Study Of The The Microstructure And Properties Of Inconel 718 Superalloy Fabricated By Selective Laser Melting

Inconel 718 alloy has good oxidation resistance,creep resistance,weldability,and high temperature strength.Thus,it is widely used as high-temperature parts such as engines,turbine disks,blades,and compressors.Generally,these parts are difficult to be manufactured by using traditional machining processes in one time because of their complicated shape.Additive manufacturing?AM?technology could break through the limitations of the shape and can directly form three-dimensional parts.However,there are some defects like microsegregation and non-equilibrium phases in the as-built sample.The subsequent heat treatment is critical for AMed parts to eliminate defects and regulate the microstructure and properties.The effect of heat treatment on the microstructure and properties of Inconel 718alloy fabricated by laser selective melting technology?SLM?was investigated.The microstructure of the as-built and conventional heat-treated SLM-Inconel 718 alloys were observed using optical microscopy,scanning electron microscopy,electron backscatter diffraction and transmission electron microscopy.The phase transition laws,creep behavior and fracture mechanism of SLM-Inconel 718 samples were discussed.Then,the heat treatment scheme was explored,and the microstructure evolution during the solution and aging heat treatment was studied.On the basis,a novel heat treatment scheme was proposed to improve the plasticity of SLM-Inconel718 alloy.The results of the experiments showed that:1.The vertical section?parallel to the building direction?of the as-built sample was composed of columnar substructures,which finally grew to dendrities.In the horizontal section?normal to the building direction?,columnar and cellular sub-structures were formed in the overlapping area,while only cellular sub-structures existed in the central fusion area.In addition to the?and Laves phases,almost no other phases were precipitated,so it exhibited low strength and high plasticity.2.When treated by the direct double aging?DA?process,?-Ni₃Nb phases mainly precipitated in the Nb-rich interdendritic regions,the?'-Ni₃?Al,Ti?and?"-Ni₃Nb were precipitated in the matrix.Homogenization plus double aging?HA?,and solution plus double aging?SA?treatments significantly eliminated microsegregation and Laves phases.The?phase dissolved in the HA sample but precipitated in the SA sample.The?'phase was prone to precipitate when treated by HA,while?"phase tended to precipitate in the SA sample.For all of the three heat-treated samples,the strength was improved greatly and the plasticity was sacrificed.3.The dendritic structures of HA and HSA samples were broken,and some fine recrystallized grains were formed,increasing the length of weak grain boundaries.In addition,the?'and?"phases with larger size in the HA and HSA samples could hinder the movement of dislocations,producing higher dislocation densities and increasing creep rates,which are harmful to creep properties.4.During the creep,the Laves and?phases at the grain boundary easily caused the formation of voids,the voids continued to accumulate along the grain boundaries and then formed the micro-cracks,which eventually led to the intergranular fracture.5.As the solution temperature increased,the micro-segregation in the sample gradually improved.when solution temperature was higher than 1100?C,the Laves phase gradually dissolved,the sample began to recrystallize,and the grain morphology and dislocation density changed greatly,but the solution treatment above1100?C had little influence on the morphology and distribution of the strengthening phase precipitated during the double aging process.6.The new heat treatment?NHT?resulted in recrystallized grains and annealing twins.The ultrafine?'and?"phases were precipitated uniformly in the matrix,and the microstructure was different from that of traditional heat treatment samples.Compared with the HA sample,the strength of the NHT sample was the same level,but the plasticity was greatly improved.