Research On Laser-TIG Hybrid Welding Of D406A Ultra High Strength Steel

D406A ultra-high strength steel is widely used in aerospace because of its high strength and good toughness and plasticity.The steel is usually welded as annealed state and heat treated after welding.The process is complex and the obtained joint toughness is not high and the conventional TIG welding is difficult to meet the requirements of its welded joint.In this paper,the laser scanning-TIG hybrid welding characteristics were studied for the first time,and the influence of different laser scanning modes on the joint performance,microstructure and properties was analyzed,so as to further improve the ultra high strength steel joint performance,refine the structure and enhance the mechanical properties of the joint.Joint performance.The strongest coupling effect between laser and arc can be obtained by matching the reasonable arc length,the distance between heat sources and the inclination angle of TIG torch.With reasonable welding speed,laser power and arc current matching,high quality welds with smooth,continuous and uniform surface and no defects such as pores and cracks can be obtained.The appropriate defocusing amount can make the weld well formed and the laser beam stirring pool has the strongest ability.Laser scanning frequency,scanning speed and scanning mode are the key factors affecting the stability of welding process and the formation of welding seam.Excessive scanning frequency or speed and inappropriate scanning mode will make the welding process unstable when heat sources interfere with each other.The smoothest weldwith uniform solidification fringes and no spatter can be obtained by adjusting them separately in a certain range.Microstructure and mechanical properties of the joint.The solidification crystalline microstructure of the weld seam shows that the fusion zone is planar and cellular,the near fusion zone is coarse cellular dendrite,the near central zone is fine dendrite,and the central zone of the weld seam is slender dendrite and equiaxed dendrite.The solid-state transformation microstructure is mainly composed of lath martensite,retained austenite and bainite in the weld zone and heat-affected zone.The martensite lath in the weld zone is thick and uneven in size,and the martensite in the heat-affected zone is short and uniform in size.Laser scanning frequency,scanning speed,scanning mode and laser scanning energy mode are the key factors affecting the solidification and crystallization microstructure of welds.When the laser scanning frequency is 50 Hz,the columnar crystals are broken and the orientations are different.When the laser scanning frequency is 100 Hz,the weld microstructure is fine and uniform.When the laser scanning frequency is200 Hz,the welding process is unstable and the microstructure is coarse.When the laser scanning speed is 50mm/s,equiaxed grains can be obtained,and the weld structure is refined uniformly.If the scanning speed is too small,the fine structure is not obvious,and if the scanning speed is too large,the welding process is unstable.A small equiaxed crystal zone appeared in the center of the weld seam during laser spiral scanning,but no equiaxed crystal zone was found in laser serration and transverse scanning.Continuous laser scanning can produce equiaxed grains in a considerable area in the middle of the weld.The mechanism of laser beam stirring.Laser scanning can significantly refine the grain size and improve the joint microstructure,and the effect of continuous laser scanning is better.Compared with the non-scanning weld,the columnar crystal of laser scanning is relatively fine,and the growth orientation of columnar crystal is deflected and somecolumnar crystal is broken in the center of the pulsed laser scanning weld.There is also an equiaxed crystal zone in the center of the weld by laser scanning.The equiaxed crystal zone by continuous laser scanning accounts for 16.9% of the total weld cross section,while that by pulsed laser scanning is only 6.9%.The size of martensite lath with or without laser scanning has little difference.In terms of mechanical properties,the average hardness of continuous laser scanning seam center is the highest,reaching 581 HV,slightly higher than that of pulsed laser scanning and non-scanning.The average hardness of continuous laser scanning seam is also the highest,reaching 590 HV,which is 162% higher than that of base metal(225HV).Because of the coupling effect of laser and arc,making the energy more concentrated,the softening zone of the heat affected zone of laser scanning is narrower,and the continuous laser scanning is reduced by50% compared with the non-scanning.The analysis of residual stress shows that the transverse residual stress of continuous laser scanning is the lowest,422 MPa,which is 40.6% lower than that of non-scanning,and the stress distribution in each region is the most uniform by continuous laser scanning.The longitudinal residual stress of continuous laser scanning in the center of weld is is the lowest,108.5 MPa,which is 47.6% lower than that of pulsed laser scanning,and the residual stress distribution uniformity in different regions with or without laser scanning is not very different.From the analysis of beam stirring mechanism,laser scanning can improve grain growth behavior by changing temperature gradient and the force of molten pool,increasing convection and stirring of molten pool,so as to improve mechanical properties of weld.