Research On Laser Oscillating Welding Process Of AA2219 Under Surface Pretreatment And Subatmospheric Pressure

Due to its high strength and low density,2219 aluminum alloy（AA2219）has become one of the most important lightweight structural materials in the aerospace field.At present,there is an urgent need for high-quality connection technology for medium and large aluminum alloy structural parts.Frequently used welding methods for AA2219 materials include arc welding,friction stir welding,etc.,but these methods are prone to defects including pores,cracks,and welding deformation.In particular,porosity defects seriously weaken the mechanical properties of welded joints.To this end,this paper proposes to weld AA2219 by using laser beam oscillation welding（LBOW）process under surface pretreatment and subatmospheric pressure environment.The specific research contents and findings are as follows:（1）In the study of the effect of alloy surface pretreatment on welding quality:the effect of laser energy density on the removal of oxide layer on AA2219 surface was carefully studied,and it was found that there are two material removal mechanisms in laser cleaning:photothermal elastic vibration mechanism and photothermal ablation mechanism.Under the optimized laser cleaning parameters,the oxygen element content of material surface significantly decreased by the photothermal ablation mechanism.Next,the consequent effect of laser beam oscillating welding（LBOW）process after laser cleaning on weld pool stability,porosity defects and microstructure formation were systematically investigated.Compared with untreated and sandpaper ground samples,the porosity of welded joint that was laser-cleaned in N₂decreased by 12.2%and 1.22%respectively,which was only 0.57%.The conclusions were drawn that the combination of laser cleaning pretreatment procedure and LBOW process enhanced the weld joint’s tensile strength of AA2219.This was due to the reduction of H element in the molten pool after the removal of pollutants on the base metal,as well as that the vortex flow were improved under LBOW,which made the pores greatly reduced;（2）In the laser beam oscillation welding under subatmospheric pressure（LOWSP）experiment:different parameters（pressure value,oscillation amplitude,and oscillation frequency）were set,and their effects on weld formation,microstructure,grain morphology,and mechanical properties were investigated.The experimental results showed that LOWSP has significantly improved the surface morphology,penetration depth,microstructure,and mechanical properties of the weld seam.The EBSD testing analysis shows that the low-angle grain boundaries（LAGBs）take up the largest proportion in the microstructure with the increased proportion of equiaxed grains,and the grain size is further refined in LOWSP.When welded at the optimal laser beam oscillation frequency of 80 Hz under the pressure of 10²Pa,the pore was almost inhibited and the tensile strength of the weld seam reached 388.62 MPa,83.16%of the base metal（the current level of the industry generally lies between300～360 MPa）.It is believed that the reduced boiling point of the aluminum alloy under subatmospheric pressure accelerated the molten metal flow rate,and the stirring effect of laser beam oscillation on weld microstructure within molten pool significantly eliminated porosity,therefore the LOWSP process generating the refined grain size and the improved mechanical properties of the weld seam.