2219 aluminum alloy is a high-strength aluminum alloy that can be strengthened by heat treatment. Owing to its excellent processability, it has become one of the most preferred structural materials in the aerospace field, and is commonly known as aviation aluminum. Laser welding, which is the most commonly used connection method for 2219 aluminum alloys, is characterized by a high welding efficiency, small heat-affected zone, and high power density. However, when a single-beam laser is used to weld the butt structure of a moderately thick plate, the penetration is too shallow, resulting in incomplete penetration. When the laser power is increased to solve the problem of incomplete penetration, large welding deformations and residual stresses caused by excessive heat input occur. Laser mirror welding synchronously operates on the plate structure through a symmetrical double-laser heat source to form a joint molten pool and keyhole, which can solve the problems of large welding deformation and welding residual stress caused by single-beam laser welding. Owing to the complex interaction between the double heat sources in the laser mirror welding process, the joint molten pool generated by the double heat sources has different cooling rates during the cooling process, which leads to uneven distribution in the microstructure. Presently, the research on the formation mechanism of the microstructural differences in laser mirror-welded joints under different welding parameters and their influence on the mechanical properties is still insufficient. Therefore, laser mirror welding experiments are performed on a 2219 aluminum alloy with a thickness of 6 mm in this study. The microstructural differences at distinct positions on the welded joint and at the same position under different welding heat inputs and their formation mechanisms are compared and analyzed. Subsequently, the difference in the mechanical properties caused by microstructural differences are explored to provide a reference for improving the quality of laser mirror welding using a moderately thick aluminum alloy plate.
In this study, 2219 aluminum alloy, which is a widely used and representative material in the aerospace field, is used. The dimensions of the welding material are 50 mm×100 mm×6 mm. The laser mirror welding experimental equipment (Fig. 1) used in this study includes two mirror symmetry laser welding heads, two high-precision robots, a gantry, a megawatt disc laser, and a splitter. The process parameter variables for this experiment are mainly the laser power (P) and welding speed (V), and the combined process parameter variables are shown in Table 2. A hardness test is performed on the laser-welded joints using a microhardness tester. The thickness of the sample used in this study is 6 mm, and an electronic universal tensile machine with a maximum tensile force of 10 kN is used for operation. Finally, the microstructures and chemical compositions of the welded joints are analyzed using scanning electron microscopy (SEM) and energy disperse spectroscopy.
A comparison of the cross-sectional areas of the weld seams under different welding heat inputs reveals that the area increases with an increase in the welding heat input (Fig. 2). When the welding heat input is 913.12 J·cm-1, the typical regions of laser welded aluminum alloy such as the non-dendritic equiaxed zone (EQZ), columnar crystal zone, and equiaxed crystal zone, appear from the heat-affected zone to the center of the weld seam. Additionally, abundant equiaxed dendrites appear in the joint part of the weld seam, and the grain sizes of the equiaxed crystals on the left and right sides of the weld seam are similar (Fig. 4). With an increase in the welding heat input, the width of the columnar crystal zone in the upper fusion line area decreases from 127 μm to 87 μm (Figs. 5-6), and the width of columnar crystal zone in the lower fusion line area becomes much smaller than that in the upper fusion line area (Fig. 7). Additionally, the width of the columnar crystal zone and number of equiaxed dendrites in the weld center area increase, which indicates that the increase in heat input refines the grains in the weld center area.
An increase in the welding heat input increases the "isosceles " ratio and size of the joint weld area. The width of columnar crystal zone in the upper fusion line area reduces from 127 μm to 87 μm, and the columnar crystal zone in the lower fusion line area is narrower than that in the upper fusion line area. Moreover, the central areas of the weld seams under different welding heat inputs have the equiaxed dendrite structure. With an increase in the welding heat input, the number of equiaxed dendrites in the central area of the weld seam increases, proving that the increase in the heat input has a positive effect on grain refinement. When the welding heat input increases, the mechanical properties of the welded joints are significantly enhanced. The maximum microhardness of the weld seam is 74.9 HV, and the tensile strength increases from 188.39 MPa to 259.47 MPa. Moreover, increasing the welding heat input can result in the formation of a more reliable joint weld seam and the enhancement of the tensile properties of the welded joints. Equiaxed dimples with different sizes exist at the fracture site, and the second phase, which is rich in Cu, exists at the bottom of the dimple. With an increase in the welding heat input, the fracture mode changes from mixed fracture to ductile fracture, the number of dimples increases, and the size becomes larger and more uniform.
