Research On Arc Characteristics And Weldline Formation Mechanism During Tig Welding Controlled By Electromagnetic Fields

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.