Welded Joint Specimens Research Articles

Study on Fiber Laser Welding of AA6061-T6 Samples through Numerical Simulation and Experiments

A three-dimensional finite element model based on thermo-elastic-plastic model is applied to simulate the temperature and residual stress of aluminum alloy joint weld during fiber laser welding. Transient temperature, residual stress field, and joint distribution are studied, and the influence of laser power influence and welding speed is analyzed. A laser welding system can automatically preheat and weld in the seal cavity full of nitrogen. The laser welding system welds 2 mm thick 6061-T6 joints with processing parameters determined by simulation. The appearance of the weld bead of the joint is smooth and continuous. The welded joint specimens can be used for low-cycle tensile-tensile fatigue tests. The fracture of fatigue test is located in the HAZ of the welded joint. Result provides support for research and technology optimization of fiber laser welding.

Influence of Hydrogenation on Residual Stresses of Pipeline Steel Welded Joints

Hydrogen embrittlement is a phenomenon that affects the performance of steel used for oil and gas pipelines. This paper presents a study of the effect of hydrogenation on the residual stresses of an API 5L X65 steel pipe. Residual stresses were analyzed by X-ray diffraction technique using the sin2Ψ method. The hydrogenation of base metal and welded joint specimens was performed by electrochemical tests in a simulated soil solution NS4. Results show that the hydrogenation led to significant changes in residual stress configuration and in the mechanical properties of steel. The hydrogenation increased the magnitude of the longitudinal residual stress of base metal and fusion zone, without changing the tensile/compressive nature. On the other hand, the hydrogenation increased the intensity of the tangential stress of base metal, and changed from compressive to tensile the residual stress of the fusion zone. The microstructural characterization by optical and scanning electron microscopy was used to complement this study.

9CrMAG鋼溶接継手のクリープ強度評価

Mod.9Cr steels are widely used as boiler materials in ultra-super critical power plants. In this study, creep tests were conducted using miniature specimens with 1mm diameter machined from a base metal, weld metal and a heat affected zone (HAZ) in a Mod.9Cr-1Mo welded joint. Creep deformation and rupture behaviors of the miniature base metal specimens were almost identical with those obtained from standard size specimens. Material constants of the Norton law were determined from obtained datum by the miniature creep specimens. Creep rupture times of the miniature welded joint specimens were much lower than those of the standard size specimens. The finite element creep analysis results indicated that the triaxiality factor in the HAZ of the miniature welded joint is smaller than that of the standard size welded joint resulting in faster creep strain rate in the miniature specimen under the same applied stress.

Low Cycle Fatigue behavior of SMAW welded Alloy28 superaustenitic stainless steel at room temperature

This paper focused on the study of Low Cycle Fatigue of welded joints of superaustenitic (Alloy28) stainless steels. Chemical composition and microstructure investigation of Base Metal (BM) and Weld Metal (WM) were identified. The results showed that both of composition is fully austenitic with a dendritic microstructure in the WM. Low cycle fatigue tests at different strain levels were performed on Base Metal (BM) and Welded Joint (WJ) specimens with a strain ratio Rε=−1. The results indicated that the fatigue life of welded joints is lower than the base metal. This is mainly due to the low ductility of the Welded Metal (WM) and the presence of welding defects. Simultaneously, Scanning Electron Microscope (SEM) observations of fractured specimens show that WJ have brittle behavior compared to BM with the presence of several welding defects especially in the crack initiation site. An estimation of the crack growth rate during LCF tests of BM and WJ was performed using distance between striations. The results showed that the crack initiation stage is shorter in the case of WJ compared to BM because of the presence of welding defects in WJ specimens.

The dependence of local deformations and internal stress fields on welding technique for grade VSt3sp structural steel: I. The influence of welding technique on the mechanical characteristics and acoustic emission parameters of grade VSt3sp steel

A staging analysis was carried out for deformation curves and the evolution of local deformation fields in defective and defect-free specimens of welded joints of grade VSt3sp steel that were made using manual arc and manual pulsed arc welding. Stages of elasticity, microplasticity, nucleation and motion of Chernov–Luder’s bands, and parabolic deformation strengthening were considered. The parameters of acoustic-emission signals during tension of specimens with a welded seam were analyzed.

Microstructural characterization and wide temperature range mechanical properties of NiCrMoV steel welded joint with heavy section

Abstract

Cyclic Stress Response and Fracture Behaviors of Alloy 617 Base Metal and Weld Joints under LCF Loading

Cyclic stress response and fracture behaviors of Alloy 617 base metal (BM) and Alloy 617 weld joints (WJ) are investigated under strain controlled low cycle fatigue (LCF) loading. Axial fully reversed total-strain controlled tests have been conducted at room temperature with total strain ranges of 0.6, 0.9, 1.2, and 1.5%. At the all testing conditions, weld joint specimens have shown higher peak stresses than the base metal specimens, whereas the plastic strain accumulation of the base metal specimens is comparatively higher than those of the weld joint specimens. The cyclic stress response behavior of both base metal and weld joint specimens revealed initial cyclic hardening during first small number of cycles followed by progressive softening to failure. Higher strain amplitudes decreased the fatigue lives for both base metal and weld joint specimens; subsequently weld joint specimens had lower fatigue resistances relative to base metal specimens. Furthermore, the cracking in weld joint specimens initiated in the weld metal (WM) region. The crack initiation and propagation showed transgranular mode for both base metal and weld joint specimens; especially weld joint specimens showed a wedge type crack initiation about 45 degrees to the loading direction because of the dendritic structure.

Strengthening Effect Analysis of Laser Shock Processing on Aluminum Alloy Welded Joint Process

High-strength aluminum alloy is one of the commonly used materials of pressure vessels. And it has the significant advantages of light-weight and cost-effective. But the strength of welded aluminum alloy greatly decreased. The welding line and heat-affected zone became high-risk area on pressure vessel. At present, it is difficult to improve the welding process, thus improving the microstructure and stress state of welded joints become the main means to improve the fatigue properties and extend the service time of aluminum pressure vessels. As a new treatment process after welding, the process parameters of laser shock processing (LSP) can be precisely controlled, and the process is non-contact. It can be adapted to deal with macro and micro scale aluminum pressure vessels, so it has broad prospects for engineering applications. Based on the introduction of principle and features of LSP welded joint process, the estimation model of laser-induced surface residual stress is analyzed. The safety lives of welded joint specimens with and without LSP are estimated by making low cycle tensile fatigue test. The effect of LSP on fatigue properties of aluminum alloy welded joint is analyzed. The fatigue fracture mechanisms of these specimens with and without LSP are studied by surface integrity and fracture surface testing. The result showed that the safety lives of these specimens with LSP are increased significantly. At the same time, Compressive residual stress and dense surface layer are induced at the surface of welded joints by LSP. After summarizing the research status about fatigue properties of aluminum alloy welded joint by LSP, the problems existing in current research of LSP welded joint process are pointed out. Finally, expectations of future studies on LSP welded joint process are brought forward.

Effect of Compressive Residual Stress on Fatigue Crack Propagation

Fatigue failures have sometimes taken place in weld joints due to stress concentration and tensile residual stress at the weld toe. Therefore managements of fatigue crack initiation and propagation contribute to the enhancement of the fatigue life of welded steel structures.The hammer peening process is well known as one of methods to improve the fatigue life of weld joints by generating the compressive residual stress field near the weld toe where recognized as the fatigue crack initiation site. With respect to the fatigue life estimation of the welded steel structure which applied the compressive residual stress field near the weld toe, it is important not only fatigue life estimation for the fatigue crack initiation but also that for the fatigue crack propagation. However there is few research work which deal with the fatigue crack estimation in the compressive residual stress field.In this study, mechanism of fatigue crack propagation in the compressive residual stress field was clarified by experimental observation of crack propagation from a surface crack after hammer peening. Morphology of a propagating surface crack in plate and gusset weld joint specimens were measured by the beach mark method. As a result, it was clarified that morphology of surface crack propagated in the compressive residual stress field was different from that in neutral stress filed. And this phenomenon was clearly observed especially under the low stress intensity factor condition.

Peculiarities of laser welding of thin parts of aerospace equipment

In this paper, the laser welding methodology technological process optimization of shallow thickness aerospaceconstructions was considered. The methodology consists in selecting of multivariate iterative processparameters. Using this methodology can significantly reduce the technological preparation time of productionand the proportion of experimental research. Experimental confirmation was work on laser butt welding parts forshallow thicknesses, material welded samples - nickel-based alloys. Work was carried out to determine thechemical composition of the alloy samples, modeling and development of a special device for welding products.Laser welding is carried out on the process plant includes a pulsed solid-state laser YAG:Nd with a wavelengthof 1,06 microns. On the basis of the developed method of optimization parameters were selected welding process.According to the results of work defines the quality of the welded joint specimens using metallographic examination of the weld, and testing of mechanical properties of butt weld specimens were performed, the tensilestrength was determined. The developed method of process optimization of laser welding of parts of smallthickness allows for high quality welds free of defects in the surface layer.

Automated evaluation of the service lives of specimens of structural materials on the basis of a micromechanical model of the time dependences of acoustic-emission parameters

A method for determining the time to the failure of specimens of welded joints on the basis of the automatic determination of the parameters of a micromechanical-destruction model and the time dependences of the number of registered acoustic-emission signals is presented. The possibilities of improving this technique on the basis of the statistical identification of the uniform-destruction stage, which determines the specimen strength, are considered.

Strengthen of Mechanism of 30CrMnSiNi2A Steel Welded Joint with Ultrasonic Impact Treatment

To analyze the strengthening mechanism of 30CrMnSiNi2A steel welded joint with ultrasonic impact treatment (UIT), the welded joint specimens were full coverage strengthened by the technology. The microstructure of the surface layer in fusion zone of the welded joint with and without UIT was investigated by optical microscopy (OM). The hardness and residual stress distributions along the thickness direction were also measured by micro-hardness tester and X-ray diffraction method respectively. The results show that the microstructure in fusion zone of the untreated 30CrMnSiNi2A steel welded joint were coarse dendrite, and there were many welding defects in this zone. UIT has the ability to achieve more compact microstructure with only small welding defects. The average hardness value of the treated specimens reached 571 HV, increased 14.4% as compared with that of the untreated specimen (499 HV). A residual compressive stress layer with thickness of 850 μm was also obtained from by UIT, and the maximum residual compressive stress was-347 MPa. The grain refinement, work hardening and residual compressive stress in fusion zone introduced by UIT increased its anti-fatigue performance.

Hot tensile and stress rupture behavior of friction welded alloy 718 in different pre-and post-weld heat treatment conditions

Alloy 718 rods in two different heat treated conditions (solution treatment (ST) and ST followed by aging (STA)) were friction welded. The weld joints were subjected to two different post-weld heat treatments (direct aging (DA) and STA). Tensile tests were carried out at 650°C with an initial strain rate of 10−4s−1. Stress rupture tests were performed at 650°C at a constant load with an initial stress level of 690MPa. The ultimate tensile strength values for all conditions, expect for base material in ST condition and weld joints with prior ST or STA in the as-welded condition, were higher than the minimum value of 1000MPa specified by the Aerospace Material Specification for the base material in STA condition. The weld joint specimen in the as-welded state with prior STA condition failed in heat affected zone (HAZ). Though strengthening precipitates (γ′′) dissolved in weld metal and HAZ during welding, HAZ was weaker than weld metal due to coarser grains and so failure occurred in HAZ. In all other conditions, samples failed in base metal. The stress rupture properties such as minimum creep rate and time to rupture of the base material in ST condition and as-welded joints with prior ST or STA condition are almost same. The sample subjected to STA both before and after welding exhibits the best stress rupture properties. It may be attributed to homogenization as well as moderate coarsening of grain structure of weld zone and fine and uniform distribution of strengthening precipitates throughout the weldment. However, to obtain the best combination of tensile and stress rupture properties, the material should be welded in ST condition and it should be subjected to direct aging after welding.

Characteristics of Fatigue Cracks Propagating in Different Directions of FSW Joints Made of 5754-H22 and 6082-T6 Alloys

The Friction Stir Welding (FSW) is a dynamically developing version of the pressure welding processes. High-quality welded joints can be created using this process for different engineering applications (e. g. automotive parts). Nowadays, the knowing of the properties and the behaviour of the welded joints is an important direction of the investigations, especially under cyclic loading. The research work aimed to demonstrate the behaviour of the friction stir welded joints under cyclic loading conditions. Fatigue Crack Propagation (FCG) experiments were performed on 5754-H22 and 6082-T6 aluminium alloys and their friction stir welded joints. The CT type specimens were cut parallel and perpendicular to the characteristic directions of the base materials and the welded joints, and the notch locations in the specimens of welded joints were different, too. Therefore, the propagating cracks represent the possible directions of the fatigue cracks both on the base materials and on the welded joints. The results of the fatigue crack propagation tests on the welded joints clearly demonstrate the different characteristics of the thermo-mechanically affected zone (TMAZ), the heat affected zone (HAZ), and the advancing (AS) and retreating sides (RS) of the weld nugget (WN). The investigations and their results were compared with each other and with the results can be found in the literature.

Influence of deformation on mechanical properties of 20 and 30HGSA steel welded joints in superplasticity regime

It may prove to be promising to use hot plastic deformation of welded joints in temperature/velocity regimes of metal superplacticity effect revealed, in order to get structural homogeneity and combination of high strength and plasticity. Isothermal rolling with hot rolls of 20 and 30HGSA steel weld joint specimens was carried out at 10, 20 and 40% deformation degrees at a temperature of 730 ± 5 oС being the optimal temperature of superplastic deformation for constructional steels, at a constant strain rate of 0.003 s-1. The rolling was performed in the laboratory six-high LIS-6/200 mill of the Institute for Metals Superplasticity Problems, Russian Academy of Sciences, with isothermal deformation conditions being provided by heating the working rolls and regulating their temperature. The minimum difference in the weld joint base metal ultimate strength values compared to the initial state is observed at 20% deformation degree and is equal to 6.3% for 20 steel and 5.2% for 30HGSA steel. The minimum difference in the weld joint base metal yield point values compared to the initial state is also observed at 20% deformation degree and is equal to 6.0% for 20 steel and 4.6% for 30HGSA steel. It is shown that isothermal rolling with hot rolls in superplastic deformation regime results in reduction of scattering of mechanical properties in different zones of 20 and 30HGSA constructional steel welded joint compared to the initial state. The comparative analysis of micro hardness and hardness values of the welded 20 and 30HGSA steel specimens after different kinds of welding treatment with equal time spent on the treatment showed that the rolling with isothermal rolls in superplastic deformation regime with 20% degree of strain has an advantage in achieving homogeneity of mechanical properties compared to annealing and thermal cycling treatment.

Very High Cycle Fatigue Behavior of 25Cr2Ni2MoV Steel Welded Joint

Conventional fatigue test is conducted in 25Cr2Ni2MoV steel welded joint specimen at room temperature. The fatigue failure mechanism in welded joint is investigated by fatigue test and microscopic analyses. Results show that the stress-fatigue life relationship presented a stepwise shape. In high cycle fatigue(HCF) regime, base metal is preferential fracture sites, which are originated at surface or subsurface inclusions. Whereas in the very high cycle fatigue(VHCF) regime, fatigue cracks are always initiated at pores in the weld metal. The fatigue life will be longer in the condition of small surface defect size or interior defect as fatigue crack initiation. Small surface defect with large interior defect has similar potential for crack initiation. Stress distribution of fatigue specimen at micro-defects are performed by finite element analyses, and it shows that the stress concentration around pore and inclusion in the weld metal are larger than inclusion in base metal. It is found that large quantities of granular particles disperse inclusions in base metal will form bigger defects and these defects are easier to initial fatigue crack than inclusions in the weld metal.

The Shielding Gas Influence on the Laser Beam Welding of 2205 Duplex Stainles Steel

Procedures for welding duplex stainless steel conventional arc welding methods were studied by several authors and nowadays and recently they are well handled. Todays practice requires application of processes providing excellent weld joint quality, quick production and possibility of automation. Therefore the application of laser beam welding has a great potential of application. This research paper describes weldability of DSS 2205 by laser beam. The main factor examined in the process of welding duplex steel is the shielding gas and his effect on weld joint properties. Welding was performed with gaseous CO2 laser machine Ferranti Photonics AF 8 having 8 kW max. output power and wave length 10.6 µm. Microstructure of weld joint specimens was analysed in order to observe the phase composition ratio. Samples are welded with same process parameters except of shielding gas. Helium and nitrogen were applied as a shielding gas to examine their influence in comparison to weld joints welded without shielding gas. Since the nitrogen promotes the austenite formation, its application brought the best results having almost same phase composition as base material.

Investigation of Mechanical Properties of Microalloyed Steels Joined by GMAW and Electrical Arc Welding

In this study, micro-alloyed steel samples were exposed to gas metal arc welding process by two welding wires coded ER100 SG and SG 3. Protection gases compositions were selected as 100% argon (Ar), 15% CO2 + 85% argon (MIG/MAG-1) and 25% CO2 + 75% argon (MIG/MAG-2) in three di erent types in order to investigate the CO2 e ect on mechanical properties. In addition, an electric arc welding electrode coded Fox EV 85 was selected for arc welding applications to research the e ect of welding method in mechanical properties. Welded joint were prepared at 130 A, 20 V; 150 A, 22 V and 170 A, 24 V welding parameters. All welded joint specimens were exposed to tensile tests, Charpy impact tests to evaluate the yields strength, maximum tensile stress, fracture stress, % elongation values and ductile-to-brittle transition temperatures values. Micro and macrostructure photos and scanning electron microscopy micrographs of specimens were obtained. The optimum parameters were advised to users at the end of this study.

The metal magnetic memory method application for online monitoring of damage development in steel pipes and welded joints specimens

The magnetic memory of metal (MMM) is an after-effect, which occurs in the form of metal residual magnetization in components and welded joints formed in the course of their fabrication and cooling in the weak magnetic field of the earth and in the form of irreversible change of components’ magnetization in zones of stress concentration and damages under working loads (ISO 24497–1:2007(E)). The results of experimental investigation by the metal magnetic memory of steel specimens in the process of their static and cyclic loading are presented. Investigation using the MMM method was carried out online in the process of cyclic loading in the synchronous manner with load frequency. Specific features of self-magnetic field’s (SMLF) intensity variation at maximum load during tensile cyclic load were revealed. The obtained results of specimens investigation experimentally confirmed the earlier drawn conclusion about the possibility of the MMM method and corresponding inspection instruments application for quick testing of mechanical properties and cyclic strength parameters at the physical level. Comparison of σ–e deformation curves and σ–ΔHx, ΔHx–e magnetograms, obtained in the static loading mode, of σ–e cyclic loading profiles and corresponding magnetograms confirms once more the earlier established correlation of magnetomechanical parameters conditioned by the “magnetodislocation hysteresis”. Fatigue curves, plotted by variation of the self-magnetic field of the specimens in the process of cyclic loading, experimentally confirm the possibility of equipment life assessment by parameters of the metal’s magnetic memory. Magnetograms, recorded online in the process of cyclic loading of specimens, allow claiming the possibility of the MMM method and the Tester of Stress Concentration type instruments application for technical state monitoring during operation of equipment under load conditions.

원격 CO2레이저 용접이음에 대한 피로시험과 해석

A remote laser system can rapidly change both the distance and the direction of the laser beam by moving a lens and rotating mirrors. It is then easy to weld complex patterns of weld lines. A conventional spot weld joint specimen and a remote laser weld joint specimen with complex weld line patterns were prepared and tested both statically and dynamically. The relationships between the fatigue strength, i. e. the maximum cyclic force, and the fatigue life were obtained. The fatigue strength of the tested welded joints at two million cycles was found to be approximately 10% of the static strength. Furthermore, it was observed that the fatigue fracture mode changed with the level of the applied cyclic force. The fatigue crack origins were confirmed as the highest stress points found in the structural analysis. The maximum cyclic stress for different weld patterns converges as the fatigue life approaches two million cycles.