| Aluminum-lithium(Al-Li)alloy is a promising light-weight structural material.With the advantages of high energy density and automation and excellent welding accuracy,laser welding has become one of the most prospective processing techniques for producing Al-Li alloy thin-walled structures.However,the property weakening,closely related to microstructure inhomogeneity,in Al-Li alloy laser-welded joints restricts their wide applications in industry.It has been well-known that the typical fine equiaxed zone(FEQZ)characteristics of Al-Li alloy fusion-welded joints are the key factors determining the joint property.It is thus of high significance to reveal the formation,distribution,and effect of FEQZ on the joint mechanical behavior under laser welding conditions for the property control and improvement of Al-Li alloy laser-welded joints.In this work,a 1.2 mm 2A97-T3 Al-Li alloy laser-welded joint with welding wires ER2319,ER4047,and ER5356 was investigated.Characterization techniques and digital image correlation(DIC)were used to analyze weld formation,microstructure,and performance of joints.In particular,the microstructure characteristics and influence of FEQZ on the joint mechanical behavior were systematically studied.Combined with finite element method(FEM),the FEQZ formation mechanism and joint microstructure evolution were discussed to establish the relationship between weld formation,microstructure,and property of laser-welded joints.The main conclusions are as follows:Under the conditions of laser welding process parameters used in this work,there were no hot cracks formed in all the welds.The hydrogen-induced pores were distributed along the fusion line,and the porosity of ER2319 weld was the lowest.Furthermore,it was found that there were differences in the geometric morphology of these three kinds of welding wire welds.The analysis showed that the weld geometric size was affected by the welding wire composition.Moreover,the joint microstructure of the three welding wire welds was similar,which were composed of weld,heat affected zone(HAZ),and base metal(BM).The weld was further divided into FEQZ,columnar zone,and equiaxed dendrite zone.Among them,the grain size of equiaxed dendrite zone in ER2319 weld was the smallest.The study showed that the distribution range of FEQZ in laser-welded joints was different from that of traditional fusion-welded joints.The FEQZ in laser-welded joints was distributed along the fusion line and the molten pool oscillation crystallization line.At the junction of the fusion line and the molten pool oscillation crystallization line,it was easy to form an aggregated distribution region of FEQZ.FEQZ consisted of fine equiaxed grains with random crystallographic orientation and the grain size was less than 20 μm.The average diameter of FEQZ did not exceed 30%of that in equiaxed dendrite zone.There were a lot of fine and dispersed precipitates and high-density dislocations in grain interiors,and grain boundaries were decorated by continuous coarse precipitates.Combined with the numerical simulation results of laser welding temperature field,the formation mechanism of FEQZ in laser-welded joints was clarified: the formation was affected by the flow behavior of molten pool and the temperature field of joint;When the regional temperature was in the range630-660 °C,it was beneficial to the formation of FEQZ.The grain nucleated on the heterogeneous phase surface at the front of the solid-liquid interface with large temperature gradient.The micro-texture of the upper surface,transverse section,lower surface,and weld longitudinal section of Al-Li alloy laser-welded joints were examined by EBSD to obtain the microstructure spatial information.HAZ showed a weakening of deformation texture and little change in shear texture of BM,which produced a weak recrystallization texture.The weld was the region containing the most significant micro-texture evolution in the joint,which completely eliminated the micro-texture characteristics of BM.FEQZ reduced the crystallographic preferred orientation intensity of columnar zone by changing the nucleation position of the columnar crystal,and increased the crystallographic orientation inhomogeneity of weld.Finally,it was found by using DIC that property of different microstructure region of the joint was axisymmetrically distributed along the weld center line,and the deformation resistance increased with the increasing of distance from weld center line.Based on the experimental results obtained by DIC,a finite element model with high accuracy was established considering the continuous inhomogeneity of mechanical properties of welded joint.The tensile behavior of joint was calculated by FEM to study the local true stress during the tensile deformation process.It was found that the fusion line was the weakest region in joint before the necking of weld.The results showed that when FEQZ with aggregated distribution was located at the edge of joints,the alternating FEQZ and columnar zone increased inhomogeneity of the microstructure and crystallographic orientation of the weld,which raised the crack tendency in fusion line.The crack was formed within FEQZ and propagated preferentially along grain boundaries of FEQZ.By controlling weld formation to reduce the thickness-to-width ratio of weld,it could effectively avoid the formation of cracks near fusion line and promote the occurrence of cracks in equiaxed dendrite zone.This was beneficial to increase the crack propagation path and delay the fracture to improve properties of Al-Li alloy laser-welded joint. |
