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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Comparative study of microstructure and properties of low-alloy-steel/nickel-based-alloy interfaces in dissimilar metal weld joints prepared by different GTAW methods

Highlights: • Number of type I and type II boundaries: H-DMWJ > C-DMWJ > W-DMWJ. • Width carbon depleted zones and carbon enriched zones: H-DMWJ > C-DMWJ > W-DMWJ. • Type I and type II boundaries are susceptible to SCC and cracked during mechanical bending. • C-DMWJ has the best performance while H-DMWJ has the worst one. - Abstract: Low-alloy-steel/buttering (or welding) metal interfaces in safe-end dissimilar metal weld joints (DMWJs) at nuclear power plants are weak parts. To optimize the quality of the safe-end DMWJs used for the construction of domestic nuclear power plants and to improve the welding productivity, a series of DMWJs in full-size have been manufactured by using different gas tungsten arc welding (GTAW) methods. The microstructure, chemical composition distribution and local properties across the SA508/52(M) interfaces in these DMWJs are studied. The high heat input and long high temperature duration during hot-wire GTAW process promote the carbon diffusion and grain boundary migration and hence the DMWJ by hot-wire GTAW has the largest amount of type I and type II boundaries and the widest carbon depleted zones and carbon enriched zones, which resulting in the worst performance among the DMWJs in simulated primary water. Type I and type II boundaries are high angle random grain boundaries (RGBs) and Cr-depleted zones are found along type II boundaries resulting in their high susceptibility to SCC. In addition, these boundaries are prone to be cracked during mechanical bending process.

36 MATERIALS SCIENCE↗
DOE OSTI2019

DeepWelding: a Deep Learning Enhanced Approach to GTAW Using Multi-source Sensing Images

Deep learning has great potential to reshape manufacturing industries. In this paper, we present DeepWelding, a novel framework that applies deep learning techniques to improve gas tungsten arc welding (GTAW) process monitoring and penetration detection using multi-source sensing images. The framework is capable of analyzing multiple types of optical sensing images synchronously and consists of three deep learning enhanced consecutive phases: image preprocessing, image selection, and weld penetration classification. Specifically, we adopted generative adversarial networks (pix2pix) for image denoising and classic convolutional neural networks (AlexNet) for image selection. Both pix2pix and AlexNet delivered satisfactory performance. However, five individual neural networks with heterogeneous architectures demonstrated inconsistent generalization capabilities in the classification phase when holding out multi-source images generated with specific experimental settings. Therefore, two ensemble methods combining multiple neural networks were designed to improve the model performance on unseen data collected from different experimental settings. In conclusion, we also found that the quality of model prediction was heavily influenced by the data stream collection environment. We think these findings are beneficial for the broad intelligent welding community.

42 ENGINEERING↗
DOE OSTI2018

Study on Microstructure and Properties of AZ31/6061 GTAW Butt Joints According to Mg-Cd-Al Ternary System

AZ31 Mg and 6061 Al were successfully joined by GTAW method with Cd as filler metal. The tensile strength of the joints reached 170 MPa through this alloying method according to Mg-Cd-Al ternary system. The microstructure, phases, element distribution and fracture mechanism of alloyed joints were studied. The results suggest that the fusion zone is mainly composed of CdMg compounds and Al-based solid solution. A transitional zone thickness of 40-80 μm with few primary Al{sub 12}Mg{sub 17} and fine Al{sub 12}Mg{sub 17}-MCMS (MCMS represents a mixture of CdMg and CdMg{sub 3} compounds) eutectic is formed between the AZ31 base metal and fusion zone. The fracture surface presents three distinct regions because of the different distribution of Al{sub 12}Mg{sub 17} compounds in the transitional zone. The alloying with Cd filler metal can disperse Al{sub 12}Mg{sub 17} intermetallic compounds and form CdMg ordered compounds with excellent plasticity. Thus, the mechanical properties of alloyed joints have been significantly enhanced.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Microstructure of a safe-end dissimilar metal weld joint (SA508-52-316L) prepared by narrow-gap GTAW

The microstructure, residual strain and interfacial chemical composition distribution of a safe-end dissimilar metal weld joint (DMWJ, SA508-52-316L) prepared by narrow-gap gas-tungsten arc welding (NG-GTAW) were studied by optical microscope (OM) and scanning electron microscope equipped with an energy dispersive X-ray microanalysis (SEM/EDX) and an electron back scattering diffraction (EBSD) system. Complex microstructure and chemical composition distribution are found, especially at the SA508-52 interface and the 52-316L interface. In brief, a complicated microstructure transition exists within the SA508 heat affected zone (HAZ); the residual strain, the fraction of high angle random grain boundaries and low angle boundaries decrease with increasing the distance from the fusion boundary in 316L HAZ; neither typical type II boundary nor obvious carbon-depleted zone is found near the SA508-52 interface; dramatic and complicated changes of the contents of the main elements, Fe, Cr and Ni, are observed at the distinct interfaces, especially at the SA508-52 interface. No carbon concentration is found at the SA508-52 interface. - Highlights: •Residual strain and GBCD change as a function of the distance from FB in 316L HAZ. •Neither type II boundary nor obvious carbon-depleted zone is found in SA508 HAZ. •No carbon concentration is found at the SA508-52 interface. •The middle part of the DMWJ has the highest residual strain.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Model Predictive Control of GTAW Weld Pool Penetration

Weld pool penetration is a major implication of weld integrity but it is not directly measurable. The key to solving this issue is to find or create observable phenomena that can be used to derive the penetration. An innovative method has recently been proposed/developed to derive the penetration from the reflection of projected laser from an oscillating weld pool surface. Yet, in addition to the penetration, the oscillation, thus the reflection determined by it, also depends on the pulse amplitude of the current, which needs to be adjusted as the control variable to feedback control the penetration as the output. This dependence imposes a constraint on the control algorithm such that the adjustment of the control variable must be relatively slow to minimize additional sources that affect the oscillation/sensor. This restriction on the control variable adjustment also brings challenges to the identification of the dynamic model of the weld process to be controlled, which has to apply a varying control signal to stimulate the dynamics. To solve these issues, this letter proposes using a low order model to catch the dominant dynamics such that a high speed variation in the control signal can be avoided when choosing the model predictive control algorithm. This allows use of relatively poor models to determine the adjustment on the pulsing amplitude, and still achieves relatively accurate control. Experimental results verified the effectiveness of the resultant penetration control system despite the restrictions imposed by the penetration sensor, which affect the identification of high order dynamics and free adjustments of control variables.

42 ENGINEERING↗
DOE OSTI2018

Dynamic Reflection Behaviors of Weld Pool Surface in Pulsed GTAW

Here in this paper, a method was studied to determine the weld pool penetration based on the dynamic behaviors of the weld pool surface. To this end, relative works were per- formed to observe, measure, and analyze the development of a weld pool through the imaged laser matrix dots and correlate the changes in the imaged dots to the weld pool penetration. It was found that during partial joint penetration, the imaged dot matrix on the weld pool was clear and bright, but became fuzzier and dimmer when critical/complete joint penetration was reached. Results showed this was related to the root damping force in partial penetration, which obstructed the oscillation of the weld pool and made the surface relatively still for imaging. When critical/complete joint penetration was reached, the root damping force was diminished, and thus the pool surface oscillation was stronger, which led to dimmer and fuzzier images. By quantifying the relation between the image grayness variations with the penetration stages, a method was found for the automated control of the welding process.

42 ENGINEERING↗
DOE OSTI2015

Gas tungsten arc welding and friction stir welding of ultrafine grained AISI 304L stainless steel: Microstructural and mechanical behavior characterization

In the present study, an ultrafine grained (UFG) AISI 304L stainless steel with the average grain size of 650 nm was successfully welded by both gas tungsten arc welding (GTAW) and friction stir welding (FSW). GTAW was applied without any filler metal. FSW was also performed at a constant rotational speed of 630 rpm and different welding speeds from 20 to 80 mm/min. Microstructural characterization was carried out by High Resolution Scanning Electron Microscopy (HRSEM) with Electron Backscattered Diffraction (EBSD) and Transmission Electron Microscopy (TEM). Nanoindentation, microhardness measurements and tensile tests were also performed to study the mechanical properties of the base metal and weldments. The results showed that the solidification mode in the GTAW welded sample is FA (ferrite–austenite) type with the microstructure consisting of an austenite matrix embedded with lath type and skeletal type ferrite. The nugget zone microstructure in the FSW welded samples consisted of equiaxed dynamically recrystallized austenite grains with some amount of elongated delta ferrite. Sigma phase precipitates were formed in the region ahead the rotating tool during the heating cycle of FSW, which were finally fragmented into nanometric particles and distributed in the weld nugget. Also there is a high possibility that the existing delta ferrite in the microstructure rapidly transforms into sigma phase particles during the short thermal cycle of FSW. These suggest that high strain and deformation during FSW can promote sigma phase formation. The final austenite grain size in the nugget zone was found to decrease with increasing Zener–Hollomon parameter, which was obtained quantitatively by measuring the peak temperature, calculating the strain rate during FSW and exact examination of hot deformation activation energy by considering the actual grain size before the occurrence of dynamic recrystallization. Mechanical properties observations showed that the welding efficiency of the FSW welded sample is around 70%, which is more than 20% higher than the GTAW welded sample. - Highlights: • Microstructure and mechanical properties of UFG 304L stainless steel were studied during GTAW and FSW. • Sigma phase formation mechanism was studied during FSW of 304L stainless steel. • THERMOCALC analysis was performed to obtain possible formation temperatures for sigma phase. • Nano-mechanical twins were found in the TMAZ region.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Welding investigation of a wrought FeMnAl steel for armor application

A new wrought FeMnAl steel contains high Mn content and high Al content. This steel has high strength. When it is compared to conventional steels with the similar strength, its density is lower. FeMnAl steels can be used in armor vehicle manufacturing, in which welding is a common practice to join the steel plates together. The welding of FeMnAl steels has not been sufficiently studied before. Here, this work studied the weldability and weld microstructure of an FeMnAl steel and provided useful data for the welding practice of FeMnAl steels. The influences of weld filler metals and shielding gases were studied, using single-pass bead-on-plate gas metal arc welding (GMAW) and gas tungsten arc welding (GTAW) two processes. The optimal filler metals and shielding gases have been determined to achieve the better weldability. Autogenous GTAW and electron beam welding were also conducted. Cracks were found in the HAZ in the GMAW welds and electron beam welds. Fewer or no cracks were found in the HAZ of GTAW welds. SEM, SEM/EDX, TEM, TEM/EDX, and EBSD were used to analyze the grain boundaries in the HAZ and the cracks. The grain boundaries of FeMnAl base metal are mostly high angle grain boundaries (HAGBs), which attract impurity and solute atoms and cause intergranular cracking. The proposed cracking mechanisms are liquation cracking and ductility-dip cracking. It was found that GTAW provides much better weldability for the FeMnAl steel than GMAW and EBW.

FeMnAl steel↗
DOE OSTI2015

A study of internal structure in components made by additive manufacturing process using 3 D X-ray tomography

Additive manufacturing methods are gaining increasing popularity for rapidly and efficiently manufacturing parts and components in the industrial context, as well as for domestic applications. However, except when used for prototyping or rapid visualization of components, industries are concerned with the load carrying capacity and strength achievable by additive manufactured parts. In this paper, the wire-arc additive manufacturing (AM) process based on gas tungsten arc welding (GTAW) has been examined for the internal structure and constitution of components generated by the process. High-resolution 3D X-ray tomography is used to gain cut-views through wedge-shaped parts created using this GTAW additive manufacturing process with titanium alloy materials. In this work, two different control conditions for the GTAW process are considered. The studies reveal clusters of porosities, located in periodic spatial intervals along the sample cross-section. Such internal defects can have a detrimental effect on the strength of the resulting AM components, as shown in destructive testing studies. Closer examination of this phenomenon shows that defect clusters are preferentially located at GTAW traversal path intervals. These results highlight the strong need for enhanced control of process parameters in ensuring components with minimal defects and higher strength.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Influence of Alloy and Solidification Parameters on Grain Refinement in Aluminum Weld Metal due to Inoculation

The goals are: (1) Establish how much Ti/B grain refiner is need to completely refine aluminum weld metal for different alloys and different welding conditions; (2) Characterize how alloy composition and solidification parameters affect weld metal grain refinement; and (3) Apply relevant theory to understand observed behavior. Conclusions are: (1) additions of Ti/B grain refiner to weld metal in Alloys 1050, 5083, and 6082 resulted in significant grain refinement; (2) grain refinement was more effective in GTAW than LBW, resulting in finer grains at lower Ti content - reason is limited time available for equiaxed grain growth in LBW (inability to occlude columnar grain growth); (3) welding travel speed did not markedly affect grain size within GTAW and LBW clusters; and (4) application of Hunt CET analysis showed experimental G to be on the order of the critical G{sub CET}; G{sub CET} was consistently higher for GTAW than for LBW.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Mechanical Behavior of Commercially Pure Titanium Weldments at Lower Temperatures

Commercially pure titanium is used for low-temperature applications due to good toughness attributed to single-phase microstructure (α). Electron beam welding (EBW) and gas tungsten arc welding (GTAW) processes have been used for welding two grades of commercially pure titanium (Grade 2 and Grade 4). Martensitic microstructure is found to be finer in the case of EBW joint as compared to GTAW joint due to faster rate of cooling in the former process. Weldments have been characterized to study the mechanical behavior at ambient (298 K) and cryogenic temperatures (20 and 77 K). Strength of weldments increases with the decrease in temperature, which is found to be more prominent in case of Grade 4 titanium as compared to Grade 2. Weld efficiency of Grade 4 is found to be higher at all the temperatures (ambient, 77 and 20 K). However, ultimate tensile strength/yield strength ratio is higher for Grade 2 as compared to Grade 4. % Elongation is found to increase/retained at cryogenic temperatures for Grade 2, and it is found to decrease for Grade 4. Electron backscattered diffraction analysis and transmission electron microscopy of deformed samples confirmed the presence of extensive twinning in Grade 2 and the presence of finer martensitic structure in Grade 4. Fractography analysis of tested specimens revealed the presence of cleavage facets in Grade 4 and dimples in specimens of Grade 2. Higher strength in Grade 4 is attributed to higher oxygen restricting the twin-assisted slip, which is otherwise prominent in Grade 2 titanium.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Electroslag additive manufacturing: A pathway for high throughput near net shape production

Electroslag Additive Manufacturing (ESAM), a new high-throughput additive manufacturing (AM) method that combines Electroslag Strip Cladding (ESC) and wire arc AM (WAAM) is introduced. This combination enables the high deposition rate of ESC (more than 20 kg/h with a 60 mm strip electrode) to benefit from the precise geometric control of WAAM. As a precursor to ESAM, the ESC process is investigated in an AM context independently by evaluating both direct and staggered bead-stacking strategies and analyzing the microstructural and mechanical properties of each. This is followed by an ESAM demonstration producing an annular geometry by pairing ESC with gas tungsten arc welding (GTAW), wherein GTAW is utilized to construct annular walls that are subsequently infilled via ESC. The microstructure and mechanical properties of ESC-only AM are compared with that of the ESAM method and it is shown that printed integral retaining walls do not impact the resulting mechanical properties of ESAM. Furthermore, results indicate that ESAM-produced Alloy 625 parts exhibit tensile properties on par with cast counterparts, supporting the method’s scalability to components exceeding one metric ton, and possibly making ESAM a viable future manufacturing approach for competitive production of large-scale components currently manufactured by casting and forging.

Additive manufacturing↗
DOE OSTI2025

Creep performance and microstructure of grade 91 steel weldments with integrated welding and thermal processing

Ferritic-Martensitic steel welds typically require post weld heat treatment (PWHT) to restore toughness and high temperature performance. This off-line thermal process reduces disparities between weld and base metal, but can cause distortion, cracking, or simply be impractical due to assembly size and joint non-uniformity. Here we show integrated welding and thermal processing applied to modified 9Cr-1Mo (Grade 91) steel, favored for advanced power generation applications, performed in real time through the addition of a secondary heat source near the primary weld head. Optimal integrated processing reduces weld fusion and heat affected zone hardness by 125 HV, approaching performance of conventional 730 °C, 60 min PWHT processing. Microstructures and mechanical performance are compared for mechanized GTAW welds, with equivalent lifetimes noted in cross-weld creep rupture tests up to 234 MPa at 550 °C, and up to 104 MPa at 650 °C. The integrated process was validated on a Grade 91 pressure vessel with multipass cold wire feed GTAW. After 550 °C, 71.4 bar thermomechanical cyclic testing, the maximum weld hardness is <350 HV.

36 MATERIALS SCIENCE↗
DOE OSTI2022

The Toughness of High-Strength Steel Weld Metals

Low-temperature phase transformation (LTPT) welding consumables are a new class of welding wires developed to mitigate hydrogen-induced cracking in the welding of high-strength steels without preheating or postweld heat treatment. LTPT weld metals have a high strength, but their toughness needs further investigation. LTPT weld metals predominately contain a martensite microstructure, which is necessary to achieve high strength; however, martensitic weld metals containing oxide inclusions have relatively poor toughness. For this study, three welding processes — gas metal arc welding (GMAW), gas tungsten arc welding (GTAW), and hot wire GTAW — were investigated. Optical microscopy, scanning electron microscopes, and transmission electron microscopes were employed for characterization. The role of the shielding gas in the formation of oxide inclusions in LTPT weld metals was investigated. The formation of oxide inclusions in the weld metals was related to the CO 2 in the shielding gas. When 100% Ar or a pure inert shielding gas mixture was used for all three welding processes, oxide inclusions were greatly reduced, and the weld metal toughness improved considerably, matching the base metal toughness. The mechanism by which inclusions promote fracture propagation in the weld metal was proposed.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Gigacycle Fatigue Strength Evaluation of Welded 316L Stainless Steels for Mercury Target Vessel

At the Materials and Life Science Experimental Facility (MLF) in J-PARC, liquid mercury target for the pulsed spallation neutron source is in operation. An enclosure vessel for the liquid mercury target made of type 316L stainless steel (SS316L) suffers two kinds of cyclic stress during operation. One is the thermal stress due to the internal heating and swings by proton beam trip. The other is the impulsive stress by the pressure waves generated by the proton beam injection. The total number of loading cycles for the former is ∼104, and the latter is ∼4 × 108 for a year operation in the J-PARC mercury target vessel. The target vessel is assembled by an electron beam welding (EBW) and a gas tungsten arc welding (GTAW). However, fatigue data of welded SS316L up to gigacycle is limited. Ultrasonic fatigue testing, applying load cycles by utilizing ultrasonic resonance, for the welded SS316L was performed to investigate the effect of welding on fatigue behavior up to a gigacycle. The result showed that the fatigue strength degradation by EBW and EBW with GTAW were not recognized up to 109 cycles. Crack initiation in welded specimens nucliated at off-center areas of the specimen whereas the cracks in base metal specimen originated at the specimen center.

Naoe, Takashi [Japan Atomic Energy Agency (JAEA)]↗
DOE OSTI2017

Monitoring Weld Pool Surface and Penetration Using Reversed Electrode Images

The three­-dimensional weld pool top surface shape provides important informa­tion about the state of weld penetration during welding. In this study, a method was developed to quantitatively relate weld pool surface height to the reversed electrode image (REI) on the weld pool surface. This new feature was extracted from the weld pool image using a passive vision-­based monitoring system during gas tungsten arc welding (GTAW). Due to the specular reflection of the weld pool top surface, the REI is visible on the weld pool surface during GTAW. The position of the REI was determined with a robust image processing algorithm. Based on the principle of light reflection, the distance between the electrode tip and the REI (DERI) was related to the weld pool surface height. By assuming the weld pool surface was a spherical mirror, a reflection model was established to calculate the surface height (SH) index based on the measurement of the DERI, arc length, and weld pool geometry. The pro­posed method was verified with bead­-on­plate welding experiments. The SH was positively related to the face reinforcement or depression of the weld bead. This method was applied to monitor the penetration state during bead­on­plate autoge­nous welding, particularly when a complete penetration weld was formed.

42 ENGINEERING↗