Phase Field Investigation On Microstructure Evolution In The Laser Welding Pool Of 2A14 Aluminum Alloy

2A14 aluminum alloy has been widely used as one of welding structure materials in the fields of aerospace,aerospace and other vehicles due to its good mechanical properites.Laser beam welding is one of the main joining methods of 2A14 aluminum alloy.Solidification behaviors of the welding pool can critically affect the performance of welded joints.Thus,it is of importance and significance to investigate solidification behaviors and resulting microstructures in the laser welding pool of 2A14 aluminum alloy.This paper takes the laser beam welding of 2A14 aluminum alloy as the research object,the dynamic solidification process during the laser beam welding of 2A14 aluminum alloy was studied by the phase field model.The microstructural evolution during the laser welding was quantitatively studied.In addition,effects of welding parameters on solidification microstructures were also quantitatively investigated.Understanding the dynamic solidification process and microstructural characteristics during the laser welding can provide useful guidance for investigating the formation of porosities and cracks during the solidification of the welding pool,and also lay a theoretical foundation for welding genetic engineering.Firstly,laser beam welding experiments of 2-mm 2A14-T6 aluminum alloy plates was conducted.Effects of welding parameters on weld formation,microstructures,porosities and mechanical properties were investigated.Best weld formation and performances have been obtained.Experimental microstructures can be used to validate simulation results.A phase field model for solidifiction of the welding pool was developed through revising a quantitative phase field model for directional solidification.The phase field model can couple with the dynamic temperature gradient and pulling velocity during the solidification of the welding pool and be used to quantitatively simulate microstructural evolution in the welding pool.Finite difference method and central difference scheme were used to solve the phase field model.Meanwhile,effects of the flow field on solute diffuse during solidification of the welding pool were considered,a microscopic flow field model was proposed and coupled in the quantitative phase field model.The modified Navier-stokes equation,staggered-grid technique and SIMPLER were used to solve the flow field model.In addition,solidification of the welding pool is a non-linear process,so the dynamic solidification conditions during the laser welding need to be established.During the solidification of the elliptical weld pool,the transient temperature gradient decreases with time,the transient pulling velocity increases with time.During the solidification of the teardrop-shaped weld pool,the transient temperature gradient decreases with time,the pulling velocity is a constant.The quantitative phase field model coupled with the flow field and the dynamic solidification conditions were used to simulate microstructural evolution during the laser welding of 2A14 aluminum alloy.Effects of welding parameters on solidification microstructures was also quantitatively investigated.The diffuse interface is solidified by cellular solidification way and the resulting columnar grains have the same characteristics of cellular morphology.The columnar grians spacing decreases with the increasing laser power,welding speed has little influence on the columnar grians spacing.The simulation results are in good agreement with experimental measurements.In order to investigate columnar grains growth orientation during the welding,the phase field model and the dynamic solidification conditions need to be revised.The columnar growth orientation angle need to be added into the models.The formation of sloping columnar grains during the laser welding and curved columnar grains during the arc welding was predicted by using the revised phase field model coupled with columnar grains growth orientation and the equations of temperature gradient,pulling velocity and columnar grains growth orientation.The columnar growth orientation angle decreases with the increase of the welding speed.Moreover,competitive growth of columnar grains with different growth orientations is also predicted by the revised phase field model.The simulation results give a good agreement with experimental findings.In this paper,the quantitative phase field model and the equations of temperature gradient,pulling velocity and columnar grains growth orientation can be used to quantitatively simulate the solidification mode,solute diffuse,columnar grains morphology,spacing and growth orientation during the welding.The simulation results are in good agreement with the classical solidification theory and experimental measurements,which lays a theoretical foundation for welding genetic engineering.