Possibilities and limitations of geometric simplifications for calculations of residual stresses and distortions

The prediction and minimization of welding distortions and the evaluation of the residual stress state after welding using numerical methods are increasingly gaining importance. These numerical models are used to optimize welding processes with respect to distortion. However, the computational time required for a transient 3-D calculation, particularly for large components, often hinders commercial usage of these approaches. Therefore, simulations have to simplify individual aspects. Due to the fact that model verification often failed according to abundant experimental research efforts, it cannot be proven whether the deformations and residual stresses calculated by those simplified models are trustworthy. With the help of the validated simulations of the IIW round robin tests and of variational calculus, this work shows the influences of sheet geometry and model simplifications, e.g. 2-D modelling, on the calculation of distortions and residual stresses. The round robin tests were performed using steel sheets made of an austenitic steel (316 LNSPH) which was bead-on-plate welded by TIG welding. The calculations for a varied sheet geometry show for the investigated process and for large components that a reduction to minimal sheet geometry is necessary and sufficient to determine the longitudinal stresses and the distortions. The transversal stresses are in general extremely sensitive to the sheet geometry.     

Application of In Situ Neutron Diffraction to Characterize Transient Material Behavior in Welding

A fundamental understanding of the transient and nonequilibrium material behavior during welding is essential in the pursuit of process control and optimization to produce defect-free, structurally sound, and reliable welds. The deep penetration capability of neutrons into most metallic materials makes neutron diffraction a unique and powerful tool in understanding the material structures and properties. However, the inadequate neutron flux limits its application in time-resolved study of transient material behavior. This article highlights recent developments toward in situ time-resolved neutron diffraction measurement of material behavior during welding with two examples: (I) measurement of the transient temperature and thermal stresses during friction-stir welding of an aluminum alloy and (II) measurement of the solid-state phase transformation behavior of an advanced high-strength steel under thermal conditions comparable to the welding processes. These newly developed experimental approaches can be broadly applied to other welding or thermomechanical processes for time-resolved measurement of the fast-changing material state in structural metals.     

Mathematical modelling of weld phenomena as a tool of modern welding technology

Summary

This article reviews state of the art mathematical modelling of welds. Examples of modelling weld phenomena in the molten pool, friction welding, residual stress and welding processes are succinctly described. The important advances achieved in weld modelling during the past decade have enabled the basic principles of various phenomena occurring in this field to be understood. In many cases, this has enabled welding processes and the behaviour of welded structures to be improved.     

Microstructure and residual stress variations in weld zone of flash-butt welded railroads

Abstract

In the weld zone and base metal contiguous to the weld zone in flash-butt welded rail, the web of the rail has high tensile residual stresses and the head and base of the rail have compressive residual stresses. The web region is susceptible to failure since most of the weld zone of the rail is coarse grained and has porosity, inclusions and defects resulting from rapid solidification of molten metal entrapment in the weld. Efforts to reduce the amount of these tensile residual stresses require recognition of their causes. In this research, microscopic and macroscopic studies were carried out on vertical and horizontal sections of the weld zone in the head, web and base of the rail. Just after flash-butt welding, the temperature of the web between the current carrying copper electrodes is higher than the temperature of the head and base of the rail. Therefore, by cooling the weld zone to room temperature, the amount of web contraction between the electrodes is higher than the amount of base and head contraction and consequently tensile residual stresses are produced in the web at and near the weld zone. In the head and base of the rail, compressive residual stresses are developed.     

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.     

Numerical Simulation and Experimental Investigation of Residual Stresses and Distortions in Pulsed Laser Welding of Hastelloy C-276 Thin Sheets

为了揭示0.5 mm厚度Hastelloy C-276薄板脉冲激光拘束焊接的热力学行为,借助ANSYS软件建立了三维有限元模型。实验测量了焊接温度历程和残余变形,验证了所建立有限元模型的可靠性。基于该有限元模型,采用改变夹具拘束条件的方法,进一步研究了拘束距离对Hastelloy C-276薄板焊接瞬态应力和塑性应变、残余应力及变形的影响规律。结果表明,温度历程和残余变形的模拟结果与实验结果吻合较好;拘束距离对瞬态塑性应变的大小有显著影响,进而改变了残余应力及变形的分布和大小。随着夹具拘束距离从20 mm减小到4 mm,除了纵向残余拉伸应力外,横向残余拉伸应力和位移的峰值以及角变形的大小都呈减小的趋势。相对较小的拘束距离可以作为抑制横向残余拉伸应力和角变形的高效低成本方法,但对纵向残余拉伸应力有不利影响。     

A comparative study of the residual deformation of an automotive gear-case assembly due to deep-penetration high-energy welding

A three-dimensional elastic–plastic finite element model was used to investigate the relative effects of different joint forms on the welding distortion and residual stresses in an automotive differential assembly due to deep-penetration high-energy welding. Numerical studies were carried out to determine optimal selections of heat generation rate and the number of weld segments to ensure both computational efficiency and accuracy of the calculation. To model the constraints and boundary conditions realistically, contact elements were used at the mating surfaces of different structural components and the shrink fit between the gear and differential case was modeled using couple sets. Two situations representing welded gear-case assemblies where the weld joints were oriented at 0° and 30° with respect to the radial direction were analyzed. Predicted welding distortions and residual stresses are compared and discussed in detail. The results indicate that the residual tensile stresses in the 0° radial joint are larger than those in the 30° angled joint and that residual distortion is sensitive to joint form.     

Contributions of numerical modelling to the optimisation of welding processes

The welding industry has not been excluded from the beneficent invasion of numerical simulation which, by allowing modelling of metal and mechanical behaviour, predicts the microstructures that will be obtained and the real residual stresses. Based on codes of calculation on finite elements, these evaluations can even establish the risk of brutal rupture or the fatigue behaviour of a given structure. As a consequence, the prediction of residual distortions opens the way for the creation of qualitatively optimised structures, created by means of optimised welding processes.     

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.     

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.     

基于双向强度理论计算钛容器的焊接残余应力

钛板具有明显的正交各向异性和双向强化效应.根据双向强度理论计算焊接钛制压力容器的残余应力,推导出了计算式并结合试验数据进行了计算分析.结果表明,焊接钛容器纵焊缝处的纵向和环向残余应力都达到钛板屈服强度的50%以上;而环向端焊缝处的纵向和环向残余应力则很不均匀,数值差别较大,某些测点甚至出现负值.对比钛容器的试验焊缝应力与计算焊接应力,可见其最重要的环向应力基本一致,最大差别在15%以内,说明文中的计算方法符合钛容器实际.     

预拉伸对铝合金焊接残余应力和变形的影响

在预拉伸应力作用下，进行了厚度为4mm的5A05铝合金试板的焊接，焊后残余应力及变形的测定结果表明．预拉伸焊接法可有效减小铝合金薄板焊后的纵向残余应力、纵向挠曲变形和平面变形。在弹性应力范由内，随着预应力的增大，试板的残余应力峰值、纵向挠曲变形及平面变形均逐渐减小。分析认为，预拉伸应力部分抵消了焊接区热膨胀产生的压缩应力，从而减小了压缩塑性变形，进而减小了冷却时焊接区域的拉伸应力水平，相应地远离焊缝区域的压缩应力也随之减小。     

Residual stresses simulation for friction stir welded joint

Abstract

As a solid state joining technique, friction stir welding (FSW) can produce high strength, low distortion joints efficiently. Compared to fusion welding, residual stresses in FSW joints are expected to be low due to a relatively low heat input. However, apart from the heat input, the force from the tool also plays an important role in the development of welding stresses. In the present paper, a semicoupled thermomechanical finite element model containing both thermal load and mechanical load was established to simulate the development of welding stresses during FSW process; an autoadapting heat source model was employed in the thermal analysis; the fixture was also included in the mechanical analysis model. The simulation results showed that due to the effect of the tool force, the longitudinal residual tensile stresses became smaller and were asymmetrically distributed at different sides of the weld centre; the peak of the tensile residual stresses at the retreating side was lower than that at the advancing side. Calculated and experimental results were compared.     

Coupled thermomechanical analysis of friction stir welding process using simplified models

Abstract

It is widely recognised that the fundamental mechanisms associated with the weld formation process and their relationships with welding parameters are complex and remain to be fully understood. The present paper reports a series of general findings based on a set of simplified numerical models that were designed to elucidate various aspects of the complex thermomechanical phenomena associated with friction stir welding. The following phenomena were investigated in separate numerical models: (i) coupled friction heat generation; (ii) plastic flow slip zone development; and (iii) three-dimensional heat and material flow. The friction induced heat generation model was used to quantify the contributions of coupled thermomechanical friction heating, including non-linear interfacial phenomena between the tooling (e.g. stir pin) and material being welded. The plastic work induced heating effects were also examined. The plastic slip formation mechanisms were then investigated by considering contributions from various heating mechanisms. Finally, a simplified three-dimensional heat and material flow model, based on the observations from the coupled friction heat generation model, was used to establish some initial insight regarding the heat and material flow. The results from the three subproblem areas were then generalised in the form of a simple parametric relationship between welding variables (i.e. travel and rotating speeds) and weld formation conditions. A series of assumptions were made in constructing these individual models since there exists little information on actual material behaviour under friction stir welding conditions. However, the findings from the present study not only illuminate some of the important weld formation mechanisms in friction stir welding, but also provide an effective framework for more focused investigations into some of the fundamental phenomena identified in the three subproblem areas: such investigations will be reported separately in a future publication.     

Vibration assisted welding processes and their influence on quality of welds

Vibration assisted welding (VAW) has emerged as a successful replacement for heat treatments and post-weld vibration treatments of arc welds to reduce residual stresses and distortions and thus to improve its mechanical properties. This review paper tries to create a knowledge platform for such a next generation research by consolidating the findings, merits, demerits and shortfalls identified hitherto in the field of VAW. This paper presents a review on the various techniques and processes of applying vibration to the welding system and their effects on microstructure, mechanical properties and residual stress of welds, and the directions for future research are presented. Vibration of workpiece during welding, oscillation of weld pool, oscillation of molten droplet, oscillation of welding arc and vibration of welding electrode were identified from the literature as the possible ways of imparting vibration. The advancement in the direction of computational work is also observed.     

Material response of GMA welded 1 mm thick DP600 overlap joints

Abstract

In a previously published model, gas metal arc welding of 1 mm thick DP600 overlap joints is validated for the transient temperature distribution, the welding distortion and longitudinal residual stresses. Tensile tests have been simulated and performed experimentally. Validations were performed for two clamping cases: an immediate release of the clamps after welding and a release of the clamps after cooling to room temperature. There is good agreement between experiments and simulations. It has been found that the temperature distribution, longitudinal stresses and welding distortions are dependent on the clamping conditions. To explain the effect of the clamping time, a bar model is proposed. It is shown that longer clamping times increase plastic deformation and hence reduce residual stresses and buckling distortion. Additionally for an overlap joint, it has been found that the longitudinal residual stresses are affected significantly by the sample's geometry.     

焊接残余应力数值模拟研究技术的现状与发展

在焊接过程中产生的动态应力应变及随后形成的残余应力，是导致焊接裂纹和接头性能下降的重要因素。因此，焊接残余应力一直是人们关注的热点问题。结合焊接残余应力的研究方法与手段。介绍近年来固内外关于焊接残余应力的研究现状与发展。