Study On The Microstructure And Mechanical Properties Of IN718 Alloy Produced By SLM Additive Manufacturing

IN718 alloy has good performance at high temperatures and is widely used in aero engine and turbine power generation industries.The use of traditional casting and forging technology to form IN718 alloy has certain limitations.Due to the presence of easily segregated elements Nb and Mo in the alloy,defects and brittle Laves phases are often generated during the casting or forging process,which all have a great impact on the properties of the material.Compared with traditional forming,selective laser melting（SLM）forming can reduce segregation and make the structure more uniform.Selective laser melting technology builds a three-dimensional model by software,and the laser will scan according to a prescribed path under the program.During scanning,the layers are superimposed and rapidly solidified.The elements that are easy to segregate during the rapid cooling process are dispersed in the matrix,and the crystal grains are too late to grow during the forming process.Therefore,it exhibits a relatively uniform and fine microstructure,which is beneficial to the improvement of performance.This technology has the advantages of short forming cycle,easy forming of parts with complex structures,and saving of raw materials.But there are also higher requirements for the forming process.Compared with traditional forming,the laser forming has significant improvement in stability and continuity and the control of the whole process.At the same time,a certain number of holes will appear.At present,there is no reliable data of nickel-based alloy material-process-performance.Therefore,this thesis adjusts the laser scanning interval process parameters to study the law between microstructure and mechanical properties,and provides effective data for the formation of nickel-based alloys.The main results of this thesis are as follows:X-ray microscopic analysis showed that with the increase of the laser scanning interval,the compactness of the IN718 alloy sample first increased and then decreased.When the laser scanning interval is 100μm,the compactness is the largest and the equivalent hole size is the smallest.The electron backscatter diffraction analysis shows that the crystal grains in the cross section parallel to the building direction are columnar crystals.When the laser scanning interval is 100μm,a strong <001> texture is formed.The grains of the cross-section perpendicular to the building direction are similar to elliptical shapes and have a certain aspect ratio.The microhardness results of IN718 alloy show that the higher the compactness,the greater the microhardness,and the microhardness of the bottom section perpendicular to the building direction is higher than that of the top.The phase structure existing in the selected laser melting IN718 alloy is analyzed.The matrix is a face-centered cubic γ phase,and a part of the segregated phase formed by element segregation is a hexagonal Laves phase.During the forming process,the dislocations are packed at the grain boundaries to form dislocation cells.The cross section perpendicular to the building direction is equiaxed dislocation cells,and the cross section parallel to the building direction is columnar dislocation cells.