Effect of helium-argon shielding gas in laser-metal inert-gas hybrid welded-brazed Al/steel dissimilar joint

Laser-metal inert-gas welding hybrid welding was applied to dissimilar butt joining of 304 stainless steel and 6063 aluminum alloy using three different shielding gas: 100% argon, 25% helium + 75% argon, and 50% helium + 50% argon. The influence of helium-argon shielding gas on the weld appearance and interfacial intermetallics layer of Al/stainless steel welding-brazing joint was discussed. The addition of helium can shrink arc, suppress laser plasma, and increase the stability of the droplet transfer process. Helium could not change the composition of the interface layer, which consists of theta-Fe(Al, Si)(3) layer and tau(5)-Al7.2Fe1.8Si layer in the top, middle, and bottom regions of the joints. However, the interfacial intermetallics layer becomes thinner and the morphology was more uniform as the helium content changed from 0% to 50%. By analyzing the tensile fracture, it was found that the thicker theta-Fe(Al, Si)(3) easily caused the fracture at the interface layer when using 100% argon. The highest tensile strength and the best ductility can be obtained by using 50% helium + 50% argon compared with that of joints obtained with 100% argon and 25% helium + 75% argon. Compared with joints welded in the pure argon atmosphere, the strength and ductility of joints increased by 18.4% and 191%, respectively.