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At least 235 records · Page 13

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DOE OSTI2017

A Monte Carlo model for 3D grain evolution during welding

Welding is one of the most wide-spread processes used in metal joining. However, there are currently no open-source software implementations for the simulation of microstructural evolution during a weld pass. Here we describe a Potts Monte Carlo based model implemented in the SPPARKS kinetic Monte Carlo computational framework. The model simulates melting, solidification and solid-state microstructural evolution of material in the fusion and heat-affected zones of a weld. The model does not simulate thermal behavior, but rather utilizes user input parameters to specify weld pool and heat-affect zone properties. Weld pool shapes are specified by Bezier curves, which allow for the specification of a wide range of pool shapes. Pool shapes can range from narrow and deep to wide and shallow representing different fluid flow conditions within the pool. Surrounding temperature gradients are calculated with the aide of a closest point projection algorithm. Furthermore, the model also allows simulation of pulsed power welding through time-dependent variation of the weld pool size. Example simulation results and comparisons with laboratory weld observations demonstrate microstructural variation with weld speed, pool shape, and pulsed-power.

36 MATERIALS SCIENCE↗

DOE OSTI2016

Definition of Beam Diameter for Electron Beam Welding

It is useful to characterize the dimensions of the electron beam during process development for electron beam welding applications. Analysis of the behavior of electron beam welds is simplest when a single number can be assigned to the beam properties that describes the size of the beam spot; this value we generically call the “beam diameter”. This approach has worked well for most applications and electron beam welding machines with the weld dimensions (width and depth) correlating well with the beam diameter. However, in recent weld development for a refractory alloy, Ta-10W, welded with a low voltage electron beam machine (LVEB), it was found that the weld dimensions (weld penetration and weld width) did not correlate well with the beam diameter and especially with the experimentally determined sharp focus point. These data suggest that the presently used definition of beam diameter may not be optimal for all applications. The possible reasons for this discrepancy and a suggested possible alternative diameter definition is the subject of this paper.

42 ENGINEERING↗

DOE OSTI2026

Integrated Simulation of Weld Residual Stress Evolution and Crack Propagation Using XFEM

Nuclear power plant components operate in environments that promote multiple degradation mecha- nisms, several of which involve crack initiation and growth. An ongoing effort in the U.S. Department of Energy’s Nuclear Energy Advanced Modeling and Simulation (NEAMS) program is developing a general capability within the Multiphysics Object Oriented Simulation Environment (MOOSE) framework for simulating three-dimensional crack growth under a range of driving conditions, including fatigue, stress corrosion cracking (SCC), brittle fracture, and stress-relaxation cracking. This report demonstrates an end-to-end workflow that uses this capability to model weld-residual-stress-driven SCC in the J-groove weld of a pressurized-water reactor control rod drive mechanism penetration in the vessel head. The workflow consists of a thermomechanical welding simulation with temperature-dependent plasticity, followed by cooldown to ambient conditions, and a restart of the simulation using the MOOSE extended finite element method (XFEM) module to propagate a three-dimensional crack through the residual stress field. New welding capabilities were developed to properly initialize newly activated elements in the weld region, and robustness improvements were made to the mesh-based algorithm for defining cutting planes in the 3D XFEM algorithm, allowing it to handle complex crack fronts and stress fields. Together these advances allowed the simulated SCC crack to grow from an initial elliptical flaw in the weld, across the weld, through the tube wall, and almost to the triple point (where the weld, tube, and reactor pressure vessel head intersect) over roughly 36 years of simulated service. These results demonstrate a workflow that can be extended to fully three-dimensional welding simulations and more complex crack interaction problems.

42 - ENGINEERING↗

DOE OSTI2019

FEAA90: Physics-based Creep Simulation of Thick Section Welds in High Temperature and Pressure Applications

Welded microstructures of ?’-strengthened, nickel-based alloy, 740H, contain significant microstructural heterogeneities that can have a strong influence on the dislocation dynamics, resulting in very different creep behavior compared to the base metal. Crystal plasticity-based finite element method (CPFEM) has been widely used to incorporate the effect of microstructural heterogeneities on deformation at the polycrystalline scale and is being utilized in this work to model the creep behavior of 740H GTA welds. Current model development is focused on secondary creep considering dislocation climb, glide, and, anti-phase boundary shearing or Orowan looping. A dislocation-density based CPFEM model addressing these mechanisms is currently being implemented in the Multi-Physics Object Oriented Simulation Environment (MOOSE: mooseframework.org) software that provides the ability to solve problems involving multiple physics concurrently and implicitly. Short term creep tests at 700-800°C of cross weld and all-weld metal samples from GTA welds in Alloy 740 will be used to determine input parameters for the model. Long term creep tests at 750°C on Alloy 740H GTA welds will be used to validate the results of modeling and simulation. The model will also be applied to welds with microstructures that develop during hybrid laser arc welding (HLAW) of Alloy 740H to evaluate the effect of weld microstructure on of the validity of the model.

20 FOSSIL-FUELED POWER PLANTS↗

DOE OSTI2015

Demonstration of friction stir Welding (FSW) Technology for packaging of used nuclear fuel - 15025

With uncertainty surrounding the disposal of US Used Nuclear Fuel (UNF), the nuclear industry is making an effort to identify an interim storage solution. One of the alternatives is to simply manage UNF on-site until a permanent repository can be established - extended storage. In an effort to provide a technical basis for extended storage, the Department of Energy (DOE) performed a gap analysis identifying several issues that need to be addressed. One of the issues relates to the performance of the UNF dry storage container (DSC). DSC container welds (fabricated using conventional fusion welding processes) are sensitive to Stress Corrosion Cracking (SCC), and their extended-term performance is questionable. Fluor, the Pacific Northwest National Laboratory (PNNL) and the Savannah River National Laboratory (SRNL) believe that Friction Stir Welding (FSW) technology may be able to address the DSC issue. The Fluor team collaborated on a project to demonstrate feasibility of FSW for DSC packaging (closure-welding). This paper reports on a 2-year project in which FSW process conditions were established (for DSC container materials and dimensions) and used in the preparation of mechanical and corrosion specimens for testing and evaluation. In addition to FSW test specimens, fusion welded specimens were prepared and tested in like manner for comparative purposes. The results show excellent mechanical properties (meeting construction code requirements) and significant improvement in corrosion performance - over the fusion welded specimens. It is believed these findings will be of significant value to efforts underway to provide a technical basis for DSC extended storage performance. FSW technology appears to be an acceptable alternative or candidate process, capable of addressing the SCC sensitivity issues associated with fusion-welded DSC fabrication welds. In addition, as an alternative to on-site, extended storage of UNF, the DOE has proposed the development and construction of a Consolidated Interim Storage Facility (CISF). UNF would be accepted from the many, local sites for consolidation at one or more CISFs until a permanent repository could be established. A recent CISF study indicates that UNF currently packaged into DSCs will likely be re-packaged into containers suitable for permanent storage. The study also indicates that re-packaging is best performed under water in a pool. FSW technology has several specific advantages for this application, 1) because it utilizes machine-tool equipment, a welding station can easily perform a cutting operation, and 2) FSW has been successfully demonstrated capable of joining container materials under water. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗

DOE OSTI2015

FSW of Aluminum Tailor Welded Blanks across Machine Platforms

Development and characterization of friction stir welded aluminum tailor welded blanks was successfully carried out on three separate machine platforms. Each was a commercially available, gantry style, multi-axis machine designed specifically for friction stir welding. Weld parameters were developed to support high volume production of dissimilar thickness aluminum tailor welded blanks at speeds of 3 m/min and greater. Parameters originally developed on an ultra-high stiffness servo driven machine where first transferred to a high stiffness servo-hydraulic friction stir welding machine, and subsequently transferred to a purpose built machine designed to accommodate thin sheet aluminum welding. The inherent beam stiffness, bearing compliance, and control system for each machine were distinctly unique, which posed specific challenges in transferring welding parameters across machine platforms. This work documents the challenges imposed by successfully transferring weld parameters from machine to machine, produced from different manufacturers and with unique control systems and interfaces.

aluminum↗

DOE OSTI2015

Physics-based process model approach for detecting discontinuity during friction stir welding

The goal of this work is to develop a method for detecting the creation of discontinuities during friction stir welding. This in situ weld monitoring method could significantly reduce the need for post-process inspection. A process force model and a discontinuity force model were created based on the state-of-the-art understanding of flow around an friction stir welding (FSW) tool. These models are used to predict the FSW forces and size of discontinuities formed in the weld. Friction stir welds with discontinuities and welds without discontinuities were created, and the differences in force dynamics were observed. In this paper, discontinuities were generated by reducing the tool rotation frequency and increasing the tool traverse speed in order to create "cold" welds. Experimental force data for welds with discontinuities and welds without discontinuities compared favorably with the predicted forces. The model currently overpredicts the discontinuity size.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2018

Residual Strain Analysis in Linear Friction Welds of Similar and Dissimilar Titanium Alloys Using Energy Dispersive X-ray Diffraction

Residual strains in linear friction welds (LFW) of similar, Ti-6Al-4V (Ti64) welded to Ti-6Al-4V, and dissimilar, Ti-6Al-4V welded to Ti-5Al-5V-5Mo-3Cr (Ti5553), titanium alloys have been characterized using energy-dispersive X-ray diffraction. For each type of LFW, one sample was chosen in the as-welded (AW) condition and another sample was selected after a post-weld heat treatment (HT). Residual strains have been separately studied in the alpha and beta phases of the material, and five components (three axial and two shear) have been reported in each case. In-plane axial components of the residual strains show a smooth and symmetric behavior about the weld center for the Ti64-Ti64 LFW samples in the AW condition, whereas these components in the Ti64-Ti5553 LFW sample show a symmetric trend with jump discontinuities. Such jump discontinuities, observed in both the AW and HT conditions of the Ti64-Ti5553 samples, suggest different strain-free lattice parameters in the weld region and the parent material, whereas the results from the Ti64-Ti64 LFW samples in both AW and HT conditions suggest nearly uniform strain-free lattice parameters throughout the weld region. Finally, the observed trends in the in-plane axial residual strain components have been rationalized by the corresponding microstructural changes and variations across the weld region via backscatter electron (BSE) images.

36 MATERIALS SCIENCE↗

DOE OSTI2019

Welding Process Development for Spent Nuclear Fuel Canister Repair

The potential for stress corrosion cracking (SCC) of welded stainless-steel interim storage containers for spent nuclear fuel (SNF) has been identified as a high priority data gap. This paper presents a fusion welding process that was developed for SNF canister repair. Submerged arc welding (SAW) was developed to weld 12.7 mm (0.5 in.) thick 304L stainless steel plates to simulate the initial welds on SNF canisters. The SAW procedure was qualified following ASME Boiler and Pressure Vessel Code requirements. During SAW, the welding temperature was recorded at various locations by using thermocouples. After SAW, weld microstructures were characterized, joint mechanical properties were tested, and the maximum tensile residual stress direction was identified. After SAW procedure qualification, artificial cracks were excavated perpendicular to the maximum tensile residual stress direction in the SAW heat affected zone. Machine cold-wire gas tungsten arc welding (CW-GTAW) was developed and used for repair welding at cracked locations.

Tang, Wei↗

DOE OSTI2019

Oxygen Effects on Solidification Behavior of Gas Tungsten Arc–Welded Laser Powder Bed Fusion–Fabricated 304L Stainless Steel

ct The laser – powder bed fusion (L-PBF) process inherently accumulates interstitial gas elements during powder fabrication and laser deposition processes. Such elements can lead to localized variations in the weld pool and affect the solidification behavior (when compared with its wrought equivalent), in addition to chemical micro-segregation within the fabricated material. This study was conducted to characterize the solidification behavior of gas tungsten arc welds made on L-PBF 304L stainless steel. The effect of surface active elements on the local solidification rates was studied. An emphasis was placed on the role local solidification rates and temperature gradients throughout the weld play on the resultant weld solidification structure and micro-segregation. It was determined that gas tungsten arc welds on L-PBF 304L stainless steel exhibited a vermicular ferrite solidification structure compared to a mix of vermicular and lathy ferrite structure in wrought 304L. The varying thermal gradients affected the solidification modes and partitioning of elements leading to fluctuations of micro-segregation in the L-PBF 304L. Macroscopically, such partitioning affected the surface tension within the weld pool, producing asymmetric weld pool geometries. The compositional differences between wrought and L-PBF fabricated 304L stainless steels resulted in irregular solidification behaviors during welding affecting the final weld microstructure.

36 MATERIALS SCIENCE↗

DOE OSTI2015

Improving Fatigue Performance of AHSS Welds

Reported herein is technical progress on a U.S. Department of Energy CRADA project with industry cost-share aimed at developing the technical basis and demonstrate the viability of innovative in-situ weld residual stresses mitigation technology that can substantially improve the weld fatigue performance and durability of auto-body structures. The developed technology would be cost-effective and practical in high-volume vehicle production environment. Enhancing weld fatigue performance would address a critical technology gap that impedes the widespread use of advanced high-strength steels (AHSS) and other lightweight materials for auto body structure light-weighting. This means that the automotive industry can take full advantage of the AHSS in strength, durability and crashworthiness without the concern of the relatively weak weld fatigue performance. The project comprises both technological innovations in weld residual stress mitigation and due-diligence residual stress measurement and fatigue performance evaluation. Two approaches were investigated. The first one was the use of low temperature phase transformation (LTPT) weld filler wire, and the second focused on novel thermo-mechanical stress management technique. Both technical approaches have resulted in considerable improvement in fatigue lives of welded joints made of high-strength steels. Synchrotron diffraction measurement confirmed the reduction of high tensile weld residual stresses by the two weld residual stress mitigation techniques.

42 ENGINEERING↗

DOE OSTI2017

Nanoindentation of Electropolished FeCrAl Alloy Welds

The present report summarizes Berkovich nanoindentation modulus and hardness measurements on two candidate FeCrAl alloys (C35M and C37M) on as-received (AR) and welded samples. In addition, spherical nanoindentation stress-strain measurements were performed on individual grains to provide further information and demonstrate the applicability of these protocols to mechanically characterizing welds in FeCrAl alloys. The indentation results are compared against the reported tensile properties for these alloys to provide relationships between nanoindentation and tensile tests and insight into weldsoftening for these FeCrAl alloys. Hardness measurements revealed weld-softening for both alloys in good agreement with tensile test results. C35M showed a larger reduction in hardness at the weld center from the AR material compared to C37M; this is also consistent with tensile tests. In general, nanohardness was shown to be a good predictor of tensile yield strength and ultimate tensile stress for FeCrAl alloys. Spherical nanoindentation measurements revealed that the fusion zone (FZ) + heat affected zone (HAZ) has a very low defect density typical of well-annealed metals as indicated by the frequent pop-in events. Spherical nanoindentation yield strength, Berkovich hardness, and tensile yield strength measurements on the welded material all show that the C37M welded material has a higher strength than C35M welded material. From the comparison of nanoindentation and tensile tests, EBSD microstructure analysis, and information on the processing history, it can be deduced that the primary driver for weld-softening is a change in the defect structure at the grain-scale between the AR and welded material. These measurements serve as baseline data for utilizing nanoindentation for studying the effects of radiation damage on these alloys.

36 MATERIALS SCIENCE↗

DOE OSTI2017

SNL/SRNL Joint Project on degradation of mechanical properties in structural metals and welds for GTS reservoirs

The objective of this project is to measure the hydrogen-affected fracture properties of structural welded metals exposed to hydrogen isotopes. The main goal of FY16 was to evaluate low-temperature effects on fracture properties of stainless steel welds pre-charged with hydrogen. Forged stainless steels consisting of 316L, 304L, and 21-6-9 welded with 308L filler metal were pre-charged and tested at 223 K at select displacement rates to evaluate fracture behavior over the lower STS temperature range. Reductions in fracture thresholds were observed for all stainless steel welds when samples were precharged with hydrogen; however, temperature effects were not observed in the 304L and 21-6-9 welds. Only 316L exhibited enhanced degradation at 223 K. In addition to fracture testing, tensile specimens were extracted from the weld region and tested at 296 K and 223 K in the hydrogen pre-charged condition. A slight increase in yield strength was measured in the pre-charged condition at 296K and 223 K for the three different welds. A reduction in total elongation of 3-11% was observed at 296 K, whereas reductions in total elongation from 50-64% were observed at 223 K. Microhardness and ferrite numbers were measured in the weld regions to try to elucidate the factors affecting fracture. Lastly, in collaboration with Savannah River National Laboratory (SRNL), weld and heat-affected zone bend specimens extracted from forged 304L and 21-6-9 stainless steel were supplied to SRNL and are in the final stages of sample preparation for subsequent tritium exposure, aging, and fracture testing. The collection of testing completed and planned between Sandia and SRNL contributes to the development of a comprehensive database of properties for materials as a function of hydrogen-isotope concentrations.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Develop a Fast Analysis Solver for Welding Sequence Optimization

During the shipbuilding manufacturing process, materials are exposed to significant stresses, as induced both thermally and mechanically, that alter the intended design and significantly affect the production schedule, labor hours (fitting, welding, rework, etc.), and material structural performance. The type and magnitude of deformation of a given structure depends on many factors such as the material, thickness and quality of components, the process heat input, preheat and inter-pass temperatures, type and size of welds, welding sequence and direction, location, sequence, and degree of fixturing. Numerical simulations using finite element analysis (FEA) have long been used to analyze welding-induced structural distortion. For large assemblies, transient thermal elastic-plastic analysis (TEPA) can take days or weeks to run, and optimization of welding sequence is not feasible. Simplified analysis methods were developed to reduce computational time. However, it is challenging to use these techniques to fully optimize welding sequencing because of their applied simplifications in modeling weld details. A fast analysis solver that could be used by the shipbuilding industry is being developed for optimizing welding sequences by taking full advantage of modern GPU-based HPC hardware and incorporating patented acceleration schemes. The accelerated processing factors are up to 2200 times greater for large, multi-pass welded structures.

Yang, Yu-Ping↗

DOE OSTI2017

Automatic monitoring of the alignment and wear of vibration welding equipment

A vibration welding system includes vibration welding equipment having a welding horn and anvil, a host machine, a check station, and a welding robot. At least one displacement sensor is positioned with respect to one of the welding equipment and the check station. The robot moves the horn and anvil via an arm to the check station, when a threshold condition is met, i.e., a predetermined amount of time has elapsed or a predetermined number of welds have been completed. The robot moves the horn and anvil to the check station, activates the at least one displacement sensor, at the check station, and determines a status condition of the welding equipment by processing the received signals. The status condition may be one of the alignment of the vibration welding equipment and the wear or degradation of the vibration welding equipment.

47 OTHER INSTRUMENTATION↗

DOE OSTI2019

Automatic monitoring of the alignment and wear of vibration welding equipment

Spicer, John Patrick↗

DOE OSTI2016

In-Situ Synchrotron and Neutron Radiation at International User Facilities Take Welding Research to a New Level

Over the past century the principal method for understanding the effects of welding on the mechanical and physical integrity of the final joint has been post-weld microstructure analysis, and it’s the interpretation of these microstructures that sets welding metallurgy apart from other related scientific fields. In conjunction with heat flow analysis, thermodynamics, and kinetics, post-weld visual observations have provided the framework for interpreting the events that lead to the final weld microstructure. However, without direct and confirming evidence of the actual phases that exist during welding, or measurement of invisible weld properties such as crystal structure and residual stress, multiple interpretations for microstructural evolution of welds are possible. For these reasons, new methods are being developed for the direct observation of phase transformations that occur both during and as a result of welding, using modern synchrotron and neutron user research facilities.

36 MATERIALS SCIENCE↗

DOE OSTI2017

Advanced Process Possibilities in Friction Crush Welding of Aluminum, Steel, and Copper by Using an Additional Wire

Joining sheet metal can be problematic using traditional friction welding techniques. Friction crush welding (FCW) offers a high speed process which requires a simple edge preparation and can be applied to out-of-plane geometries. In this work, an implementation of FCW was employed using an additional wire to weld sheets of EN AW5754 H22, DC01, and Cu-DHP. The joint is formed by bringing together two sheet metal parts, introducing a wire into the weld zone and employing a rotating disk which is subject to an external force. The requirements of the welding preparation and the fundamental process variables are shown. Thermal measurements were taken which give evidence about the maximum temperature in the welding center and the temperature in the periphery of the sheet metals being joined. The high welding speed along with a relatively low heat input results in a minimal distortion of the sheet metal and marginal metallurgical changes in the parent material. In the steel specimens, this FCW implementation produces a fine grain microstructure, enhancing mechanical properties in the region of the weld. Aluminum and copper produced mean bond strengths of 77 and 69 pct to that of the parent material, respectively, whilst the steel demonstrated a strength of 98 pct. Using a wire offers the opportunity to use a higher-alloyed additional material and to precisely adjust the additional material volume appropriate for a given material alignment and thickness.

36 MATERIALS SCIENCE↗