Inherent Strain Theory Research Articles

Effect of Welding Gap of Thin Plate Butt Welds on Inherent Strain and Welding Deformation of a Large Complex Box Structure.

In this study, an effective numerical model was developed for the calculation of the deformation of laser-welded 3 mm 304L stainless steel plates with different gaps (0.2 mm, 0.5 mm, and 1.0 mm). The welding deformation would become larger when the welding gaps increased, and the largest deformation values along the Z direction, of 4 mm, were produced when the gap value was 1.0 mm. A larger plastic strain region was generated in the location near the weld seam, since higher plastic deformation had occurred. In addition, the tensile stress model was also applied at the plastic strain zone and demonstrated that a larger welding gap led to a wider residual stress area. Based on the above results, inherent deformations for butt and corner joints were calculated according to inherent strain theory, and the welding formation for the complex structure was calculated with different gaps. The numerical results demonstrated that a larger deformation was also produced with a larger welding gap and that it could reach the highest value of 10.1 mm. This proves that a smaller welding gap should be adopted during the laser welding of complex structures to avoid excessive welding deformation.

Estimating welding deformation of ultra-thin mild steel bead-on-plate joints by means of inherent strain method

The inherent strain method as an efficient computational tool has been widely used to predict welding deformation for large and complex structures with thin and medium-thick plates. For ultra-thin plate welded structures, geometric non-linear phenomena often arise during welding because of the low stiffness. In this study, the welding deformation of 0.8 mm thick bead-on-plate mild steel joints was studied by both experimental method and thermal-elastic-plastic finite element method (T-E-P FEM), and the influences of weld lengths on Tendon force, transverse shrinkage and angular distortion were examined experimentally and numerically. Meanwhile, a non-linear elastic finite element method based on the inherent strain theory using ABAQUS software as a platform was developed to reproduce the welding deformation of the ultra-thin bead-on-plate joints with different weld lengths, and the adaptability of the inherent strain method to ultra-thin plate weldments was clarified.

Suppressing welding deformation instability of vacuum vessel considering external restraint and welding sequence

The vacuum vessel has a double-layer shell structure with complex ribs and high-density, full-penetration welded joints. Suppression of welding deformation is very critical to the improvement of manufacturing accuracy. In this paper, the welding experiment is studied for a double-layer shell pre-developed part. The welding deformation measured after welding is compared with the simulation results based on the inherent strain theory, which verified the effectiveness of the numerical simulation method. The welding deformation of the vacuum vessel poloidal segment is simulated numerically. The influence of different welding sequences on welding deformation without constraints is analyzed, and the position where large deformation occurs is fixed by multiple constraints to restrain deformation. The influence of several restraint schemes and fixture removal sequences on deformation suppression is compared. The results show that the optimum welding sequence can reduce 13.6 % of the welding deformation, the minimum welding deformation under free conditions is 14.52 mm, and the final deformation is effectively reduced to 3.6 mm by fixed constraints, which can provide technical guidance for engineering manufacturing.

Numerical analysis and experimental investigation of residual stress and properties of T-joint by a novel in-situ laser shock forging and arc welding

A novel technique, referred to as Weld-LSF, has been proposed for the low-stress manufacturing of T-joints through in-situ synchronous assisted arc welding, employing laser shock forging. This innovative process introduces plastic deformation in the deposited weld seam at elevated temperatures, effectively reducing the tensile residual stresses (TRS) associated with welding. The thermal-mechanical coupling finite element model for Weld-LSF has been established, combining the classical bar-frame model and the inherent strain theory. The mechanism behind achieving low-stress welding of T-joints through in-situ LSF has been analyzed. Subsequently, experimental investigations were conducted using Q345 base metal and ER60–5 welding wire to study the influence of in-situ LSF on the microstructure and mechanical properties of the specimens. The results demonstrate that by introducing in-situ LSF at higher temperatures in the weld seam, tensile residual stresses are transformed from high to low levels, with the average TRS reduced from 342 MPa to 185 MPa, representing a significant reduction of 45.9 %. In-situ LSF forms a hardened layer with a depth of approximately 0.5 mm on the surface of the weld seam, albeit with certain depth limitations. Furthermore, the in-situ LSF treatment addresses the issue of poor ductility caused by stress concentration at the weld toe of T-joints, achieving a desirable combination of strength and ductility. The feasibility of in-situ LSF technology in welding fabrication has been established. In comparison to traditional welding techniques, the in-situ LSF treatment offers promising prospects for achieving low-stress welding, thereby enhancing the mechanical performance and fatigue life of the welding process.

Study on welding buckling evaluation method based on inherent strain theory and eigenvalue buckling analysis

Welding buckling deformation is one of the foremost failure modes of thin plate welded structures. It is important to estimate welding buckling for structural design and process optimization. Inherent strain-based welding buckling evaluation methods have been proved to be valid, but the existing methods have low efficiency due to requiring multiple inherent forces. In this study, a novel welding buckling evaluation method based on longitudinal shrinkage force was established. The effectiveness of the proposed method was validated using a butt welded joint. In addition, this method was applied in a large-scale thin plate structure for evaluating the structural stability, the effects of heat input, welding manner, and external constraints on welding deformation and buckling were also discussed. The results reveal that the proposed inherent strain-based welding buckling evaluation method can be used to judge whether welded structures are buckled. The reduction of welding heat input and the use of single-sided welding can mitigate welding deformation. Increasing external constraints can not only reduce the welding deformation, but also improve the welding critical buckling load for preventing welding buckling.

Laser shock forging—a novel in situ method designed towards controlling residual stresses in laser metal deposition

This paper presents a novel hybrid in situ additive manufacturing (AM) method–laser shock forging (LSF), which combines laser shock peening (LSP) with laser melting deposition (LMD). Based on the classical bar-frame model and inherent strain theory, the mechanisms of the effects of pretreatment and posttreatment on AM process have been elaborated for the first time. Towards controlling tensile residual stress (TRS) in the as-built (AB) state of AM parts which has a detrimental effect on their fatigue life, we then developed LSF to introduce tensile inherent strains in LMD built parts in an in situ manner, which will convert TRS to compressive residual stress (CRS). The laser beam used for shock peening can be adjusted to move synchronously with the laser beam used for metal deposition and keep a certain distance, ensuring the laser shock peening to act on the region where the material temperature cools down to the forging temperature range. Then, experimental works have been conducted on 316L stainless steel; residual stress distributions of the AB, LSP, and LSF treated specimens were compared; results show that LSF increases both the magnitude and depth of CRS compared with conventional LSP treatment, thus providing a promising application in enhancing fatigue life in AM process.Graphical Abstract

Investigating influence of external restraint on welding distortion in LAHS steel thin-plate structures by means of integrated computational approach

In order to ensure the dimensional accuracy during welding assembly, external restraints are often used to control welding-induced distortion. The objective of this study is to investigate the effectiveness of external restraints on the reduction of welding distortion in low alloy high strength (LAHS) steel thin-plate panel structures with different plate thickness by means of an integrated computational approach. In the current study, the total deformation of LAHS steel thin-plate welded structure was computed by a two-step integrated computational approach which combines the thermal elastic plastic finite element method (FEM) with the elastic FEM based on inherent strain theory. In the integrated computational approach, the thermal elastic plastic FEM was employed to obtain the inherent deformations of each typical joint involved in the thin-plate welded structure at the first step, and then the elastic FEM based on the inherent strain theory was used to estimate the total deformation of the whole thin-plate welded structure. Firstly, the integrated computational approach was used to simulate the buckling distortion in a thin-plate panel structure, and the corresponding experiment was carried out to verify its computational accuracy. Then, two finite element models with plate thickness of 5 and 12 mm were established to clarify the effectiveness of external restraint on the reduction of welding distortion of the thin-plate welded structures. The simulated results suggest that external restraint could not effectively prevent the occurrence of buckling distortion for the thin-plate structure with 5 mm thickness, while it can reduce the out-of-plane deformation to a large extent for the thin-plate welded structure with 12 mm thickness.

Numerical and experimental investigation on the mechanism of synchronous trailing cold air heat sink in eliminating the deformation during laser welding SUS301L thin sheet

In order to control and eliminate the welding induced welding deformation of thin stainless steel a laser welding method assisted by a new type of synchronous trailing cold air heat sink is first proposed in this paper. This kind of heat sink is obtained by using vortex tube compressed air refrigeration (VTCAR), which is safe, of low cost and easy to be implemented compared with other commonly used heat sinks such as argon-water mixture, liquid nitrogen, etc. Then, a thermo-elastic-plastic multi physical field coupling numerical model of laser welding with a cold air heat sink located at the rear of laser beam is established, and the effects of the places where the VTCAR heat sink is exerted and heat sink parameters including the heat sink strength and the distance between the heat sink and the laser beam on the welding induced welding deformation are numerically studied. Such two cases as the heat sink is located on the top and bottom surface of the sample respectively are considered. The simulation results show that the cold source exerted on the top surface has the more significant effect than that on the bottom surface under the same heat sink parameters, and the position of heat sink, exactly speaking, the distance between the cold source and the laser beam, has a decisive effect on reducing welding induced buckling. The smaller the distance, the smaller the welding induced buckling. At the heat sink strength of 200 ml and the distance of 5 mm, the equivalent residual stress decreases by 8.09%, and the maximum welding induced deformation decreases by 31.62%. Thirdly, experiments of laser welding with a synchronous trailing VTCAR heat sink are carried out onSUS301L thin sheet. It can be seen that the specimen appears a saddle shape after welding. The coordinate values of the points on the outer plane of the post weld specimen are measured by three coordinates measuring machine, from which the actual welding deformation can be obtained. Both the specimen shape and the measured welding deformation value are in good agreement with the simulation results based on the large deformation theory, which verifies the correctness and the accuracy of the above simulation model. Finally, the mechanism of the synchronous trailing VTCAR heat sink in eliminating the deformation is discussed by inherent strain theory. In the cooling stage during laser welding, the heat sink not only reduces the temperature field of the base metal near the weld, but also makes the typical comet characteristics at the tail of the weld pool disappear and become a flat temperature profile, which effectively reduces the temperature gradient in the weld area and weakens the longitudinal tensile plastic strain of the weld specimens, and eventually reduces Tendon force and the welding inherent strain. There is a high consistency between Tendon force and the welding deformation, so Tendon force can be taken as the basis for evaluating the welding induced welding deformation.

Analysis and mitigation of the bending deformation in girth-welded slender pipes with numerical modelling and experimental measurement

Girth welded pipes are extensively used in the components of energy and chemical industries. The welding deformation modes depend on various influential factors, such as process schemes and geometric dimensions. The objective of this study is to investigate the characteristics and mechanism of the bending deformation generated in the girth welded slender pipes, and to develop mitigation schemes. Firstly, several 304-type stainless steel pipes were girth welded using four different welding schemes, and the 3D shape measuring system was employed to capture the basic welding deformation modes. Numerical models were developed to simulate the corresponding welding processes with experimental verification. And the mechanism of bending deformation was further clarified based on inherent strain theory. The results show that obvious bending was observed in the pipe of continuous full turn welding due to the non-uniform distribution of the axial residual plastic strain on the circumference. Welding of half turn tends to generate a bending from the weld starting position to the finish position besides the bending toward the weld side, which brings about the mutual elimination of the bending deformation with same direction half-turns and the superposition with reverse direction half-turns. Furthermore, a dual-torch scheme was proposed for girth welding of slender pipes and presented a great potential to mitigate the bending deformation with an improved welding efficiency.

Size effect in selective laser melting additive manufacturing of 700 mm large component

The complex structure with large size (e.g., >500 mm) in aerospace, mould, nuclear power industry, has been urgently required for production with selective laser melting additive manufacturing (AM). Revealing the effect of the component size on the distortion could contribute to better understanding of AM and suggest control strategy for distortion reduction. Here, a method combining elastic-plasticity theory and inherent strain theory is proposed to study the size effect in AM of large size component. Firstly, the traditional thermo-mechanical coupled model based on the thermo elastic-plasticity theory is used to calculate the inherent strain of AMed parts. And a single tracked experiment was performed to confirm the availability of thermo-mechanical coupled model. Secondly, the aircraft blade of 700 mm is used to verify the availability of simulating large component deformation based on the inherent strain. Finally, the size effect of distortion and effective stress of large-size components is briefly illustrated. This manuscript provides a powerful simulation framework for predicting the distortions of large structure produced by thousands and thousands of melts in selective laser melting, and this framework could also be extended to apply in the other AM technologies, e.g., laser metal deposition, wire-arc deposition, and so on.

Stiffener Design to Maintain Line Heating Efficiency during the Lifting Process Considering Phase Transformation.

In the shipbuilding industry, welding is the main technique used to join steel structures. There is a lifting process, post-welding, that can eliminate the correction effect of line heating. Line heating is reperformed after the lifting process. This can significantly delay the ship assembly process. Herein, we present a design method for installing a permanent stiffener to avoid the disappearance of the line heating effect during the lifting process. The change in physical properties due to heating and cooling of the line heating is calculated. The limiting stress, at which the effect of the line heating completely disappears, based on the inherent strain theory, is obtained. The phase fraction by the cooling rate is calculated using the continuous cooling transformation diagram and the Kiustinen–Marburgerm equation. Physical properties affected by the phase transformation are calculated, considering the physical properties and fraction of each phase. The square plate theory and superposition principle are used to construct a local model, with a stiffener, of the ship block. The stress caused by the shape of the stiffener and the distance between the stiffeners were calculated for the local model. The calculated stress and the limiting stress were compared to determine, for the expected line heating efficiency, the most acceptable stiffener design. Finally, to confirm the elimination of the problem, the designed stiffener is analyzed using the finite element method.

Welding Distortion Prediction for Multi-Seam Welded Pipe Structures using Equivalent Thermal Strain Method

Distortion prediction of a welded structure is an important way to improve work efficiency and ensure product quality during the fabrication process. Recently, the widely used approaches for evaluating residual deformation include the thermo-elastic-plastic finite element method (TEP-FEM) and the inherent strain method (ISM). The former can accurately simulate the entire welding process with excessive computational time, whereas the latter has great potential in the fast prediction of residual distortion in complex welded components. In this study, an equivalent thermal strain method based on inherent strain theory was proposed to predict residual deformation of the multi-seam welded pipe structures. Moreover, a full 3D TEP-FEM model was also developed to estimate the distributions of inherent strain in longitudinal (L-seam) and circumferential (C-seam) welds. To validate the accuracy of the proposed ISM, the welding distortion predicted by the proposed ISM is compared with the experimental data and TEP-FEM simulation results. It is discovered that the proposed ISM can be used to accurately predict welding-induced deformation in multi-seam welded pipe structures. Furthermore, compared with TEP-FEM simulation, the significant advantage of this approach is that the computing efficiency can be increased by about 50 times.

Mechanism clarification of mitigating welding induced buckling by transient thermal tensioning based on inherent strain theory

Welding induced buckling in thin plate structures significantly affects productivity and fabrication cost due to post-weld correction works. Transient thermal tensioning (TTT) as an in-process treatment was employed for mitigating welding induced buckling, and its mechanism was clarified by inherent strain theory. Butt welding experiment with 304 L stainless steel was conducted. A finite element (FE) model was established to examine thermal-mechanical response during the entire welding process. Computed results were validated by the measured out-of-plane welding distortion, which was obtained using a three-dimensional optical scanning system. Then, two additional heat sources on both sides of the welding line were applied to achieve TTT treatment with the established numerical model. Moreover, the influence of heating location on mitigating welding induced buckling was discussed. The results show that the maximum deflections along and perpendicular to welding direction were reduced by 82.6% and 74.7%, respectively. The heating location has a decisive influence on the mitigation of welding induced buckling, which should be kept away from the welding line with appropriate distance. The thermal stretching from additional heat sources enhances the self-constraint of the base material on the welding region during the cooling process, resulting in more tensile plastic strain generated to reduce welding inherent strain and tendon force.

Numerical simulation of the effect of ultrasonic impact treatment on welding residual stress

Ultrasonic impact treatment (UIT) is an effective method that has been widely applied in welding structure to improve the fatigue properties of materials. It combines mechanical impact and ultrasonic vibration to produce plastic deformation on the weld joints surface, which introduces beneficial compressive residual stress distribution. To evaluate the effect of UIT technology on alleviating the residual stress of welded joints, a novel numerical analysis method based on the inherent strain theory is proposed to simulate the stress superposition of welding and subsequent UIT process of 304 stainless steel. Meanwhile, the experiment according to the process was carried out to verify the simulation of residual stress values before and after UIT. By the results, optimization of UIT application could effectively reduce the residual stress concentration after welding process. Residual tensile stress of welded joints after UIT is transformed into residual compressive stress. UIT formed a residual compressive stress layer with a thickness of about 0.13 mm on the plate. The numerical simulation results are consistent with the experimental results. The work in this paper could provide theoretical basis and technical support for the reasonable evaluation of the ultrasonic impact on residual stress elimination and mechanical properties improvement of welded joints.

Artificial thermal strain method: A novel approach for the analysis and fast prediction of the thermal distortion

Abstract Understanding the thermal distortion induced by arc welding, laser, or induction heating is a challenge in academia and industry. This study presents the artificial thermal strain (ATS) method which was modified from strain as boundary (SDB) method, based on inherent strain theory, and its equivalent mechanical model to analyse the distortion mechanism. Experiments and numerical simulation of multi-pass laser bending of alloy 304 L were conducted, wherein the various kinds of inherent strain distributions and the boundary of effective inherent strain were examined. Further, the mechanism of bending angle reduction under lower line energy was evaluated by considering the inherent strain redistribution at each pass. The results indicate that the bending angle reduction in multi-pass laser scanning is determined by the size of the inherent strain area at low line energy. Moreover, a simplified plastic finite element model based on the ATS method was developed to predict the welding deformation of plate curvature induced by parallel laser scanning. Additionally, only 210 elements and 7 s was cost to perform with the deformation analysis, which indicates that the ATS model is effective in predicting thermal distortion. Meanwhile, the ATS method can provide theoretical guidance and fast prediction for the actual thermal processing in large welded structure and additive manufacturing in the future.

Optimal simultaneous welding to minimise welding deformation of a general ship grillage structure

ABSTRACT A precise estimation of the welding deformation is an essential process in improving steel fabrication processes for building ships and offshore structures. This study validated the effect of the optimal simultaneous welding on the mitigation of welding displacements. Most previous studies on the optimised welding sequence for the reducing welding displacements have focused on one welding line at each order. However, in heavy industries, several welders or robotic welding machines generally work together to efficiently spend work time. Herein, inherent strain theory is introduced to calculate the complex mechanical behaviour during a welding operation using the elastic finite element method (FEM). Multipoint constraint function and the interface element theory are employed to consider the relationship of the different welded parts. The impact of the optimal simultaneous welding in the sequence on the reduction of welding displacement is validated by using the proposed numerical method in the present study.

Numerical prediction of deformation in thin-plate welded joints using equivalent thermal strain method

Large welding-induced distortions are inevitably generated in a thin-plate welded joint due to its weak stiffness. In this study, an equivalent thermal strain method based on inherent strain theory is proposed to predict the welding deformation of thin plates. An artificial temperature and an orthotropic thermal expansion coefficient are adopted to directly simulate the welding shrinkage and bending behavior of the cooling phase in all directions. To describe the distribution of transverse inherent strain through thickness more accurately, a composite shell element is employed, which can divide the thickness into multiple layers and set different material properties. A three-dimensional (3D) thermo–elastic–plastic finite element method (TEP-FEM) model is also developed to simulate the welding process of a thin plate and compute the inherent strain. Moreover, an experiment was performed to verify the accuracy of the numerical model. Comparing the results from experimental data, TEP-FEM, and the proposed method, it is discovered that the new method can predict the deformation of a thin-plate welded joint with good computational accuracy.

Reconstructing a 3D map of longitudinal residual stress by combining the inherent strain theory and contour method

Reconstructing a 3D map of longitudinal residual stress by combining the inherent strain theory and contour method

Analysis and control of welding deformation for CFETR vacuum vessel PS2

Abstract The large scaled vacuum vessel (VV) of CFETR is designed as a water-cooled, double-walled structure with a D-shaped poloidal cross-section. One sub-sector of VV contains four poloidal segments (PS) assembled by welding. Because of the large size of the VV, the distortion caused by welding will make the assembling difficult. Controlling the deformation during the welding process is a significant technological challenge. This article gives the analysis of the deformation of the component during the welding process, the optimization of the welding process and the design of the welding tooling and clamping positions firstly. Based on the inherent strain theory, the welding deformation analysis of the five different welding sequences of PS2 is performed, and the corresponding welding tooling and clamping positions are designed in accordance with the deformation results. The methods provide highly valuable information for the actual processing, and the welding deformation of PS2 in the actual production has been well controlled, all of which are within the tolerance requirements.

Systematic method for positioning clamps and strongbacks based on their influence on welding displacements

To control welding displacements, mitigation methods such as clamps and strongbacks are widely used in heavy industries. It can be easily concluded that providing for as many clamps and strongbacks as feasible on welded structures to minimize welding displacements is common knowledge, but this may not always be feasible due to restrictive work environments as well as cost factors and interference from other portions of the structure. Currently there is not a distinct system to efficiently position clamps and strongbacks at welded structures. Based on understanding of how clamps and strongbacks effect on the reduction of welding displacements, a systematic method to efficiently position them will enable improvements to the welding process. In the present study, several cases which have differently positioned clamps and strongbacks at welded structures were numerically simulated by the elastic Finite Element Method (FEM) using inherent strain theory to investigate the influence of clamps and strongbacks on the reduction of welding distortions. According to the simulation data, the applicable systematic method for efficiently positioning clamps and strongbacks for minimizing welding deformations is proposed herein.