Temperature Field Simulation And Experimental Investigation Of Laser Welding Of AZ31B Magnesium Alloy Using Power Modulation Adjustable-Ring Fiber Laser

Magnesium alloys are known as "21st century green engineering materials".They are widely used in aerospace,automotive and other fields.However,magnesium alloys tend to form welding defects such as collapse and pores during laser welding,resulting in poor joint performance.In order to improve the weld quality of magnesium alloys after laser welding,combined with the new technology of power modulation laser welding and annular spot fiber laser welding,a power modulation beam adjustable mode fiber laser welding technology is proposed in this paper.the changing law of welding temperature field is studied,and the weld formation is discussed in depth.rules and mechanisms.The specific work carried out is as follows:(1)The laser welding experiment of sinusoidal power modulation of AZ31B magnesium alloy was carried out,and the influence of modulation frequency and modulation amplitude on weld formation,microstructure and mechanical properties was studied.The results shown that compared with constant power laser welding,power modulated laser welding can improve the surface collapse and spatter to a certain extent,but the effect of suppressing porosity is not obvious and the process window is very narrow.When the modulation frequency is 200 Hz and the modulation amplitude is 750 W,the weld surface forming is the best.As the modulation frequency increases,the top surface of the weld becomes poorer.The modulation frequency has a great.influence on the width of the columnar grain region near the weld fusion line,while the modulation amplitude has a great influence on the average size of the equiaxed grains.The tensile strength and elongation of the joint were not significantly improved by the modulation frequency,while the tensile strength of the joint was improved by the modulation amplitude.Compared with constant power laser welding,when the modulation frequency is 200 Hz and the modulation amplitude is 300 W,the tensile strength and elongation of the joint are increased by 13.4%and 41.2%,respectively,and the fracture mode of the joint changes from brittle fracture to ductile-brittle hybrid fracture.(2)The three-dimensional temperature field simulation model of AZ31B magnesium alloy power modulation adjustable ring laser welding was established,the influence of the center beam sinusoidal power modulation on the temperature field of adjustable ring laser welding was studied,and the thermal cycle curves at different positions of the weld were analyzed.The results shown that the sinusoidal power modulation of the central beam can reduce the temperature gradient at the center of the annular spot laser welding seam.The temperature reaches the maximum at 0.005s after the heat source reaches the cross-section during the central beam sinusoidal power modulation adjustable ring laser welding process.During welding,the peak temperature at the center of the molten pool varies sinusoidally with the instantaneous laser power of the central beam.(3)The AZ31B magnesium alloy center beam sinusoidal power modulation adjustable ring laser welding test was carried out,the influence of the center beam power modulation parameters on the weld formation was studied,the dynamic behavior of the welding keyhole and the molten pool was analyzed,and the weld formation mechanism was revealed.The results shown that the sinusoidal power modulation of the central beam has an excellent effect on improving the welding surface formation of adjustable ring laser welding.The fish scale pattern on the weld surface is fine and uniform,and the process window is wide.When the modulation frequency of the central beam is in the range of 200 Hz to 600 Hz,the surface of the weld is well formed.Then,with the increase of the modulation frequency,the collapse of the top surface of the weld increases slightly,but the surface fish scale pattern is still well formed.When the modulation amplitude of the central beam is small,the top surface of the weld is slightly collapsed,and then the modulation amplitude is in the range of 300 W to 900 W,the surface of the weld is well formed,and there is no collapse on the top surface of the weld.The sinusoidal power modulation of the central beam can improve the fluctuation along the wall and the unstable flow of the molten pool on the top surface after the adjustable ring laser welding of keyholes,thereby improving the quality of weld formation.(4)The porosity distribution,microstructure and joint mechanical properties of central beam sinusoidal power modulated adjustable ring laser welding were studied.The results show that the central beam power modulation has an obvious inhibitory effect on the porosity of the magnesium alloy adjustable ring laser welding,and the porosity is mainly distributed in the upper part.The central beam power modulation can reduce the width of the cellular grain region near the weld fusion line and the size of the equiaxed grain in the center of the weld.Under the optimized parameters,the maximum tensile strength of the joint is 233 MPa,which is about 82.3%of the base metal,and the elongation is 9.5%,which is about 76%of the base metal.