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At least 721 records · Page 40
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Mechanism of Nonequilibrium Formation of Novel Interface Structures in Explosive Welding

Novel structures are commonly observed in explosive welding products. This paper presents the nonequilibrium mechanism of the welding interface. A theoretical model was established first to deduce the motion equation of the rapid accelerated flyer plate. Numerical simulations of the welding interface were carried out with the smooth particle hydrodynamic method. Both the theoretical and numerical results were compared with experimental phenomena and showed that under the very rapid application of an impact load usually associated with high-strength plastic deformation, a significant strain rate, and temperature rise, the material volumes of the welding interface are in the viscoelastic state and prone to rotation, detrusion, and dilatation. In addition, the extreme values from the nonequilibrium development of the interface structure are a consequence of the competition between the loading and unloading stress waves during the explosive welding process.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructure Evolution, Structural Integrity, and Hot Corrosion Performance of Nitrogen-Enhanced Stainless Steel Welds

This work articulated the multi-pass, gas tungsten arc welding by adopting pulsating current for joining nitrogen-enhanced, low-carbon austenitic stainless steel, AISI 316L(N). An austeno-ferrite stainless steel (ER2594) and an over-alloyed (ERNiCrMo-3) fillers were employed to join 6-mm-thick plates of AISI 316L(N). Acicular and lathy ferrite in the weld zone of ER2594 and the occurrence of secondary phases in the fusion zone of over-alloyed filler were observed. The notch tensile strength data attested that the joint strength was greater for the welds employing an over-alloyed filler. This study also affirms that the welds obtained with ER2594 filler resulted in better impact toughness (144 J) compared with ERNiCrMo-3 filler and base metal. Also the notch impact toughness of ER2594 weldment is increased to 60% in comparison with that of candidate metal. Corrosion studies were also performed by exposing the coupons in a eutectic salt mixture comprising of 40%K{sub 2}SO{sub 4}-60%NaCl for 50 cycles at 650 °C. It is observed that the weld zone employing over-alloyed filler experienced better high-temperature corrosion resistance compared to the base metal. The specific results of this investigation will be of greater demand to the nuclear and marine applications utilizing these joints.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of Tool Offset and Rotational Speed in Dissimilar Friction Stir Welding of AISI 304 Stainless Steel and Mild Steel

In the present study, dissimilar friction stir welding was carried out between stainless steel (UNS S30400) and mild steel (UNS G10080) plates of 4 mm thickness using a tungsten carbide tool. The influence of tool rotational speeds (600, 875 rpm) and tool offsets (0.6, 1.2 mm) on mechanical properties, i.e., hardness, tensile strength, and impact toughness of welded joints was investigated. Maximum tensile strength of the joint was about 107.6% of the mild steel under rotational speed of 875 rpm and tool offset of 1.2 mm. The maximum hardness reached in the stir zone was about 281 HV{sub 0.5} due to the phase transformations and grain refinement. Charpy’s notch toughness of the welded joints was observed lower than the base materials. The microstructural characterizations were carried by using an optical microscope, and FESEM–EDS analysis which revealed the complex material mixing and material movement during the welding. Tungsten-rich bands were observed in the weld micrograph especially toward the advancing side. During this study, various wear mechanisms like oxidation wear, abrasive wear, and adhesion wear were responsible for the degradation of tungsten carbide tool.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Friction Stir Lap Welding 0.8-mm-Thick 2024 Aluminum Alloy with the Assistance of Stationary Shoulder

In this work, 0.8-mm-thick 2024 aluminum alloy was friction stir lap welded with the assistance of a stationary shoulder system. The joint surface formation, microstructure and mechanical properties of the welded joints were studied. The results showed that sound joints without defect could be obtained at a wide parameter range because stationary shoulder increased the material flow during welding. The stationary shoulder decreased the joint roughness from 125.5 to 58.9 µm. A thickness reduction of 4% was achieved at a welding speed of 200 mm/min. The joint fabricated by the stationary shoulder system had superior lap shear properties compared to the joint fabricated using the conventional tool. A high failure load of 7440 N was achieved at welding speed of 200 mm/min. The joint presented tensile fracture mode and the fracture morphology showed typical ductile fracture.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Understanding formation mechanisms of intermetallic compounds in dissimilar Al/steel joint processed by resistance spot welding

Here, this paper confirmed the formation mechanism of intermetallic compounds (IMCs) in Al/steel resistance spot welds with transmission electron microscopy, electron backscatter diffraction, nanoindentation and thermodynamic calculations. In particular, the formation of AlFe with BCC_B2 structure, which is not commonly seen in welds, was identified. The formation mechanism of IMCs at the high welding energy region is described as follows. Al 13 Fe 4 first nucleates from the Al side, followed by Al 5 Fe 2 growth with Fe atoms accumulating at the Al 13 Fe 4 grain boundaries. Then, Al 5 Fe 2 grains grow continuously to coarse columnar grains, and AlFe forms at the interface of Al 5 Fe 2 grains and the ferrite phase. Lastly, the needle-like Al 13 Fe 4 forms in the cooling process. At the middle welding energy region, only equiaxed Al 5 Fe 2 and small Al 13 Fe 4 grains are formed at the interface because of the lower diffusion rates of Al and Fe, hence postponing the growth of Al 5 Fe 2 and Al 13 Fe 4 . At the low welding energy region, only sporadic Al 5 Fe 2 and Al 13 Fe 4 grains are formed surrounded by the Al phase.

42 ENGINEERING↗
DOE OSTI2025

Laser repair welding of irradiated alloy 182

Welding repair of irradiated nickel-based alloys, such as Alloy 182, poses a significant challenge due to helium-induced cracking (HeIC) and grain boundary degradation (GBD) in the heat affected zone, driven by helium accumulation at grain boundaries and welding-induced tensile stresses. This study investigates the weldability of irradiated Alloy 182 up to 15 wppm doped boron using both conventional laser welding and the Auxiliary Beam Stress Improved (ABSI) laser welding technique. While HeIC was observed at the weld toe of the entry pass in both methods due to the higher effective heat input associated with the initial pass directly on the base metal, no additional cracking occurred elsewhere, even at elevated helium concentrations. Optical and scanning electron microscopy analysis revealed that the ABSI technique, which introduces additional compressive stresses to counteract solidification-induced tensile stresses, significantly reduced GBD formation, lowering its total count from 1,230 to 339 and decreasing both average and maximum GBD lengths. In conclusion, these results demonstrate that the ABSI technique is a promising approach to mitigate helium-induced damage and improve the weldability of irradiated Alloy 182, offering a viable solution for structural repairs for long-term operation of existing nuclear reactors.

grain boundary degradation↗
DOE OSTI2013

Precipitation of sigma and chi phases in δ-ferrite of Type 316FR weld metals

The decomposition behavior and kinetics of δ-ferrite are examined using aging treatments between 873 and 1073 K for Type 316FR stainless steel weld metals with different solidification modes (316FR AF, 316FR FA). The dominant precipitates are sigma, chi, and secondary austenite nucleated at δ-ferrite/austenite interfaces or in the interior of the ferrite grains. These precipitates consume all the ferrite during isothermal aging in both 316FR AF and FA weld metals. Differences in the precipitation behavior (precipitation initiation time and precipitation speed) between weld metals can be explained by i) the degree of Cr and Mo microsegregation within δ-ferrite or austenite near ferrite and ii) the nucleation sites induced due to the solidification mode (AF or FA), such as the ferrite amount. For both weld materials, a Johnson–Mehl-type equation can express the precipitation behavior of the sigma + chi phases and quantitatively predict the behavior at the service-exposure temperatures of a fast breed reactor. - Highlights: • Precipitation of σ and χ phase in Type 316FR welds (two solidification modes) • Different precipitation behaviors: precipitation initiation time and growth speed • Johnson-Mehl–type equation is the most applicable to the precipitation behaviors • Precipitation behaviors are predicted under service conditions of FBRs.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Effect of long-term aging on microstructure and local behavior in the heat-affected zone of a Ni–Cr–Mo–V steel welded joint

Evolution of microstructure, micro-hardness and micro-tensile strength behavior was investigated in the heat-affected zone of a Ni–Cr–Mo–V steel welded joint after the artificial aging at 350 °C for 3000 h. After detailed characterization of microstructures in optical microscopy, scanning electron microscopy and transmission electron microscopy, it is revealed that the change of martensite–bainite constituent promotes more homogeneous microstructure distribution. The aging treatment facilitates redistribution of carbon and chromium elements along the welded joint, and the micro-hardness is increased slightly through the welds due to enrichment of carbon. The types of precipitates in the weldment mainly include M{sub 3}C, MC, M{sub 2}C and M{sub 23}C{sub 6}. The carbides in base metal, weld metal and coarse-grained heat-affected zone are prone to change from ellipsoidal to platelet form whereas more uniform spherical carbides are observed in the fine-grained zone. Precipitation and coarsening of M{sub 23}C{sub 6} near the fusion line, and formation of MC and M{sub 2}C, are responsible for the tensile strength decrease and its smooth distribution in the aged heat-affected zone. This implies that the thermal aging can relieve strength mismatch in the weldments. - Highlights: • Microstructure homogeneity improved in HAZ after long-term aging. • Tensile strength decreased in HAZ due to precipitation and coarsening of M{sub 23}C{sub 6}. • Strength mismatch in NiCrMoV steel welds was relieved after aging at 350 °C × 3000 h.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Microstructural characterization of dissimilar welds between Incoloy 800H and 321 Austenitic Stainless Steel

In this work, the microstructural character of dissimilar welds between Incoloy 800H and 321 Stainless Steel has been discussed. The microscopic examination of the base metals, fusion zones and interfaces was characterized using an optical microscope and scanning electron microscopy. The results revealed precipitates of Ti (C, N) in the austenitic matrix along the grain boundaries of the base metals. Migration of grain boundaries in the Inconel 82 weld metal was very extensive when compared to Inconel 617 weldment. Epitaxial growth was observed in the 617 weldment which increases the strength and ductility of the weld metal. Unmixed zone near the fusion line between 321 Stainless Steel and Inconel 82 weld metal was identified. From the results, it has been concluded that Inconel 617 filler metal is a preferable choice for the joint between Incoloy 800H and 321 Stainless Steel. - Highlights: • Failure mechanisms produced by dissimilar welding of Incoloy 800H to AISI 321SS • Influence of filler wire on microstructure properties • Contemplative comparisons of metallurgical aspects of these weldments • Microstructure and chemical studies including metallography, SEM–EDS • EDS-line scan study at interface.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Investigation of temperature dependent microstructure evolution of pure iron during friction stir welding using liquid CO2 rapid cooling

Highlights: • Microstructure evolution during the FSW of pure iron was investigated under low and high heat input conditions, respectively. • Rapid cooling, tool stop action, marker insert and short-time annealing were employed to clarify the microstructure evolution. • Continuous and discontinuous dynamic recrystallizations occur during the microstructure evolution under both the conditions. • Incomplete and complete recrystallized microstructures were developed severally under the low and high heat input conditions. • Grain growth occurs during the normal cooling stage but it is very limited under the low heat input condition. - Abstract: The microstructure evolution of pure iron during friction stir welding was reconstructed by an ingenious experimental design, in which the rapid cooling friction stir welding combined with the tool “stop action” technique and the subsequent short-time annealing were adopted to “freeze” the transient microstructure during the stirring and reproduce the normal cooling during conventional friction stir welding, respectively. The microstructure evolution during the stirring and normal cooling was investigated along the material flow path and in the annealed “frozen” weld zone by high-resolution electron-backscatter-diffraction technique. The results show that the continuous and discontinuous dynamic recrystallizations occur simultaneously at the severe deformation stage in front of the tool both under low and high heat input conditions. However, during the material flow, the microstructure evolution involves the plastic deformation, recrystallization, high angle boundaries migration and dynamic recovery under the low heat input condition, while in a dynamic balance of deformation, recrystallization and grain growth under the high heat input conditions. At the cooling stage, normal grain growth occurs both for the low and high heat input welding conditions, while it is very limited for the low heat input condition.

36 MATERIALS SCIENCE↗
DOE OSTI2018

On the microstructure characterization of the AA2098-T351 alloy welded by FSW

Highlights: • The microstructural characterization of the AA2098-T351 Al-Cu-Li alloy welded by FSW was carried out. • There was strong correlation between the results of the various techniques. • DSC was an effective technique for characterization of GP zones complementing the microstructural characterization by TEM. - Abstract: The complex structure of the AA2098-T351 alloy welded by friction stir welding (FSW) was investigated by transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and microhardness measurements. Thermal modelling process of the FSW process was carried out by soldering thermocouples at distances of 6, 9 and 12 mm from the weld centerline, and thermocouple measurements were used as input data into the model. Finite element software COMSOL v5.2 was used for data analysis. The prevailing phases in the base metal (BM) are T1 (Al{sub 2}CuLi) θ′ (Al{sub 2}Cu), δ′/β′(Al{sub 3}(Li,Zr)) and Ω (Al{sub 2}Cu). In the heat affected zone (HAZ), either in the retreating or advancing sides, θ′ phase was not identified. In the thermomechanical affected zone (TMAZ), T1, δ′/β′, GP zones phase were detected in the retreating side, whereas T1 and Guinier-Preston (GP) zones were not observed in the advancing side. This result supports the asymmetric behavior observed in the microhardness profile of the weld. In the stir zone (SZ), GP zones, T1, δ′/β′ (Al{sub 3}(Li,Zr)) and Ω were identified.

36 MATERIALS SCIENCE↗
DOE OSTI2018

In situ analysis of plasticity and damage nucleation in a Ti-6Al-4V alloy and laser weld

Highlights: • EBSD in situ tensile test on a Ti-6Al-4V alloy and laser weld • Mainly prismatic, basal and pyramidal a slip activity in the base metal • Damage nucleation in the vicinity of the β phase and at α grain boundaries • Alignment of 10-10 crystallographic direction with the tensile axis in the α′ laths of the weld metal • Crack initiation along specific laths interfaces, former β grain boundary or twinned martensite laths in the weld metal The present work addresses the mechanisms of plasticity and damage nucleation in a Ti-6Al-4V alloy and its laser weld. To this end, in situ tensile tests are performed in a Scanning Electron Microscope (SEM) equipped with a Electron BackScatter Diffraction (EBSD) camera. The slip activity is automatically analyzed using a modified Schmid analysis. It is shown that the slip activity of the base metal consists mainly in prismatic, basal and pyramidal a slip. Damage nucleation in the vicinity of the β phase and at α grain boundaries is also evidenced in the base metal. In the α{sup ′} weld metal (a microstructure similar to the one often obtained in additive manufacturing Ti-alloy parts), plastic deformation results in a crystallographic rotation of the laths tending to align the 〈10 1-bar 0〉 direction with the tensile axis. The rare events of crack initiation in the martensite are shown to occur along specific laths interfaces, former β grain boundary or twinned martensite needles.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Microstructure and mechanical properties of friction stir weld performed on neutron-irradiated 304L steel with helium

As nuclear power plants (NPPs) approach or exceed 40–60 years of service, it may become necessary to repair damaged neutron-irradiated components to prolong longevity. However, fusion welding repair of irradiated steels and metallic materials is challenging because of helium-induced cracking. This study used friction stir welding (FSW) to address helium-related issues, such as helium bubble formation and grain boundary cracking. The microstructure, helium-induced degradation, and mechanical properties of a friction stir weld produced on neutron-irradiated 304L stainless steel with approximately 5.2 appm of helium were characterized. The analysis focused on variations in grain size, texture, and the morphology of helium-induced damage in the friction stir weld. Mechanical properties were characterized on the irradiated base metal (BM) and metallurgical zones of the friction stir weld: the stir zone (SZ), the thermo-mechanically affected zone (TMAZ), and the heat affected zone (HAZ). Only minor scattered porosity in the SZ and TMAZ and a few short microcracks (below 20 μm in length) in the TMAZ were observed, indicating limited helium-induced degradation. Tensile tests revealed good mechanical properties and fractography analysis demonstrated predominantly ductile fracture. In conclusion, the results highlight the immediate and substantial benefits of the FSW approach for repairing or joining helium-containing irradiated materials in NPPs.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Characterization of Residual Stress as a Function of Friction Stir Welding Parameters in Oxide Dispersion Strengthened (ODS) Steel MA956

This article characterizes the residual stresses generated by friction stir welding of oxide dispersion strengthened steel MA956 over a series of welding conditions. A plate of MA956 steel was friction stir welded at three conditions: 500 rpm/25 millimeters per minute (mmpm), 400 rpm/50 mmpm and 400 rpm/100 mmpm. The residual stresses across these welds were measured using both x-ray and neutron diffraction techniques. Longitudinal residual stresses up to eighty percent of the yield strength were observed for the 400 rpm/100 mmpm condition. Increasing the traverse rate while holding the rotational speed fixed increased the residual stress levels in the stir zone and at the stir zone-thermomechanically affected zone interface. The stress profiles displayed the characteristic M shape, and the asymmetry between advancing and retreating stress peaks was limited, occurring mainly on the root side of the weld. The large magnitude of the stresses was maintained throughout the thickness of the plates.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Effects of low-temperature neutron irradiation on the microstructure and tensile properties of duplex 2304 stainless steel and its electron-beam welds

A lean duplex stainless steel material (2304-grade) in its base metal and electron beam (e-beam) welded conditions were studied microstructurally and mechanically as a function of irradiation conditions to evaluate its use as a structural material at low temperatures (60–100 °C). Neutron irradiation up to a fluence of 1.40 × 10 19 n/cm 2 (E > 0.1 MeV) or ~0.011 dpa decreased the total elongation of both base metal and e-beam welded samples. Overall, radiation hardening was observed in all the samples. The transversely cut irradiated samples showed some nonuniform quasi-cleavage and shearing in their fracture surfaces, indicating the variance of ductile nature of the two-phased (deformable austenite and harder ferrite) duplex structure. The e-beam welded samples also showed quasi-cleavage fracture, which is a characteristic of radiation-induced embrittlement. Furthermore, these observations of the e-beam welded samples were attributed to the formation of coarse ferrites, grain boundary and intragranular phases such as γ 2 and γ 3 , and minor impurity phases such as CrN and Cr 2 N in the weld pool and/or heat-affected zone of the samples. Radiation-induced elemental segregation was also identified in the post-irradiated base metal.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Indentation investigation of 304L stainless steel friction stir weld simulated crack repair

Simulated cracks were repaired in 304L stainless steel using low temperature friction stir welding. Indentation studies were carried out to understand the effect of microstructural features on the mechanical property variation across the weld and to measure the size of the weld zones with a quantitative technique. Microhardness and nanoindentation hardness profiles were constructed on a transverse section across the weld. The data obtained were correlated by extrapolating the nanoindentation hardness to greater depths which showed that the nanoindentation hardness closely reflects the microhardness values throughout the weld. Finally, grain size in the stir zone (SZ) was found to vary with the tool temperature which, in turn, alters the nanoindentation modulus variability and higher tool temperature resulted in widening of the SZ.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Electric field enhanced diffusion welding of alloy 617: Microstructural characteristics and mechanical properties

This study investigated the microstructural characteristics and mechanical behavior of diffusion welded nickel-based Alloy 617 obtained by electric field-assisted sintering (EFAS) using various parameters. The interfacial microstructure exhibited different characteristics including good grain boundary (GB) migration across the interface in the samples diffusion-welded at 1100 °C and a flat interface in the samples joined at 1000 °C and 1050 °C. The interface consisted of fine Al 2 O 3 oxides, while precipitation of interfacial M 23 C 6 carbides was not observed. Grain boundaries migrated across the Al 2 O 3 oxides, leaving these oxides within the grains. Graded grain size was observed, with grain coarsening being more significant near the sample surface due to the temperature gradient induced by EFAS. Tensile testing revealed that the specimens fractured in the matrix away from the interface, indicting strong diffusion-welded joints. Further, the peak tensile strength of 807 MPa was obtained in the samples welded at 1000 °C due to minimal grain growth. The materials obtained at 1100 °C exhibited reduced tensile strength but improved ductility. Strain maps revealed by digital image correlation showed alternating high and low strain segments in the samples produced at 1000 °C and 1050 °C, indicating that the flat interfaces with no GB migration were less ductile compared to the matrix. A greater strain uniformity was observed along the bond interfaces with improved GB migration. The hardness reduced near the sample surfaces due to enlarged grains induced by temperature gradient. This study demonstrates that GB migration and enhanced mechanical strength can be achieved in diffusion-welded Alloy 617.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Unveiling mechanisms and onset threshold of humping in high-speed laser welding

The fabrication of fuel cells relies on a rapid laser welding process. However, challenges arise with the occurrence of humping when the welding speed surpasses a critical threshold, which poses difficulties in achieving a smooth surface finish and a consistent weld strength. This study aims to elucidate the humping mechanisms by analyzing the morphology of molten pool and the characteristics of melt flow at varying welding speeds via in situ synchrotron high-speed X-ray imaging and computational fluid dynamics simulations. Our findings indicate that the short keyhole rear wall, the high backward melt velocity, and the prolonged tail of molten pool are the primary factors contributing to the onset of humping. Furthermore, a dimensionless humping index (π h ) was introduced, which successfully captured the onset threshold of humping across different literatures. This index not only provides a quantitative description of the humping formation tendency but also serves as a valuable tool for optimizing the laser welding process.

42 ENGINEERING↗