Effect of oxygen content in He–O2 shielding gas on weld shape in ultra deep penetration TIG

Abstract

The effect of the shielding gas concentration on the weld shape was studied for the moving bead on plate TIG welding of SUS304 stainless steel under He–O₂ mixed shielding. The small addition of oxygen to the helium base shielding gas can precisely control the oxygen content in a liquid pool and the weld shape. Oxygen is a surface active element for stainless steel. When the oxygen content in the liquid pool is above the critical value of ～ 70 ppm, the weld shape suddenly changes from a wide shallow type to a deep narrow one due to the change in the Marangoni convection from the outward to inward direction on the liquid pool surface. Weld shape variations influenced by the welding parameters including welding speed, welding current and electrode tip work distance under pure He and He–0.4%O₂ mixed gas shielding were systematically investigated. The investigation results showed that the final shape of the TIG weld depends to a large extent on the pattern and magnitude of the Marangoni convection on the pool surface, which is governed by the combined effect of the oxygen content in a liquid pool, temperature coefficient of the surface tension (dσ s/dT) and the temperature gradient on the pool surface (dT/dr, r is the radius of the weld pool surface). It is considered that the change in welding parameters alters the temperature distribution and gradient on the pool surface, and thus, affects the magnitude of the Marangoni convection and final weld shape.     

Mechanisms increasing welding efficiency during new development of double shielded TIG process

Abstract

A new welding method has been developed for the tungsten inert gas process which can both improve the weld depth/width (D/W) ratio and protect the electrode from oxidation during the welding process. The effects of the welding parameters and the Marangoni convection patterns on the weld pool shapes are discussed. Results showed that the change of welding parameters directly change the temperature distribution on the pool surface and the concentration addition of the surface active element, hence, eventually affect the pattern and strength of the Marangoni convection. For the liquid pool on stainless steel, the weld shapes depends to a large extent on the pattern and strength of the Marangoni convection. It is possible to generate a heavy oxide layer on pool surface under a relatively high oxygen content to affect the surface tension stress. The heavy oxide layer may inhibit the heat flow on the weld pool surface, and reduce the D/W ratio.     

Numerical simulation for welding pool and welding arc with variable active element and welding parameters

Abstract

A numerical modelling of the welding arc and weld pool is established for moving argon shielded gas tungsten arc welding to systematically investigate the effect of the active element oxygen and the welding parameters on the Marangoni convection and the weld shape using FLUENT software. The different welding parameters will change the temperature distribution and gradient on the pool surface, and affect the strength of Marangoni convection and the weld shape. Under high oxygen content, the weld depth/width (D/W) ratio substantially depends on the welding parameters. A high welding speed or large electrode gap (arc length) will make the weld D/W ratio decrease. The weld D/W ratio initially increases and then remains constant around 0·5 with the increasing welding current. When the oxygen content is lower, the weld D/W ratio decreases with the increasing welding current. However, the weld D/W ratio is not sensitive to the welding speed or electrode gap. The predicted weld D/W ratio agrees well with the experimental results.     

TIG焊接过程中熔池对流形式的示踪分析

为验证传统钨极惰性气体保护焊接过程和双层气流保护焊接过程的熔池表面对流形式,确定双层气流保护焊接方法的熔池深度增加机理,试验研究以钨颗粒为示踪相,焊前将300～500μm的钨颗粒均匀涂覆于焊件表面,同时在熔池底部两侧插入钨板,将焊接熔池中心区域和边缘区域阻隔开.传统钨极惰性气体保护焊接方法在熔池表面产生外向对流,钨颗粒...     

焊接电弧与活性组元对TIG焊熔池形貌影响的数值模拟

通过对FLUENT软件进行二次开发,建立了焊接电弧和焊接熔池模型,模拟分析了不同活性组元O元素含量下定点和移动TIG焊熔池形貌变化,对比了氩弧和氦弧的电弧参量及其对熔池形貌的影响.结果表明,由活性组元O元素含量变化导致的熔池内Marangoin对流变化是熔深增加的主要因素;在氩弧下,来自于电弧的气体剪切力对熔池形貌有较大影响;与氩弧相比,氦弧明显收缩,电流密度更大,更多的热量传递到熔池,增大了电磁力引起的内对流运动,可获得更深熔深.焊缝深宽比的模拟结果与试验结果吻合较好.

Abstract：

Welding arc and weld pool modeh were established by FLUENT software for spot and moving TIG welding of SUS304 stainless steel to investigate the effect of the surface-activating element oxygen on the weld shape and analyze the properties of argon arc and helium arc and their effects on the weld shape. The results show that the change of the Marangoni convection induced by different oxygen contents can be considered as one of the principal factors to increase penetration. The plasma drag force from the argon arc has obvious effect on the weld shape. Compared with the argon arc, the hehum arc is more constricted, the welding current density is much greater and the much more heat flux is transferred into the weld pool, which increase the inward convection induced by the electromagnetic force, thus the deeper weld depth can be obtained.The calculated weld D/W ratio agrees with that of the experiment.     

He与He＋CO_2双层气流保护TIG焊工艺

以0Cr13Ni5Mo水轮机转轮用低碳马氏体不锈钢为母材,开展了内层为氦,外层为He＋CO2的双层气流保护TIG焊工艺研究.内层通纯氦避免电极直接接触外层CO2而氧化烧损,外层添加活性气体CO2向熔池溶解活性元素氧改变熔池表面张力对流模式,进而增加熔深和焊缝深宽比.研究了外层气体中CO2含量的变化对电极保护和焊缝形貌的...     

Weld shape variation and electrode protection under Ar–(Ar–O2) double shielded GTA welding

Abstract

Double shielded gas tungsten arc welding (GTA welding or TIG welding) of an SUS304 stainless steel with pure inert argon as the inner layer shielding and the Ar–O₂ active gas as the outer layer shielding is proposed in this study in order to investigate its effect on the tungsten electrode protection and the weld shape variation. The experimental results show that the inner inert argon gas can successfully prevent the outer layer active gas from contacting and oxidising the tungsten electrode during the welding process. The active gas, oxygen, in the outer layer shielding is decomposed in the arc and dissolves in the liquid pool, which effectively adjusts the active element, oxygen, content in the weld metal. When the weld metal oxygen content is over 70 ppm, the surface tension induced Marangoni convection changes from outward into inward, and the weld shape varies from a wide shallow one to a narrow deep one. The effect of the inner layer gas flowrate on the weld bead morphology and the weld shape is investigated systematically. The results showed that when the flowrate of the inner argon shielding gas is too low, the weld bead is easily oxidised and the weld shape is wide and shallow. A heavy continuous oxide layer on the liquid pool is a barrier to the liquid pool movement.     

微量活性组元氧对焊接熔池Marangoni对流和熔池形貌影响的数值模拟

针对SUS304不锈钢的定点钨极惰性气体保护焊过程,建立三维瞬态定点热源作用下的焊接熔池数学模型,系统研究了不同氧含量下的熔池温度场、速度场以及熔池形貌演变过程.结果表明:随着熔池中活性组元氧的增加,熔池内的对流模式经历了以外对流为主、内外对流共存到以内对流为主的演变过程,熔池形貌由浅且宽形、"勺"形变成深且窄形.熔池中微量氧直接影响熔池表面张力温度梯度系数,改变熔池表面Marangoni对流模式和熔池最终形貌.当氧含量低于80×10-6时,熔池表面以外向Marangoni对流为主,熔池形貌宽且浅;当氧含量超过120×10-6时,熔池表面以内向Marangoni对流为主,熔池形貌窄且深;当氧含量处于(80-120)×10-6之间时,熔池形貌为"勺"形,并且随时间的增加,熔池内外对流区域逐渐变小,内对流区域逐渐变大.定点联合保护焊实验结果表明,熔池形貌变化规律的模拟结果与实验结果吻合.     

Weld shape comparison with iron oxide flux and Ar–O2 shielding gas in gas tungsten arc welding

Abstract

The influence of iron oxide flux and O₂–Ar mixed shielding gas on weld shape and penetration in gas tungsten arc welding is investigated by bead-on-plate welding on SUS 304 stainless with low oxygen and low sulphur contents. The oxygen content in the weld metal is measured using a HORIBA EMGA-520 oxygen/nitrogen analyzer. The results show that both the iron oxide flux and the O₂–Ar mixed shielding gas can significantly modify the weld shape from shallow wide to deep narrow. A large weld depth/width ratio around of 0.5 is obtained when the oxygen content in the shielding gas is in the range of 3000–6000 vol. ppm. Oxygen over a certain critical value, i.e. 70 wt. ppm, in the weld pool alters the temperature coefficient of the surface tension on the pool surface, and hence changes the Marangoni convection. A thick oxide layer on the weld pool surface is generated when the oxygen content in the shielding gas is over 6000 vol. ppm, which becomes a barrier for the oxygen conveyance to the liquid pool and prevents the liquid pool from freely moving, and therefore, decreases the intensity of the Marangoni convection on the pool surface.     

氧元素分布模式与AA-TIG焊熔池形貌的数值模拟

文中结合电弧辅助活性TIG焊熔池氧元素分布的试验研究结果,提出焊接熔池表面氧元素分别与熔池表面温度和位置相关两种分布模式,建立了更完善的电弧辅助活性TIG焊熔池模型,求解结果与已有的试验结果和理论研究吻合良好. 结合求解结果,利用格拉晓夫数G_r,磁雷诺数R_m和表面张力雷诺数M_a分析了浮力、电磁力和表面张力的相对作用大小; 利用Peclet数分析了熔池热对流和热传导的相对强弱. 结果表明,电弧辅助活性TIG 焊熔池表面张力作用远大于电磁力和浮力,并决定熔池流动形式;熔池热对流主导熔池的传热过程,揭示了不锈钢活性TIG焊活性元素决定深而窄的熔池形貌的内在本质.     

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.     

Highly efficient TIG welding of Cr13Ni5Mo martensitic stainless steel

A double-shielded TIG welding process using pure He gas as the inner shielding layer and He and CO₂ mixed gas as the outer shielding layer was proposed for the welding of Cr13Ni5Mo martensitic stainless steels. This proposed welding process can successfully address the problem of electrode oxidation with mixed-gas TIG welding and the issue of low weld depth and welding efficiency of traditional TIG welding. A change in the direction of the surface tension convection mode was the primary mechanism that affected the fusion zone profile. When the oxygen content in the weld pool was in the range of 80–120 ppm, the surface tension convection direction changed from outward to inward, resulting in both a larger weld depth and a larger weld depth/width ratio. This process not only allows for a high welding efficiency comparing with traditional TIG welding but also produces better weld impact properties than those of MAG welding (metal active gas welding).     

Quantifying residual stresses in resistance spot welding of 6061-T6 aluminum alloy sheets via neutron diffraction measurements

Residual strains of resistance spot welded joints of 6061-T6 aluminum alloy sheets were measured in three different directions denoted as in-plane longitudinal (σ₁₁), in-plane transversal (σ₂₂), and normal (σ₃₃). The welding process parameters were established to meet or exceed MIL-W-6858D specifications (i.e., approximately 5.7 mm weld nugget and minimum shearing force of 3.8 kN per weld confirmed via quasi-static tensile testing). Electron backscatter diffraction (EBSD) and optical microscopy (OM) were performed to determine grain size and orientation. The residual stress measurements were taken at a series of points along the weld centerline at depths corresponding to the weld mid-plane and at both 1 mm below the top surface of the plate and 1 mm above bottom surface. The residual stresses were captured on the fusion zone (FZ), heat affected zone (HAZ) and base metal (BM) of the resistance spot welded joint. Neutron diffraction results show residual stresses in the weld are approximately 40% lower than yield strength of the parent material. The maximum variation in residual stresses occurs, as expected, in the vertical position of the specimen because of the orientation of electrode clamping forces that produce a non-uniform solidification pattern. Despite the high anisotropy of the welding nugget and surrounding area, a significant result is that σ₃₃ measured stress values are negligible in both the horizontal and vertical directions of the specimen. Consequently, microstructure–property relationships characterized here can indeed inform continuum material models for application in multiscale models.     

不同驱动力对熔池表面变形行为影响的数值模拟

基于流体动力学方程，采用焓-孔隙度法来处理液-固相变，采用VOF方法追踪熔池自由表面变形，建立了固定电弧下的三维瞬态TIG焊熔池数学模型，求解获得了在浮力、Marangoni力、电磁力和电弧压力单独作用时的熔池表面变形行为及其温度场与速度场的分布.模拟结果表明，在大电流（I≥250 A）时，在浮力、表面张力温度系数为正时的Marangoni力、电磁力单独作用于熔池上表面将会产生凸起变形，在电弧压力、表面张力温度系数为负时的Marangoni力单独作用下，熔池上表面将会产生凹陷变形.在大电流下，TIG焊和活性TIG焊熔池均产生凹陷变形.TIG焊熔池的中心区域形成向内的涡流，边缘部位形成向外的涡流，而活性TIG熔池在熔池中心和熔池边缘则分别形成两种成因不同的内向涡流.熔池表面变形量并不是各个驱动力作用的简单叠加.     

基于Fluent熔池内部受力的数值分析

根据TIG焊熔池的实际情况,建立了定点热源作用下TIG熔池的二维轴对称数学模型.利用FLUENT软件及其UDF进行二次开发,同时考虑了相变潜热、材料热物理性参数随温度变化等问题,对熔池中的流场及温度场进行了数值模拟,分析了熔池内部所受到的浮力、Marangoni力、电磁力和电弧力等对其流场及温度场的影响规律,特别是针对...     

CuZnSnSi合金钎料相变过程的热分析动力学

CuZnSnSi合金钎料在钎焊钢工件中具有良好的综合性能.为进一步探究其合金性能,借助差示扫描分析(DSC)和热重分析(TG)技术,分别采用微分非等温法和积分非等温法分析了CuZnSnSi合金钎料相变过程的热分析动力学.结果表明,CuZnSnSi合金的相变温度范围为1150.5~1221.5 K,吸热峰温度为1174.46 K,在相变过程中没有化学反应,但出现升温段的吸热峰温度滞后于降温段的放热峰,说明了合金钎料在结晶过程需要一定的过冷度;非等温分析法计算得到合金钎料相变表观活化能为615.72 kJ/mol;由Arrhenius公式得出合金钎料相变速率常数k的变化规律为1.71×1027exp(-6.16×10⁵/RT).     

PA position full penetration high power laser beam welding of up to 30 mm thick AlMg3 plates using electromagnetic weld pool support

Abstract

Full penetration 15 kW Yb fibre laser butt welding of thick AlMg3 (AW 5754) plates was performed in PA position. A contactless inductive electromagnetic weld pool support system was used to prevent gravity dropout of the melt. The welding speed needed to achieve 20 mm penetration was ～0·5 m min^?1. An ac power supply of ～244 W at 460 Hz was necessary to completely suppress gravity dropout of the melt and eliminate sagging of the weld pool root side surface. The oscillating magnetic field can suppress the Marangoni convection in the lower part of the weld pool. The system was also successfully used in the full penetration welding of 30 mm thick AlMg3 plates.     

Performance of activated TIG process in austenitic stainless steel welds

Five kinds of oxide fluxes, MnO₂, TiO₂, MoO₃, SiO₂, and Al₂O₃, were used to investigate the effect of activated tungsten inert gas (activated TIG) process on weld morphology, angular distortion, delta-ferrite content, and hardness of Type 316L stainless steels. An autogenous TIG welding was applied to 6 mm thick stainless steel plates through a thin layer of flux to produce a bead-on-plate welded joint. The oxide fluxes used were packed in powdered form. The experimental results indicated that the SiO₂ flux facilitated root pass joint penetration, but Al₂O₃ flux led to the deterioration in the weld depth and bead width compared with conventional TIG process. Activated TIG welding can increase the joint penetration and weld depth-to-width ratio, thereby reducing angular distortion of the weldment. On the basis of the present results, it is considered that the centripetal Marangoni convection and constricted arc plasma as a mechanism in increasing the penetration of activated TIG joint.     

Effect of active flux addition on laser welding of austenitic stainless steel

Abstract

The use of active flux in tungsten inert gas (TIG) welding is known to increase its weld depth. The present paper involves study of active flux laser beam welding (ALBW) of austenitic stainless steel sheets with respect to its effect on plasma plume, microstructure and mechanical properties of the resultant weldments. ALBW performed with SiO₂ as the flux significantly modified shape of the fusion zone (FZ) to produce narrower and deeper welds. Plasma plume associated with the process was considerably smaller and of lower intensity than that produced during bead on plate laser beam welding (LBW). Flux addition during LBW produced thin and rough weld bead associated with humping. The development of such a weld bead is cause by reversal in the direction of Marangoni flow by oxygen induced inversion of surface tension gradient, widely fluctuating plasma plume and presence of oxides on the weld pool surface preventing free flow of the melt. Active flux laser weldments exhibited lower ductility than that of bead on plate laser weldments.     

激光辅助活性焊接法

提出了激光辅助活性焊接的方法,研究了焊接参数对激光熔化处理后TIG焊接熔深、熔宽的影响.首先,使用极小功率激光在氧气保护下熔化焊件表面,使焊缝表面的氧含量增加.然后使用普通的TIG焊接,覆盖激光焊缝,达到增加熔深的目的.利用此方法,不使用活性剂就可以使TIG焊焊接熔深增加约2倍,与A-TIG焊相比没有表面熔渣,表面成形较好.小功率激光处理后焊缝中O元素含量增加,熔池表面张力温度系数由负变正,导致激光辅助活性焊熔深增加.

Abstract：

On the bases of the study for the mechanism of the increasing of A-TIG welding penetration, a new method of activating TIG welding, the laser aided activating TIG welding was proposed. At first, the surface of weld was melt by the mini power laser protected by oxygen. As a result, the oxygen content increases in the weld pool surface. Then the conventional TIG welding was used to cover the weld. The oxygen could change the direction of fluid flow in molten pool, which leads to the weld beads with narrower width and deeper penetration. Without the activating flux, the penetration can be increased by 2 times. There is no slag in the surface of weld and the appearance is good. The oxygen can change the temperature dependence of surface tension gradient from a negative value to a positive value; and cause the significant changes in the weld penetration. Fluid flow could be inward along the surface of the weld pool toward the center and then down. This fluid flow pattem transfers heat to the weld root and produces a relatively deep and narrow weld efficiently. This change is the main cause to increase the penetration.