Hierarchical Pore Defects and Cumulative Effect between Powder Bed and Melted Area in Electron Beam Powder Bed Fusion of Tungsten

Abstract

In this article, meso-scale simulations and modelling on the multi-layer spreading and printing of tungsten (W) through electron beam powder bed fusion (EB-PBF) were conducted by three-dimensional discrete element method (DEM) coupled computational fluid dynamics (CFD) approach, in which the recurring pore defects were explored, and the cooperative effects of parameters during powder spreading (spreading velocity Vs, spreading direction) and printing (melting power Pm, melting velocity Vm) on the formation of pore defects in end of the melted area were analyzed. Based on the simulation of the repetitive spreading/printing process, the mechanism of pore defect formation was revealed, and the occurrence frequency and pore size were quantified (including the average number of layers required for pore occurrence Nr, the number of the pores np, the average equivalent radius of pores rap, the equivalent radius of the amount area of the pore rp). Results show that in the printing stage, due to the withdraw of the heat source, there is always a keyhole in the end of the melted area, and increasing Pm or decreasing Vm will enlarge the keyhole. In the powder spreading stage, reverse powder spreading will enlarge the pores between particles in the keyhole, and the decrease of Vs will slightly increase the local density of particles in the keyhole. In the multi-layer printing process, the presence of keyholes will cause periodic pore defects at the end of the melted area. Increasing Pm and decreasing Vm will reduce Nr and rp, and increase rap and np. Increasing Vs and adopting reverse powder spreading have little effect on Nr, but can slightly increase rp, rap, and np. The obtained highlighted results are not only of theoretical significance, but also of practical value for parametric setting and optimization in the actual EB-PBF of high-performance W material.

Keywords: Electron beam powder bed fusion of tungsten, DEM-CFD simulations, Pore defects, Cumulative effects, Mechanism analysis