【作者】 杨旭；

【导师】 胡壮麒； 常云龙；

【作者基本信息】 沈阳工业大学 ， 材料加工工程， 2012， 博士

【摘要】 钨极惰性气体保护弧焊（简称TIG焊）是使用纯钨或活化钨（如钍钨、铈钨等）作为非熔化电极，采用惰性气体（如氩气、氦气等）作为保护气体的电弧焊方法，在现代工业中应用广泛，它的优点是焊接接头质量高，几乎可用于所有金属的连接，特别适合薄板或薄壁管件的焊接。但TIG焊熔深小、生产效率低。为了提高TIG焊接效率，人们采取了许多有效的方法，如活性剂TIG焊（A-TIG）、超声波TIG复合技术（U-TIG）、热丝TIG焊和高速TIG焊等，其中高速TIG焊是采用提高焊接速度的方法来提高焊接效率，焊接速度一般都在1m/min以上。但焊速提高会导致TIG电弧阳极斑点滞后，电弧后拖，进而产生咬边、驼峰等焊接缺陷。焊速越高，电弧后拖越严重，咬边和成形不良加剧。如何改善TIG电弧形态，提高TIG电弧的能量密度，解决电弧的挺直度（即电弧沿电极轴向挺直的程度）问题成为高速TIG焊的关键技术。论文查阅了大量国内外文献资料，提出了一种在TIG焊接区外加磁场的方法。针对磁控电弧特性以及磁场对焊缝成形的影响规律进行了深入研究，丰富了外加磁场作用下TIG焊接电弧理论，为扩展TIG电弧的应用范围提供一种有效途径。论文研制了可调节参数范围广的TIG焊电磁控制设备，设计技术参数为：输入电源电压220V/50Hz；空载直流电压：60V；电流调节范围0~10A；负载持续率：100％；低频范围：2~500 Hz；中高频0.5~5 kHz；占空比调节范围：30~70％；具有直流及间歇交变方波输出功能。设计了与TIG焊焊枪配套的磁头结构，磁头类型有双L型的横向磁场磁头与螺线管绕制的纵向磁场磁头。采用高速摄像对磁场控制下的焊接电弧行为进行了观测，拍摄了不同外加磁场条件下的焊接电弧形态变化。结果表明：在纵向直流和低频交流磁场作用下，电弧形态由锥形变为钟罩形；在高频纵向磁场作用下电弧形态由锥形变为柱形。采用电弧压力传感器测量外加不同磁场作用下焊接电弧压力值，结果表明：在纵向直流和低频交流磁场作用下，电弧压力随着磁感应强度以及磁场频率的增加而呈现环形双峰分布，且当磁感应强度为50mT时，电弧中心出现负压；在焊接电流100A，气流量10L/min，磁感应强度30mT，磁场频率1.5kHz，激磁电流5A，电弧中心压力由不加磁场时的280Pa增大到370Pa，增长幅值32.1%。采用研制的测量装置进行了磁场作用下的电流密度测量，电流密度与电弧压力分布呈现相同变化趋势。电弧等离子体受到高频磁场的磁压力拘束作用，产生磁压缩；磁场频率越高，电弧压缩越明显，在磁场频率为1.5kHz时，压缩效果最好。论文分析了磁压缩机理。对磁场作用下的TIG焊焊缝进行一系列性能试验，结果表明：不加磁场时，焊接速度在高于4.5m/min条件下，焊缝出现开裂；外加横向直流磁场和高频纵向磁场将提高TIG焊接速度；在焊接电流100A,气流量10L/min，激磁电流4A时，焊接速度将达到5.5m/min并且焊接质量最佳；焊缝成形情况测试表明：高速焊时，不加磁场将出现开裂；当施加磁场后焊缝成形良好。力学性能试验表明：外加横向直流磁场作用下抗拉强度较不加磁场时提高20MPa，伸长率增加5.5%，并建立了磁场作用下抗拉强度和焊缝成形系数回归模型。论文对磁控TIG焊焊缝成形机理进行了理论分析，外加磁场可以改变液态金属表面张力的差值，进而改变熔池的流动方向。当eσ/eT>0时，熔池液态金属凝固时容易产生咬边；eσ/eT<0时，咬边倾向减小。理论分析表明：外加磁场时TIG焊电弧阳极斑点的有效直径与无磁场时相比大，这将有助于减小表面张力温度系数，从而有助于解决高速焊接时出现的咬边与驼峰等问题。更多还原

【Abstract】 Tungsten Inert Gas Arc Welding (TIG for short) is a method that uses pure tungstenor active tungsten such as thorium-tungsten and cerium-tungsten as the non-consumableelectrode, uses inert gas such as argon or helium as the shielding gas. It is very popular inmodern industry. It has many advantages of high quality in welding joint and fitting foralmost all metal materials joining, especially in sheet steels or thin-walled tubes.Nevertheless, the TIG welding has a smaller weld penetration and a lower productivity. Alot of effective methods such as active TIG (A-TIG), ultrasonic TIG (U-TIG), heatedwire TIG and high-speed TIG are adopted to increase the welding productivity.High-speed TIG welding is the way to improve efficiency by enhancing the weldingspeed which is usually above 1m/min. Welding speed increasing, however, causes anodespot delayed and arc dragged backwards seriously. As a result, the defects such asundercut and hump seam are generated. The higher the welding speed, the more seriousthe defects. Hence, the measures of improving TIG arc shape, raising the energy density,and making the arc straight in the axial direction become the key technology of highspeed TIG welding.A large number of references at home and abroad were searched. The way ofmagnetic fields added in TIG arc welding were put forward. The arc characteristics andlaws of the influence of magnetic fields on weldline formation were investigated deeply.The TIG arc theory under the external magnetic fields was enriched and an effective wayof extending the range of TIG welding application was developed.An electromagnetic field equipment with a wide range of adjustable parameters forTIG welding was developed. The technical parameters are as below:Input voltage of power source: 220V/50Hz, no-load voltage: 60V (DC), directcurrent range: 0~10A, load duration rate: 100％, low frequency scope: 2~500Hz,intermediate and high frequency scope: 0.5~5 kHz, duty cycle: 30~70％. The equipment has functions of outputting direct current and intermittent alternative square waveform.Magnetic heads matching with TIG welding gun were designed. There were two kinds ofmagnetic heads, one was transverse in double L type, the other longitudinal wrapped withspiral tube and coil.The high speed camera was used to observe the welding arc behaviors under themagnetic fields. The variation of arc shape was shot with different conditions ofmagnetic fields. The experiment results indicated that the arc configuration changed fromcone form to bell shape under direct current longitudinal magnetic fields and alternativecurrent low-frequency magnetic fields, and from cone form to cylinder shape inlongitudinal high-frequency magnetic fields.The sensor was used to measure the arc pressure under different magnetic fields.The results showed that the arc pressure took on the annular bimodal distribution withthe increasing of magnetic induction and low-frequency under direct current longitudinalmagnetic fields and alternative current low-frequency magnetic fields. The center arcpressure was negative when the magnetic induction reached 50mT. The center arcpressure raised from 280Pa without magnetic fields to 370Pa with parameters as follow,welding current 100A, gas flow rate 10L/min, magnetic induction 30mT, magnetic fieldfrequency 1.5kHz, exciting current 5A. And the increasing amplitude of center arcpressure was 32.1%. Aself-developed equipment was used to measure the distribution ofwelding current density under magnetic fields, which had the same variation trend withthat of arc pressure. The arc plasma produced magnetic compression under theelectromagnetic pinch caused by high-frequency magnetic fields. The higher themagnetic frequency, the heavier the magnetic compression. When the magneticfrequency was up to 1.5kHz, the best result was achieved. The mechanism of magneticcompression was analyzed.A series of performance tests on the TIG welds were conducted under magneticfields. The results showed that weld cracks were produced without a magnetic field whenthe welding speed increased higher than 4.5m/min. The TIG welding speed could beenhanced under direct current transverse magnetic fields and high-frequency longitudinalmagnetic fields. The best weld performance could be obtained when the speed was5.5m/min with the parameters as follow, welding current 100A, gas flow rate 10L/min,exciting current 4A. The test on the weldline formation indicated that cracks could takeplace without a magnetic field at a high speed. The mechanical tests showed that the tensile strength was increased by 20MPa, and had an elongation of 5.5% compared withthe situation without a magnetic field. The regression models of tensile strength andcoefficient of weldline formation were established under magnetic fields.The TIG welding weldline formation mechanism controlled by the electromagneticfields was analyzed theoretically. The external magnetic fields could change the distanceof surface tension of liquid metal, which may lead to the change of flow direction ofmolten pool. A undercut could emerge when the liquid metal was freezing with thecondition ofeσ/eT >0, on the other hand , ifeσ/eT< 0, the trend of undercutgenerating decreased. The theoretical analysis showed that: the effective diameter of theanode spot of the TIG welding arc under a magnetic field was longer than that ofsituation without a magnetic field, which could benefit for reducing the temperaturecoefficient of surface tension. And in this way, the problems such as undercut and bumpseam in the process of high speed TIG welding could be solved satisfactorily.更多还原