Research On Narrow Gap Laser Welding With Filler Wire And Laser-MAG Hybrid Welding Of The 301L Austenitic Stainless Steel

Based on the excellent performance of corrosion resistance,plasticity and weldability,austenitic stainless steel is widely applied in the manufacture of metro's stainless steel car-body.This thesis takes 8mm thickness SUS301L-MT as research objective to study process characteristics and joint performance of Laser-MAG hybrid welding and narrow gap laser welding through the analysis of wire melting behavior,arc droplet's character,welding process character,microstructure of welded joint and integrated mechanical performance.EBSD was used to explore welded joint's microcosmic forming mechanism.This thesis provides the experimental foundation and theoretical basis for the further engineering application of Laser-MAG hybrid welding and narrow gap laser welding technology.This thesis applies high speed video technology to study the fusion of welding wire of laser welding with filler wire and the law that exert the influences on the weld pool by laser thermal source;analyzes the laser arc's coupling mechanism.The result indicates:wire distance,welding speed,wire feed speed,laser power and defocusing amount exert significant influences on droplet transition and formation of weld.Only when previous parameters get a reasonable matching within the certain range,can the good forming welded joint appears.Compared with laser weld with filler wire,laser-MAG hybrid welding owns worse droplet transition stability and formation of weld but the wider controllable range of process parameters.After analyzing the weld joint's microstructure and welding thermal cycle of laser-MAG hybrid welding and narrow gap laser welding with filler wire with 301L stainless steel respectively,the study gets a result:weld joint's microstructures in hybrid weld and filler wire weld are similar,their weld zone are Austenitic columnar crystal and dendritic ferrite;the heat affected zone and base metal are Austenite and strain-induced Martensite;The filler wire weld's thermal cycle owns low peak temperature but the high cooling speed so that the condenser depression is strong,and the crystallization temperature is low,the grain is small.The thesis compares the integrated mechanical performance of laser-MAG hybrid welding and narrow gap laser welding with filler wire with 301L stainless steel respectively and draws the stress cloud of longitudinal residential stress,the result is shown as follows:the tensile tests of filling wire welding and composite welding are broken in the weld,the fracture mechanisms are dimple fracture;the yield strength of weld joint in filler wire welding is 576MPa;the tensile strength is 824MPa,elongation is 27.9%.The three process parameters are higher than in hybrid weld.The hardness of filler wire weld's joint is greater than hybrid weld's,the joint soften zone is small.The distributions of residential stress in two weld process are similar that the "stress dip" appears around the weld and fusion line.The hybrid weld's peak of longitudinal residential stress is 600MPa,which is higher than filler wire weld's 304MPa.Based on the EBSD technology,this study analyzes the weld joints molten pool solidification oriented feature,grain morphology and grain size.From the perspective of material microstructure,the study tries to explain the differences of property between the joints of laser-MAG hybrid welding and narrow gap laser welding with filler wire.The result is shown as follows:weld grain tends to the growth with planes of<100>//<100>,{100}//{100} and form the{100}<100>texture;narrow gap laser welding with filler wire joints's grain size is smaller than laser-MAG hybrid's,in the condition of uniaxial tensile load,the Schmid factor of filler wire weld's grain in {111}<110>sliding pattern is higher than hybrid weld's and the dislocation slide mechanism is easier to be activated so that narrow gap laser welding with filler wire joints strength and plasticity are better than laser-MAG hybrid joints.