Controlling Dendritic Structure Of Laser Additively Manufactured Inconel 718 For Oxidation Property

Laser additive manufactured Inconel 718 own a mechanical property as high as wrought,which build the foundation for its widely application.While the application of laser additive manufactured Inconel 718 on the aircrafe engine as the material of hot components such as turbine disc requires an excellent high-temperature oxidation properties.However,the interdendritic microstructure such as segregation and constituent phase reduce the whole oxidation performance through affecting the local oxidation behavior,which would restrict the application of laser additive manufactured Inconel 718 on the area of aerospace and nuclear.In this thesis,the preferential interdendritic oxidation behavior and mechanism were investigated.The microstructure controlling method for quasi-continuous laser additive manufacturing and homogenization heat treatment process were established to prepare a high oxidation property laser additive manufactured Inconel 718.Moreover,the microstructure controlling method of quasi-continuous laser manufacturing were further applied in the laser surface melting to obtain a super corrosion resistance alluminum alloy surface.Firstly,observed the interdendritic microstructure and preferential oxidation behavior and characterize the microstructure of oxide scale.Calculated the thermodynamics and mass gain coefficient of interdendritic preferential oxidation.Preferential interdendritic oxidation of as-built additively manufactured Inconel718 is observed.Elemental segregation and primary precipitates are responsible for the formation of a deformed and fine-grain-sized Cr₂O₃ external scale and a discontinuous NbO interlayer that promote countercurrent diffusions of oxygen and metal cations.A low oxygen partial pressure gradient is established and subsequent complex NbO₂,NbO and(Cr,Al)O oxides are formed along the interdendritic region.Preferential interdendritic oxidation dramatically promotes both the transient and the diffusion-controlled oxidation kinetics.The mass gain coefficient of preferential interdendritic oxidation reaches 70.12%.Measures to homogenize the composition and to control the primary precipitates are desired to improve the oxidation performance.In addition,the controlled microstructure and oxidation behavior of quasi-continuous laser additive manufactured Inconel 718 was observed.Characterized the microstructure of intragranular oxide scale.The alleviated Nb segregation promote the formation of partial-oxidized NbO and?-Ni₃Nb mixed interlayer,improving the oxidation resistance of dendritic core by inhibiting the diffusion of O and Cr.While the fast diffusion of O and alloying elements through the Nb-depleted gap further reduce the oxygen partial pressure to 2.46×10⁴ atm/?m,resulting in the formation of Cr₂O₃internal oxidation zone.The short-circuit diffusion of O and alloying elements along the grain boundaries is the mechanism for preferential intergranular oxidation.Controlling the interdendritic structure for high oxidation performance should also keep the grain sizes.Further,the influence of heat treatment process on the as-built microstructure is studied.The microstructure controlling criteria for high oxidation performance is established as reducing segregation,decreasing constituent particle size and inhibiting recrystallization.Through the combination of quasi-continuous laser additive manufactured process and 1060?for 1h homogenization heat treatment,the Nb segregation is reduced to 6.53%,constituent particle size decrease to 0.497?m.the preferential intergranular oxidation is almost eliminated.The formation of continuous Nb-rich interlayer improve the oxygen partial pressure to 1.61×10⁵ atm/?m,oxidation mass gain k_p reduce one magnitude to 2.04×10^-7 mg².cm^-4.s^-1.The oxidation properties of laser additive manufactured Inconel 718 is successfully prepared as high as wrought.Finally,a quasi-continuous-wave laser surface melting process is performed on2219 aluminium alloys to improve the corrosion resistance.Observed the microstructure and corrosion properties of the base metal and laser-treated layer with different laser modes.The fast cooling rate and the multi-directional solidification behavior of the process transform large and long-chained constituent particles into fine and isolated circular precipitates with alleviated solute segregation.The improved corrosion resistance is due to the formation of low-flaw-density passive film,the reduction of the driving force for corrosion and the improvement of the re-passivation capability of the modified surface.The large and long-chained constituent particles are the preferred sites for corrosion initiation and propagation.