Mechanical Activation Process Of Al-PTFE Composites Based On CFD-DEM Coupling

Aluminum(Al)powder is frequently added as a combustion promoter in explosives and propellants due to its unique exothermic properties.However,during burning,micron-sized Al powder will agglomerate,decreasing the effectiveness of energy release.The aluminum-polytetrafluoroethylene(Al-PTFE)composite powder prepared by high-energy stirring ball milling can solve the agglomeration problem in the combustion of micron-sized Al powder.Multi-scale modeling was used in this research to simulate the high energy ball milling of AlPTFE composite powder in order to study the powder’s deformation mechanism and identify the key factors influencing it.The simulation results are used to guide the structure and process optimization of the ball mill in reality,and provide suggestions for the optimization of process parameters for pilot scale-up.The main research contents and conclusions are as follows:(1)The computational fluid dynamics-discrete element method(CFD-DEM)coupling method was used to implement the macro-scale simulation modeling of high-energy ball milling.The process of grinding ball collision is summed up by looking at the motion law of grinding balls in fluid.The force of the fluid,the friction created by contact with the vertical wall of the tank,and the supporting force created by contact with the bottom surface of the tank are all applied to the grinding ball when the movement in the ball mill is steady.The complete milling ball movement maintains a stable level because their vector sum along the z-axis is equal to the milling ball’s own gravity in the opposite direction.Following statistical analysis,it is discovered that the frequency distribution of the normal component of the relative velocity exhibits a power law distribution law during the stable period of the grinding ball movement.The low-speed collision in the collision of grinding balls with each other accounts for the main body.The distribution of the tangential component is indirectly impacted by the relative velocity normal component.In high relative velocity normal component intervals,the tangential component values are typically larger.It is also more evenly distributed numerically.(2)Using a dense discrete phase model(DDPM),meso-scale simulation simulates the effect of grinding balls on powder.The position distribution of the powders in the fluid and the flow field at various relative speeds were investigated.This leads to the conclusion that the relative velocity and the quantity of powders impacted are related.The normal component of the relative velocity will rise when the grinding ball strikes the powder,which will cause a more noticeable change in the flow field’s velocity gradient close to the collision contact point and increase the number of powders struck.The velocity of the flow field(perpendicular to the direction of the relative velocity of the grinding ball)on either side of the collision contact point will reduce as the tangential component of relative velocity increases,as does the number of powders being impacted.The scatterplot of the ratio of the number of impacted powders to the tangential and normal components of the relative velocity at various relative speeds is fitted to produce an exponential curve.The curve closely correlates the relative velocity with the amount of powder affected.The curve calculates the action over a certain period of time and the stress energy distribution on each powder.(3)To acquire stress,strain,and energy changes in the powder process,micro-scale powder microcompression simulation(micro-scale simulation)is introduced.In a short amount of time,the powder morphology distribution is determined by comparing the powder energy change acquired by micro-scale simulation with the powder stress energy distribution calculated by macro-scale and meso-scale simulation.The results show that the stress energy distribution interval of the powder within 0.02s is wide,resulting in a large difference in the morphology of the powder in a short time.Among all powders affected by impact,only plastic deformation occurred in the number of powders accounted for 45.63%.54.37%of the powder was crushed or broken to varying degrees.However,the number of powders affected by impact in a short period of time accounted for too little in the total number of powders,indicating that the morphology of the powder formed by the stirring ball mill in a short period of time was widely distributed.An average-sized composite powder can be produced after a protracted time of mechanical impact.(4)In this paper,the volume of the vertical stirring ball mill was magnifended to 8 times according to the geometric similarity.By comparing the simulation results before and after amplification,the process parameters of the ball mill were optimized.While keeping the process parameters unchanged,model amplification increases parameters such as collision force,collision frequency,and stress energy of the grinding ball.The strain velocity of the powder at high impact energy is accelerated.The deformation mechanism of the powder changed.After adjusting the mixing shaft of the amplified model to the appropriate speed,the same relative velocity normal and tangential component distributions as the model before amplification were obtained,and the same powder deformation mechanism was obtained.Through data comparison,the average impact frequency of a single grinding ball after magnification is increased by 1.14 times that before amplification.Ball milling efficiency has improved.