Construction Of Dynamic Recrystallization And Grain Growth Models For Ni-based Superalloy As Well As Its Application

Bad working conditions have made strict demands on the mechanical properties of exhaust valve.The mechanical properties of material with certain chemical compositions are determined by the microstructure to a great extent.Therefore,it is very important to study the microstructure of exhaust valve material.For this purpose,in this research,the dynamic recrystallization and grain growth behaviors were studied according to the conducted isothermal compression and grain growth experiments of Nimonic 80 A superalloy.Subsequently,the dynamic recrystallization model and grain growth model were constructed.Finally,the microstructure models were integrated with the finite element software to develop the finite element model for the electric upsetting process and study the microstructure evolution rules during this process.The main work and conclusions were summarized as follows.1.Gleeble-3500 thermal-mechanical simulator and M.MF.03000 numerical-controled and digital-display metal melting furnace were employed to conduct the isothermal compression tests and grain growth experiments of Nimonic 80 A superalloy respectively,which can provide experimental and data basis for the studies of dynamic recrystallization and grain growth behaviors.2.The flow stress level of Nimonic 80 A increases with the increase of strain rate or the decrease of deformation temperature.The main softening mechanism is dynamic recrystallization.Moreover,the dynamic recrystallization fraction and dynamic recrystallization grain size increase with the increase of temperature or the decrease of strain rate.According to the experimental results,the critical strain model,dynamic recrystallization fraction model and dynamic recrystallization grain size model were calculated.3.The grain of Nimonic 80 A superalloy would grow under high temperature.At a certain temperature,the grain size increases with the increase of the holding time,but the grain growth rate decreased.Under certain holding time,the grain size increases with increasing temperature,and higher temperature would result in greater grain growth velocity.According to the experimental results,he grain growth model of Nimonic 80 A superalloy was calculated.4.Based on the finite element software DEFORM-2D,the multi-scale finite element model involving macroscopical deformation and microscopic grain size for the electric upsetting process of Nimonic 80 A superalloy was developed.Based on this finite element model,the microstructure evolution as well as the effects of dynamic recrystallization and grain growth on the grain size during electric upsetting process were systematically analyzed.The results show that the final grain size was determined by the synthetic effects of coexisting grain coarsening induced by grain growth and grain refinement induced by dynamic recrystallization.But over all,the grain refinement effect of dynamic recrystallization plays a more dominant role in the present electric upsetting process.