| Meeting modern welding requirements of high efficiency,high quality,low energy consumption and low deformation,double-sided arc welding has been applied in many fields,such as shipbuilding,pressure vessel,nuclear power equipment and ocean engineering etc.However,studies concerning the double arcs influence mechanism,molten pool forces law,weld formation principle,and heat transfer characteristics are rare.Therefore,the medium-thick(5?10mm)aluminum alloy and high nitrogen steel(HNS)were welded by double-sided coaxial TIG welding(DSCTW)process to study the arc shape,penetration mode,molten pool forces,and heat transfer characteristics systematically.A dual-robots cooperative DSCTW system was established,and the welding experiments of aluminum alloy were carried out.The influences of welding speed,double arcs spacing and heat distribution on aluminum alloy welding formation were analyzed.The results show that DSCTW process can effectively increase the weld penetration,greatly improve the energy utilization,and have a good inhibition on the porosity and deformation of the weld.The DSCTW process characteristics for HNS were studied.It is found that the viscosity of the HNS molten pool is large and the wettability is poor.Using constant current welding can easily lead to the hump defect,while the stable forming quality can be obtained by using pulse welding.There are almost no pores in the weld with pure argon shielding.However,when the nitrogen gas is added to the shielding gas,the pores appear and increase with the increase of the nitrogen ratio.It is found that there are two kinds of penetration modes of fusion and keyhole welding during DSCTW.When keyhole welding occurs,the heat input is large,and the double arcs impact each other through the keyhole,resulting in a continuous "plume reflection"phenomenon.The welding stability and forming controllability are poor.The heat input during fusion welding is small,the welding process is stable,and the high-quality weld is easy to get.The weld forming mechanism in different positions is revealed through the molten pool forces analysis.Gravity is the main driving force of the molten pool flow in DSCTW.In double-sided coaxial TIG flat-overhead and transverse welding,the gravity direction is perpendicular to the welding direction,so the negative influence of gravity on the molten pool formation is remarkable,improving the formation difficulty.The flat-overhead welded joint is like an inverted arch bridge,and the transverse welding seam is easy to deviate from the center.The gravity direction opposite to the welding direction in double-sided coaxial TIG vertical welding,the negative influence of gravity on the molten pool formation is relatively small,so it is easier to obtain high quality joints.Based on numerical simulation,melting efficiency calculation and heat transfer theory analysis,the internal mechanism of increasing weld penetration in DSCTW is clarified.The simulation results show that the temperature field of the joint are symmetrically distributed with equal welding currents on both sides of the workpiece,while the temperature field is asymmetrically distributed in one-side welding.On the center line of double arcs,the temperature of DSCTW is symmetrical in the form of "U",but the temperature distribution of one-side welding decreases monotonously as the arc goes away,and the peak temperature is lower than that of DSCTW.Compared with the two sides of the workpiece,the heat flux of DSCTW molten pool confluence zone is larger,which is beneficial for heat entry.The melting efficiency of DSCTW is greatly improved compared with one-side welding.With the increase of the double arcs spacing and welding speed,the melting efficiency decreases.The fusion area of DSCTW is not a direct summation of two one-side welding,and there is a strong interaction between double arcs heat,which makes the heat produce agglomeration and gain effect,thereby greatly improving the penetration ability and saving energy.In order to examine the quality of DSCTW joints,the microstructure and mechanical properties of typical aluminum alloy and HNS joints were characterized and tested respectively.The results show that the microstructure of aluminum alloy joint is uniform with nearly no element segregation.The ultimate tensile strength(UTS)of double-sided TIG vertical welded joints can reach 96.2%of base metal,and the maximum elongation reaches 86.6%.The UTS and elongation of flat-overhead welded joints can reach 90.7%and 53.1%of base metal respectively.The aluminum alloy joint does not show obvious softening sign.The UTS of HNS joint reaches 81.9%of base metal,and the joint ductility substantially decreases.With the increase of nitrogen gas ratio in the shielding gas,the arc voltage increases,the weld nitrogen increases,the ferrite decreases,and the dendrite arm spacing increases.The joint microhardness is greatly improved when the nitrogen gas is added to the shielding gas,whereas the joint hardness has little change with increasing the nitrogen addition ratio. |
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