Numerical Simulation And Experimental Study On The Packing Densification Of Ti-6Al-4V Powder

Titanium alloys are widely used in aviation,aerospace and other fields due to a series of excellent and comprehensive properties.Ti-6A1-4V(TC4 or Ti64)is the first titanium alloy to be put into production and widely used.However,due to its high processing difficulty,it is difficult for traditional plastic processing method to manufacture Ti64 product.Powder metallurgy(PM),as a near net forming technology has been recognized as a promising and effective approach due to the advantages of low cost,precise structure control and high utilization rate.In PM process,the lower initial packing density of powder and the nonuniformity of pore distribution will have negative effects on the properties of final products.High density and uniform initial packing structure can eliminate macropores from the root,reduce the binder content and reduce the size shrinkage,which has an important impact on the performance of components.Therefore,how to improve the packing density and uniformity of Ti64 powder in PM filling process needs to be further studied.In this paper,discrete element method(DEM)is used to simulate the densification of Ti64 powder in the mold.Not only the macroscopic properties(such as packing density)and microscopic properties(such as coordination number(CN),radial distribution function(RDF),contact between powder particles,force distribution and transition)of the obtained packing structures were characterized and compared,but also the densification mechanism of Ti64 powders with different particle sizes and compositions was studied from the perspectives of mechanics and kinetics.Besides,the related physical experiments are carried out to verify the numerical simulation.The innovative results are as follows.Firstly,the densification of three groups of single size Ti64 powders with different particle sizes under vibration was numerically simulated by DEM method.The evolution of macro and micro properties of each group of powders before and after vibration was compared and analyzed,and the densification mechanism of powders was studied from the perspectives of mechanics and pore filling.The results show that mechanical vibration can effectively improve the packing density of Ti64 powder.The initial packing density and the final packing density decrease with the decrease of particle size.The analysis of contact force shows that the normal force between particles increases and the orientation of most forces tends to be horizontal due to the influence of vibration,and the distribution of strong normal force between particles decreases exponentially before and after vibration.Secondly,the densification of multi-sized Ti64 powder under vibration was simulated by DEM,and the simulation results were verified by physical experiments.In addition to the macroscopic properties,the micro properties of the powders obtained before and after vibration were quantitatively characterized and compared,including CN,contact orientation,radical polyhedron(RP)structure,force distribution and transition,etc.Radical polyhedron analysis shows that the relative volume of pores in RP decreases after vibration.The force analysis shows that the introduction of vibration field increases the contact force between the particles,and makes the strong force mainly concentrated in the vertical direction.The analysis of contact types shows that the contact between small particles decreases obviously,and the contact between small particles and large particles increases.It shows that the contact between the small particles is dispersed after vibration,and more small particles fill the pores around the large particles to form contact with the large particles,which leads to the decrease of pores in the packing system.