Effect Of Heating Treatment On Microstructure And Properties Of SLM Formed Inconel 738 Alloy

Inconel 738 is a precipitation-reinforced cast nickel-based high-temperature alloy containing refractory elements.It is widely used.Its working temperature is less than 900℃.It has good durability,creep resistance,oxidation resistance,and high corrosion resistance.The alloy haves the best corrosion performance.The Selective laser melting（SLM）is used to product Inconel 738 alloy parts.Due to the characteristics of rapid melting and solidification during the SLM process,the formed alloy has the disadvantages of high residual stress,easy deformation and cracking,and insufficient precipitation of precipitated phases.The formed parts cannot be directly used.In this paper,the three-step heat treatment:stress relief annealing,solution,and aging is used to adjust the microstructure and properties of SLM formed Inconel 738 alloy,a new heat treatment process is developed,and the evolution of the structure,the behavior of precipitated phases and the rel ationship with the properties are studied.The evolution of microstructure and mechanical properties of SLM formed Inconel 738 alloy after holding at different annealing temperature（700⁹00℃）for 24hours was studied.Studies have shown that after performing stress-relief annealing at different temperatures（700⁹00℃）,the micro-melt pool and sub-crystalline structure are maintained.Continuous dynamic recrystallization and discontinuous dynamic recrystallization occurred during the deposition state,and the dislocation migration rearranged after annealing,and the mechanism of tissue evolution was recovery.When the annealing temperature is 800℃,the residual stress decreases from 310.4 MPa（deposited state）to-66.7 MPa,and the residual stress is eliminated.Form a bimodal distribution of spherical（50²50 nm）and short rod-like（240⁴40nm）γ’phase.MC transition to M₂₃C₆ occurred,and the grain boundary carbides grew continuously.When the annealing temperature is 800℃,the alloy has high hardness and tensile strength（581.2 HV,1330 MPa）which are 1.42 times and 1.41 times the casting process performance（410 HV,945 MPa）,but the elongation is lower（3.78%）.The evolution of the microstructure and mechanical properties of SLM formed Inconel 738 alloy after solid solution treatment at different temperatures（1000¹270℃）for 2 hours on the basis of the annealing treatment（800℃×24 h/AC）formulated earlier was studied.Studies have shown that alloys are affected by the solute atoms and carbide,the recrystallization temperature is lower than direct solution.The static recrystallization mechanism is the nucleation of a bow in the grain boundary and a small number of subcrystal nucleation.The grain boundaries migrate and the recrystallized grain grows,the recrystallization ratio and the recrystallization texture are enhanced.no solid solution occurred at 1000℃.Incomplete solid solution occurs between 1120¹220℃.A microstructure with coarse regular square（0.3⁰.5um）and uniform fine spherical（100²00 nm）,two shapes and sizesγ’were formed,γ’square positive degree is the best at 1170℃.,complete solid solution occurs at 1270℃,and a uniform and fine（200-300 nm）squareγ’phase precipitates.At 1170℃,M₂₃C₆ begin to dissolve,and the square MC carbides are distributed in chains along the grain boundaries.With the increase of solution temperature,the carbides in grain boundary gradually change from continuous to intermittent and then to point distribution.When the temperature is higher than 1170℃,M₂₃C₆ dissolves and MC grows up.After treatment at 1170℃,the tensile strength and elongation（1361 MPa,17.2%）of the alloy are optimized coMPared to the annealing performance（1330 MPa,3.78%）,and have the best comprehensive mechanical properties.But The alloy is a supersaturated solid solution,the structure is metastable,and still needs further heat treatment after stress relief annealing+solution treatment.The Evolution of microstructure and mechanical properties of SLM forming Inconel 738 alloy after aging treatment at different temperatures（650⁹50℃）for 24hours was studied.It is based on stress relief annealing+solution solution（800℃×24 h/AC+1170℃×2 h/AC）formulated in the early stage.Studies have shown that:after aging,the fine grain structure and subcrystalline structure are maintained.There is no obvious change in theγ’phase at low temperature at650⁸50℃.As the aging temperature increases,theγ’phase gradually grows.,theγ’phase is obviously coarsened,theγ’phase degenerates into a spherical shape at950℃.There is no obvious change in MC at low temperature between 650⁸50℃.With the increase of aging temperature,M₂₃C₆ gradually precipitated around MC.At950℃,the grain boundary M₂₃C₆ begins to dissolve.The cracks start from the inside of the carbide or the interface between carbide/γmatrix phase,and expand along the grain boundary direction.The multi-point initiating micro-cracks gradually expand and connect to each other,which eventually leads to the fracture of the specimen.After the alloy undergoes stress relief annealing+solution+agi ng treatment,the hardness first increases and then decreases,the tensile strength gradually decreases,and the elongation rate decreases first and then increases with the aging temperature increases.When aging at 750℃,A small amount of M₂₃C₆ was precipitated,obtain aγ’matching structure with two types of shapes and sizes,which has the best structure and comprehensive mechanical properties（1345 MPa,16.2%）coMPared to other aging temperatures,which is the casting properties（945 MPa,7.5%）1.42 times and2.17 times.A new heat treatment system（800℃×24 h/AC+1170℃×2 h/AC+750℃×24 h/AC）for SLM forming Inconel 738 alloy was selected.Inconel 738 alloy formed by SLM has achieved the goal of stabilizing microstructure and improving mechanical properties after the heat treatment.