Study On The Hot Tensile Deformation Behaviors Of A New Hot Isostatic Pressing Nickel-based Powder Metallurgy Superalloy

With high thrust-weight ratio of the aeroengine turbine disk,nickel-based powder metallurgy superalloy becomes the preferred material.The third generation of nickel-based powder superalloy adopts the technical route of "hot isostatic pressing → hot extrusion →isothermal forging → dual property heat treatment".Due to the hot extrusion cracking problem in the actual production process,it seriously affects the development and production process of turbine disk.Therefore,through the hot tensile deformation experiment,the hot deformation and fracture behaviors of the new hot isostatic pressing nickel-based powder superalloy are studied.And the hot extrusion process of the alloy is optimized by finite element simulation and industrial test.The main research contents and conclusions of this paper are as follows:(1)The effects of hot tensile deformation process parameters on the deformation behaviors and tensile properties of the alloy were analyzed by high temperature tensile deformation experiments.The results show that the rheological curve of hot isostatic pressing nickel-based powder metallurgy superalloy shows obvious recrystallization characteristics.With the increase of deformation temperature and the decrease of strain rate,the recrystallization dynamic softening effect of the alloy is strengthened and the rheological resistance of the alloy is reduced.In the rheological softening stage of the alloy,there is a competitive evolution process of powder particle strength,prior particle boundary strength,grain strength and grain boundary strength in the alloy structure.With deformation temperature is 1070℃,the grain strength and grain boundary strength are higher than the powder particle strength and the prior particle boundary strength.The powder particles and the original particle boundaries are more likely to break.When deformation temperature is1130℃,the alloy structure is more prone to recrystallization.At this time,it is easier to slip at the grain boundary and the plastic deformation ability of the alloy becomes stronger.In addition,a constitutive model which can predict the rheological behaviors of the alloy is established based on the tensile data.(2)The microstructure evolution behaviors of the alloy during deformation were analyzed by metallographic microscope experiment,scanning electron microscope experiment and electron backscatter diffraction experiment.The results show that the powder particles are broken,split and recrystallized.With the increase of deformation temperature and the decrease of strain rate,the crushing effect of powder particles decreases.During alloy deformation,γ’ phases do not dissolve at1070℃ and γ’ phases dissolve at 1130℃.However,large size γ’ phases transfer to small γ’ phases during the cooling process under all deformation conditions.The recrystallization degree of grain structure after deformation is relatively high.And grain structure is composed of equiaxed grains.The alloy is more prone to recrystallization with the increase of deformation temperature.The increase of strain rate is conducive to obtain more uniform and fine grain structure.(3)The effects of deformation parameters on the morphology distribution of holes at the fracture of HIPed nickel-based powder superalloy and the nucleation mechanisms of holes during hot tensile deformation were analyzed by metallographic microscope experiment,scanning electron microscope experiment and energy spectrum analysis experiment.The results show that the number of holes at the fracture of the alloy increases obviously with the increase of deformation temperature and the decrease of strain rate.Under the condition of high temperature and low strain rate deformation,necking phenomenon appears obviously with the increase of pore content.There are few holes under the condition of low temperature and high strain rate,but obvious cracks appear directly.The powder particles,prior particle boundaries,non-metallic inclusions,metal oxides and powder particle debris are reasons for inducing pore nucleation during hot tensile deformation.(4)Combined with finite element simulation and meso-damage mechanics theory,a finite element damage model which can describe hot tensile deformation damage behaviors of HIPed nickel-based powder superalloy was established.Based on the finite element simulation results,the damage evolution behaviors of alloy during high temperature tensile deformation were analyzed.The results show that there is almost no void nucleation and growth in the early stage of deformation.The rate of void nucleation and growth increases with the increase of strain in the middle stage of deformation.And the void content suddenly increases to the maximum in a very short time in the fracture stage of the alloy.(5)Combined with the damage model,the hot extrusion finite element model of alloy was established.The distributions of equivalent stress,equivalent strain and equivalent strain rate of billet during hot extrusion were studied.The results show that the microstructure flow of billet is uneven in the process of hot extrusion.Although the extrusion process can be completed within a certain extrusion speed range,it is not conducive to the smooth entry of the blank into the extrusion die when the extrusion speed is fast or slow.Considering equivalent stress,equivalent strain and equivalent strain rate,the initial temperature of the blank should be controlled at about 1110℃ and the extrusion speed should be controlled in the range of 35mm/s to 45mm/s.