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At least 1,441 records · Page 80
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Friction stir welding and self-ion irradiation effects on microstructure and mechanical properties changes within oxide dispersion strengthened steel $\mathrm{MA956}$

We report the joining process for oxide dispersion strengthened (ODS) alloys remains a key challenge facing the nuclear community. The microstructure and mechanical properties were characterized in the base material and friction stir welded ODS MA956 irradiated with 5 MeV Fe 2+ ions from 400 to 500°C up to 25 dpa. Nanoindentation was performed to assess changes in hardness and yield stress, and the dispersed barrier hardening (DBH) model was applied to described results. A combination of scanning transmission electron microscopy (STEM) and atom probe tomography (APT) were used to assess evolution of the microstructure including dispersoids, network dislocations and dislocation loops, nanoclusters, and solid solution concentrations. Overall, softening was observed as a result of increased dose, which was exacerbated at 500°C. The formation and coarsening of new dispersoids was noted while nanoclusters tended to dissolve in the base material, and were not observed in the stir zone. Solute nanocluster evolution was identified as a primary driver of the changes in mechanical properties.

36 MATERIALS SCIENCE↗
DOE OSTI2017

The significance of intermetallic compounds formed during interdiffusion in aluminum and magnesium dissimilar welds

Diffusion couples between Al-AA6111 and Mg-AZ31 have been used to perform a comprehensive review of the IMC phases that develop in this system, including; their growth kinetics, texture, residual stresses, and mechanical properties. The new observations made have been compared to data in the literature with the aim of providing an in-depth understanding of the detrimental effects of interfacial reaction in aluminum–magnesium dissimilar welding. Three Al–Mg binary IMC phases were identified, γ-Al{sub 12}Mg{sub 17} and β-Al{sub 3}Mg{sub 2}, as well as ε-Al{sub 30}Mg{sub 23}, which has not been previously reported in inter-diffusion studies. The β phase had the highest growth rate and became the dominant constituent of the IMC layer at longer reaction times. The ε-phase only appeared after extended heat treatment, via transformation at the Al{sub 12}Mg{sub 17}/Al{sub 3}Mg{sub 2} interface. Intrinsic diffusion coefficients of Al and Mg, calculated in the Al{sub 3}Mg{sub 2} phase, confirmed that Al is the faster diffusing species (by an order of magnitude) during IMC growth. Indentation techniques showed that all the IMC phases had extremely low toughness (1/2 ). FIB-assisted ring core milling revealed that both the β-Al{sub 3}Mg{sub 2} and γ-Al{sub 12}Mg{sub 17} phases contained large residual stresses. Their in-situ fracture toughness was consequently found to be asymmetric with lower values parallel to the joint interface. Greater tensile residual stress was measured in the β-Al{sub 3}Mg{sub 2} phase, which, combined with a lower toughness and more rapid growth kinetics, explains why it is generally the ‘weak link’ in Al–Mg dissimilar joints.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Precipitation behavior of stainless steel weld overlay cladding exposed to a long-term thermal aging

Highlights: • Silicide phase formation with G-phase was first time to be observed. • Precipitation behavior discrepancy existed at different locations of ferrite phase. • G-phase and silicide phase formed along dislocations and within ferrite matrix. • Heterogeneous distribution of elements induced silicide phase formation. - Abstract: Thermal aging behavior of stainless steel weld overlay cladding aged at 400 °C for 10,000 h was investigated. The results showed that the precipitates along the dislocation and within ferrite matrix contained the face-centered cubic structural G-phase and silicide phase. Silicide phase, supposed to be (Fe, Mn){sub 3}Si phase, preferred to form with G-phase along the dislocations and within ferrite matrix. Silicide phase showed a cube-on-cube relationship with ferrite phase and G-phase. Heterogeneous distribution of elements within Ni-Mn-Si clusters was responsible for silicide phase formation during thermal aging process, and the corresponding precipitation behavior discrepancy at different forming sites (dislocations, ferrite matrix and phase boundary) was discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of thermal aging on oxide film of stainless steel weld overlay cladding exposed to high temperature water

Highlights: • Oxide film of aged cladding material in high temperature water was studied. • Thermal aging contributed to high corrosion susceptibility of the ferrite phase. • Highly corrosion of aged ferrite phase was caused by spinodal decomposition. • Carbides along phase boundary facilitated the corrosion of the cladding material. - Abstract: Characteristics of the oxide film formed on stainless steel weld overlay cladding aged at 400 °C for different times in high temperature water were studied. The results showed that thermal aging showed no obvious effect on the volume fraction of ferrite phase but resulted in high corrosion susceptibility of the aged ferrite phase mainly. The oxide film formed on the unaged cladding material consisted of 100-nm-thick inner oxide layer (FeCr{sub 2}O{sub 4} and Cr{sub 2}O{sub 3}) and Fe-rich outer oxide particles (Fe{sub 3}O{sub 4}) with a size of 2 O4, Cr{sub 2}O{sub 3} and Fe{sub 3}O{sub 4}) and outer oxide particles (Fe{sub 3}O{sub 4} and FeCr{sub 2}O{sub 4}) with a size of <700 nm formed on the aged ferrite phase. Spinodal decomposition was considered as the main factor to promote the corrosion of the aged ferrite phase, and existence of carbides along phase boundary facilitated corrosion of the cladding material in high temperature water.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Comparison of microstructural and mechanical properties of friction stir spot welded ultra-high strength dual phase and complex phase steels

Highlights: • The microstructure and mechanical properties of FSSW of ultra high strength DP and CP steels are compared. • The stir zone (SZ) of CP steel mainly consists of bainite and martensite; no bainite is identified in the SZ of DP steel. • The cooling was not fast enough for the CP steel to suppress the effect of chemical composition on phase transformation. - Abstract: The microstructure and mechanical properties of friction stir spot welding of ultrahigh strength dual phase (DP) and complex phase (DP) steels are compared based on the experimental results. For both DP and CP steels, FSSW joints are successfully fabricated using the same process parameter set. While the microstructure of joints are characterized in detail using scanning electron microscopy, electron back scattered diffraction, and transmission electron microscopy, the mechanical property of joints are evaluated by shear tensile tests. The result of the microstructural analysis shows that the stir zone (SZ) of CP steel mainly consists of bainite and martensite, while no bainite is identified in the SZ of DP steel. The existence of bainite in the SZ of CP steel suggests that the cooling rate, which is nearly identical for both CP and DP steels, was not fast enough to suppress the effect of the alloying composition, particularly that of Mn, on the phase transformation. The result of the present study contributes to the development of solid state joining process of the ultrahigh strength steels.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of grain boundary character distribution on weld heat-affected zone liquation cracking behavior of AISI 316Ti austenitic stainless steel

Highlights: • Grain boundary engineered AISI 316 was produced through thermomechanical processing. • Grain boundary engineering resulted in random high angle boundary network disruption. • Liquation cracks are arrested at triple junctions having one or more CSL boundaries. • Grain boundary engineering resulted in improvement in liquation cracking resistance. - Abstract: In this work, the role of grain boundary engineering (GBE) on the weld heat-affected-zone (HAZ) liquation cracking resistance of austenitic stainless steel AISI 316Ti was investigated. Standard wrought-processed alloy 316Ti was cold rolled to 5% strain and subsequently annealed at 1373 K for 30 min to achieve the optimum grain boundary character distribution (GBCD). The GBE samples were found to consist of a significantly higher fraction (72%) of low Ʃ coincident site lattice (CSL) boundaries as compared to the as received (AR) samples (45%). To study the liquation cracking behavior, single-track longitudinal varestraint tests was performed on AR and GBE samples. Microstructural examination revealed that the grain coarsening and the liquation events were less prominent in the GBE samples as compared to the AR samples. It is believed that the reduced segregation of impurities to the special boundaries (coherent Ʃ3 twins, in particular) present in the GBE samples is responsible for the observed improvement in the HAZ liquation cracking resistance.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of tailored martensitic transformation in a thick weld: Residual stresses mitigation, heterogeneous microstructure, and mechanical properties

The residual stresses, microstructural heterogeneities, and mechanical properties were extensively examined in a 25 mm thick weld consisting of two regions which respectively undergo phase transformation at relatively low (100 °C) temperature (LTT) and high (670 °C) temperature (HTT) during cooling. Due to the LTT martensitic transformation, which features a transformation strain of ~4000 με, large compressive residual stress (−510 MPa) was generated as confirmed by neutron diffraction and contour method. Meanwhile, significant heterogeneities were observed in terms of the chemical composition, yield strength, and microhardness across the interface between the LTT and HTT regions. The dependency of martensitic transformation starting temperature (M{sub s}) on chemical composition is empirically formulated, and it reveals that the dependency of M{sub s} on the Ni and Cr compositions becomes stronger when the temperature is lower than 200 °C. The maximum change of residual stresses (σ{sub x}{sup max}) exponentially decreases as M{sub s} decreases in the manner of σ{sub x}{sup max} = 97.5 exp(M{sub s}/227)-649. The fracture behavior was found highly dependent upon the volume fraction of retained austenite influenced by M{sub s}. While a ductile fracture mode was found in the cellular LTT region containing relatively higher amount of retained austenite (~10%), the interface shows transgranular brittle fracture features and sub-cracking due to the relatively small amount of retained austenite and the predominant martensite constituent.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Solid state welding of medium-entropy CrCoNi with heterogeneous, partially recrystallized microstructures

We report that heterogeneous, partially recrystallized (PRX) microstructures have recently been used to improve strength-ductility combinations in high-entropy alloys. However, these microstructures are incompatible with conventional joining processes that require melting or prolonged exposure to elevated temperatures. This work presents an initial exploration of solid state joining in this challenging condition using vaporizing foil actuator welding (VFAW) applied to PRX equiatomic alloy CrCoNi.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Statistical analysis of S—N type environmental fatigue data of Ni-base alloy welds using weibull distribution

In this study, the probabilistic fatigue life model for Ni-base alloys was developed based on the Weibull distribution using statistical analysis of fatigue data reported in NUREG/CR-6909 and the new fatigue data of Alloy 52M/152 and 82/182. The developed Weibull model can consider right-censored data (i.e., non-failed data) and quantify the improved safety (or reliability) based on the level of failure probability. The overall margin in the current fatigue design limit model (ASME design curve + NUREG/CR-6909 F en model) is similar to that of the Weibull model with a cumulative failure probability of approximately 2.5%. The margin in the current fatigue design limit model demonstrated inconsistencies for the Ni-base alloy weld data, whereas the Weibull model showed a consistent margin. Therefore, the Weibull model can systematically mitigate the excessive safety margin.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Room temperature corrosion behaviour of plastically deformed AISI 304 stainless steel by friction stir welding in neutral and acidified chloride solutions

Severe plastic deformation of 304L stainless steel plate was carried out by friction stirring under an isothermal tool temperature at 825°C. The friction stirred zone showed extensive grain refinement, high fraction of low angle grain boundaries due to dislocation rearrangement, Σ3 and Σ9 special grain boundaries and discrete Fe–Cr–Mn-rich particles. Electrochemical polarisation measurements were done on the base metal (BM) and friction stir welded (FSW) specimens in the neutral and acidified 3.5% NaCl solutions. The BM showed marginally better pitting resistance in the neutral chloride solution than the FSW because of a larger fraction of special grain boundaries, and larger grain size. Whereas in acidified 3.5% NaCl solution, the FSW showed better corrosion resistance than the BM. Furthermore, the surface film of the FSW had a lower impedance and a lower defect concentration than the BM indicating higher diffusivity of point defects in the FSW.

36 MATERIALS SCIENCE↗
DOE OSTI2023

A Novel Design for Improving the Control on the Stainless-Steel Vessel Welding Process for Superconducting Magnets

Stainless steel vessels see widespread use in superconducting magnets for particle accelerator applications. Their function varies in different magnet designs: they always provide the necessary liquid helium containment, but in some cases are also used to provide azimuthal prestress and can also be welded to the magnet end plate to provide additional longitudinal stiffness. A magnet designed with the bladder and key technology does not rely on the structural role of the vessel. They are structurally supported using azimuthally prestressed aluminum shells, and the longitudinal constraint by rods. In this case, the magnet designer would generally like to minimize the interaction between the magnet and the stainless-steel vessel and to minimize the coil stress variation due to the vessel. The stress state in the vessel and in the coil is a function of the circumferential interference, defined as the vessel azimuthal length minus the magnet circumference. The vessel and the magnet azimuthal length machining tolerances are relatively large resulting in significant stress variations in the superconducting coils. Here in this paper we introduce an interference-control shim, which can significantly limit the stress variation of the coils for a given variation of the interference. The effectiveness of the interference-control shim is evaluated numerically on the MQXF, the low-β quadrupole for the High Luminosity LHC.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Weld Development for Aluminum Fission Chamber

The Sigma welding team was approached to help fabricate a small fission chamber (roughly ½ inch dia. x ½ inch tall cylinder). These chambers are used as radiation sensors that contain small traces of radionuclides (Cf 252, U 235, and U 238) that serve to ionize gas atoms in addition to external radiation. When a voltage is applied within the chamber, the resulting ion flow can be calibrated and monitored. Aluminum has the advantage of not forming radioactive compounds when exposed to high external radiation (except from minor Na alloy content). Since aluminum has not been used before in this application, this presented an unexplored challenge.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Laser confocal microscope for analysis of 3013 inner container closure weld region

As part of the protocol to investigate the corrosion in the inner container closure weld region (ICCWR) a laser confocal microscope (LCM) was used to perform close visual examination of the surface and measurements of corrosion features on the surface. However, initial analysis of selected destructively evaluated (DE) containers using the LCM revealed several challenges for acquiring, processing and interpreting the data. These challenges include topography of the ICCWR sample, surface features, and the amount of surface area for collecting data at high magnification conditions. In FY17, the LCM parameters were investigated to identify the appropriate parameter values for data acquisition and identification of regions of interest. Using these parameter values, selected DE containers were analyzed to determine the extent of the ICCWR to be examined.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Quarterly Management Document – FY17, 3rd Quarter, Physics-based Creep Simulations of Thick Section Welds in High Temperature and Pressure Applications

Progress on modeling the the creep of thick sections welds in Alloy 740H is reported. Climb of dislocations during creep has been added to the model. Creep testing is continuing and stress drop testing has been initiated to determine the threshold stress of this particle-strengthened alloy. Work has also begun on generating synthetic microstructures, representative of experimental microstructures, from data collected with EBSD. This will enable incorporation of experimental microstructures into the modeling and simulation effort.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Quarterly Management Document – FY18, 1st Quarter, Physics-based Creep Simulations of Thick Section Welds in High Temperature and Pressure Applications

During the 1st quarter of FY18 modeling and simulation of the creep of thick section welds continued although some workscope from FY17 still needs to be completed. Four stress drop tests to determine the threshold stress for creep in Alloy 740H were carried out. One has finished and post-test analysis shows a threshold creep stress of about 67 MPa which is lower than that found in a previous program for Alloy 617 at the same temperature. Alloy 617 is not as creep resistant as Alloy 740H and the reason for the lower than expected threshold stress for Alloy 740H is being investigated further. Also analysis of the gamma prime particle size in an aging study closely follows the aging characteristics of gamma prime in Alloy 617. Further analysis of other aged materials will be carried out during the 2nd quarter of FY18 to confirm that the aging characteristic of Alloy 740H are the same as that for Alloy 617.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Quarterly Management Document – FY19, 3rd Quarter, Physics-based Creep Simulations of Thick Section Welds in High Temperature and Pressure Applications

During the 3rd quarter of FY19, a strategy was developed to incorporate creep damage arising from diffusional processes into a computational model describing creep behavior of thick section welds in Alloy 740H. This approach will be followed in the 4th quarter modify the existing creep model to include diffusional damage processes in its predictions of creep behavior and especially the transition to tertiary creep behavior which will allow the prediction of creep rupture time.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Quarterly Management Document – FY19, 4th Quarter, Physics-based Creep Simulations of Thick Section Welds in High Temperature and Pressure Applications

During the 4th quarter of FY19, diffusional creep effects, based on the local stress tensor, was incorporated into the modeling and simulation efforts on thick section welds of Alloy 740H. The model is now complete and addresses both dislocation-based creep as well as diffusional creep mechanisms. Preliminary result indicate the model is now capable of predicting a transition from secondary creep to tertiary creep. Calibration of the model using experimental creep data remains to be performed and will be done in the first quarter of FY20.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Ultrasonic Welding of Dissimilar Plastics

The ultrasonic welding of dissimilar plastics is considered. This process depends on the diffusion of molecules to the surface of polymers at temperatures corresponding to the viscoplastic state. The weldability of a pair of dissimilar plastics depends on the compatibility of their acoustic properties, assessed in terms of the density and viscosity of the melts.

42 ENGINEERING↗