Both 2060-T8 and 2099-T83 are new third-generation aluminum-lithium (Al-Li) alloys that have been applied in the aviation industry. These two types of Al-Li alloys have excellent specific strength, elastic modulus, and fatigue resistance. Owing to the addition of lithium in the alloy, the structural weight can be further reduced by 10%?15%. Compared with conventional aluminum alloys used in aviation, however, laser-welded Al-Li alloys are more likely to form hot cracks and pores. In addition to welding parameters, changes in weld composition are closely related to the formation of hot cracks and pores. In particular, the composition and precipitation phase of weld grain boundaries are closely related to the formation of hot cracks. When welding wires with high Si content such as ER4047 are used, Al₂Cu, LiAlSi, and Al-Si divorced eutectics are primarily formed at the grain boundaries. However, studies showed that 4047 welding wires cannot completely suppress the generation of hot cracks. By contrast, an excessively grown LiAlSi phase will deteriorate the grain-boundary performance. In this study, for the double-sided laser-beam welding of Al-Li alloy T-joints, we use five types of welding wires with different Si and Cu contents. Thus, the grain-boundary microstructure is effectively controlled by the composition of the welding wire. Hot cracks and porosity defects in the weld are suppressed effectively. Additionally, the transverse tensile and longitudinal compression properties of the T-joints improve significantly. This study may serve as a reference for the engineering application of double-sided laser-beam welded 2060/2099 Al-Li alloy T-joints.

In this study, 2-mm-thick 2060-T8 and 2099-T83 Al-Li alloys are used. Optimized parameters are used for the double-sided laser-beam welding of Al-Li alloy T-joints. The double-sided laser-beam-welding system platform comprises two fiber lasers and two wire feeders controlled by two 6-axis industrial robots. The fiber lasers, which feature an emission wavelength of 1.07 μm, can be delivered in the continuous wave (CW) mode. The laser beam passes through a focusing mirror with a focus length of 192 mm and is finally focused as a spot measuring 0.26 mm in diameter. The optimized mixing parameters are as follows: laser wavelength, 1.07 μm; laser power, 3000 W; laser scanning speed, 10.0 m·min^-1; wire-feeding speed, 4.3 m·min^-1; wire extension, 8.0 mm; and shielding-argon-gas flow rate, 15.0·L·min^-1. No heat treatment is performed on the welded T-joints post welding. Five types of welding wires with different Si and Cu contents are evaluated for the grain-boundary alloy regulation of the double-sided laser-beam welded Al-Li alloy T-joints. Prior to analysis and testing, the surface of the T-joint weld is first treated with Keller reagent for corrosion. The macrostructure characteristics of the weld are observed using an optical microscope (OM). The microstructure characteristics of the weld are observed using scanning electron microscope (SEM). The transverse tensile and longitudinal compression performances of the T-joints are evaluated.

Based on SEM observation, the T-joint weld comprises columnar dendrites, equiaxed dendrites, and an equiaxed grain zone (EQZ), whereas the weld grain boundary primarily comprises Al₂Cu, LiAlSi, and Al-Si divorced eutectics. When the Si mass fraction is extremely low (4.18%) in wire 1# or extremely high (7.23%) in wire 3#, cracks distributed along the transverse and longitudinal directions are observed in the T-joint welds. Meanwhile, when the Cu mass fraction (7.13%) in the wire is extremely high, crack and porosity defects are observed in the welds (Fig. 5). The cracks and pores are located in the columnar- and equiaxed-dendrite zones, and the cracks are formed along the grain boundaries (Fig. 6). When the Si and Cu mass fractions in wire 4# are 5.41% and 6.17%, respectively, Al₂Cu fully precipitates and presents a grid-like distribution at the grain boundaries, whereas LiAlSi maintains a small size of less than 1 μm. However, further increasing the Si and Cu contents in wire 5# disrupts the grid distribution of Al₂Cu and significantly expands the LiAlSi phases, thus causing cracks and porosity defects to reappear (Fig. 7). The mechanical properties of the T-joints are closely related to the microstructure and defects of the weld. When using wire 4#, the T-joints achieve a maximum average transverse tensile strength of 406 MPa and a maximum average longitudinal compressive load of 95 kN (Figs. 8 and 9).

When using Si-Cu wires for welding, the double-sided laser-beam welded Al-Li alloy T-joints comprise an EQZ adjacent to the fusion line, a columnar-dendrite zone adjacent to the weld surface and EQZ, and an equiaxed dendrite zone at the center of the weld. The weld grain boundaries primarily comprise an θ phase (Al₂Cu), a T phase (LiAlSi), and Al-Si divorced eutectics. The different Si and Cu contents in the welding wire affect the formations and microstructures of the θ phase, T phase, and Al-Si divorced eutectics. Excessively low or high Si and Cu contents can result in the formation of hot cracks along the grain boundaries in the columnar- and equiaxed-dendrite zones. When the Si and Cu mass fractions in the welding wire are 5.41% and 6.17%, respectively, the θ phase on the weld grain boundary is distributed in a grid-like manner and the T-phase size is controlled below 1 μm, which resultes in a significant second phase-strengthening effect. Furthermore, the cavities formed by the solidification shrinkage of the intergranular liquid phase can be filled and repaired by the Al-Si divorced eutectics, thereby suppressing hot cracks and porosity defects. When the Si and Cu mass fractions in the welding wire are 5.41% and 6.17%, respectively, the maximum average transverse tensile strength of the T-joints is 406 MPa, which is 80% of the skin tensile strength. The maximum average longitudinal compressive load of the T-joints is 95 kN. These test results based on the optimal composition of wire 4# are the best among the five tested wires.