Study On The Laser Weldability And Welding Technology Of High Strength Aluminum-Lithium Alloy 2195

Aluminum-lithium alloy,due to its low density,high specific strength,high specific stiffness advantages,has been considered as an ideal light-weight and high-performance material in aerospace application.Using conventional joining methods such as TIG and MIG faces many problems,including the presence of porosities and hot cracking,high structural distortion,and serious mechanical properties degradation which caused by great heat input,making the welded Al–Li structure not meet the requirement of aircraft performance.Compared with arc-welding process,laser welding is a high-power-density,high-welding-speed and low-heat-input welding process that produces high aspect ratio welds with a relatively narrow heat affected zone,low structural distortion and good mechanical properties.Consequently,people show more and more attention on the development of it in recent years.In this paper,high strength aluminum-lithium alloy 2195-T8 with thickness of 3.7 mm is welded by using high power CO₂ laser and fiber laser welding method.The effects of welding process parameters on weld formation and defects of porosity and hot cracking are analyzed,also differences of these two welding methods on weld formation are compared.Based on this,through process optimization,well-formed and defect free butt joints can be obtained.The microstructure and mechanical properties of typical joints are studied.Meanwhile,the microstructure and mechanical properties of joints after post-weld heat treatment are investigated.This work will provide practical experience and technical support for Al-Li alloys expansion application in laser welding.Systematic investigation and analysis were carried out on the effects of laser power,welding speed on the weld front width,back width,cross-section,also weld porosity and hot crack.On this basis,through process optimization experiments of preheating before welding,reducing the cooling speed,good appearance of welded joint without defects are obtained.Comprehensive comparison of CO₂ laser and fiber laser welding are analyzed,CO₂ laser welding is better for weld formation due to its broader parameters window,more stable welding process,more homogeneous penetrating in the back and less surface defects.Systematic observation and characterization were carried out on microstructure and mechanical properties of joints.The fusion line area consists fine equiaxed grains with size of 3⁵ microns,which is related with Li,Zr,Ti etc alloy elements.The microstructure in weld seam transforms from columnar grains to dendritic grains to equiaxed grains.Lots of??Al?+???Al₂Cu?eutectic phase forms on the grain boundary,whereas little amounts precipitation of S'?Al₂CuMg?and T_B(Al_7.5.5 Cu₄Li)precipitates in the fused zone.EDS analysis results show that the main solute elements Cu,Mg exhibits a obvious macro-segregation and micro-segregation in the weld seam.Microhardness results show that microhardness of weld seam is lower than that of HAZ and base metal.The microhardness of weld seam has dropped 32.3%of that of the base metal.Tensile test results indicates that tensile strength of weld joint has decreased to 296 MPa,which is 51%of the base metal and ductility decreased to 0.93%.The fracture occurred in weld seam,shows evident characteristic of intergranular brittle rupture.Comprehensive comparison of microstructure and mechanical properties of welded joints by CO₂ laser,fiber laser and traditional arc welding methods shows that the difference between two laser welding methods is not big,whereas they are superior to arc welding.Systematic investigation and analysis were carried out on the microstructure and mechanical properties of joints by PWHT.The weld seam grain size grew up,dendritic grains transformed into equiaxed grains.And lots of fine acicular phase T₁?Al₂CuLi?precipitates out at the grain boundary and internal grain,which makes the hardness and strength of weld joint has obviously improved.Compared to the as-welded joints,the hardness has increased by 25.6%,the tensile strength welding has increased by 13.7%,the fracture shows the same intergranular fracture characteristic.With the increase of aging time,the size of grain in the weld seam has grown up,precipitated phase gathered to coarsen,which makes the hardness of weld seam increased,but the amount of increase is not much.