Study On Microstructure Evolution And Ultrasonic Constitutive Model Of 9310 Steel During Ultrasonic Vibration-Assisted Hot Forming

Compared with the traditional forming technology,the ultrasonic vibration-assisted plastic forming technology can reduce the load and interface friction coefficient,prolong the service life of the die,and obtain better surface quality and dimensional accuracy of the workpiece.However,the mechanism and theory of ultrasonic action have not formed a unified understanding.In this paper,the ultrasonic vibration-assisted hot forming technology is taken as the research object.The microstructure simulation model and ultrasonic constitutive model of ultrasonic vibration-assisted hot forming process are established by the experimental research and cellular automaton（CA）method,and the reason of flow stress reduction of 9310steel is quantitatively analyzed from the macroscopic and microscopic scales.The main research contents are as follows:（1）Based on the critical dislocation density model,dislocation density model,nucleation and growth model and dynamic recrystallization growth rule,the CA method was used to establish the microstructure simulation model.The ultrasonic energy is introduced into the hot forming process,and the activation energy and dynamic recovery coefficient were modified to establish the theoretical model of microstructure simulation of ultrasonic vibration assisted hot upsetting process.（2）Based on the theoretical model,the CA method was used to simulate,and the reason of flow stress reduction is studied from the micro scale.The simulation results show that when the vibration amplitude is less than 9.96μm,when the vibration amplitude increases,the ultrasonic energy increases,the average grain size decreases faster,the recrystallization volume fraction increases faster,the reduction of stress increases,and the ultrasonic softening phenomenon becomes more obvious.When the strain rate is in the range of 0.0003s^-1to 0.3s^-1,when the strain rate is smaller,the average grain size decreases faster,the recrystallization volume fraction increases faster,and the reduction of stress decreases.（3）The thermal compression test is carried out.Based on the classical Taylor relation and the"two-stage"dislocation density model,the flow stress model is established by using the experimental data.The model accurately describe the flow stress and strain relationship during traditional hot forming process.The ultrasonic vibration-assisted tensile is carried out.The results show that"acoustic softening"occurred after ultrasonic vibration is applied,and within the scope of the study,the acoustic softening effect increases with the increase of ultrasonic vibration amplitude and decreases with the increase of strain rate.Based on crystal plasticity theory,an ultrasonic constitutive model considering the acoustic softening effect is established.The prediction accuracy of the microstructure simulation model of the ultrasonic vibration-assisted hot forming process is analyzed by comparing the simulation results,the predicted results of ultrasonic constitutive model and the experimental results of reduction of flow stress.In this paper,the CA simulation model of ultrasonic vibration-assisted hot forming is established to further discuss the reason of flow stress reduction at the micro scale.An ultrasonic constitutive model is established to quantitatively describe the relationship between ultrasonic amplitude and strain rate and acoustic softening effect.