Dynamic behavior of plume and its effect on weld formation in laser welding of high-nitrogen austenitic stainless steel under various shielding gas types and pressures

High-nitrogen austenitic stainless steel (HNASS) is susceptible to defects such as nitrogen loss during conventional fusion welding, which compromises the service performance of the joints. Atmosphere-pressurized laser welding can effectively suppress nitrogen escape and improve microstructural properties. However, the dynamic behavior of the plume is a critical factor influencing laser energy transmission and weld formation. This study investigates the effects of shielding gas type (N 2 , Ar) and pressure (0.1, 1.0 MPa) on the weld morphology, plume behavior, and physical properties of the plasma. The results indicate that as the atmospheric pressure increases from 0.1 MPa to 1.0 MPa, the weld penetration depth decreases by approximately 60%, and the solidification mode of the joint microstructure transitions from the FA mode to the A mode. Under high pressure (1.0 MPa), the plume shifts from a stable eruption mode at atmospheric pressure (0.1 MPa) to a periodic “expansion-contraction” eruption mode, with its average area increasing by 26%-38%. In terms of shielding mechanisms, the scattering, absorption, and refraction of the plume plasma collectively contribute to laser energy attenuation. The elevated pressure intensifies the Rayleigh scattering of particles. Under the N 2 atmosphere, the dissociation of nitrogen molecules leads to a 74.4% increase in electron density, thereby enhancing inverse bremsstrahlung absorption. Conversely, under the Ar atmosphere, beam divergence becomes more pronounced due to a “negative lens effect” induced by refraction.