Investigation on laser-MIG hybrid-welded joint for 5A06 aluminum alloy: effect of the laser heat input on grain size and microhardness

Abstract

5A06 aluminum alloy plates with a thickness of 6.9 mm are welded by laser-metal inert gas (MIG) hybrid welding. To investigate the influence of laser heat input on grain size and microhardness, optical microscope observation, scanning electron microscope (SEM) and microhardness test are conducted. According to the microstructure distribution characteristics, the weld seam can be divided into arc-affected zone and laser-affected zone. The crystalline morphology from the weld boundary to the weld center is coarse columnar dendrites, finer columnar dendrites and equiaxed dendrites. In addition, the calculation and comparison of the grain size indicate that, the grain size Zl of the coarse columnar dendrites and the fine columnar dendrites in the laser-affected zone has increased, compared to both of the arc-affected zone. Due to the mechanical stirring effect on the upper molten pool, the finer equiaxed dendrites are formed at the center of arc-affected zone. It is worth noting that the grain size inhomogeneity has been significantly improved due to the decrease of laser heat input. The average microhardness of the arc-affected zone in case 1 is significantly lower than that of the other three cases due to the higher laser heat input. The laser line energy of 90 J/mm is more suitable for achieving the excellent microhardness property of laser-MIG hybrid welded joint for the 6.9 mm 5A06 aluminum alloy.