Welding Behavior Research Articles

A molecular dynamics simulation on tunable and self-healing epoxy-polyimine network based on imine bond exchange reactions

ABSTRACT In this work, a post-crosslinking method is used to construct crosslinked epoxy-polyimine networks (C-PIs) based on the imine bond. Due to the dynamic nature of the imine bond, bond exchange reactions (BERs) and rearrangements would take place in polyimine segments under heat or water external stimuli. Molecular dynamic (MD) simulations are performed to investigate imine polycondensation and post-crosslinking procedures. The results indicate that C-PIs exhibit different glass transition temperatures (Tg s) and crosslinking densities by tuning the ratio of epoxy and imine groups, and the Tg values of C-PIs increase linearly with the crosslinking density. In addition, the interface welding behaviour of different C-PIs is investigated, and our results show that BERs can improve the diffusion rate and the final penetration depth. By tracking the microstructures and examining the mechanical properties of samples varying with healing time, it is revealed that the C-PI with lower crosslinking density is easy to self-heal. Furthermore, MD results also imply that the internal stress of the C-PI can be eliminated under the coupling effect of chain relaxation and BERs. This study proves the reliability of a new crosslinking method and provides some useful guidance for designing new materials.

Indentation creep deformation behavior of local zones for X70 girth weld

Creep failure of in-service pipes is a fundamental engineering issue. Since the girth weld joint is a weak link in the pipeline, the creep deformation behavior of the girth weld must be carefully researched. This research sheds light on determining the local creep properties of girth welds. The creep deformation behavior of local zones for the X70 girth weld was studied using nanoindentation creep experiments with maximum nano-indenter loads of 10 mN, 50 mN, and 100 mN. The load dependency and creep mechanism of local zones for the X70 girth weld were investigated through creep properties such as creep strain rate, creep stress, creep stress index, and strain rate sensitivity index. In addition, the microstructures of base material (BM), root welding zone (RWZ), and heat-affected zone (HAZ) were studied to determine the differences in creep properties for the girth weld's local zones. The creep displacement of BM, RWZ, and HAZ follow the RWZ > BM > HAZ sequence as the maximum load increases. The creep stress, on the other hand, exhibits HAZ > RWZ > BM sequence. Furthermore, RWZ and HAZ have a higher load dependency of creep stress index and strain rate sensitivity than BM.

Hydrogenation behavior of Cr-coated laser beam welds of E110 zirconium alloy

The influence of chromium coating on hydrogenation behavior of laser beam welds (LBW) of E110 zirconium alloy was studied at 360–900 °C and hydrogen pressure of 2 bar. The deposition of Cr coating can slow down hydrogen absorption rate of the LBW samples as activation energy for hydrogen absorption (70 kJ/mol) is higher compared to uncoated ones (61 kJ/mol). The growth of hydrides depends on the hydrogenation conditions and original microstructure of welded alloy in different zones. Nanoindentation and three-point bending tests show that hydride precipitations can cause hardening of LBW E110 alloy samples and loss of its plasticity. Some additional aspects of applying Cr coatings for protection of LBW Zr alloy in hydrogen atmosphere are presented.

Low-Temperature Deformation Mechanism and Strain-Hardening Behaviour of Laser Welded Dual-Phase Steels

This paper analysed the change in microstructure after laser welding DP800 and DP1000, the effect of the laser welds on low temperatures deformation, and strain hardening behaviour when loaded at temperatures between −40 °C and 20 °C using quasi-static strain rates (1.7 × 10−2 s−1). The results showed that the fusion zone (FZ) was fully martensitic due to the rapid cooling during welding. Owing to the severity of the heat-affected zone, the joint efficiencies of DP800-DP800 and DP1000-DP1000 welds were 99.0% and 88.7%, respectively. The UTS, YS, and work hardening exponents of the welded joints increased slightly, while the strain hardening capacity of the base metals was much higher than those of the welded joints with decreasing temperatures. The evaluated work hardening exponents of the welded joints were determined using the Hollomon equation, Afrin equation, and Crussard-Jaoul analysis are in the range of 0.2–0.47, 0.24–0.59, and 0.45–0.71, respectively. The welded joints and the base metals demonstrated only stage III strain hardening, with DP800 joints exhibited excellent uniform and total elongation ranging between 8.0–8.7% and 10.4–14.2%, respectively. Fractures were located in the base metal of welded DP800 and SCHAZ of DP1000 welds, respectfully. The fracture surfaces demonstrated characteristic dimple fractures. The uniqueness of this study is found in its design, as there is currently no known literature on the low-temperature deformation mechanism and strain-hardening behaviour of similar DP800 and DP1000 welds.

Mechanical behavior of austenitic stainless-steel welds with variable content of [formula omitted]-ferrite in the heat-affected zone

δ-ferrite solidified in the fusion zone (FZ) mitigates the microfissuring susceptibility in multi-pass welded joints performed on austenitic stainless steel (ASS). The herein investigation studied welded joints between two 304L grade ASS with different chemical compositions, which varied δ-ferrite content in the heat-affected zone (HAZ). The pulsed-current gas tungsten arc welding (GTAW-P) process was used along with filler metals ER 308L and ER309L. Additionally, an autogenous joint was made with stainless steel (SS) of high δ-ferrite content (10%). A thermo-mechanical finite element (FE) model was applied to estimate the thermal history, residual stress (RS) and distortion distributions on SS joints. Then, numerical results were correlated with the fracture analysis. The tensile test corroborated the increment in ductility due to δ-ferrite content increasing in the HAZ. The enhancement of mechanical strength observed in the autogenous welded joint was associated with the pinning effect produced by δ-ferrite stringers formed in the HAZ, which inhibited grain growth. Despite the high joint efficiency of SS 304L weldments, the FZ was fractured due to the microfissuring and precipitated particles. The FZ rupture starts in the FZ-HAZ interface of the first weld bead related to the ductility loss with the heat input. Microfissures found in the FZ of all weldments were related to the tensile stress concentration caused by the uneven distortion distribution and the ductility increment (up to 39%) provided by the high δ-ferrite content. Some chromium carbides in the FZ acted as stress raisers.

Cold welding of ultrathin metallic glass nanowires with side-to-side contact using molecular dynamics simulations

Cold welding can effectively join a wide variety of components, forming high quality assemblies. In this work, the cold welding behaviors of Cu-Zr metallic glass nanowires (MGNWs) in the form of side-to-side contact are investigated by molecular dynamics simulation. The mechanical performance of the welded MGNW is inspected by uniaxial tensile tests. Cold welding processes of MGNWs with various sizes and under different welding conditions are explored. Our results show that the strength of the welded MGNW at 50 K is less than 60% of that of the crude nanowire. At the same welding velocity, the change of welding time within a certain range does not influence the welding quality obviously. The increase in overlap width makes the welded MGNW stronger. The enhancement of welding temperature facilitates the atom diffusion activity, and thus strengthens the contact regions, improving the welding quality. We also compare the side-to-side contact and head-to-head contact cold welding. It is found that the former endows the welded MGNW with lower strength but better plasticity. This work provides a preliminary theoretical guideline on the cold welding of MGNWs for further experiments and to facilitate the engineering practice of cold welding.

Investigations of Welding Parameters Impacts on Mechanical & Corrosion Behavior of Steel Welds

Now days while the welded product of duplex stainless steels (DSS) is to be used for Cl- containing environment & sour service applications for example petrochemical, off-shore platforms, oil, gas, paper & pulp industries etc. The resistance to chloride stress corrosion cracking (CSCC) during service application has always been a threat for the manufacturer & the end user of the product. Resistance regards SCC under chloride existing environment based on current ferrite content under carefully welded duplex (ferrite-austenitic) stainless steel framework. DSS ferrite specifications required by various organizations are 30 to 60%, 30 to 70 Ferrite Number, 35 to 75%, etc. In this study, DSS 2205 material all weld metal consumable test coupons within ferrite content range 14% to 30% or 20 FN to 40 FN have been prepared & investigated. Investigations have shown that yield strength & tensile strength needs regards base metal & filler metal classifications was higher at every ferrite levels observations & no SCC cracking analysed after 1000 hours exposure at ferrite level below 15%.

Hydrogen trapping and desorption affected by ferrite grain boundary types in shielded metal and flux-cored arc weldments with Ni addition

Hydrogen trapping behavior and diffusion induced by the microstructure of shielded metal and flux-cored arc weldments (SMAW and FCAW) were characterized using a combination of high-resolution microstructural characterization methods, hydrogen trap site studies, and a modeling technique. H trapping by HAGBs that was found by TDS was confirmed by NanoSIMS with a cryogenic stage. Cellular automaton modeling results showed that in grain sizes smaller than a critical grain size, the hydrogen diffusion coefficient decreases with decreasing grain size, indicating that H trapping dominates short-circuit diffusion mechanism along high-angle grain boundaries (HAGBs). These results firstly show that smaller grain size and high HAGB density in the FCAW specimen results in a lower H diffusion coefficient and higher density of relatively strong HAGB traps, and a lower total desorbed hydrogen content in the FCAW specimen. Also, it was suggested that the fraction of acicular ferrite grains can define the HAGB content in the alloy, and can be a determinant factor in the behavior of weldments in H-containing media.

Optimization of aluminum-to-steel friction stir lap welding for the fabrication of high-integrity structural components

Friction stir lap welding is a solid-state joining technique used to effectively fabricate high-quality lightweight and energy-efficient hybrid structures by overlapping thin parts of Al and other similar or dissimilar alloys. In this study, friction stir lap welding between 6061-T651 Al and JAC270 45/45 galvanized steel sheets was investigated, considering both single-pass and double-pass protocols to optimize the process and quality of the resulting welds. The single-pass welding was first studied by conducting experiments at constant tool rotation and traverse speeds (1200 RPM and 1 mm/s) and various tool plunge depths (0.05, 0.2, and 0.4 mm). The microstructures and Al-steel interfaces of the welds were characterized using microscopy and energy-dispersive X-ray spectroscopy, and the weld strength was evaluated by lap shear tensile testing. The defects and intermetallic compounds that form at the Al-steel interfaces were identified as the limiting factors for the weld strength. The intermediate tool plunge depth of 0.2 mm resulted in the highest weld strength due to the least amount of interfacial defects and intermetallic compounds amongst the studied single-pass welds. A double-pass welding method was then developed to optimize the strength of the Al-steel lap welds. The selection of the processing conditions for the double-pass welding was guided by the behavior and properties of the single-pass welds. The double-pass welding successfully eliminated the defects and intermetallic compounds from critical locations, creating a robust Al-steel interfacial bond and significantly improving the weld strength. The knowledge and findings in this study can be applied to optimize the design, material integration, and mechanical properties of lap welds for high-integrity structural components.

A Numerical Simulation of UIC60 Rail-Weld's Fatigue and Crack Growth under Wheel Frictional Contact and Bending

In service condition rail joints, especially the weldments are under the action of various loadings which are not only working in multiple axis direction but also time-dependent having a cyclic and mixed-mode in nature and non-relative to each other. The surface of the rail and its weldment is acted by very high repetitive stress through the wheel and because of this contact stress the running surface or subsurface may have cracks or fractures due to fatigue. This work is based on numerical simulation of an aluminum thermite weldment on a UIC 60 rail under multi-axial fatigue crack propagation under the friction with surficial interaction between weldment and wheel with bending load due to vertically applied load through the wheel on the weld. Since contact is highly influenced by vertical load and also for minimizing the simulation time the lateral and longitudinal traction forces are not included in this study. The work formulation and discretization have been done with the finite element method and a non-linear lagrangian algorithm solver is applied. A 3-D rail-weld wheel model assembly and a semi-elliptical crack as a flaw on the weld surface are used to identify 3-Modes of SIFs along with its graphical plot generation. Simulation is performed under multi-axial weld wheel surface contact at different locations on weld running surface, taking into account varying position of fracture crack on weld 3-D model to calculate fracture life of weld joint and observation of fatigue crack propagation. This work involves the numerical and theoretical approach of fracture mechanics on created FE fatigue model using the Linear Elastic Fracture Mechanics (LEFM) method following Paris law for fracture mechanics. All the numerical simulation for critical fracture dimension and cycle count with stress intensity factor for weld failure data is estimated using software ANSYS 2020 academic and plotted, then comparison of predicted and observed transverse crack growth behavior and fatigue life of weld, based on Millions Gross Tonnes (MGT) is discussed.

Instance selection-based dissimilar weldment design prediction for resistant spot welding

Resistance spot welding (RSW) is a critical joining method in sheet-metal industries. The machine-learning technique fueled by the historical experimental data of the existing materials has been used to build the data-driven model (DDM). The DDM is expected to be a promising tool to investigate a new material and its welding behavior because DDM can narrow the range of the test matrix and can thus reduce the number of necessary physical experiments and the cost. However, one of crucial data quality problems with machine learning is that training data sets’ lack of descriptability for test sets causes poor prediction. This research starts by indicating that such data quality problems that exist in the context of weldment design. To resolve this problem, the presented study introduces a novel approach named Similar Weldment Case Selection (SWCS), which predicts the key parameter, the nugget size, of spot welding results of a new material by selecting the most similar one among the existing welding cases and then constructing a prediction model to generate the results. In order to overcome the difficulties with defining the selection criteria only with the material properties and geometric features, this study has come up with another factor, nugget-size weld-current series (NWS), to consider; the NWS is a factor that describes the shape of the relation between weld-current and nugget size. The similarity between two NWSs of different materials is calculated (quantified) with the dynamic time warping (DTW) method. Initially, the twelve conventional algorithms are tested for varying degrees of descriptability between the two weldment designs for test and train datasets; the prediction accuracies are found to be proportional to the train set’s descriptability on the test set. The results are then compared with those from the SWCS. The SWCS yields superior accuracy than the twelve algorithms do when the two materials are similar or different. However, the superiority disappears when the two are the same.

Hot-Plate welding behavior of Sisal and Jute Polypropylene composites

ABSTRACT The recent global pandemic has created a grave concern and awareness about the importance of the environment. The researchers/engineers/scientists are now focusing their efforts on the development of technologies that can ensure the sustainable growth of communities. Materials are at the core of every technological advancement. Natural fiber-based composites are a class of sustainable materials being used in diverse engineering applications, ranging from automotive to household goods. The processing methods, such as hand lay-up, injection, and compression molding are used extensively to fabricate simple composite parts. However, for complex/intricate product designs, the parts/components are made independently and these are assembled to get the final product. Joining thus becomes an inevitable process ensuring the assembly of parts/components. In the current experimental investigation, the hot-plate welding (HPW) behavior of sisal/jute fiber (both woven and short) reinforced polypropylene (PP) composite specimens fabricated using direct compression molding(DCM) and Extrusion injection molding (EIM) have been analyzed. The process parameters optimization has been achieved using the Taguchi experimental design approach. The hot plate welded joints of jute/sisal/PP composites were further investigated on the basis of their flexural and tensile properties. The failed joint specimens were characterized using TGA and XRD characterization techniques to analyze any changes occurred in their thermal degradation and crystalline behavior during welding process. The failure mechanisms of the joints have been thoroughly analyzed using SEM micrographs.

Study of weld formation characteristics in laser-arc hybrid welding

The laser-arc hybrid welding was carried out on the marine-use steel, and the effects of primary parameters on weld behavior were investigated. The results show that the weld penetration increased gradually with the increase of laser power. Meanwhile, superior weld formation, penetration and reinforcement were achieved when laser and wire distance was 2 mm. The weld width, penetration and reinforcement were increased with the increase of current, while they were decreased with the increase of welding speed. The influence of laser power on the generated microstructure was analyzed by the observation in junction of weld and heat affected zone (HAZ). The microstructure of HAZ coarsened with the increase of laser power, which is consistent with the hardness distribution trend in HAZ.

Numerical simulation of fracture behavior for the pipeline with girth weld under axial load

The failure accidents of the pipeline caused by girth weld problems occur frequently in recent years. However, the fracture mechanism and behavior of girth weld in pipeline are not clear at present. Accordingly, based on Gurson-Tvergaard-Needleman (GTN) damage model, a three-dimensional full-scale model of the X80 pipeline with girth weld was established in this paper, in which the differences of material properties in base, weld, and heat-affected zone (HAZ) were considered. Subsequently, the fracture process of the pipeline with girth weld under uniaxial tensile load was obtained. On this basis, the fracture behavior and the causes of failure of the pipeline were analyzed. The elongation and fracture strain were respectively defined to quantitatively describe the fracture toughness of the pipeline. The results showed that the crack initiated at the junction of the weld and HAZ in the inner wall of the pipeline due to material softening of HAZ, and then the crack propagated from the inner wall to the outer wall until forming a penetrated crack. Lastly, the crack grew rapidly along the circumferential direction, resulting in the overall failure of the pipe. Compared to the pipeline without weld, there was a smaller value of the elongation and fracture strain in the pipeline with girth weld. The elongation and fracture strain of the pipeline without weld was 26.49% and 0.78, and that of the pipeline with girth weld was 4.73% and 0.18, which was reduced by 82% and 77% respectively.

New insights into the fracture behavior of advanced high strength steel resistance spot welds

While the automotive industry is striving for the reduction of car body weight to increase the fuel efficiency and to reduce CO2 emission without compromising the safety and crashworthiness of vehicles, a new generation of advanced high strength steels (AHSS) have emerged as excellent candidates to meet these requirements. However, their integration into the car body structure is associated with welding-related problems. This research utilizes a novel approach to establish a fundamental correlation between welding parameters, microstructure and mechanical performance of AHSS resistance spot welds. In-situ micro-cantilever bending experiments are executed and analyzed in a quantitative manner to evaluate the effect of texture and post-welding heat treatment on the local fracture toughness of spot welds. A striking finding is that, through a switch from single to double pulse weld scheme, the texture of martensite formed in the fusion zone becomes responsible for a significantly higher fracture toughness of the area in front of the pre-crack. In addition, it is found that paint baking heat treatment also results in a much enhanced fracture toughness through tempering of the martensitic microstructure. A quantitative correlation is made between the micro-scale fracture toughness and macro-scale mechanical performance of advanced high strength steel welds.

Welding behavior of an ultrahigh-strength quenching and partitioning steel by fusion and solid-state welding methods

How the quenching and partitioning (Q&P) steel with an ultrahigh strength (>1 GPa) behaves during the welding process is a considerable issue for its practical application, but the systematical evaluation on the welding behavior of this new steel has not yet been concerned. Here the structure-property relationships of three typical Q&P 1180 steel joints were comparatively studied by using laser welding (LW), tungsten inert gas (TIG) welding, and friction stir welding (FSW). Weld metal (WM) region and heat-affected zone were generated in the welded joints and their widths were mainly related to the welding linear energy (LW < FSW < TIG welding). During LW and TIG welding, the parent metal (PM) was totally destroyed and coarsened in the WMs whereas ultrafine microstructures with high hardness were obtained after FSW by means of the low peak temperature and severe plastic deformation. The TIG welding produced serious material softening, while the softening degree could be alleviated via LW and FSW. Excellent tensile strength as high as that of the PM was achieved in both the LW and FSW joints by means of the suppressed material softening as well as small soft zone width. However, dramatic losses of strength and ductility were found in the TIG welded joint owing to the premature strain localization and fracture in the soft zone.

Evaluation of thermal aging activation energies based on multi-scale mechanical property tests for an austenitic stainless steel weld beads

The long-term thermal aging behavior of a 316L austenitic stainless steel welds containing ∼12% of δ-ferrite was investigated using the nano-scale (nanopillar compression), micro-scale (micro-hardness and small punch), and macro-scale (tensile and J-R) mechanical property testing methods. Specimens were aged at 343, 375, and 400 °C for up to 20,000 h. The thermal aging activation energies were estimated based on the various mechanical test results using two fitting methods. The activation energies vary from 124 to 300 kJ/mol, depending on fitting and mechanical testing methods. Among the mechanical properties, the nanopillar and J-R test results showed similar activation energies lower than those from other mechanical properties. The similarities and discrepancies among the estimated activation energies were discussed in view of the contribution of embrittled δ-ferrite to the fracture and deformation of test specimens.

Self-bending extrusion welding behavior of streamlined function die

Self-bending extrusion welding behavior of streamlined function die

Adaptable Phosphate Networks towards Robust, Reprocessable, Weldable, and Alertable-Yet-Extinguishable Epoxy Vitrimer.

Covalent adaptable networks (CANs) combine the uniqueness of thermoplastics and thermosets to allow for reprocessability while being covalently crosslinked. However, it is highly desirable but rarely achieved for CANs to simultaneously demonstrate reversibility and mechanical robustness. Herein, we report a feasible strategy to develop a novel epoxy vitrimer (EV) composed of adaptable phosphate networks (APNs), by which the EVs exhibit promising mechanical properties (tensile strength of 62.5 ~ 87.8 MPa and tensile modulus of 1360.1 ~ 2975.3 MPa) under ambient conditions. At elevated temperatures, the topology rearrangement occurs relied on phosphate transesterification, which contributes to the shape memory performance, self-healing, reprocessing, and welding behaviors. Moreover, the incorporation of APNs allows for improvements in anti-ignition and also the inhibition of both heat release and smoke generation to avoid empyrosis, asphyxiation, and toxication during burning, showing expected intrinsic fire safety. Thermal, mechanical properties, and flame retardancy of the reprocessed EVs after hot pressing are very close to those of the original EVs, which is attributed to the sufficient reversibility of APNs. Accordingly, combining the aforementioned features, EVs are manufactured as flame-triggered switches for fire alarms, which symbolizes the innovative development of high-performance covalent adaptable polymeric materials.

Effect of specimen configuration and notch root angle on fatigue behavior of novel dissimilar resistance spot welds of AA5754 to HSLA steel

Facing stringent requirements for fuel economy and regulations for greenhouse gas emissions, structural lightweighting using multi-material solutions has become commonplace in the automotive industry. When joining dissimilar materials such as aluminum to steel by resistance spot welding (RSW), a thin layer of brittle intermetallic compound forms at the aluminum-steel interface and dominates mechanical behavior of the joint. In this contribution, RSW of 1.1 mm thick AA5754 sheet to 2.0 mm thick high-strength low-alloy (HSLA) steel sheet was performed using multi-ring domed (MRD) electrodes and multiple solidification weld schedules to achieve acceptable static joint strength. Load-controlled fatigue tests were conducted and the results show that the fatigue life is longer in the AA5754 to HSLA steel spot welds than that of the 1.1 mm thick AA5754 joined to itself (aluminum-aluminum). Structural stress analysis revealed that all fatigue life data points from both the lap shear and coach peel configurations fall onto a single master curve indicating that the weld nugget diameter is the controlling parameter for fatigue life. Finite element simulation considering material inhomogeneity in the weld further confirms that a large notch root angle at the weld nugget is beneficial to yield longer fatigue life as less maximum principal strain occurs in the aluminum sheet in the AA5754-HSLA steel spot welds.