Uneven hardness mechanism and related fracture characteristics in laser-MIG hybrid welded Al–Mg–Si alloy butt

In this paper, a 6082-T6 high-strength aluminum alloy with 5 mm thickness was taken as the research object. The TRUMP Trudisk 10002 disc laser and the Transpils synergic 4000 welder were used to weld it by laser-Metal-Inert-Gas composite welding. The property transition and uneven hardness of the base metal (BM), the heat affected zone (HAZ), and the partially melting zone (PMZ) were analyzed by a transmission electron microscope, a scanning electron microscope, and an energy dispersive spectrometer. The results showed that: grain coarsening was not the main reason for softening in the HAZ but the evolution of precipitates is the main reason. Precipitates annexation in HAZ was the main reason for hardness decrease in the HAZ. Solid diffusion occurred in PMZ, and the enrichment of main strengthening elements at grain boundary played a positive role in hardness recovery. A large number of spherical GP zones and the β″ phase had been found in the PMZ, which played a major role in strengthening and increased the hardness of the PMZ. The tensile results showed that the BM had the highest elongation and tensile strength. The strength and plasticity of the HAZ were lower than those of the BM. A lamellar second phase with lamellar tearing was found at the bottom of the fracture in HAZ. The softening fracture morphology was different from that of the BM. This paper mainly focuses on the softening of HAZ of welded joints and the hardness bump in PMZ. The distribution of precipitates, grain size, alloy element diffusion distribution, and mechanical properties were analyzed. The uneven hardness behavior of the joint and its influence on mechanical properties were further revealed.