Study On High Velocity Constitutive Equation Of Copper Based On Electromagnetic Ring Expansion

High velocity forming (HVF) is the shaping method of materials by impulse impact pressure with rapidly conveying energy to workpiece during very short durations. The material formability is largely improved compared to static forming method. As the high strain rate about 10⁴s^-1 obtained with high velocity forming, flow stress level exponentially increased when comparing to quasi-static constitutive model. The improved formability was mostly influenced by the changing of constitutive model. And the strain of workpiece had obvious characteristic of dynamic plasticity. So it is indispensable to study the HVF constitutive model to guide metals' transformation. The dynamic plasticity was also the important point in analyzing HVF characteristic.Dynamic analysis is an important method to study the material deformation under instantaneous impulse impact pressure. The deformation movement of workpiece is setup with dynamic method and the displacement, yelocity fields are obtained to figure out the relationship between flow stress and strain. As the constitutive model needs to reflect the dynamic plastic response, the model must be based on dynamic characteristic. The excessive stress model takes plastic strain-rate as function of excessive stress to reflect the strain-rate effect. With the dynamic plastic characteristic the HVF constitutive relation is setup based on excessive stress model.High velocity forming was studied based on electromagnetic forming method. It was summarized the change of constitutive relation to be important factor effecting the material high velocity formability. The strain-rate improvement determined the constitutive change. Effect of strain-rate was obvious and the high velocity constitutive relation had dynamic plasticity trait.The HVF constitutive model of copper was researched based on the electromagnetic ring expansion with accessorial tube method, which had uniform electromagnetic force. The deformation of metal's was undergone four phases, such as elastic load, plastic load, plastic unload and consecutive elastic phases. With analyzed the deformation phases, the displacement field, speed field and acceleration filed was setup. So the constitutive relation between stress and strain was built. And the copper HVF constitutive equation was obtained.The HVF constitutive model of copper was setup based on the excessive stress model to describe the strain-rate effect and the change of flow stress. As the displacement, strain of workpiece and electromagnetic force were obtained in the copper ring electromagnetic expansion experiment. And the field of displacement and strain-rate was also deducted. The material parameters were calculated by numerical method and the HVF constitutive model of copper based on electromagnetic forming was acquired. The result of simulation declared that the HVF constitutive model of copper could express rightly on the formability of metals in HVF compared to constitutive model based on quasi-static forming.