Characteristics of arc in variable polarity plasma arc welding of aluminum alloy

A double invert variable polarity plasma arc （VPPA） power source based on 16-bit MCU applied for aluminum alloys was developed. Mechanics, electrical and the produced heat mechanism of VPPAW arc were tested and analyzed. Results indicate that during the VPPA welding procedure of aluminum alloy, the arc of electrode negative （EN） has more effect on force, whereas the arc of electrode positive （ EP ） has more effect on heat. It should be noted that keeping the balance of the force and heat is the critical element of VPPAW. This power source had been successfully used to weld aluminum alloy with a 15 mm thickness in vertical welding. The conclusions are applicable to the variable polarity plasma arc welding technique used in the aerospace industry.

Formative mechanism of the undercut defect in horizontal variable polarity plasma arc welding

Abstract Horizontal welding is important for heavy or huge welding structures. Keyhole mode variable polarity plasma arc welding of aluminum alloy plates with medium thickness was carried out in horizontal position. The characteristic of welding defects was introduced. Preliminary experiments indicated that the undercut defect could not be eliminated easily. The relationship between welding parameters and the undercut defect showed that this deject could be lessened by using higher heat input. The fluid flow of weld pool was observed by a high speed camera. The fluid flow in weld pool was not symmetric and much of molten metal gathered in the lower part. The fluid flow velocity in the lower part was bigger than that in the upper part. To this end, the formative mechanism of the undercut defect was proposed. The flowability of the molten metal was an influential factor for the undercut defect. A preheating method was designed to verify the formative mechanism.

Mechanical properties of welded joints in vertical-up variable polarity plasma arc welding of 2219 aluminum alloy

The mechanical properties of the 2219 aluminum alloy welded joints by vertical-up variant polarity plasma arc welding (VPPAW) with keyhole were analyzed. The tensile strength of welded joints can reach 55% of that of base metal. The microstructure of weld joints was studied to explain the mechanical properties of the weld. The results show that the intensity of weld center is less than that of HAZ, and the intensity of weld center is the worst. So an advanced welding procedures is presented to reduce the heat input and increase the welding speed. The results in this procedures show that the tensile strength of welded joints reaches more than 60% of that of the parent metal, and the specific elongation does not descend.

Cathodic cleaning in variable polarity plasma arc welding of aluminum alloys

Variable polarity plasma arc welding (VPPAW) is one of the most excellent processes used for welding aluminum alloys recently. It combines the advantages of variable polarity welding and plasma arc welding, and can achieve the most rational heat distribution and cathodic cleaning. With the VPPAW equipment developed by authors, the cathodic cleaning regularity that is one of the most important problems in VPPAW is investigated in this paper. The results will be helpful in realizing the real cleaning mechanism.

Experimental Analysis on the Variable Polarity Plasma Arc Pressure

Arc pressure is one of the key factors for variable polarity plasma arc（VPPA） and welding pool formation. In this paper, VPPA pressure is measured by pressure transducer and U-tube barometer methods, and advantages and disadvantages of the two methods are compared. The effects of welding parameters, including with straight polarity（SP） current, reverse polarity（RP） current, time ratio of SP to RP, plasma gas flow rate, on VPPA pressure are investigated by using an orthogonal design. The experimental results indicate that the influencing degree of the welding parameters are in the order of plasma gas flow rate, SP current, time ratio of SP to RP, RP current. These results are important to researches of VPPA welding process and its mechanism. The physics behavior of VPPA is taken into account when the above influence mechanisms are analyzed. Firstly, according to the mechanism of the cooling compression to the arc, the compression to VPPA is enhanced with the increase of plasma gas flow, so the VPPA pressure would increase obviously. Secondly, although the temperature of VPPA is as a function of the welding current, the radius of VPPA is also enhanced. So the effects of SP current on VPPA pressure are inferior to the effects of plasma gas flow. Thirdly, VPPA pressure increases as a function of time ratio of SP to RP because the frequency of welding current influences the arc pressure to the some degree; Finally, the RP intervals are farther less than the SP intervals, so the influence to the pressure is minimal.

Effect of variable polarity frequency on 2A14-T6 aluminum alloy welds using HPVP-GTAW process

Hybrid ultrahigh frequency pulse variable polarity gas tungsten arc welding （HPVP-GTAW） for 2A14-T6 high strength aluminum alloy was carried out and the effects of variable polarity frequency with constant pulse current frequency 40 kHz on weld bead geometry, microstrueture and microhardness were analyzed. Experimental results indicate that, compared to that of the conventional VP-GTAW process, the weld depth and ratio of weld depth to width are improved significantly by the variable polarity frequency in the HPVP-GTAW process, which the ratio of weld depth to width is improved by 36% at equal variable polarity frequency of 100 Hz, and improved by 55% with that of 200 Hz. Weld microstructure and microhardness distribution are changed obviously with the increase of variable polarity frequency. In the conventional VP-GTA W process, the grains in weld central zone are coarser, and the microhardness in weld central zone and fusion zone is about 95 HV and the lowest 82 HV, respectively. The microhardness is enhanced to a certain extent both in the weld central zone and fusion zone with the variation of variable polarity frequency in the HPVP-GTAW process due to the refinement and uniformity of weld microstructure. With the variable polarity frequency of 600 Hz, the microhardness in weld central zone and fusion zone reaches nearly 110 HV and 97 HV, respectively.

Approximate entropy analysis of arc stability in VPPA-GMAW hybrid welding

Variable polarity plasma arc-gas metal arc welding(VPPA-GMAW)integrates the advantages of VPPA and GMAW,and it is particularly applied to weld thick-plates aluminum alloys.High-speed camera and data acquisition system were used to analyze the arc shape and the welding process electrical signal.According to the analysis of arc swing amplitude and the approximate entropy of arc voltage signal denoised by wavelet threshold method,the influence of VPPA frequency on the arc stability was studied.The results show that the approximate entropy of GMAW arc voltage decreases with the increase of VPPA frequency in a certain range,and the stability of the hybrid arc is significantly improved.The spectral analysis shows that the arc stability is reduced due to the resonance effect between the VPPA and the GMAW arc when the VPPA frequency closes to the GMAW arc pulse frequency.The results are helpful to understand hybrid welding mechanism and the selection of welding process parameters.

Effects of pulse parameters on arc characteristics and weld penetration in hybrid pulse VP-GTAW of aluminum alloy

A novel ultrafast-convert hybrid pulse variable polarity gas tungsten arc welding process （HPVP-GTAW） is developed. High frequency pulse square-wave current which has a frequency of more than 20 kHz is exactly integrated in the positive polarity current duration. The effects of pulse current parameters on arc characteristics and weld penetration have been studied during the HPVP-GTAW process using Al-5. 8 Mg alloy plates. The arc characteristics studied by arc voltage and its profile, weld penetration noted by the ratio of weld depth to width have been found to be influenced significantly by the pulse current. The experimental results show that the HPVP-GTA W process can improve the arc profile predominantly and obtain the higher weld penetration with lower heat input. The observation may help in understanding the weld characteristics with respect to variation in the pulse current parameters which may be beneficial in using the novel HPVP-GTAW process to produce the better weld quality of aluminum alloy plates.

铝合金VPPAW穿孔焊接匙孔闭合处的微观组织与力学性能

铝合金因其密度低,比强度高等特性,被广泛应用到航空航天、军事工业等领域.与其他焊接工艺相比,变极性等离子弧焊接在大型铝合金结构件加工方面具有突出优势.针对轻金属合金等离子弧焊接过程中匙孔难以自由闭合的问题,提出了气电分离时序脉冲等离子弧环焊缝收弧方法,对比研究了环焊缝收弧方法与手工钨极惰性气体保护焊(tungsten inert gas welding, TIG焊)填补法焊缝匙孔闭合处的微观组织与力学性能.结果表明,通过调节焊接起弧、收弧策略,实现了等离子弧匙孔自由闭合,焊缝收弧区成形良好,组织为均匀的等轴晶;抗拉强度为329.9 MPa,断后伸长率为16.3%,比TIG填补法的抗拉强度提高了22.91%,断后伸长率提高了55.24%.平均硬度值为78.9 HV0.2,热影响区硬度呈下降趋势,熔合线处硬度呈升高趋势,焊缝区硬度波动范围为75~81 HV0.2.

2219铝合金FSW/VPPAW交叉接头组织和性能

通过沿垂直于搅拌摩擦对接焊缝的方向进行变极性等离子弧对接立焊获得交叉焊缝,研究了6mm厚的2219铝合金交叉焊缝的微观组织和拉伸性能,并利用扫描电镜对焊缝断口的形貌进行了观察。结果表明,交叉焊缝呈现出铸态组织的特征,但是与单纯变极性等离子弧焊焊缝相比组织更不均匀。交叉焊缝的力学性能具有方向性,沿变极性等离子弧焊方向抗拉强度为254.5 MPa,沿搅拌摩擦焊方向的性能是207.53MPa,仅为母材的50%,满足使用要求。气孔是造成焊缝性能降低的主要原因。

VPPAW工艺的变极性焊接电流受控稳定性

在铝合金的变极性等离子弧焊(variable polarity plasma arc welding,VPPAW)工艺中,变极性焊接电流的受控稳定性是确保焊接成形质量的前提.然而,等离子电弧形态会随焊接电流极性的切变而变化,使得等离子电弧作为VPPAW电源的输出负载具有显著的非线性特性,常规比例积分微分(proportional integral derivative, PID)控制策略无法确保VPPAW电源输出电流波形畸变,而输出焊接电流波形的畸变又会进一步加剧电弧负载的非线性特性,降低变极性焊接电流的受控稳定性.为了改善VPPAW工艺的变极性焊接电流的受控稳定性,研究了常规PID控制策略下铝合金的变极性等离子弧穿孔立向上焊接工艺的焊接电流受控稳定性以及等离子电弧的动态稳定性,引入了模糊控制理论,基于现场可编程门阵列(field programmable gate array, FPGA)的并行逻辑时序设计方法,设计出了一种具有并行运算能力的模糊PID控制器.验证试验结果表明,具有并行运算能力的模糊PID控制器能够有效地改善VPPAW工艺的变极性焊接电流波形的受控稳定性.

VPPAW在铝合金焊接中的应用

变极性等离子弧焊是新型高效的焊接方法,其焊接效率高、成本低、焊缝质量好,但焊接稳定性稍差。在变极性等离子弧小孔法焊接工艺试验研究的基础上,确定最佳的焊接工艺,并应用于铝合金螺旋焊管以及铝合金封闭电器筒体的焊接。

Optimization of HPVP-GTAW aluminum alloy process parameters for Al-Mg welded joints

Optimization of hybrid ultrahigh frequency pulse variable polarity gas tungsten arc welding （HPVP-GTAW） process was carried out to obtain the optimum weld characteristics of 5AO6-HX4 aluminum-magnesium alloy sheets of 3 mm thickness. The square butt joints were produced by HPVP-GTA W process, the microstructure and mechanical properties were systematically investigated. It is observed that the process parameters influence the weld microstructure and mechanical properties significantly. Weld microstructure is mainly composed of grid a （A1） solid solution, with a large number of intermetallic （ Mg2Al3 ） precipitates distributed in the matrix. The amount of ~ （ Mg2Al3 ） precipitates has an obvious difference with different parameters. Compared with that of the base material, tensile strength of 5AO6-HX4 alloy welded joints has a certain reduction, as well as the elongation and reduction of area. The optimized welding process parameters for 5A06-HX4 alloy sheets are presented.

航天铝合金变极性等离子弧焊焊接接头的腐蚀与疲劳交替研究

随着可重复使用航天器理念的提出,可重用航天器多次空天往返和地面修复过程的腐蚀与疲劳交替问题不可忽视。本工作以5A06铝合金变极性等离子弧焊焊接接头为研究对象,对焊接接头进行腐蚀与疲劳交替试验,研究不同单位腐蚀时间对腐蚀与疲劳交替工况下疲劳寿命的影响。通过扫描电子显微镜和激光共聚焦显微镜对其断口进行表征,分析了腐蚀与疲劳交替后焊接接头的损伤特性。结果表明,随着单位腐蚀时间的延长,腐蚀与疲劳交替寿命呈现先下降后平稳的趋势。这主要归因于腐蚀会钝化疲劳裂纹尖端,形成新的点蚀坑,同时钝化疲劳过程中的侵入/挤出和微裂纹降低了表面应力的集中程度,使疲劳寿命相对延长,从而反映出寿命保持稳定的状态。其中,断裂位置均处于热影响区,这主要因为焊接热影响区耐腐蚀性差,易造成严重腐蚀损伤,较大的腐蚀坑引起严重的应力集中,从而加速裂纹萌生。

铝合金变极性等离子弧焊接阴极斑点动态行为

【目的】变极性等离子弧(VPPA)焊接作为铝合金高效焊接的优质方法,逐渐受到越来越多的关注。正极性(EP)阶段电弧特性是决定铝合金稳定无缺陷焊接的关键,阴极斑点的动态行为及其热力作用是EP阶段最主要特征。该文主要针对VPPA阴极斑点运动规律及其影响机理开展研究。【方法】搭建试验平台,利用高速摄像拍摄阴极斑点动态过程,采用MATLAB对图像进行处理,得到清晰的斑点移动过程,分别以单个斑点和斑点簇为研究对象综合分析其运动规律。【结果】结果表明,阴极斑点的运动区域主要在距熔池中心5~16 mm环状区域内,且阴极斑点是从里向外清理氧化膜的;新的阴极斑点大概率出现在上一个阴极斑点附近,原因可能是金属蒸气形成的等离子云团促使电子发射所致。以宽度为1 mm的环状斑点簇为观测目标,可近似得到其从中心向外围扩展速率范围大致在1.75~2.25 m/s之间,而单个斑点的速度会远远大于这个斑点簇向外沿伸扩展的速度。【结论】斑点高电流密度、高热量的特性对铝合金VPPA焊接的成形有着重要的影响。

变极性等离子弧焊技术发展及其在航天制造领域应用现状

变极性等离子弧(VariablePolarityPlasmaArc,VPPA)焊是一种高效的铝合金和镁合金焊接方法,其主要优势在于等离子电弧具有高能量密度,可以在焊接时穿透熔池形成小孔,大幅度降低焊接变形和残余应力,提高焊接质量。了解熔池小孔行为、电弧物理特性和控制方法对于优化和提高焊接过程的稳定性至关重要。本文综述了VPPA焊接技术发展的最新情况,对VPPA焊接过程信息的检测研究为深刻认识焊接稳定性机理提供了有力保障,尤其是利用X射线成像系统观测熔融金属流动的研究,对提升VPPA穿孔熔池物理本质的认识做出了重要贡献。此外,针对不同位置VPPA焊接的研究增加了这一焊接工艺的灵活性,由VPPA焊接衍生的多种拓展工艺拓宽了该焊接方法的适用范围。VPPA穿孔焊接技术以其焊接变形小、焊后无缺陷、单面焊接双面一次成形等优势,成为航空航天领域铝合金中厚板材焊接制造的优选工艺。

能量配比对6 mm 7A52铝合金板材VPPA-MIG复合焊接残余应力分布的影响

VPPA-MIG复合焊接的热源由VPPA和MIG两种电弧热源混合而成,该焊接方法集合了MIG焊接效率高、参数区间宽以及VPPA焊接能量集中、穿透深度大等优点,能够实现不同厚度铝合金板材的高效率和高质量焊接。在铝合金板材VPPA-MIG复合焊接过程中,VPPA与MIG两种电弧不同能量配比会影响焊后接头的残余应力分布。以6 mm厚7A52铝合金板材为试验材料,设置VPPA电弧能量配比不同的焊接参数进行VPPA-MIG复合焊接试验,焊后采用钻孔法测试并分析了接头残余应力分布及峰值大小。结果表明,复合焊接接头的熔合区和热影响区出现拉应力,母材区出现压应力。在复合焊接热源总能量不变的情况下,残余应力峰值随VPPA电弧能量配比的增加而增大,且当VPPA电弧能量配比从35%增大至40%时,可以获得成形效果良好的复合焊缝,横向残余应力峰值不超过73 MPa,纵向残余应力峰值不超过232 MPa,不会出现过大的残余应力峰值。与单MIG焊接相比,VPPA-MIG复合焊的残余应力峰值比单MIG焊大,而高应力区要比单MIG焊窄。

基于弧压反馈的叶片微束等离子焊接修复弧长跟踪控制

在损伤航空发动机涡轮叶片微束等离子增材修复过程中,多层堆焊后熔覆层面不平整导致凸起形貌产生,使得待修复表面与熔覆层上表面一致性差。提出一种基于弧压反馈的弧长跟踪控制方法,提高弧长稳定性,以解决上述问题。利用电弧电压信号间接表征电弧弧长,并提出变分模态分解与中位值平均滤波融合算法以去除噪声信号。在此基础上,设计一种变论域模糊PID与PID并联的控制器,调节焊枪的高度以实现弧压稳定,并进行仿真优化。最后搭建弧压采集与弧长控制平台,进行弧压信号去噪实验和表面凸起试片焊接实验。结果表明,此方法能显著提高弧长稳定性,且凸起表面与熔覆层上表面一致性好。

镁合金的活性电弧焊接

在不同的电流条件下,研究了9种常见氧化物、氟化物、氯化物活性剂在镁合金交流氩弧焊及变极性等离子焊弧中的行为。结果表明:活性剂TiO2、Cr2O3、AlF3、NiCl2、CdCl2、ZnCl2、MgCl2使镁合金交流氩弧焊焊缝熔深增加;MgF2、SiO2使镁合金交流氩弧焊焊缝熔深减小;TiO2、SiO2、Cr2O3、AlF3、NiCl2、CdCl2使镁合金变极性等离子弧焊焊缝熔深增加;MgF2、ZnCl2、MgCl2使镁合金变极性等离子弧焊焊缝熔深减小;活性剂对镁合金交流氩弧焊焊缝熔深增加的作用大于对镁合金变极性等离子弧焊的;电弧收缩是活性剂使得镁合金电弧焊焊缝熔深增加的原因之一;活性剂的加入使得焊缝的晶粒比未涂敷活性剂时粗大。

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.