Research On In-situ Preparation Of Titanium Matrix Composites By Selective Laser Melting In Nitrogen Atmosphere

Selective laser melting（SLM）technology is a metal additive manufacturing technology that has developed rapidly in recent years.It has a wide range of application prospects due to its advantages of designability,rapid net shaping of complex components,and high surface quality.When using SLM to prepare titanium matrix composites,using nitrogen as an active atmosphere source to react with the clad titanium alloy can generate nitrides with higher hardness and improve the wear resistance of the materials.However,there are few reports on the effects of N₂ concentration and laser energy density（LED）on the microscopic morphology and physical properties of the samples when the SLM process is used to prepare Ti6Al4V-based composites in N₂ atmosphere.In this paper,the Ti6Al4V spherical powder was used as the original powder,and the titanium matrix composites were formed by SLM in a mixed atmosphere of N₂/Ar.The reinforcement phase reaction mechanism and reinforcement mechanism are explained.The research results are mainly summarized as:1.In this thesis,it is found that the optimal LED window of preparing of Ti6Al4V alloy by SLM is 84.8-163.6 J/mm³ under a pure Ar atmosphere.Within this window,the sample has no obvious defects,and the sample has high density and the best mechanical properties.2.When the concentration of N₂ remains unchanged,with the increase of LED,the degree of nitridation of the sample is deepened,and the nitrides in the microstructure gradually increase.When LED reaches above 218.2 J/mm³,large-sized nitrides appear in the sample,and the plastic loss of the sample is serious.3.When LED was constant,the N₂ concentration in the SLM forming environment was changed,and a uniformly distributed nano-scale enhanced phase was generated in the sample in a low-concentration N₂ atmosphere（3 vol.%,10 vol.%）.The hardness,ultimate strength,and yield strength of the samples were improved,but plasticity was not lost.4.During the SLM process,a Ti-based composite material with Ti N as the reinforcing phase was formed in situ through a gas-liquid reaction.The enhancement mechanism relative to titanium matrix composites lies in that the nano-scale Ti N particles generated by the in-situ reaction are uniformly distributed in the matrix structure,and the grains of the titanium matrix are refined due to the"pinning"effect.Part of the N solid solution enters into the Ti lattice,which plays the role of solid solution strengthening.The nitride precipitation phase formed by the gas-liquid reaction has good dispersibility,which can play the role of precipitation strengthening and dispersion strengthening;and the phase interface is clean,which is conducive to the transfer of load to the reinforcing phase,and can give full play to the strengthening effect of the reinforcing phase.