| Narrow gap welding method adopts deep and narrow groove instead of the large angle groove,and the filling area is only 1/4 to 1/2 that of the conventional welding method,which greatly increases welding efficiency and improves joint quality.Narrow gap laser welding with heating source concentration,micro-angle groove and high speed has a wide application prospect in the welding of thick plate.As for lack of sidewall fusion,pores,uneven microstructure and performance,using oriented magnetic field and alternating current,the new technique of electromagnetic(EM)-assisted narrow gap laser wire feeding welding(NG-LWFW)is proposed to drive hot wire to swing laterally.The energy distribution and molten pool dynamic behavior were studied.The mechanisms of improved sidewall fusion were clarified,which is of significance to the application of thick-wall components.Circular laser welding process can improve the adaptability of laser heat source to narrow gap welding.The laser heating source was changed and the central region had the minimum energy,while the maxmum energy peak appeared in the two sides of the region,presenting a "concave" energy distribution mode.As beam wobble amplitude and frequency increased,the peak energy was reduced and the area of laser heat source was increased.Weld deep-width ration was decreased and "finger-shaped" penetration was avoid.When beam wobble amplitude and frequency were 2.0 mm and 150 Hz,the flowing of met pool surface was changed with turbulent flowing mode,increasing weld width and wetting behaviour.Meanwhile,the melting filler wire distributed evenly and reduced hydrostatic pressure on keyhole wall.The high-speed movement of laser beam can make keyhole overlapped,enlarge keyhole area and stabilize keyhole shape,which can restrain weld pores.However,due to the restraint of groove width,laser energy was difficult to radiate directly on sidewall,and there was still a poor fusion of sidewall.The EM-assisted NG-LWFW was proposed to adapt narrow space and laser energy constraint.The results indicated that hot-wire current reduced the dependence of filler wire melting on laser energy.And as filler wire offset increased,the percentage of heat conduction of molten pool was increased.In comparison to conventional laser welding,the eruption direction of plasma was changed and the electron density in laser irradiation direction was reduced,increasing laser absorptivity.The expansion of plasma increased heating area at groove sidewall,which heated groove sidewall as an effective heating source.High-temperature melting metal distributed evenly and increased energy input to sidewall.The forced convection increased the momentum of the lateral flow of the liquid metal,which promoted high-temperature weld metal flow to sidewall.The holding time at sidewall was increased to improve sidewall wetting and fusion.With increasing EM amplitude and frequency to 6.0 mm and 30 Hz,the stirring velocity of filler wire increased and forced convention played a dominant role in molten pool flowing.When EM frequency and circular laser beam realized the coupling regulation of molten pool flowing,the stability of molten pool and the sidewall wetting were the best.The welding heat transfer with EM-assisted welding was changed and process window was enlarged.The results indicated that the peak temperature of molten pool was decreased by 7.6 %,while sidewall temperature was increased by 10.7 %.The temperature of molten pool was more even,temperature gradient of molten pool front decreased and cooling rate increased.EM-assisted welding improved sidewall temperature by increasing heat conduction between high-temperature molten pool and sidewall.Groove width and weld surface curvature increased,and weld symmetry improved with "crescent" bead.Multi-layer welding of 316 L stainless steel with 40 mm thickness was realized by one laser autogenously layer and eleven filling layers.The defect-free narrow-gap welded joint was obtained.Simulation results of multilayer narrow gap welding showed that the thermal characteristics of filling layers were similar.As filling layer increased,heating accumulation of molten pool increased,while the heat dissipation was weak.The longer holding time of molten pool increased heat conduction into sidewall and decreased cooling rate of molten pool.The post-weld had a re-melting and heat-treatment effect on former weld,causing the decrease of peak temperature and cooling rate.The weld microstructural uniform was improved with EM-assisted welding.Stirring effect on molten pool increased weld solidification rate and reduced temperature gradient of molten pool front.The transverse oscillation of filler wire drove molten pool flowing,which would flash the solidified dendrites at the solid/liquid interface,promoting even diffusion of weld elements and weakening elements segregation.The growing of coarser columnar crystal was inhibited along optimal direction and dendrite spacing was reduced.The width of heat affected zone and grain size were decreased.The grain refinement and orientation diversification of 316 L welded joint improved joint performance.The welded joint with electromagnetic assisted welding had excellent strength and plasticity. |
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