Microstructure and mechanical properties of laser-MIG hybrid welding of 1420 Al-Li alloy

This paper investigates the microstructure and mechanical properties of 1420 aluminum-lithium (Al-Li) alloy joints before and after heat treatment by CO2 laser-metal inter gas (MIG) hybrid welding. The 5-mm-thick 1420 Al-Li alloy plates were welded by CO2 laser-MIG hybrid welding. Full penetration joints without any defects were produced. Optic and scanning electron microscopy were used to study the microstructure and fractograph characteristics. The results show that the microstructures of the heat-affected zone (HAZ) and fusion zone exist as a predominantly discontinuous equiaxed dendritic structure and as a fine cellular dendritic structure, respectively. After heat treatment, the microstructures change from dendritic structure to a spheroidal crystal; the grain size of fusion zone is obviously larger than that of the base metal and the HAZ. Furthermore, the hardness recovers substantially to a level similar to that of the parent material. The tensile strengths of the joints in the as-welded condition and after heat treatment are 223 and 267 MPa, reaching up to 57 and 68 % of the parent materials' strength, respectively. The fractographs show that the joint as-welded condition exhibits the characteristics of dominated dimples and a small amount tear ridges, which are associated with the mixed ductile and brittle facture mechanisms. The fracture mode transforms from a transgranular to an intergranular after heat treatment; cleavage cracking coupled with an intergranular microvoid coalescence fracture mechanism occurs.