外加直流磁场下微束等离子电弧行为与焊缝成形研究

为实现薄壁件金属在增材制造中成形及成性的精确控制,基于带电粒子在直流磁场作用下的运动规律,研究了外加直流磁场作用下电弧行为,得到电弧偏转程度、偏转方向与外加磁场之间的关系.在电弧偏转的基础上分析了焊缝形貌、焊缝晶粒变化原因.结果表明:外加直流磁场作用下电弧发生偏转,在试验参数范围内随磁场强度增加,电弧偏转程度增大,且电弧偏转方向与外加磁场方向相关;外加正向直流磁场时熔池向焊接方向后方偏移,焊缝余高相对增加,焊缝晶粒较无外加磁场细化;外加反向直流磁场时熔池位于电弧下方,焊缝余高相对降低,焊缝晶粒较外加正向磁场更加细化.外加正向直流磁场控制焊缝形貌,具有"控形"效果;外加反向直流磁场有明显的细化晶粒作用,可以达到"控性"目的.     

磁场辅助电弧焊接旋转等离子体行为的数值模拟

目的 研究外加纵向磁场对倾斜电极TIG焊接的电弧温度分布、流动模式和工件所受热力作用的影响.方法 建立磁场-电弧复合焊接热、电、磁、流动的三维数学模型.通过数值模拟和高速摄像实验,揭示倾斜电极电弧在外加磁场作用下的流动、形貌及温度演化机制.结果 外加纵向磁场后,电弧流动速度明显增加,流动模式由沿电极方向喷射变为近似沿竖直方向旋转向下的流动模式;电弧对工件的热作用均匀性提高,热作用中心向电极正下方靠近,但在焊接横向方向上存在偏离;工件受到表面的电弧旋转拖拽力和内部的旋转洛伦兹力作用,最大洛伦兹力可达50000 N/m3.结论 基于所建立数学模型的模拟结果与实验电弧形貌吻合良好,结果表明,外加纵向磁场能够显著改变电弧的形态及流动模式,提高电弧热流密度的均匀性,并能够对熔池产生有效的搅拌作用.     

嵌合型杯状横-纵磁触头结构磁场分析

以高压直流真空断路器(HVDC-VCB)灭弧系统触头结构为研究对象,基于杯状横磁(TMF)与杯状纵磁(AMF)触头结构与磁场特性分析,建立了嵌合型杯状横-纵磁触头模型,并对其稳态磁场与剩余磁场进行有限元仿真模拟。分析表明,在稳态磁场下触头中部区域的横向磁场增强,中心区域纵向磁场提升;而当电流过零时,剩余磁场值远小于传统杯状触头,有利于金属蒸气的扩散以及电弧能量的有效逸散。     

外加纵向交变磁场下的TIG焊电弧数值模拟

在局部热平衡状态假设前提下，以磁流体动力学为基础，建立二维轴对称TIG焊电弧模型，研究外加纵向交变磁场下电弧所产生的周期性变化。当磁场强度为0.03 T,交变频率为1 000 Hz时，与无外加磁场的电弧相比，电弧发生明显的收缩现象。根据进一步的模拟研究，发现当磁场方向改变，电弧会随之发生收缩或扩散现象，但相较于无外加磁场的电弧，总体上还是呈现收缩现象。靠近阳极区域的电弧温度及阳极所受压力会随磁场方向的变化而发生周期性的单/双峰转变。     

纵向磁场对直流真空电弧等离子体发展进程的影响研究

从微观角度出发,数值模拟直流真空电弧燃弧初期电弧的发展进程,研究纵向磁场对电弧等离子体参数输运特性的影响。建立电弧等离子体流体-化学混合模型,仿真研究纵向磁场作用下的电弧运动特性。数值模拟电弧等离子体中粒子输运特性,以得到电弧等离子体参数随磁场变化规律。仿真结果表明:纵向磁场分布对直流真空电弧放电初期粒子的数密度与温度等参数的作用,直接影响电弧的发生与发展。研究并分析了直流真空电弧燃弧初期的过程。     

等离子喷枪内电弧动力学与等离子体特性的三维数值模拟

基于局域热力学平衡假设,建立三维非稳态等离子电弧的湍流模型,应用计算流体力学软件ANSYS CFX模拟了氩/氢等离子喷枪内阳极弧根的再附着过程、电弧与气流的相互作用以及等离子体温度和速度分布。结果表明,在冷气流推动下,阳极弧根向喷嘴下游移动,电弧被拉长,弧电压升高,电场强度增大。当电弧向下游移动到电场强度足够大时,电弧击穿并形成新的弧根。喷枪内电弧波动导致喷枪出口等离子体最大温度和最大速度发生波动,滞后时间为10μs。喷枪内的流动与传热具有明显的三维特征,与温度分布相比,速度分布的三维特征更明显且更向中心处集中。     

电磁场作用下的粉末成形固结技术研究进展

外加电场或磁场作用下粉末成形固结技术是快速制备高性能材料的新途径。应用前景广阔。重点综述了外加电场作用下放电等离子烧结技术和外加磁场作用下动磁压制技术的原理、特点及应用情况。并展望了今后粉末成形固结技术的研究方向。     

变极性等离子弧穿孔熔池受力及焊缝成形稳定性

通过YB005-01型压力变送器测定相同参数条件下正极性等离子电弧力大于反极性等离子电弧力,并建立了铝合金VPPA焊接穿孔熔池受力模型,分析了在不对称正、反极性等离子电弧力的作用下,穿孔溶池稳定性及其焊缝成形机理.同时进一步分析铝合金VPPA力学特性,掌握了焊接电流和离子气流量等重要焊接参数对其影响.经穿孔焊工艺实验,...     

一种适用于大面积加热的磁摆动电弧

研究了横向交变磁场作用下的磁摆动电弧,在LTE的前提下用谱线绝对强度法测定了磁摆动电弧的时均温度场,实验研究了磁摆动电弧的动态特性.结果表明,磁摆动电弧具有明显的均温效果,对表面清理及大规模表面处理工艺具有明显的优越性.     

超音速电弧加热等离子发动机的数值研究

为了研究电弧加热等离子发动机的内外流场特性,采用基于磁矢量势的磁流体力学控制方程,在计算流体力学软件Fluent中添加用户自定义函数(UDF)实现添加特殊源项和自定义标量,对自主设计的超音速电弧加热等离子发动机流动特性进行数值研究。通过数值模拟,获得不同工作电流下发动机内外的温度场和速度场,以及中心轴线上电压和电流密度分布。算例验证表明在Fluent中添加UDF程序能够较为准确的模拟等离子体射流。计算结果表明:工作电流对等离子体的温度场和速度场有重要的影响,增加工作电流大小能够明显增加流场的温度和速度,但工作电流对等离子体射流的马赫数影响较小,细喷嘴电弧加热等离子发动机得到上升的伏安特性。为电弧加热等离子发动机的应用提供了理论基础。     

厚板钛合金磁控窄间隙TIG焊接技术发展现状

磁控窄间隙TIG焊接技术能有效解决厚板钛合金侧壁熔合不良的问题,其工艺稳定性好、设备成本较低,能为大型钛合金海工结构和装备制造提供有力支撑。围绕厚板钛合金磁控窄间隙TIG焊接技术及其研究现状展开论述。重点阐述了外加横向磁场对电弧摆动的作用机理,并讨论了主要焊接工艺参数(窄间隙坡口设计、焊接电流强度、外加磁场强度和频率及电极位置)对焊缝成形的影响。结合自研结果探讨了不同规格钛合金磁控窄间隙TIG焊接接头的显微组织和力学性能,并进一步概述了焊后残余应力分布特征。研究结果表明,利用外加磁场实现电弧周期摆动能有效调控热输入分布,进而改善侧壁熔合不良的问题;相比其他窄间隙焊接技术,磁控窄间隙TIG焊接能获得更加均匀的焊缝组织和优异的接头力学性能;磁控窄间隙TIG焊接后,接头表面存在较大拉应力,结合合适的热处理工艺能有效降低接头残余应力。最后,对磁控窄间隙TIG焊技术发展和前景进行了展望。     

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.     

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.     

Numerical Analysis of Energy Effect in Laser-TIG Hybrid Welding

The hybrid source that combined CO2 laser with TIG arc to proceed welding was analyzed. Based on an energymodel, the temperature field and weld shape were calculated numerically. The heat transfer characteristic of thehybrid heat source to workpiece and i     

Robust sensing of arc length

During arc welding, the arc heats and melts the workpiece as heat flux. When the welding current is given, the distribution and the intensity of the heat flux are determined by the length of the are. The measurement and control of the are length are fundamental in robotic and automated welding operations. Length of welding arc determines the distribution of the arc energy and thus the heat input and width of the weld. This work aims at improving the measurement accuracy of arc length using the spectrum of are light at a particular wavelength during gas tungsten arc welding (GTAW) with argon shield. To this end, effects of welding parameters on spectral distributions were studied. To verify the effects of base metal and arc length, the arc column was also sampled horizontally as Layers for spectral analysis. Results show that spectral lines of argon atoms are determined by are length, independent of welding parameters other than the current. Based on these findings, a compact arc light sensor has been designed to measure the arc length with adequate accuracy. A closed-loop arc length control system has been developed with the proposed sensor     

TC4合金等离子–MIG复合焊过程温度场的数值模拟

目的 研究TC4合金在等离子-MIG复合焊(Plasma–MIG Hybrid Welding)过程中的温度场特性,探究不同电弧功率对熔池形貌的影响。方法 进行了2组4 mm TC4合金板堆焊试验,根据实验结果提出了一种改进的复合热源模型并进行了相应的仿真分析。结果 仿真与实验获得的焊缝截面相吻合;等离子电流的增大使熔池尺寸增大且余高减小,等离子电弧功率的变化对熔池宽度的影响相对较小。结论 等离子–MIG复合电弧对工件的热作用非常集中,更易实现深熔焊、焊接效率更高;所提出的热源模型适用于TC4合金等离子–MIG复合焊温度场模拟。     

Failure Analysis of HAZ Cracking in Low C–CrMoV Steel Weldment

This case study describes the failure analysis of steel nozzle in which cracking was observed after a circumferential welding process. The nozzle assembly was made from low C–CrMoV alloy steel that was subsequently single pass butt welded using gas tungsten arc welding. No cracks were found in visual inspection of the welds; however, X-ray radiography showed small discontinuous cracks on the surface in the area adjacent to weld bead, i.e. heat affected zone. The welding of nozzle parts made of same material was a routine process and this type of cracking did not occur in the past. Therefore, it became essential to determine the root cause of the failure. A detailed investigation including visual examination, non-destructive testing, optical microscopy, microhardness measurements and residual stress measurements were carried out to find out the primary cause of failure and to identify actions required to avoid its reoccurrence in future. Results of the investigation revealed that the principal cause of failure was the presence of coarse untempered martensite in the heat affected zone due to localized heating. The localized heating was caused by high welding heat input or low welding speed and resulted in the high transformation stresses. These transformation stresses combined with the thermal stresses and the constraint conditions to cause intergranular brittle fracture.     

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.