晶粒尺寸对双激光束双侧同步焊接接头力学性能的影响

Effect of Grain Size on Mechanical Properties of Double Laser-Beam Bilateral Synchronous Welding Joint

用ER4047焊丝对厚度为2.0 mm的2060/2099铝锂合金进行双激光束双侧同步焊接实验,定量分析了晶粒尺寸对铝锂合金T型焊接接头力学性能的影响。实验结果表明,等轴细晶区内晶粒近似为圆形,当焊接热输入为46.16 J/mm时,晶粒的尺寸最小。随着随焊接热输入的增大,等轴细晶区的宽度逐渐增大,柱状晶尺寸逐渐减小,铝锂合金T型接头的焊缝熔合线区域发生局部软化现象,导致该区域内的显微硬度最低。T型环向拉伸实验结果表明,下熔合线区域为接头薄弱区,该区域内晶粒尺寸对焊接接头力学性能的影响也最大。焊趾处为应力集中区域,也是拉伸断裂的起始点。拉伸断裂过程中,裂纹由焊趾处逐渐扩展至下熔合线,最终在下熔合线的蒙皮处断裂。研究结果表明,熔合区内的晶粒尺寸越小,焊接接头的力学性能越好。

Objective Compared with the traditional Al-Li alloy,the Al-Li alloy weighs less and has high stiffness,making it more conducive for manufacturing aerospace components.However,because of its low boiling point,high thermal expansion,and high thermal conductivity,a heat source with concentrated energy is more suitable for welding Al-Li alloys.Based on the unique welding structure of aircraft fuselage panels,a novel technology of dual laser-beam bilateral synchronous welding(DLBSW)is proposed and applied in the manufacturing process to ensure the fuselage shape and improve the welding efficiency and quality.During the DLBSW process,improper heat input affects the temperature gradient and solidification speed of the metal in the molten pool,resulting in coarse grains of the T-joints,which is unconducive to improve the macroforming and overall mechanical properties of welded components.Here,we analyze the grain morphologies and sizes in different regions of the joints under different welding parameters and explored their influence on the mechanical properties of welded joints,providing reference and guidance for further improvement of the mechanical properties of welded joints.Methods Here,2060(500 mm×125 mm×2 mm)and 2099 Al-Li alloy sheets(650 mm×28 mm×2 mm)are used as the skin and stringer,respectively.A 1.2 mm diameter ER4047 Al-Si welding wire is used as the filler material.The chemical compositions of the base metal and welding wire are shown in Table 1.Before welding,the sample surface should be chemically cleaned to remove the oxide film and oil stain.The welding experiment is conducted using a double laser-beam welding system(Fig.1).Based on the preliminary welding test and comprehensive analysis of the welding seam forming quality,four better welding parameters are selected in the experiment(Table 2).After welding,the metallographic sample of the cross-section of the T-joints is cut by wire cutting technology and inlaid with epoxy resin.Next,the metallographic sample is polished and etched with Keller's reagent.Furthermore,the microstructure and grain size of the joints are analyzed using a metallographic microscope.Also,tensile tests are performed on the specimen until fracture,and the fractured specimen surface is observed by transmision electronic microscopy(SEM).Results and Discussions Based on the T-joint microstructure,the grain morphology from the upper fusion line to the weld center in the solidification process are equiaxed fine grains,columnar dendrites,and equiaxed dendrites(Fig.3).The average width of EQZ WEQZ increases with the increase in welding heat.With a heat input of 43.64 J/mm,the equiaxed fine crystal band narrowed down,its width is 2--3 times as long as the grain size,whereas with a heat input of 48.00 J/mm,the width of the crystal band increases to 4--5 times as long as the grain size(Fig.4).When the heat input is low,because of a decrease in the temperature gradient,the dendrites of equiaxed fine grains grow and become columnar crystals,decreasing the WEQZ,even though it cannot be observed.Also,with low input,the ratio of the temperature gradient to the crystallization speed in the columnar crystal is larger,and the growth of the columnar crystal nucleus becomes difficult.Thus,with low heat input,the columnar crystal grains are smaller.From the microhardness distribution results,the microhardness values of 2060 and 2099 Al-Li base metals are the highest,followed by the heat-affected zone and weld center.The microhardness in the fusion zone is the lowest(Fig.5).No EQZ exists at the weld toe,which is composed of coarse and short columnar crystals.The stress concentration in the weld toe because of the structural mutation and the grain size is more significant than that in the EQZ zone,where dislocation slip is more likely to occur under the action of an external force.Therefore,the weld toe is the starting point of the T-joint fracture(Fig.6).Conclusions In this study,the heat input in the range of 43.64--48.00 J/mm of DLBSW welding is investigated to explore the mechanical properties of 2060 and 2099 Al-Li alloy T-joints affected by different grain morphologies and size characteristics.The mechanical properties of the weld can be controlled by changing the welding process parameters,which affect the grain structure characteristics of the welding joint,especially near the fusion zone.With the increase of heat input,the heat-affected and partially molten zones in the T-joint widens,and the width of equiaxed fine-grained zone increases;the grain size of equiaxed fine-grained zone first decreases and then increases with the increase in welding heat input,and the grain size of columnar dendrite increases with the increase in heat input.Based on the average hardness,tensile properties,and fracture morphology of the joint,it can be concluded that with a heat input of 46.16 J/mm,the mechanical properties of T-joint are the best,and the tensile strength can reach 335.7 MPa.Hence,reducing the grain size in the EQZ of welded joints can significantly improve the mechanical properties.Therefore,the mechanical properties of the joint can be improved by changing the welding process parameters to limit the grain growth trend.