Research On The Mechanism Of High Speed Laser-arc Hybrid Welding Of High-strength Steel

Different from laser welding and conventional arc welding,laser arc hybrid welding not only reduces the gap sensitivity,but also ensures the stability of the arc in the high speed welding.Under the characteristics of low assembly precision and high efficiency,the application of hybrid welding in the industrial field is becoming more and more extensive.The fiber laser assisted by MIG source was used to weld 3 mm thick high-strength steel(HSS)in butt configuration.The influences of laser power,arc voltage,wire feeding rate and welding direction on the weld appearance and droplet transition were studied at high welding speed(4?6m/min),and then compared with the medium/low speed laser-arc hybrid welding process.The results indicated that the increase of laser power would improve concentration of the laser plasma firstly,affecting energy absorption,and then expanded outward by heat;increasing the wire feeding speed,for the laser leading,the arc force was increased,and the keyhole radius was enlarged,which helped to stabilize the welding process.For arc leading,the laser couldn't effectively attract the arc column,and the weld bead was not continuous;the increase of welding speed reduced the residence time of the plasma,but leading to the decrease of linear heat input;as the arc voltage increased,the arc heat source radius increased and the arc pressure decreased,leading to the stability of molten pool;in the different welding direction,the weld transverse sections showed shallow Y-shape and deep Y-shape.In addition,the coupling of the laser and arc,droplet transition and the stability of the keyhole would also change.In the laser leading mode,compared with the high speed welding,the weld width of the medium speed welding increased,and the reinforcement decreased;the frequency of droplet transition was similar;the micro-hardness distribution of the upper and lower areas of the weld joint was more uniform.In the arc leading mode,compared with the high speed welding,the interface between weld joint and base metal was more smooth in the medium speed welding process;the frequency of droplet transition reduced;the micro-hardness distribution of the lower area was uneven.At high speed welding,the weld porosity in the arc-leading mode was lower;the situation was reversed in the case of medium speed welding.When the welding speed was 6m/min,the scope of the process parameter window was determined: P=3.4k W?5k W,WFS=7?11m/min,U=24?30v.In high-speed welding,the microstructure of arc zone was mainly martensite and bainite,and the crystal shape was more uniform when the arc was leading.The microstructure of laser zone was respectively ferrite weiss(laser leading)and lath martensite/bainite(arc leading).In medium speed welding,due to the large linear heat input,the cooling rate of the molten pool was slow,resulting in coarse grains.When the laser was leading,the hardness distribution of the upper and lower welds obtained by the medium speed welding was relatively uniform;and when the arc was leading,the overall hardness distribution obtained by the high speed welding was relatively uniform.This was mainly affected by factors such as the range of the arc force and the solidification speed of the molten pool.The welding wire element(Mn,Si,etc.)dissolved in the molten pool could improve micro-hardness and tensile strength of welded joints(404HV/613 MPa,both better than the mechanical properties of base metal).