Numerical Simulation Of Metal Powder During Mold Pressing Based On Macroscopic Scale And Meso-scale

Cu-Al alloys are reported to possess good mechanical performance,high strength and high temperature resistance properties,which have been used extensively in automotive,aerospace,electronic and machine tool industry.They.However,due to the high melting point and wide crystallization temperature range of cu-al alloy,they are frequently synthesized by conventional powder metallurgy methods.Cold compaction is an important procedure in the manufacture of compacts with excellent performance,high efficiency,and low cost,which has been widely used in fabricating metal fuel propellant,polymer bonded explosive and medicine field.Therefore the densification of mold compaction were mainly analyzed by experiment and numerical simulation from the macroscopic scale and the mecroscopic scale.Firstly,the influences of different pressing parameters during mould compaction of Cu-Al composite powders on the distribution of relative density and equivalent von Mises stress in the compact were analyzed by experiments and finite element method based on continuous medium mechanics from macroscopic scale.Compaction experiments of Cu-Al composite powders were performed to obtain the relationship between compaction and average relative density of the compact.And the validity of the modified Shima model and the correctness of the results are verified.On this basis,the effects of initial packing density,metal ratios,compaction pressure,roughness of die wall,dwell time and height-diameter ratio on the properties of the compact were analyzed.And the compaction methods that can improve the densification of composite metal powder and decrease the equivalent von Mises stress are given.According to the study of die wall friction,a compaction equation based on Van Der Zwan-Siskens including the friction coefficient effect was proposed to reflect the relationship between densification properties and compaction pressure of the compact under different friction conditions.In order to reveal the densification mechanism of metal powder die forming from the meso-scale,discrete element method based on the molecular dynamics was combined with finite element method to simulate the compaction process.The variation of average relative density of the compact,equivalent von Mises stress and force chain structure in the particle were analyzed with the same particle diameter,fixed size ratio and normal distribution conditions.Furthermore,the influence of compaction pressure,packing structure and aluminum content were discussed.The results show that when the copper powder with the same particle size and the fixed particle size ratio are regularly arranged,the contact normal force between the particles are mainly located on the contact surface of the particles in the same adjacent layer,and is smaller between the particles in the same layer.In the same compaction condition,the packing structure has little effect on the average relative density during the middle and late compaction process.When the distribution of "hard" metal particles within the compact are more dispersed,and the number of the "soft" metal particles are higher,the forced chain formed between the "hard" metal particles are weakened,which can reduce the obstruction during the compaction process,and slow down the stress concentration phenomenon.With the addition of the “soft” metal powder,the densification behavior improved.The equivalent von Mises stress inside the “hard” particles are higher than that in “soft” particles.In addition,within a given range of compaction pressure,the higher the dispersion degree and the larger the difference of particle size,the better the densification behavior and the higher the average relative density.While when the pressure continue to increase,the effect of size ratio decreases.The friction coefficient of the die wall only have an obvious impact on the particles near the wall.The larger the friction coefficient,the larger the gradient of the equivalent von Mises stress is.However,it has little effect on the powder particles far away from the die wall.