Research On Microstructure And Numerical Simulation Of Ti-6Al-4V Alloy Fabricated By Electron Beam Selective Melting

Additive manufacturing(AM),commonly known as 3D printing technology,is an advanced manufacturing technology based on discrete and stacked forming principle.It has the characteristics of high freedom of design and manufacturing,short production cycle,batch customization manufacturing,etc.The electron beam selective melting(EBM)technology is suitable for the production of small batch metal parts with complex shapes and is now widely used in key fields such as aerospace,medical and health due to its high energy efficiency,good mechanical properties,and good density of the parts.With the wide application of parts in key fields,higher performance requirements are put forward for parts fabricated by EBM technology,such as high machining accuracy,high strength and no defects,and the microstructure and forming defects of parts fabricated by EBM technology have important effects on their performance.In this paper,Ti-6Al-4V alloy bone shank samples fabricated by EBM technology were studied and the microstructure and phase of the samples were analyzed by OM,SEM and XRD.The process of melting the Ti-6Al-4V alloy powder bed by EBM technology was simulated numerically and the forming defects in the forming track were simulated,and the mechanism of the defects was studied deeply.The main contents are as follows:Firstly,the bone shank samples perpendicular to the substrate direction were fabricated by EBM technology using Ti-6Al-4V alloy powder as the raw material,and the microstructures of the cross and longitudinal sections of the bone shank at different positions were characterized by OM,SEM and XRD and the microstructure evolution information was obtained.In the cross and longitudinal sections,α phase and β phase are the main microstructure,and the longitudinal section has the original β columnar crystals growing along the construction direction,with relatively complete grain boundary α.A net basket organization with staggered distribution of α lamellae based on β phase is formed in the cross-section.The size of α lamellae increases with the increase of the cross-section,and the pore defects of different shapes and sizes appear in the microstructure.The diffraction pattern obtained by XRD shows that HCP-α(α’)phase is the main component of the sample.Secondly,the Ti-6Al-4V alloy powder bed model was established by using EDEM software based on DEM,and the powder bed model was imported into the simulation software FLOW-3D.The control equation and heat source model controll the melt pool flow and heat input,and the volume of fluid method(VOF)capture and construct the dynamic surface of the melt pool to reproduce the process of electron beam melting metal powder bed.The process of melting and forming a single track with different process parameters(power,scanning speed,powder layer thickness)and the influence law of process parameters on the width and height of the melt pool were studied.Finally,based on the analysis of the previous microstructure and simulation,the formation mechanism of each defect was analyzed through the melt pool cross-section by studying the balling defects and nonuniform track that appear in the forming single track process and the irregular porosity defects that appear in the forming double track process.For the balling defects,it is due to the lack of energy input resulting in the substrate not melting,the melt and the substrate were not wet enough,and finally the melt towards the direction of minimum surface energy(spherical);The nonuniform track was attributed to the difference of partial melting and complete melting of the powder on both sides of the single track;The irregular pore defects were attributed to the irregular pores formed by the lack of melting between the tracks due to the improper selection of the scanning spacing.