Effect of Heat Input on the Microstructure and Mechanical Properties of Electron Beam-Welded AW2099 Aluminium-Lithium Alloy

The paper focuses on the investigation of the effect of heat input on the microstructure and mechanical properties of welded joints produced by electron beam welding of 4.0 mm-thick AW2099 aluminum-lithium alloy. This type of alloy is intended for application in an airplane fuselage. Information on electron beam welding of such type of materials is up to now is very limited. Non-dendritic equi-axed zone (EQZ) was observed at the heat-affected zone–weld metal interface. The higher heat input (HHI) led to the development of EQZ with a larger width. The thickness of EQZ was non-uniform across the base material thickness. EQZ was characterized by the presence of higher amounts of elements at the grain boundaries due to segregation. Eutectics based on α-aluminum + θ-Al 2 Cu were detected in those areas. Transmission electron microscopy detected the presence of AlLi and Al 2 Li 3 intermetallic phases in the weld metal. Dissolution of the low-temperature δ'-Al 3 Li phase was observed by differential scanning calorimetry (DSC). Higher peak temperatures of a thermal cycle were measured during HHI welding. A peak temperature of 451 °C at a distance of 1.5 mm from the weld centerline was measured. The dissolution of precipitate particles caused by a thermal welding cycle resulted in the drop of microhardness in the fusion zone. Mean microhardness was slightly higher in the case of lower heat input (LHI) welding, i.e., 73% of that of the base material. The maximum weld tensile strength reached more than 83.8% of that of base materials. The fracture surface revealed the presence of dimples and bright brittle surfaces along with the microcracks and grain boundary eutectics.