Objective Compared with the traditional Al-Li alloy, the Al-Li alloy weighs less and has high stiffness, making it more conducive for manufacturing aerospace components. However, because of its low boiling point, high thermal expansion, and high thermal conductivity, a heat source with concentrated energy is more suitable for welding Al-Li alloys. Based on the unique welding structure of aircraft fuselage panels, a novel technology of dual laser-beam bilateral synchronous welding (DLBSW) is proposed and applied in the manufacturing process to ensure the fuselage shape and improve the welding efficiency and quality. During the DLBSW process, improper heat input affects the temperature gradient and solidification speed of the metal in the molten pool, resulting in coarse grains of the T-joints, which is unconducive to improve the macroforming and overall mechanical properties of welded components. Here, we analyze the grain morphologies and sizes in different regions of the joints under different welding parameters and explored their influence on the mechanical properties of welded joints, providing reference and guidance for further improvement of the mechanical properties of welded joints.

Methods Here, 2060 (500 mm×125 mm×2 mm) and 2099 Al-Li alloy sheets (650 mm×28 mm×2 mm) are used as the skin and stringer, respectively. A 1.2 mm diameter ER4047 Al-Si welding wire is used as the filler material. The chemical compositions of the base metal and welding wire are shown in Table 1. Before welding, the sample surface should be chemically cleaned to remove the oxide film and oil stain. The welding experiment is conducted using a double laser-beam welding system (Fig. 1). Based on the preliminary welding test and comprehensive analysis of the welding seam forming quality, four better welding parameters are selected in the experiment (Table 2). After welding, the metallographic sample of the cross-section of the T-joints is cut by wire cutting technology and inlaid with epoxy resin. Next, the metallographic sample is polished and etched with Keller's reagent. Furthermore, the microstructure and grain size of the joints are analyzed using a metallographic microscope. Also, tensile tests are performed on the specimen until fracture, and the fractured specimen surface is observed by transmision electronic microscopy(SEM).

Results and Discussions Based on the T-joint microstructure, the grain morphology from the upper fusion line to the weld center in the solidification process are equiaxed fine grains, columnar dendrites, and equiaxed dendrites (Fig. 3). The average width of EQZ W_EQZ increases with the increase in welding heat. With a heat input of 43.64 J/mm, the equiaxed fine crystal band narrowed down, its width is 2--3 times as long as the grain size, whereas with a heat input of 48.00 J/mm, the width of the crystal band increases to 4--5 times as long as the grain size (Fig. 4). When the heat input is low, because of a decrease in the temperature gradient, the dendrites of equiaxed fine grains grow and become columnar crystals, decreasing the W_EQZ, even though it cannot be observed. Also, with low input, the ratio of the temperature gradient to the crystallization speed in the columnar crystal is larger, and the growth of the columnar crystal nucleus becomes difficult. Thus, with low heat input, the columnar crystal grains are smaller. From the microhardness distribution results, the microhardness values of 2060 and 2099 Al-Li base metals are the highest, followed by the heat-affected zone and weld center. The microhardness in the fusion zone is the lowest (Fig. 5). No EQZ exists at the weld toe, which is composed of coarse and short columnar crystals. The stress concentration in the weld toe because of the structural mutation and the grain size is more significant than that in the EQZ zone, where dislocation slip is more likely to occur under the action of an external force. Therefore, the weld toe is the starting point of the T-joint fracture (Fig. 6).

Conclusions In this study, the heat input in the range of 43.64--48.00 J/mm of DLBSW welding is investigated to explore the mechanical properties of 2060 and 2099 Al-Li alloy T-joints affected by different grain morphologies and size characteristics. The mechanical properties of the weld can be controlled by changing the welding process parameters, which affect the grain structure characteristics of the welding joint, especially near the fusion zone. With the increase of heat input, the heat-affected and partially molten zones in the T-joint widens, and the width of equiaxed fine-grained zone increases; the grain size of equiaxed fine-grained zone first decreases and then increases with the increase in welding heat input, and the grain size of columnar dendrite increases with the increase in heat input. Based on the average hardness, tensile properties, and fracture morphology of the joint, it can be concluded that with a heat input of 46.16 J/mm, the mechanical properties of T-joint are the best, and the tensile strength can reach 335.7 MPa. Hence, reducing the grain size in the EQZ of welded joints can significantly improve the mechanical properties. Therefore, the mechanical properties of the joint can be improved by changing the welding process parameters to limit the grain growth trend.