Electron Beam Additive Manufacturing of Ti6al4v: Importance of Powder Reuse on Metal Porosity, Pore Size and Spatial Distribution

Abstract

Porosity is one of the primary concerns in metal powder bed fusion Additive Manufacturing (AM). While investigations concerning defects in metal AM are common, there is limited understanding concerning the spatial distribution of pores in the metal, and how they evolve with powder reuse. In this study, the porosity in Ti6Al4V produced by Electron Beam Melting (EBM) AM was characterized by X-ray Micro Computed Microtomography (μCT) over a series of 30 build cycles (~480 hr cumulative build time). Investigated were the volume fraction of pores (i.e. the porosity), their size, shape, and location distribution. The most prevalent pores identified were those originating from the gas atomized powder and pores caused by incomplete fusion of the melt pool. The overall average volumetric porosity was 0.10±0.02%, with no significant dependence on powder reuse. A general tightening of the pore size distribution was seen with reuse including slight increases in the 10th and 50th percentile and a reduction in the 90th percentile. However, some consistency in the size distribution was noted, with some builds developing extreme size pores. An inverse relationship between pore diameter and sphericity was found, with large pores posing greater effective stress concentration. Regarding spatial distribution, the largest pore density developed around 0.5 mm adjacent to the surface of parts, at the transition between the contour and melt beams. Based on results of this study, µCT is recommended for quality control of metal parts, and models that account for these defects in defining the damage tolerance of components are needed.