Research On The Superplastic Deformation Behavior And Microstructure Evolution Of Electron Beam Welded 5A90 Aluminum Lithium Alloy

Welding/superplastic deformation combination techniques has a great advantage in the manufacture of lightweight aluminum-lithium alloy multilayer hollow structures.Diffusion bonding/superplastic forming(SP/DB) combination techniques have used more often at Present. But the SPF/DB technology have many problems in the application of aluminum-lithium alloy, such as connection difficulties and the mechanical properties of formed parts decline. Vacuum electron beam welding can overcome the difficulties in lithium aluminum alloy diffusion bonding and form good performance joint. Electron beam welding/superplastic deforming technology is expected to become a new welding/superplastic combination technology which has a good processing performance. In this thesis, the superplastic deformation behavior and microstructure evolution of electron beam welded 5A90 aluminum-lithium alloy sheet was investigated, and reached the following conclusions.1)The electron beam welded joints of 5A90 aluminum lithium alloys present good superplasticity at temperature range of 425℃ ~ 500℃ and strain rate range of5×10-4s-1~1×10-2s-1. The elongation of longitudinal joints increased first and then decreased with increase of the deformation temperature and initial strain rate, and the maximal elongation of 171.1% was obtained at the optimal parameter 450℃, 5?10-3s-1;the deformation rate of the transverse joints increased with the initial strain rate increase,and increased first and then decreased with the increase of temperature. Deformation coefficient K was introduced to characterize the superplasticity of longitudinal joints and the K values were beyond 70% at all deformation parameters.2)The metallographic microstructure observed that the fine equiaxed and equiaxed dendrite of weld bead transformed into coarse equiaxed at the initial stage, the grain boundary migration that caused by diffusion is the main mechanism of superplastic deformation. The coarse equiaxed start to refine at the late deformation, and the superplastic deformation mechanism is dominated by dynamic recrystallization. The average grain size of HAZ decreased continuesly with the increase of the deformation,and dynamic recrystallization is the main superplastic deformation mechanism.Organization uniformity coefficient K was introduced to characterize the uniformitydegree of the organization of weld bead and heat affected zone. The K increase rapidly with strain increasing at the initial strain stage, when the strain reaches 0.4, the K increased to 0.82 from 0.36, and the K is still increased slowly with the deformation continuing.3)There is a serious deformation inhomogeneity in the superplastic deformation of5A90 aluminum lithium alloy electron beam welded joint. The sectional shrinkage of specimen decreases from the specimen fracture to a shoulder. The sectional shrinkage near the fracture reaches 84.9%, while The sectional shrinkage 4mm from the fracture just was 61.1%. The joint formed a large number of cavity after superplastic deformation, and the cavity mainly concentrated in the heat affected zone and base metal. The number of cavity in the heat affected zone was maximum and the size of cavity was larger, while the number of cavity in the weld zone was smaller and the size of cavity was smaller. The number and size of the cavity increases with the strain increasing. The shape of cavity in the weld zone were nearly circular, while the shape of cavity in the heat affected zone were nearly circular when the cavity was small and became irregular shape with the cavity growing.