Effects of Ar-N2-He shielding gas on microstructure, mechanical properties and corrosion resistance of the Laser-MIG additive manufacturing 316L stainless steel

To eliminate excessive δ-ferrite in the 316L stainless steel fabricated by wire arc additive manufacturing (WAAM) under traditional Ar-20%CO2 shielding gas, a novel Ar-N2-He mixed shielding gas with laser intervention was first applied in laser-MIG hybrid additive manufacturing 316L. The results show that the residual δ-ferrite can be completely removed, and the highest fraction of γ-austenite can be obtained under 80%Ar-10%N2-10%He, which is in total contrast with the amounts of skeletal/dendritic δ-ferrite in Laser-MAG 316L shielded by 80%Ar-20%CO2. Meanwhile, the Laser-MIG 316L has significantly higher mechanical properties and corrosion resistance than the Laser-MAG 316L due to the combined effects of the interstitial solid solution strengthening of N, inhibition of Cr-Mo-Ni atomic segregation at the δ-γ interface, and substantially greater enrichment of Cr-Mo-Ni-N passivating species in the passive film. In particular, the higher fractions of Cr2O3, MoO3 and NH4+ were identified to reduce the oxygen vacancy density defects and suppress the film dissolution. Furthermore, the allowable ratio of N2 can be increased to 15% during the Laser-MIG depositing 316L process, which can never be achieved in the traditional MIG 316L process due to the more adequate conversions among the excited N2, N, and N+ species within the higher stabilized Laser-MIG arc plasma. This study confirms the feasibility of applying Ar-N2-He shielding gas in eliminating δ-ferrite and improving the service performance of the 316L stainless steel fabricated by Laser-MIG additive manufacturing without post-heat treatment.