Study On Microstructure And Fatigue Properties Of IN 718 Alloy Based On Selective Laser Melting

IN 718 alloy has been widely used in aerospace and other fields due to its excellent mechanical properties,but the traditional processing methods have long cycle,high cost,and complex structure is difficult to manufacture.The emergence of additive manufacturing technology solves this problem well,but there are significant differences microstructure and performance of the samples fabricated by additive manufacturing and traditional manufacturing.In this paper,optical microscopy（OM）,scanning electron microscopy（SEM）,electron backscatter diffraction（EBSD）,X-ray diffraction（XRD）and some other techniques were used to study the microstructure,texture and mechanical properties of IN 718 alloy built at different building orientations using different laser energy density.The fatigue properties of laser selective melted and heat treated IN 718alloy were examined by high cycle fatigue test and the fatigue fracture mechanism was clarified.With the decrease of laser energy density,the axial ratio of the columnar grains decreases and the grains become small due to the lack of epitaxial growth.When using high energy density,a strong<001>fiber texture parallel to the building direction is promoted.However,with the decrease of energy density,the texture strength gradually weakened,and the grain orientation gradually showed diversity.Tensile specimens constructed along the 45°direction have the highest strength due to their high Taylor factor.In addition,with the decrease of laser energy density,grain refinement also plays a strengthening effect.The anisotropy of yield strength becomes more significant with the increase of laser energy density due to texture.Through the high cycle fatigue test,the fatigue limit of the sample without heat treatment is 285 MPa,and the relationship between the fatigue limit and the tensile strength is:sw（28）0.24s_uts.The near surface defect is the main fatigue source.As the fatigue source,the crack originates and expands from the defect,which may lead to secondary cracks and finally the quasi-cleavage fracture.After heat treatment,the main strengthening element Nb,Al,Ti and other elements in IN 718 alloy are redistributed,resulting in a large number of precipitates,mainly the short rod-likeδphase（Ni₃Nb）distributed along grain boundaries,and the sphericalγ′phase（Ni₃（Al,Ti））and the flake（needle-like）γ″phase（Ni₃Nb）dispersed in the grain.After heat treatment,the fatigue limit of the sample is 362MPa.Due to the strengthening effect ofγ′phase andγ″phase and the hindering effect ofδrelative crack propagation along the crystal,the fatigue limit is increased by 27%compared with that without heat treatment.The relationship between the fatigue limit and the tensile strength is:σ_w（HT）=0.2 3σ_uts（HT）.In addition to the near surface defects,other types of fatigue sources are also observed on the fracture surface of the specimens after heat treatment,such as the second phase particles,unfused particles,spheroidizing particles,and circular pores.Using the method of extreme value statistics combined with Murakami model,OM and SEM were used to calculate the defects size of block samples and fatigue fracture surface,so as to predict the fatigue limit.It is found that the fatigue limit prediction results of the samples without heat treatment are reliable.However,for the samples after heat treatment,the sensitivity of the material to small defects increases due to the Vickers hardness over 400 HV,so the model should be modified accordingly.