MICROSTRUCTURES AND MECHANICAL PROPERTIES OF DUAL-BEAM LASER KEYHOLE WELDED JOINTS OF ALUMINUM ALLOYS TO STAINLESS STEELS

Aluminum alloy and steel thin sheets have been mostly used in the automotive industry to get a lightweight car body. Nowadays several studies are focused on the joining of aluminum alloy to steel by new welding methods especially by laser welding. In this work dual-beam fiber laser keyhole welding was introduced to joining of 1.5 mm-thick aluminum alloys to 1.8 mm-thick 304 stainless steels in an overlap joint configure. The influences of different laser focusing positions on the weld appearance, interface microstructures and tensile mechanical resistance of the welded joints were studied. As a result, the good weld appearance of the aluminum alloy to stainless steel joints were obtained by dual-beam fiber laser keyhole welding process without any filler materials. The thickness of the intermetallic compound layer of the joint interface is comparatively thin when the laser beam with low energy is focusing on the front. The nano-hardness testing results show that the average hardness of intermetal-lic compound layer is 9.61 GPa, which is significantly higher than that of the parent stainless steel of 4.12 GPa and aluminum alloy of 1.09 GPa. The fracture of the welded joints occurs on the aluminum alloy/stainless steel interface layer. The highest mechanical resistance of 131 N/mm can be obtained by the low energy laser beam focused on the front.