Physical Experiments And DEM Numerical Simulation On The Vibrated Packing Densification Of Aluminum And Titanium Alloy Powders

In the study of particle packing,the particle size plays an important role in determining the packing structure and properties.When the particle size is lower than about 100?m,the cohesive force between particles is dominant,and compared with the non-cohesive "coarse"particles,this kind of powder shows unique packing behavior like high porosity of the packing structure,which limits its development in many areas of industry(especially for the final quality of selective laser melting production).Therefore,how to effectively improve the initial packing density of powders(especially fine powders)and how to improve the powder packing structure and properties are the focus in people's research.With the problems encountered in current research on the powder packing densification,the project proposed the combination of physical experiments and DEM(discrete element method)numerical simulations,where the frequently used Al powder(Al)and Ti alloy powder(Ti6A14V)in selective laser melting(SLM)whose size distributions range from 20 to 130 ?m were selected as the research target and systematic physical experiments and numerical simulations were carried out to study their packing densification under vibrations.In physical experiments,the influences of vibration parameters(e.g.vibration time,amplitude,frequency,and vibration intensity)on the packing densification of each powder were studied.Meanwhile,based on the experiments obtained optimal processing parameters,corresponding to numerical models were established and the macro property such as the packing density and various micro properties such as coordination number,contact types,force distribution and radial distribution function etc.obtained in the simulation were characterized.On this basis,the void in the powder packing structure during vibrated packing densification was quantitatively analyzed using.Voronoi/Radical Tessellation.And the relationship between the external mechanical vibration and powder packing structure was established.The main findings are as follows.(1)In physical experiments,for Al powder(Al)and Ti alloy powder(Ti6Al4V)whose size distributions range from 20 to 130?m,the larger the size of powder particles,the higher the initial or vibrated packing density.The relationship between the vibration parameters and the packing density follows near quadratic function.By using vibration,the transition of packing structure from random loose packing(RLP)to random close packing(RCP)can be realized.For example,for Al-20 powder system,its RLP density is 0.462,while under optimal vibration condition,its RCP density can reach 0.551 with 10%improvement.(2)In numerical simulation,when the packing density of the powder system increases and reaches about 0.6,typical packing structure as shown in "coarse" particle system can be identified in the powder packing structure.The normal contact force involving cohesive force plays a critical role in the packing structure,which decreases with the increase of the average particle size.While during vibration,for a certain packing system,the normal contact force involving cohesive force increases with packing density.(3)The DEM based JKR model can effectively simulation the powder particles with cohesive force,and the packing density agrees well with that obtained from physical experiments,which further proves the effectiveness of the utilized numerical method and the reliability of the simulation results.