Microstructure and mechanical properties of laser-MIG hybrid multi-layer welded joints for 20-mm thick aluminum alloy plates

Laser-MIG hybrid multi-layer welding was performed upon the 20-mm thick 6082-T6 aluminum alloy butt-joints. The weld formation, microstructure, and mechanical properties of the welded joints were studied in details. The results indicated that the well-formed weld without obvious incomplete fusion and cracks could be obtained by using the optimal welding parameters, only very few porosities appeared in the filling layer and covering layer. The equiaxed crystals and columnar crystals were respectively observed in the weld center and near the fusion in the weld metal; their sizes and widths of each layer were different. The microhardness values of the weld metal and heat-affected zone are lower than those of the base metal; the lowest microhardness value appeared in the heat affected zone. The order of microhardness values in the weld center from high to low was filling layer, backing layer, and covering layer; their microhardness values were 74 HV, 70 HV, and 67 HV, respectively. The average tensile strength of the joints reached up to 235.2 MPa, which was 79.7% of the base metal. The tensile specimen fractured near the fusion line in the heat affected zone and the fracture propagated approximately parallel to the fusion line, and the tensile fracture showed a typical plastic fracture mode. The median fatigue limit and safety fatigue limit of the welded joints were 99 MPa and 93 MPa, respectively. The fatigue specimen fractured in the weld metal, and the crack initiated in the backing layer.