Modeling the keyhole shape and dimension in plasma arc welding

It is of great significance to model the keyhole shape and dimensions to optimize the plasma arc welding process parameters. In this study, through employing a combined volumetric heat source mode, the weld pool in keyhole plasma arc welding is determined firstly, and then the dynamic force-balance condition on the interface between the plasma jet and the molten metal is dealt with in describing the keyhole formation inside the weld pool. The effects of welding current on the shape and size of keyhole are numerically analyzed. The sharp transformation from a partial keyhole to a full-penetration keyhole is quantitatively demonstrated.     

Measurement method of the arc welding current and voltage

Plasma arc welding can realize high-speed welding, deeper weld penetration and, furthermore, smaller thermal distortion. For these reasons, plasma arc welding is employed for keyhole welding. However, it has been pointed out that it is difficult to obtain the stability of the welding. In this study, we have developed a unified plasma arc welding model for analysing the welding mechanism and influence of the torch design and operation conditions of the arc on the welding process. In this paper, the keyhole welding of a thick aluminium plate employing the plasma arc was numerically analyzed for clarifying the mechanism determining the keyhole size which is important for improving the stability of the process. As a result, it was found that the keyhole size is determined mainly by a force balance between the surface tension of a weld pool and metal vapour pressure.     

小孔等离子弧焊接热场瞬时演变过程的数值分析

根据小孔等离子弧焊接的工艺特点,考虑等离子弧对熔池的“挖掘”作用,提出了一种新的焊接热源模型（TPAW）,以描述和反映等离子弧作用下“倒喇叭”状焊缝横断面的特点以及等离子弧热流沿工件厚度方向的分布．在此基础上,对小孔等离子弧焊接热场进行了有限元计算,分析了引弧后熔池及周围温度场的瞬时演变过程．计算出的小孔等离子弧焊缝形状与实验结果基本吻合,达到准稳态的时间与实验测试值一致．     

铝合金变极性等离子弧穿孔横焊焊缝成形规律分析

以穿孔等离子弧焊接过程中形成的穿孔熔池为研究对象,根据熔池热源形态的特点,采用数值模拟与试验相结合的手段研究横焊位置下的铝合金变极性等离子弧焊缝成形.由于焊接速度波动和工件厚度的影响,体热源作用下的穿孔熔池背面存在最高温度点和最大熔宽截面相背离的现象;因此通过对穿孔熔池背面进行分区和定义,提出温宽偏离度概念,即熔池背面最高温度点和最大熔宽截面的偏离程度,用以描述穿孔熔池状态及焊缝成形;通过调节焊枪角度来改变焊接过程中的温宽偏离度,在其它参数不变的情况下减轻重力在焊接过程中对焊缝成形的影响,实现变极性等离子弧穿孔焊接在横焊位置上的良好成形.     

NTWV-based sensing keyhole dimension in plasma arc welding

During stable keyhole plasma arc welding, the pilot arc and the transferred arc exist at the meantime, and the arcs can be considered as a composition of two parts inside and outside the nozzle, respectively. Under the mechanical constriction and thermal contraction effects, the inside arc has certain arc length, electron density and arc profile etc. inducing constant tungsten-to-nozzle voltage. However, the arc outside the nozzle diverges at about 5 degrees and has certain characteristics similar to the free arcs. The nozzle-to-workpiece voltage （NTWV） depends mainly on the length of the arc, which gets bigger as increasing of the weld penetration and keyhole size. The NTWV sensor is developed for monitoring NTWV in real time. The welding experiments are designed to get different penetrations and keyhole sizes. It is found that as the weld penetration and the keyhole size increase, NTWV also increases linearly. The NTWV signals can be used as the feedback variable in automatic control of keyhole plasma arc welding.     

铝合金变极性等离子弧穿孔焊过程控制

分析了铝合金变极性等离子弧穿孔立焊工艺特点,提出了通过对焊接参数的精确控制,实现变断面铝合金变极性等离子弧穿孔立焊工艺的方法,并将焊接电流、离子气流量和焊接速度确定为变断面铝合金变极性等离子弧穿孔立焊过程的被调节参数.保持穿孔熔池上"热"和"力"的动态平衡是调节焊接参数的根本依据,是实现变断面试件自动焊接的关键所在.采用单片机为核心的控制器对焊接参数进行实时调节,动态保持穿孔熔池上热和力平衡,实现了变断面铝合金变极性等离子弧穿孔立焊工艺.     

Plasma welding of thin plate

The plasma arc has a large arc force (regarded as the plasma force) and keyhole welding is generally performed by the formation of a small hole at the weld pool of a butt joint by making use of this plasma force. Under these circumstances, due to the small footprint area of the plasma heat source and welding by the formation of a keyhole, there is little melting at the joint zone and welding with a narrow bead width becomes feasible. Recently, a new type of plasma welding process has been proposed such that the welding tungsten electrode tip is brought close to the tip of the cooling chip and keyholeless welding is performed, similarly to TIG welding and this process has been made practicable.¹ This process solves the disadvantages of keyhole welding while retaining the advantages of plasma welding and is capable of meeting the requirements of high speed welding. However, it is difficult to apply either method to fillet joints where keyhole formation is difficult due to the problems of the large plasma force and the torch structure. In either case, the large plasma force due to the plasma passing through the cooling chip small hole is surmised to be intimately associated with the weld morphology due to plasma welding.     

小孔型等离子弧焊接条形气孔形成机理

等离子弧焊接中厚钢板时,常存在焊接工艺窗口窄问题. 在中厚不锈钢板小孔型等离子弧焊(keyhole plasma arc welding,K-PAW )中发现,离子气体流量较弱而使匙孔未打开,形成盲孔时,会在焊缝内产生平行于焊接方向的长条形气孔,形成条形气孔时,等离子弧形态发生明显变化,电弧面积增加且波动更为剧烈,出现向后的反射. 使用高速摄影装置研究了条形气孔的形成过程.结果表明,中厚板等离子弧焊中,条形气孔是在一定的焊接速度和较低的电弧能量下,熔池前壁倾角较大,等离子弧受熔池前壁反射作用冲击熔池后壁,使熔池后壁发生弯曲,并在一定的凝固条件下保留,产生平行长气孔. 对条形气孔的研究有助于为匙孔等离子弧焊接中匙孔形成条件及机制提供新认识.     

穿孔法变极性等离子立焊的收弧过程

研究了穿孔法等离子弧焊的收弧规律。发现焊接电流、离子气流量和焊接速度对收弧过程影响较大。在收弧阶段采用适当焊接电流，增大送丝速度，减小离子气流量和焊接速度，以增加焊接热输入，可以实现收弧弧坑的填充。对于不同厚度的铝合金，收弧阶段的规范参数有一最佳区间。研究了收弧坑的接头性能，其抗拉强度和伸长率均不低于中间焊缝。     

Soft variable polarity plasma arc horizontal welding technology and weld asymmetry

Abstract

Soft variable polarity plasma arc welding technology has been proposed to solve the problems in horizontal position welding for aluminium alloys. Arc thermal field shape, arc pressure distribution and the characteristic of the weld pool, which are important for keyhole stability, have been changed compared with the ordinary variable polarity plasma arc. Its stability in application can be evaluated by the operational window. The thermal cycle curves, microstructures and microhardness of the horizontal position weld indicate the weld asymmetry. The influence of asymmetry as well as welding defects on tensile strength and fracture behaviour has been studied. The fundamental reason for the asymmetry was considered as the asymmetric fluid flow around the keyhole in the weld pool.     

铝合金交流脉冲双面弧焊工艺试验及特点分析

通过工艺试验,详细分析了LYl2CZ铝合金板交流脉冲等离子弧（PA）-钨极氩弧（GTA）双面弧焊（DSAW）的工艺特点。通过与常规钨极氩弧焊和等离子弧焊工艺比较,发现该工艺可以显著增加熔深,减小焊后热变形,提高焊接生产效率。分别采用多孔喷嘴和单孔喷嘴进行了对比试验,其中采用多孔喷嘴能够有效防止双弧的产生,采用单孔喷嘴能够获得较大熔深。当采用小孔型交流脉冲双面弧焊工艺焊接铝合金时,由于小孔的存在,焊接过程中阴极雾化效果降低,影响焊缝成形及质量。     

等离子弧焊小孔行为的反翘电压检测

在穿孔等离子弧焊接过程中,小孔稳定性对焊接过程的稳定和优质的焊缝成形是非常关键的。文中采用无源探针对由等离子弧梯度电压和等离子鞘层电压构成的等离子反翘(等离子云)电压进行检测。根据等离子鞘层理论,随等离子反翘长度和角度变化的反翘电压可以反映小孔的状态信息及其形成、坍塌过程。文中讨论了检测信号的特征,以及材料、探针位置、焊接电流,等离子气流等因素对检测信号的影响。试验证明,该方法可以有效的预测小孔信息和焊缝熔透情况。     

Sensing controlled pulse key-holing condition in plasma arc welding

According to the strategy of controlled pulse key-holing, a new sensing and control system was developed for monitoring and controlling the keyhole condition during plasma arc welding (PAW). Through sensing and processing the efflux plasma voltage signals, the quantitative relationship among the welding current, efflux plasma voltage and backside weld width of the weld was established. PAW experiments show that the efflux plasma voltage can reflect the state of keyhole and backside weld width accurately. The closed-loop control tests validate the stability and reliability of the developed keyhole PAW system.     

Numerical analysis of keyhole establishment time in keyhole plasma arc welding

Numerical analysis of keyhole shape and keyhole establishment time is of great significance for selection and optimization of the process parameters in keyhole plasma arc welding. In this paper, a three-dimensional transient model is developed to analyze the evolutions of keyhole shape and keyhole establishment time in continuous current plasma arc welding process. Firstly, a combined volumetric heat source model is used to simulate the transient variation of temperature field. And then the surfaced formation equation is adopted to calculate dynamic features of the keyhole shape and keyhole establishment time inside weld pool, in which the force action on weld pool surface is considered. Experiment is conducted to validate the numerical simulation results. The predicted keyhole size and keyhole establishment time are in agreement with the experimental measurement. And the calculated fusion zone geometry is consistent with the measured one.     

Detecting and tracking of welding line in visual plasma robotic welding

The formation of stable back beads in joining of the thick materials is important in order to achieve high-quality welded metal joints. Plasma welding uses the high welding current density, which is suitable for thick materials. The keyhole in the plasma welding depends on the pilot gas and the welding current. The voltage behaviour depends on the keyhole situation. If the torch is moved away from the welding line in conventional GMA welding, the welding voltage and the welding current are changed due to variations of the arc length. But the welding voltage does not change with the arc length in plasma welding, because the welding voltage depends on the situation of the keyhole. The authors tried to observe the weld pool on the top side by using a CCD camera. The timing of the shutter in the CCD camera is investigated to take clear images of the weld pool. As a result, the clear images of the weld pool were taken when the welding current was reduced to 30 A and an interference filter of 950 nm was attached to the CCD camera. The weld pool shape was changed with the torch position in the groove. The image-processing method was developed to detect the top of the weld pool. The torch position was estimated by processing the weld pool images. The digital control was designed to trace the welding line. The performance of the controller was verified by carrying out tracking experiments.     

Determining the keyhole geometry based on double-elliptic distribution of plasma arc pressure

The plasma arc pressure plays an important role in determining the keyhole formation and size. So it is of great significance to choose adaptive distribution mode of the plasma arc pressure for determining keyhole shape and size. In this study, through employing a double-elliptic distribution mode of plasma arc pressure, three-dimensional keyhole was numerically simulated. The unsymmetric feature of the keyhole inside the weld pool was described. The development of keyhole was demonstrated under different levels of welding current. The critical current required to form an open keyhole was obtained for the study cases.     

焊丝熔化方式对激光焊接过程的影响

研究了两种焊丝熔化方法(电弧预熔丝激光焊、激光填丝焊)激光焊接过程对匙孔稳定性以及焊缝成形的影响,进一步研究了焊丝熔化方法对焊接接头质量的影响,并对比分析了两种焊丝熔化方式对焊接速度的适应性. 结果表明,电弧预熔丝激光焊过程中,熔池表面匙孔开口尺寸变化不大,匙孔较为稳定;激光填丝焊方法由于熔化的液态金属距离匙孔边缘很近,焊接过程中熔池表面匙孔开口尺寸变化较大,而且容易出现熔池表面匙孔的闭合. 与激光填丝焊相比,电弧预熔丝激光焊熔化的焊丝端部可以沿熔池边缘流入,与匙孔边缘的距离较远,匙孔稳定性较好,焊缝气孔数量较少. 当焊接速度为8 m/min时,电弧预熔丝激光焊的焊缝成形良好;而激光填丝焊焊缝背面成形不连续,并且出现了未焊透的缺陷.     

等离子电弧力的研究

从等离子电弧力的角度 ,探讨了等离子弧焊接工艺参数及其匹配的内在联系与规律性。在不同工艺条件下 (焊接电流、等离子气流量、电极内缩量以及喷嘴高度 )对等离子电弧轴向力进行了测量。并通过显著性分析 ,定量地认识和描述了各工艺参数对电弧轴向力的作用和影响。进而指出了获得稳定的小孔型等离子弧焊接过程的必要条件。本文的工作对于等离子弧焊接工艺参数的匹配及其过程的优化有重要的指导意义。     

穿孔等离子弧焊接热场和流场的数值模拟

考虑熔池和小孔的耦合作用,建立了穿孔等离子弧焊接热过程的三维瞬态模型.采用焓孔隙度法处理了凝固熔化过程中相变潜热以及动量损耗问题.基于流体体积函数法（VOF）对小孔界面实施追踪.对等离子弧焊从小孔形成到穿孔的瞬态演变行为、熔池流场的动态变化过程进行了数值模拟.开展了穿孔等离子弧焊接试验,对数值模拟结果进行了试验验证.结...     

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.