Multi-Scale Simulation Of The Mircrostructure Evolution During Solidification In Laser Welding Of Aluminum Alloy Sheet

Aluminum alloy is currently the most widely used light alloy material due to its advantages of low density,high specific strength,etc.Laser welding is an ideal technology for welding thin-walled aluminum alloy because of its advantages,such as low heat input,high welding speed,good flexibility,etc.At present,the laser welding technology of aluminum alloy sheet has been gradually applied to the manufacturing fields of automobile,aerospace,rail transportation,etc.,and has been favored by the industry.The performance of the joints is closely related to the solidification microstructure of the weld.An in-depth understanding of the solidification process in the molten pool is of great significance for optimizing the welding process and improving the quality of joints,and is the forefront of international research.However,the rapid solidification process in laser molten pools is a complex process involving macro-scale mass/heat transfer processes and micro-scale dendritic nucleation-growth processes.It is currently difficult to study this complex,transient process by experimental methods.This work aims to use the multi-scale modeling methods and numerical simulation techniques to conduct in-depth research on the heat transfer and flow behavior,the distribution of solidification parameters ahead of the molten pool,and the evolution of solidification microstructures during the laser welding of aluminum alloy sheets.The main conclusions are given as follows:(1)A multi-scale model,which combined the macro-scale model for heat transfer and fluid flow and the micro-scale model for dendritic nucleation-growth,was developed for laser welding of aluminum alloy sheets.In view of the multi-scale characteristics of the solidification process in the molten pool,a macroscopic heat transfer and fluid flow coupling model for laser welding of aluminum alloy sheet and a microscopic quantitative phase field model for alloy solidification were established.The relationship between the macroscopic model and the microscopic model was established.Finally,a multi-scale model considering the macro-scale heat and mass transfer behavior and the micro-scale dendritic nucleation-growth behavior was built.The model was solved using multi-node multi-GPU parallel acceleration technology.(2)The macroscopic heat transfer and fluid flow behavior during laser welding of aluminum alloy sheet and its effects on the solidification parameter distribution at the solidification front and the evolution of the microstructure were studied.It was found that the main heat transfer mechanism in the molten pool during laser welding of aluminum alloy sheet was thermal diffusion(the Peclet number is approximately 1.9).Meanwhile,the three-dimensional heat flow distribution was analyzed,and results showed that the heat flow along the thickness direction was negligible due to the small thickness.Hence,the heat transfer can be approximated as a two-dimensional heat conduction process on the horizontal section.The solidification parameters at the solidification front of the molten pool were calculated quantitatively,and the size and morphology of the solidification microstructure under different laser welding processes were analyzed qualitatively/semi-quantitatively.It is found that high laser power matching high speed welding process can reduce the columnar grain region and refine the microstructure(the secondary dendritic arm spacing decrease from 4.1?m to 3.4?m).(3)The microstructure evolution from planar grains,cellular grains to columnar grians in the initial solidification stage of the molten pool was studied.It was found that the instability of the planar solidification interface was primarily caused by the rapid increase of the crystal growth rate(0.04 mm/s?9.41 mm/s)during the solidification process,which made the liquidus temperature gradient(59.14 K/mm?-98855.44 K/mm)at the solidification front much steeper than the applied temperature gradient(483 K/mm?336K/mm),which led to the larger constitutional undercooling.The grain orientation or misorientation angle can significantly affect the interface stability.As the misorientation angle increased,the stiffness of the solid-liquid interface became larger,the stability effect of surface tension increased,and the solid-liquid interface became more and more stable.Subsequently,the cellular-to-columnar transition was studied,and two important solidification phenomena were found,namely,tertiary dendritic growth and tip splitting.Finally,the competitive growth of cellular/columnar grains was analyzed,and two different grain competitive growth mechanisms were revealed.(4)The evolution of solidification microstructure in the entire laser weld was studied.Firstly,it was proved that the nucleation mechanism for equiaxed grains in the welds was heterogeneous nucleation using the"overlap welding"procedure.Furthermore,it was found that the nuclei were Al₃Ti phase.Then,the evolution of the temperature field,the solute field as well as the constitutional undercooling field during the solidification of the molten pool were analyzed.The dynamic evolution of the solidification microstructure in the entire weld was illustrated.Subsequently,the solute segregation behavior in the entire weld was analyzed.Results showed that there was no obvious macro-segregation in the weld.Near the fusion line,there was a fluctuation in solute content,bahving like the banding segregation.There was signigicant micro-segregation,and the degree of micro-segregation varied in different regions in the weld.Finally,the effects of the maximum nucleation density on the weld structure was analyzed.It was found that increasing the maximum nucleation density can significantly reduce the columnar grain region,expand the equiaxed grain region,and refine the weld grain.The average diameter of the equiaxed grains decreases from 47.4?m to 26.2?m(about 45%)as the maximum nucleation density increases from(1.0×10¹¹ m^-3 to 5.0×10¹¹ m^-3).(5)Based on the above researches,two approaches for optimizing the weld microstructure were proposed,namely,high laser power matching high speed welding and Ti element addition welding.It was found that,in the premise of the acceptable weld formation,appropriate high-power(?3000 W)matching high-speed(?120 mm/s)laser welding process can achieve the refinement of the solidification structure from the grain structure level(witdh of columnar region:420?m?330?m)and sub-grain structure level(secondary dendrite arm spacing:4.1?m?3.4?m),which is beneficial to improve weld hardness and tensile strength.On the other hand,adding an appropriate amount of Ti element(about 1.7 wt%)to the weld during welding can lead the formation of Al₃Ti heterogeneous nuclei.This can promote the nucleation of the equiaxed grains in the weld,reduce the width of the columnar grain region(210?m?150?m)and expand the equiaxed grain region,which was conducive to improving weld hardness and tensile strength.