Objective 2060 aluminum-lithium alloy is a third-generation aluminum-lithium alloy, which has excellent performance, such as low density and high specific stiffness. It has broad application prospects in the aerospace field. Fiber laser welding technology is rapidly developing because of high welding speed and high efficiency. The lightweight material of the aluminum-lithium alloy, combined with fiber laser welding, can satisfy the lightweight requirements of aircraft. Because of the large thermal expansion coefficient of the aluminum-lithium alloy and easy burning of elements, the problems of cracks, pores, and mechanical properties are concerning. Although the crack-assisted process can effectively suppress cracks, the pores and mechanical properties still need to be improved. Moreover, the filler wire welding process causes problems such as high light wire matching and complicated melting and solidification behavior of the wire. In this study, focus rotation and filler wire were adopted in the fiber laser welding (denoted as laser welding-FRFW) of 2 mm-thick 2060 aluminum-lithium alloy to analyze the impact of laser focus rotation on the weld formation, porosity, distribution of microstructures in the weld, and the mechanical properties of the welded joint.

Methods The test specimen was a piece of 2060-T8 aluminum-lithium alloy. We used 4047 welding wires with a diameter of 1.2 mm. The YLS-6000 fiber laser was used. The core diameter of the transmission fiber was 200 μm, the focal length of the collimating lens was 200 mm, and the focal length of the focus lens was 300 mm. The wedge angle of the wedge prism was designed to obtain the required laser focus rotation radius. A laser focus rotating device was used to regulate the rotational speed of the wedge prism. We adopted the process parameters as follows: laser power, 3.8 kW; welding speed, 3 m/min; wire-feeding angle, 45°; wire-feeding speed, 3 m/min; laser-wire distance, 0 mm. Furthermore, a color high-speed camera was used to observe the droplet transfer behavior in the welding process. When the welding was completed, we prepared metallographic specimens for analyzing the weld morphologies and porosity. Scanning electron microscopy was used to observe the microstructures of welded joints. A scanning electron microscope and an energy dispersive spectrometer (EDS) were used to analyze the ingredients of any selected area. Further, X-ray diffraction (XRD) was used to analyze phase compositions in different areas within the weld. We used a hardness tester to measure the microhardness of the welded joints with the load of 0.98 N loaded for 15 s. The tensile properties of welds were tested based on the ASTM E8m standard.

Results and Discussions The weld morphology considerably changed after applying laser welding-FRFW (Fig. 1). The surface of the weld was smooth with shallower fish scale-shaped ripples; the width of the entire weld became more uniform; and the spatters around the weld were effectively suppressed. The laser focus periodically acted on the weld pool and the end of the welding wire, and the weld pool was stable with a small fluctuation range and no spatter (Fig. 3 and Fig. 4), and there were only tiny pores around the fusion line (Fig. 5). The laser focus periodically acted on the welding wire and weld pool. This action can make the weld pool longer, making it easier for bubbles to move upwards and escape the weld pool.

There were four zones distributed from the fusion line to the center of the weld: HAZ, PMZ, EQZ, and CGZ (Fig. 6). When focus rotation was applied, the obvious thick grain boundary could still be observed in the PMZ near the fusion line. Further, the width of the EQZ was reduced, and the sizes of column grains near the equiaxed grain zone became smaller [Fig. 7(a) and Fig. 7(b)]. The main precipitated phases are α (Al) solid solution, θ phase (Al₂Cu), and T phase (AlLiSi).

The low hardness appeared in the PMZ in both cases(Fig. 13). In filter wire laser welding, the strength at the weld center was reduced to approximately 115 HV0.1. When focus rotation was applied, the strength of the entire welded became more uniform and increased to approximately 123 HV0.1. Compared with that of the filler wire laser welding without focus rotation, the microhardness of the welded joint prepared by filler wire laser welding with focus rotation was increased by 6.9%. According to the tensile test results (Fig. 14), the tensile strength of the welded joint of laser welding-FRFW was 365.0 MPa, which is slightly higher than 349.4 MPa (the joint of filter wire laser welding). Fracture of each welded joint occurred in the area near the fusion line of the welds. Fractures of the welded joints obtained in both welding processes were dimple-aggregation type intergranular fractures with features of mixed fractures.

Conclusions Because of the high-frequency rotation of the laser focus, the laser focus could act periodically on the weld pool and one end of the welding wire. Based on the laser welding-FRFW, the weld pool was longer and more stable, which can improve the weld morphology, suppress spatters and reduce the number of pores. The width of the EQZ and sizes of the grains in EQZ as well as the sizes of the columnar grains near the equiaxed grain zone on the weld were reduced in the laser welding-FRFW. The microhardness near the fusion line and tensile strength of the welded joint were slightly increased compared with the filter wire laser welding without focus rotation. Moreover, the fracture of the welded joint, with features of mixed fractures, occurred near the fusion line.