Magnesium is the lightest metal with a very high specific strength. However, its practical applicability is limited by its toughness and reliability. Mg, being HCP has low ductility. This makes the improvement of toughness a grand challenge in Mg alloys. Friction stir processing (FSP) is a thermomechanical technique used to effect microstructural modification. Here, FSP was utilized to affect the toughness of WE43 sheets through microstructural modification. Room temperature Kahn-type tests were conducted to measure the toughness of WE43 sheets. Microscopic techniques (SEM, TEM) was utilized to study the effect of various microstructural factors like grain size, texture, constituent particles, precipitates on crack initiation and propagation. Tensile properties were evaluated by mini-tensile tests. Crack growth in WE43 sheets was also affected by mechanics and digital image correlation (DIC) was utilized to study the plastic zone size. The underlying mechanisms affecting toughness of these sheets were understood which will help in formulating ways in improving it. WE43 nanocomposites were fabricated via FSP. Uniform distribution of reinforcements was obtained in the composites. Improved mechanical properties like that of enhanced strength, increased hardness and stiffness were obtained. But contrary to other metal matrix composites which show reduction in ductility with incorporation of ceramic reinforcements, the nanocomposites showed good strength-ductility combination. The composites were precisely characterized and mechanisms governing this property were studied. The nano-length of the reinforcements was observed to be the main criteria and the dislocation-particle interaction, the main reason behind the strength-ductility property.

Temperature control has been implemented in friction stir processing and has demonstrated the ability to give improved process control. In order to have optimal control of the process, the parameters of the system to be controlled must be accurately identified. The system parameters change with tool geometry and materials, workpiece materials, and temperature. This thesis presents the use of the relay feedback test to determine the system parameters. The relay feedback test is easy to use and promotes system stability during its use. The results from the relay feedback test can be used to determine controller gains for a PID controller. The use of this method, as well as the quality of the resulting control is demonstrated in this paper.

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This dissertation presents an assortment of research aimed at understanding the composition-dependence of deformation behavior and the response to thermomechanical processing, to enable efficient design and processing of low stacking fault energy (SFE) high entropy alloy (HEAs). The deformation behavior and SFE of four low SFE HEAs were predicted and experimentally verified using electron microscopy and in-situ neutron diffraction. A new approach of employing a minimization function to refine and improve the accuracy of a semi-empirically derived expression relating composition with SFE is demonstrated. Ultimately, by employing the minimization function, the average difference between experimental and predicted SFE was found to be 2.64 mJ m-2. Benchmarking with currently available approaches suggests that integrating minimization functions can substantially improve prediction accuracy and promote efficient HEA design with expansion of databases. Additionally, in-situ neutron diffraction was used to present the first in-situ measurement of the interspacing between stacking faults (SFs) which were correlated with work hardening behavior. Electron transparent specimens (< ~100 nm thick) were used in order to resolve nanoscale planar faults instead of the thicker sub-sized specimens (on the order of millimeters in thickness) which exhibit the classical stages III work hardening behavior characteristic of low SFE metals and alloys. The present study demonstrates these characteristic dimensions of SFs can be tracked in real-time using neutrons or high-energy x-rays. SFs have also been shown to act as barriers to dislocation motion and thus contribute to strengthening and sustained work hardening during deformation.

Al 7050-T7451 alloy with good combinations of strength, stress corrosion cracking resistance and toughness, is used broadly in the aerospace/aviation industry for fatigue-critical airframe structural components. However, it is also considered as a highly anisotropic alloy as the crack growth behavior along the short transverse direction is very different from the one in the long transverse direction, due to the inhomogeneous microstructure with the elongated grains distributed in the work material used in the sheet/plate applications. Further processes on these materials are needed to improve its mechanical and material properties and broaden its applications. The material with ultra-fine or nano grains exhibits improved wear and corrosion resistance, higher hardness and better fatigue life, compared to the one with coarse grains. In recent times, the development of novel processing technologies has gained great attention in the research community to enhance the properties of the materials employed in the aerospace, biomedical, precision instrument, automotive, nuclear/power industries. These novel processing technologies modify the microstructure of this alloy and improve the properties. The aim of this dissertation is to investigate the effects of cryogenic processes, including friction stir processing (FSP), machining and burnishing, on Al 7050-T7451 alloy to solve the inhomogeneity issue and improve its surface integrity. FSP is applied to modify the microstructure of Al 7050-T7451 alloy for achieving more homogeneous structure with near ultra-fine grains (UFG) which were less than 2 µm, particularly in cryogenic FSP with liquid nitrogen as the coolant. Approximately 10% increase could be observed from the hardness measurement from the samples processed by cryogenic FSP, in contrast to dry FSP. Also, the texture change from Al (200) to Al (111) could be achieved in all the samples processed by dry and cryogenic FSP. Cryogenic machining and burnishing processes were also applied to enhance the surface integrity of the manufactured components with near-UFG structure. The highest cutting temperature was reduced by up to 44.7% due to the rapid cooling effect of liquid nitrogen in cryogenic machining, compared with dry machining. Nano grains were produced in the refined layers induced by cryogenic burnishing. And, up to 35.4% hardness increase was obtained within the layer depth of 200 µm in the cryogenically-burnished surface. A numerical finite element method (FEM) model was developed for predicting the process performance in burnishing. Less than 10% difference between the experimental and predicted burnishing forces was achieved in the simulation of cryogenic burnishing, and reasonable predictions were also achieved for temperatures, severe plastic deformation (SPD) layers.

Commercially pure titanium can take up to six months to successfully manufacture a six-inch in diameter ingot in which can be shipped to be melted and shaped into other useful components. The applications to the corrosion-resistant, light weight, strong metal are endless, yet so is the manufacturing processing time. At a cost of around $80 per pound of certain grades of titanium powder, the everyday consumer cannot afford to use titanium in the many ways it is beneficial simply because the number of processing steps it takes to manufacture consumes too much time, energy, and labor. In this research, the steps it takes from the raw powder form to the final part are proposed to be reduced from 4-8 steps to only 2 steps utilizing a new technology that may even improve upon the titanium properties at the same time as it is reducing the number of steps of manufacture. The two-step procedure involves selecting a cylindrical or rectangular die and punch to compress a small amount of commercially pure titanium to a strong-enough compact for transportation to the friction stir welder to be consolidated. Friction stir welding invented in 1991 in the United Kingdom uses a tool, similar to a drill bit, to approach a sample and gradually plunge into the material at a certain rotation rate of between 100 to 2,100 RPM. In the second step, the friction stir welder is used to process the titanium powder held in a tight holder to consolidate into a harder titanium form. The resulting samples are cut to expose the cross section and then grinded, polished, and cleaned to be observed and tested using scanning electron microscopy (SEM), electron dispersive spectroscopy (EDS), and a Vickers microhardness tester. The results were that the thicker the sample, the harder the resulting consolidated sample peaking at 2 to 3 times harder than that of the original commercially pure titanium in solid form at a peak value of 435.9 hardness and overall average of 251.13 hardness. The combined results of the SEM and EDS have shown that the mixing of the sample holder material, titanium, and tool material were not of a large amount and therefore proves the feasibility of this study. This study should be continued to lessen the labor, energy, and cost of the production of titanium to therefore allow titanium to be improved upon and be more efficient for many applications across many industries.

Friction stir processing (FSP) has been developed based on the basic principles of friction stir welding (FSW), a solid-state joining process originally developed for aluminum alloys. What is attractive about FSP is that it can be incorporated in the overall manufacturing cycle as a post-processing step during the machining operation to provide localized modification and control of microstructures in near-surface layers of metallic components. FSP has emerged as an important post-processing technique, and has been identified as a process that may have a high impact, and perhaps is a disruptive manufacturing process. In this study, FSP has been applied to Al cast alloy A206, which is a high strength, widely used cast alloy in the manufacturing industry. Motivations behind this work are to (1) investigate the feasibility of FSP on manipulating the cast microstructure and strengthening the material, and (2) to explore the viability of FSP to produce a localized particle reinforced zone in cast A206 aluminum components. The thesis will show that we have optimized FSP for processing of Al alloys to locally manipulate the cast microstructure, eliminate casting defects, and attain grain refinement and second phase homogenization. We have established the mechanism leading to the microstructure evolution and have evaluated the resultant mechanical properties, i.e. hardness, tensile property and fatigue properties. We have also synthesized a localized composite material in the A206 work piece with three different reinforcement materials via FSP. These results will be presented and discussed.

In this thesis, the relationship among friction stir processing (FSP) parameters, microstructure evolution, texture development, and mechanical hehavior of AZ31B Mg alloy was investigated. First of all, in order to reveal the correlation among the deformation conditions, dynamic recrystallization (DRX) mechanisms, and microstructure evolution in the Mg alloy, hot compression tests at a wide range of Zener-Hollomon parameter (Z) values were conducted. Through optical microscopic examination, it was found out that above a critical Z value, twinning influences the DRX process resulting in a more effective grain refinement, which is manifested in a significant change in the slope of the Z-drec relationship, where drec is the recrystallized grain size. Moreover, EBSD examination revealed that the twinning also contributed to a distinct change in the recrystallization texture. Compression tests were performed along both through-thickness and in-rolling-plane directions of the plate to study the orientation dependency of twinning activities and its influence on the DRX process. X-ray line profile analysis (XLPA) provides further insights by highlighting the differences in the dislocation density/types, subgrain sizes, and twin densities during the DRX processes operating with or without the twinning. Secondly, the constitutive behaviour study was applied to the investigation of microstructure evolution during FSP. By varying the key FSP parameters systematically, i.e. rotation and travel rates of the tool, a series of FSP specimens were prepared with a wide range of thermo-mechanical inputs in terms of Z. The resulting tensile behavior in the stir zone (SZ) showed a dramatic change as a function of Z, caused by a systematic change in the texture within SZ measured by neutron diffraction. A three-dimensional transient model was developed to investigate the detailed deformation history including the temperature and strain rate profiles and material flow pattern during FSP of the Mg alloy. Such deformation history can be combined with the constitutive study from the compression tests in order to analyze the developments of micro-texture and DRX grains during FSP, which will, in turn, dominate the mechanical properties. Based on the studies above, new fundamental understandings were gained on the governing mechanisms for the deformation and recrystallization processes during FSP and the influence of thermo-mechanical input during FSP on ductility enhancement in the Mg alloy.

In this study, mechanical properties of Al-Zn alloys were conducted, with various parameters including Zn contents, grain size, and tensile strain rate. Experimental samples were all manufactured with friction stir processing method. Samples of Al-Zn alloys with the grain size of 1.5£gm, 1£gm, or 0.5£gm and five Zn concentration were pulled in tension at strain rate of 10-3s-1,10-4s-1 and 10-5s-1 . The data set were then used to draw engineering and true tensile stress vs. strain curves , flowing stress vs. Zn contents curves, Hall-Petch equation curves, m vs. Zn contents curves and m vs. grain size curves. Quantitative analysis were conducted to discover that solid solute softening and inverse Hall-Petch relation were present in Al-Zn alloys, which were more prominent at slower tensile strain rate when grain size was less than 1£gm and the Zn contents was higher than 10wt%. Quantitative analysis of strain rate sensitivity (m) showed the trends of increasing value of m with higher Zn contents and smaller grain sizes when solid solute softening and inverse Hall-Petch relation were present. The high grain-boundary diffusion coefficient of Zn which accelerates the efficiency of dynamic recovery are considered the main reason. The effect gets more prominent with increasing Zn contents , smaller grain size , and slower tensile strain rate. For Zn concentration higher than 10wt%, dynamic recovery may drive inverse Hall-Petch relation to appear when grain size is about 1£gm large.

Friction stir processing (FSP) is applied to produce particulate-reinforced aluminum matrix composites with ultrafine grained structure from elemental powder mixtures of Al-Cu, Al-Ti and Al-Si. The microstructures of the composites were characterized by the use of XRD, SEM and TEM. Microhardness, tensile and compressive tests were performed to evaluate the mechanical properties of these composites. The mechanisms of microstructure evolution in FSP and the strengthening mechanisms in these composites are discussed. In the Al-Si system, the Si particles were broken and uniformly distributed in the stir zone by the application of multiples-pass FSP. The average size of Si particles and Al grains were refined to below ~2