Research On Microstructure And Properties Of In-situ Nitrogen Strengthening Ti6Al4V Via Selective Laser Melting

As one of the most attractive metallic additive manufacturing technologies,Selective laser melting(SLM)process has great advantages in the structural design and manufacturing of metal parts due to unique characteristics of layer-by-layer additive manufacturing.Therefore,SLM process is widely applied in many fields,making it become a popular research direction of advanced manufacturing technologies in recent years.Due to high strength and low density,titanium alloy is widely used in aerospace,and biomedical and other fields.With the increasing complicated conditions,the mechanical performance of titanium alloy is also put forward more harsh requirements.However,because of the great flexibility in the design of material composition,SLM process has great advantages in the alloy strengthening as well as the preparation of metal matrix composites.This dissertation proposed a method to regulation the manufacturing atmosphere during the SLM process of Ti6Al4V alloy.By using nitrogen-argon mixed atmosphere during the SLM manufacturing of Ti6Al4V alloy,the process parameters and volume ratio of nitrogen were adjusted to achieve the preparation of in-situ nitrogen strengthening Ti6Al4V alloy.The microstructural evolution and the strengthening mechanism of mechanical performance were studied.The main research contents and conclusions as follows:(1)The effect of in-situ nitrogen solid solution strengthening of SLM-manufactured Ti6Al4V alloy under the low volume ratio of nitrogen atmospheric condition(5 vol.%N₂)was researched.According to XRD and TEM results,it is found that the?-Ti lattice is distorted due to the solid solution effect of nitrogen atoms,which caused that the(0002)crystal interplanar spacing increased.The microstructure characterization showed that martensites of SLM-manufactured Ti6Al4V alloy was coarsened under the 5 vol.%nitrogen atmospheric condition.It is obvious that nano?phases has increased as well as the more uniform distribution.Thanks for the multiple effect of in-situ nitrogen solid solution,the coarsening of martensites,and the increasing of nano?phases,the mechanical performance of SLM-manufactured Ti6Al4V alloy was extremely improved(?_0.2?1300 MPa,e_f?6%).(2)The in-situ Ti N reinforced of SLM-manufactured Ti6Al4V alloy under the higher volume ratio of nitrogen atmospheric condition(15?25 vol.%N₂)was studied.Under the higher volume ratio of nitrogen atmospheric condition,the nitrogen content of Ti6Al4V alloy further increased.But the lattice distortion did not aggravate significantly.According to the results of microstructure,it is suggested that the obvious molten pool boundary appeared due to the more intense thermal history.The martensites were significantly refined under the 25 vol.%N₂ atmospheric condition.The extremely fine lamellar microstructures and precipitated dendritic Ti N were formed.The hardness and wear resistance of SLM-manufactured Ti6Al4V alloy under high nitrogen atmosphere are also improved compared to those under pure argon atmosphere.(3)The mechanism of in-situ nitriding SLM-manufactured Ti6Al4V alloy under argon-nitrogen atmosphere was demonstrated.During the SLM process under the nitrogen-argon atmosphere,nitrogen was ionized and generated plasma after being heated by laser source.Nitrogen plasma with most absorbed heat collided the surface of molten pool at extremely rapid speed.This process sharply promoted the heat transmission as well as the dissolution of nitrogen during the additive manufacturing process,which resulted in increasing the peak temperature of the molten pool.(4)The microstructure evolution of SLM-manufactured Ti6Al4V alloy under argon-nitrogen atmosphere was discussed.Under the low volume ratio of nitrogen atmospheric condition,nitrogen is mainly dissolved into the octahedral interstice of the?-Ti lattice in the form of interstitial solid solution,causing the distortion of lattice.With the increasing of nitrogen content,the solid solution of nitrogen in the?-Ti lattice reached the limitation.Ti N dendrites were precipitated during the solidification process.Due to the interaction between the laser and nitrogen,the peak temperature in the molten pool is higher than that in the pure argon atmosphere during the SLM process.The intrinsic heat treatment was more severe during the SLM process.Under the low volume ratio of nitrogen atmospheric condition,the limited effect of intrinsic heat treatment could not enough to reach the?transform temperature.This effect only promoted the growth of microstructures,which corresponded to the coarsening of microstructures.Under the higher volume ratio of nitrogen atmospheric condition,the peak temperature of the molten pool further rose,which led to a sharper temperature gradient and cooling rate.Because the intrinsic heat treatment was more intense,the temperature of molten pool reached the?transform temperature.Under the effect of the higher cooling rate,the significant refinement of microstructures was occurred.(5)The mechanism of in-situ nitrogen strengthening was revealed.The mechanical performance of SLM-manufactured Ti6Al4V alloy gradually increased with the increasing of the nitrogen content.Under the low volume ratio of nitrogen atmospheric condition,the hardness of SLM-manufactured Ti6Al4V alloy increased linearly with increasing nitrogen content.It is demonstrated that the strengthening contribution was mainly attributed to the solid solution strengthening.Such a linear trend was not maintained as nitrogen increased further.It is indicated that the solubility of nitrogen almost reached limitation.The contribution of solid solution strengthening no longer increased.The further improvement of hardness was mainly attributed to the grain refinement strengthening and the second phase strengthening.By studying the mechanism of in-situ nitrogen strengthening Ti6Al4V alloy in the SLM process,the in-situ nitriding process,microstructure evolution and strengthening mechanism are revealed.The relationship among nitrogen content,microstructure,and mechanical performance was established.This provided a strengthening method for SLM-manufactured titanium alloy.