Study On The Characteristics Of Ultra Narrow Gap Oscilating Laser Welding Of Thick Aluminium Alloy

Thick aluminum alloy structures have a wide application prospect in aerospace equipment frame,engine and fuel tank,armored car shell,hull structure of large warships and so on.Laser welding has certain advantages in the welding of thick aluminum alloys due to its low heat input and welding distortion.Narrow gap laser multi-layer filler wire welding is always used in welding of thick structures.However,lack of inter-run and side-wall fusion often appears in narrow gap multi-layer filler wire laser welding.With the development of laser technology,oscillating laser has been attracted more and more attention.It was found that the oscillating laser beam significantly solved the problems of lack of inter-run and side-wall fusion.Therefore,narrow gap oscillating laser welding of thick aluminum alloy plates has been investigated in this paper.The weld appearances,microstructures and mechanical properties were analyzed.In order to obtain the correlation of oscillating parameters,keyhole and molten pool,the periodic variation of laser induced plasma,keyhole and molten pool behaviors were studied.The narrow gap oscillating laser welding of aluminum alloy thick plates was carried out.It was found that the oscillating laser welding could improve the weld appearances,and the narrow gap weld joint of 18 mm and 50 mm thick 5A06 aluminum alloys with sound weld performance was obtained.The microstructure results showed that the weld structure near the fusion line was equiaxed zone,and gradually transferred into columnar crystals.The growth direction of the columnar crystals was toward the center of the weld.In the tensile test of different layers,the average tensile strength was 321 MPa,which was 94% of the strength of the base metal.The fracture location was on the weld seam.The average tensile strength of the complete 50 mm thick weld joint was 296 MPa,which was 86% of the base material strength.In order to study the differences between the oscillating laser welding and the conventional laser welding,welding plasma,the keyhole and the weld pool were observed using high speed camera,and the corresponding features of laser induced plasma,keyhole and the weld pool was analyzed.It was found that a complete plasma dynamic period of the oscillating laser welding included eruption,attenuation,deviation from the laser incident direction and disappearance,and the dynamic period was consistent with period of the laser beam oscillation.Keyholes existed in two modes in oscillating laser welding: non-overlapping keyholes and superimposed keyholes.The size of the non-overlapped keyhole was approximately 2.5mm~2,the keyhole moved along with t oscillating laser beam,and the forward metal counterclockwise was pulsated.The overlapped opening area of the keyhole increased to about 4.5 mm~2.Oscillating laser beam caused the liquid metal to periodically flow toward the tail of the pool.The oscillating trajectory mainly affected the position of plasma generation,dynamic behaviors and molten pool flow.Oscillating amplitude and frequency affected the presence of keyholes and the mode of molten pool flow.When the oscillating frequency was small,the superposed keyhole could not form and the opening area of the keyhole was equivalent to that in the conventional laser welding.With a large oscillating frequency and a small oscillating amplitude,an overlapping keyhole formed,and the opening area of the keyhole became larger.