The heat-treatable aluminum alloy is widely used in the high-speed train body manufacturing industry owing to its low density and high specific strength. In the current high-speed train manufacturing industry, metal inert gas (MIG) welding and friction stir welding are the most commonly used welding techniques for medium-thick aluminum alloys. However, these two welding methods exhibit certain limitations. Laser-MIG hybrid welding combines the advantages of laser and MIG welding, and is a promising welding technology for joining aluminum alloy components. Many investigations have shown that it has several typical technical advantages such as faster welding speed, deeper weld penetration, lower heat input, smaller welding deformation, narrower heat-affected zone, and better mechanical properties. Currently, there are few research reports on laser-MIG hybrid multipass welding for medium-thick aluminum alloys; however, this welding technique is urgently needed in the high-speed manufacturing industry. Laser-MIG hybrid backing welding is a key step in laser-MIG hybrid multipass welding technology and has crucial effects on the weld formation and mechanical properties of weld joints. Therefore, in this study, the laser–MIG hybrid backing welding of a 20 mm thick 6082-T6 aluminum alloy is studied.

Laser-MIG hybrid backing welding is performed on 20 mm thick 6082-T6 aluminum alloy butt joints. The influence of the groove shapes on the arc behavior, droplet transfer, and weld formation is studied using a high-speed camera. The influence of groove form and size on weld formation is analyzed. Based on the optimized groove form, an orthogonal test is used to optimize the welding process parameters, and the primary and secondary orders of influence of the process parameters on weld formation are obtained. Finally, the microstructural characteristics and mechanical properties of the joints are investigated under the optimal parameters.

The U-groove and V-groove have almost similar arc behaviors and droplet transitions, and their droplet transition periods are both 2.5 ms. Compared with the U-groove, the arc length of the V-groove is larger, and it is worth noting that some small welding spatters are found on the V-groove edge. In contrast to the absence of porosity defects on the weld cross section of the U-groove, some porosity defects appear on the weld cross section of the V-groove, which is probably caused by the unstable molten pool and droplet transfer disturbance. In addition, the transition between the weld and upper groove wall is smooth for the U-groove but concave for the V-groove, which is unfavorable for the interlayer cleaning (Figs. 4 and 5). Almost no porosity defects can be observed in the weld seam when the blunt edge height is 10 mm. With an increase in the height of the blunt edge, the number and size of porosity defects in the weld increase sharply (Fig. 6). The main reason is that a larger blunt edge height requires a higher laser power and a smaller laser focusing diameter, which is more likely to cause the keyhole to become unstable and close during the welding process, preventing the pores in the molten pool from escaping and causing porosity defects in the weld seam. An orthogonal test is performed based on the optimized U-groove, and the weld joint is scored using a comprehensive weighted scoring method. The primary and secondary orders of influence of crucial factors on the weld quality from strong to weak are as follows: the welding speed, arc current, laser power, and arc length correction. Because of the lower cooling rate and longer high-temperature residence time in the arc zone, the width of the columnar crystal zone in the arc zone is smaller than that in the laser zone, and the width of the partial melting zone in the arc zone is larger than that in the laser zone (Fig. 7). The stress corrosion resistance susceptibility index of the weld joints is 0.024. The fracture locations of the tensile and stress corrosion resistant specimens occur in the heat-affected zone, the fracture paths are parallel to the fusion line, and their fracture morphologies present typical plastic fracture features.

The optimal groove type is a U-groove with a blunt edge height of 10 mm, which is conducive to obtaining high-quality welds without porosity defects and stable welding processes without spatter. For the laser-MIG hybrid backing welding of a 20 mm thick aluminum alloy butt joint, the primary and secondary orders of the effects of crucial factors on the weld quality from strong to weak are the welding speed, arc current, laser power, and arc length correction, and their optimized values are 0.6 m/min, 6.5 kW, 300 A, and -5 %, respectively. The other welding parameters are as follows: the gap size is 2 mm, laser focusing diameter is 1.0 mm, heat source distance is 3 mm, and defocusing amount is 2 mm. The widths of the columnar and partially melted zones in the arc zone are narrower and wider, respectively, than those in the laser zone. The microhardness values in the weld metal and heat-affected zone are lower than those in the base material, and the lowest microhardness value is obtained far from the fusion line in the heat-affected zone. The weld joints exhibit good stress corrosion resistance, with a stress corrosion resistance susceptibility index of 0.024. The fracture locations of the tensile and stress corrosion resistant specimens occur in the heat-affected zone and the fracture paths are parallel to the fusion line. Their fracture morphologies exhibit typical plastic fracture features.