| As a metal additive manufacturing technology,wire and arc additive manufacturing(WAAM)has the advantages of high degree of design freedom,short production cycles,low material and equipment costs,high deposition rate and efficiency,and low environmental pollution.It is especially suitable for manufacturing and repairing large parts with moderate geometric complexity and has great potential for applications in various industrial fields.However,the deposition process of WAAM involves complex cyclic heating processes,including rapid heating and cooling,repeated melting and solidification,and directional heat dissipation.It generates complex structures and non-equilibrium microstructures in as-built parts with anisotropic properties.In this study,the as-built and heat-treated WAAM 316L block parts deposited with optimal deposition parameters were investigated for their organization,mechanical properties and deformation behavior,corrosion and stress corrosion properties,and their dependence.The structure and microstructure of each surface of the as-built and heat-treated samples were observed and analyzed by metallographic microscopy,X-ray diffraction spectroscopy,scanning electron microscopy and transmission electron microscopy.The relationship between the anisotropic microstructure and properties of the as-built parts,as well as the influence of heat treatment on the properties,were revealed by tensile tests with various-sized samples and electrochemical corrosion experiments.The in situ tensile tests were used to study the deformation and fracture behavior of various regions in the microstructure.The oxide film structure and the susceptibility of the microstructure to stress corrosion were revealed by corrosion weight gain tests and slow-strain rate tensile experiments.The main conclusions obtained were listed as follows:(1)The different multilayered and periodic structures in each surface and nonequilibrium microstructures in the layers in WAAM 316L block parts led to anisotropic mechanical and corrosion resistance properties.The multilayered structure mainly exhibited the changes in the ferrite morphology in the weld layer in the building direction,and the periodic structure mainly exhibited alternating remelting areas and overlapping areas in the transverse direction(perpendicular to the scanning direction in the layer).In the same weld layer,the remelting areas had a lower ferrite content,smaller austenite dendrite size,more dispersive orientation and lower residual stress than those of the overlapping areas.The elongated austenite columnar crystals in the section and side surfaces(parallel to the building direction)made the tensile strength anisotropic.And the different deformation behaviors of the remelting areas and overlapping areas in the periodic structure severely reduced the elongation in the section and top surface(perpendicular to the scanning and building directions,respectively).Fewer track-track fusion lines with weaker element segregation on the top surface led to its best pitting corrosion resistance.But the narrower ferrite primary dendrite arm spacing,lower ferrite content and more uniform element distribution in the remelting areas on the section surface provided its best general corrosion resistance.(2)High-temperature solution annealing heat treatment(above 1000℃)homogenized the microstructure within the layers and destroyed the periodic structure,which significantly improved the elongation and corrosion resistance,and eliminated their anisotropy.After the recovery and homogenization annealing heat treatment below 1000℃,the periodic structure and columnar austenite grains were unaffected,and only the morphology and volume fraction of ferrite and σ phase within the grains were changed.With the increase of heat treatment temperature,the ferrite gradually transformed into σ phase,the volume fraction of the second phase gradually decreased,the elongation slightly increased,the material strength and its anisotropic degree decreased,and the corrosion resistance decreased.After solution annealing heat treatment above 1000℃,the ferrite and σ phase were almost dissolved in the austenite matrix,and the austenite grains recrystallized,destroying the periodic structure.With the increase of heat treatment temperature,the volume fraction of recrystallized austenite grains increased,the strength of the material decreased,the elongation increased significantly,the corrosion resistance increased,and the anisotropy was eliminated.(3)The deformation and fracture of the remelting and overlapping areas in the periodic structure depended on the competition between material hardening and deformation fracture.When the tensile direction was perpendicular to the periodic structure,the deformation of the periodic structure followed the equal load model.The dislocation packing and grain rotation were induced by the higher proportion of high-angle grain boundaries activated many slip systems in the remelting areas,which made the hardening rate of the remelting areas higher than that of the overlapping areas.And the strain was gradually concentrated in the overlapping areas,where the local necking and fracture occurred preferentially.When the tensile direction was parallel to the periodic structure,the deformation of the periodic structure followed the equal strain model.The rotation of the grains in the remelting areas induced many slips and promoted the formation of twins,which made the plasticity of the remelting areas higher than that of the overlapping areas,and necking and fracture preferentially took place in the overlapping areas.(4)Different from that at the room temperature environment,in the hightemperature and high-pressure water environment,the deformation behavior of the periodic structure with equal strain deformation contributed to stress redistribution,making the direction parallel to the periodic structure less sensitive to stress corrosion while the other directions remained very sensitive to stress corrosion.The type of oxide film on the surface of the as-built and heat-treated samples in the hightemperature and high-pressure water environment was the same.The oxide films were composed of FeCr2O4 and Fe3O4,and the surface oxide films of the heattreated samples were denser than those of the as-built samples.In the hightemperature and high-pressure water slow-strain rate tensile experiments,when the tensile direction was parallel to the periodic structure,the periodic structure helped to eliminate the local stress concentration formed by the stress corrosion crack and promoted the overall uniform deformation;while in the tensile tests with the other directions,the local necking increased stress corrosion crack initiation and reduced the elongation.As a result,the direction parallel to the periodic structure was less sensitive to stress corrosion,and the anisotropy of elongation in each surface was the opposite. |
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