| The integration of structural and functional realization of complex components has gradually become an important means of improving the performance and innovation of aero-engines and gas turbines.Laser Solid Forming?LSF?can digitally manufacture the complicated component without die in a short time,making it possible to manufacture the complicated part of aeroengine,which has gradually become an important way to realize the integrated manufacturing of aero-engine and gas turbine complex components.However,for nickel-base superalloys,which are widely used in aero-engine and gas turbine hot-end components,most of them exhibit a relatively high thermal cracking sensitivity during LSF process.And the microstructure and properties of laser solid formed nickel-base superalloys are often not fully utilized.Therefore,this paper focuses on a solid solution strengthened nickel-base superalloy,Inconel 625 superalloy,which is widely used in aero-engines and gas turbines,and has relatively low thermal cracking sensitivity.The microstructural evolution during LSF and subsequent heat treatment process and the corresponding mechanical properties of the room temperature were studied.The deformation behavior and microstructural evolution mechanism during the tensile and high temperature compression were investigated.Additionally,the crack formation mechanism and cracking control of Inconel 625 during LSF was analyzed.This research would lay the solid foundation for the theory of laser solid formed?LSFed?Ni-based superalloy component,the following conclusion can be drawn:?1?The microstructural evolution of LSFed Inconel 625 has been obtained.LSFed Inconel625 mainly consist of epitaxial columnar grain.The equiaxial grain formed by columnar-equiaxial transition?CET?mainly locates in the top part of the specimen.The segregation of Nb,Mo would result in the formation of interdendritic??+Laves?eutectic crystal.During the multi-layer and multi-pass processing,temperature gradient in the molten pool would gradually decrease,while the dendritic spacing would increase,as the increment of deposition height.Secondary dendritic arm would become more and more obvious and the size of interdendritic Laves phase would also increase,from the bottom zone to the top zone.In addition,in the deposited specimen,it has the strong<100>texture because of the combing effect of heat flow and preferential growth.?2?The stability of LSFed Inconel 625 microstructure is relatively high.When the solution temperature increases from 900°C to 1200°C,the diffusion rate of alloying elements such as Nb and Mo increases gradually,the Laves phase remelting rate increases significantly,and the volume fraction of Laves phase decreases gradually in the sample.And the texture becomes weaker.Meanwhile,the as-deposited sample fully recrystallizes when the solution treatment temperature reaches 1200°C,and the epitaxial growth columnar grains are replaced by equiaxed grains.Only a small amount of columnar grains recrystallized during the solution treatment when the solution temperature is lower than 1200°C.?3?With the increment of solution temperature,the yield strength of LSFed Inconel 625superalloy gradually decreases,the elongation gradually increases,and the tensile strength changes little.When the solution treatment temperature reaches 1200°C,the yield strength decreases from 500 MPa to 360 MPa,and the elongation is doubled compared with the as-deposited sample,reaching 60%.However,the tensile strength is maintained at about 750 MPa.The main reason is that as the solution treatment temperature increases,the dislocation density in the samples decreases,resulting in a decrease in yield strength.Meanwhile,the strain hardening coefficient of Inconel 625 superalloy gradually increases,and the uniform plastic deformation ability of the sample becomes stronger during the tensile process.And the plasticity of the alloy has been significantly improved,while the enhancement of the work hardening effect during the plastic deformation process results in little change in the tensile strength of the alloy.?4?The constitutive equation of LSFed Inconel 625 under high temperature deformation(950-1150?,0.001-1 s-1)has been established:?(5=?exp?442110/?=2.2×1015×[sinh?(0.0047)]4.24,and then the thermal deformation active energy of LSFed Inconel 625(442.11 k J?mol-1)is lower than that of the forged Inconel 625(522.5 k J?mol-1).With the increasing deformation temperature and decreasing strain rate,the critical stress and peak stress of LSFed Inconel 625 would gradually decrease and the softening effect is not so obvious under high temperature deformation.?5?As the deformation temperature increases and the strain rate decreases,the volume fraction of dynamic recrystallized grains and grain size would increase.Under low deformation temperature and high strain rate,the recrystallization mechanism of LSFed Inconel 625 is primarily continuous dynamic recrystallization;In contrast,under high deformation temperature and low strain rate,the recrystallization mechanism would be discontinuous dynamic recrystallization.Compared with forging parts,the kinetic process of dynamic recrystallization of LSFed Inconel 625 is relatively slow and its dynamic process is insufficient.?6?Crack initiation mechanism of LSFed Inconel 625 has been acquired.The crack in the deposited specimen of LSFed Inconel 625 is mainly made up of solidification crack,distributing in the interdendritic along the deposition direction,which results from the formation of eutectic membrane caused by the segregation of Nb,Mo,Si etc in the interdendritic zone.The crack would be easily initiated in such eutectic membrane under thermal stress.Also,the crack would be left in the deposited specimens because of the inadequate supply of liquid metal form molten pool.?7?The increase of titanium content can effectively inhibit the occurrence of solidification cracks in Inconel 625 during LSF process.The increase of titanium content can reduce the solidification temperature range of Inconel 625 alloy and reduce the sensitivity of the alloy to solidification cracking.In addition,the increase of titanium content can promote the formation of more eutectic liquid phase between the dendrites,which is conducive to the cohesion of solidification cracks.Considering the influence of the addition of titanium on the solidification crack and mechanical properties of LSFed Inconel 625 alloy,the addition of 3%titanium content is suitable. |
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