Multi-Scale Mechanics and Mechanism of Densification in High Velocity Compaction of Metal Powder

Author:Zhang Zuo



Degree Year:2019





The high velocity compaction(HVC)of metal powder,which rapidly applies a highspeed impact load to the powder and makes powder to achieve densification in a very short time,is a relatively new dynamic pressing forming method.Compared with the conventional compaction(CC)technology,the HVC has the higher pressing efficiency,higher density and more uniform distribution of density.However,the mechanism of densification in HVC is not yet fully understood.The metal powder belongs to the granular matter,and the multi-scale mechanisms included microscopic,mesoscopic and macroscopic characteristics of powder can help us to further reveal its densification mechanism.In this paper,combined with the theory of granular matter and the theory of HVC,the mesoscopic force chain quantitative characteristics,macroscopic stress propagation characteristics,the relationship between force chain and stress and the influence of force chain on densification process are analyzed.Furthermore,experiments of HVC are carried out to investigate the dynamic evolution of the stress and the frictional characteristics of the die walls during the densification process.Above works can further expand the theoretical basis of the metal powder densification in HVC.First,the simulation model is established by using DEM.Aimed at the mesoscopic force chain,the force chain information in HVC is extracted based on the chaining criterion and the identifying algorithm of force chains.The force chain length,strength,direction coefficient and collimation coefficient are used to quantitatively analyze the force chain characteristics.Furthermore,the effects of friction coefficient,impact velocity and initial packing density on the dynamic evolution of force chain are analyzed.The relationship between quantitative characteristics of force chain and the density of green compacts is also discussed.The study focuses on the relationship between force chain length and other quantitative characteristics of force chain.Secondly,in order to investigate the macro stress distribution in the system,the dynamic measurement circle is used to capture the internal stress’ change of the powder,and the overall stress magnitude,distribution and direction of the principal stress are analyzed.The variation of the forward and reverse stress propagation velocity under different impact velocity,initial porosity and friction coefficient between particles are obtained by using the stress propagation velocity analysis method,and the mechanism of stress propagation evolution in HVC is analyzed.Moreover,the relationship between the mesoscopic force chain and macroscopic stress is investigated,and the interaction mechanism between distribution and quantitative characteristics of force chains and distribution,propagation and directional characteristics of stress is analyzed,too.Thirdly,the relationship between evolution of force chains and densification in HVC is investigated.The structure characteristics of powder are characterized by porosity,coordination number and radial distribution function.Combined with the distribution and direction of force chains,the influence of mesoscopic force chains on densification behavior of powder is analyzed.Finally,in order to analyze the quick and dynamic evolution of wall’ frictional characteristics and stress characteristics,the HVC experiments are carried out to obtain the density of green compacts.Combined with Janssen-Walker model and dynamic strain acquisition technology,the evolutions of stresses at top,bottom punches and die wall and friction coefficients of die wall are analyzed,and the influence of impact heights is also considered.Moreover,the relationship between peaks of friction coefficient and densification is discussed initially.