| Hastelloy X,a Ni-Cr-Fe-Mo solid-solution-strengthened Ni-based superalloy,is widely employed in the aerospace,petroleum and chemical industries because of its outstanding weldable properties,exceptional corrosion resistance,high oxidation resistance and high strength.It is difficult to meet the requirements for fabricating components with complex structures and high precision during the traditional manufacturing process.Selective laser melting(SLM)technology has been widely applied to form components with complex degrees of freedom structures.At present,the mechanical properties of SLM-fabricated high-temperature nickel-based alloy components are weaken by their poor surface quality,which is difficult to meet the high surface quality standards for components in the aviation and aerospace fields.Electrochemical Machining(ECM)has the advantages of non-contact,high efficiency,and high-precision material removal.It is expected to develop a composite processing technology of additive and subtraction materials with high accuracy and high efficiency,combining SLM and ECM.The microstructure inhomogeneity and surface quality of SLM-fabricated Hastelloy X alloy are different from the traditional cast,wrought and rolled Hastelloy X alloy,which will affect the electrochemical behavior of the anodic material in the ECM process,further affecting the dimensional accuracy and surface quality of the ECM workpiece and processing efficiency.Therefore,the influence mechanism of the microstructure characteristics of SLM-fabricated Hastelloy X alloy on its electrochemical behavior was systematically studied to become an important scientific basis problem to be solved urgently,promoting the process optimization of SLM/ECM additive and subtractive composite processing technology of Hastelloy X alloy.In this dissertation,SLM-fabricated Hastelloy X alloy is taken as the research object.Aiming at the relative basic science problems of SLM/ECM additive and subtractive material combined machining,the research on the surface quality,microstructure and mechanical properties of the deposited Hastelloy X alloy was investigated.The influence mechanism of SLM process parameters on the surface quality of as-deposited Hastelloy X alloy is clarified.The effect of microstructure characteristics on yield strength anisotropy at room temperature and high temperature plastic deformation is analysized,guiding the regulation of microstructure and preparing components to meet the requirements of mechanical properties.The anodic passivation behavior,passive film characteristics,anodic dissolution behavior and leveling mechanism of the deposited Hastelloy X alloy on different surfaces were systematically studied in the ECM process.The influence mechanism of the microstructure characteristics on the electrochemical behavior of the deposited Hastelloy X alloy is clarified.The anodic passivation behavior,passive film characteristics,anodic dissolution behavior and leveling mechanism of the deposited Hastelloy X alloy on different surfaces were systematically studied in the ECM process.The influence mechanism of the microstructure characteristics on the electrochemical behavior of the deposited Hastelloy X alloy is clarified.The main research results are as follows:(1)Based on the response surface method and variance analysis,the regression prediction model of the molding parameters and the density and surface roughness of the specimen is established,and the influence mechanism of laser power,scanning speed and scanning distance on the surface quality is revealed.The research results show that the density mainly depends on the laser power,while the surface roughness is mainly determined by the laser power and scanning speed.The optimal combination of multi-objective forming parameters was obtained by Design-Expert.The difference in microstructure characteristics between the specimen formed with the optimal process parameters and the wrought specimen is compared and analyzed,revealing its impact on different surface mechanics at room temperature and high temperature tensile state influence law of property anisotropy.The quantitative analysis of yield strength strengthening contribution shows that dislocation strengthening is the main reason for the anisotropy of yield strength of specimens parallel to(XOY surface)and perpendicular to the tensile direction(YOZ surface),and the strengthening contribution of wrought specimens is mainly composed of solid solution strengthening and grain boundary strengthening.A large number of deformation twins and more slip deformation sites are the main factors,leading to the significant increase in the elongation of wrought specimens during high temperature tensile.(2)Based on the above,the effect of micro structure on the anodic passivation behavior of Hastelloy X alloys prepared by two different processes is analyzed.The effective mechanism of different processes on the passivation behavior of Hastelloy X alloys is systematically discussed in combination with microstructural factors such as grain size,second phase distribution,and dislocation density.The research result shows that the anodic passivation characteristics decrease in the following order:XOY surface>YOZ surface>wrought surface.Higher passivation behavior of the XOY surface is mainly due to its higher dislocation density and finer grains.M23C6type carbide/matrix interface potential difference(50 mV)in the wrought Hastelloy X alloy contributes to the galvanic coupling between the precipitates and the y matrix,accelerating the dissolution rate of the passivation film.(3)A(semi)quantitative model of the influence of the microstructure characteristics of Hastelloy X alloys fabricated by two different processes on the formation of passive film is established to reveal the nucleation and growth mechanisms of passive film.The mechanism of SLM process on the performance of passive film for the Hastelloy alloys was systematically elucidated from microstructural evolution,compositional change,nucleation kinetics and dynamics,as well as film electronic properties.The research result shows that all the passive film exhibit n-type semiconducting behaviour.The vacancy diffusion rate at the interface between the passive film and the substrate is mainly determined by the diffusion coefficient D and oxygen vacancy diffusion coefficient Do.Steady-state passivation is mainly depended on the lower formation potential of passive film,while the interfacial equilibrium during passive is dominated by oxygen vacancy diffusion.During diffusion-controlled nucleation process,more active sites for the growth and nucleation of passive films on the XOY surface are provided due to the refined grain size and high density of low-angle grain boundaries.The lower oxygen vacancy diffusion rate contributes to forming a thicker and denser passive film on the XOY surface.(4)The influence mechanism of microstructure characteristics on the passivation behavior,anodic dissolution behavior and leveling process regarding the surface of Hastelloy X alloy fabricated by two different processes is studied.The breakdown process map of the passivation film is developed to clarify the breakdown behavior of the passivation film at low current density,including electrochemical breakdown reactions and mechanical breakdown reactions.The evolution process of the anodic leveling interface structure is mainly determined by the current density,action time and the difference in the morphology of the local corrosion pit.Pitting corrosion occurs preferentially at the region in the segregation zone around the M23C6-type carbide of the wrought samples,further developing into localized corrosion.In the state of high-speed scouring electrolyte,the current efficiency of the wrought specimen is always higher than that of SLM-fabricated Hastelloy X alloy.The current efficiency at a low current density is higher than that at high current density,indicating the stray corrosion occurs serious. |
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