Microstructure And Electrochemical Anodic Behavior Of Inconel 718 Fabricated By Laser Solid Forming

Laser solid forming?LSF?technology has been successfully applied to form small-scale parts with high precision and large-scale parts with high efficiency.However,a basic contradiction that efficiency is inversely proportional to precision during LSF process,making it difficult to give consideration to high efficiency and high precision for large-scale component fabricated by LSF.At present,combining the high-efficiency near net shape forming of LSF with near net material removal of mechanical processing and electrochemical machining?ECM?can solve this contradiction.Compared to mechanical processing,ECM possesses many advantages of no tool wear,high material removal rate and good surface quality.In addition,this technology is free of the mechanical properties of the anodic materials,such as strength and hardness.Based on this,Inconel 718 nickel-base superalloy,which is widely used in the field of aero-engine and gas turbine,is taken as the object in this work.Aiming at the relative basic science problems of laser/electrochemical hybrid precision forming Inconel 718,this paper investigated the surface morphology and microstructure of LSF Inconel 718,revealed the influencing mechanism of processing parameters on surface morphology,the correlation between laser solidification conditions with the as-deposited microstructure,investigated the passive behaviors and transpassive behaviors of the as-deposited Inconel 718 on different sections,mastered the anodic dissolution behavior of the constituent phases in the as-deposited Inconel 718,and developed the relationship between laser processing parameters,microstructural characteristic and electrochemical anodic behavior of the as-deposited Inconel 718 finally.Main conclusions were detailed as follows:?1?The influencing mechanism of laser processing parameters on the surface morphology of the LSF-fabricated Inconel 718 was revealed.The overlapping ratio during LSF process determines the top surface morphology of the LSF-fabricated Inconel 718;while the layer thickness and scanning strategy influence the side surface morphology.In addition,with the increasing of laser volumetric energy density,the top surface smoothness of the LSF-fabricated Inconel 718 improves.?2?The microstructure of LSF-fabricated Inconel 718 mainly consists of columnar grains along the build direction.A fiber texture exists in the as-deposited Inconel 718,and one of the<001>directions is parallel to the bulid direction.The columnar dendrite inside the grain is composed of?matrix,Nb-rich region,?/Laves eutectic and little?/Carbides eutectic phases.In addition,there exists microsegregation in the dendrite.The elements of Ni,Fe and Cr are rich in dendritic trunk,while Nb,Mo and Ti are rich in interdendritic regions.The columnar equiaxed transition?CET?occurs at the top of the as-deposited specimen,and the thickness of the discontinuous equiaxied grains zone is ranging from 0 to 500?m.The primary dendritic arm space increases linearly with the increasing of the laser linear energy density.With the increasing of cooling rate,the coarse?/Laves eutectic phase changes into discrete and fine particle.?3?The passive behaviors and transpassive behaviors of the LSF Inconel 718 on different sections were cleared.The polarization curves of the as-deposited Inconel 718 in quiet 10wt.%Na NO₃ solution?exposed to the air?is a typcial of?activation-passivation-transpassivation?.With the increasing of the anodic potential,the resistance of the passive film presents a type of?increment-stabilizaiton-decrement?.The passive film formed at 0.9 V?vs.SCE?for 10 h,was made of the oxides of Ni,Fe,Cr,Ti,Mo and Nb,including Ni₂O₃/Ni OOH/Ni?OH?₂,Fe OOH,Cr₂O₃,Ti O₂,Mo O₂,Nb O₂ and Nb₂O₅.The film possessing a p-type semiconductor electronic characterstic,is very thin.Although the as-deposited microstructure is anisotropy,the passive film leads to the dissolution rate isotropy.?4?The rules obeyed by anodic current from low current density to high current density were clarified and the current-potential selective map of LSF Inconel 718 Ni-based superalloys in high-flowing 10wt.%Na NO₃ electrolyte was developed.At the beginning of the anodic process,the reaction is controlled by activation polarization?7?9)?????1?.In passive region,as for the passive film with a p-semiconductor charge characterstic,the passivation current also obeys Tafel relationship?7?9)????(6?1?.Arrariving at transpassive region,the passive film become thin,and the oxygen evolution and anodic dissolution dominate.When the passive film ruptures,anodic current obeys the tafel relationship again.Until the diffusion speed of the metal ions reaches the limit,the anodic current is controlled by mass transmission process and the anodic current keeps constant.?5?The dissolution behavior of the constituent phases in LSF Inconel 718 was mastered.Nb-rich?phase dissolves faster than?matrix due to the higher lattice distortion energy of the Nb-rich?phase,the low electrode potential of Nb,and the atoms in the phase boundary of Nb-rich?phase.When the applied current density is 40 A cm^-2,the dissolution rate of?phase is 29?m s^-1,while the Laves phase is almost 0.High current density improves the surface micro-morphology smoothness.The current efficiency values of the as-deposited Inconel 718on different sections are the same.At low current density,the current efficiency is higher than that at high current density,indicating that the stray corrosion occurs early.