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At least 703 records · Page 39
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Microstructure evolution in the conventional single side and bobbin tool friction stir welding of thick rolled 7085-T7452 aluminum alloy

Highlights: • Comparison of microstructure evolution via EBSD of SS-FSW and BB-FSW was investigated. • Friction heat, strain rate and degree of recrystallization affected grain size. • Discontinuously dynamic recrystallization occurred in the weld nugget zone. • The degree of recrystallization was mainly controlled by the strain rate. - Abstract: The microstructure evolution during the conventional single side friction stir welding (SS-FSW) and the bobbin tool FSW (BB-FSW) of 7085-T7452 alloy thick plate was extensively studied by using the state-of-the-art materials characterization techniques. The Electron backscattering diffraction results reveal that a significant grain refinement and a larger percentage of high angle grain boundaries (HAGBs) exist in the weld nugget zone (WNZ) and the thermal-mechanical affected zone (TMAZ) compared with the base material. Meanwhile, more obviously inhomogeneous refined grains and HAGBs in WNZ along the thickness direction and a larger percentage of low angle grain boundaries (LAGBs) were observed in SS-FSW joint than that in BB-FSW joint. The change of the average grain size is the result of friction heat and strain rate of plastic deformation combined with a certain degree of recrystallization. Moreover, the recrystallization is mainly controlled by the strain rate during the FSW process. The presence of Cube and Cube ND after welding shows that the discontinuously dynamic recrystallization (DDRX) is a major factor in the final microstructural response. The research and findings help the understanding of the mechanism of microstructure homogenization and further optimizing the welding process for application.

36 MATERIALS SCIENCE↗
DOE OSTI2014

In mold laser welding for high precision polymer based optical components

To assemble a complete subsystem as a rear lamp, is necessary to have different machines and to perform several tasks. This necessity obliges the companies to have large structures to support all the assembling process. These huge structures are very costly and have as a consequence the reduction of the competitiveness of the companies. The process presented in this document has the intention of reducing the number of tasks needed to produce the final subsystem/product. To achieve this goal were combined several technologies, as in-mould assembling, laser welding and LEDs (light-emitting diode). One of the advantages of this process was the utilization of only one injection molding machine with three injection units to do all the assembling process. To achieve the main objective, firstly, the rear lamp was designed according to with the legislation of UNECE Vehicle Regulations - 1958 Agreements; Regulation No. 50 -Rev.2 - Position lamps, stop lamps, direction indicators for motorcycles. Posterior several polymeric materials were studied at different levels. Initial were studied several concentrations of carbon nanotubes mixed with PC (polycarbonate). This had the objective of determine, if these materials are suitable to conduct the necessary electric current to turn on the different LEDs. One of the main advantages of this process is the use of the laser transmission welded process. Since, with this welding technology is possible reduce the complexity of the final part. To understand the potentialities of this technology a combination of two materials was studied. The studied showed that all materials presented a high transparency to the laser beam. In terms of weld process, the study showed that the best welding conditions are the lowest velocity, diameter and power. With these studies was possible conclude that this new process is suitable to be implemented at the industrial level.

42 ENGINEERING↗
DOE OSTI2016

Structure of Ti-6Al-4V nanostructured titanium alloy joint obtained by resistance spot welding

The structure of weld joints of the titanium alloy Ti-6Al-4V in the initial ultrafine-grained state, obtained by resistance spot welding, is studied using the optical and scanning electron microscopy method and the X-ray structure analysis. The carried out studies show the relationship of the metal structure in the weld zone with main joint zones. The structure in the core zone and the heat affected zone is represented by finely dispersed grains of needle-shaped martensite, differently oriented in these zones. The change in the microhardness in the longitudinal section of the weld joint clearly correlates with structural changes during welding.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Plasmonic welded single walled carbon nanotubes on monolayer graphene for sensing target protein

We developed plasmonic welded single walled carbon nanotubes (SWCNTs) on monolayer graphene as a biosensor to detect target antigen molecules, fc fusion protein without any treatment to generate binder groups for linker and antibody. This plasmonic welding induces atomic networks between SWCNTs as junctions containing carboxylic groups and improves the electrical sensitivity of a SWCNTs and the graphene membrane to detect target protein. We investigated generation of the atomic networks between SWCNTs by field-emission scanning electron microscopy and atomic force microscopy after plasmonic welding process. We compared the intensity ratios of D to G peaks from the Raman spectra and electrical sheet resistance of welded SWCNTs with the results of normal SWCNTs, which decreased from 0.115 to 0.086 and from 10.5 to 4.12, respectively. Additionally, we measured the drain current via source/drain voltage after binding of the antigen to the antibody molecules. This electrical sensitivity of the welded SWCNTs was 1.55 times larger than normal SWCNTs.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Evaluating the Potential for Remote In-Process Monitoring of Tool Wear in Friction Stir Welding of Stainless Steel

A wear characterization study was performed to determine the useful lifetime of Polycrystalline Cubic Boron Nitride tooling for the friction stir welding (FSW) of stainless steel samples in support of a nuclear repair welding research and development program. In-situ and ex-situ laser profilometry were utilized as primary methods of monitoring tool geometry degradation, and volumetric defects were detected through both non-destructive and destructive techniques, as repeated welds of a standard sample configuration were produced. The spectral content of weld forces were examined to search for indications of evolving material flow conditions, caused by significant tool wear, that would result in the formation of defects, and an artificial neural network was trained and evaluated as a means of automatically detecting these conditions. Here, the resulting performance constituted a successful demonstration of in-process monitoring of tool wear and weld quality in FSW of a high melting temperature, high hardness material.

36 MATERIALS SCIENCE↗
NASA NTRS2026

Modeling Study of Hatch Spacing’s Effect on Grain Morphology in Repairing Damaged Metal Parts with Welding and Hatch Spacing’s Potential Use in Lunar Exploration

The welding process is a potential way of repairing a damaged metal component, especially cavity damage caused by a harsh environment like the lunar environment, which is characterized by large temperature differences and reduced gravity. The adjustment of welding parameter (e.g., hatch spacing) can improve production efficiency in the repair process. Seen from the microstructural level, hatch spacing sensitivity affects the metallic grain evolution and morphology in the welding process, which can further influence a repaired part’s mechanical properties; however, the study of hatch spacing’s effect on microstructure is challenging. Traditional experimental procedures are costly and time-consuming, and any change in hatch spacing value needs roll-back of experimental procedure. A modeling study can address the above challenges in experimental observation. In this research, a modeling approach based on the Kinetic Monte Carlo (KMC) Potts theory was used to simulate grain evolution and morphology with three hatch spacings. Through quantifying and analyzing the predicted grain morphologies, the effect of hatch spacing on microstructure in a welding-fabricated part was investigated. The predicted grain morphologies were validated with an EBSD image of welding microstructure, which has been published before. The primary grain morphologies were columnar grains with a small amount of fine equiaxed grains formed in the scanning path centerline. When increasing the hatch spacing, the columnar grains become larger and more lengthy, while the effect of hatch spacing on the equiaxed grains is not obvious.

Welding for repairing↗
DOE OSTI2017

Weld Development of FeCrAl Thin-Wall Cladding for LWR Accident Tolerant Fuel

The development of new cladding materials to replace Zr-based cladding for light water reactors (LWRs) has been actively pursued with improved high-temperature oxidation resistance to significantly suppress hydrogen production from an accident such as a loss of coolant event. FeCrAl alloy with a typical composition of approximately Fe-15Cr-5Al has been selected as a candidate material because of its improved high-temperature oxidation resistance over the present LWR zircaloy cladding. As a result of higher thermal neutron absorption for FeCrAl alloy, the cladding wall thickness needs to be reduced by approximately half compared to that of Zircaloy cladding to be comparable to the current LWR fuel performance under the normal operating condition. New weld techniques for joining the endplug to thin-walled cladding need to be developed. Both laser beam weld and pressure resistance weld were employed. This paper summarizes the recent progress on the laser weld development for FeCrAl cladding with 350 µm wall thickness. The results of microstructural characterization and mechanical property testing of the weld for the cladding-endplug set are discussed.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2022

Laser Welded Edge Seals for Glass/Glass Modules

Polymers used in module construction are often involved in degradation mechanisms and hamper recycling. This project explores femtosecond (fs) laser welding of glass/glass modules to enable polymer-free modules. We are utilizing the knowledge base of the industrial laser community to apply glass/glass fs laser welding technology to solar modules. Experimentally measured stress intensity factors for glass welds are input into a COMSOL model of a full-size module to test weld failure under static loading. If successful, fs laser welding could enable modules that are polymer-free that will extend module warrantees, allow hermetically sealed glass/glass modules for moisture-sensitive semiconductors, and improve module recyclability.

COMSOL finite element model↗
DOE OSTI2018

Remote Evaporator Vessel Repair by Robotics and Powder Injection Laser Welding - 18107

In February 2016 a leak was identified in the 25H evaporator at the Savannah River Site (SRS). The 25H evaporator is used to manage water volume in high level waste (HLW) tanks during solid salt cake and sludge removal. The location and the cause of the leak were unknown. Orano Federal Services (formerly AREVA Federal Services) applied its robotics expertise and commercial nuclear expertise to perform the inspection. Orano and Framatome (formerly AREVA NP INC) combined a BROKK, a commonly used tracked vehicle with remote control, with the KUKA, an automated robotic arm, commonly used in automobile manufacturing, to perform the partial dismantlement and inspection. Once inspection was complete and the leak was identified, Orano conducted a study on two welding processes: Gas Tungsten Arc (GTA-Tig) and Powder Injection Laser Welding. Based on the inspection of the SRS HLW evaporator's material type being Hastelloy{sup R}, along with the remote repair and material thin wall condition, the Powder Injection Laser Welding was chosen for this application. Orano has developed the remote powder injection laser welding and delivery process that will be used to conduct the repair of the SRS HLW evaporator in the fall of 2017. The process includes robotic delivery of components and robotic performance of the weld. Specialized robotic equipment were developed or adapted from other industries to perform the high hazard job of repairing an eroded section of the evaporator pot. The integration of multiple innovative remote techniques provides a unique solution in a highly radioactive environment. These techniques and innovations can be capitalized upon to solve many different remote repairs on equipment resulting in time savings, reduced dose to personnel, and recovery of initial investment in equipment. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2021

Oxidation Behavior of Welded Fe-Based and Ni-Based Alloys in Supercritical CO 2

Next-generation supercritical CO 2 (sCO 2 ) power cycles will require different classes of alloy throughout the operational temperatures to optimize tradeoff of creep strength, oxidation performance and cost. This will necessitate joining methods such as welding, which might pose compatibility concerns at the joined interfaces. In this study, similar and dissimilar metal welds were generated from a variety of candidate alloys for sCO 2 systems including ferritic/martensitic steels, austenitic steels, and Ni-based superalloys. Samples were extracted from different regions of the welds and exposed to sCO 2 at 550 °C and 20 MPa for 2500 h, then characterized to understand their behavior in this environment. Unsurprisingly, the local oxidation behavior was largely dictated by the Cr content in the underlying metal. High-Cr austenitic steels and Ni alloys formed slow-growing Cr-rich oxide scales with minimal carburization of the underlying metal, while low-Cr ferritic/martensitic steels formed fast-growing Fe-rich oxide scales with significant carburization. Most welds did not show any unusual oxidation behavior at the interfaces, considering the local Cr content. The one exception was the 347H similar metal weld, where a larger grain size and complex grain structure in the fusion zone led to a significantly higher rate of Fe-rich oxide nodule formation compared to the base metal. This suggests that microstructural changes at joined interfaces can play an important role on the oxidation-limited lifetimes in future sCO 2 systems. The composition changes across the interfaces enabled study of the effect of Fe on the growth rate of Cr-rich oxides and of the origins of the sub-surface recrystallization zone that forms beneath them.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Titanium Alloys Thin Sheet Welding with the Use of Concentrated Solar Energy

The present study deals with the welding of titanium alloys thin sheets 1.3 mm thick, with the use of concentrated solar energy. The experimental part of the work took place at a medium size solar furnace at the installation of the Centre National de la Recherche Scientifique, at Odeillo, in Southern France, where similar and dissimilar defect-free welds of titanium Grades 4 and 6 were achieved, in the butt joint configuration. After the determination of the appropriate welding conditions, the optimum welded structures were examined and characterized microstructurally, by means of light optical microscopy, scanning electron microscopy, and microhardness testing. In addition, test pieces extracted from the weldments were tested under uniaxial tensile loading aiming to the estimation of the strength and the ductility of the joint. The analysis of the experimental results and the recorded data led to the basic concluding remarks which demonstrate increased hardness distribution inside the fusion area and severe loss of ductility, but adequate yield and tensile strength of the welds.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Nugget Structure Evolution with Rotation Speed for High-Rotation-Speed Friction-Stir-Welded 6061 Aluminum Alloy

High-rotation-speed friction stir welding (HRS-FSW) is a promising technique to reduce the welding loads during FSW and thus facilitates the application of FSW for in situ fabrication and repair. In this study, 6061 aluminum alloy was friction stir welded at high-rotation speeds ranging from 3000 to 7000 rpm at a fixed welding speed of 50 mm/min, and the effects of rotation speed on the nugget zone macro- and microstructures were investigated in detail in order to illuminate the process features. Temperature measurements during HRS-FSW indicated that the peak temperature did not increase consistently with rotation speed; instead, it dropped remarkably at 5000 rpm because of the lowering of material shear stress. The nugget size first increased with rotation speed until 5000 rpm and then decreased due to the change of the dominant tool/workpiece contact condition from sticking to sliding. At the rotation speed of 5000 rpm, where the weld material experienced weaker thermal effect and higher-strain-rate plastic deformation, the nugget exhibited relatively small grain size, large textural intensity, and high dislocation density. Consequently, the joint showed superior nugget hardness and simultaneously a slightly low tensile ductility.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure and Mechanical Properties of Dissimilar Joints of Al-Mg{sub 2}Si and 5052 Aluminum Alloy by Friction Stir Welding

Al-Mg{sub 2}Si alloy and 5052 Al alloy were welded successfully by friction stir welding (FSW) in this study. The results show that the alloy consists of three distinct zones after FSW: the base material zone (BMZ), the transitional zone, and the weld nugget (WN). The morphologies of the primary Mg{sub 2}Si phases are identified as coarse equiaxed crystals for Al-Mg{sub 2}Si alloys in the BMZ. The WN is a mixture of rich Al-Mg{sub 2}Si and rich 5052 alloy, and a banded structure is formed in the zone. Interestingly, in the WN, the equiaxed crystals changed to polygonal particles with substantially reduced sizes in the rich Al-Mg{sub 2}Si zone. However, in addition to the white rich Mg phase appearing in the rich 5052 zone near the interface, the 5052 alloy does not show obvious changes. The hardness gradually increases from the BMZ of the 5052 to the welded joint to the Al-Mg{sub 2}Si BMZ. In addition, the ultimate tensile strength (UTS) of the welded joint is higher than that of the base material of the Al-Mg{sub 2}Si, whereas it is lower than that of the 5052 base alloy. The results of the elongation are similar to the UTS results. The fracture mechanism is also investigated.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Inverse Thermal Analysis of Ti-6Al-4V Friction Stir Welds Using Numerical-Analytical Basis Functions with Pseudo-Advection

Inverse thermal analysis of Ti-6Al-4V friction stir welds is presented that demonstrates application of a methodology using numerical-analytical basis functions and temperature-field constraint conditions. This analysis provides parametric representation of friction-stir-weld temperature histories that can be adopted as input data to computational procedures for prediction of solid-state phase transformations and mechanical response. These parameterized temperature histories can be used for inverse thermal analysis of friction stir welds having process conditions similar those considered here. Case studies are presented for inverse thermal analysis of friction stir welds that use three-dimensional constraint conditions on calculated temperature fields, which are associated with experimentally measured transformation boundaries and weld-stir-zone cross sections.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Strain Rate Dependency of Simulated Welding Residual Stresses

In the present work, the influence of taking into consideration the strain rate-dependent nature of steel S355 during a calculation of the welding residual stresses through a finite element simulation is investigated. The Perzyna material model is calibrated using experimental values found in the literature and is introduced to a validated weld simulation model, where the strain rate dependency had not been taken into consideration before this study. The calculated profiles of the welding residual stresses, for strain rate-dependent and independent behavior, are then compared with experimentally measured profiles. The results of this first-step analysis show that taking into consideration strain rate dependency during a welding simulation of the investigated S355 has non-negligible influence on the calculated welding residual stresses.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Ti Addition on the Microstructure and Mechanical Properties of Weld Metals in HSLA Steels

Acicular ferrite (AF) can significantly improve the mechanical properties of steel welds. One practical approach to enhance the formation of AF is to provide the heterogonous nucleation sites such as Ti oxides. In this study, Ti was added to different conventional welding processes including shield metal arc welding (SMAW), submerged arc welding (SAW) and tandem SAW (T-SAW). In the SMAW process, TiO{sub 2} particles as a source of Ti were inserted into the weld groove, while in the SAW and the T-SAW processes, the Ti-enriched S2MoTB wire was used as the filler metal. The microstructural evolution of weldments was characterized by employing optical and scanning electron microscopes. In addition, microhardness (Vickers, HV), Charpy impact and tensile tests were carried out to investigate the mechanical properties of weldments. Although the microhardness measurements of all weldments did not vary significantly and were in the range of 205-252 HV, there was a considerable difference in tensile and impact properties of the SAW and the T-SAW weldments. In the SMAW process, the addition of TiO{sub 2} results in no significant enhancement in tensile and impact toughness. This can be attributed to the inhomogeneous distribution of TiO{sub 2} particles as well as the formation of large inclusions in the structure. On the other hand, Ti addition to WM increased the yield strength from 489 to 552 MPa for the SAW process, and in contrast, it decreased the impact toughness from 75 to 33 J. This detrimental effect can be related to the higher deposition of other alloying elements in the WM and the formation of more ferrite side plate phase. By applying the T-SAW process, more Ti in WM led to a higher content of AF in the microstructure and increased both yield strength and impact toughness from 528 to 595 MPa and 100 to 180 J, respectively.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructure and Mechanical Property Improvement in Laser-Welded TC4 Titanium Alloy and 301L Stainless Steel Joints Without Filler Metal

Pulsed laser welding of Ti alloy to stainless steel (SS) has been applied without filler metal. When the laser beam was focused on the Ti alloy–SS interface, large volumes of Ti-Fe intermetallics were formed in the joint. These Ti-Fe intermetallics were highly brittle, causing the joint to crack spontaneously after welding. The brittle coarse Ti-Fe intermetallics seriously deteriorates the joint strength of Ti alloy and SS; therefore, it is necessary to control the quantities and distributions of brittle Ti-Fe intermetallics in the joint. The amount of melted base materials was controlled by changing laser offset to control the melted proportion and thus control the quantity and distribution of brittle Ti-Fe intermetallics in weld. When laser beam was on the SS side, melting amount of Ti alloy in weld was decreased. When laser beam was on the Ti alloy side, melting amount of SS in weld was decreased. When the laser offset was 0.35 mm away from SS side, the sample realizes the effective connection between Ti alloy and SS. A compound layer was formed at the SS side with the main microstructure of β-Ti and TiFe{sub 2}. The joint fractured at the compound layer with the maximum tensile strength of 182 MPa. Microstructures and mechanical properties of joints were improved by changing laser offset.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Statistical Strength Analysis of Dissimilar AA2024-T6 and AA6061-T6 Friction Stir Welded Joints

Friction stir welding has been emerged as one of the most reliable processes for joining aerospace-grade aluminum alloys. However, limited studies have been focused so far on the reliability of these joint types. In this study, the effect of welding parameters (traverse and rotation speeds) on the fracture strength of dissimilar AA2024-T6 and AA6061-T6 friction stir welded joints was statistically analyzed by the Weibull approach. Based on the results, the Weibull approach was found to be much more accurate than widely used standard deviation for evaluating the scatter of the fracture strength data. The improper selection of the process parameters not only decreased the average fracture strengths of the joints, but also scattered them around the mean value, and increased the failure probability under the same applied tension. According to the microscopic and macroscopic examinations and microhardness measurements, although the microstructural inhomogeneities caused by the improper setting of the welding parameters negatively affected the joints hardness and fracture strength, the welding defects were likely to be the most responsible factor affecting the fracture strength and its reliability.

36 MATERIALS SCIENCE↗