Study On Laser Welding Technology Of LZ91 Mg-Li Alloy

With the development of society and economy,the country has also paid more and more attention to saving resources and protecting the environment.The concept of“lightweight”came into being.Magnesium alloy has the advantages of small density,high specific strength,high rigidity,easy recovery,etc.It has been widely used in the automotive,railway and aerospace industries,and with the increasing demand for environmental protection,especially the demand for carbon dioxide emission reduction,magnesium The application of alloys and other environmentally friendly lightweight materials is also actively progressing,and as the world's lightest alloy,magnesium-lithium alloy has a wide range of applications and prospects.To truly realize industrial applications,its connection technology is the most important thing.However,the melting point of magnesium and lithium is extremely low,and the metal is highly active.Evaporation loss and oxidation easily occur during the welding process.Laser welding has the advantages of small heat input,fast cooling speed and high degree of automation.Therefore,using a laser as a light source can be an effective method to solve the above problems.In this paper,the full-factor experiment of flat-panel laser welding of Mg-9Li-Zn Mg-Li alloy was first performed,and the welding process window was established.The effects of welding power,welding speed,shielding gas flow rate,defocusing amount and line energy on the welding seam formation of laser welding magnesium-lithium alloy plate splicing welding were studied in detail.It was found that good welding joints could be obtained only when the welding power was 1000~1600 W,the welding speed was 2~4m/min,the protection gas flow was 0.6~0.9 m~3/h,and the line energy was between 24~48kJ/m.Through the high-speed imaging of the keyhole plasma,it was found that when the on-line energy is between[14 KJ/m,24 KJ/m]and power[700,900],the molten pool just reached the bottom of the test plate,the weld width is small,and the bottom of the weld will be The collapse occurs because the surface tension is difficult to support the gravity of the weld metal.Once the keyhole passes through the test plate,a well-formed weld seam is formed,the melt width is significantly increased,and the force of the liquid metal reaches a stable balance.In the following,the microstructure of the weld is studied in detail in this paper.There are heat-affected zone,columnar crystal zone and equiaxed crystal zone in the laser welded magnesium-lithium alloy welding joint.The base material is in the as-rolled state,and the?-phase growth strips are distributed in the?-matrix phase.The center of the weld is equiaxed,which is the result of the rapid cooling of the laser welding.The center component of the weld is extremely cold.After the eutectic transformation of the?and?phases,a supersaturated solid solution is formed.Part of the?phase precipitates at the grain boundary first..Then,the supersaturated solid solution continues to cool and precipitates into fine needle-like structures uniformly distributed within the crystal grains.The alpha phase size at the heat affected zone is also larger but less than the parent material size.This is because there is a liquefaction zone in this area,and a portion of the alpha phase is liquefied to form dendrites and broken into the matrix.The XRD patterns of the base metal and the weld were analyzed.It was found that the phase(?)of the softening phase(MgLiZn)originally existing in the base metal after laser welding disappeared.This obviously increases the hardness and strength of the weld.Further,the microhardness of the weld was measured.From the center of the weld to the heat affected zone and to the base metal,the microhardness showed a decreasing trend.The weld center can be up to 73.75 HV,an increase of 37.2%over the parent material53.75 HV.After laser welding,after the joints with good cold wind shape were stretched,they were all broken in the base material zone,indicating that the strength of the weld was greater than that of the parent metal.After the beveling of the weld seam,the fracture is in the heat-affected zone after stretching.The final UTS of the joint can reach 188.47 Mpa,and the UTS of the parent material is 155.58 Mpa.The corrosion resistance of the weld and the base metal was investigated.It was found that the self-corrosion potential of the base metal was-1.62 V,and the corrosion potential of the weld was-1.25 V.After laser welding,the weld microstructure is more uniform and refined,which can significantly improve the corrosion resistance of the structure.Finally,the magnetic field-assisted laser welding of LZ91 magnesium-lithium alloy welding experiments.It was found that for a 2 mm thick Mg-Li alloy,the magnetic field can significantly improve the weld formation when the magnetic field strength is?200mT.For the previous single laser 800 W,the 2 m/min welding was found to have collapsed,and it was found that with the increase of the magnetic field strength,the amount of the fall was relatively reduced,and after 200 mT,there was almost no collapse.Observing the microstructure,it was found that after the magnetic field was added,the weld area was more uniform,and the fishbone-like structure appeared in the parent material no longer appeared,and the grain orientation at the grain boundary became disorganized.The standard deviation of the microhardness of the weld is reduced,indicating that the microstructure is more uniform,which is the result of the effect of the magnetic stirring force.