Study On The Behavior And Control Of Nitrogen During Selective Laser Melting Forming Of High-nitrogen Steel

Substitution of nitrogen for nickel in high-nitrogen stainless steel facilitates inexpensive manufacturing.However,the conventional high pressure melting procedure for high-nitrogen stainless steel manufacture is intricate,necessitates expensive equipment,and presents substantial machining challenges,thereby impeding its production and application.As an essential metal additive technology,laser selective melting does not necessitate molds and can swiftly produce parts,facilitating"net forming"and subsequent machining reduction.Its potential integration into high-nitrogen steel formation is being explored.Presently,efficacy-related research is nascent.Therefore,this article utilizes laser selective melting technology for high-nitrogen steel formation.Following high pressure nitrogen addition thermodynamics and micro melt pool nitrogen addition kinetics analysis,a high-nitrogen steel melt thermodynamics model is developed,with the experimental plan underpinned by phase diagram calculation.The dissolution and precipitation dynamics of nitrogen during laser selective melting and forming,the effects of nitrogen pressure,Cr N nitrogen increasing agent,and nano Ti N modified powder on high-nitrogen stainless steel sample nitrogen content,density,microstructure,and mechanical properties,are examined,culminating in gaining insight into nitrogen dissolution and precipitation dynamics,and laser selective melting and forming high-nitrogen steel process.These findings have considerable academic and commercial importance in furthering high-nitrogen steel development.The specific research contents and main conclusions are as follows:Firstly,the nitrogen solubility in Fe-18Cr-11Mn-3Mo-x N（wt%）high-nitrogen steel under various nitrogen pressure conditions was deduced.It revealed that nitrogen solubility was notably elevated with increasing nitrogen pressure.At 1.5 MPa nitrogen pressure,the nitrogen content in the micro melt surpassed 1.6 wt%,aligning well with the calculated data from Fe-N binary phase diagram.Examination into nitrogen addition and evolution dynamics in laser selective melting micro melt pools suggested that nitrogen additives facilitated nitrogen introduction into steel melts.Meanwhile,modeling calculations of ideal arrangement laser absorption rate of high-nitrogen steel powder showed that the inclination between high-nitrogen steel powder impacts the number of laser reflections between the powder,thereby determining powder laser absorption efficiency.Hence,controlling the laser selective melting process for variable powders,particularly overmixed ones,proves necessary to yield an optimal process.Secondly,Fe-18Cr-11Mn-3Mo-（0.32-0.41）N high-nitrogen steel powder enabled successful preparation of high-nitrogen steel via atmospheric and high-pressure laser selective melting processes.The impact of nitrogen pressure on the nitriding mechanism and microstructure properties of high-nitrogen steel was scrutinized and compared.Under varying laser energy densities from 69.44 to 182.29 J/mm3,nitrogen content of atmospheric pressure samples varied from 0.249 to 0.361 wt%（with an initial nitrogen content of powder as 0.41 wt%）,featuring predominantly ferrite with minor austenite.Its microhardness range was 263.8 to 423.14HV_0.3;The nitrogen content of high-pressure samples could reach0.384～1.07 wt%（with an initial nitrogen content of powder at 0.32 wt%）,and the structure was a uniform austenite with a microhardness within the range of 245-281HV_0.3.Elevating nitrogen pressure notably boosted the nitrogen content of laser selectively melted high-nitrogen steel.Subsequently,employing the atmospheric pressure laser selective melting forming method,Cr N nitrogen increasing agent was mixed with high-nitrogen steel powder to generate distinct overmatch powder.This study evaluated the effect of varying overmatch powder on the microstructure and properties of high-nitrogen steel.The findings suggest that high-nitrogen steel with superior nitrogen content can be fabricated through selective laser melting at atmospheric pressure with overblended powder.The initial nitrogen content,physical attributes of overblended powder,forming parameters,and alloy constituents significantly affect the final nitrogen content of high-nitrogen steel.Boosting the initial content of the overblended powder,enhancing powder fluidity and particle size uniformity,reducing laser energy density,augmenting chromium and manganese content,and diminishing nickel content positively impact the final nitrogen content.High-nitrogen steel retains an austenitic phase primarily but exhibits escalating ferrite constituent with increased energy density,demonstrating ideal comprehensive mechanical properties.The N1 powder forming sample demonstrates the highest tensile strength of 1216 Mpa,the N2 powder forming sample yields the highest yield strength of 971 Mpa,and the L1 powder forming sample presents the highest elongation of 21.56%.Given that these samples exhibit superior mechanical properties,optimal process conditions include laser power of 250W,scanning speed of 800mm/s,scanning spacing of 0.08mm,powder thickness of 0.03mm,and laser energy density of 130.21 J/mm³.Ultimately,to augment the mechanical attributes of high-nitrogen steel,a combination of outperforming powder and tin nanoparticles was employed for modification.The effect of such tin nanoparticle modification on the structure and characteristics of high-nitrogen steel caused by laser selective melting of outperforming powder were examined.The findings indicate that tin nanoparticles efficiently refine the grain size of high-nitrogen steel,enhancing its strength and toughness.The resultant microstructure of tin-containing high-nitrogen steel consists mainly of fine equiaxed austenite and ferrite due to considerable grain refinement.The alloy’s strength and ductility are substantially enhanced with the addition of tin nanoparticles.Under identical processing conditions,the maximum tensile strength of tin-added high-nitrogen steel increases by 127 MPa,the maximum yield strength ascends by206 MPa,and the average elongation grows by 4.6%.Figure 101;Table 23;Reference 224...