New technique to control welding buckling distortion and residual stress with non-contact electromagnetic impact

Abstract

A new technique using non-contact electromagnetic forces has been proposed for controlling welding buckling distortion and residual stresses in welded thin plates. The experimental results show that the method can successfully eliminate the buckling distortion and reduce the residual stresses. Three-dimensional finite element modelling has been developed to study the evolution of the stress and strain throughout the welding and electromagnetic impacts. The predicted welding distortion and residual stresses are in good agreement with the experimental results. The numerical analyses show that the reduction in distortion and stress is a result of the change of the plastic strain field in the weld region: electromagnetic impacts reduce longitudinal compressive plastic strain in the local region near the weld, and even produce the tensile plastic strain. Moreover, it is found that the residual stress can promote the changes of the longitudinal plastic strain state under electromagnetic impact.     

A mechanism of formation of a single section of a vacuum discontinuous coating

A welding distortion prediction method based on the inherent strain concept is presented. In the proposed method, welding distortion of large-welded structures could be estimated by elastic analysis using the result of thermal-elastic–plastic analysis and the result of smaller welded joints or components. Thermal-elastic–plastic analysis is performed to calculate residual plastic strain distribution, which is the input data for the elastic analysis of welding distortion. The obtained residual plastic strain distribution is mapped to non-deformed finite element models to calculate welding distortion by elastic analysis. The mapping procedure is done in different ways for welding start/end parts and the rest of the weld length in order to take into consideration the unsteady strain distribution at the start/end of welds. For start/end parts, strain distribution used is identical with thermal-elastic–plastic analysis. For the part except start/end parts, strain distribution obtained by thermal-elastic–plastic analysis is extracted from the centre of weld length and is extruded along the welding direction.

The proposed method was applied to the welding distortion prediction of joints with weld length of 900 and 1200 mm based on the thermal-elastic–plastic analysis result of a joint with weld length 600 mm. The estimated results were in good agreement with the thermal-elastic–plastic analysis results of models of corresponding weld length to show the validity of the proposed method.     

Evaluation of 2D, 3D and applied plastic strain methods for predicting buckling welding distortion and residual stress

Abstract

Welding induces residual stresses which in thin section structures may cause buckling distortion. The magnitude of longitudinal residual stress is critical in the prediction of buckling distortion, which affects numerous welding applications in the ship building, railroad and other industries. The objectives of this paper are to overview and evaluate modelling procedures for bucking distortion. Moving source two-dimensional (2D), three-dimensional (3D) small deformation, 3D large deformation, and 2D–3D applied plastic strain analyses are evaluated by comparing computed residual stress and distortion against experimental measurements. Guidelines for modelling welding distortion are developed along with an assessment of the efficiency and limitations of the various analysis methods.     

Mechanical characteristics of resistance multispot welded joints

Abstract

In the present study, three-dimensional thermal elastoplastic analysis has been carried out in order to clarify the mechanical phenomena of the thermal elastoplastic behaviour of multispot welded joints. As the shape of multispot welded joints is not axisymmetrical, unlike single spot welded joints, the solution domain for simulation should be three-dimensional. Therefore, the present study first developed three-dimensional heat conduction and thermal elastoplastic programs using the isoparametric finite element method. Second, from the results analysed by the developed programs, thermal and mechanical characteristics and their production mechanisms on single and multispot welded joints were clarified. Moreover, effects of pitch length on the temperature distribution, welding residual stresses and plastic strain of multispot welded joints were evaluated, indicating that a pitch of 30 mm was more advantageous compared to a pitch of 15 mm.     

Influence of local heat treatment on residual stresses in electron beam welding

Abstract

Electron beam welding (EBW), as a high performance welding method, is also subject to requirements of high integrity welds which particularly include residual stresses affecting distortion and fatigue behaviour. In this context multiple beam technique may provide new applications of specific thermal weld treatment. The present work introduces a promising method of local post-heating in a certain distance to the current welding location applying multiple beams for the reduction in residual stresses in EB welded sheet metals. The conducted investigations include finite element analysis (FEA) and stress measurements in the weld seam area. Using a systematic approach within the FEA decisive process parameters are optimised with reference to the achievable reduction in residual stress. All simulation results are validated by experiments applying strain gauges for the stress measurement. Both simulations and experiments revealed a considerable decrease in residual stress achievable by appropriate positioning of additional heat sources combined with the right power input and beam focusing.     

Sensitivity analysis of history dependent material mechanical models for numerical simulation of welding process

Abstract

Yield stress of 6013-T6 aluminium alloy was tested on Gleeble 1500D thermal–mechanical system at predesigned temperatures during different typical thermal cycles, in order to accurately reflect the influence of weld thermal history on material properties. The typical thermal cycles were referred to the temperature field simulation results of real welding process. The changes of yield stress were obtained directly from the stress–strain curves generated by the tensile tests. The tests were more accurate than previous publications, where only the yield stresses at room temperature after thermal history were tested or calculated from microstructure evolution model. Experimental results showed that the changes of yield stress during the cooling stage of typical thermal cycles followed one set of curves. These yield stress–temperature curves were different from those during the heating stage. Temperature and temperature history dependent material model M2 and M3 were established based on the experimental results. M2 model was perfectly plastic model while work hardening effect was considered in M3 model. Compared with conventional temperature dependent material model M1, the distributions of longitudinal residual stress and strain obtained with temperature and temperature history dependent models fit better with published results. Yield stress of the material at the weld zone decreased a lot after having experienced weld thermal history and longitudinal compressive plastic strain at the weld zone recovered to some extent during the cooling stage in M2 and M3 models. These were the main causes for lower peak longitudinal residual tensile stress in M2 and M3 models.     

Transferability of critical condition for ductile cracking in bent small scale specimen to evaluation of critical internal pressure for ductile cracking in axially notched linepipe

Abstract

For two types of API 5L X65 linepipes, the critical conditions for ductile cracking of the linepipe steel and their applicability to the evaluation of the ductile cracking of an axially notched linepipe were investigated. Static three point bending tests and finite element (FE) analyses for a Charpy V notch specimen were conducted to evaluate the critical conditions for ductile cracking from the notch tip. At the position of ductile cracking for the Charpy specimen, the calculated stress triaxiality was almost constant for both linepipe steels; however, the equivalent plastic strain for each linepipe steel was different. Hydrostatic burst tests were then conducted for internally patched linepipes with an axial through wall notch. The results of FE analyses for the hydrostatic burst tests indicated that the maximum equivalent plastic strain value within the wall thickness was almost the same as that obtained from the three point bending test in the Charpy V notch specimen. It was therefore ascertained that the critical conditions for ductile cracking of linepipes with an actual notch can be predicted from the results of a small scale test and FE analysis, which are used to evaluate the relationship between the stress triaxiality and the equivalent plastic strain.     

Effects of processing conditions on the mechanical properties and deformation behaviors of plasma-sprayed thermal barrier coatings: Evaluation of residual stresses and mechanical properties of thermal barrier coatings on the basis of in situ curvature measurement under a wide range of spray parameters

A modified analytical model has been developed to describe the nonlinear elastic response and residual stress in plasma sprayed thermal barrier coatings (TBCs) on the basis of the measured curvature–temperature plot during the spraying process. Evolution of residual stresses and the nonlinear stress–strain relation of the coating during the cooling stage after deposition were identified and the strain-dependent coating modulus was obtained. Wide ranges of deposition temperature from 200 to 850 °C as well as that of passage thickness from 6 to 58 μm were explored in the experiments because significant changes in the TBC microstructure could be expected. The analyzed results were related to the processing parameters in spraying, such as the substrate temperature and passage thickness. A complicated interplay among the coating microstructure, residual stresses and mechanical properties was identified. Generally with increasing deposition temperature or passage thickness, denser microstructures were observed with an increase in elastic modulus. The nonlinear strain–stress curves of TBCs indicated that the coating modulus increased with compressive residual stress due to closing of microcracks and inter-splat sliding. Moreover, the coating modulus depended not only on the magnitude of residual stress but also on the coating thickness and it was found that the axial force, which is the product of the residual stress and coating thickness, could be used to express their synergistic effect.     

Themo-elasto-viscoplastic modelling of friction stir welding

Abstract

A coupled two-dimensional Eulerian thermo-elasto-viscoplastic model has been developed for modelling the friction stir welding process. First, a coupled thermo-viscoplastic analysis is performed to determine the temperature distribution in the full domain and the incompressible material flow around the spinning tool. Next, an elasto-viscoplastic analysis is performed outside the viscoplastic region to compute the residual stress. Both frictional heat and plastic deformation heat generation are considered in the model. Furthermore, this is the only known model computing residual stress accounting for plasticity caused by both thermal expansion and mechanical deformation due to material spinning. The computed residual stress is verified by comparing to experimentally measured data.     

铀表面Al/Ti复合镀层热应力的有限元分析

对铀表面Al/Ti复合镀层的热应力进行了热弹塑性有限元分析,表明Ti镀层内为压应力,Al镀层内为拉应力,并达到铝的屈服强度,靠近试样侧边,存在边缘效应引起的应力分布不均匀性,离试样侧边2倍镀层厚度处,不均匀性逐渐消失,试样侧边U-Al界面剪切应力大于中部区域.对沉积温度、镀层厚度及镀层力学性能对镀层热应力和塑性应变的研究表明,随着沉积温度升高,镀层内热应力和塑性应变明显增大,减薄Al镀层和增厚Ti镀层可降低镀层内热应力和塑性应变,Al镀层屈服强度及Ti镀层弹性模量对镀层热应力和塑性应变有重要影响.     

Welding of corrosion-resisting steels to titanium alloys

Summary

This paper describes a new method for measurement of the three‐dimensional residual stresses in fillet welds using inherent strains, which are considered to be a source of residual stresses. The new method implies first the inherent strains being determined from a few measured elastic strains with the aid of a proposed inherent strain distribution function, and second the residual stresses being calculated through an elastic analysis when the inherent strains are applied to welded joints.

The measurement method is called the TL_yL_z method when three types of T, L_y and L_z specimen are cut from the fillet weld, and the T method when only one specimen T is cut from the weld.

The TL_yL_z and T methods are respectively used to measure three‐dimensional residual stress distributions on the transverse sections of single‐pass and multipass T‐type fillet welds. The residual stresses measured by the TL_yL₂ and T methods show good agreement with the ones directly measured on the surfaces of welded joints, thus demonstrating the validity and practical applicability of the proposed method.     

Stress and distortion mitigation technique for welding titanium alloy thin sheet

Abstract

A stress and distortion mitigation technique for Gas Tungsten Arc Welding (GTAW) of titanium alloy Ti–6Al–4V thin sheet is presented. The proposed welding technique incorporates a trailing heat sink (an intense cooling source) with respect to the welding torch, and it is also called the Dynamically Controlled Low Stress No-Distortion (DC-LSND) technique. The development of this mitigation technique is based on both detailed welding process simulation using the advanced finite element technique and systematic laboratory experiments. The finite element method is used to investigate the detailed thermomechanical behaviour of the weld during conventional GTAW and DC-LSND GTAW. With detailed computational modelling, it is found that by the introduction of a heat sink at some distance behind the welding arc, a saddle shaped temperature field is formed as a result of the cooling effects of the heat sink; the lowest temperature exists in the zone where the heat sink is applied. High tensile action on the surrounding zone is generated by abrupt cooling and contraction of the metals beneath the heat sink, which increases the tensile plastic strain developed during the cooling process and decreases the compressive plastic strain developed in the heating process, and therefore mitigates the residual stresses and plastic strains within and near the weld. The experimental results confirmed the effectiveness of the DCLSND technique and the validity of the computational model. With a proper implementation of the DC-LSND technique, welding stress and distortion can be reduced or eliminated in welding titanium alloy Ti–6Al–4V thin sheet, while no appreciable detrimental effects are caused on the mechanical properties of welded joints by applying the heat sink in the GTAW process.     

焊接应力变形原理若干问题的探讨(二)

提出焊接残余应力形成和消除原理:焊接残余应力不是压缩塑性应变引起的,而是由于焊缝和近缝区金属在"力学熔点"及以下温度冷却收缩受阻产生的;消除焊接残余应力不是产生拉伸塑性应变以减少、抵消和补偿压缩塑性应变,而是将残余弹性应变转变为塑性应变;消除焊接残余应力并不是必须去除固有应变,部分去除或完全不去除固有应变也能完全消除残余应力.提出随焊后热精确控制应力变形焊接法,既可实现无应力焊接和无应力无变形焊接,也可实现适当压应力无变形焊接和较大压应力微变形焊接;并对传统方法与有限元法进行了分析比较.     

TA15钛合金中纳米压痕附近残余应力-应变场及几何必须位错密度的分布

结合纳米压痕及高分辨电子背散射衍射技术(EBSD)测定了TA15钛合金中α及β相的弹性模量和纳米硬度,揭示了纳米压痕附近应力-应变场及几何必须位错(GND)密度的非均匀分布情况.利用高分辨EBSD测试过程中同步保存的背散射电子衍射花样,并基于cross-correlation的处理方法,计算得出了纳米压痕附近区域的残余弹性应力-应变场分布.结合应变梯度场理论,计算分析了纳米压痕附近区域的几何必须位错密度分布,进而对合金的微观塑性变形机制进行了讨论与分析.结果表明,α相的弹性模量及纳米硬度分别为129.05 GPa和6.44GPa,而β相的相应值为109.80 GPa和4.29 GPa.纳米压痕附近区域的残余Mises应力呈现明显的非均匀分布并受到相邻较软β相的显著影响.压痕附近的低残余应力区域伴随有显著较高的<α>形和柱面型几何必须位错密度分布.     

Separation of macroscopic, elastic mismatch and thermal expansion misfit stresses in metal matrix composite quenched plates from neutron diffraction measurements

Neutron diffraction measurements of the strain profile in a quenched plate of an aluminium-dash;silicon carbide particle-reinforced metal matrix composite are reported. The results have been used to evaluate the efficacy of an analysis technique which allows distinction of the stiffness mismatch and shape misfit stresses between the matrix and reinforcement, as well as between these and any macrostress present. The analysis is presented for measurements made on a metal matrix composite plate which, as a consequence of quenching from elevated temperature, shows large variations in residual stress as a function of position through the plate thickness. The measurements illustrate the additional insight which can be obtained through the separation of the elastic mismatch and thermal misfit stresses. The stress components thus obtained show good agreement with calculated long-range residual and mismatch stresses.     

Elasto-visco-plastic analysis of welding residual stress

Abstract

In the past three decades, extensive research has focussed on the application of numerical methods for the computation of residual stress. Most commonly, the simulations involved performing weakly coupled thermal mechanical finite element analyses in Lagrangian reference frames assuming rate independent elasto-plastic material response. Nearly all approaches assumed rate independent elasto-plastic material response, which is most appropriate at low to moderately elevated temperatures. At, the high temperatures near the fusion zone, the material response becomes rate dependent and an elasto-visco-plastic model would be more suitable. In 1989, Brown et al. (Int. J. Plast., 1989, 5 , 95–130) proposed a rate dependent constitutive equation (commonly known as Anand's model) to describe the plastic evolution of metals at high temperatures. The objective of this work is twofold: evaluate the suitability of Anand's elasto-visco-plastic model in computing welding residual stress and investigate the feasibility of an Eulerian implementation of Anand's model in modelling welding residual stress. Such an implementation has the potential to reduce computational cost in modelling welding processes, since it is a steady state analysis as compared to the common time incremental Lagrangian analyses. An Eulerian reference frame is also more advantageous in modelling processes with large deformation such as friction stir welding, rolling and extrusion since excessive mesh distortion and re-meshing are no issues as the case of Lagrangian models (Int. J. Mater. Form., 2008, 1 , 1287–1290).     

Thermodynamic analysis of shape memory phenomena — I. Effect of transformation plasticity on elastic strain energy

The effect of plastic strains accompanying the thermoelastic transformation on shape memory phenomena is evaluated. It is shown that transformation plasticity can be formally included in Eshelby's macroscopic elastic analysis by defining a net transformation strain equal to the difference of the crystallographic transformation strain and the associated plastic strain. A two variant analysis for a thermoelastic martensite transformation is developed which enables calculation of the variant structure along the path of minimum elastic energy. It is shown that plastic strain accompanying the formation of a martensitic plate reduces the elastic energy stored during the forward transformation. The equilibrium variant structure is shown to be determined by both the applied stress and the fraction of martensite. If the plastic strains accompanying the forward and reverse transformations do not cancel, residual elastic stress fields remain after completion of the reverse transformation. The residual elastic strain fields influence subsequent transformation behaviour and provide the driving force for two-way shape memory behaviour.     

Finite element analysis of controlling the TC4 thin plate weldment wave-like deformation by welding with impacting rotation

The new technology of welding with impacting rotation is put forward to decrease the wave-like deformation of the TC4 thin plate weldment.The thermal stress and strain are vital to understand the mechanism of controlling the wave-like deformation.In order to know the development of internal thermal stress and strain,finite element method is utilized for the stress and strain are difficult to be investigated by experimental methods during the welding process.Temperature field,thermal stress evolution and distortion of thin plate are compared with the test results such as weld thermal cycle,residual stress sectioning measurement,and the deflection of the thin plate respectively.By the finite element analysis and test results verification,the mechanism of the technology to control the wave-like deformation is brought forward,non-uniform thermal elastic strain between compressive plastic region and elastic extensive region is diminished by a certain amount of extensive plastic deformation by welding with impacting rotation process.     

Modifying residual stress levels in rail flash-butt welds using localised rapid post-weld heat treatment and accelerated cooling

Abstract

Flash-butt welding is used in the manufacture of continuously-welded rails. Finished welds typically exhibit high tensile residual stresses in the rail web and at the upper surface of the rail foot, which may increase the risk of fatigue failure in service. An understanding of the influence of the welding process, including post-weld cooling, on the residual stress distribution is necessary to improve the performance of flash-butt welds by post-weld heat treatment (PWHT), since incorrect treatment may have adverse effects on both residual stress and weld material characteristics. A finite element model has been developed to simulate post-weld cooling in flash-butt welded AS60 kg m^–1 rail. Computed thermal histories for normal (air) cooling, rapid PWHT, and accelerated cooling (water spray) were used as inputs to calculate sequentially coupled stress–time histories, including phase transformations. In addition, the localised influence of the initiation time for rapid PWHT, after final upset, on the reduction of tensile residual stresses was investigated. Heating the rail foot immediately after final upset reduced tensile residual stresses in the web region of the weld. Preliminary numerical predictions showed that water quenching the entire weld region too soon after the austenite–pearlite transformation is completed can induce further tensile residual stresses without affecting the microstructure. The results of the numerical analysis can be used to modify the flash-butt welding procedure to lower residual stress levels, and hence improve weld performance.     

Comparative study on evaluation of tendon force for welding distortion prediction in thin plate fabrication*

Tendon force is an essential concept to predict welding distortion such as longitudinal shrinkage and welding induced buckling in thin plate fabrication.In this study,three approaches with experimental,theoretical and computational analysis,are examined to evaluate the magnitude of tendon force.In detail,inherent deformation theory is introduced first,the theoretical analysis to obtain the inherent strain solution is also reviewed;and then analytical solution for tendon force is achieved.Also,the theory of FE analysis for welding is introduced and implemented in a computation to obtain the transient temperature distribution,plastic strain,residual stress and welding distortion in a bead-on-plate welded joint with 2.28 mm in thickness.The longitudinal displacement is employed to evaluate tendon force directly,and these computed inherent strain and inherent stress can also be employed to evaluate tendon force by integration approach later.All the evaluated magnitudes of tendon force have a good agreement with each other.