Medium-thick aluminum alloys have a wide range of application prospects in railway transportation; therefore, studying the welding of medium-thick aluminum alloys is crucial. Laser-MIG hybrid welding (MIG, metal-inert gas) combines the advantages of laser heat source and arc heat source while compensating for their individual shortcomings, making it particularly suitable for welding medium-thick plates. To enhance welding efficiency, it is essential to analyze the single-layer, single-pass welding process. This study focuses on the microstructures and mechanical properties of laser MIG-welded joints of 6005A aluminum alloys with different groove dimensions.

In this study, a 10 mm thick 6005A aluminum alloy is welded using the laser-MIG hybrid welding technique in a single-layer, single-pass manner. Welding tests are conducted under a constant laser power, welding speed, and wire feeding speed of 5000 W, 16 m/min, and 8 mm/s, respectively, using different groove dimensions, including a root thickness of 5 mm with groove angles of 50°, 60°, and 70° and a groove angle of 60° with root thicknesses of 4 mm and 6 mm. The microstructures of the welded joints are analyzed using optical microscope (OM) and electron backscatter diffraction (EBSD). Tensile tests are performed on the specimens at a rate of 2 mm/min, and the tensile fracture morphology is observed using scanning electron microscope (SEM). The hardnesses of the welded joints are measured using a Vickers hardness tester. Finally, the weld temperature field is investigated using finite element analysis, and time?temperature profiles are extracted for specific locations.

A metallographic analysis reveals that successful weld connections are achieved for plates with different groove dimensions under constant processing conditions. Because of the similar heat input during welding, the microstructure of the welded joint exhibits the analogous performance. The grains in the center of the weld zone (WZ) are equiaxed dendrites, whereas those near the fusion zone are columnar dendrites, oriented perpendicular to a fusion line. The heat-affected zone (HAZ) shows randomly distributed, large black precipitate phases, which are significantly larger than the uniformly distributed point-like precipitate phases in the base metal (BM). The grain diameter in the WZ is about 140 μm, while the grain diameters in the HAZ and BM are 73 μm and 76 μm, respectively. The grain diameter in the WZ is much larger than those in the HAZ and BM, whereas the grain diameter in the HAZ is similar to that in the BM (Fig. 7). The tensile strengths of the specimens with different groove dimensions are 75%?79% of that of the BM, indicating that the welded joints have good bond strengths (Table 4). Because all the tensile fractures are in the HAZ, the HAZ is the weakest region (Fig. 8). This is mainly because of the severe softening behavior that occurs in the HAZ during welding. Fracture morphologies indicate that the fracture type is ductile (Fig. 9). The hardnesses of the welded joint in the HAZ and WZ are significantly lower than that of the BM, with the minimum hardness observed in the HAZ (Fig. 10). This reduction is mainly due to the transformation of a strengthening β phase into a β '' phase during thermal cycling. A finite element analysis shows that the region with a peak temperature of 480 °C in the HAZ corresponds to the location of minimum hardness (Fig. 13).

The paper studies the single-layer, single-pass laser-MIG hybrid welding process of a 6005A aluminum alloy with a thickness of 10 mm, and the relevant properties of the welded joints with different groove dimensions are analyzed. Plates with different groove dimensions are successfully joined at a laser power of 5000 W, wire feed rate of 16 m/min, and welding speed of 8 mm/s. The grains in the WZ are equiaxed at the center and columnar at the fusion line. The material in the WZ undergoes repetitive melting and solidification to form larger grains, whereas the grains in the HAZ and BM remain almost unchanged. The HAZ is the weakest region of the welded joints of the aluminum alloy 6005A. The tensile properties of specimens with different groove dimensions are similar, and their fracture characteristics are mainly ductile. Softening behavior occurs in all the welded joints, and the heat-affected zone exhibits minimum hardness. An analysis of the corresponding welding temperature field confirms that the region with a peak temperature of 480 °C is the softest region of the welded joints.