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.     

单电弧TIG焊咬边影响机理的数值模拟研究

避免咬边是提高焊接速度的关键问题.通过对TIG焊温度场进行数值模拟,找到TIG焊在不同焊接速度时熔池成形规律.研究表明:焊接熔池中熔宽最大的位置滞后于熔深最大的位置,影响咬边产生的关键在于动态深宽比.在对模型校核的基础上,从动态深宽比的角度分析了TIG高速焊接时的咬边问题,指出在焊接电流相同的条件下,随着焊接速度的提高,熔深和熔宽都将减小,但熔深减小更多,所以动态深宽比减小,熔池表面张力向内的合力增大,容易产生咬边.即使在保证熔深相同的前提下,随焊接速度的提高,熔池的动态深宽比也会越来越小,导致咬边倾向增加.     

氧元素和硫元素对熔池影响的数值模拟

为了探究氧元素和硫元素分别对熔池的影响,建立了数值模型并计算出含有不同的氧和硫的304不锈钢钨极氩弧焊的瞬态温度场及速度场。模拟结果显示,硫元素和氧元素改变了熔池的表面张力,增加熔深进而影响熔池形貌。元素含量较低时(5×10-5),含有硫的熔池深度大大超过含有氧的熔池深度;在高元素含量时(15×10-5),分别含有硫和氧的熔池熔深没有差别。当氧含量或硫含量超过20×10-5时,熔池的形貌将不再发生变化。     

脉冲激光离散熔凝加工的熔池几何形态研究

研究了Ｎｄ∶ＹＡＧ脉冲激光离散加工时功率密度和作用时间对熔池几何形态的影响。５ＣｒＭｎＭｏ钢的实验结果表明：在一定加工条件下,熔池存在热传导型和深熔型两种形态。激光作用时间小于４ｍｓ时,随着激光功率密度的增加,熔池深宽比显著增加;作用时间超过４ｍｓ后,功率密度对深宽比的影响减弱。基于线性热传导模型的计算结果表明：熔凝起始阶段固态加热过程可忽略;熔池几何尺寸在激光脉冲结束后某个时刻达到最大值;当平均功率密度大于５×１０５Ｗ／ｃｍ２,同时激光作用时间大于２ｍｓ时,热传导型激光熔池向深熔型激光熔池转变     

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。     

峰值功率和脉冲宽度对激光焊接质量的影响

基于有限元法,以峰值功率和脉冲宽度为参数,建立了304不锈钢薄板激光焊接有限元模型,并进行瞬态热分析,获得了各参数对熔池形状和温度的影响规律。基于正交试验设计,分析了各参数对熔池形状和温度的影响程度,并得出最优参数组合。结果表明:随着峰值功率和脉冲宽度的增加,熔宽、熔深、深宽比和熔池温度增加,当峰值功率为280 W,脉冲宽度大于或等于4 ms时,熔池温度超过材料沸点;峰值功率对熔池形状和温度的影响较脉冲宽度大,最优参数组合为峰值功率为240 W,脉冲宽度为5 ms。分析结果可为激光焊接304不锈钢工艺参数的选择提供理论依据和技术指导。     

激光-等离子弧复合焊接熔池流动和传热的数值分析

建立了激光-等离子弧复合焊接过程中液相区、糊状区和固相区的统一模型,在模型中同时考虑了表面张力、热浮力和电磁力的作用,通过数值计算得到了熔池的速度场和温度场,重点研究了电磁力对熔池流动和传热的影响.结果表明,在熔池形成的初始阶段,表面张力对熔池中的对流和传热起主要作用,而电磁力的作用受到抑制,随着熔深增加,电磁力作用增强并引起涡旋,导致熔深和下表面熔宽增加.针对1420铝锂合金的复合焊工艺试验表明计算结果与实际相符合.     

考虑自由表面的TIG焊熔池行为数值模拟

根据TIG焊接过程中的实际情况,利用VOF方法追踪自由表面,建立了包括自由表面的三维瞬态TIG焊熔池模型。采用FLUENT软件,考虑浮力、电弧压力、电磁力以及表面张力,对不同焊接电流下的TIG焊熔池行为进行数值模拟,得到了熔池的温度场、流场和自由表面变化情况。结果表明:所建立的TIG焊熔池模型是正确的,随着焊接电流的增大,熔池最高温度升高,熔深、熔宽以及自由表面变形程度也随之增大。     

激光-等离子弧复合焊接熔池流动和传热的数值分析

建立了激光一等离子弧复合焊接过程中液相区、糊状区和固相区的统一模型，在模型中同时考虑了表面张力、热浮力和电磁力的作用，通过数值计算得到了熔池的速度场和温度场，重点研究了电磁力对熔池流动和传热的影响。结果表明，在熔池形成的初始阶段，表面张力对熔池中的对流和传热起主要作用，而电磁力的作用受到抑制，随着熔深增加，电磁力作用增强并引起涡旋，导致熔深和下表面熔宽增加。针对1420铝锂合金的复合焊工艺试验表明计算结果与实际相符合。     

双丝埋弧焊在造船业的应用和探索

造船业采用高效的双丝埋弧焊法,在焊接线能量一定的条件下,针对性很强地匹配各焊接参数以获得符合规范要求的焊接接头,同时满足CCS有关焊接工艺认可要求,并适用于车间大生产。通过单、双丝焊接接头金相对比观察到熔池液体金属结晶角度方向产生很大变化,认为其原因在于焊接熔池的表面张力与温度为函数关系,熔池表面张力大小取决于熔池表面上的温度分布,而熔池表面张力是影响熔池内液体金属流动的主要驱动力,当大电压、小电流的后丝电弧叠加时的温度场形成新的热循环改变了整个熔池液体金属结晶角度方向,不同于单丝埋弧焊焊接电流熔深(熔深系数)和单丝熔池的观点。     

Theoretical study of Marangoni convection and weld penetration under influence of high oxygen content in base metal

Abstract

A 3D mathematical model is developed to simulate the weld pool development in a moving weld pool of 304 stainless steels with different oxygen concentrations. It is shown that the oxygen can cause significant changes in fluid flow patterns and temperature fields. When oxygen content is <280 ppm, the weld penetration and depth/width ratios increase sharply with increasing oxygen content; positive and negative temperature coefficients of surface tension coexist and cause shallow and wide weld pool. When oxygen content is in the range of 280–500 ppm, positive temperature coefficient of surface tension dominates the flow patterns and causes deep and narrow weld pool. The weld penetration and depth/width ratios remain nearly constant. When oxygen content is >500 ppm, the weld penetration and depth/width ratios decrease again. Vortexes that have different positions, strength and directions may be found in the pool. The vortexes with opposite directions caused by positive temperature coefficient of surface tension can efficiently transfer the thermal energy from the arc, creating a deep weld pool.     

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.     

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.     

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.     

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.     

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.     

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.     

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.