| INCONEL718(IN718)alloy is widely used in the aerospace industry due to its excellent high temperature mechanical properties,corrosion resistance,oxidation resistance and fatigue resistance.Some aerospace components are often designed as thin-walled structures or complex curved surfaces and high-diameter ratios in order to ensure its dynamic performance,which will cost more money and time by using traditional manufacturing process.Laser Additive Manufacturing(Laser Additive Manufacturing,LAM)technology has the characteristics of high molding freedom,short molding cycle,less following manufacturing steps.It shows great application potential in manufacturing such structures.However,due to the fast solidification rate of LAM technology,it is easy to cause microstructural defects such as lacking of strengthening phases and severe microsegregation.So it is necessary to carry out the subsequent heat treatment process to deal with the original microstructures.During the heat treatment process,the grain boundary charicteristics and precipitate will change significantly,which will affect the mechanical property.Thus it is of great importance to research on this issue.In this article,IN718 alloy was fabricated by Slective Laser Melting,and processed by the following solid solution and aging heat treatment.The multi-scale structure characterization methods such as X-Ray diffraction technology,electron backscatter diffraction technology,scanning electron microscope technology and the transmission electron microscope technology are employedto analyze the microstructure and crystal structure of the as-deposited and heat-treated alloy.The growth mechanism,grain boundary characteristics,precipitates formation and evolution law have been studied deeply.The research results show that:1.The solidified microstructure of the IN718 alloy formed by LAM shows anisotropy.In the section parallel to the deposition direction,there are a large number of columnar grains that enlongate along the building direction,and a small amount of equiaxed grains are distributed around the columnar grains.The epitaxial growth between the layers is epitaxial growth in[001]direction,and the epitaxial growth between tracks is 900 epitaxial growth in the form of[001]?[100].The cross-section.<001>{100} texture is caused by the two epitaxial growth modes.After the crystal grains pass through the molten pool,Competitive growth mechanism launches.The smaller the angle between the preferred crystallographic orientation and the direction of the steepest temperature gradient is,the faster the growth rate is.After solution heat treatment of the deposited structure,static recrystallization begins to occur at 1100?,and complete at 1120?.After recrystallization,the grain morphology is a large number of twins.The driving force for recrystallization is the residual stress caused by the fast melting and solidification during deposition.2.The grain boundary of the as-deposited IN718 alloy is characterized by the high angle grain boundary with high local misoritation.The internal cellular grain boundaries form Low-angle grain boundaries.Low-angle grain boundaries account for 38.26%of the total grain boundaries.After solid solution of the as-deposited alloy,the low-angle grain boundariesbegin to decrease and transforms into sub-grain boundaries when the temperature exceeds 1090?.The content of low-angle grain boundaries is less than 5%at 1120?,and 65%of large-angle grain boundaries are ?3 twin boundaries.The mechanism of nucleation in the initial recrystallization stage is the nucleation and sub-crystal merging mechanism.In the later stage of recrystallization,The stacking faults caused by high angle grain boundary migration form a large number of twin boundaries When the solution temperature is 1100?,75%of the twin boundaries are incoherent ?3 twin grain boundary,which has strong mobility.As the temperature increases,collisions occur during the migration of the grain boundaries,resulting in ?9 and ?27 grain boundaries,thus forming ?3n grain boundary clusters which effectively block the connection of the High-angle grain boundary.When the solid solution temperature reaches 1120?,the coherent ?3 twin boundary is the main twin boundary,and the mobility becomes worse,and the total amount of ?9+?27 grain boundaries decreases.There are coherent twins and incoherent twins on the interface.The incoherent twins segment is composed of dozens of atomic layers.The formation of twins undergoes stacking fault?Transition from nano twins?non-coherent twins?coherent twins.3.The phase composition of the as-deposited IN718 alloy is:matrix ? phase,MX phase,and Laves/? eutectic phase.There is a segregation zone of Nb and Ti elements with a thickness of 50nm at the grain boundary and subgrain boundary of the matrix.MX and Laves phases are distributed in this segregation zone.After solution treatment,the segregation band disappears when the temperature is above 980?.The relationship between minimum solid solution time and temperature are expressed as:t=1/3169 exp(21597/T).The MX and Laves cannot be completely re-dissolved.After aging treatment,only nano-level ?" phase is precipitated under 720?/8h and uniformly distributes inside the crystal grains,and the size is about 50nm long,10nm wide.?" is enriched at the sub-grain boundaries due to the high concentration of Nb element in this area,forming a ?" precipitation zone.at 800?/8h,8 and coarsened ?" phases are precipitated at the same time.There are 12 variants of 8 phase and 3 variants of ?" phase,both of which are similar in morphology.Both have plate-like,disc-like and needle-like morphologies.The length is about 0.5?1?m.The Angle of ? phase variant is about 710 and that of ?"phase variant is 900 which can be determined the trace analysis method.The precipitation type and size of strengthening phase are determined by temperature,but the amount of strengthening phase is determined by element enrichment4.The 8 phase precipitates during the medium and high temperature aging.Before the formation of ? phase,the modulation structure is formed in the matrix.With the increase of time,the modulation structure is transformed into a long-period structure,and finally the ? phase is formed.With the increase of temperature,the transformation rate from modulated structure to long-period structure is accelerated,and the precipitation rate of 8 phase is accelerated.The size of 8 phase precipitated is larger.The needle-like 8 phase is more likely to be precipitated in high temperature aging,and the plate-like and block 8 phases are more likely to be formed in medium temperature aging,? "phase precipitates in the middle and low temperature aging,?" coarsely forms micron scale precipitates in the middle temperature aging,and ? "fine and dispersed in the matrix to form nanometer scale precipitates in the low temperature aging,and no modulating structure is formed before precipitates.?-Cr phase precipitates at medium and low temperature,and the fastest precipitates at 700?.There are two variations in this phase.The crystal relationship between ?-Cr and the matrix is:[111]Cr ?[001]?;(101)Cr ?(110)?[111]Cr ?[001]?;(011)Cr ?(110)?,In the rapid solidification process of LAM,the supersaturated solid solubility of Cr element increases,which promotes the precipitation of Cr in the aging process. |
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