Study On CMT Arc Additive Manufacturing Process Of High Nitrogen Steel For Quaternary Shielding Gas

This paper aimed at the problems of unstable cladding process,nitrogen loss,and nitrogen holes in the high-nitrogen austenitic stainless steel arc additive manufacture process.This topic is based on the robot CMT arc additive manufacturing technology to study the effect of quaternary shielding gases on the high nitrogen steel additive manufacture process,and optimized the distribution ratio of the high nitrogen steel CMT arc additive manufacture protective gases.Firstly,a single-channel additive manufacture test for CMT was performed.Through high-speed camera observed the droplet transfer under different protective gases.It was found that the main reason for the instability of the CMT droplet transfer process in high nitrogen steel is the explosion splash during the droplet formation process.Collect the corresponding arc shape and electrical signal parameters,and analyze the changes.It was found that when the N₂ content was between 0%and 10%,the O₂ content was between 0%and 2%,and the He content was between 0%and 30%,a more stable cladding process could be obtained.Secondly,through systematic study of single-pass additive manufacture surface forming,internal porosity content,nitrogen content,microstructure and composition,and microhardness.It was found that the addition of N₂ helps to balance the nitrogen partial pressure inside and outside the molten pool,suppresses the formation of pores.Besides,molten pool solidification mode changed from FA solidification mode to A solidification mode.The addition of He will help the internal pores to float and reduce the formation of internal pores,but if the content is too high,it will prolong the existence of the molten pool,resulting in a greater loss of nitrogen content.And will promote the further growth of ferrite dendrites,forming secondary dendrites or even tertiary dendrites.The addition of O₂ can reduce the tendency of nitrogen atoms to combine with each other to generate nitrogen,and inhibit the formation of pores,and increase the supersaturation of nitrogen in the liquid molten pool.But when the content is too high,the burnout of Mn is exacerbated,the formation of austenite structure is suppressed,and the number of stripe and granular ferrite increases.Finally,based on the developed shielding gas process window,the surface forming,internal defects and tensile properties of the additive manufacture components were studied.It was found that the surfaces of members 2 to 5 are well formed,and the flatness is about 1.3mm;the cracks existing in the channels of the additive manufacture components and the inclusion defects in the layers are caused by the Mn oxide generated during the additive manufacture process.By optimizing the protective gas composition,it is possible to obtain an additive member with fewer pores and a lower defect rate.There is obvious plastic deformation in the tensile macro fracture of the additive member,and there are a large number of dimples and spherical second-phase particles in the micro fracture.The fracture mode is microporous aggregate fracture.And the maximum tensile strength reaches 870MPa and the elongation after breaking reaches 30%.In the end,the optimized optimal multiple protective gas ratio was73.5%Ar+5%N₂+1.5%O₂+20%He.