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At least 577 records · Page 32
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Neutron diffraction illustrates residual stress behavior of welded alloys used as radioactive confinement boundary

Corrosion-resistant welded alloys are frequently used as a leak-tight boundary in critical applications that require confinement of hazardous and/or radioactive substances, including an increasing population of spent nuclear fuel (SNF) canisters. The behavior of residual stresses generated as a result of irregular elastic–plastic deformation during processes such as welding is one of today's key issues to a full understanding of the aging mechanisms that may compromise the confinement boundary. Whether such processes and any subsequent weld repairs, not subjected to post-weld heat treatment, would negatively affect the initial material by introducing through-thickness tensile stresses remains an open question. Here we report the first residual stress measurements using neutron diffraction on the welded joints of a SNF canister. We found significant tensile residual stresses in the as welded sample, indicating that initiation and through-thickness growth of cracks may be possible. Following repair, we observed a stress redistribution and introduction of beneficial compressive stresses. We anticipate our results will improve understanding of confinement susceptibility to aging and guide improvements in repair techniques.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Effect of laser welding parameters on the austenite and martensite phase fractions of NiTi

Although laser welding is probably the most used joining technique for NiTi shape memory alloys there is still a lack of understanding about the effects of laser welding parameters on the microstructural induced changes: in both the heat affected and fusion zones martensite may be present, while the base material is fully austenitic. Synchrotron X-ray diffraction was used for fine probing laser welded NiTi joints. Through Rietveld refinement the martensite and austenite phase fractions were determined and it was observed that the martensite content increases towards the weld centreline. This is related to a change of the local transformation temperatures on these regions, which occurs due to compositional variation in those regions. The martensite phase fraction in the thermally affected regions may have significant implications on functional properties on these joints. - Highlights: •Synchrotron X-ray diffraction was used for fine probing of the microstructure in laser welded NiTi joints. •Rietveld refinement allowed to determine the content of martensite along the heat affected and fusion zones. •The martensite content increases from the base material towards the weld centreline.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Formation mechanisms of periodic longitudinal microstructure and texture patterns in friction stir welded magnesium AZ80

Many studies of friction stir welding have shown that periodicity of metal flow around the tool pin may result in the formation of periodic differences in microstructure and texture in the weld nugget area correlated with the weld pitch. The current work investigates the periodicity of magnesium weld microtexture in the nugget region and its association with material flow using optical and electron microscopy. Two welds created in AZ80 at different processing conditions are presented in detail, one illustrating periodic longitudinal texture change, and one showing for the first time that periodic variations in texture, grain size, or composition are not defining features of periodic nugget flow. While nugget texture is dominated by shear deformation, it was found here to be affected to a lesser degree by compaction of material behind the welding tool, which led to reduction in intensity of the shear texture fiber. The decreased tendency for magnesium based alloys to form periodic patterns as compared to aluminum based alloys is explained with reference to the shear textures. - Highlights: •It is shown here that periodic material flow in the nugget does not necessitate longitudinal texture patterns. •Longitudinal texture patterns are shown to be present or absent in Mg AZ80 based on processing conditions. •Texture in the nugget is mainly dictated by shear deformation, but has measurable effects from other deformation modes. •Explanation of why longitudinal texture change is frequently reported in aluminum but not magnesium alloys is provided. •A new vector visualization of material flow based on EBSD data analysis is shown.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Interfacial structure and bonding mechanism of weld seams during porthole die extrusion of aluminum alloy profiles

Highlights: • Interfacial structures and bonding mechanisms of extrusion welds were studied. • Interfacial grain boundaries, amorphous layers and voids were found. • Bonding mechanisms in the absence/presence of a gas-pocket were proposed. • Voids closure, oxide films breaking and grain boundaries migration were discussed. - Abstract: Fine nanostructures of bonding interfaces of weld seams formed by porthole die extrusion in the absence/presence of a gas-pocket behind the bridge of the extrusion die were systematically studied to understand the underlying interfacial bonding mechanisms. Interfacial grain boundaries, nanoscale amorphous layers, and three kinds of new interfacial structures were found. Specifically, it was found that, in the absence of a gas-pocket behind the bridge, there are two distinctly different interfacial structures. For the first kind of bonding interface, interfacial grain boundaries exist in contact areas and micro-voids exist in non-contact areas. For the second kind of bonding interface, there are no interfacial grain boundaries in contact areas and only nanoscale micro-voids exist in non-contact areas. In the presence of a gas-pocket behind the bridge, nanoscale voids and amorphous layers exist at the bonding interface. It was also found that the formation of gas-pockets can be avoided by increasing the depth of the welding chamber, and the increase of the welding chamber's depth and extrusion speed also contributes to the volume reduction of micro-voids and the migration of grain boundaries at the bonding interface, so as to improve the welding quality of weld seams. Based on the experimental findings, two interfacial bonding mechanisms corresponding to the absence/presence of a gas-pocket are proposed. The specific behavior of micro-asperities contact, micro-voids closure, oxide films breaking and interfacial grain boundaries migration are described, and the solid-state bonding process during porthole die extrusion is revealed from the micro-nano scales.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Evaluation of Zn-rich coatings for IMC reaction control in aluminum-magnesium dissimilar welds

Highlights: • Thermodynamic and diffusion simulations have estimated the Zn conc. required to change the IMC reaction in Al-Mg dissimilar welding. • The selected coating alloy was effective in changing the Al-Mg reaction path by forming τ-(Al,Zn){sub 49}Mg{sub 32} as a substitute for β-Al{sub 3}Mg{sub 2}. • Refill™ friction stir spot welds produced with the coating alloy exhibited a thinner reaction layer and an increase in lap shear strength. - Abstract: The dissimilar welding of Al to Mg is very challenging owing to the rapid formation of an intermetallic (IMC) reaction layer at the joint interface, typically comprised of the binary phases γ-Al{sub 12}Mg{sub 17} and β-Al{sub 3}Mg{sub 2}. A potential metallurgical solution has been explored that aimed to change the reaction path by using Al-Zn alloy coatings to promote alternative IMCs with a slower growth rate. To determine the optimum composition for the coating, combined CALPHAD thermodynamic calculations and diffusion simulations were used to estimate the minimum Zn concentration (20 at.%) required to suppress the formation of the faster growing Al{sub 3}Mg{sub 2} phase. This prediction was verified by both static diffusion and Refill™ friction stir spot welding (FSSW) experiments. In each case the selected coating alloy was effective in changing the Al-Mg reaction path by forming the τ-(Al,Zn){sub 49}Mg{sub 32} phase as a substitute for β-Al{sub 3}Mg{sub 2}. However, in static heat treatments there was no reduction in the IMC reaction layer thickness due to the new phase having similar growth kinetics to the β-Al{sub 3}Mg{sub 2} phase it replaced. Nevertheless, Refill™ friction stir spot welds prepared with the new AL-Zn coating alloy exhibited a thinner reaction layer and an increase in lap shear strength. This benefit was attributed to the Zn addition reducing the joint interface liquation temperature, which resulted in a lower peak weld temperature and less IMC reaction; although an increase in local eutectic melting limited the improvement observed.

36 MATERIALS SCIENCE↗
DOE OSTI2014

On the development of nugget growth model for resistance spot welding

In this paper, we developed a general mathematical model to estimate the nugget growth process based on the heat energy delivered into the welds by the resistance spot welding. According to the principles of thermodynamics and heat transfer, and the effect of electrode force during the welding process, the shape of the nugget can be estimated. Then, a mathematical model between heat energy absorbed and nugget diameter can be obtained theoretically. It is shown in this paper that the nugget diameter can be precisely described by piecewise fractal polynomial functions. Experiments were conducted with different welding operation conditions, such as welding currents, workpiece thickness, and widths, to validate the model and the theoretical analysis. All the experiments confirmed that the proposed model can predict the nugget diameters with high accuracy based on the input heat energy to the welds.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Radiographic detection of defects in friction stir welding on aluminum alloy AMg5M

In order to reveal weld defects specific to friction stir welding we undertook radiographic inspection of AMg5M aluminum alloy welded joints. Weld defects in the form of voids have been revealed in the weld obtained under the non-optimal rotation and feed rate. Both shape and size of these defects have been confirmed by examining metallographically successive sections prepared in the weld plane as well as in the plane transversal to the tool feed direction. Linear defects have been also found in the sections that are not seen in the radiographic images. Both the preferable localization and origination of the defects have been analyzed.

36 MATERIALS SCIENCE↗
DOE OSTI2015

The microstructure of aluminum A5083 butt joint by friction stir welding

This study presents the microstructure of the aluminum A5083 butt joint surface after it has been joined by friction stir welding (FSW) process. The FSW process is a unique welding method because it will not change the chemical properties of the welded metals. In this study, MILKO 37 milling machine was modified to run FSW process on 4 mm plate of aluminum A5083 butt joint. For the experiment, variables of travel speed and tool rotational speed based on capability of machine were used to run FSW process. The concentrated heat from the tool to the aluminum plate changes the plate form from solid to plastic state. Two aluminum plates is merged to become one plate during plastic state and return to solid when concentrated heat is gradually further away. After that, the surface and cross section of the welded aluminum were investigated with a microscope by 400 x multiplication zoom. The welding defect in the FSW aluminum was identified. Then, the result was compared to the American Welding Society (AWS) FSW standard to decide whether the plate can be accepted or rejected.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Influence of deformation on structural-phase state of weld material in St3 steel

The structural-phase condition of the weld material subjected to the plastic deformation was investigated using the translucent diffraction electron microscopy method. The investigations were carried out near the joint of the weld and the base metal. The seam was done by the method of manual arc welding without artificial defects. The St3 steel was taken as the welded material. Influence of the plastic deformation on morphology, phase composition, defect structure and its parameters of weld metal was revealed. All investigations were done at the distance of 0.5 mm from the joint of the weld and the base metal at the deformation degrees from 0 to 5% and after destruction of a sample. It was established that deformation of the sample did not lead to qualitative changes in the structure (the structure is still presented by ferrite-pearlite mixture) but changed the quantitative parameters of the structure, namely, with the increase of plastic deformation a part of the pearlite component becomes more and more imperfect. In the beginning it turns into the destroyed pearlite then into ferrite, the volume fraction of pearlite is decreased. The polarization of dislocation structure takes place but it doesn’t lead to the internal stresses that can destroy the sample.

36 MATERIALS SCIENCE↗
DOE OSTI2017

High Strain Rate Testing of Welded DOP-26 Iridium

The iridium alloy DOP-26 is used to produce Clad Vent Set cups that protect the radioactive fuel in radioisotope thermoelectric generators (RTGs) which provide electric power for spacecraft and rovers. In a previous study, the tensile properties of DOP-26 were measured over a wide range of strain rates and temperatures and reported in ORNL/TM-2007/81. While that study established the properties of the base material, the fabrication of the heat sources requires welding, and the mechanical properties of welded DOP-26 have not been extensively characterized in the past. Therefore, this study was undertaken to determine the mechanical properties of DOP-26 specimens containing a transverse weld in the center of their gage sections. Tensile tests were performed at room temperature, 750, 900, and 1090°C and engineering strain rates of 1×10 -3 and 10 s -1 . Room temperature testing was performed in air, while testing at elevated temperatures was performed in a vacuum better than 1×10 -4 Torr. The welded specimens had a significantly higher yield stress, by up to a factor of ~2, than the non-welded base material. The yield stress did not depend on the strain rate except at 1090°C, where it was slightly higher for the faster strain rate. The ultimate tensile stress, on the other hand, was significantly higher for the faster strain rate at temperatures of 750°C and above. At 750°C and above, the specimens deformed at 1×10 -3 s -1 showed pronounced necking resulting sometimes in perfect chisel-edge fracture. The specimens deformed at 10 s -1 exhibited this fracture behavior only at the highest test temperature, 1090°C. Fracture occurred usually in the fusion zone of the weld and was, in most cases, primarily intergranular.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Report on the Progress of Weld Development of Irradiated Materials at the Oak Ridge National Laboratory

This report summarizes recent welding activities on irradiated alloys in the advanced welding facility at the Radiochemical Engineering Development Center of Oak Ridge National Laboratory and the development of post-weld characterization capabilities and procedures that will be critical for assessing the ability of the advanced welding processes housed within the facility to make successful repairs on irradiated alloys. This facility and its capabilities were developed jointly by the U.S. Department of Energy, Office of Nuclear Energy, Light Water Reactor Sustainability Program and the Electric Power Research Institute, Long Term Operations Program (and the Welding and Repair Technology Center), with additional support from Oak Ridge National Laboratory. The significant, on-going effort to weld irradiated alloys with high Helium concentrations and comprehensively analyze the results will eventually yield validated repair techniques and guidelines for use by the nuclear industry in extending the operational lifetimes of nuclear power plants.

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
DOE OSTI2013

Towards Real Time Diagnostics of Hybrid Welding Laser/GMAW

Methods are currently being developed towards a more robust system real time feedback in the high throughput process combining laser welding with gas metal arc welding. A combination of ultrasonic, eddy current, electronic monitoring, and visual techniques are being applied to the welding process. Initial simulation and bench top evaluation of proposed real time techniques on weld samples are presented along with the concepts to apply the techniques concurrently to the weld process. Consideration for the eventual code acceptance of the methods and system are also being researched as a component of this project. The goal is to detect defects or precursors to defects and correct when possible during the weld process.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Monitoring and Control of the Hybrid Laser-Gas Metal-Arc Welding Process

Methods are currently being developed towards a more robust system real time feedback in the high throughput process combining laser welding with gas metal arc welding. A combination of ultrasonic, eddy current, electronic monitoring, and visual techniques are being applied to the welding process. Initial simulation and bench top evaluation of proposed real time techniques on weld samples are presented along with the concepts to apply the techniques concurrently to the weld process. Consideration for the eventual code acceptance of the methods and system are also being researched as a component of this project. The goal is to detect defects or precursors to defects and correct when possible during the weld process.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Aluminum Tailor-Welded Blanks for High Volume Automotive Applications

A Design of Experiment based approach is used to systematically investigate relationships between 8 different welding factors (4 related to tool geometry, 4 related to weld process control) and resulting weld properties including strength, elongation and formability in 1.2mm-2mm thick friction stir welding of AA5182-O for TWB application. The factors that result in most significant effects are elucidated. The interactions between several key factors like plunge depth, tool tilt, pin feature and pin length on the overall weld quality is discussed. Appropriate levels of factors that lead to excellent weld properties are also identified.

Aluminum↗
DOE OSTI2019

A dual-mesh method for efficient thermal stress analysis of large-scale welded structures

Transient thermo-mechanical analysis of welding problem requires tremendous computation cost. To accelerate the thermal analysis of large-scale welded structures, an efficient computation scheme based on heat transfer localization and dual meshes was proposed. The computation accuracy is guaranteed by a local fine mesh model with size determined by a theoretical solution and a global coarse mesh model with equivalent heat input. The validity and accuracy of the dual-mesh method were verified using an experimental bead-on-plate model. By extending the weld length, the computation time of the proposed method was proved to be almost linearly dependent on the model scale. The thermal analysis of fillet welding of a large panel structure with 6-m-long weld was accelerated by 10 times over conventional finite element analysis and 2.2 times over adaptive mesh method. Meanwhile, the physical memory consumption was also greatly reduced by the dual-mesh method. Such efficient computation method enables fast evaluation of welding stress and distortion which are vital for manufacturing process and structure performance.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure, Chemical Composition and Local Corrosion Behavior of a Friction Stud Welding Joint

In this paper, friction stud welding technology was used to join a stud and base metal, which were composed of X65 steel. The scanning vibrating electrode technique (SVET) and localized electrochemical impedance spectroscopy (LEIS) were used to investigate the localized corrosion behaviors of the welded joint. Scanning electron microscopy, metallographic microscopy and a micro-hardness tester were used to observe the microstructure and measure the hardness of the welded sample. Raman spectrometry and energy-dispersive spectrometry were used to measure the composition of the weldment before and after corrosion, respectively. The results show that there are the maximum micro-hardness and the densest microstructure in the welded zone compared with the other zones. In addition, α-Fe{sub 2}O{sub 3} and Fe{sub 3}O{sub 4} are present in the welded zone. The SVET and LEIS data indicate that the welded zone has the lowest current density and the largest impedance due to the presence of iron oxides and the densest microstructure, thus showing the excellent corrosion resistance. The relationship among microstructure, micro-hardness, chemical composition and local electrochemical behavior was discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2018

A Comparative Study of Fracture Toughness at Cryogenic Temperature of Austenitic Stainless Steel Welds

The ITER magnet system is based on the “cable-in-conduit” conductor (CICC) concept, which consists of stainless steel jackets filled with superconducting strands. The jackets provide high strength, limited fatigue crack growth rate and fracture toughness properties to counteract the high stress imposed by, among others, electromagnetic loads at cryogenic temperature. Austenitic nitrogen-strengthened stainless steels have been chosen as base material for the jackets of the central solenoid and the toroidal field system, for which an extensive set of cryogenic mechanical property data are readily available. However, little is published for their welded joints, and their specific performance when considering different combinations of parent and filler metals. Moreover, the impact of post-weld heat treatments that are required for Nb{sub 3}Sn formation is not extensively treated. Welds are frequently responsible for cracks initiated and propagated by fatigue during service, causing structural failure. It becomes thus essential to select the most suitable combination of parent and filler material and to assess their performance in terms of strength and crack propagation at operation conditions. An extensive test campaign has been conducted at 7 K comparing tungsten inert gas (TIG) welds using two fillers adapted to cryogenic service, EN 1.4453 and JK2LB, applied to two different base metals, AISI 316L and 316LN. A large set of fracture toughness data are presented, and the detrimental effect on fracture toughness of post-weld heat treatments (unavoidable for some of the components) is demonstrated. In this study, austenitic stainless steel TIG welds with various filler metals have undergone a comprehensive fracture mechanics characterization at 7 K. These results are directly exploitable and contribute to the cryogenic fracture mechanics properties database of the ITER magnet system. Additionally, a correlation between the impact in fracture toughness and microstructure resulting from the above treatment is provided.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure and Mechanical Characterization of Friction-Stir-Welded Dual-Phase Brass

Friction stir welding (FSW) is an ideal process to join brass to avoid the evaporation of zinc. In the present investigation, 6-mm-thick dual-phase brass plates were joined efficiently using FSW at various tool rotational speeds. The microstructures were studied using optical microscopy, electron backscattered diffraction and transmission electron microscopy. The optical micrographs revealed the evolution of various zones across the joint line. The microstructure of the heat-affected zone was similar to that of base metal. The weld zone exhibited finer grains due to dynamic recrystallization. The recrystallization was inhomogeneous and the inhomogeneity reduced with increased tool rotational speed. The dual phase was preserved in the weld zone due to the retention of zinc. The severe plastic deformation created a lot of dislocations in the weld zone. The weld zone was strengthened after welding. The role of tool rotational speed on the joint strength is further reported.

36 MATERIALS SCIENCE↗