Study On Dynamic Mechanical Response Of Metal Powder Under Impact Loading Condition Based On 3D Discrete Element Method

Under the condition of impact loading,metal powders will undergo many different and traditional pressing special phenomena,and their forming mechanisms will become very different.In order to study the forming process of metal powder under impact loading conditions,the dynamic response process of particle systems And the densification mechanism of metal powders.Based on the principle of discrete element,this paper attempts to use PFC3 D software to simulate the three-dimensional numerical model of impact loading of metallic powders.The threedimensional numerical model is close to the actual situation of engineering application to the greatest degree,and analyzes the situation of particle movement,disturbance propagation,etc.The process of formation of the three-dimensional network of the force chain in the particle system,the internal structure of the three-dimensional force chain network,and the force condition of the single particle were studied and analyzed during the single loading process.(1)According to the theory of granular material mechanics,the disadvantages and applicable scope of the soft ball numerical simulation model of the two-particle contact basic contact force theory(both Hertz contact theory and Mindlin-Deresiewicz contact theory)and particle discrete element method are compared.Use a soft ball model for numerical simulation.The model can not only calculate the tangential force and normal force existing between the powder particles,but also simulate the relative rotation between the particles.(2)Use PFC3 D software to establish a three-dimensional model of metal powder impact compression.In a cylindrical shape having a width of 10 mm and a height of 5 mm,13845 circular particles having a diameter of 0.15 mm were established,and the pressing plate was moved down along the Z axis at a speed of 10 sm /.The simulated transmission waveform conforms to the general characteristics of the transmitted wave waveform under the impact loading conditions of metal powder.And the internal structure of the powder model has a high degree of similarity with the microscopic metallographic map,which can reflect the real project situation more realistically.(3)From the aspects of particle motion and particle contact force,the dynamic mechanical response of the granular system during loading was analyzed.When the metal powder is subjected to impact loading,the particles are disturbed by vibration and the particles have greater momentum,which is beneficial to the rearrangement of the particles.During the loading process,a shear band is formed due to the flow of the particles,which is favorable for grain gaps.In the elimination of air,the time of action of a single particle is extremely short,which greatly reduces the energy consumed by the particles due to friction,thereby improving the forming efficiency.Vibration,shear bands,greater momentum and friction loss,these factors are conducive to the degree of densification of green compaction.(4)By observing the waveform of the transmitted wave of the stress wave,the influence of the impact loading speed on the forming action is analyzed: With the increase of the loading speed,the vibration intensity in the early stage of loading is gradually reduced,and therefore the movement of the particles may be weakened,and the gap between the powders may be adversely affected.The filling and air removal,but the increase in loading speed will also increase the force of the metal powder,can provide greater mechanical meshing force for the compact.When using this mathematical model,the distribution of the pressure on the upper side of the mold wall that could not be obtained by applying the traditional experimental method is also obtained: the maximum lateral pressure received by the mold wall increases gradually with the increase of the height.The staggered arrangement of the inner strength chains.(5)Using a split Hopkinson pressure bar,the metal powder was subjected to a high strain rate impact loading experiment and it was found that there was a secondary loading phenomenon in the impact loading.The influence of the impact velocity on the transmitted wave of the stress wave is studied.With the increase of the impact velocity,the peak value of the transmitted wave also increases linearly,and the overall vibration amplitude decreases.This phenomenon is the same as the numerical simulation result.The influence of preload on the impact loading was also studied.It was found that the increase of the pre-pressure can increase the amplitude of the transmitted shock wave and promote the particles to enter the deformation stage more quickly.