Laser Oscillating Welding Of High-strength Aluminum Alloy With Root Locking Joint Type

Laser welding has been seldom used for aluminum(Al)alloys,especially the structures with partial penetrated weld because of poor stability,high porosity,and bad mechanical properties.Laser oscillating welding is a new technique by using galvanometer to control beam oscillating.Except for the advantages of laser welding,it can suppress the weld defects,and reduce the assembly precision requirements.That is,it has the potential to improve the weld quality of Al alloys.However,relevant research,especially about the root locking joint with partial penetration weld has not been carried out.This thesis then studied the influences of oscillating parameters on the morphology,porosity,microstructure and mechanical properties of the partial penetrated welds of Al alloys.The main conclusions are presented as follows.The width,especially the root width of the welds of 6063 Al alloy can be increased by beam oscillating.It changes the weld profile features.The weld cross-section shape varies from the‘nail'shape of the welding without beam oscillation to"V"shape of longitudinal oscillation,the‘W'shape at transversal oscillation and‘U'shape at circular oscillation.Under given parameters,the maximum area of keyhole inlet of laser welding with beam oscillation is about 0.5 mm~2,3 times higher than that without oscillation.This increased keyhole inlet is beneficial to increase welding stability,reduce the spatters,and improve the weld morphologies.It was found that the circular oscillating is the best oscillation pattern for weld morphologies,the optimized parameters of which are the oscillating amplitude of 1.2-1.6 mm and the oscillating frequency of 200-400 Hz.The weld porosity can be suppressed by laser beam oscillating.During circular oscillation welding,the weld porosity of 5A06 aluminum alloy firstly increases but then reduces gradually to be eliminated with the increase of the oscillating amplitude or frequency.The frequency threshold to eliminate the porosity was obtained,which decreases with increasing the amplitude.At the laser power of 3.5 kW and the welding speed of 2.0 m/min,corresponding to the amplitude of 0.8,1.2,1.6,2.0 and 2.4 mm,the frequency threshold is 500,400,175,125 and 75 Hz in the sequence.The effects of beam oscillation on porosity suppression are attributed to two reasons.Firstly,the beam high-speed movement enlarges the keyhole inlet.It reduces the tendency of bubble formation by stabilizing the keyhole.Secondly,the beam oscillation widens the volume of the pool at lower part,and enhances the stirring effect in molten pool.It promotes the escaping of the bubbles from the molten pool to the air.The ultimate tensile strength(UTS)and the elongation(EL)of the root locking weld of 5A06 Al alloy both increase with increasing either the amplitude or the frequency.The UTS and the EL are up to 310 MPa and 18%,which are 97%and 87%of the base metal,respectively,and 40%and 200%higher than those of the welds without beam oscillation.The results demonstrated that the mechanical properties is dependent on the weld porosity rather than the change of microstructure.