Welding Stresses Research Articles

Preventing Crack in an Aluminum Alloy Complex Structure during Welding Process Based on Numerical Simulation Technology

Aluminum alloy structures are widely used for weight reduction in aviation, shipbuilding, rail vehicles and automotive industries. Fusion welding technology is one of the most important joining methods for lightweight structure assembly due to its advantages such as flexibility in design, high production efficiency, and low cost. However, the local centralized heating during fusion welding inevitably produces residual stress and welding deformation. For actual engineering structures, if the product design is unreasonable or the external restraint is inappropriate, the transient stress or residual stress become a key factor resulting in cracking during the assembly process. In the current study, an effective computational approach was developed based on the MSC Marc software to simulate transient and residual stress fields for complex aluminum alloy structures during the welding process. In the developed computational approach, according to the location and arrangement of welding lines, an instantaneous heat source model was used to replace the traditional moving heat source model, and as a result significanlty improved the calculation efficiency to meet actual engineering needs. The welding stresses, including transient and residual stress, of an A6061 aluminum alloy complex structure were calculated by the developed numerical simulation technology. The simulation results indicated that the cracking was produced by excessive transient stress during welding process. Subsequently, the effect of external restraint intensity on welding stress at the key location was examined numerically. Based on the simulation results, measures to reduce welding stress and cracking risk were put forward based on adjusting the external restraint intensity.

Stress evolution mechanisms under different SSPT-related effects during laser welding

In this study, a thermal-metallurgical-mechanical (TMM) model was developed for finite element (FE) analysis in laser welding of ultra-high-strength steel (UHSS). The solid-state phase transformation (SSPT) model of austenization was improved by introducing a heating rate-dependent coefficient. Seven cases considering different SSPT-related effects (Case 1–5) and different austenization models (Case 5–7) were considered for the simulation of welding stress to study the stress evolution mechanism during welding. The effects include changes in the yield strength, coefficient of thermal expansion, volume strain, and plastic strain, all of which are induced by the SSPT. By comparing the predicted and measured partial austenitized zone (PAZ), it was found the proposed model had good accuracy for the ultra-high heating rates during laser welding. The predicted transverse/longitudinal welding stresses were also in good agreement with the measurements. The results show that each effect can dramatically change the evolution process and the final magnitude and sign of stress. For example, the SSPT-induced plastic strain always results in a plastic strain having the same sign as the stress, and, in return, gives rise to a decrease in the magnitude of stress. In addition, the SSPT-related effects are weakened when the austenization model underestimates the austenite fraction, leading to a larger error in the predicted stress.

Distribution of Cutting and Welding Thermal Residual Stress in Steel Bone with and without Holes in the Partially Encased Composite Wall

In order to ensure the mechanical reliability of a partially encased composite wall (the PEC wall), the distribution law of the thermal residual stress of the steel bone of the PEC wall needs to be deeply studied. In this study, we first conducted a cutting thermal conductivity analysis and stress analysis of the flange and webs. Second, based on the cutting residual stress of different sheet steel, the welding thermal conductivity analysis and stress analysis of I-shaped calculation units with and without a hole in the web were carried out. Finally, the difference in the distribution of the stress on the section during welding (or cutting) and after the cooling of the welding (or cutting) was clarified, the shape and size of the residual stress distribution in the two types of steel bone formed by the cutting and welding thermal processing were evaluated, and the approximate distributions of the residual stress of the flange and webs were obtained, respectively.

Calculation-experimental procedure for determining welding deformations and stresses based on a digital image correlation method

The object of this study is determining the stressed-strained state (SSS) of a welded article by applying quantitative non-destructive testing. The relevance of the study is associated with the need to devise a universal methodology for the non-destructive quantification of SSS using the simplest approaches and means of provision. To solve this task, an estimation-experimental procedure has been developed. This procedure is based on comparing digital stereo images of the individual sections (spatial primitives) of an article before and after its welding, followed by computer processing. To validate the developed procedure, the SSS of a cylindrical article made of aluminum alloy 7005, at the end of which two flanges were welded laserly with ring seams, was determined. It was established that after performing four diametrically opposed point tacks, the residual deformations of the ends of the article can reach 0.02–0.05 mm, and after performing continuous ring seams – to decrease to 0.01–0.02 mm. The calculation showed that the residual deformations of the end of the article after welding a ring seam are at the level of 0.02 mm, and the residual stresses in the same zone – in the range of 50–60 MPa. The deviation in the coincidence of residual deformations is in the range of 10–20 %, which is a satisfactory result and can be considered as an error in the results of determining SSS in general. Based on the developed methodology for determining SSS, an experimental industrial complex has been created that allows TIG and PAW to perform welding of objects from steels and alloys with the ability to determine the resulting stressed-strained state of these objects. The procedure devised and the equipment designed can be used for to non-destructively determine SSS of spatial structures made of steels and alloys

Quantitative study on magnetic-based stress detection and risk evaluation for girth welds with unequal wall thickness of high-grade steel pipelines

The magnetic-based stress detection technology has a great application potential in the field of girth weld stress detection. However, this technology lacks an effective theoretical model as a scientific guide. Therefore, to investigate the quantitative relationship between the magnetic gradient signal and weld stress and quantitatively evaluate the stress status of girth welds. In this paper, a numerical simulation model of stress-induced magnetic signals of girth welds with unequal wall thickness (UWT) is first established. Then, the model is used to calculate and analyse the quantitative variation law of the magnetic gradient signal of the girth weld with stress and detection height. Moreover, a magnetic-based stress detection and risk evaluation method is established to assess the stress failure risk of girth welds with UWT, whose accuracy is experimentally validated. The results indicate that the residual strength ratio RSR exponentially reduces from 0.83 to 0.49 as the Gmax increases from 373 to 542 μT/m. Moreover, the goodness of fit of the experimental data based on this relationship mentioned above reaches 0.98. The magnetic signal also exhibits a decaying exponential trend with detection height (0.1 m–0.3 m) when the internal pressure varies within 3 MPa–9 MPa. The numerical range of the RSR of seven girth welds is 0.31–0.95, which shows good agreement with the contact inspection results.

Fatigue strength assessment of complex welded structures with severe force concentrations along a weld seam

This paper investigates weld root fatigue strength of complex structures including severe force concentrations along a weld seam. Fatigue tests were carried out for a pillar assembly with various weld configurations. A comparative analysis was carried out with local nominal weld stress (LNWS), structural weld stress, and effective notch stress (ENS) approaches. The study shows that the LNWS approach gives the best prediction, with a scatter range index of 1.26 to 1.68 depending on the weld type. The accuracy of the structural weld stress approach and ENS was significantly lower, with a scatter range index of 4.98 and 5.40, respectively.

Methods and Equipment for Experimental Evaluation of the Performance of Shell and Hull Structures

Introduction. Pressure-operated thick-walled hull structures are the most common type of high-duty welded structures. When these structures are loaded with internal pressure, a complex biaxial stress field arises in them, which is summed up with the fields of residual welding stresses. Therefore, when selecting a technology for manufacturing critical welded structures, the results obtained during conventional uniaxial tests of samples are insufficient. The variety of factors affecting the performance of structures, and the difficulties of separate assessment of their influence, caused the need to maximize the approximation of experimental conditions to the real working conditions of the structure.Materials and Methods. Testing of full-scale structures has a number of advantages, but they are extremely expensive, and, as a rule, only one, the weakest link, is identified, the bearing capacity of the other structural elements remains unclear. For testing, UDI radiometric installations designed for different sample sizes were used. The presented installations allow testing samples of various shapes, types of welded joints (butt, T-bar), changing the position of welded parts.Results. Without rejecting the results obtained during the testing of full-scale structures in the works of the Bauman Moscow State Technical University, DSTU, NRC «Kurchatov Institute» – CRISM «Prometey», and the authors proposed to conduct the basic scope of the research on individual structural elements that would reflect the characteristic features of loading, manufacturing technology, and operating conditions. The design of the «fitting-sheet» connections was applied, which made it possible to increase the indicators of the failure initiation and propagation to the level of the base metal.Discussion and Conclusions. Schemes of structures for obtaining a biaxial tension or bending field in samples were presented. Samples tested according to the proposed schemes allowed us to draw conclusions about the performance of welded joints under conditions close to the actual operation of the structures under study. The proposed test scheme is used by research laboratories in our country and around the world.

Heat generation, plastic deformation and residual stresses in friction stir welding of aluminium alloy

The interactions among thermal history, plastic deformation and residual stresses in the friction stir welding (FSW) process under different welding parameters have been widely considered a crucial issue and still not fully understood. In the present study, a novel three-dimensional fully coupled thermo-mechanical finite element (FE) model based on Coupled Eulerian-Lagrangian approach (CEL) has been developed to simulate the FSW process of aluminium alloy AA 6082-T6 and to analyse the thermo-mechanical interaction mechanisms under different welding conditions. The numerical model successfully simulates the plunge, dwell, and welding steps in FSW and captures the evolution of temperature, plastic deformation, and residual stresses in the welded joint. The obtained results were validated by experimental testing with observed cross-weld thermal history, optical macrography and residual stress measurement using the neutron diffraction technique. The results reveal that the tool rotation speed governs the temperature evolution; the peak temperature increased from 740 to 850 °K when the tool rotation speed rose from 800 to 1100 rpm. The rotational speed also affected the plastic deformation, material flow, and the volume of material being stirred during the welding process. Higher plastic deformation is formed in the stirring zone by increasing the tool angular velocity. This behaviour led to an increase in the stirring effect of the welding tool, reduction of the tunnel defect size and enhancing the quality of weldments. The distribution of residual stresses in different zones of the FSW joints has been found to have an M-shaped profile. A significant tensile residual stress is characterised in the edge of the nugget zone in both longitudinal and transverse directions, balanced by compressive stresses in the thermo-mechanically affected zone, heat-affected zone and base metal. The presented FE modelling provides a reliable insight into the effects of the welding parameters on the weld quality of FSW joints and process optimisation with minimised experimental trials.

Investigation of Distribution of Residual Stress in Swing arc Narrow Gap MAG Welding by Numerical Simulation and Experiments

Investigation of Distribution of Residual Stress in Swing arc Narrow Gap MAG Welding by Numerical Simulation and Experiments

Effects of PWHT on the Residual Stress and Microstructure of Bisalloy 80 Steel Welds

Quenched and tempered (Q & T) steels have numerous applications, particularly in the defence industry with welding as the main fabrication route. Since welding imparts stresses due to thermal gradients development during welding, plus the fact that the Q & T fabricated structures are expected to function in a complex loading environment, it is critically important to relax the welding stresses before exposing the parts to service conditions. The present study reports on the generated residual stresses when Bisalloy 80 is welded by pulsed gas metal arc welding (GMAW-P) and verifies the effects of post-weld heat treatment (PWHT) on the microstructural changes, removal or reduction of residual stresses and the resulting mechanical properties of the welded Q & T steel joints. Neutron diffraction was utilized to measure the residual stresses in the as-welded and after PWHT of the Bisalloy 80 steel weldments. High levels of tensile residual stresses reaching to the yield strength of the weld metal were present (642 ± 24 MPa) in the as-welded joints but were substantially reduced after PWHT (145 MPa ± 21 MPa, which is ~23% of the yield strength of the weld metal). PWHT led to microstructural changes in different regions of the parent and weld metals, including the formation of coarsened polygonal ferrite grains and bainitic ferrite laths. This finding is in line with hardness measurements, where hardness reductions were evident in the heat-affected zone (HAZ) and the weld metal (WM) of the heat-treated specimens.

Study on Welding Deformation and Optimization of Fixture Scheme for Thin-Walled Flame Cylinder.

In this paper, the best fixture scheme for the TIG welding torch of nickel-base solid solution superalloy GH3536 in the welding process is explored. First of all, to meet the extremely high-dimensional accuracy requirements of the flame cylinder, a multifield coupling analysis model based on the flame cylinder is established on SYSWELD software. By studying the stress and deformation of welded parts under different line constraint positions and applied pressure, the trend of welding deformation is obtained, and the relevant mathematical model is established based on this. Finally, the particle swarm optimization (PSO) algorithm is used to calculate the best fixture scheme to make the welding stress and deformation better. The simulation results show that the welding deformation is negatively related to the line constraint distance and positively related to the applied pressure. According to the optimized clamping scheme of PS0, through simulation calculation, the average axial deformation is reduced by 82.5%, the maximum radial shrinkage deformation is reduced by 60.6%, and the maximum residual stress is reduced by 60.3%. Finally, it is verified by the flame barrel experiment that it meets the acceptance requirements and successfully solves the problem of serious axial shrinkage during the TIG welding of the outer ring of the flame barrel.

Effect of fluxes on weld penetration during TIG welding – A review

Tungsten Inert Gas (TIG) welding, also known as Gas Tungsten Arc Welding (GTAW), is a common welding procedure which produce high quality weld with composition as close to that of parent metal. Wide range of materials like, ferrous alloys, magnesium alloys, Aluminum alloys and Titanium alloys can be welded using TIG welding process. Conventional TIG welding can only produce a maximum penetration of 3 mm in a single pass. Alloys with higher thickness require multi-pass TIG welding which involves preparation of joint and additional filler. This leads to the increase in production cost and higher heat input which leads to increased thermal stress and distortion of weld. This paper reviews recent novel developments to improve the penetration depth in TIG welding of various materials. The application of Activated Flux induced TIG welding (A-TIG), Flux Bounded TIG welding (FBTIG) and Flux cored wire TIG of A-TIG on Ferrous alloys, Titanium alloys, Magnesium alloys and Aluminum alloys are discussed. The paper also discusses about the mechanism involved in improving the penetration of weld in these three processes. It also discusses the future scope of application of these processes.

A Neutron Diffraction Study of the Effect of Holdings and Heat Treatment on Residual Stresses in a Steel Weld

A Neutron Diffraction Study of the Effect of Holdings and Heat Treatment on Residual Stresses in a Steel Weld

Residual stress and material flow prediction in Friction Stir Welding of Gr2 Titanium T-joints

Friction Stir Welding is nowadays an established technique successfully used by many industries. However, most of the research and, consequently, most of the applications regard aluminum alloys and butt joints. T-joints are of high interest for different industrial sectors as aeronautical, aerospace, naval and ground transportation, for which joint integrity and low residual stress are extremely important. In this study, an experimental and numerical approach is proposed with the aim to study the peculiarities of the residual stress distribution and material flow occurring in FSW of CP-Ti T-joints. Experiments were carried out to assess the feasibility of the process and to acquire the temperature data needed for the validation of a specifically set-up numerical model. Peculiar numerical strategies have been defined in order to calculate the residual stress with affordable CPU time and to highlight the bonding surface between the skin and the stringer. Main findings include a longitudinal residual stress profile different from the one usually found for butt joints. The reasons for this behavior were explained through a deep investigation on the temperature distribution in space and evolution in time. Finally, the bonding surface between skin and stringer was highlighted together with the possibility of flow defects in the fillet area.

Investigation of welding temperature field and residual stresses of corrugated steel web girders

This paper proposes a new approach for obtaining the welding temperature field and stress distribution in a corrugated steel web girder. This approach was validated by comparing the control temperatures (obtained from finite element modelling of CO2 gas shielded welding) of a corrugated steel web girder with experimental findings. Moreover, the effects of different welding speeds on welding residual stress were analysed. The control point temperatures obtained using modelling corresponded closely with those of the observed data. The welding residual stress was primarily affected by the longitudinal residual tensile stress and increased as the distance to the weld decreased. The welding speed had an insignificant effect on the residual stress distribution but a significant effect on the residual stress peak. The findings of this study can serve as a reference for the analysis of the welding temperature and stress fields of corrugated steel web girders.

The effect of oscillating capping TIG welding on residual stresses of 2219-T8 butt joint

Oscillating capping TIG welding (OCW) was proved valid to increase joint properties of the 2219-T8 aluminium alloy. Its effects on the welding residual stresses (WRS) should also be clarified. In this study, the residual stresses of the 2219-T8 plate butt welding of capping welding (CW) + OCW and CW technologies were investigated by simulation and experiment. The results showed that OCW reduced the peak value of the tensile longitudinal residual stress (LRS) in the joints. And its mechanism was discussed. Differences in bead geometry, heat input and number of welding passes of OCW affected LRS. Among them, wide bead geometry resulted in lower residual plastic strain, thus decreasing LRS peak value in a great manner.

Assessing and mitigating the distortion and stress during electron beam welding of a large shell-flange structure

Electron beam (EB) welding can efficiently join large-scale components using one single autogenous pass, but it still faces challenges associated with weld-induced distortion and stress. This study investigates EB welding in a low-alloy steel thick-section shell-flange structure for a small modular reactor. A 3D thermal-metallurgical-mechanical model is developed to assess the weld-induced distortion and stress, as well as the strategy to mitigate them. When no restraint is imposed on the circumferential weld plane, an opening and sliding gap develops during the EB welding, which can cause weld defects and even process failure. Restraint through tack welds can effectively mitigate the gapping distortion, but it generates high transient tensile stress in the tack weld. Circumferentially continuous tack weld is preferential over circumferentially discrete tack welds to minimise the tensile stress. The final residual stress is insensitive to the tack-weld restraint, and the stress distribution in the steady-state welding region is broadly similar to that found in plate butt welds. However, concentration of residual stresses with high triaxiality occurs in the weld stop region, with high tensile stresses generated just behind the beam stop location, which cannot be diminished by overlap welding or change of weld stop position. The mechanisms responsible for the distortion and the transient and residual stresses are analysed. This study could provide rational basis for designing weld restraint to control distortion and guiding stress mitigation strategy for crack-susceptible region in EB weldments.

A computational framework based on FEA, ML and GA for estimation of welding residual stresses

The present work describes a computational framework based on finite element (FE) analysis and machine learning (ML) and genetic algorithm (GA) to accurately estimate and minimize the residual stresses in welding. The FE analysis comprises of sequentially coupled thermal and mechanical analyses that are nonlinear due to temperature-dependent thermal/mechanical properties. In the FE analysis, the effective stress function (ESF) algorithm is used for accurate computation of the temperature-induced elasto-plastic stress field during welding. The ESF algorithm accounts for the large variation of yield curves due to temperature changes in welding. To improve the accuracy of the thermal and mechanical analyses, an optimization process combining supervised machine learning and binary coded genetic algorithm is utilized. This optimization technique is implemented to derive an accurate phase-change model for the thermal FE analysis of SS304, which is otherwise a burdensome and costly task to obtain from calorimetric experiments. The results obtained through the derived phase-change model are validated through welding time-temperature distribution reported for a welding experiment in literature. The same optimization process is further used to obtain a set of weld operating conditions (such as weld voltage, current, welding speed, and the gap between plates) for SS304 that reduce the tensile residual stresses. It is found that the accurate estimation and reduction of welding residual stresses can be realized by using the present computational framework. • A detailed FE analysis is presented for the estimation of residual stresses in welding. • ESF algorithm implemented in the FE analysis is found very useful. • An optimization methodology is proposed based on machine learning and genetic algorithm. • Proposed optimization scheme is used to estimate the phase-change model and weld parameters. • Accuracy in estimation of welding temperature field is improved and tensile residual stress is minimized to a certain extent.

The Construction Technology of Freeform Surface Spatial Bending-Torsion Steel Structure

Taking the construction project of the National Cybersecurity Talents and Innovation Base as an example, this paper studies the construction technology of freeform surface spatial bending-torsion steel structure based on structural design model transformation, including the parametric modeling of deepening design model, computer-aided bending and torsion frame production, flexible docking in the bending-torsion combination unit, and welding stress deformation, in hope to provide reference for similar projects in the future.

NODES OF HOLLOW CORE PROFILE TRUSS BRIDGES WITH INCOMPLETE WELDS

Among the various types of steel bridge girders, trusses seem one of the most powerful and reliable. The individual members may be simple, but the nodes are the complex parts, especially if more than 4 members are connected. In view of the complexity, internal welds become inevitable and are inaccessible after finishing, which is overlooked easily during construction. The proposal is to eliminate those of the inside welds that may be dispensable. If some of them would be eliminated, the consequences may be acceptable, or require minor modifications only. The assumption is tried out on the example of a structure replacing a historic bridge. This structure is composed of rectangular hollow profiles, connected at nodes. Member forces and bending moments, have been determined. If the internal welds are about to be eliminated, an alternative load path is found. The calculation of stresses in the various fillet welds is based on the assumption of evenly distribution of forces. The weld stresses can be calculated by the derived formulas. The results show some welds require strengthening, due to the elimination of the more complicated internal welds. This concerns ultimate limit state. Thus, the possibility is confirmed that some of the internal welds may be eliminated, although there still is no proof the method may be applied to more complex nodes. Research must be continued to clarify this. For the type of hollow core members being considered, omitting of some internal welds may well be an acceptable alternative.