Microstructure And Mechanical Behavior Of Selective Laser Melted Inconel 718

In recent years,selective laser melting(SLM)has gained wide attention from multiple industries,such as the aerospace,energy,biomedical and automotive industries due to its numerous advantages,including the unrivalled design freedom,customized production,near-net shape manufacturing for geometrically complex components as well as reduction in R&D time and cost.However,there are still many key issuses need to be addressed before the widespread industrial application of SLM process.In this thesis,SLM-fabricated Inconel 71 8 using in the gas turbine blades was chosen.A series of key scientific issues related to the fabrication,the post treatment as well as the qualification and certification of SLM parts have been systematically investigated.This study has theoretical and practical significance for clarifying the process-structure-properties-performance relationships,exploring the possible strategies to optimize the mechanical properties,and establishing the qualification and certification standard for the mechanical properties of SLM parts.The important results were drawn below:1.Two types of scanning strategies(bidirectional scanning with 0°(SS X)and 90°(SS XY)rotation between the successive layers)were adopted to study the effect of scanning strategy on the microstructure and mechanical properties of SLM-fabricated Inconel 718.Under a fixed energy density(74.2 J/mm3),a weak texture changed to a strong cube texture when the 90°-rotation was applied,which is attributed to the change in the cell growth direction and the competitive grain growth mechanism by the scanning strategy.A criteria of cell growth direction within the melt pool was proposed:0°???54.7°.Moreover,the tensile and fatigue strengths at room temperature are effectively improved without significantly reducing the tensile plasticity by tailoring the scanning strategy.2.The anisotropic tensile properties of SS X and SS XY specimens were systematically investigated.It is found that the yield strength of these two types of specimens exhibits obvious anisotropy behavior,which is mainly caused by the non-uniform distribution of residual stress parallel to and perpendicular to the building directions.Compared with SS XY specimen,only SS X specimen shows anisotropic strain hardening behavior.The proposed equal stress-equal strain model indicates that the inhomogeneous plastic deformation between the adjacent grains of SS X specimen is a function of relative grain sizes and relative grain orientations.The anisotropic strain hardening behavior of the SS X specimen is mainly controlled by the relative number of activated slip systems between the adjacent grains.3.Three types of heat treatment routes were newly designed to tailor the microstructure of SLM-fabricated Inconel 718.After heat treatment,tensile and fatigue strengths are improved dramatically due to the elimination of Laves phases and the precipitation of ?" strengthening phases.Besides,the strength,plasticity and fatigue limit are found to be simultaneously enhanced by tailoring the size and distribution of acicular ? phases.In the case of the basically same volume fraction,the smaller the space of acicular ? phase is,the more significant its contribution to the strain hardening rate is.The nanoscale acicular ? phase can effectively hinder the dislocation movement,and even if it breaks,such tiny crack can hardly propagate into the surrounding matrix,resulting in the suppression of degraded effect of voids on the fatigue limit.4.The tensile and fatigue properties of SLM-fabricated Inconel 718 specimens with the thickness ranging from 0.1 mm to 1 mm were investigated systematically.The"microstructure unit" reflecting the microstructure characteristic of SLM-fabricated materials was defined.As the ratio(t/d)of the specimen thickness to the "microstructure unit" size reduced below one,the occurrence of premature necking and dramatic decrease in the tensile plasticity is mainly attributed to the change of strain localization behavior.A probabilistic statistical model between fatigue limits and the specimen thickness was preliminarily established that could be used to predict the fatigue limits of the thin-wall metallic parts.To ensure stable mechanical properties,the thickness of miniature test specimens or the minimum wall thickness of SLM parts should meet the requirement of t/d?4.5.Effects of the surface roughness and the build thickness on fatigue properties of SLM-fabricated Inconel 718 were investigated at 650?.The result indicated that fatigue strength was enhanced by?50%after surface machining and polishing.The thinner un-machined specimens exhibit a longer fatigue lifetime than the thicker ones,while the opposite trend appears in the machined specimens.Finite element simulations reveal that the defect shape has more evident influence on fatigue properties than the defect depth when it is less than 200 ?m.Fatigue safety thresholds of the stress concentration factor and the defect depth are 2 and 50 ?m,respectively.