Microstructure and properties of laser-MIG hybrid welded joints for aluminum alloy extrusions with different gap sizes

Four-millimeter thick 6005A-T6/6106-T6 aluminum alloy extrusions were laser-MIG hybrid welded with different gap sizes. The influences of the gap size on the weld formation, microstructure, and mechanical properties were studied. When the gap sizes were 0 mm and 0.7 mm, the weld formation, microstructure, and mechanical properties were almost the same. The welding speed should be reduced and the arc current should be increased when the gap sizes increased to 1.0 mm and 1.5 mm, resulting in the linear energy and arc energy ratio increased. The size of the cellular dendrites increased and the grain boundaries became apparent when the gap size exceeded 1.0 mm, even cracks appeared near the fusion line when the gap size increased to 1.5 mm. The lowest microhardness existed in the heat affected zone near the fusion line when the gap size smaller than 1.0 mm, but which appeared in the weld zone when the gap size was 1.5 mm. The tensile strength slightly decreased as the gap size increased. The tensile specimens fractured near the fusion line when the gap sizes smaller than 1.0 mm and characterized by ductile fracture. The tensile specimens fractured in the heat affected zone when the gap size was 1.5 mm and characterized by ductile-brittle mixed fracture. When the gap size increased to 1.5 mm, the median fatigue limit and safe fatigue limit respectively decreased by about 26% and 30% due to the cracks existing near the fusion line in the heat affected zone.