| Due to the advantages of high strength and low specific gravity,2024 aluminum alloy has a wide range of applications in aerospace industry.Laser welding has the advantages of small heat input,fast speed,good flexibility,small deformation and high precision.It is an important process for aluminum alloy welding.However,the mechanical properties of welded components are reduced by the coarse microstructure.It is of great significance to study the quantitative relationship between the microstructure and mechanical properties of welded joints.For the porpose of optimizing the welding process and improving the overall mechanical properties of welding joints,it’s important to understand the mechanical properties of joints under different processes.However,it is difficult to obtain the phase properties of aluminum alloy weld joint.The distribution of microstructure is random and the characteristics are difficult to extract.Therefore,it is difficult to understand the quantitative influence of microstructure on mechanical properties through experiments.Combined with welding process analysis and experimental characterization techniques,a mesoscopic finite element model based on the microstructure of laser welded joint of aluminum alloy is established in this paper.The mesoscopic stress-strain evolution law of weld under external load and its influence on macroscopic mechanical properties are systematically studied.The optimization methods of microstructure and mechanical properties of weld are proposed.The main conclusions are provided as follows:(1)A mesoscopic finite element model based on realistic duplex microstructure characteristics and phase mechanical property of 2024 aluminum alloy laser welded joint was developed.(1)Considering the scale effect and the influence of residual stress,the stress-strain curves of aluminum matrix and eutectic phase were obtained by reverse analysis based on the nano-indentation test.(2)A mesoscopic finite element model was established based on the actual weld microstructure,and the characteristics of strain inhomogeneity of joints under different external loads were analyzed.(3)The accuracy of the prediction results was verified by mesoscale local misorientation and macroscopic tensile test.The error of simulation result was less than 3.019%.(2)The influence mechanism of laser welding residual stress on mechanical properties of aluminum alloy plate was studied.Through the comparison experiment of laser welding of 2024 aluminum alloy of T4 state and O state,the welds with the same microstructure characteristics were obtained.The simulation results showed that the longitudinal residual stresses were respectively 220.8 MPa and 122.7 MPa.(1)The effect of residual stress on dislocation density and mechanical properties of phase were analyzed.It was found that the decrease of residual tensile stress led to the decrease of dislocation density and the increase of strength of the meso-phase.(2)The evolution characteristics of meso-strain distribution and macroscopic deformation behavior of welds with different residual stresses under external load were studied.It was found that the increase of strength of phase with low residual stress led to the increase of macroscopic yield strength and tensile strength of weld.The residual stress of the weld decreased by 98.1 MPa,resulting in an increase of 18.3 MPa in the strength of the welded joint.(3)The effect of microstructure distribution on mechanical properties of aluminum alloy weld was revealed.(1)The microstructure and macroscopical mechanical properties of equiaxed and columnar grain zones in typical aluminum alloy welds were analyzed.It was found that the eutectic phase stress in equiaxed grain zone was great under the same longitudinal external deformation load.(2)By comparing the meso-strain distribution of weld with different secondary dendrite spacing,it was found that the strain uniformity of refined secondary dendrite was improved,and the strain concentration area was reduced.(3)The meso-strain evolution of welds with different eutectic phase widths and the global average deformation behavior of representative volume elements were studied.It was found that the decrease of width of eutectic phase led to the increase of the bearing capacity and the tensile strength for the weld.The width of eutectic phase decreased from 0.95 μm to0.86 μm,resulting in the increase of the tensile strength from 316.6 MPa to 335.4 MPa.(4)Based on the above research,two optimization methods for weld microstructure and mechanical properties were proposed including optimization of laser welding process parameters and optimization of laser welding energy trajectory.The optimization mechanism was clarified through simulation analysis and experimental verification.(1)By opyimizing the laser welding process parameters,the following relationships were found.When high laser power matching with high welding speed,the welding heat input decreases.This can promote the increase of cooling rate of weld,decrease the secondary dendrite spacing and eutectic phase width of weld.It was revealed that the strain inhomogeneity of the weld with low heat input was reduced and the strength of eutectic phase was increased,thus the tensile strength of the weld was increased.When the heat input decreased from 50J/mm to 35.7 J/mm,the macroscopic tensile strength of the weld increased by 13.3%.(2)By adjusting the energy trajectory with the optimized laser oscillating welding process,dendrite fragmentation and grain nucleation in the melting zone could be promoted,and the secondary dendrite spacing and eutectic phase width could be reduced.Through macroscopic thermal and mechanical simulation,it was found that the heat source energy deposition model of laser oscillating welding process showed double peaks distribution,which reduced the average longitudinal residual stress in the weld center.The above synergistic optimization of microstructure and residual stress improved the tensile strength of weld under laser oscillating welding process.When the laser power was 4200 W,the welding speed was 45 mm/s,and the vibration amplitude was 2 mm,and the oscillating frequency was 100 Hz.Under the conditions of above process parameters,the tensile strength of the weld could be increased to 373.0 MPa. |
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