电弧热流分布模式对GMAW焊接温度场的影响

提出了ＧＭＡＷ熔池表面产生较大变形时的电弧热流分布模式,以此为基础并考虑熔滴过渡过程及焊缝余高,建立了焊接温度场的数值分析模型,通过数值模拟,定量分析了焊接工艺参数－ＧＭＡＷ熔池表面变形－电弧热流分布－熔池形态及其温度是之间的相互影响。焊接工艺试验结果,与高斯热源模型相比,采用本文给出的ＧＭＡＷ电弧热液分布模型的计算结果更符合实际。     

Three-dimensional turbulent weld pool convection in gas metal arc welding process

Abstract

A three-dimensional model has been developed to study turbulent fluid flow and heat transfer in a gas metal arc weld pool. The phase change process during melting and solidification is modelled using the enthalpy–porosity technique. Mass and energy transports by droplet transfer are considered through a thermal analysis of the electrode. The droplet heat addition into the molten pool is considered to be in the form of a volumetric heat source distributed in an imaginary cylindrical cavity within the weld pool ('cavity' model). A two-equation k-ε model capable of addressing turbulent weld pool convection, taking into account the morphology of the phase change, is presented. The weld pool dynamics and geometry (shape and size) in a moving gas metal arc welding (GMAW) process are studied and the effects of enhanced diffusivities on the turbulent weld pool are discussed. The predicted weld pool geometry using laminar and turbulent models is also compared with corresponding experimental post-weld sections.     

Analytical thermal model of conduction mode double sided arc welding

Abstract

An analytical thermal model of conduction mode double sided arc welding (DSAW) has been derived and used to predict the weld pool dimensions and shapes and temperatures within 2˙5 and 1˙15 mm thick AA5182 Al alloy sheets as functions of the primary DSAW parameters. Separate Gaussian distributed arc heat sources from a plasma arc welding and gas tungsten arc welding torch were assumed to act on the top and bottom surfaces of the sheets. There was excellent correlation between observed and predicted DSAW weld pool dimensions and shapes provided that suitable values for arc efficiencies and distribution coefficients for the two separate arcs were used in the model. The model is capable of predicting weld pool dimensions and shapes of both full and partial penetration conduction mode DSAW welds made in Al alloy sheet, the welding speed at which there is a transition from full to partial penetration welding and the speed above which no melting occurs.     

Neural network model of heat and fluid flow in gas metal arc fillet welding based on genetic algorithm and conjugate gradient optimisation

Abstract

Although numerical calculations of heat transfer and fluid flow can provide detailed insights into welding processes and welded materials, these calculations are complex and unsuitable in situations where rapid calculations are needed. A recourse is to train and validate a neural network, using results from a well tested heat and fluid flow model to significantly expedite calculations and ensure that the computed results conform to the basic laws of conservation of mass, momentum and energy. Seven feedforward neural networks were developed for gas metal arc (GMA) fillet welding, one each for predicting penetration, leg length, throat, weld pool length, cooling time between 800°C and 500°C, maximum velocity and peak temperature in the weld pool. Each model considered 22 inputs that included all the welding variables, such as current, voltage, welding speed, wire radius, wire feed rate, arc efficiency, arc radius, power distribution, and material properties such as thermal conductivity, specific heat and temperature coefficient of surface tension. The weights in the neural network models were calculated using the conjugate gradient (CG) method and by a hybrid optimisation scheme involving the CG method and a genetic algorithm (GA). The neural network produced by the hybrid optimisation model produced better results than the networks based on the CG method with various sets of randomised initial weights. The CG method alone was unable to find the best optimal weights for achieving low errors. The hybrid optimisation scheme helped in finding optimal weights through a global search, as evidenced by good agreement between all the outputs from the neural networks and the corresponding results from the heat and fluid flow model.     

Role of Oxygen as Surface-Active Element in Linear GTA Welding Process

Although the surface-active elements such as oxygen and sulfur have an adverse effect on momentum transport in liquid metals during fusion welding, such elements can be used beneficially up to a certain limit to increase the weld penetration in the gas tungsten arc (GTA) welding process. The fluid flow pattern and consequently the weld penetration and width change due to a change in coefficient of surface tension from a negative value to a positive value. The present work is focused on the analysis of possible effects of surface-active elements to change the weld pool dimensions in linear GTA welding. A 3D finite element-based heat transfer and fluid flow model is developed to study the effect of surface-active elements on stainless steel plates. A velocity in the order of 180 mm/s due to surface tension force is estimated at an optimum concentration of surface-active elements. Further, the differential evolution-based global optimization algorithm is integrated with the numerical model to estimate uncertain model parameters such as arc efficiency, effective arc radius, and effective values of material properties at high temperatures. The effective values of thermal conductivity and viscosity are estimated to be enhanced nine and seven times, respectively, over corresponding room temperature values. An error analysis is also performed to find out the overall reliability of the computed results, and a maximum reliability of 0.94 is achieved.     

Modeling the keyhole shape and dimension in plasma arc welding

It is of great significance to model the keyhole shape and dimensions to optimize the plasma arc welding process parameters. In this study, through employing a combined volumetric heat source mode, the weld pool in keyhole plasma arc welding is determined firstly, and then the dynamic force-balance condition on the interface between the plasma jet and the molten metal is dealt with in describing the keyhole formation inside the weld pool. The effects of welding current on the shape and size of keyhole are numerically analyzed. The sharp transformation from a partial keyhole to a full-penetration keyhole is quantitatively demonstrated.     

Modeling of the Weld Shape Development During the Autogenous Welding Process by Coupling Welding Arc with Weld Pool

A numerical model of the welding arc is coupled to a model for the heat transfer and fluid flow in the weld pool of a SUS304 stainless steel during a moving GTA welding process. The described model avoids the use of the assumption of the empirical Gaussian boundary conditions, and at the same time, provides reliable boundary conditions to analyze the weld pool. Based on the two-dimensional axisymmetric numerical modeling of the argon arc, the heat flux to workpiece, the input current density, and the plasma drag stress are obtained. The arc temperature contours, the distributions of heat flux, and current density at the anode are in fair agreement with the reported experimental results. Numerical simulation and experimental studies to the weld pool development are carried out for a moving GTA welding on SUS304 stainless steel with different oxygen content from 30 to 220 ppm. The calculated result show that the oxygen can change the Marangoni convection from outward to inward direction on the liquid pool surface and make the wide shallow weld shape become narrow deep one. The calculated result for the weld shape and weld D/W ratio agrees well with the experimental one.     

GTA焊接电弧与熔池系统的双向耦合数值模拟

建立了GTA焊接过程电弧与熔池双向耦合统一数学模型．该模型考虑了熔池自由表面变化对电弧和熔池的影响,并通过不断更新自由表面形状实现了电弧与熔池相互耦合．电弧和熔池的两组控制及辅助方程采用有限差分法进行求解．计算中采用了适体坐标系以确定不断变化的自由表面形状．用所建模型对304不锈钢材料的定点GTA焊接过程进行了数值计算分析,取得了良好效果．     

Numerical analysis of heat transfer process for double sided tungsten inert gas – metal inert gas weld pool

Abstract

Double sided arc welding is a new type of technology developed in recent years. Many experiments show that this technology has great advantages over single arc welding for the joining of intermediate thickness stainless steel and aluminium alloy base metals. In the present work, a three-dimensional transient numerical model is created to reveal the heat transfer process for a double sided tungsten inert gas (TIG) - metal inert gas (MIG) weld pool from the viewpoint of heat transfer literature and hydrodynamics. Considering of the features of the model, effective calculation software using finite element technology is adopted. The temperature fields in the weld pool for double sided TIG - MIG welding are successfully calculated; in addition, the configuration of the weld pool is also calculated.Comparisons show that thecalculatedresults agree approximately with the experimentally measured results. STWJ/310     

Study on heat source model in twin-arc GMAW with a common weld pool

The heat input from arcs to weld pool in twin-arc gas metal arc welding (GMAW) with a common weld pool is investigated by high-speed photography. The characteristics of arc shapes and droplet transfer are studied and then the models for heat flux distribution on top surface of weld pool and enthalpy distribution of metal droplets transferred into weld pool are established. By using the model, 3-D geometries of weld pools in twin-arc GMAW with a common weld pool are predicted. Corresponding welding experiments on mild steel plates are carried out and the results indicate that the predicted shape of weld bead on cross section shows good agreement with measured one.     

ForceArc双面焊熔池流场与温度场的数值模拟

基于超威弧(ForceArc)焊接时电弧的形态特征,及超短电弧下实现强制熔滴喷射过渡的特点,采用双椭圆分布热源模型及峰值指数熔滴热焓衰减分布模型. 通过分析工件坡口形貌对ForceArc焊接物理过程的影响,建立贴体坐标系下的ForceArc焊接过程三维瞬态数值分析模型. 模拟了不同预热温度及层间温度对焊接过程的影响,分析了工件上下坡口形貌对熔池传热、传质的影响,并与试验结果进行了对比. 结果表明,文中所建模型能够较好地模拟ForceArc焊接过程,对优化其焊接工艺参数具有一定的指导意义.     

Analysis of submerged arc welding process by three-dimensional computational fluid dynamics simulations

The effect of torch angle and current polarities on the convection heat transfer in single wire submerged arc welding is analyzed. To develop arc models such as arc heat flux, arc pressure and electromagnetic force, this study adopts the Abel inversion method with CCD camera images for direct and alternating current polarities. The heat transfer by molten slag from the flux consumption is considered as an additional boundary heat source in the numerical simulation. The variation of arc forces, the direction of droplet flight with polarity and the torch angle significantly affect the molten pool flow and the resultant weld beads. The simulated weld pool profiles are validated with corresponding experimental results and found to be in good agreement.     

Molten pool behavior in the tandem submerged arc welding process

A three-dimensional numerical heat transfer and fluid flow model is developed to examine the temperature profiles, velocity fields, weld pool shape and size in a two-wire tandem submerged arc welding process. The model solves the equations of the conservation of mass, momentum, and energy along with the volume of fluid method. The volume of fluid method is used to track the shape of the free surface. Further, a novel scheme is proposed to handle the arc interaction and its influence on the molten droplet transfer direction. Using the computational fluid dynamics simulations, it is found that the droplet movement and arc forces from the leading electrode heavily affect the molten pool flow patterns and the resultant bead shapes, even though the same heat inputs are applied. The computed weld width and penetration are in fair agreement with the corresponding experimental results.     

GTAW熔池形状数值模拟精度的改进

采用高斯分布或双椭圆分布热源模式对GTAW熔池形状的数值模拟中发现,计算得到的熔池尾部形状与实际情况有较大差别.文中分析了误差产生原因,提出了数值模拟程序设计中如何确定热源在工件上作用区域计算半径的三个原则.解决了计算结果中熔池尾部后拖不足的问题,提高了GTAW熔池形状数值模拟精度.     

A study on V-groove GMAW for various welding positions

This study performed three-dimensional transient numerical simulations using the volume of fluid method in a gas metal arc V-groove welding process with and without root gap for flat, overhead, and vertical welding positions. The elliptically symmetric arc models for arc heat flux, electromagnetic force and arc pressure were used to describe the more accurate molten pool behaviors. The numerical models not only formed a stable weld bead but also simulated the dynamic molten pool behaviors such as overflow which was not described before. This study analyzed these molten pool flow patterns for various welding positions and validated the numerical models used by comparing the simulation results with experimental ones.     

Numerical analysis of keyhole establishment time in keyhole plasma arc welding

Numerical analysis of keyhole shape and keyhole establishment time is of great significance for selection and optimization of the process parameters in keyhole plasma arc welding. In this paper, a three-dimensional transient model is developed to analyze the evolutions of keyhole shape and keyhole establishment time in continuous current plasma arc welding process. Firstly, a combined volumetric heat source model is used to simulate the transient variation of temperature field. And then the surfaced formation equation is adopted to calculate dynamic features of the keyhole shape and keyhole establishment time inside weld pool, in which the force action on weld pool surface is considered. Experiment is conducted to validate the numerical simulation results. The predicted keyhole size and keyhole establishment time are in agreement with the experimental measurement. And the calculated fusion zone geometry is consistent with the measured one.     

激光+GMAW复合热源焊接过程热-力耦合数值分析

从宏观的焊接热过程出发,根据激光+GMAW复合热源焊接的特点,提出了适用于复合热源焊接的“双椭球体＋峰值递增圆柱体”组合式体积热源分布模式;建立了激光+GMAW复合热源焊接过程的有限元模型,数值计算了焊接温度场和焊缝横截面的形状尺寸,计算结果与试验结果吻合良好,证明了组合式体积热源模型的合理性和适用性. 采用焊接温度场的计算结果,进一步对复合热源焊接和GMAW的焊接变形和残余应力进行了数值模拟和对比分析. 结果表明,在焊缝熔深基本相同的情况下,复合热源焊接的焊接热输入、焊缝熔宽、焊接变形和高应力区域范围等均比GMAW小. 研究结果印证了激光+GMAW复合热源焊接工艺的优越性,并为焊接工艺参数的优化提供了基础理论数据.     

Analytical studies on thermal behaviour and geometry of weld pool in pulsed current gas metal arc welding

In view of the criticality of pulsed current gas metal arc welding (P-GMAW) due to simultaneous influence of the pulse parameters on thermal and metal transfer behaviour of the process an analytical model has been developed for predicting the temperature and geometry of the weld pool by appropriately considering two types of heat sources of different nature. The model considers the impact of heat in droplets of filler metal depositing in the weld pool in addition to initial arc heating. The model assumes the primary heat transfer to weld pool is the initial arc heating considered as continuous heat source (arc heat source) of double ellipsoidal nature followed by deposition of superheated filler metal considered as point heat source of interrupted nature superimposed on the first one. The dissimilar nature of the two heat sources is treated by different analytical techniques to estimate their temperature distribution in weld pool and HAZ at its vicinity. The geometry of the weld pool has been estimated by evaluation of the weld isotherms causing melting of the base metal under the influence of two heat sources acting on the weld pool. The impact of impinging droplets on weld pool has been considered to determine the depth at which the droplets transfer their heat in it. The predicted temperature and geometry of the weld pool as well as the temperature of HAZ are found well in agreement to the experimental values with a deviation of the order of ±10% in case of the weld deposition of Al–Mg alloy and commercial aluminium especially at high mean current of the order of 180 A and beyond the transition current of the filler wire. However, prediction of weld pool temperature and weld geometry is relatively different at comparatively lower mean current of 150 A below the transient current of the filler wires is not up to the mark.     

Numerical analysis of dynamic variation of weld pool geometry in fully-penetrated TIG welding

A mathematical model is developed for numerical analysis of thermal process in TIG welding with a moving arc, which is considered the double-elliptic distribution for both arc heat flux and arc pressure. An adjusting factor is introduced into the expression of arc pressure. The domain within which the arc heat flux is distributed non-symmetrically due to arc moving is selected appropriately, and three conditions for the domain to meet are described. The latent heat is taken into consideration by liquid fraction method. The dynamic development of weld pool geometry during TIG welding is analyzed numerically, and the effect of arc moving on the weld pool geometry is discussed. The experimental results show that the numerical analysis accuracy is obviously improved through taking the above-mentioned measures.     

Numerical simulation of heat transfer and fluid flow in GTA/Laser hybrid welding

Abstract

In order to understand the temperature fields, cooling rates and mixing in the weld pool, a comprehensive, three-dimensional heat transfer and fluid flow model is developed and tested by comparing model predictions with two sets of experimental data. The first set of data was taken from the literature. The experiments varied the separation distance between the heat sources for three arc current levels at a constant laser power. The second set of experiments analysed the effect of varying laser power for a constant heat source separation distance. The results demonstrate that the distance between the two heat sources significantly affects the cooling rates. The calculated results showed that the hybrid weld pool was very well mixed with strong convection currents resulting from the interaction between the electromagnetic and Marangoni forces. The calculated and experimental results showed that hybrid welding increases the weld pool width and gap bridgability when compared with laser welding. The weld pool depth in hybrid welding was affected mainly by the characteristics of the laser beam. Hybrid weld pool penetration depth is maximised at an optimal distance between the arc electrode and laser beam. The cooling rate increases significantly when the heat sources are separated beyond a critical distance. At close separation between arc and laser, calculations show that the arc radius must be decreased to achieve the observed weld depths.