Electron Beam Welding Technology Of Al-Cu-Li Alloy And Microstructure And Properties Of Welded Joint

Nowadays, with the rapid development of aerospace industry, aluminum-lithium alloys as a structure material with excellent properties, have increasingly received considerable attention as replacement of conventional aluminum alloys. At present, Al-Li alloys applied in engineering are the third-generation alloys. Compared with the second-generation Al-Li alloys, the new generation Al-Li alloys have excellent comprehensive properties, such as the well balance between strength and toughness, good damage tolerance, low anisotropy of mechanical properties, thermal stability, corrosion resistance and well formability, etc. However, due to the high chemical activity of Li element, weld defects such as weld porosity and hot crack will appear when the conventional welding methods are used to weld these novel Al-Cu-Li alloys. So the welded joints with better quality are difficult to be obtained. Compared with the common welding methods, electron beam welding has many advantages including high energy density, a narrow heat-affected zone, low distortion and good atmosphere as welding is performed in a high vacuum.Therefore, it is superior to weld Al-Cu-Li alloys. Based on these, in this paper, the electron beam welding(EBW) of Al-Cu-Li alloys with 2.5mm thickness is investigated. The effect of electron beam current on the weld appearence, microstructure and mechanical properties of welded joints are studied. Moreover, in order to improve the microstructure and mechanical properties of welded joints, the post-weld heat treatment(PWHT) including solution and aging is carried out to the joints. Besides, the corrosion resistance of welded joints is also systematically evaluated.The microstructure observation of welded joints shows that as-welded Al-Cu-Li alloys EBW joints exhibit three regions, namely, fine equiaxed grains zone(EQZ), columnar crystals and equiaxed crystals respectively from fusion zone to the center of weldment.The equiaxed grains zone formed near fusion line is related to Zr and Li element in Al-Li alloys. At the same time, the united recrystallization in the edge of molten pool is restrained by EQZ. There are a lot of eutectic structures distributed in the dendrites boundary of weld center. EDS analysis and XRD test of weldment indicate that the eutectic in grains boundary is mainly composed of α+Al2Cu phase.And further analysis by transmission electron microscopy(TEM) verifies that there is little strengthening phase in the joints under the as-welded state and precipitation free zone(PFZ) occurred along the grain boundary.Results of tensile test show that the microhardness of heat affected-zone(HAZ) and weld metal(WM) is lower than that of base metal(BM). It is because that softening phenomenon occurs in HAZ and underaged condition occurs in WM during welding. Consequently, by the comparison of Al-Li alloy itself, the tensile strength of welded joints is reduced to a certain degree. With the increase of welding heat input, the tensile strength of joints decreased. SEM observation demonstrates that the fractograph of welded joint presents the obvious characteristic of ductile fracture at room temperature.After post-weld heat treatment of solution and aging, the microstructure of joints is obviously changed. The fine EQZ along fusion boundary disappeared and the dendritic structure in the center of weldment transformed into equiaxed grains. Moreover, the segregation of grain boundary also eliminated. TEM observation is conducted to weldment after PWHT, results show that many strengthening phases precipitate in the weldment, such as spherical δ’ phase and needle-like T1 phase. The underaged condition of WM is effectively improved after PWHT, and the tensile strength and mirohardness of the joints obviously increased. The tensile strength reaches from 348 MPa in as-welded state to 423.1MPa after PWHT. And a large number of dimlpes uniformly distribute in the fracture surface.The immersion method and electrochemical test are used to evaluate the corrosion resistance of Al-Cu-Li alloy welded joints respectively. Results indicate that, when the joint sample is immersed in intergranular corrosion(IGC) solution for 24 h, great pitting corrosion occurs to base metal. The local intergranular corrosion with reticulation occurs to weld metal and a little intergranular corrosion is developed within HAZ along the rolling direction. Joint sample is immersed in exfoliation corrosion(EXCO) solution, the corrosion gradually becomes severely with the prolonging of corrosion time. The morphology of exfoliation corrosion first appeard in BM, HAZ is the second. When joint sample is immersed in EXCO solution for 96 h, the base metal shows severe exfoliation corrosion, the obvious exfoliation corrosion is not found to weld metal. Electrochemical corrosion results show that the corrosion resistance of welded joint is better than that of Al-Cu-Li alloys.