Research On Arc Characteristics And Melting Behaviors Of Ternary Gas Shielded Narrow GAP GMA Welding

For thick plates welding,narrow gap welding is an efficient welding method,in which a narrow and deep gap is applied instead of wide-angle groove.Narrow gap GMA welding has a higher welding efficiency due to the fewer cost of filler metal,and it can provide a good welded joint with excellent mechanical properties,so it has a good application prospect.The most common welding defect in narrow gap GMA welding is the lack of the sidewall penetration.To solve this problem,welding methods have been developed to ensure the sidewall penetration depth by increasing the arc heating zone.In arc welding,the arc plasma is formed after the shielding gas is ionized,so the shielding gas decides the arc characteristics.The arc characteristics can affect the metal melting behaviors directly.Hence,it will be an effective method to improve the welding process and weld quality by optimizing the shielding gas composition.The frequently used shielding gases for narrow gap GMA welding of thick steel plates are Ar-CO2 binary mixtures.Helium has its special physical properties,such as high thermal condutivity and high ionization energy.The arc energy and the amount of the fusion metal will increase with the addition of helium to the shielding gas.In this study,helium was added to the shielding gas and Ar-CO2-He ternary shielding gas was used for narrow gap GMA welding of thick plates,in order to ensure the narrow gap GMA welding sidewall penetration.In this dissertation,ternary gas shielded beads-on-plate GMA welding,narrow gap GMA welding and tandem narrow gap GMA welding tests were conducted to study the effects of shielding gas composition,especially the helium content,on the arc characteristics.The influence mechanisms of arc characteristics on the droplet transfer and weld formation were revealed.Finally,the ternary shielding gas for narrow gap GMA welding was optimized.This research is helpful to clarify the influence mechanism of shielding gas composition on welding process and also to further improve the weld quality of narrow gap GMA welding.In beads-on-plate GMA welding process,the helium or CO2 content was varied.It was found that arc contracts and the arc length decreases with the increase of the helium or CO2 content.Two-demensional numerical simulation model were established to study the arc characteristics.It was found that,as the helium content increases,the temperature field constricts but the high-temperature zone in the arc expands.The current distribution concentrates and the current density around the arc center increases,and the heat flux on the cathode increases.The forces acting on the droplet were analyzed,and the forces changes with the shielding gas composition and it affects the droplet transfer process.The droplet transfer frequency decreases and the droplet size increases as the helium or CO2 content increases.The droplet transfer affects the molten pool flow and the weld shape,the weld profile transforms from finger-like penetration to bowl-like penetration with the increase of helium content.In single-wire narrow gap GMA welding,when the welding voltage is constant,the arc length evidently decreases with the increase of CO2 content,and the arc core expands as the helium content increases.The shielding gas composition affects the arc static working point.In pulsed GMA welding,the arc length dreases as the CO2 content increases,and the arc width at pulse peak increases as the helium content increases.The droplet transfer frequency decreases as the helium or CO2 content increases.When the wire feed speed is 9 m/min,the droplet transfer mode transforms from pulsed spray to one pulse-one droplet with the increase of helium or CO2 content.The sidewall penetration increases with the proper increase of helium content or CO2 content,and increased helium content has a more significant effect on the side-wall penetration.When the helium content is 10%,the sidewall penetration depth increases by 60%.In tandem narrow gap GMA welding,the droplet transfer processes are special.When the shielding gas is 90%Ar-5%CO2-5%He,because the lead arc has a preheating effect on the trail wire,the trail arc welding current is higher than that of the lead arc,although the welding parameters for two arcs are same,The droplet transfer of the two wires are different.When the wire feed speed is 8 m/min,the lead wire droplet transfer mode is one pulse-one droplet,while the trail wire droplet transfer mode is spray.The welding currents of the two arcs both decrease as the CO2 content increases,but increase as the helium content increases.With the increase of helium content,the lead wire droplet transfer mode transforms from pulsed project to pulsed spray,and the trail wire droplet transfer mode remains pulsed spray.As the CO2 content increases,the trail wire droplet transfer transforms from pulsed spray to on pulse-one droplet,but the lead wire remains pulsed project transfer.The weld width is decided by the arc heat transfer characteristics and the arc length.As the helium content or CO2 content increases,the weld width increases firstly,then decrsases because the decreased arc length results in the decrease of the arc heating zone.According to the tandem narrow gap GMA welding process,a three-dimensional numerical model for molten pool behaviors was estabilished.Compare the molten pool temperature field with the weld profile,and adjust the heat source model,it was proposed that the ternary shielding gas composition affects the arc characteristics and the heat model.The arc concentration coefficient of heat source decreases as the helium content increases and more welding heat transfers to the edge of the heat source,and it is good for the increase of the sidewall penetration.The statistical models of narrow gap weld bead geometry with the shielding gas composition were developed by response surface methodology,and the models' accuracy and reliability were checked by analysis of variance(ANOVA).The optimized ternary shielding gas for narrow gap GMA welding was obtained and the ternary mixture is 79%Ar-10%CO2-11%He.The ternary gas mixture was used for tandem narrow gap GMA welding of Q690 steel with a thickness of 60 mm.The mechanical properties of the welded joints were investigated.The mechanical properties of the welded joints meet the application requirements of pile leg of marine drilling platform.