Controlling Corrosion Behavior of Inconel 718 via Additive Manufacture Based on Grain Boundary Engineering
Inconel 718 is a typical γ"phase precipitation strengthened superalloys,it is widely used in gas turbine,marine,nuclear power plant and aerospace industry.In this environment,corrosion resistance has become an important index for the engineering appli-cation of Inconel 718 alloy.Corrosion is a complex electrochemical process,and the corrosion behavior of polycrystalline alloys is closely related to grain boundaries.Therefore,it is necessary to explore the special relationship between grain boundary characteristic distribution and corrosion properties.Grain boundary engineering refers to optimizing the grain boundary characteristic distribution of through thermomechanical treatment process,so as to improve the properties related to grain boundary.It is found that the proportion of special grain boundaries is greatly increased and the effect of optimization for grain boundary character is better in fine-grained sam-ples after grain boundary engineering.Nickel-base superalloys formed by selective laser melting(SLM)usually have very fine grains.Therefore,grain boundary engineering can be used to optimize the grain boundary characteristic distribution of SLM-built alloy to fur-ther improve the material properties,which also expands the application scope of SLM.It is found that grain boundary engineering is conducive to the complete recrystallization of the alloy manufactured by SLM to optimize its mechanical properties.However,there are few studies on the corrosion properties of SLM-built alloys by grain boundary engineering.In view of this,this paper took SLM-Inconel 718 alloy as the research object and prepare SLM-Inconel 718 alloy sample with different grain boundary characteristic distribution by different grain boundary engineering treatment.The purpose was to find a suitable process route to optimize the grain boundary charac-teristic distribution,so as to improve the corrosion performance of the alloy.SLM parameters for Inconel 718 alloy were:laser power of 280 W,scanning speed of 950 mm-s-1,scanning spacing of 110 μm,layer thickness of 40 μm,rotation angle of 67° between layers and stripe scanning strategy.SLM-Inconel 718 alloy samples were cut into Φ6 mm×9 mm cylindrical samples by wire cutting.After so-lution treatment at 1150 ℃/1.5 h/air cooling(AC),the samples were compressed to 5％,10％and 20％at a deformation rate of 0.1 s-1 at room temperature(25 ℃),and then annealed at 1150 ℃ for 0.5 h.Electron back scattered diffraction(EBSD)data were collected by scanning electron microscope(SEM)equipped with HKL-EBSD probe.The step size of EBSD test was selected as 1 μm,then ∑(reciprocal of density of coincidence site lattice)values were determined by Channel 5 software according to Palumbo-Aust standard.Low ∑CSL(coincidence site lattice)and random grain boundaries were classified,with the grain boundary ratio as a percentage of length.The results showed that the microstructure of SLM-Inconel 718 alloy treated with 10％deformation+1150 ℃/0.5 h was com-posed of fine recrystallized grains and annealing twins,which indicated that recrystallization was basically completed under this pro-cess.The proportion of high angle grain boundarys(HAGBs)and tiwn boundarys(TBs)in the alloy was the highest.Meanwhile,the proportion of low ∑CSL grain boundary in 10％deformation+1150 ℃/0.5 h SLM-Inconel 718 alloy was the highest(64.23％),where the proportion of ∑3 grain boundary was 50.03％,and the proportion of ∑9+∑27 grain boundary was 13％.When the proportion of(∑9+∑27)/∑3 was high,multiple twins developed more fully,forming more mutual"3"oriented grain boundary clusters".At 10％de-formation,the proportion of(∑9+∑27/∑3 was 0.26 also much greater than 0.015 and 0.025 under 5％and 20％deformation,which meant that 10％deformation was more conducive to the formation of grain boundary clusters in SLM-Inconel 718 alloy.The results of Tafel polarization curve and impedance curve measured by electrochemical workstation were used to characterize the change of corro-sion resistance.It could be seen from the polarization curve that the potential of 10％deformation+1150 ℃/0.5 h SLM-Inconel 718 al-loy was the most positive(-0.225 V),and the corrosion current density was 2.732×10-8 A·cm-2.Because the corrosion current density was directly proportional to the corrosion rate,the average corrosion rate was calculated as 1.8×10-4 mm·year-1.It could be seen from the impedance diagram that the diameter of Nyquist diagram of the sample with 10％deformation is larger than that of the sample with 20％and 5％deformation,which proves that the corrosion sensitivity of 10％deformation+1150 ℃/0.5 h SLM-Inconel 718 alloy was much lower than that of the sample with 20％and 5％deformation.The amplitude and phase angle of impedance in bode diagram were usually used to characterize the stability of sample passivation film.For 10％deformation+1150 ℃/0.5 h SLM-Inconel 718 alloy,the proportion of J3 grain boundary as nearly 30％,followed by J2,J1 and J0.The original random grain boundary network with high con-nectivity had been interrupted,so that the corrosion resistance of samples with 10％deformation had been significantly improved.
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