Study On Microstructure Evolvement And Forging Technology Of Great Size Bar For GH4720Li Alloy

GH4720Li is a classical difficult deformation nickel-base superalloy,this mainly embodies that thermal deformation temperature window for the alloy is narrow and hot-deformation resistance is great and thermal plasticity is bad.The hot-working character of GH4720Li alloy is between in the wrought and unwrought forging.As the sizes of forging basic become bigger,the sizes of ingot and bar of GH4720Li alloy is becoming bigger too.These put forward a number of new problems in smelting and forging of ingot,one of the problems is the control of microstructure and properties for the great size forging bar.The fast-radial forging process of GH4720Li alloy is selected as the subject investigated in this dissertation.In this work,the hot deformation behavior,microstructure evolution and the influence of microstructure on mechanical property of GH4720Li alloy have been systematically studied.The mathematical models of flow stress has been well established,the maps for workability analysis has been product.Putting the research conclusions into use,the fast-radial forging process of GH4720Li alloy bar is formulated and implemented.The main contents are summarized as follows:(1)The samples cutting from 1/2 radial position of forging bar are regarded as the analytical objects.Through many sorts solution treatment experiments,the growth of grain in isothermal condition and the dissolution behavior of ?' have been researched,the results show that the rate of grain growth changes at 1160?(dissolution temperature of ?'),the grain growth activation energy of the grain growth change between 684.0 kJ/mol and 2040.4kJ/moL The grain grows slowly as the solution temperature is below 1160?,the grain grows quickly as the solution temperature is above 1160?.As the solution temperature is belong 1130? to 1150?,the primary ?' phase occurs to dissolve in partial position on the boundary of grain,grain growth rate of GH4720Li alloy in the different part are different,as result that the grain appears belt distribution.The data of grain growth is applied to construct the mathematical models.(2)The samples cutting from 1/2 radial position of forging bar which are solution treated at 1170? have been used to aging treated in different processes.Through these test,the influence of aging treatment on the mechanical property and the precipitation behavior of ?'phase for GH4720Li alloy have been researched.The results show that the hardness of alloy continuously raise when the temperature is between 650 ? to 760? when the temperature is between 850? to 1050? the hardness of alloy happens to raise at first and decline after reaching the peak hardness.With the aging treatment time extension,the size of y' increases,the shapes of ?' phase change from circle to square.(3)The samples cutting from different position of forging bar which are heat treated by typical heat treatment have been used to tensile experiment.The influence of microstructure on the mechanical property for GH4720Li alloy has been researched.The results show that the quantity and plasticity of the primary ?' phase is the main reason for plasticity of the alloy.Boundary of the primary ?' phase and base is the main source of crack.Because the precipitation temperature is high,and cooling rate is slow,the primary ?' phase in the position of bar center has good plasticity.This makes the alloy in the bar center have has good plasticity too.As a result,a method to increase the plasticity of GH4720Li alloy can be obtained.This process can improve the plasticity of GH4720Li alloy,the result is proved by hot compress test.(4)The samples with large size grain have been used to do thermal simulation experiment on the MMS-200.Through the single and double compression,the behavior of dynamic re crystallization,static recrystallization and the evolution of ?' phase for GH4720Li alloy are researched.The results show that behavior of dynamic recrystallization is the main softening mechanism for the GH4720Li alloy.When the formation temperature is below 1160?,the behavior of precipitation and recrystallization for the ?' phase cause the great elevation of peak stress and the thermal deformation activation energy for the alloy.The average of thermal deformation activation energy for GH4720Li alloy is 1240.7kJ/mol.Thermal deformation activation energy is increased first then decreased with the elevation of formation temperature.Thermal deformation activation energy is increased first then decreased with the elevation of formation rate too.As the formation temperature is at 1160 ?,and the strain rate is 0.1 s-1,the thermal deformation activation energy for GH4720Li alloy obtains minimum 602 kJ/mol.On the base of experimental data,the thermal deformation constitutive equation considering formation temperature and strain rate and evolution of micro structure was established,and the hot processing maps of GH4720Li alloy have been developed based on above-mentioned condition.(5)On the base of rigid(stick)plastic finite element theory,the cogging model of DEFORM-3D software have been used to study the influence of deformation temperature and reduction per pass and feed rate and strain rate on the temperature and strain field and evolution of micro structure in forging billet.The results show that deformation temperature and reduction per pass are the main influence factor for the temperature and strain field of billet,the heating temperature and strain rate are the minor influence factor for the temperature and strain field of billet.According to the result of numerical simulation,the ruler of forging has been established.In the fast-forge process,the heating temperature cannot be higher than 1150?,compression rate waves between 40 and 80mm/s,reduction per pass is not bigger than 50mm per pass.In the radius forge process,the heating temperature cannot be higher than 1120 ?,reduction per pass is not bigger than 40mm per pass.(6)According to the research result of grain growth and dynamic recrystallization and static recrystallization and hot processing maps and numerical simulation,the process of forging GH4720Li alloy has been established.The process has been implemented on 3150 ton fast forging machine and 1800 ton radial forging machine for GH4720Li alloy ingot with diameter ?508mm.Through testing the microstructure and mechanical property of the fast forging billet and radial forging billet,the rationality and feasibility for the process of forging have been validated.