Numerical Simulation And Joints Performance Study Of K-TIG Welding Of 2205 Duplex Stainless Steel

Keyhole Tungsten Inert Gas(K-TIG)welding is a new type of high-current "keyhole" welding method,which can be used to weld stainless steel materials below12 mm in a single pass without filler welding material and opening groove.It has the advantages of low cost,high welding efficiency and high quality.It has broad application prospects in various fields such as vessel manufacturing,ships,ocean engineering,oil pipelines and construction.However,most of the current research was focused on the microstructure and performances of K-TIG welded joints,and there is a lack of relevant research on the formation mechanism and dynamic behavior of "keyhole" in the process of K-TIG welding by using simulation methods.Therefore,the study combined experiments and simulations to investigate the morphology evolution law,microstructure regulation,joint performance strengthening mechanism and "keyhole" stability of K-TIG welded joints of duplex stainless steel.In this paper,three combined heat sources were used to simulate the morphology of K-TIG welded joints of 8 mm thick 2205 duplex stainless steel,calculated the welding temperature field at different welding speeds and optimized the welding process parameters.The results showed that the combined heat source model with a double ellipsoid heat source as the upper part and gaussian conical heat source as the lower part is more suitable for the numerical simulation of K-TIG welding.When the welding speed was 280-340 mm/min,the simulated cross section presented a goblet shape.The peak temperature in the center area of the weld is the highest and its cooling rate is the fastest.With the increase of welding speeds,the peak temperature in the center area of weld gradually decreased,and the cooling rate increased.At the same time,the center area of the weld was closest to the center of the heat source,thus its temperature gradient is higher,and gradually became lower towards ends of the weldment.Meanwhile,the temperature field on the workpiece surface appeared tailing phenomenon.A mathematical model of molten pool of K-TIG welding was established considering electromagnetic force,arc shear force,surface tension,buoyancy and gravity.The free interface of the pool was captured by volume of fluid(VOF)method.The basic data of the flow characteristics of the pool and the dynamic behavior of the "keyhole" in K-TIG welding process were obtained based on simulated results.The results of flow field simulation showed that the flow trend of molten metal in the process of keyhole formation was mainly from the center of the keyhole to around the keyhole under the action of electromagnetic force,surface tension and other driving forces.Among them,the high-temperature liquid metal in the upper part of the molten pool flew in the opposite direction of welding,and the liquid metal in the inner and bottom of the molten pool flew diagonally downward toward the center of the keyhole.At the same time,when the welding speed was reduced,the maximum flow velocity of molten metal in the molten pool increased under the action of Marangoni force,which promoted the formation of keyhole exit.At the same time,K-TIG keyhole exit is mainly formed under the balance of surface tension,gravity and arc pressure,and would be in a dynamic equilibrium state with the change of keyhole shape and fluid flow state.When the welding speed was 280-340 mm/min,the proportion of Σ3 coincident site lattice grain boundary(CSLGB),austenite content,and the grain size of ferrite and austenite in welded metal zone of K-TIG welded joints gradually decreased with the increase of welding speed.The mechanical properties of K-TIG welded joints of2205 duplex stainless steel were closely related to microstructure.The microhardness and tensile strength of K-TIG welded joints were inversely proportional to austenite/ferrite ratio.At the same time,the tensile strength of the welded metal zone was also related to the grain size,but independent of low angle grain boundary(LAGB)ratio and Σ3 CSLGB ratio,and the tensile fracture surface of the welded metal zone had pits and torn edges.The low temperature impact performance of welded joint was closely related to austenite/ferrite ratio,grain size,grain boundary characteristics and other factors.The low temperature impact performance of welded metal zone was the best when the welding speed was 280mm/min.