Double Ellipsoidal Heat Source Research Articles

Cover plate welding sequence and shape rationality simulation analysis on wall cracks of stainless steel spent fuel pool

The wall of spent fuel pools is prone to pitting or electrochemical corrosion owing to a long-term service in acidic media, where a penetrating cracking may occur in severe cases. Moreover, totally replacing the spent fuel pool causes huge economic losses. Thus, cover plate welding is used to repair the cracked region of spent fuel pools using the ‘fitness for service’ principle. The finite element method is used to simulate and analyse the temperature and stress fields during the repair process of cover plate welding to guarantee that the repair process does not cause new dangerous effects on the cracked region. In this study, we used self-fusion gas tungsten arc welding (GTAW) to cover defects with square and circular patch plates. The modified double-ellipsoid heat source model is derived for cure welding. The effects of two patch plate shapes and different welding sequences on the temperature and stress fields during the welding process were compared using numerical analyses. Finally, a reasonable patch board shape and welding sequence were determined.

Molten pool behavior and solidification characterization in steady magnetic field assisted laser-MIG hybrid welding of aluminum alloy

Microstructure evolution is highly correlated with the melt flow characterization in welding. In this study, a 3D heat flow model with consideration of magnetic field was constructed to describe the temperature field, the flow and keyhole behaviors in the molten pool during the laser-MIG hybrid welding. The role of magnetic flux density in the molten pool dynamics and microstructure evolution was explored. A moving hybrid heat source model including Gaussian surface heat source model and Gaussian rotary body heat source model as well as double ellipsoidal heat source model was carried out to elucidate the laser and MIG arc energy distribution. Decent agreement is realized between the measured weld geometry and predicted weld pool dimension. Numerical results indicate that an expanded molten pool is clearly observed under magnetic field. As the magnetic flux density increases, the downward flow velocity of the molten pool is enhanced. Moreover, visible difference of oscillation growth period for keyhole depth is produced and this leads to a variation in the time it takes for the molten pool to stabilize. Significant grain refinement appearing in both arc-affected zone at the weld upper part and laser-affected zone at the weld lower part is documented although the refinement degree varies at different zones. This is attributed to the formation of many initial nuclei or crystallites, originated from the enhanced flow of the molten pool with the assistance of magnetic field. This study provides a perspective in understanding the physical phenomena involved in magnetic field assisted laser-MIG hybrid welding and lays the foundation for weld microstructure optimization.

Residual Stress Simulation of Heterogeneous Cladding Layer Between SiC/Ti6Al4V

SiC cladding on the alloy surface can significantly improve the high temperature resistance and oxidation resistance of the material. However, heterogeneous cladding usually produces large residual stress and affects the service life of the material. In the process of laser cladding simulated by finite element method, the mechanism of residual stress during SiC deposition on alloy surface is studied by using moving double ellipsoid heat source model. The simulation results show that the residual stress is unevenly distributed near the cladding layer, and the residual stress presents a compression-tension-compression distribution in the thickness direction due to the mixing law of material properties, which provides a reliable numerical prediction method for the residual stress of SiC cladding layer cladding on the alloy base.

Numerical simulation for determination of temperature field and residual stress of stainless steel butt joints with and without clamping

Many welded structures are fabricated from stainless steels because these steels have good mechanical properties and good corrosion resistance. It is necessary to predict the residual stress after welding in order to evaluate the performance of the welded joint. In this paper, the butt welded joint of AISI 316L stainless steel plates is studied. The Metal Inert Gas (MIG) welding process is selected for this material. A SYSWELD software based on the Finite Element Method (FEM) is used to determine the temperature field and residual stresses of two stainless steel plates. The welding simulation includes a sequential coupled thermomechanical analysis. The elemental generation and death technique is utilized to simulate metal deposition in welding. The double ellipsoidal heat source model is used for the heat input of the MIG welding process. The temperature distribution for various time steps at some important points is presented. Cooling times and cooling rates over a temperature range of 800 oC to 500 oC at these points are determined. The residual stress distribution in the longitudinal and transverse direction in two cases with and without clamping is obtained and compared. Obtained results show that: the temperature field in the case of clamping is the same as in the case of no clamping; only the longitudinal stress and transverse stress components are important, the other stress components are not important; the longitudinal stress at the middle of weld line is very high; the transverse stress in the case of clamping is greatly increased, so it is necessary to limit clamping to reduce residual stress after welding

Analysis of element loss, densification, and defects in laser-based powder-bed fusion of magnesium alloy WE43

It is well known that laser-based powder-bed fusion (L-PBF) additive manufacturing of magnesium (Mg) and its alloys is associated with high Mg loss due to vaporization (Mg Loss ) and high incidence of many types of defects in the manufactured parts/samples. Despite this, Mg Loss , densification, and defect characteristics have not been holistically considered in the determination of the optimal values of L-PBF processing parameters for Mg and its alloys. This study presents a combined modeling and experimental approach applied for a widely used Mg alloy (WE43) to address this shortcoming in the literature. First, an experimentally calibrated model is proposed to determine Mg Loss as a function of the L-PBF processing parameters. The model couples the temperature profile using a double ellipsoidal heat source with a Langmuir vaporization model and is calibrated using the width of the single-track L-PBF process and the measured Mg loss using inductively coupled plasma mass spectrometry (ICP-MS). Second, the densification of the samples is determined using a modification of the Archimedes method that considers the amount of Mg Loss in the calculation of the relative density. Third, a comprehensive and quantitative study is conducted on the relationships between the characteristics of porosity defects and the L-PBF processing parameters. Finally, the optimized L-PBF processing parameters are determined by considering the Mg Loss , densification, and the characteristics of defects. The present study yields 0.23 wt.% Mg Loss compared to 2 wt.% Mg Loss that was reported in the previous studies. Furthermore, more than 99.5% densification is achieved while only ∼2% and ∼0.5% of the total defects are characterized as keyhole and lack of fusion defects, respectively.

Effect of Residual Stress on Hydrogen Diffusion in Thick Butt-Welded High-Strength Steel Plates

Thick high-strength steel plates are increasingly being used for ship structures. Moreover, hydrogen enters the process of manufacturing and service, and large residual tensile stress occurs near the weld. Such stress can facilitate the diffusion and accumulation of hydrogen in the material, leading to hydrogen embrittlement fracture of the shell. Therefore, residual-stress-induced diffusion and accumulation of hydrogen in the stress concentration region of thick butt-welded high-strength steel plate structures need to be studied. In this study, manual metal arc welding was realized by numerical simulation of residual stress in a thick butt-welded high-strength steel plate model using the thermoelastic–plastic theory and a double ellipsoidal heat source model. To analyze residual stress, a set of numerical simulation methods was obtained through comparative analysis of the test results of relevant literature. Residual and hydrostatic stress distributions were determined based on these methods. Then, hydrogen diffusion parameters in each region of the model were obtained through experimental tests. Finally, the results of the residual stress field were used as the predefined field of hydrogen diffusion to conduct a numerical simulation analysis. The distribution of hydrogen diffusion under the influence of residual stress was obtained based on the theory of stress-induced hydrogen diffusion. The weak area of the welding joint was found to be near the weld toe, which exhibited high hydrostatic stress and hydrogen concentration. Further, the maximum hydrogen concentration value of the vertical weld path was approximately 6.1 ppm, and the maximum value of the path parallel to the weld centerline and 31 mm away from the weld centerline was approximately 6.22 ppm. Finally, the hydrostatic tensile stress in the vertical weld path was maximized (~345 MPa), degrading the material properties and causing hydrogen-related cracking. Hence, a reliable method for the analysis of hydrogen diffusion according to residual stress in thick high-strength steel plates was obtained. This work could provide a research basis for controlling and eliminating the adverse effects of hydrogen on the mechanical properties of ship structures and ensuring the safe service of marine equipment.

Investigation of Laser Butt Welding of AISI 304L and Q235 Steels Based on Numerical and Experimental Analyses

The fiber laser butt welding of AISI 304L and Q235 low-carbon steel is performed in this study. An integrated heat source combined with an asymmetric double-ellipsoidal heat source and a cylindrical heat source is designed to perform the numerical simulation of the laser butt welding process. With the established numerical simulation model, the formation of the welded joint is studied by investigating the thermal process. The effects of the laser power and laser beam offset to the sides of the center of the welding gap on the welded joint shape and strength are experimentally investigated, and the optimum laser power and laser beam offset are determined via tensile strength and hardness tests of the weldment. The numerical simulation results based on the asymmetric heat source agree well with the experimental results and are applied to investigate the mechanisms for forming different welded joint shapes in AISI 304L and Q235 steels. The different thermal conductivity and melting temperatures of the two dissimilar metals result in the different shapes of the welded joint.

Numerical and experimental investigation for the cladding of AISI 304 stainless steel on mild steel substrate using Gas Metal Arc Welding

In the present study, Gas Metal Arc Welding (GMAW) is used to deposit AISI 304 stainless steel layer on the mild steel substrate. The cladding parameters like welding voltage and feed rate have been varied to identify the deposited AISI 304 stainless steel bead profile and factors affecting the bead deposition on the mild steel substrate. Microscopic and EDS analysis confirms the successful deposition of the stainless steel layer on the mild steel substrate. The GMAW parameters like welding voltage and wire feed rate (WFR) govern the AISI 304 clad bead quality like clad profile, heat affected zone (HAZ), and clad penetration. A numerical model has also been developed using a double ellipsoidal heat source to identify the effect of welding parameters on the clad bead. The developed numerical model identifies the temperature distribution along with the clad bead profile and the model is identical with the experimental results. The experimental results showed that a non-uniform weld bead formation has been observed at lower voltage (14 V) and low WFR (3 m/min). The clad bead quality has improved with an increase in the welding voltage (16 V & 18 V) and wire feed rate (5 m/min & 7 m/min). However, cladding at a high wire feed rate (7 m/min) results in low dilution of the clad material with the substrate.

A novel systematic numerical approach on determination of heat source parameters in welding process

The double-ellipsoidal heat source concept, established by Goldak, has been extensively employed to represent the energy distribution in a broad range of arc welding simulation processes. However, the Goldak's parameters need to be exactly and efficiently defined for accurate arc welding simulation. In this study, a novel procedure was proposed to accurately predict the Goldak's parameters in Gas Tungsten Arc (GTA) Welding simulation. A developed three dimensional (3D) Finite element (FE) analysis was performed to generate thirty sets of normalized input (welding pool characteristics) and outputs (Goldak's parameters). The relevance between Goldak's parameters and welding pool characteristics were established using two regression models and Artificial Neural Network (ANN) computing systems. Linear and quadratic regression models and ANN were compared for evaluation of accuracy of the parameters. Analysis of the results indicated that ANN slightly transcends both regression models, even though the regression models and ANN were able to suitably predict Goldak's parameters for welding simulation. Hence, Goldak's parameters for the welding numerical model were estimated and employed from the ANN model. 3D FE analysis based on thermal-elastic-plastic model using predicted Goldak's parameters were then conducted and validated by the experimental tests in terms of the size of welding pool, temperature distribution and induced residual stress. In the proposed procedure, the data set in training process was obtained using an efficient FE model analysis, which eliminates the cost and time associated with plenty of experimental tests.

Numerical Simulation of 50 mm 316L Steel Joint of EBW and Its Experimental Validation

The 316L thick plate electron beam welding (EBW) has been widely used in fusion test reactor manufacturing. Therefore, the numerical simulation of the 50 mm 316L austenitic stainless steel by two heat sources and experimental on microstructure and residual stress have been studied in this article. In the simulation study, the traditional heat source model (3D Gaussian heat source) and composite heat source (double ellipsoid heat source superimposed on the 3D Gaussian heat source) were proposed to simulate the welding of local joint. Weld cross-section, temperature curve, and residual stress after welding obtained by simulations were investigated. The experimental study involved residual stress tests and microstructure analysis. It turned out that the result of the composite heat source was closer to the actual joint. The residual stress distribution of simulation was validated and in accordance with experimental measurement. Moreover, the microstructures were studied by electro backscattered diffraction (EBSD) and compared with the temperature curve. The formation mechanism of microstructural heterogeneity was caused mainly by different thermal cycles at different positions of the thick plate. The top of the joint was more prone to stress concentration.

Numerical and experimental investigations on the welding residual stresses and distortions of the short fillet welds in high strength steel plates

The current study investigates the influence of different welding sequences and external constraints on development of welding- induced angular distortion and residual stresses in the short fillet welds made of the high strength steel (HSS), S700, using the finite element (FE) method. Three-dimensional thermo-elastic-plastic FE modeling was performed in the ABAQUS FE software based on the double-ellipsoidal heat source model, temperature-dependent material properties, and considering geometrical non-linearity. The accuracy of the FE models were verified by comparing the simulation results and experimentally measured data. Both the numerical method and measurements show that external constraint has relatively larger influence than welding sequence on the development of angular distortion and the peak magnitude of residual stresses. To determine the significance of short fillet welds in terms of induced distortion and residual stress fields, the results captured by simulation and validated by measurements were compared to those of continuous fillet welds. From a numerical point of view, the results of this study are meaningful to understand the reasonable accuracy required to capture the necessary details of the welding process. From an engineering prospect, the results of this study can be important as they indicate that transverse residual stress fields due to short fillet welds are localized with considerably larger peak magnitudes compared to continuous fillet welds. With respect to the angular distortion, the results of this study show that stiffness of external constraint has a greater impact on prevention of angular distortion in continuous fillet welds than in short fillet welds. In the case of fillet welds with small lengths, beside proper clamping, other deformation control techniques such as pre-alignment of the welded members need to be considered.

Using double ellipsoid heat source model for prediction of HAZ grain growth in GTAW of stainless steel 304

The grain growth in HAZ of austenitic stainless steels during welding is an important metallurgical change that affects the mechanical properties. In this study, the finite element simulation of heat transfer in GTAW of stainless steel 304 is investigated using double ellipsoid heat source model. The simulation results are compared with the results obtained from the analytical Rosenthal model and also experimental data. The simulated peak temperatures are close to the experimental results at the measurement points (the maximum relative difference is 10%), while the Rosenthal model has a maximum relative difference of 34%. It is also observed that the simulated weld pool size is also better compatible with the experimental results than the Rosenthal model. Additionally, the calculated grain size based on the simulation results is also more consistent with the experimental data than the Rosenthal model, particularly near the fusion boundary.

Finite element analysis and experimental investigation of moving heat source model for GMAW deposited mild steel weld bead

An investigation has been done to identify the heat source model suitable for numerical modeling of the Gas Metal arc Welding (GMAW) welding process. In the present investigation, a numerical model was constructed to simulate the GMAW welding process through transient thermal analysis for different thermal heat source models for weld deposition of Mild steel considering semi-cylindrical weld bead geometry. The Gaussian heat source with radial coordinate system and double ellipsoidal thermal heat source have been considered to develop a Finite Element Analysis (FEA) model. Validation of numerical analysis results has been confirmed through experimental investigation. Numerical modeling confirms the utilization of both heat sources, i.e. Gaussian and Double ellipsoidal for the prediction of the weld bead deposition profile. However, the numerical modeling developed using a double ellipsoidal heat source has more resembles experimental investigation due to the utilization of bead profile data. However, the Gaussian heat source is found to be suitable in the case of the unavailability of experimental data. The experimental studies show a variation in the weld bead geometry, along its welding direction due to multiple environmental factors. The microstructural investigation also shows the development of a fine columnar grain structure at the welding bead & weld penetration zone, and the grain structure became coarser at the Heat Affected Zone (HAZ).

Numerical Analysis of Electron Beam Welding Deformation for the Vacuum Vessel Lower Port Stub of 316L Stainless Steel

The China Fusion Engineering Test Reactor vacuum vessel has strict tolerances requirements and a large number of fully penetrated joints in the manufacturing and assembly processes. As the most complicated component of vacuum vessel, port stubs have stricter tolerance requirements, to guarantee the high assembly accuracy with the vacuum vessel. Due to the small deformation of electron beam welding, this method is adopted for the welding of port stub. However, welding deformation cannot be avoided, thus it is necessary to carry out finite element analysis to analyze and control the welding deformation of the port stub. In this paper, based on thermo-elastic–plastic theory, the double ellipsoid and cone-shaped hybrid heat source can accurately outline the electron beam weld pool contour of the 316L stainless steel with 50 mm thickness, which is further verified by the welding process experiment. The above hybrid heat source is applied to analyze the welding deformation of the lower port stub, and according to the deformation simulation results, the welding fixture is designed to reduce the welding deformation. The welding deformation analysis can effectively provide theoretical data support and practical guidance for the actual electron beam welding process of port stub.

Welding-induced stresses and distortion in high-strength steel T-joints: Numerical and experimental study

The main objective of this study is to develop a computational approach based on the finite element (FE) method to efficiently predict welding deformations and residual stresses of fillet welded T-joints made of high strength steel (HSS), S700, using different welding sequences and external constraints. With this aim, thermo-elastic-plastic FE models were developed in ABAQUS FE code based on Goldak's double ellipsoidal heat source model, material non-linearity and geometrical non-linearity. The results of the FE models in terms of temperature fields, angular distortion and transverse residual stress were verified against measurements. The results showed that angular distortion and transverse residual stress were significantly impacted by configuration of the external constraints, while longitudinal stress were less affected. It was found that the welding sequences had a smaller effect on the sequential and cumulative welding distortions and final residual stresses than the configuration of the external constraints. The results of this study are meaningful for understanding of the calibration and accuracy of FE computational approaches to simulate welding processes. Additionally, from a practical point of view, the results are important to understand the distortions and residual stress control measures of structural members made from HSSs.

Selective Laser Melting of Pre-Alloyed NiTi Powder: Single-Track Study and FE Modeling with Heat Source Calibration.

Unique functional properties such as the low stiffness, superelasticity, and biocompatibility of nickel–titanium shape-memory alloys provide many applications for such materials. Selective laser melting of NiTi enables low-cost customization of devices and the manufacturing of highly complex geometries without subsequent machining. However, the technology requires optimization of process parameters in order to guarantee high mass density and to avoid deterioration of functional properties. In this work, the melt pool geometry, surface morphology, formation mode, and thermal behavior were studied. Multiple combinations of laser power and scanning speed were used for single-track preparation from pre-alloyed NiTi powder on a nitinol substrate. The experimental results show the influence of laser power and scanning speed on the depth, width, and depth-to-width aspect ratio. Additionally, a transient 3D FE model was employed to predict thermal behavior in the melt pool for different regimes. In this paper, the coefficients for a volumetric double-ellipsoid heat source were calibrated with bound optimization by a quadratic approximation algorithm, the design of experiments technique, and experimentally obtained data. The results of the simulation reveal the necessary conditions of transition from conduction to keyhole mode welding. Finally, by combining experimental and FE modeling results, the optimal SLM process parameters were evaluated as P = 77 W, V = 400 mm/s, h = 70 μm, and t = 50 μm, without printing of 3D samples.

Effects of laser and GMA hybrid welding parameters on shape, residual stress and deformation of HSLA steel welds

Abstract As a high-efficiency and high-quality welding process, hybrid laser-MAG welding (HLMW) has significant potential of application in welding thick plates. In the present study, based on thermal elastic–plastic theory, a three-dimensional finite element model is developed to predict the weld shape characteristics, residual stress and distortion in HLMW for a butt joint of 12-mm-thick high strength steel plate. Metal active gas arc welding (MAG) heat input and laser energy are modeled as one double-ellipsoid body heat source and one cone body heat source with enhanced peak density along the central axis, respectively. The comparison between calculated molten pool shapes and those obtained by the experiment shows a good agreement. Then weld shape characteristic, residual stresses and distortions are calculated in four different welding process parameters. The results show that the increase of laser power and current can effectively increase the weld penetration width when the welding speed is fixed. At the top surfaces of weldment, the peak stress of high laser beam power is more significant than that of low laser beam power. A high compressive transverse stress of low laser beam power can be found at the welding zone and the surrounding heat affected zone. However, at the bottom surfaces of weldment, the peak stress of low laser beam power is larger than that of high laser beam power. The peak stress of low laser beam power is much larger than that of high laser beam power. A high compressive transverse stress of low laser beam power can be found at the welding zone. The vertical deformation in low laser beam power and low arc current welding has the lowest value.

Welding-induced Residual Stresses in U-Ribs of a Steel Bridge Deck

To calculate the welding-induced residual stresses in U-ribs of the steel deck plate and conduct quantitative analysis of influential factors, the U-ribs of steel deck plate of Xinghai Bay Bridge was taken as the research object. In the ABAQUS finite element software, the local models of U-ribs of steel deck plate were established. Nodal body force loads, i.e., heat generation rate, of the double ellipsoidal heat source models were applied via the compiled subsidiary Dflux program. The welding process of the v-groove welds was simulated, to obtain the residual stresses distribution in the top plate and U-rib plates. The influence of thickness of top plate and angle of welding groove on the residual stresses in the U-ribs were studied. The results show that the welding-induced residual stresses calculated by the numerical method proposed in this paper agree well with the experimental data. The maximum residual stresses in the top plates and the U-rib plates all occur near the welds, which exceeds the yielding limitation of the material. As the thickness of top plate increases, the maximum values of residual stresses in the top plates and U-ribs increase. However, with the increase of groove angle, the maximum values of residual stresses in the top plates and U-rib plates decrease.

Dynamic evolution simulation of sintering pool in selective laser sintering walnut shell/Co-PES composite

In view of the difficulties in the observation of sintering pool and too many factors influencing the dynamic evolution process of sintering pool during the selective laser sintering (SLS) process of biomass composite materials, the structural morphology of sintering pool during the SLS process of walnut shell/Co-PES (WSPC) powder and its dynamic evolution process were studied. Based on the DFLUX subprogram, a moving double ellipsoid heat source model was established. Considering heat conduction, heat radiation, heat convection and thermophysical parameters changing with temperature, the three-dimensional finite element model of WSPC powder was numerically simulated in the process of SLS. Through numerical simulation analysis, the surface shape of sintering pool is circularand the cross section one is deep bowl, and the sintering forming process is a process in which the sintering pool is continuously superposed along the sintering direction. By comparing the experimental results with the numerical simulated results, the experimental results are in good agreement with the simulated results. The dynamic evolution law of sintering pool during SLS process of WSPC powder and the change law that sintering pool size increases with the increase of preheating temperature and laser power and the decrease of scanning speed were obtained.

Welding simulation of circumferential weld joint using TIG welding process

Circumferential weld joints are commonly used in marine engineering, nuclear power plant, aerospace industry. In this welding simulation of circumferential weld joint using Tungsten inert gas welding (TIG) process was done. In TIG welding joint the high temperature is induced due to which there is high distortion and residual stress. For that developing a good welding joint in controlled residual stress and deformation we are using the Welding simulation Finite element methodology. A SS304 material with TIG welding is selected as experimental and combination of Finite Element Analysis (FEM) using the MSC marc software to predict the temperature, residual stresses and distortion with varying cylindrical thickness, voltage, pressure and efficiency. Using finite element simulations, the welding heat input is varied by modifying welding parameters (such as welding current and welding speed) to study the effect on residual stresses. The aim of this research is to provide information to verify the validity of current process circumferential manufacturing technology for thin-walled pipes, in order to prevent the failure of these structures in operation due to these inherent stresses. The study's welding heat source model is the Double Ellipsoidal Heat Source Model, which is the best heat source model of welding simulation.