Study On The Reduction Of The Forging Mixed Grains And Its Effect On Mechanical Properties Of GH4169 Alloy

With excellent high temperature performance,GH4169 alloy is widely used in the manufacture of key hot end parts of aeroengines.In the die forging process of GH4169 alloy,due to the strong deformation resistance,the complex geometry of the workpiece and the cooling effect of the die,the mixed grain structure often appears,which will cause the deterioration of service performance and lead to the shortening of the service lifetime.Therefore,in order to improve the microstructure and improve the properties of GH4169 alloy,this paper adopts the method of aging + recrystallization annealing to eliminate the forging mixed grain structure,and evaluate the mechanical properties of the alloy after heat treatment,the simulation of the deformation behavior and microstructure evolution of the alloy during high temperature tensile process was also executed.The main research contents and conclusion as follows:(1)In order to study the method and mechanism of eliminating the mixed grain structure of the forged GH4169 alloy,the heat treatment with aging + annealing + single stage cooling was carried out on the alloy after hot deformation.The results show that there is no obvious recrystallization in the aging process before annealing,but static recovery occurs in the alloy.During isothermal aging process,the δ phase mainly precipitates at grain boundary with the shape of short-rod like,while that precipitates intergranularly in short-rod shape and intragranularly in needle-like shape in variable temperature aging process.The "weak precipitation zone" is difficult to appear in the alloy with variable temperature aging,but the "dense precipitation zone" appears in the alloy aged with long time at isothermal temperature or variable temperature,and both the two δ phase distribution states are not conducive to the uniform refinement of grain structure.Increasing the annealing temperature,prolonging the annealing time,and slowing down the cooling rate can promote the dissolution of δ phase and increase the degree of recrystallization.During annealing,the long needle-like δ phase can effectively promote recrystallization nucleation,while the short rod-like δ phase mainly inhibits grain boundary migration.The alloy with variable temperature aging has uniform nucleation during annealing and small energy difference between grains,which is beneficial to promote nucleation and avoid abnormal grain growth.(2)In order to improve the service performance of forged GH4169 alloy,it is necessary to further control the δ phase and twin content after accomplish the uniform refinement of mixed grain structure.Therefore,a heat treatment method of variable temperature aging + annealing + segmented cooling was proposed,and the hardness and tensile properties of the alloy after heat treatment were tested.The results show that reasonable segmented cooling process after annealing can control the uniform refinement of mixed grain structure and adjust the distribution of δ phase and twin.However,decreasing the annealing temperature/slow cooling termination temperature or shortening the slow cooling time will lead to a large amount of residual δ phase and decrease the degree of recrystallization.On the contrary,a large amount of δ phase dissolved and recrystallized grains merged with each other to form annealed coarse grains.However,when the cooling time was prolonged,the precipitation of δ phase was also induced.The annealing twins tend to nucleate and expand near the δ phase and at the triple grain boundary.When there is a large amount of residual δ phase or almost no δ phase distribution in the alloy,the alloy hardness will be increased.When the alloy grains are uniform and fine,the γ’’ phase is fully precipitated,and only a small amount of residual δ phase is distributed at grain boundaries,the tensile properties of the alloy can be improved.(3)For study the effect of microstructure on tensile behavior of GH4169 alloy at the micro scale,a finite element model of crystal plasticity was established.The results show that the plastic deformation mainly occurs after 4%-6% strain during tensile process.The alloy with smaller grain size can bear higher stress,and plastic deform more uniform.In the alloy with mixed grain structure,the deformed grain will bear more plastic deformation,so the alloy is easy to initiate cracks at the boundary between fine recrystallized grain and deformed grain.A large amount of δ phase will cause insufficient precipitation of γ’’ phase,resulting in reduced deformation resistance and shortened service life of the alloy.The ununiform distribution of δ phase will lead to higher plastic deformation in the region with more δ phase,which will eventually lead to the initiation of cracks in the alloy.When the grain size is small,the stress that the alloy can withstand is increased,the crack initiation resistance is also increased,and the crack propagation path is tortuous.Moreover,the damage evolution speed and the inhomogeneity of the damage evolution can be reduced by the rotation of more grains.(4)According to the above study,the GH4169 alloy with a thermal deformation of 0.1s-1 to 50% deformation amount at 950℃ was aged for 9h at variable temperature,and then annealed at 1020℃ for 10 min.After annealing,it was cooled to 960℃ at a uniform speed within 10 min,and then immediately cooled by water,the heat treatment routes can effectively reduce the mixed grain structure and promote the uniform and refinement of grains.For the the heat-treated alloy,the recrystallization degree is 86.55%,the average grain size is 13.93μm,and the grain size inhomogeneity factor is 0.50.The δ phase fraction is 2.19%,and the twin content is 46.71%.