The Microscale Simulation Of Structure Evolution During Solidification Of 2A12 Aluminum Alloy Laser Welding Molten Pool

2A12 aluminum alloy has high specific strength and good performance in high and ultra-low temperature.It is usually used to fabricate aerospace structures,such as aircraft skin and purlin.Laser welding is widely applying in the joining of 2A12 aluminum alloy due to high energy density,small heat input and little deformation.However,jointing softening and tensile reduction occur after 2A12 aluminum alloy laser welding.During the solidification process of laser welding molten pool,the microstructure evolution siginificantly affects the morphology and size of fusion zone microstructure,which affect the jointing softening and tensile reduction.Therefore,understanding the microstructure evolution during solidification process in the molten pool and figuring out the influence factors are important for improving quality of laser welded joint of 2A12 aluminum alloy.High-speed photography of molten pool and observation of fusion zone microstructure are applied to indirectly study the solidification of molten pool.However,there is still a lack of effective experimental means to direct observe the solidification of molten pool.With the rapid development of computer hardware and computational materials science,numerial simulation develops as a new method for studying the microstructure evolution during the solidification of the molten pool.Based on the above background,laser welding of2A12 aluminum alloy was chosen as the object of study in this work.Phase-field model was applied to study the critical factors that affect epitaxial growth,competitive grain growth and CET(columnar-to-equiaxed transition).Finally,the fusion zone microstructure of 2A12 aluminum alloy laser welding was predicted.The main results in this work are as follows:(1)The quantitative multi-scales phase-field model for nucleation and growth of dendrites during the solidification of 2A12 aluminum alloy laser welding molten pool was established.Considering crystalline orientations and nucleation of equiaxed grains,the microscopic model for dendrite nucealtion and growth was established according to the solidification condition in the molten pool.By the macro-micro mapping of temperature distribution,the macroscopic microstructure evolution model was established.(2)The influence of crystalline orientation on the dendritic growth of a single grain during the solidification of 2A12 aluminum alloy laser welding molten pool was studied.It was found that the deflection angle of tiled dendrites was equal to misorientation angle(the angle between crystalline orientations and the maximum temperature gradient).The results showed the crystalline orientation changed the growth direction of dendrites.The dendrites were titled.Besides,the deflection angle equaled to the misorientation angle.When the misorientation angle exceeded a threshold value,the dendrite tip kept splitting during the dendrite growth and the dendrite morphology turned into seaweed-like.(3)The influence of crystalline orientations on the competitive grain growth during the solidification of 2A12 aluminum alloy laser welding molten pool was studied.The mechanisms of dendrite lateral growth and grain boundary deflection to change sizes of grains were revealed.The law of grain boundary migration was studied.The results showed that the variation of grain size during the solidification was affected by 3 factors in the competitive dendritic growth between different grains:(a)the lateral growth of the leading dendrite at the competitve growth stage;(b)the deflection angle of converging grain boundary at the relatively steady growth stage;(c)the development of sidebraching of denrites at the diverging grain boundary.(4)The influence of nucleation parameters on the CET during the solidification of 2A12 aluminum alloy laser welding molten pool was investigated.The mechanical blocking mechanism was verified as the major blocking meachanism for the CET.When the solute concentration of the columnar dendrite tip was enriched to a certain extent after the solute layers of equiaxed grains and columnar grains interacted with each other,the dendrite tip velocity of the columnar dendrite started to decrease.However,the columnar dendrites kept growing.When there was no space for columnar dendrite growth due to the blocking of equiaxed dendrite,the columnar dendrites stopped growing.Mechanical blocking stopped the columnar dendrite growth.The small nucleation undercooling was and the bigger nucleation density was,the more likely the CET occurred.Therefore,choosing the appropriate nucleating agent and increasing the amount of nucleation agent promote the CET and refine the microstructure of fusion zone.(5)The microstructure of the whole macroscopic fusion zone of 2A12 aluminum alloy laser welding was predicted.The microstructure evolution druing the solidification of the macroscopic laser welding molten pool could be divided into 2 stage:(a)planar crystal stage;(b)competitive dendritic growth stage.During the solidification process of molten pool,nucleation occurred all the time.The number of nucleation increased with time.However,at the planar crystal stage and early competitive dendritic growth stage,all the formed nuclei were finally melted in the liquid.After completely entering the competitive dendritic growth stage,the nuclei formed and grew which finally formed a equiaxed dendrite belt.The belt prevented the dendrite growth and the CET occurred.With the increasing of number of surviving equiaxed dendrites,the equiaxed dendrites were refined.Comparing with increasing the number of nucleating agent,changing the kind and size of nucleating agent is more efficient to refine microstructure of fusion zone.