Fabrication Mechanism And Technology Of Inconel718 Superalloy With Gradient Microstructure By Laser Solid Forming

The property of the alloy which is influenced by microstructure determines the application.The hot-aisle components of the aero-engine which is serviced at high temperature?high stress gradient and oxidation corrosion environment should have high application and process performance.The temperature in aero-engines decreases quickly from turbine blade tip to turbine bore,while the stresses are increasing.Turbine disk and blade are mostly made of precipitation strengthened nickel-base superalloys,but modern turbine blade is generally made of directionally solidified or single crystal superalloy and the turbine disk is made of equiaxed grains(the rim part of the dual-performance turbine disk is mostly consisted of coarse grains,while the bore part is mostly consisted of fine grains).The traditional process requires separately preparing the turbine disk and the blade and then assembling them together.The manufacturing process is complicated,the process precision is high,the yield is low,and the production is high cost.Laser solid forming(LSF)technology can realize the preparation of the turbine disk and blade at the same time due to its high flexibility,high forming efficiency,saving time,without dies,high density and high performance.In present work,Inconel718(IN718)superalloy was used to prepare the specimen by LSF.The effects of forming process parameters and heat treatment on the microstructure and properties of IN718 superalloy with gradient microstructure consisted of fine grains,coarse grains and columnar grains were analyzed.The aim was to improve the performance of the rapidly fabricated parts and to promote the final realization of superalloy part with gradient composition and microstructure.Main conclusions were as follows:(1)All the as-deposited IN718 superalloys are consisted of obvious columnar dendrites under different process parameters.There are a few equiaxed grains in the columnar dendrites.The volume fraction of equiaxed grains is the highest of all the IN718 superalloy under laser power of 1000 W(laser beam diameter 2 mm).Fine columnar dendritic microstructure of as-deposited IN718 superalloy part is obtained under low laser power of 600 W(laser beam diameter 1 mm).High laser power of 2000 W(laser beam diameter 4 mm)results in the formation of coarse columnar dendrites.The as-deposited IN718 superalloy under laser power of 1000 W(laser beam diameter 2 mm)has the highest ultimate tensile strength and yield strength which are 936 MPa and 729 MPa.The columnar dendrites could not completely convert into equiaxed grains with increasing solution treated temperature.(2)The volume fraction of columnar to equiaxed transition zone is increasing with increasing laser power(laser beam diameter 1 mm).The morphologies of Laves phase are discrepant under different laser power.The microstructure of the IN718 superalloys are significantly affected after solution treated at 1100?.The residual stress acts as the driving force during the static recrystallization process which lead to the variation of the grain size.Fine and relatively uniform grains of IN718 superalloy are obtained under laser power of 1000 W after solution treated at 1100? for 60 min,and the average grain size is 60?m.(3)The microstructures of as-deposited LSF IN718 superalloys are consisted of non-uniform columnar dendrites under different laser power(laser beam diameter 2 mm).The microstructure of solution treated LSF IN718 superalloy obtained under laser power of 1350 W possesses the smallest grain size.The ultimate tensile strength and yield strength of solution treated IN718 superalloys are relatively low,but the elongations are higher than 50%.Compared with the solution treated IN718 superalloy,both the ultimate tensile strength and yield strength are strongly improved after homogenization + ? aging + double aging treatment,and the corresponding values are 1342 MPa and 1129 MPa.The mechanical properties of IN718 specimen under laser power of 1350 W are better than the others.(4)The residual stress of the as-deposited IN718 alloy under laser power of 1850 W(laser beam diameter 3 mm)is the largest.The percentage of twin boundaries grain is the highest while the percentage of low angle boundary is the lowest after solution treatment.The ultimate tensile strength and yield strength after homogenization + ?aging + double aging treatment are the highest of all the samples under laser power of 1850 W(laser beam diameter 3 mm),and the corresponding values are 1318 MPa and 1107 MPa.The residual stress of the as-deposited IN718 alloy under laser power of 2300 W(laser beam diameter 4 mm)is the largest.There still exists a certain percentage of columnar dendrites in the solution treated IN718 superalloy.(5)The microstructures of as-deposited IN718 superalloy with gradient microstructure are consisted of non-uniform columnar dendrites under different process parameters.The microstructure of solution treated IN718 superalloy are consisted of fine grains in the zone under laser power of 1000 W(laser beam diameter 1 mm),coarse grains in the zone under laser power of 1850 W(laser beam diameter 3 mm)and columnar dendrites in the zone under laser power of 2000 W(laser beam diameter 4 mm).The fracture positions of the homogenization + ? aging + double aging treated IN718 superalloy are in the zone under laser power of 1850 W(laser beam diameter 3 mm)and 2000 W(laser beam diameter 4 mm)during the tensile tests.