Study On The Morphology And Formation Mechanism Of Microstructures In Al-Li Alloy Laser Welded Joints

With the rapid development of contemporary aircraft technology,structural weight reduction has become an important tendency in aviation manufacturing industry.Great demands and challenges of aircraft materials and structures have been proposed.The Al-Li alloys are widely adopted in aircraft structural materials due to their excellent properties such as high specific strength,perfect corrosion resistance,wonderful thermal stability,etc.Meanwhile,the rapid development of laser welding technology leads to its extensive employment in aluminum alloy connections.However,there are still some difficulties in Al-Li alloy laser welding technology on account of the characteristics of Al-Li alloys and the requirements of aircraft service performance.In this paper,laser welding experiments of 2060 and 2099 Al-Li alloy based on two kinds of typical joint forms are carried out.The morphologies and formation mechanisms of microstructures in welded joints are mainly studied.Firstly,preliminary laser welding experiment of 2060 Al-Li alloy butt joint and dual laser-beam bilateral synchronous welding(DLBSW)experiment of 2060-2099 Al-Li alloy skin-stringer T-joint are performed,aiming to verify the accuracy of the temperature field simulation results.Then,considering the physical dimensions of the weldments and the validated heat source models comprehensively,the finite element models for laser welding of the two structures are established.The temperature field simulation results of the two structures reproduce the molten pool morphologies under different process parameters,from which the optimal process parameters are obtained to guide the experiments.Secondly,laser welding experiments of 2060 Al-Li alloy butt joint are designed and implemented based on the temperature field simulation results.The effects of welding parameters on the joint macroscopic morphology and microstructures are quantitatively analyzed.While the distribution characteristics of elements in welded joints are also investigated.On these basis,the hardness distribution and the tensile fracture characteristics of welded joints are discussed.The experimental results indicate that the mechanical properties of the weld seam without filling wire are reduced because of the burning loss of the beneficial elements and the segregation of heavy elements.Furthermore,the DLBSW experiments of 2060-2099 Al-Li alloy T-joint are designed and conducted.Through the detection and analysis about the macroscopic features of the welded joints,the microstructure characteristics in different sections,the distribution characteristics of the elements as well as the welding defects and their formation mechanism,the reasons for the differences inmechanical properties between the two types of welded joints are analyzed.The T-structures possess the better mechanical properties compared with the butt structures,owing to the element supplement for the weld seam and introduction of a large amount of silicon-rich hard phase by adopting the filling material.Finally,the equiaxed grain zone(EQZ)along the fusion boundary in Al-Li alloy laser welded joint is emphatically concerned in this paper.The effects of alloying elements,welding thermal process and molten pool flow are considered to explore the formation mechanism and influence law of the EQZ.The results show that the existence of Li and Zr is vital for the formation of EQZ.During the laser welding process,the constitutional supercooling on the frontier of solid-liquid interface at the molten pool edge and the flow characteristics of the pool are of great significance to the EQZ microstructures.