Experimental Research On Adjustable Rring Model Fiber Laser Welding Of AZ31B Magnesium Alloy

Magnesium alloys have the advantages of high specific strength,high specific stiffness and easy recycling,and have great application potential in aerospace,transportation and defense equipment components.However,magnesium alloys are prone to defects such as collapse,hump and pores during laser welding.Aiming at the problems of poor weld formation and low joint performance of magnesium alloys,butt welding experiments were carried out on AZ31B magnesium alloy sheets with a thickness of 5 mm using adjustable ring mode fiber laser.The formation of fiber laser weld with adjustable ring mode of magnesium alloy,porosity,and microstructure and mechanical properties of joint were studied.The main work is as follows:(1)The influence of the laser power ratio of the central beam on the macro-forming of the weld was studied.The research shows that only when the laser power ratio of the central beam is 55%,the cross-section of the weld is smooth without obvious expansion,and the overall shape is narrow "V".Compared with single laser welding,magnesium alloy adjustable ring mode fiber laser welding can improve weld formation,but there are strict requirements for the power ratio of center beam and ring beam.The laser power ratio of the central beam is 100%,the cross-section of the weld is "I"-shaped,and there are serious depressions and hump defects on the surface of the weld.When the laser power ratio of the central beam is 55%,the weld formation is the most stable,and there are no obvious depressions and humps on the weld surface.When the central beam laser power ratio increases or decreases,the hump and pit defects increase,resulting in poor forming.In addition,when the proportion of the laser power of the central beam is low,the melting width of the upper surface of the weld is large.(2)The influence of the laser power ratio of the central beam on the porosity was studied,and the dynamic behavior of the molten pool and the porosity suppression mechanism were analyzed.The study found that when the laser power ratio of the central beam is 100%,the number of pores is the largest,the porosity is the highest,the volume of a single pore is larger,and the shape is irregular.As the proportion of the laser power of the central beam decreases,the number of pores decreases,the porosity decreases,and the volume of a single pore decreases.When the laser power ratio of the central beam is 55%,the number of air holes is the smallest,and the volume of a single air hole is the smallest.When the laser power ratio of the central beam continues to decrease,the porosity increases instead and the volume of a single pore is larger.The high-speed camera observation found that compared with only the center beam laser welding,when the center beam laser power accounted for 55%,the fluctuation of the depth direction of the small hole during the welding process was small,the opening of the small hole was larger and the outline was clear;Visible spatter,very stable melt width,and therefore low porosity.In addition,because the molten pool on the upper surface is wider,the cooling rate is relatively slow,and the pores have more opportunities to escape from the molten pool,which also reduces the porosity to a certain extent.(3)The microstructure and joint properties of the welds with different center laser power ratios of the ajustable ring mode fiber laser were studied.The results show that the grain size is smaller when only the center beam is welded.During the ajsutable ring mode laser welding,the grains in the area where the ring-beam acts are slightly coarsened.During pure central beam welding,the hardness value of the weld zone is higher than that of the base metal,and the microhardness value decreases with the reduction of the laser power ratio of the central beam.The joint tensile test shows that when the central beam laser power accounts for 55%,the tensile strength of the welded joint can reach 230.3 MPa,reaching 85.3%of the base metal,and the elongation is 7.2%,reaching 58.5%of the base metal.The fracture position of the adjustable annular spot welded joint is on the center side of the weld,showing a brittle-ductile mixed fracture.(4)The ajustable ring mode fiber laser double-sided welding experiment was carried out,and the surface forming,metallographic structure and joint performance of the double-sided welding were analyzed.The results show that the upper and lower welds are completely connected by double-sided welding,and the surface of the weld is in a typical fish scale shape with good consistency.There are mainly columnar crystals near the fusion line of the weld boundary,and equiaxed crystals are mainly distributed in the center of the weld.The tensile strength of the adjustable ring mode laser double-sided welding head can reach 244.9 MPa,which is 90.7%of the base metal,and the elongation is 7.6%,which is 62.0%of the base metal.