Study On Behavior Of Dissimilar Steel TIG-MIG Hybrid Narrow Gap Welding By Double Layer Shielding Gas

304 austenitic stainless steel with its good performance is widely used inthe production and life.However,the global shortage of nickel resources leads its use limited.443 ferritic stainless steel contains a very small amount of nickel with its superior corrosion resistance,which is good austenitic stainless steel alternative.The connection of dissimilar stainless steel metal between ferrite and austenitic will make full use of the advantages of the two kinds of metal materials,which will enable the full use of resources.TIG-MIG hybrid welding is a highly efficient and low-heat input welding method in the field of welding and the welding cost is low.Different from the traditional arc welding,the mechanism of TIG-MIG hybrid welding arc morphology and droplet transfer mode is not clear.According to the relevant research,in the welding process,double-layer shielding gas plays more superior role in arc stability,droplet transfer and welding quality.Nitrogen is strong austenitizing element,so adding a certain amount of N2 to the shielding gas in the austenite or duplex steel welding can promote the increase of the austenite phase ratio.At the same time,the impact toughness of the heat affected zone of ferrite stainless steel is improved.In this paper,firstly,the arc morphology and droplet transfer mode of TIG-MIG hybrid welding under different welding parameters such as welding current,welding voltage,tungsten-workpiece height,tungsten-wire distance and shielding gas are seized by means of high speed camera system and electric signal acquisition system.Then,according to the comparative analysis ofIV welding stability,the dissimilar stainless steel welding experiment underdifferent kinds of shielding gas and welding speed was carried out.The morphology,microstructure and mechanical properties of joints were analyzed.The result shows that with the increase of the welding current,the droplet transfer cycle and size decreased.The "current of water" sputtering transition occurs at 260 A.The arc volume and the brightness increased with the increases of welding voltage.When the tungsten-workpiece height decreased,the droplet transition stability and double arc coupling were poor.When the height increased,the droplet transition is expressed as a hybrid transition,which generated poor droplet directivity.The welding arc is a single arc,and droplet transition frequency is slow with the increase of the tungsten-wire distance.Ar(inner)+ N2(outer layer)double layer shielding gas led to the arc shrinkage significantly,droplet transition frequency and droplet size decreased.Under double arc state,the welding current through the wire end is divided into two streams back to the welding power.With single arc,the current flows through the position of molten wire end contacting with the base metal through the GTA flowing to the tungsten returning welding power negative.Compared with the traditional arc welding,the arc of TIG-MIG hybrid welding increased the area of action on droplet and enhance its electromagnetic shrinkage force.The double-layer protective atmosphere reduced the droplet surface tension and improves the plasma flow to promote the droplet transition to the molten pool.The fine grain transition is more conducive to the promotion of TIG-MIG hybrid narrow gap dissimilar steel welding to obtain a benign welded joint morphology.When the gap between the dissimilar test-plate is 1.2mm,the shape of the joint showed that the shape of the top side and the backside weld bead is poor,when the gap is 1.5mm or 1.8mm,shape of the top side and the backside weld bead is both excellent.The double-layer protective atmosphere facilitates the flow of the molten pool to the bottom of the test plate during the welding process,facilitating the formation of the weld bead.The welding joints obtained a smaller weld width,a smaller weld reinforcement and a wider width of theweld under the double-layer shielding gas.The microstructures of welding joints indicated that,the HAZ grain size of304 austenitic stainless steel was not significantly increased.The HAZ grain size of 443 ferrite stainless steel was coarse.Under the condition of pure argon atmosphere,the weld structure was biphase organization contains of austenite phase and bone shape ?-ferrite phase.Under double-layer protective atmosphere,the weld structure was biphase structure contains of austenite phase and wormlike ?-ferrite phase,and the proportion of austenite phase increased.The grain size of 443 ferrite stainless steel HAZ decreased.The mechanical properties show that the microhardness of the 443 HAZ is lower than that of the base metal,and the microhardness of 304 HAZ is not changed.The hardness of the weld zone decreases with the double-layer protective atmosphere,and the microhardness of the 443 HAZ is slightly improved.The welded joints of the tensile fracture position are located in 443 ferritic stainless steel base metal.The weld zone and the 443 ferrite stainless steel HAZ under the double protective atmosphere obtained higher impact toughness and little change was produced on austenite HAZ.