Research On Metal Transfer And Vertical-down Welding Process Of Tandem Narrow Gap GMAW

As a high efficient welding technology, narrow gap welding has been widelyemployed in the industry to manufacture thick plates or pipelines because of its low heatinput, high efficiency, low thermal stress and weak damage to the microstructure ofweld beads. Tandem narrow gap GMAW is able to improve the efficiency of metaldeposition and availably solve the problem of poor sidewall fusion which stems fromthe traditional narrow gap GMAW. In this paper the characteristics of metal transfer andarc behaviors in tandem NG-GMAW have been successfully observed and studied.Moreover, the tandem NG-GMAW process and the single-molten-pool behaviors invertical position have also been researched.The previous tandem NG-GMAW torch was modified in the terms of innerstructure, insulation performance and air tightness. New water-cooled copper nozzlewas designed and made to replace the former nozzle whose cooling effect was bad inthe welding mode of single-molten-pool. Additionally, a specialized nozzle with twoinert gas channels was also fabricated to serve the purpose of double-molten-pool modewelding and a tandem NG-GMAW torch with compact structure and stable performancewas finally obtained.Two methods were used to analyze the characteristics of metal transfer and the arcbehaviors, in which one is backlit and the other is based on positive vision. Due to therestriction of narrow gap and the mutual heat complementation of double electrodes,better metal transfer mode was prone to take place under the condition of narrow gapwelding than that of plate welding. Moreover, the transfer mode of leading wire wasdifferent from that of trailing wire and the peak pulse voltage, pulse frequency and pulsetime all had effects on the metal transfer and arc shape. In vertical position, propermode such as globular transfer could be obtained with higher welding current, or higherpulse frequency or longer pulse time since the gravitational force had failed to promotethe droplets breaking away from the tip of the consumed electrodes in the axial directionof feeding wires.By employing larger welding current, the effects of welding parameters such asdistance between two electrodes, angle of the bending contact tube, matching of leadingwire rate and trailing wire feeding rate, travel speed, and peak time voltage on thewelding process, were studied detailedly. Then, a sound welded joint of Q235lowcarbon steel with a thickness of30mm was made in vertical position. By means of highspeed photography, the asymmetry of the single-molten-pool which originated from the system which applied bending contact tubes to promote the fusion of sidewall, had beenfirstly discovered and it exerted directly negative impacts on the stability of dropletstransfer and caused other problems, thus disturbing the welding process.Due to the instability and the narrow range of single-molten-pool mode,double-molten-pool mode was employed and welding parameters such as wire feedingrate, travel speed, peak time voltage and distance between two electrodes were alsostudied in the vertical-down welding process. Meantime, another sound Q235lowcarbon steel welded joint with a thickness of30mm was achieved resorting todouble-molten-pool mode. Eventually, further problems which needed to be studiedwere pointed out according to the characteristics of double-molten-pool mode.