Research On The Microstructure,Properties And Heat Treatment Of Inconel 718 Alloy Fabricated By Selective Laser Melting

Inconel 718 alloy,as a nickel-based superalloy with the largest amount of consumption,is widely used in aerospace,nuclear industry,energy and other high temperature and high load fields due to its good mechanical properties,excellent corrosion and oxidation resistance under high-temperature environments.However,Inconel 718 parts formed by traditional casting and forging processes are prone to serious element segregation,coarse grains,shrinkage cavity or simple structure,which limits the application of Inconel 718 alloy.However,Selective laser melting technology breaks through the limitation of traditional casting and forging process on product structure,and has unique advantages in preparing Inconel 718 alloy parts with complex structure,fine crystal grains,high density and performance.In this paper,a quantitative analysis of domestic atomized Inconel 718 alloy powder is carried out in combination with a number of powder performance detection indexes,and the optimal SLM process parameters for forming good performance samples are optimized.Based on the traditional heat treatment scheme of Inconel 718 alloy forming through casting and forging,the influence of heat treatment process on the microstructure and properties of SLM-formed Inconel 718 alloy was studied,and a heat treatment scheme which can significantly strengthen SLM-formed Inconel 718 alloy was optimized.Based on the actual application conditions of Inconel 718 alloy parts,the effects of heat treatment process on mechanical properties of Inconel 718 alloy parts are analyzed in detail from room temperature tension,high temperature tension and ultra-high symmetrical bending fatigue.Analysis of the particle size,shape,fluidity and morphology of Inconel 718 alloy powder shows that the particle size distribution of Inconel 718 alloy powder is reasonable,84.6%of the powder particle size is concentrated in the range of 15.9?44.8?m,D₁₀=17.4?m,D₅₀=27.6?m and D₉₀=44.2?m.Powder particles with circularity greater than 80%account for 96%of the total number of particles,the aspect ratio of 85%powder particles is greater than 70%,particles with bluntness greater than 70%account for 80%,and nearly 70%powder does not undergo satellite formation.The Hall flow rate of the powder was 3.13 g/s and the compressibility was14.3%.The powder has high sphericity and good fluidity and is suitable for SLM forming.The main phase of the powder is matrix?phase,and the surface of the powder is mainly cellular crystal and dendritic crystal structure.The effects of laser power,scanning speed and energy density on the surface roughness,compactness and hardness of SLM formed parts were compared and analyzed by single factor variable method.The optimal process parameters for forming Inconel 718 alloy by SLM were obtained as follows:laser power 260 W,scanning speed 650 mm/s,layer thickness 30?m,scanning spacing 100?m,and energy density 133 J/mm³.Under this parameter,the upper surface roughness of the sample is Ra3.44?m,the density is 99.15%,and the hardness is 304HV_0.2.The dendrite and cellular crystal structure in the crystal grain of the sample formed by the optimal parameters are obvious and mainly consist of matrix?phase and Laves phase.A large number of brittle Laves phases can be detected in the interdendritic phase.At the same time,the yield strength of the sample is 700 MPa,the tensile strength is 993 MPa,the elongation after fracture is 20.4%,the elongation after fracture in the deposited state is 4 times that of the standard casting and 1.7 times that of the standard forging,but the strength of the sample is insufficient.By comparing the effects of laser parameters on the SLM forming Inconel 718 alloy,it is found that the matching mode of low laser power,high scanning speed or high laser power and low scanning speed easily leads to larger fluctuation,poor continuity of melt path,as well as the increase of holes and large volume spherical particles in the sample,thus increasing the surface roughness of the formed part.At the same time,it will also promote the formation of irregular holes or large spherical holes in the material and reduce the density and hardness of the sample.Reasonable laser energy density can reduce nodularization defects,improve the lap ratio of melt paths,avoid over-burning of alloy and reduce porosity.When the energy density is in the range of 110?140 J/mm³,the overall forming effect of the sample is better in quality.The microstructure and phase transformation of the samples before and after heat treatment were studied.The results show that the homogenization temperature can affect the recrystallization degree of the alloy,the dissolution amount of Laves phase and dendrite.At the same time,increasing the homogenization temperature can significantly affect the number and size of short rod-like?phase precipitated at grain boundaries after solid solution aging treatment.Aging treatment can effectively promote the precipitation of strengthening phase,but the precipitation amount and size of strengthening phase are related to Nb content in matrix and are affected by high temperature homogenization.After homogenization treatment under three different conditions?980°C×2 h,1080°C×2h,HIP×2h?,the hardness of the sample generally decreases,but after solution aging treatment,the strength and hardness of the sample are significantly higher than that of the deposited state.The yield strength,tensile strength and hardness of solution+double aging?SA?state reach 1243MPa,1375 MPa and 504 HV_0.2 respectively,and the elongation after fracture is 10.4%.The comprehensive mechanical properties and economy are the best among the three heat treatments.Through the analysis of microstructure,it is found that the strength of the material is affected by the content of?"phase,the dissolution degree of Laves phase,?precipitation,grain size and microscopic defects.According to the tensile fracture morphology at room temperature,the fracture dimples of the samples in different heat treatment states are uniform and fine,which are mixed fracture modes of transgranular fracture and intergranular fracture.The effects of the best heat treatment process on the high temperature mechanical properties and ultra-high cycle symmetric bending fatigue properties of the specimens are analyzed.The tensile strength,yield strength and elongation after fracture of of SA-state high-temperature stretching?650°C?are 1086 MPa,955 MPa and 11.4%,respectively.Compared with room temperature tensile,the yield strength and tensile strength are decreased by 21.0%and 19.9%,respectively,but the elongation after fracture is increased by 9.6%;The tensile strength at high temperature is higher than that of the standard forging as deposited,and the plasticity is equivalent to that of the standard forging.The ultra-high cycle symmetric bending fatigue test results show that under low stress condition,the fatigue life of SA state sample can reach 2.67×10⁸ cycles,which is 6 times higher than that of deposited state.When the cycle times are reach to 10⁶ and 10⁷,the fatigue strength of SA state is increased by 63.7%and 21.2%respectively compared with that of deposited state.