Study On Microstructure And Properties Of IN718/CoNiCrAlY Dissimilar Materials Fabricated By Laser Powder Bed Fusion

The commonly used preparation methods of IN718/CoNiCrAlY superalloy dissimilar material parts exist some problems,such as thin protective layer thickness,poor interfacial bonding strength,and difficult preparation of complex structure,which is gradually unable to meet the preparation requirements of superalloy dissimilar materials with high performance and complex structure.Laser powder bed fusion（LPBF）technology shows significant advantages in the low-cost and efficient fabrication of high-performance dissimilar materials with complex geometry.In order to overcome the limitations of traditional methods for preparing superalloy dissimilar materials,a new process for preparing IN718/CoNiCrAlY dissimilar material was proposed in this study.The influence of LPBF and post-treatment process on microstructure and mechanical properties of IN718 alloy were studied,and the parameters of LPBF in preparing CoNiCrAlY alloy were optimized.The interfacial microstructure and mechanical properties of IN718/CoNiCrAlY dissimilar material were analyzed.The oxidation kinetics of single IN718 alloy and IN718/CoNiCrAlY dissimilar materials were studied,and the oxidation mechanisms of the corresponding materials were revealed.Firstly,the LPBF process optimization of IN718 alloy was completed.The optimal laser volumetric energy density（VED）for IN718 alloy was 67.5 J/mm³,when the relative density of the alloy is 99.76%.The grain morphology of the alloy is composed of columnar structure parallel to the deposited direction and equiaxial structure perpendicular to the deposition direction.The microstructure is mainly composed ofγmatrix phase and Laves phase,and its tensile properties at room temperature reach the level of casting.Furthermore,the post-treatment processes of LPBFed IN718 alloy were studied.After the hot isostatic pressing（HIP）treatment,the relative density of LPBF specimens can increase to about 99.99%.HIP process also results in complete recrystallization of grains and promotes the dissolution of Laves phase.After solution-aging（SA）heat treatment of LPBF and HIP specimens,the density and grain morphology of the two specimens have not changed significantly.A great number ofγ’andγ′′phases have precipitated out of theγmatrix,and some Laves andδphases exist near the grain boundary.Meanwhile,the mechanical properties of the two specimens are higher than that of forging specimens.In addition,after HIP or SA treatment,the friction coefficient and wear rate of LPBF specimens are significantly reduced.Secondly,the LPBF process in preparing CoNiCrAlY protective layer was studied.Based on the hot crack sensitivity of CoNiCrAlY alloy,the effects of laser rotation angle and preheating temperature of substrate on the crack defects were studied.It was determined that the least crack defects were obtained when the laser rotation angle was 20°and the preheating temperature of substrate was 180℃.Subsequently,the effects of laser VED on the porosity,surface roughness,microstructure and mechanical properties of CoNiCrAlY alloy were investigated.The results show that excessive VED lead to high surface roughness,coarse grain size and poor mechanical properties of the specimens,while too low VED lead to high porosity and poor mechanical properties of the specimens.CoNiCrAlY alloy exhibits excellent comprehensive properties under the optimum VED（75.8 J/mm³）condition:relative density is 98.55%,microhardness is 345±6 HV,tensile strength is 931±14 MPa,yield strength is 767±9 MPa,elongation is 2.1±0.5%and friction coefficient is 0.385±0.018,respectively.The microstructure of CoNiCrAlY alloy fabricated by LPBF is composed ofγ-Co（Cr,Ni）phase,β-NiAlphase and Ni₂Y phase.Based on the study of microstructure and properties of IN718 and CoNiCrAlY alloys fabricated by LPBF,the interfacial microstructure and mechanical properties of IN718/CoNiCrAlY dissimilar material fabricated by LPBF were further studied.The results show that the IN718/CoNiCrAlY dissimilar material fabricated by LPBF exhibits good metallurgical bonding at the interface,and no obvious defects such as cracks and non-fusion are found.The higher grain average orientation difference in the interfacial region indicates an obvious stress concentration at the interface,which is related to the differences in lattice parameters of the dissimilar materials.The microstructure of interfacial transition region is mainly composed ofγ-Ni（Co,Cr）phase,β-NiAlphase,Ni₂Y phase andη-Ni₃Al_0.5Nb_0.5precipitate phase.The dissimilar material shows high bond strength（871.4±13.0 MPa）and good elongation（8.3±0.5%）.In addition,the ultimate tensile strength and bending strength of the dissimilar material are lower than those of the single IN718 and CoNiCrAlY alloys.Finally,isothermal oxidation tests of IN718/CoNiCrAlY dissimilar material（IN718 as the matrix）fabricated by LPBF process were carried out at 1100℃for different time（1-100h）.The high temperature oxidation mechanism and kinetics of IN718 alloy and IN718/CoNiCrAlY dissimilar material were compared.The results show that the oxides of IN718 alloy exhibit sandwich structure,in which the outermost layer isNiCr₂O₄,Fe₂O₃,Ti O₂and Nb₂O₅,the middle layer is Cr₂O₃layer,and the inner layer is Ti Nb O₄ thin film layer.The oxidation resistance mainly depends on the middle Cr₂O₃ layer.However,the oxidation reaction of dissimilar material only occur in the CoNiCrAlY alloy region,and the IN718 alloy region below it does not occur.The oxides of CoNiCrAlY alloy region are divided into two layers,the outer layer is mainly Cr₂O₃,（Co,Ni）Cr₂O₄ and CoNiO₂,the inner layer is dense continuousα-Al₂O₃ layer.The high temperature oxidation resistance of the dissimilar material is obviously better than that of IN718 alloy.This is because the denseα-Al₂O₃ film with excellent high temperature stability is formed on the surface of CoNiCrAlY alloy,which hinders the diffusion of elements and the oxidation reaction.In addition,the increases of oxide thickness of IN718 alloy and CoNiCrAlY side of dissimilar material both follow Parabolic law.