A-TIG焊中氧含量对熔池流动方式影响的数值模拟

建立了三维移动热源作用下焊接熔池的数学模型,模拟了不同氧含量下熔池中的速度场和温度场.结果表明,氧化物活性剂中的氧元素改变了熔池中的表面张力温度系数,从而影响熔池中的流动方式,是熔深和深宽比增加的重要原因;随着氧含量的增加,深宽比和熔深急剧增加,熔宽减小.当氧含量超过150×10-6时,增加氧量,熔池表面最高温度减小,并逐渐趋于一定值;当氧含量超过200×10-6时,深宽比、熔深和熔宽趋于一定值;氧含量小于300×10-6时,熔池表面最高温度高于正表面张力温度系数作用的温度范围,正、负表面张力温度系数在熔池中同时存在.当氧含量超过300×10-6时,熔池表面最高温度处于正表面张力温度系数作用的温度范围之内,熔池中的表面张力温度系数为正值;随着氧含量的增加,在熔池中出现数目、大小、方向、位置不同的涡流,当两个涡流的方向均由熔池边缘流向熔池中心的环流时,涡流有效地把电弧能带到熔池底部,产生较大的熔深和深宽比.     

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.     

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.     

The study of minor elements and shielding gas on penetration in TIG welding of type 304 stainless steel

The effects of minor elements and shielding gas on the penetration of TIG welding in type 304 stainless steel have been studied. The bead-on-plate test was performed, then the depth and width of the weld were measured using an optical projection machine. The arc voltage was measured with an arc data monitor. In addition, the metallurgical characteristics of weld were examined using OM and SEM. The results show that oxygen and sulfur are beneficial in increasing a depth/width ratio because of the increased surface tension/temperature gradient. Elements, such as aluminum, that have a deleterious effect on the depth/width ratio will combine with oxygen and reduce the soluble oxygen content in the weld pool. On the other hand, silicon and phosphorus have a minor effect on the depth/width ratio. Shielding gas using Ar + 1% O₂ or Ar + 5% H₂ can significantly promote the depth/width ratio. The former contains increased soluble oxygen content in the weld pool, and the latter produces an arc that is hotter than that produced by pure argon.     

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.     

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.     

低碳钢激光预熔活性焊接法

以8 mm厚的低碳钢为研究对象,在氧气的保护下,用小功率光纤激光在待焊焊件表面进行预熔处理,使表面熔化生成一层氧化层,然后用TIG焊或激光电弧复合焊覆盖氧化层,研究工艺参数对焊缝成形的影响.结果表明,激光预熔后进行覆盖焊接,电弧没有收缩,熔深增加到1.5倍左右,表面成形良好.随着电流的增加,熔深增加,但激光预熔后的焊道增加更快.随着焊接速度的增加,熔深减小.随着激光预熔功率的增加,熔深增加.随着复合焊接中激光功率的增加,熔深增加.焊缝含氧量的增加,使得表面张力温度系数由负变正,是低碳钢激光预熔活性焊接熔深增加的主要原因.     

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).     

Study of the law between the weld pool shape variations with the welding parameters under two TIG processes

A double-shielded TIG method was proposed to improve weld penetration and has been compared with the traditional TIG welding method under different welding parameters (i.e., speed, arc length and current). The strength of the Marangoni convection was calculated to estimate the influence of the welding parameters on the variations in weld pool shapes. The results show that the changes in the welding parameters directly impact the oxygen concentration in the weld pool and the temperature distribution on the pool surface. The oxygen content and heat distribution on the weld pool surface are determinants of the pattern and strength of the Marangoni convection. For a negative temperature coefficient of surface tension (∂σ/∂T < 0), an outward Marangoni convection leads to a wide and shallow weld pool shape. The narrow and deep weld pool shape occurs when the Marangoni convection flows along an inward direction (∂σ/∂T > 0). The oxide layer that may appear with the relatively high oxygen content in the weld pool is harmful for the heat flow along the pool surface so as to reduce the welding efficiency especially in the double shielded TIG process.     

Effect of Active Gas on Weld Shape and Microstructure of Advanced A-TIG-Welded Stainless Steel

Advanced A-TIG method was conducted to increase the weld penetration and compared with the conventional TIG welding process.A two-pipeline setup was designed to apply Ar + CO_2 mixed gas as the outer layer,while pure argon was applied as the inner layer to prevent any consumption of the tungsten electrode.The results indicate that the presence of active gas in the molten pool led to the change in the temperature coefficient of surface tension so that the Marangoni convection turns inward and forms a deep weld zone.The increase in gas flow rate causes a decrease in the weld efficiency which is attributed to the increase in oxygen content in the weld pool and the formation o f a thicker oxide layer on the weld surface.Moreover,the stir and the temperature fluctuation,led by double shielding gas,create more homogeneous nucleation sites in the molten pool so that a fine grain micros true ture was obtained.     

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.     

活性元素氧对AA-TIG焊熔池传输行为影响的数值模拟

电弧辅助活性TIG焊(arc assisted activating TIG welding,AA-TIG焊),采用辅助电弧以Ar+O₂作为保护气体预熔待焊母材表面以形成氧化层,再进行常规TIG焊,可使熔深明显增加.文中结合AA-TIG焊熔池氧元素分布的实验研究,提出焊接熔池表面氧元素的两种不均匀分布模式,考虑浮力、电磁力和表面张力,建立了更完善的电弧辅助活性TIG焊熔池模型,模拟研究氧元素在熔池表面呈不均匀分布时,AA-TIG焊瞬态熔池中动量及能量的传输行为.假设熔池内部液态金属是湍流、不可压缩Newton流体,使用FLUENT RNG k-ε湍流模型进行处理.结果表明,当氧在熔池上表面呈非均匀分布,并且氧的不均匀分布模型为低氧模型时,熔池内部仍然以内对流流动为主.     

表面张力对激光深熔焊熔池流动的影响

为研究激光深熔焊中表面张力对熔池流动行为及熔池形貌的影响,获得活性剂对Q235低碳钢和304L不锈钢焊缝截面形貌的影响规律. 基于有限元计算软件Fluent,计算分析了激光深熔焊时表面张力在改变熔池流动行为中的重要作用. 结果表明,未添加活性剂焊接时,熔池液态金属的表面张力温度系数为负,熔池表面熔融金属以匙孔为中心由内向外流动,焊缝横截面形貌宽且浅. 添加活性剂焊接后,表面张力温度系数为正值,在熔池表面形成了由外向内的反向流动,形成了窄且深的焊缝形貌. 随表面张力温度系数绝对值的增大,表面张力变大,流体流动速度增大.     

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.     

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.     

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.     

钛合金激光表面氮化层成形特征及分析

利用半导体激光器在氮气气氛中对钛合金表面进行氮化,为了研究线能量,激光功率和扫描速度对Ti-6Al-4V合金氮化层表面成形,熔池几何形貌,熔宽,熔深及成形系数的影响,采用光镜,扫描电镜拍摄氮化层表面成形及其横截面形貌,用显微硬度计测量氮化层硬度值。结果表明氮化层表面形貌由表面光滑和表面粗糙型两种区域组成。熔池表面温度较低时,增加激光束与基材的相互作用时间,对表面成形影响不大。相同线能量下随激光功率增大,氮化层熔深熔宽均呈增大趋势,成形系数减小;当激光功率相同时,随扫描速度减小,熔深熔宽均呈增大趋势,成形系数减小;成形系数很小时氮化层会产生沿纵向分布的裂纹。对流形式及强弱导致形成不同的界面形态,如：半球形界面,指状界面和W形界面等,而且熔合界面会发生多次弯曲。     

YAG laser welding with surface activating flux

YAG laser welding with surface activating flux has been investigated, and the influencing factors and mechanism are discussed. The results show that both surface activating flux and surface active element S have fantastic effects on the YAG laser weld shape, that is to obviously increase the weld penetration and D/W ratio in various welding conditions. The mechanism is thought to be the change of weld pool surface tension temperature coeffwient, thus, the change of fluid flow pattern in weld pool due to the flux.     

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.     

不锈钢PA-GTA双面弧焊工艺特点分析

详细分析了不锈钢等离子弧(PA)-钨极氩弧(GTA)双面弧焊(DSAW)的工艺特点,该工艺可以增加熔深,减小焊后热变形,尤其适用于中厚板的焊接.当小孔效应建立后,PA-GTA双面弧焊过程中的电弧均得到了不同程度的压缩,两焊枪之间的电弧电压出现下降趋势,节省了能源.形成小孔后,电弧穿过工件从工件内部进行加热,提高了热效率.表面张力、电弧吹力和电磁搅拌力均有利于获得较大的熔深,浮力有利于增加焊缝中间部位的宽度.