Research On The Characteristics Of Ultrasonic Assisted Laser Welding Of Magnesium Alloy

As the lightest metal structure materials used in engineering application,the magnesium alloy has high strength,light and beautiful,easy to recycle,and many other advantages,has been widely used in the aerospace,automobile manufacturing,3C products,and some other fields.It will cause lots of problem such as porosity,coarse grains and other issues in laser welding process because of the special physical properties of magnesium alloy.The ultrasonic assisted processing has been widely studied in casting,hardfacing and welding field,implemented the grain refinement,eliminating porosity and cracks,so that improve the mechanical properties.This paper proposed ultrasound assisted laser welding process.Used the cavitation and acoustic streaming effect as well as mechanical and thermal effect of ultrasound to eliminate the porosity and improve the microstructure and mechanical properties,and studied its mechanism,lay a theoretical basis for high quality welding of magnesium alloys.Weld appearance and cross section morphology were analyzed.Compared with laser welding,the surface of ultrasonic assisted laser welded joint became smooth,but this method could not solve the collapse of weld surface.Studied the effect of various parameters on the welding process.With the increased of laser power or decreased welding speed,the weld width has been increased and the collapse of weld surface became worse.Only when the defocusing amount was a negative could the welding process be stabled.DL-U and F mainly affected the amplitude ultrasonic energy spread into the magnesium alloy molten pool.With the increased of DL-U,the ultrasonic energy has been seriously attenuated,the influence of ultrasound on weld appearance became smaller.The same as F,when the F was small or large,ultrasonic energy loss is bigger,caused bad weld.By scanning electron microscope and EDS spectrum analysis.The porosity has been studied.It found that there were two typies of porosity in magnesium weld.They were Hydrogen porosity and the porosity due to the keyhole instability.The ultrasound could increase the porosity when the welding speed was low.Used the tracer element method to study the molten pool flow characteristics.It found that the ultrasound could enhance the molten pool flow intensity.But this effect would disappear when increase the welding speed.Combined with characteristics of molten pool by ultrasonic assisted laser welding and the effect of ultrasonic,we concluded the mechanism: When apply the ultrasonic into the molten pool,the cavitation and acoustic streaming effect as well as mechanical and thermal effect of ultrasound would happen,it could promote the bubble nucleation,growth and evolution process so that increase the porosity defects.The microstructure and mechanical properties of ultrasonic assisted laser welded joint were analyzed.It consist of α-Mg and β-Mg17Al12,and the β phase distribution in the grain boundary and the interior of the grains.And the columnar garins near the fusion line were large.The cavitation,acoustic streaming as well as mechanical and thermal effect of ultrasound would happen when apply the ultrasound into the molten pool.It can improve the wettability of liquid metal,promote to the nucleation of molten metal and the flow of the molten pool,made it easier for has crystal grain was volume flow of metal into the molten pool,Helped to form the equiaxial grains.And because of the assist of ultrasound,the mechanical properties of welded joint had been improved.