Modeling of transport phenomena in hybrid laser-MIG keyhole welding

Mathematical models and associated numerical techniques have been developed to investigate the complicated transport phenomena in spot hybrid laser-MIG keyhole welding. A continuum formulation is used to handle solid phase, liquid phase, and the mushy zone during the melting and solidification processes. The volume of fluid (VOF) method is employed to handle free surfaces, and the enthalpy method is used for latent heat. Dynamics of weld pool fluid flow, energy transfer in keyhole plasma and weld pool, and interactions between droplets and weld pool are calculated as a function of time. The effect of droplet size on mixing and solidification is investigated. It is found that weld pool dynamics, cooling rate, and final weld bead geometry are strongly affected by the impingement process of the droplets in hybrid laser-MIG welding. Also, compositional homogeneity of the weld pool is determined by the competition between the rate of mixing and the rate of solidification.     

Multivariable adaptive control of the bead profile geometry in gas metal arc welding with thermal scanning

This research implements the newly introduced scan welding technique in processes with material deposition. A laboratory station is developed with robotic plasma-arc welding under infrared pyrometry sensing for thermal scanning, and with gas metal arc welding under laser profilometry feedback for material transfer. The dynamics of the weld profile geometry (i.e. the bead width and reinforcement height) are modeled experimentally with respect to the process conditions (weld speed and wire feed). This model serves as the basis for the development of a simple geometry feedback control scheme. A multivariable adaptive control system, based on a generalized one step ahead regulation algorithm is also established and validated experimentally. Application of such bead profile regulation is explored in multipass weld joining, orbital welding, coating hardfacing and rapid manufacturing methods.     

3D heat transfer model of hybrid laser Nd:Yag-MAG welding of S355 steel and experimental validation

A three-dimensional heat transfer model was developed to predict the temperature fields, the weld geometry and the shape of the solidified weld reinforcement surface during hybrid laser-MAG arc welding of fillet joints. Melt pool deformation due to arc pressure was calculated by minimizing the total surface energy. A series of hybrid welding experiments was conducted on S355 steel for different welding speeds and wire feeding rates. A high speed video camera was used to measure weld pool depression and surface weld pool geometry. Visualization of the weld pool during welding has also allowed for a better understanding of the interaction between the keyhole and droplets. The various weld bead shapes were explained through these observations. The arc pressure, the surface energy distribution, and arc efficiency were evaluated by comparing experimental data and numerical results for a wide range of welding operating parameters. Good correlation was found between the calculated and experimental weld bead shapes obtained for the hybrid laser-MAG arc welding process as well as for laser or MAG alone.     

Numerical Analysis of Transient Temperature Field and Keyhole Geometry in Controlled Pulse Key-Holing Plasma Arc Welding

A three-dimensional transient model is developed for numerical analysis of the temperature field, the weld pool, and keyhole shape during the controlled pulse key-holing plasma arc welding process. With considering the coupled thermal-mechanical actions exerted on the keyhole wall from the arc plasma, the periodic variation of the keyhole undergoing “establishing – expanding – contracting – closing” is numerically simulated in a pulse cycle. The computed accuracy is improved by accounting for the thermal-balance condition on the keyhole wall. The predicted keyhole sustaining period and fusion zones are in agreement with the experimentally measured ones.     

核电管系的焊接自动化研究

核电再热管及辅助管系为公司重要生产部件,焊接量大,形状复杂,现多采用手工氩弧焊打底、半自动气保焊填充方法焊接,生产效率低、焊接质量不高。文章通过对产品结构分析,提出各自适合于再热管及辅助管系适合的自动焊方法,逐步实施,效果显著。     

不锈钢中厚板激光全熔透焊的有限元模拟

运用有限元计算软件ABAQUS对16mm厚不锈钢板的激光全熔透焊的温度场和应力场进行了模拟.采用一体两面的复合焊接热源模型来刻画激光全熔透焊过程中的热输入特征,以柱状体热源代表焊接小孔传热模式,以2个超高斯面热源代表等离子体/金属蒸气云对熔池的辐射传热模式.结果表明:温度场模拟结果得到了与实验结果相一致的"沙漏状"焊缝;钢板内纵向残余应力最大,横向应力次之,板厚方向横向应力最小;纵向拉应力主要分布在焊缝两侧约25mm的区域内,最大值已超过材料的屈服强度;经测算,钢板焊后的角变形量仅为0.35°,这是由于激光焊接能量输入高且集中、可不用开坡口而一次性将钢板焊透.     

Prediction of residual stresses and distortions due to laser beam welding of butt joints in pressure vessels

A two-level three-dimensional Finite Element (FE) model has been developed to predict keyhole formation and thermo-mechanical response during Laser Beam Welding (LBW) of steel and aluminium pressure vessel or pipe butt-joints. A very detailed and localized (level-1) non-linear three-dimensional transient thermal model is initially developed, which simulates the mechanisms of keyhole formation, calculates the temperature distribution in the local weld area and predicts the keyhole size and shape. Subsequently, using a laser beam heat source model based on keyhole assumptions, a global (level-2) thermo-mechanical analysis of the LBW butt-joint is performed, from which the joint residual stresses and distortions are calculated. All the major physical phenomena associated to LBW, such as laser heat input via radiation, heat losses through convection and radiation, as well as latent heat are accounted for in the numerical model. Material properties and particularly enthalpy, which is very important due to significant material phase changes, are introduced as temperature-dependent functions. The main advantages of the developed model are its efficiency, flexibility and applicability to a wide range of LBW problems (e.g. welding for pressure vessel or pipework construction, welding of automotive, marine or aircraft components, etc). The model efficiency arises from the two-scale approach applied. Minimal or no experimental data are required for the keyhole size and shape computation by the level-1 model, while the thermo-mechanical response calculation by the level-2 model requires only process and material data. Therefore, it becomes possible to efficiently apply the developed simulation model to different material types and varying welding parameters (i.e. welding speed, heat source power, joint geometry, etc.) in order to control residual stresses and distortions within the welded structure.     

脉冲埋弧焊技术在膜式水冷壁管屏拼接中的试验及应用

使用埋弧焊(SAW)进行锅炉水冷壁管屏生产时,如何在保证焊接质量的前提下,提高焊接速度是需要解决的技术难点.上海锅炉厂有限公司采用脉冲埋弧焊[1～2]技术,对水冷壁管屏拼接工艺进行了焊接试验和产品试生产.实践证明,脉冲埋弧焊相比常规的直流埋弧焊能提高焊接速度13%～25%,改善水冷壁管屏生产中因速度提高而产生的咬边等成形不良问题,使焊缝质量更加优良,有效地提高了焊接生产率.     

基于响应曲面法的涡轮增压器废气阀门激光焊接工艺优化研究

激光焊接技术凭借其高效、高精度的特点已广泛应用在汽车制造产业,在涡轮增压器的生产中,激光焊接也逐步被应用于实现废气阀门与阀杆的连接。虽然当前对于激光焊接的工艺研究很多,但对工艺参数的优化也一直缺少一个科学有效的方法。本文以熔接深度、熔接宽度和内部缺陷作为响应变量,选取焊接功率、焊接速度作为因子,运用响应曲面法(RSM)设计实验方案,通过对响应结果进行分析,研究焊接参数对焊接质量的影响,拟合能够预测最优参数的数学模型,最终将获得的最优参数运用到生产中进行验证。     

Thermomechanical modelling of weld microstructure and residual stresses in P91 steel pipe

Abstract

A multipass circumferentially butt welded P91 steel pipe, typically used for high temperature applications in power plants, has been numerically analysed to determine residual stresses, induced by the process of welding, as well as microstructural regions in the weld, caused by thermal cycles. The finite element (FE) method has been applied to simulate residual stresses generated in the weld region and heat affected zone (HAZ), which are then validated by published experimental data. The axisymmetric FE simulation incorporates solid state phase transformation by allowing for volumetric changes and associated changes in yield stress and hardening behaviour due to austenitic and martensitic transformations. The thermal cycles during welding cause different microstructural regions to emerge within the weld metal and HAZ. Columnar and equiaxed microstructural zones have been numerically modelled in the weld region of the pipe. The predicted FE microstructural regions have been corroborated by columnar and equiaxed zones that have been mapped out on a cross-sectional macroimage of the weld.     

Sensitivity to welding positions and parameters in GTA welding with a 3D multiphysics numerical model

A three-dimensional numerical model of Gas Tungsten Arc welding has been developed to predict weld a bead shape, fluid flow in the weld pool, as well as thermal field in the workpiece. This model accounts for coupled electromagnetism, heat transfer, and fluid flow with a moving free surface to simulate different welding positions. The solution strategy of the coupled non-linear equations that has been implemented in the Cast?M finite-element code is also discussed. The capabilities of our numerical model are first assessed by comparison to the experimental results. Then, as fluid flows in a weld pool play a prominent role in the weld quality as well as in the final shape of the weld bead seam, the effect of various welding positions on the weld pool shape has been investigated. This constitutes the main novelty of this work. The performed computations point out a strong sensitivity to gravity on the weld pool shape depending on assisting or opposing the weld direction with respect to gravity. This study contributes to assessing the model capabilities that provide a deeper physical insight into a more efficient optimization of welding processes.     

Optimal parameters for pulsed gas tungsten arc welding in partially and fully penetrated weld pools

In the present paper, a numerical model of spot pulsed current GTA welding for partially and fully penetrated weld pools is presented. Heat transfer and fluid flow in the weld pool driven by the combination of electromagnetic force, buoyancy force, surface tension gradient and latent heat are included in our model. A new formulation of the electromagnetic problem is introduced to take into account eddy current in the weld pool. The shape of the free deformable surface under the action of pulsed arc force is also handled after the magneto-hydrodynamic calculation.The numerical model was applied to 304 stainless steel welding. We compare the influence of various pulsed welding parameters such as pulse frequency and current ratio on the weld quality. Experimental study is conducted to compare our numerical prediction with welding macrographies. It shows a good agreement of the model.     

Temperature distribution and residual stresses due to multipass welding in type 304 stainless steel and low carbon steel weld pads

Welding is a reliable and efficient metal-joining process widely used in industry. Due to the intense concentration of heat in the heat source of welding, the regions near the weld line undergo severe thermal cycles, thereby generating inhomogeneous plastic deformation and residual stresses in the weldment. Plates of different thickness are used in industry and these plates are normally joined by multipass welding. In a multipass welding operation, the residual stress pattern developed in the material changes with each weld pass. In the present experimental work, thermal cycles and transverse residual stresses due to each pass of welding have been measured in the weld pads of AISI type 304 stainless steel and low carbon steel with 6, 8 and 12 mm thickness. X-ray diffraction method was used for residual stress measurements. The welding process used was the Manual Metal Arc Welding (MMAW) process. In this paper, the peak temperatures attained at different points during deposition of weld beads in stainless steel and low carbon steel weld pads are compared. The residual stress patterns developed, the change in the peak tensile stress with the deposition of weld beads, and the relation between the peak temperatures and the residual stresses in the weld pads are discussed.     

电站锅炉异种钢焊接接头失效问题浅析

本文对电站锅炉受热面部件中奥氏体与铁素体异种钢焊接接头的失效案例进行了总结分析,认为设计结构因素、焊接质量问题、运行环境是促进异种钢失效的主要外因,并从全过程质量控制的角度提出了降低失效风险的应对方法。     

Effects of electromagnetic force on melt flow and porosity prevention in pulsed laser keyhole welding

Porosity formation in pulsed laser keyhole welding was found to be affected by two competing factors: (1) the solidification rate of molten metal and (2) the back filling speed of molten metal during the keyhole collapse process. Porosity (pores/voids) was found in welds when the solidification rate of molten metal exceeds the back filling speed of molten metal. In this study, the use of electromagnetic force was proposed to control the back filling speed of molten metal, and a mathematical model was developed to investigate the effects of electromagnetic force on the transient melt flow, keyhole dynamics, and porosity formation. The results demonstrate that porosity in pulsed laser welding can be prevented by an applied electromagnetic force. Parametric studies to determine the desired strength of the electromagnetic force and its duration were also conducted to achieve quality welds.     

含沙水流对水轮机过流部件焊缝的影响

含沙水流对水轮机过流部件焊缝有很强的磨蚀破坏力,其主要原因是焊缝材料的疲劳强度很低,在相同的磨蚀强度下,焊缝材料最先被磨蚀破坏。研究结果表明,在焊接过程中,采取适当的表面强化工艺,提高焊缝材料的抗疲劳特性．是提高水轮机过流部件焊缝表面抗磨蚀能力的有效途经。     

锅筒焊接热裂纹原因分析及建议

在某厂余热发电锅炉首次检验时，发现锅筒内部环焊缝多处表面裂纹，通过化学成分、金相、硬度分析等方法确定裂纹的性质是焊接热裂纹．制造焊接过程中，化学成分不合格的工艺垫板熔入焊缝并未能消除干净，是造成锅筒内部封底焊缝表面热裂纹的主要原因针对制造工艺和质量控制出现的问题，提出改进措施和意见     

0Cr18Ni10Ti与（SA387Gr11c12＋堆焊层）异种钢的焊接工艺

针对产品加强管（0Ci118Ni10Ti）与筒体（SA387Gr11C12＋堆焊层）角焊缝异种钢焊接接头焊接问题，提出了先在筒体开孔厚度上焊接隔离层，然后再焊接角焊缝实践证明，采用这种焊接工艺进行焊接，可以获得优良的焊缝，能够保证设备的正常运行、     

全位置TIG对接焊机工艺试验研究

为提高锅炉弯管(外径φ30 mm～φ80 mm)焊接自动化程度及接头质量,上海锅炉厂有限公司引进了2台全位置TIG对接焊机.通过对几种材料、几种规格、几种不同坡口型式的管子进行的大量焊接试验,最终确定了最佳的坡口型式及焊接工艺规范参数,且获得了焊缝表面成型美观、根部焊透性好、接头质量可靠等性质优良的焊接接头,为工厂生产应用打下了基础.     

A heat transfer model for deep penetration laser welding based on an actual keyhole

Using a specially designed experimental setup, we obtained a clear stable keyhole with a high-speed camera. A heat transfer model with a cylindrical surface heat source has been developed under the assumption of the keyhole per thin layer being cylindrical. The model is numerically solved by finite element method, the temperature field around the keyhole and the heat flux lost to the keyhole wall can be obtained. The effects of such factors as the shape and the size of the keyhole, the welding speed on the shape of the melt pool are studied. By comparing the laser intensity absorbed on the keyhole walls with the heat flux lost there, the mechanism of energy balance on the keyhole walls is investigated.