deformation regions, and their microstructures, and evolving features are discussed. Vickers microhardness testing are performed to examine the differences in hardness values between different heat treatments, and for correlation with the level of deformation and grain refinement in the microstructure. SEM imaging was used in BSE mode to correlate microhardness variation to the different regions within the CS microstructure.

Industrial products of high-strength Al-alloys are currently manufactured by thermo-mechanical processes, which are only applicable in the integrated plants requiring high investment cost. Moreover, reduction of the average grain size not less than 10 &mu / m and re-adjustment of process parameters for each alloy type is evaluated as disadvantage. Therefore, recently there have been many research studies for development of alternative manufacturing techniques for aluminum alloys. Research activities have shown that it is possible to improve the strength of Al-alloys remarkably by severe plastic deformation which results in ultra-fine grain size. This study aims to design and manufacture the laboratory scale set-ups for severe plastic deformation of aluminum alloys, and to characterize the severely deformed samples. The stages of the study are summarized below: First, for optimization of die design and investigation of parameters affecting the deformation finite element modeling simulations were performed. The effects of process parameters (die geometry, friction coefficient) and material properties (strain hardening, strain-rate sensitivity) were investigated. Next, Equal Channel Angular Pressing (ECAP) system that can severely deform the rod shaped samples were designed and manufactured. The variations in the microstructure and mechanical properties of 2024 Al-alloy rods deformed by ECAP were investigated. Finally, based on the experience gained, a Dissimilar Channel Angular Pressing (DCAP) system for severe plastic deformation of flat products was designed and manufactured / then, 6061 Al-alloy strips were deformed. By performing hardness and tension tests on the strips that were deformed by various passes, the capability of the DCAP set-up for production of ultra-fine grain sized high-strength aluminum flat samples were investigated.

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