Microstructure and corrosion resistance of laser-MIG 316L stainless steel under the nitrogen assistance

In order to enhance the MIG arc stability, improve the internal microstructure and strengthen the corrosion resistance of 316L stainless steel weldments manufactured by MIG under the pure argon gas, a 1 200 W low power laser was introduced to induce compression on the MIG arc, with N₂ mixed into Ar to explore the effect of Ar-N₂ mixed shielding gas with different flow rates on the microstructure and corrosion resistance of the 316L welding seam. Experimental observations display that the MIG arc became more stable under the induced effect of 1 200 W laser. With the increase of N₂ gas flow rate, the fusion line of the molted pool become smoother and the internal porosity defects are significantly reduced. XRD tests and microstructure observations indicate that the content of internal γ-phase increase significantly. It can be clearly seen that most fine cellular γ phase distributed uniformly in the lower middle regions of the molted pool, and the upper middle regions were dendritic γ phase, with its primary dendrite spacing gradually decreased. As the N₂ gas flow rate increase to 5 L/min, the micro-hardness of the welding seam could be enhanced by 20 HV. Electrochemical polarization tests revealed that the Laser-MIG 316L welding seam formed under the Ar-N₂ mixed gas exhibit stronger corrosion resistance. Above experiments confirmed that the N₂-assisted laser-MIG hybrid welding technology can improve the microstructure and corrosion resistance of 316L stainless steel weldments, and when the Ar : N₂ gas flow rate is 20 : 5, the strengthening effect of γ phase is most significant and the best corrosion resistance can be achieved comprehensively.