LAZ931 Magnesium-lithium Alloy TIG Welding And Laser Welding Welding Processes And Properties Study

Magnesium-lithium（Mg-Li）alloy is currently the lightest metal structure material,and it has the characteristics of high specific stiffness,low density,and high specific strength,which has the ideal effect in equipment weight reduction.To realize the industrial application of Mg-Li alloy,its connection technology is very critical.At present,there is less research on laser welding and TIG welding of Mg-Li alloy,its related research is still in the beginning stage,and TIG welding heat-affected zone is wider,for the properties of the joints weak areas,the two welding methods to obtain the properties of the joints has decreased,the welding method for Mg-Li alloy needs more in-depth and systematic research.In this paper,laser welding and TIG welding process exploration and microstructure properties study of 2.15mm thick LAZ931magnesium-lithium alloy plate,in order to obtain excellent properties of the welded joints,the main research content and results are as follows:In the LAZ931 Mg-Li alloy laser welding process exploration,the use of single-sided welding double-sided forming method,when the heat input at 25-40KJ/m,it can get well formed,defect-free welded seam zone;In the TIG welding experiment of double-sided welding with double-sided forming,different pulse parameters（base current,peak current,pulse frequency）are used to obtain specimens with smaller deformation and cracking tendency,high formability,and excellent welded seam zone.The results show that the melt width of the welded seam zone tends to increase as the base current increases;when the peak current is too high,the melt width of the welded seam zone increases significantly;as the pulse frequency increases,the melt width of the welded seam zone decreases.Laser welding research shows that the welded seam zone is composed of fine needle-likeα-phase,β-phase and granular precipitation phase;the heat-affected zone is composed of coarseβ-phase,incompletely meltedα-phase and irregular precipitation phase,with the increase of heat input,theαphase andβphase in the welded seam zone and heat-affected zone become coarse,and the size of irregular precipitation phase particles in the heat-affected zone becomes smaller.After laser welding,the over-aged softened phases Li Al and Li Mg Zn phases in the base metal disappeared under different heat inputs,and new phases Mg Zn₂,Li Mg₂Zn₃and Al₃Mg₂phases were formed in the welded seam zone.The microhardness test showed that the microhardness value at the welded seam zone was the highest,the heat-affected zone was the second highest,and the microhardness at the base metal was the lowest.With the increase of heat input,the microhardness values at the welded seam zones reached 112.3 HV,107.5 HV,84.8 HV,showing a decreasing trend,and the microhardness value at the welded seam zones under different heat inputs were much higher than the base material;the tensile strength and elongation after break showed a decreasing trend,the tensile strengths were 177 MPa,173 MPa,and 166 MPa,respectively,and the elongations after break were 38%,34.5%,and26%,respectively,fractured in the base metal region.The pulse TIG welding experiments on LAZ931 Mg-Li alloy show that the welded seam zone organization are cytosolicα-phase dendrites,β-phase equiaxed crystals and granular precipitation phases,and after pulse TIG welding,the content of the over-aged softened phase Li Al phase in the base metal is reduced,and a second phase Li Mg Al₂phase is generated;Depending on the distance to the center of the welded seam zone,the heat-affected zone exhibits two different morphologies:Theα-phase in the heat-affected zone near the base metal is partially ellipsoidal,and there is a needle-likeα-phase distributed in theβ-phase,and theβ-phase shows equiaxed crystal morphology,and theβ-phase is coarser than that in the base metal;Theβ-phase in the heat-affected zone near the welded sea zone becomes coarse equiaxed crystal morphology,and the precipitatedα-phase is coarser than that in the heat-affected zone near the base metal.Studies have shown that the pulse parameters have a greater impact on the performance of the welded joint.Microhardness curves are"M"type,the highest microhardness values at the heat-affected zone.When the base current increases from10 A to 30 A,the tensile strength and elongation after fracture of the welded joints increased,when the base current of 30 A welded joints fracture in the base metal location;when the peak current increases from 50 A to 70 A,the tensile strength and elongation after fracture of the welded joints appeared to be enhanced and then weakened,when the peak current was 60 A,the welded joint fractured at the base metal location.When the pulse frequency was increased from 2 Hz to 6 Hz,the tensile strength of the welded joint increased from 73.8 MPa to 181.95 MPa,the elongation after break increased from 8.3%to 27.6%,and the welded joint fractured at the base metal location when the pulse frequency was 6 Hz.It was found that when the peak current is too high,an appropriate increase in pulse frequency can make a significant improvement in the property of the welded joint.