| With the increasing improvement of aero engine thrust-to-weight ratio,the materials of key parts for turbofan aeroengine flame tube has changed from the stainless steel AISI310 to the solid solution strengthened nickel-based superalloy GH3625 sheet.If GH3625 flame tube end cover is made by cold stamping process,it is easy to crack and difficult to control the shape due to springback during forming.The hot stamping process is proposed by enterprises to form the GH3625 parts,which require that grain size of the parts must be less than 60μm and the tensile strength of the parts should not be less than 1300MPa.In order to meet the requirements of enterprises,it is necessary to study the microstructure&properties control and hot stamping application of GH3625 sheet,the technical issues involved are:1)Grain growth and grain boundary control during heating and heat preservation.The effects of heating temperature and holding time on grain growth behavior and grain boundary distribution of GH3625sheet were studied to realize the control technology of grain size and grain boundary;2)Constitutive modeling with high precision,and plastic deformation at high temperature and fracture control technology for GH3625 sheet metal.The high temperature constitutive model of GH3625 sheet was established and modified to improve the simulation accuracy,and obtain the optimal range of plastic deformation parameters to prevent high temperature fracture.3)Microstructure control and grain coarseness control in hot forming process.The effects of different forming temperature and strain rate and strain on dynamic recrystallization,grain boundary,grain size and grain boundary growth mechanism were analyzed to determine the optimal forming parameters to prevent grain coarsening.4)The technology of preventing coarse grain and forming process by using double hot stamping.The effect of the second heating on the grain growth and microstructure and properties of GH3625 sheet,and the matching problem of the second and the first movement distance,which were studied to prevent the coarse grain size and hot stamping cracking.5)Application validation and evaluation.The process experiment and microstructure and properties test of GH3625flame tube end cover were carried out,and the technical specification of double hot stamping for GH3625 sheet was established.Based on the above technical issues,relevant research is carried out.The specific research contents and conclusions are as follows:（1）The effects of heating temperature and holding time on grain growth behavior and grain boundary distribution of GH3625 sheet metal were studied,and the grain growth model of GH3625 sheet was established,the grain growth and grain boundary can be controlled during the heating and insulation process.The optimum heating temperature range is 950～1050℃,and holding time is 5min.The results show that the average grain size are uniform and fine,and varies from 42μm to 58μm when the heating temperature ranges from 950℃to 1050℃;the average grain size are uniform and fine,and varies from 61μm to 65μm when the heating temperature is 1100℃;when the heating temperature is 1150℃,the grain size increases rapidly and the mixed grain structure with large difference in grain size appears,and the average grain size varies from 72μm to 100μm;when the heating temperature increased to 1200℃and 1250℃,the grain size further increased and the mixed grain structure disappeared,and the average grain size varied from 144μm to 334μm.（2）the constitutive model and the optimum process parameter range of high temperature plastic deformation were obtained by studying the mechanical behavior of GH3625 sheet at high temperature.The main conclusions are as follows:1)The high temperature constitutive model of GH3625 sheet is constructed and modified,and the modified model has good prediction accuracy.2)It is found that this alloy has the best plasticity when the temperature is 800～850℃and the strain rate is 0.1～10s-1,and the elongation at break is about 49%～56%,which satisfied forming requirement.（3）The effects of different deformation temperature,strain rate and strain on dynamic recrystallization,grain boundary size and grain boundary growth mechanism were studied.The microstructure control and grain coarse control in hot forming process were fulfilled,and the optimal parameters range of hot forming process was obtained.The main conclusions are as follows:1)Sigma3 grain boundaries are mainly formed by accidental growth mechanism,sigma9 and sigma27 grain boundaries are formed by regeneration mechanism;2)GH3625 alloy has fine grains and more twin boundaries after deformation when the temperature is 800～850℃and strain rate is0.1～1s-1,the average grain size varies from 42～52μm.（4）Through numerical simulation optimization and process test of hot stamping forming process for flame tube end cover,the influence rule of blank shape size and forming process parameters on formability of end cover are obtained,and the optimal blank shape size and process parameters:the cutting angle size is 135mm,the diameter of the center hole is 100mm,the friction coefficient is 0.15,the blank holder force is144k N,and the forming speed is 100mm/s.When the single hot stamping process was adopted,cracks appeared at the round corners of the stamping parts.At last,the reason of cracking was analyzed.（5）The double hot stamping process at the same die was put forward,the effects of twice heating on grain growth and microstructure properties were analyzed and obtained:in comparison with single heating,the effect of twice heating on grain size is very small,the volume fraction of dynamic recrystallization and substructure are more than single heating,and the plasticity is increased by about 10%after twice heating,which lays a theoretical foundation for the development of twice hot stamping process.The deformation amount of the first and second hot stamping was determined by the numerical simulation and process test of the flame tube end cover.Finally,the hot stamping parts which satisfied the shape size,grain size and mechanical properties were obtained.The microstructure observation and mechanical properties testing at different positions of the stamping parts show that the grains in different parts are fine and uniform,the average grain size is about 45μm;for the strength and plasticity of hot stamping part,the yield strength is 544MPa,the tensile strength is 1556MPa and the elongation at break is 60%,which meet the assembly requirement of the microstructure and performance indexes.Finally,the double hot stamping process of GH3625 sheet was formulated,and microstructure transformation diagram of GH3625 sheet during hot stamping was obtained. |
