Gotowa bibliografia na temat „AA 2219”

Limitation in penetration depth is a concern in conventional TIG welding process. To improve penetration capability of TIG process, both Activated TIG (ATIG) and Flux Bounded TIG (FBTIG) are investigated in aluminum alloy AA 2219 T87. Undesirable arc wandering and cracking tendency are observed in ATIG welds. Microstructural investigation reveals ATIG welds are prone for liquation cracks. Morphology of the cracks along with the attributable factors are explained with optical and SEM (Scanning Electron Microscope) micrographs. Energy Dispersive Spectroscopy (EDS) results are also presented to explain the solute enrichment in the grain boundaries of the ATIG welds. FBTIG is found to produce good quality welds and is more suitable for welding aluminum alloys. Key words: Flux Assisted TIG; ATIG; FBTIG; Penetration Improvement; Microstructure; AA2219.

This study presents new findings on the underlying failure mechanism of thick dissimilar electron-beam (EB) welds through a study on the AA 2219-AA 5083 pair. Contrary to the prior studies on EB welding of thin Al alloys, where liquation in the grain boundaries (GBs) in the partially melted zone (PMZ) was not observed, the present investigation for thick EB welds reports both liquation and increased segregation of Cu in the PMZ. The work is thus directed towards understanding the unusual observation in the PMZ of thick EB weld through investigation of the microstructural variation across the various regions of the produced weld. The microstructural results are correlated with the mechanical properties of the weld, i.e., hardness variation and tensile response. Results of this investigation suggest that unlike the convention that EB welding produces sound dissimilar Al welds, the weld performance for thick EB Al welds is affected by the heat input, the associated cooling rates, and most importantly by the base material thickness. Extensive liquation and Cu segregation induced failure in the PMZ on the AA 2219 side of the dissimilar weld. The underlying failure mechanism is explained through a heat-transfer analysis. Beyond a certain plate thickness, the heat transfer changes from two to three-dimensional. As a result, retarded cooling promotes liquation and Cu segregation in thick EB welds.

Friction Stir Processing (FSP) is emerging as one of the most competent Severe Plastic Deformation (SPD) methods for producing bulk ultra-fine grained materials with improved properties. The significant advantage of FSP is that it can be used for localized microstructural modification which is not possible with the other common SPD techniques such as Equal Channel Angular Processing (ECAP), High Pressure Torsion (HPT), and Accumulative Roll Bonding (ARB). The process is derived from the basic principles of Friction Stir Welding (FSW), a solid state welding technique developed for the high strength aluminium alloys used in structural applications. In FSP, a non-consumable rotating tool with a shoulder and a pin is traversed along the region on the work-piece which is to be modified. In the present investigation on FSP, two heat treatable aluminium alloys with different hot deformation behaviour, 2024 (Al-Cu-Mg) and 2219 (Al-Cu) and a strain hardenable alloy 5086 (Al-Mg) has been considered. FSP involves complex thermo-mechanical interactions and hence the optimization of process parameters is an important aspect to be considered for a successful processing. The number of process parameters involved is more in FSP and the three most important parameters are tool rotational speed, tool plunge depth (normal load on the work piece) and the tool traverse speed. These parameters are varied for a fixed tool geometry and tool tilt angle in a custom-built FSW/FSP machine. A parametric study has been carried out in order to have a clearer picture on the relative importance of various parameters by using tools of different pin lengths. The tool plunge depth and tool rotational speed are also varied in the parametric study. It has been observed that tool plunge depth is the most important parameter for the FSP of high strength aluminium alloys and the first parameter to be optimized. As per the inputs obtained from this parametric study, a systematic experimental procedure has been developed (a bottom-up approach) for optimizing the most important process parameters of FSP. The optimal process parameters obtained from the experimental bottom-up approach has helped in achieving bulk tensile strength higher than the starting material strength for the strain hardenable alloy 5086-O. In heat treatable alloys, due to the presence of a weaker heat affected zone the achievable strength in a single pass FSP were 93% and 80-85% of the starting material strength in the alloys 2024 and 2219 respectively. Micro-tensile testing of the samples taken from the nugget zone of the alloy 2024 indicated an ultimate tensile strength of 1.3 times the starting material strength. This strength increase is attributed to the combined effects of grain size strengthening and precipitation hardening. FSP has been perceived as a grain refinement technique and hence the most important region in any processed sample is the nugget zone. Due to the continuous stirring of the tool pin at high rotation rates, it is possible that different regions in the nugget zone can develop varied microstructure and crystallographic texture. The nugget zone of the optimally processed samples are characterized in detail using the advanced characterization techniques such as Scanning Electron Microscopy (SEM), Electron Back-Scattered Diffraction (EBSD), X-Ray Diffraction (XRD) and Electron Probe Micro-Analyzer (EPMA) in order to understand the underlying micro-mechanisms of microstructure and texture evolution. Micro-texture studies on the alloys revealed gradients in textures across the thickness with the dominance of shear texture components. The bulk texture is weaker in all the three alloys. Bulk texture measurements revealed that the texture development during FSP is an alloy independent phenomenon. The dominant texture component observed is different in heat treatable and strain hardenable alloys. The dominant component of texture is identical in both the heat treatable alloys irrespective of the differences in optimal process parameters and the thickness of the plates used. Microstructural evolution during FSP is more of an alloy dependent phenomenon. Particle Stimulated Nucleation (PSN) and Strain Induced Boundary Migration (SIBM) are observed as the dominant nucleation mechanisms of Dynamic Recrystallization (DRX) in the heat treatable and strain hardenable alloys respectively. Normal grain growth through the Burke and Turnbull mechanism is observed with the presence of few larger grains in the microstructure caused by geometrical coalescence. DRX has been observed to occur through separate nucleation and grain growth stage in all the three aluminium alloys and hence indicative of a discontinuous process. Bulk texture development during FSP has been correlated to the microstructure evolution with the mechanisms of PSN and SIBM both weakening the textures in all the alloys. In order to expand the understanding as a commercially viable technique and studying the stability of FSP microstructure and texture, multiple processing routes have been employed. In the Multi-Pass FSP (MP-FSP), the processing is carried out at the same location and the objective is to study the stability of the processed samples under extreme conditions of strain and temperature. In Multi-Track FSP (MT-FSP), an overlap ratio of 0.33 is selected for the successive passes which will allow partial nugget zone penetration. MT-FSP can be used for producing large volume of fine grained materials. It is observed that the microstructure and crystallographic texture is stable under the mild and extreme conditions of strain and temperature. Subsequent heat treatment studies after FSP in the alloy 2024 confirmed that the processed microstructure is stable up to temperatures as high as 723K (450°C). These results are indicative of the advantage of FSP as a successful materials processing technique in which the retained lower strain energies leading to the development of a stable microstructure and texture. Compressive residual stresses are observed at different regions in the nugget zone of all the alloys after FSP. This is attributed to the combined effects of a solid state processing route and the optimal selection of process parameters.

A reduction in mechanical properties has been observed in Friction-Stir-Welded (FSW) Aluminum panels. This reduction in strength has generally been attributed to Residual Oxide Defect (ROD). From NASA experience it was also found that certain processing parameters would yield these reduced mechanical properties. The strength of FSW Aluminum panels generally decreases with increasing tool travel rate, decreasing rotation speed, and offset of the weld seam to the retreating side of the FSW tool. The microstructure of welds exhibiting these strength reduction as well as welds that behaved as expected were examined to determine microstructural effects of processing parameters. Therefore the evolutions of microstructural properties are immensely important to understand and evaluate to avoid any catastrophic failures due to the defects arising from welding operations. Scanning Electron Microscopy shows that these weld conditions are accompanied by large precipitates along the grain boundary for both AA-2219 and AA-2195 FSW welded samples. Transmission Electron Microscopy (TEM) also shows the precipitates to be “theta particles (Al2Cu)” and intermetallics in the AA-2219; and T1 (Al2CuLi), and TB particles in the AA-2195. The large size and heavy distributions of these precipitates, especially on the advancing side of the weld seam may influence these properties. It is determined that the existence of ROD in the samples must be analyzed systematically and carefully through the evolutions of microstructures, if catastrophic failures are to be avoided during service conditions. A more complete understanding of this phenomenon is necessary to ensure consistent and predictable weld properties thereby reducing or eliminating the risk of unforeseen failures.

The suitability of aluminum alloys in a vast majority of engineering applications forms the basis for the need to understand the mechanisms responsible for their deformation and failure under various loading conditions. The material investigated in this study is AA 2219-T8 aluminum alloy. Supplied by the NASA Research Center, with high strength to weight ratio and corrosive resistance. Containing a unique mixture of aluminum, copper, and other trace elements, this alloy has potential applications in multiple fields including aerospace, defense, and commercial industries. In this paper, the dynamic high strain rate impact deformation of the AA2219-T8 aluminum alloy was performed using the split Hopkinson pressure bars. The evolution of localized strain in the aluminum samples during the deformation process obtained using high speed digital cameras is reported. Microstructural analysis of deformed aluminum samples was also performed using optical microscopes in order to determine the influence of impact strain rate on localized strain along narrow adiabatic shear bands in the AA2219-T8 aluminum alloys. Results obtained indicate that peak flow stress in the deformed aluminum sample depends on the strain rate at which the deformation test was performed. The non-uniformity of the strain obtained using the digital image correlation as deformation time progresses shows two distinct areas of non-uniform strains that may be indicating potential sites for the formation of adiabatic shear bands in the tested samples.

A revolutionary hydrothermal steam generator is being developed by a federal, state university and industry partnership in the US to enhance economic growth and trade. The new generator is designed to accept solutions and slurries without corrosion and deposition on heat transfer surfaces up to the supercritical conditions of water, above 221 bar (3205 psia) and 374 C (705 F). The generator will produce steam from low quality water, such as from geothermal sources, for increased electric power generation. Water treatment costs and effluents will be eliminated for “zero discharge.” To improve efficiency and limit carbon dioxide and other emissions, the new steam generator will be tested for converting wastewater slurries of low-cost fuels and “negative value” wastes such as hazardous wastes, composted municipal wastes and sludges, to clean gas turbine fuel, hydrocarbon liquids, and activated carbon. Bench-scale results at sub- and supercritical conditions for lignite, refuse derived fuel, tire rubber and activated carbon are presented. An advanced continuous-flow pilot plant is being designed to test the generator over a wide range of operating conditions, including slurry feed up to 30 percent solids. Demonstration of the hydrothermal steam generator will be followed by design and construction of combined-cycle energy systems.