| With the continuous development of aeroengine technology,the performance of the aircraft is constantly improving,and the requirements for engine materials are also getting higher and higher.Traditional superalloys can no longer meet the temperature requirements.GH720Li nickel-based superalloy is a new age-strengthened deformation superalloy.Compared with other nickel-base superalloys,GH720Li alloy is the superalloy with the highest content of titanium and aluminum in the deformed nickel-base superalloy in China,and the largest content of reinforcing phase.Compared with the conventional deformed nickel-base superalloy,the use temperature of the GH720Li alloy can reach 750°C.Therefore,it is an ideal high-temperature aerospace structural material and can be widely used in the new generation of aeroengine turbine disks.However,due to the severe alloying and the large number of reinforcing phases,the GH720Li alloy has poor plastic processing ability and thermal it is easy to crack during high temperature deformation.Cogging is an important method to improve the microstructure and improve the thermal processing plasticity.This cogging method can break the coarse casting structure in the organization,refine the grain,make the organization more uniform,and can also eliminate the hollow,shrinkage and other defects inside the ingot.Therefore,it is necessary to study the cogging process of this alloy and investigate its deformation behavior under high temperature conditions.In this paper,the GH720Li alloy ingots were sampled at different positions.The hot compressing experiment was carried out at a temperature range of 1080-1180°C and a strain rate range of 0.01-10 s-1 to find out the influence of the process parameters on the flow stress.By introducing the work hardening index,a critical dynamic recrystallization model and dynamic model are established.The effect of initial microstructure,temperature,and strain rate on the mechanical properties and microstructure evolution was studied.It was found that the increase in temperature and decrease in strain rate caused a decrease in peak stress and steady state stress,and also resulted in larger grain size.The initial grain size has little effect on the steady state stress.When the temperature exceeds 1140°C,crystal grains begin to grow significantly and secondary recrystallization occurs.An increase in temperature causes dissolution of the smaller-sized reinforcements,and larger phase remains in the microstructure.Based on the dynamic material model(DMM)as well as destabilization criteria as the theoretical basis,a three-dimensional thermal processing map was built.It was found that the strain rate sensitive factor was not only affected by the temperature,but also increased first and then decreased as the strain rate increased.Through analyzing the Instability,safety,high and low power dissipation areas in the thermal processing map,process parameters are optimized and the area with fine grain and no defects is 1120-1130°C/0.01-0.1s-1.The finite element simulation technique was used to simulate the cogging process under different anvil types and three important cogging process parameters.The effect of process parameters and anvil shape on the stress,strain,temperature distribution and damage values were analyzed to determine the appropriate ones.The optimized reduction amount was 50 mm,the feed amount was 100 mm,and the pressing speed was 30 mm/s.The actual cogging test was conducted and it was found that the microstructure was distributed uniformly,the grain size was small,the original as-cast structure had disappeared,the voids in the billet and other defects were basically eliminated,the forgeability of the billet was good and the surface had no cracks.The results of the cogging test have effectively verified the accuracy and reliability of the numerical simulation process parameters. |
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