Research On Process,Microstructure And Properties Of Aluminum-lithium Alloy Sheet With Back Reflection Induced Synergistic Laser Welding

Lightweight manufacturing of aviation aircraft has always been a research hotspot and an unremitting pursuit goal in the field of aerospace.The deep application of new materials represented by Al-Li aloy and the replacement of traditional riveting technology by laser welding technology are two trends that have attracted much attention in the lightweight manufacturing of aviation aircraft.However,there are some problems such as low energy utilization and welding defects in laser welding of Al-Li aloy at present.Back reflection induced synergistic laser welding is a new technology of high-efficiency and low-threshold laser welding proposed by the research group.It can achieve high quality near-X-shape weld forming under low energy threshold condition that conventional laser welding can only achieve near-V-shape weld forming.Aiming at the existing problems of laser welding with Al-Li al oy,this paper explores the feasibility of high-efficiency and high-quality weld forming of 2198 AlLi aloy sheet used back reflection induced synergistic laser welding.In the study,by using numerical analysis and experimental research methods,the temperature field distribution,microstructure and properties,weld forming of Al-Li aloy sheet with back reflection induced synergistic laser welding was studied systematically.Based on this,the heat source model parameters which have important influence on the numerical analysis of back reflection induced synergistic laser welding are further explored.The purpose of this study is to provide reference for the realization of lightening of aircraft by laser welding of Al-Li aloy and further enrich and develop the theories related to back reflection induced synergistic laser welding.The main research work and the main research results are as follows:(1)Numerical simulation and analysis of the temperature field of Al-Li aloy sheet with back reflection induced synergistic laser welding were carried out.Based on the back reflection induced synergistic laser welding heat source model parameters estimated from previous research results,a three-dimensional transient finite element analysis model of the welding process was established and verified by experiments.The temperature field distribution of the melting pool of 2198 Al-Li aloy sheet under different laser power densities and different line energies was analyzed.The results show that with the increase of the laser power density,the welding mode changes from thermal conduction welding to deep fusion welding,and at the same time from the edge to the weld center,the temperature gradient on the back of the weld shows a change trend of first increase and then decrease,and the temperature gradient at the center of the weld is the smallest.Meanwhile,the weld width and depth are proportional to the line energy.With the decrease of the line energy,the weld pool on the front and back of the weld decreases,and the corresponding weld width decreases accordingly.(2)Research on the forming characteristics of Al-Li aloy sheet with back reflection induced synergistic laser welding.Base on numerical simulation analysis and exploratory experiment,the effects of laser power density and linear energy density on the weld formation of 2198 Al-Li aloy sheet with back reflection induced synergistic laser welding were studied.The results show that the experimental and simulated results of the effect of laser power density and linear energy density on weld forming are basically consistent.Meanwhile,under the experimental conditions in this paper,the optimal process parameters are laser power 1300 W,welding speed 2600mm/min,and defocusing amount 0mm.In addition,the metalographic morphology of the cross section of weld shows the existence of the reflective and synergistic energy field induced by the reflective gasket on the back of the weld.(3)Research on the microstructure and properties of joints of Al-Li aloy sheet with back reflection induced synergistic laser welding.The microstructure,hardness distribution,tensile strength and fracture morphology of the welded joints in different regions were investigated by taking the welded joint under the optimal process parameters,and compared with conventional laser welding under the same process parameters.The results show that the microstructure,hardness distribution and tensile strength of welded joints obtained by the two methods are basically same under the optimal process parameters.However,the molten pool area of back reflection induced synergistic laser welding is 26.57% larger than that of conventional laser welding under the same process parameters,which indicates that back reflection induced synergistic laser welding can realize high-efficiency laser welding of Al-Li aloy sheet.According to the analysis,microstructure and properties of the weld joints with back reflection induced synergistic laser welding under the experimental conditions in this paper did not show a good improvement effect,which may be due to the limited guiding significance of the assumed heat source model parameters based on the test results of pre-welding titanium alloy in the simulation analysis.(4)The parameters of double heat source model for back reflection induced synergistic laser welding based on inversion algorithm were explored.The mathematical description equation of double gaussian composite heat source model for back reflection induced synergistic laser welding was established.The inversion algorithm is introduced and the mode search method is used to calculate the energy ratio coefficients of the two heat sources under different reflection energy fields in the dual gaussian composite heat source model.The simulation results and experimental results under different reflection energy fields are compared and analyzed.The results show that effective radius of the surface heat source and the body heat source remain unchanged but the surface heat source energy ratio decreases gradually with the increase of reflected energy field distance.In other words,the thermal effect of the reflected energy field on the back will gradually decrease or even disappear with the increase of the reflected energy field distance.This result quantifies the energy proportion of double heat sources in the process of back reflection induced synergistic laser welding and provides a more accurate heat source model parameter for the numerical analysis of back reflection induced synergistic laser welding in the future.