Numerical and experiment analysis of residual stress on magnesium alloy and steel butt joint by hybrid laser-TIG welding

Hybrid welding technology has received significant attention in the welding of dissimilar materials recently. While, great welding residual stress and deformation often result by the difference of coefficient of thermal expansion This study describes the thermal elastic–plastic analysis using finite element techniques to analyze the thermo mechanical behavior and evaluate the residual stresses and welding distortion on the AZ31B magnesium alloy and 304L steel butt joint in laser-TIG hybrid welding. A new coupled heat source model was developed which combined by double-elliptic planar distribution, double-ellipsoid body distribution and Rotary–Gauss body distribution model. From the results, it can be concluded that the temperature distribution at the hybrid weld region is exposed to faster rate of heating and cooling in hybrid welding than TIG. Furthermore, compared to the welding stress distribution on the TIG weld, residual stress σ_y is found about 20% higher on hybrid weld joints, and the residual stress on the 304L steel plate is lower than that on the AZ31B magnesium plate.     

Influence of focusing thermal effect upon AZ91D magnesium alloy weld during vacuum electron beam welding

The influence of focusing thermal effect upon the weld shape, microstructure and alloying elements distribution in the welded joints during vacuum electron beam welding on AZ91D magnesium alloy was studied. The results show that the focus state affects the offset of DOF, and further significantly affects the actual welding heat input in the process of vacuum electron beam welding. The sharp focusing state is characterized with higher welding energy density, but the welding energy density of defocusing state is lower. Therefore, the welding process with sharp focusing state and smaller calculation welding heat input can obtain the same weld penetration as the welding process is the conditions of defocusing state and larger calculation welding heat input. And the welding process of sharp focusing state and smaller calculation welding heat input can induce more strongly burning loss of Mg element than the conditions of defocusing state and larger calculation welding heat input. Then, which will affect the distribution of alloy elements in weld seam.     

Numerical simulation of variable polarity vertical-up plasma arc welding process

Three-dimensional transient governing equations were developed based on conservation laws of energy, momentum and mass. These equations described physical phenomena of convection in weld pool and heat transfer in workpiece during variable polarity vertical-up plasma arc welding process. Boundary conditions for the developed governing equations were given. Welding energy input for variable polarity vertical-up plasma arc welding process was quantitatively expressed. Free surface deformation of the keyhole molten pool was coupled into calculation. Effect of wire filling on the geometry of molten pool and weld reinforcement was considered in the simulation. Correlations of temperature and thermophysical properties for aluminum alloy 2219 were quantitatively established. A control volume based finite difference method was used to solve the discrete governing equations. Moreover, dynamic evolutions of geometrical profile, dimension and fluid flow for the molten pool and keyhole were simulated through the developed computational routines, which achieved transient solution of fluid flow field coupling with thermophysical properties, temperature field and weld pool free surface deformation. Besides, the effect of the workpiece thickness on the moments of keyhole formation and stable keyhole establishment was analyzed, and thermal cycles for the main welding stage were calculated. In addition, experiments via variable polarity vertical-up plasma arc welding technique were conducted, and the established models were experimentally verified through weld cross-section profiles.     

Solidification behaviour in plasma are welding

Melting and solidification behaviour in the deep penetrution welding process is different from that in conventional welding process in deep penetration processes there is keyhole formation and the full thickness of the plate receives the are heat input unlike conventional processes in which the heat input is received only by the surface nodes. In the present study, the thermal analysis of molten pool formation and solidification keyhole welding using plasma are welding has been done using the finite element method. The model accounts for the several phenomena associated with welding, like the distributed are heat input over the top surface and along the thickness, the temperature-dependent material properties. convection and radiation heat losses etc. The analysis is performed for different combinations of parameters. viz welding current and welding speed, which have the maximum influence on molten pool shape and solidification behaviour. The model has also been validated by conducting experimental measurement of thermal cycles experienced by the plate for different welding parameters. The weld pool dimensions. viz. the length and widlh are found to increase with inincreasing current and decereasing welding speed. Thermal cycles at locations close to the weld reach a higher value of temperature and the time for peak temperature is also less but at farther locations the peak temperature reached is lower and the time for peak temperature is higher. Details of the model, the experimental results obtained and the solidifications charateristics of the pool are discussed in this paper.     

Melt pool vorticity in deep penetration laser material welding

In the present study, the vorticity of melt motion in the keyhole and weld pool has been evaluated in case of high power CO₂ laser beam welding. The circulation of vorticity is obtained as a function of Reynolds number for a given keyhole volume which is linked to Mach number variation. The shear stress and thermal fluxes present in the turbulent pool are linked to diffusivity and Prandtl number variation. It was shown that below a critical value of Rayleigh number, the conduction mode of melt transfer signifying beam absorption becomes dominant. Above this value, convective heat transfer indicates melting and evaporation occurring in the weld pool during laser welding. The evaporative recoil pressure expels the liquid while surface tension and hydrostatic pressure help to retain the melt in the keyhole cavity in this high power laser beam welding. The understanding of several hydrodynamic phenomena occuring in the weld pool is valuable not only for understanding basic mechanistic aspects but also for process optimization involved in laser beam welding.     

AC TIG welding with single-component oxide activating flux for AZ31B magnesium alloys

Magnesium-based alloys are finding extensive applications foreground in aerospace and automotive applications. Weldability of magnesium alloys has recently been investigated with a variety of processes. In this article, the activating flux TIG (ATIG) welding of magnesium alloys with three single-component fluxes (TiO₂, Cr₂O₃ and SiO₂) under alternating current (AC) mode was studied. The effects of welding speed, weld current and electrode gap on the weld shape and the weld arc voltage in AC TIG welding with oxide fluxes were investigated on an AZ31B magnesium alloy substrate. The mechanisms of oxide fluxes on the arc shape and the arc voltage on the weld shape are discussed. The result showed that the TiO₂ and Cr₂O₃ increase the weld penetration of AC TIG welding of magnesium with good bead cosmetics. The SiO₂ increased the weld penetration with very poor formation of the weld surface. However, the arc voltage decreased with the used of TiO₂ flux, and increased with the used of Cr₂O₃ flux. The mechanism of TiO₂ and Cr₂O₃ fluxes increasing penetration should not accord with the “arc constriction”. It would comply with some potential effects of the flux interacting with the liquid metal of fusion zone.     

Numerical simulation of fluid flow and temperature field in keyhole double‐sided arc welding process on stainless steel

Double‐sided arc welding process powered by a single supply is a type of novel high‐production process. In comparison with the conventional single‐sided arc welding, this process has remarkable advantages in enhancing penetration, minimizing distortion and improving welding production. In this paper, a three‐dimensional steady numerical model is developed for the heat transfer and fluid flow in plasma arc (PA)–gas tungsten arc (GTA) double‐sided keyhole welding process. The model considers the surface tension gradient, electromagnetic force and buoyancy force. A CCD camera is used to observe the size and shape of the keyhole and weld pool. The acquired images are analysed through image processing to obtain the surface diameters of the keyhole on the two sides. A double‐V‐shaped keyhole geometry is then proposed and its characteristic parameters are derived from the images and cross‐section of weld bead. In the numerical model, the keyhole cavum within the weld pool is treated as a whole quality, whose temperature is fixed at the boiling point of the workpiece material. The heat exchange between the keyhole and weld pool is treated as an interior boundary of the workpiece. Based on the numerical model, the distributions of the fluid flow and temperature field are calculated. A comparison of cross‐section of the weld bead with the experimental result shows that the numerical model's accuracy is reasonable. Copyright © 2005 John Wiley & Sons, Ltd.     

基于复合热源的铝合金激光-MIG复合焊接的有限元模拟

针对15mm厚Al-Mg-Zn铝合金试板多层多道激光-MIG复合焊接过程,综合考虑了脉冲MIG焊接过程大量焊丝的填充带入熔池的熔滴热量和激光焊接过程中的小孔能量沉积效应,建立了复合热源模型。使用ABAQUS有限元软件对上述焊接过程进行了温度场模拟,同时采用非线性弹性边界条件来模拟真实的工装约束作用;通过与实验结果进行对比,模拟的焊缝形貌与试验吻合得较好。     

A study on the metal flow in full penetration laser beam welding for titanium alloy

A mathematical model for flow simulation of full penetration laser beam welding of titanium alloy is presented. In this model, the heat source comprises a plane heat source on the top surface and a cylindrical heat source along the z-direction, which takes into account the plasma effect and the keyhole absorption. By solving the conservation equations of energy, momentum and mass, the temperature and flow fields are obtained. The momentum interpolation scheme with under-relaxation parameter is used to simplify the calculation algorithm and save the storage space of computer. The mushy region is introduced to provide a simple method to dispose of the pressure and velocity boundary conditions. Results calculated from the models are found to agree with the experimental results for the geometry profile of weld. The calculated results indicate the metal flow is the main reason for forming the typical “hourglass” cross-section profile.     

Numerical and experimental investigation on temperature distribution of the discontinuous welding

The main objective of this study is to generate new understanding and improve computer methods for calculating the thermal cycles and temperature distribution of 5A06 aluminum alloy structure during discontinuous welding. Predicting the thermal cycle also provides an estimate of the weld penetration and weld width. The submodeling technique was used to save computing time and improve calculation accuracy. The arc heat input was applied in the weld zone using different forms of surface, volumetric and combined heat flux distribution functions. In order to validate the thermal simulation model, temperature distribution at the weld backside was precisely measured by infrared thermography during the welding process. The simulation results showed that the model is quite sensitive to the energy distribution during the welding process, the effects of the pre-heating and re-heating are significant. The present simulation model can be used as a proper tool to investigate the effect of different metal inertia gas (MIG) process parameters.     

等离子弧焊接熔池和小孔形成过程数值分析

目的 研究等离子弧焊接穿孔过程中熔池内部的金属流动情况和小孔动态变化过程。方法 通过“传热-熔池流动-小孔”之间的相互耦合关系,建立了等离子弧焊接穿孔过程的数值分析模型,通过VOF方法追踪了小孔界面,采用FLOW-3D软件模拟了等离子弧焊接熔池和小孔的形成过程,定量计算了等离子弧焊接温度场、熔池流场及小孔形状;分析了等离子弧焊接熔池和小孔行为;并通过等离子弧焊接实验数据验证了模拟结果。结果 当焊接时间为0~1.0 s时,小孔深度曲线与熔深曲线几乎相同,小孔底部紧贴熔池底部;在2.8 s以后,小孔深度曲线与熔深曲线有一定距离,小孔深度曲线在一定范围内波动,等离子弧焊接电弧挖掘作用到达极限,电弧压力与其他力达到平衡状态。模拟的焊缝熔深为8.04 mm、熔宽为13.20 mm,实验测得的焊缝熔深为8.00 mm、熔宽为13.42 mm。结论 构建的随小孔动态变化的曲面热源模型和电弧压力模型可以描述等离子弧焊接过程中的电弧热-力分布;模拟出了等离子弧焊接熔池和小孔动态演变过程;模拟得到的等离子弧焊接焊缝形貌与实验测得的焊缝形貌基本吻合。     

Microstructural evolution in AZ91D magnesium alloy during electron beam welding

Vacuum electron beam welding can have a low heat input, which means there is a minimum heat affected zone during welding of AZ91D magnesium alloy. From the observed microstructure, the weld of the AZ91D magnesium alloy can be divided into four regions, which are the weld metal zone, a partially-melted zone adjacent to the fusion boundary, a partially-melted zone adjacent to the base metal and the base metal zone. A sharp transition from the fusion zone to the non-melted zone, especially the characteristic partial melting microstructure and nature of the alloy elements, was observed. It was found that significant partial melting had taken place in the very narrow region around the weld metal of the AZ91D magnesium alloy. The Al content of eutectic β-Mg₁₇Al₁₂ in the partially-melted zone adjacent to the fusion boundary was close to the content in the continuously precipitated eutectic β particles in the fusion zone and much lower than the eutectic β in the base metal. The fully melted eutectic β-phase coexisted with the partially melted eutectic β phase in the partially-melted zone adjacent to the base metal.     

Numerical analysis of weld pool geometry in globular-transfer gas metal arc welding

The weld pool geometry and its dimension in the globular-transfer mode during gas metal arc welding (GMAW) were numerically analyzed by using the thermal conduction model, which considered the influence of the deformation of weld pool surface on heat flow in the quasi-steady state. According to the features of the globular-transfer mode, the additional heat energy from molten metal droplets was treated as a plane or volumetric heat source term to correspond to different welding conditions. The weld pool surface profile was predicted while considering the effect of droplet impingement on the depression of the weld pool. The bead-on-plate GMAW experiments were performed under different welding conditions to validate the model of numerical analysis. It has been found that the predicted results agree well with the measured ones.     

粉煤灰复合活性剂在A-TIG焊中增加熔深的机理研究

为实现对工业废弃物粉煤灰的剩余价值利用,尝试以粉煤灰作为主要原料制备焊接复合活性剂,并在AZ91镁合金板上进行A-TIG焊.利用焊缝的电特性实时采集、焊接温度场采集、电弧力测试等手段研究活性剂对电弧影响,通过熔池Bi粒子示踪实验探究活性剂对表面张力温度梯度影响.结果 表明:与常规TIG焊相比,粉煤灰复合活性剂可以使焊缝熔深增深1.4倍,熔宽减小,深宽比是常规TIG焊的1.43倍.粉煤灰复合活性剂中氟化物的解离和电离吸热过程、带电粒子的电子扩散和复合过程可以促进电弧收缩,使焊接电压升高,热输入量提高.而活性剂中的氧化物既可以通过对电弧的机械压缩作用强迫电弧收缩,又可以通过电离产生的氧元素实现对熔池液态金属表面张力温度梯度系数的改变,提高熔池中心热输入.A-TIG焊AZ91镁合金熔深增加是电弧收缩理论和表面张力温度梯度改变理论共同作用的结果.     

Numerical investigation of droplet impact on the welding pool in gas metal arc welding

A transient three‐dimensional model that describes physical phenomena inside a welding pool during gas–metal arc welding process is presented. The model considers such phenomena as heat‐mass transfer, electromagnetics, hydrodynamic processes and deformation of the weld pool free surface. The fluid flow in the weld pool is induced due to the presence of the mechanical impact of the droplets, thermo‐capillary surface tension, thermal buoyancy and electromagnetic forces. The weld pool surface deformation is calculated by considering arc pressure and droplet impact force. A comparative analysis of the impact of the electric current of the welding arc and different force factors causing the motion of liquid metal in the weld pool on the shape of the welded seam was carried out and discussed.     

Understanding the solidification and microstructure evolution during CSC-MIG welding of Fe–Cr–B-based alloy

The present is a study of the solidification and microstructure of Fe–28.2%Cr–3.8%B–1.5%Si–1.5%Mn (wt.%) alloy deposited onto a 1020 plain carbon steel substrate using the controlled short-circuit metal inert gas welding process. The as-solidified alloy was a metal matrix composite with a hypereutectic microstructure. Thermodynamic calculation based on the Scheil–Gulliver model showed that a primary (Cr,Fe)₂B phase formed first during solidification, followed by an eutectic formation of the (Cr,Fe)₂B phase and a body-centered cubic Fe-based solid solution matrix, which contained Cr, Mn and Si. Microstructure analysis confirmed the formation of these phases and showed that the shape of the (Cr,Fe)₂B phase was irregular plate. As the welding heat input increased, the weld dilution increased and thus the volume fraction of the (Cr,Fe)₂B plates decreased while other microstructural characteristics were similar.     

Study on welding process and prosperities of AA5754 Al-alloy welded by double pulsed gas metal arc welding

In this paper, the effect of double pulsed gas metal arc welding (DP-GMAW) on metal droplet transfer, weld pool profile, weld bead geometry and weld joint mechanical properties of Al alloy AA5754 are presented. A high speed camera was utilized to reveal the metal transfer behavior and weld pool profile. A self-developed electrical signal acquire system was adopted to record the current waveform during welding process. The results indicated that the metal transfer, weld pool profile and weld bead geometry in DP-GMAW significantly differ with P-GMAW. The microstructure showed that grain size of the weld bead decreased with increasing of thermal pulse frequency, and the eutectic precipitates Mg₂Si were homogeneously distributed at fusion zone. The mechanical properties of welded joints were improved.     

基于ANSYS 的D500 钢激光焊接温度场数值模拟

目的研究D500钢激光焊接温度场的变化。方法运用ANSYS有限元分析软件,以5 mm厚D500钢为研究对象,采用均匀分布的柱体热源与椭球热源组合的方法,建立了激光焊接热源模型。对D500钢激光焊接温度场进行了模拟计算,并与实验所得焊缝形状及尺寸进行了比较分析。结果结果表明,数值模拟所得焊缝截面尺寸与实验结果一致性达到95%以上。结论验证了柱体热源与椭球热源的组合热源模型在D500钢激光深熔焊接温度场模拟中的适用性,从而为不同焊接工艺条件下D500钢激光焊接焊缝形状和尺寸的预测,提供了一种有效的途径。     

Laser welding of AZ31B magnesium alloy to Zn-coated steel

The characteristics of laser lap welding of AZ31B magnesium alloy to Zn-coated steel were investigated. Welding was difficult when the laser beam was irradiated onto the AZ31B alloy and the processing parameters were set to obtain a keyhole welding mode. The difference in the physical properties between the two materials resulted in unstable welding process particularly when the laser beam penetrated into the steel specimen and a keyhole was formed therein. By switching to a conduction mode, the process stability was improved and successful welding could be achieved because the liquid metal film remained unbroken and the laser beam did not penetrate into the material. A 25 mm wide joint failed in tensile shear testing at loads exceeding 6000 N. This high joint strength was attributed to the formation of a 450 nm thick layer of Fe₃Al intermetallic compound on the steel surface as a result of the interaction between Al from the AZ31B alloy and Fe. The presence of Zn-coating layer was essential to eliminate the negative effects of oxides on the joining process.     

An analytical model and tomographic calculation of vacuum electron beam welding heat source

According to the analysis to the characteristics of welding heat source and thermal effect, a mathematic model of rotary Gaussian body heat source with incremental power-density-distribution was developed, which was in line with the characteristics of heat source during vacuum electron beam welding. The affecting radius of source model decreases progressively with the law of Gaussian function and the power density varies gradually with the law of exponential function in depth direction. The evaluation of peak-power-density coefficient β and the tomographic calculation of the source model in different focused conditions were discussed. The results showed that the focused conditions, which were the deviation of depth of field in electron beam, were dependent on the coefficient β in the source model. Simulation of thermal effect and the analysis of weld formation in vacuum electron beam welding validated the feasibility of the model.