Powder reuse has become a central issue in the pursuit to industrialize metal Additive Manufacturing (AM). Furthermore, build porosity is a critical concern to component reliability and damage tolerance of the metal. However, the contributions of powder reuse to metal porosity has received very limited attention. In this study, the porosity resulting from Powder Bed Fusion-Electron Beam Melting (PBF-EB) AM of Ti6Al4V was characterized over a series of 30 build cycles (consisting of similar to 480 h cumulative build time) using X-ray Micro Computed Tomography (mu CT). Investigated were the volume fraction of pores (i.e. the porosity), as well as the pore size, shape, and spatial distribution. The most prevalent pores identified were: i) those originating from the gas atomized powder, and ii) those caused by incomplete fusion of the melt pool. Although there was a minor reduction in the pore size distribution with powder reuse, the overall average volumetric porosity was 0.10 +/- 0.02% and there was no significant change with increasing reuse. An inverse relationship between pore diameter and sphericity was found, with large pores posing greater effective stress concentration. Whereas the greatest pore density was located at the transition between the contour and melt beams, approximately 0.5-1.0 mm adjacent to the surface of parts, the largest pores were located within the interior hatching region. Hence, despite progressive deformation of the particles and an increase in oxidation, there was no discernable change in metal porosity with powder reuse in PBF-EB AM of Ti6Al4V.

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.

Additive Manufacturing (AM) processes for metals are advancing at a rapid pace. Among many attractive qualities, AM relaxes design constraints and can significantly reduce material waste in comparison to subtractive manufacturing processes. However, there are some fundamental issues that must be addressed for metal AM to become prevalent in aerospace. In powder bed fusion AM, powder reuse from previous build cycles is desired to improve process economics. However, there is limited understanding of the contributions from powder reuse to particle and part quality. The present study investigates this topic in electron beam melting (EBM) powder fusion AM of a titanium alloy (Ti6Al4V) over 30 cycles of build and powder reuse. Results show that nearly all aspects of the process are influenced by powder reuse. Specifically, the particle size distribution tightens, largely due to fewer with small diameter. Particle damage increases with reuse, which includes surface deformation (reduction in sphericity), partial melting and/or particle fusion and fracture. In regard to the built metal, the microstructure exhibits increasingly finer basket weave and greater surface area to volume ratio of a with reuse. Yet, there are no apparent trends in a-lath thickness or volume. In the analysis of composition, while substantial changes in the Al and V content are apparent, or in Fe, H and N, the O concentration of the powder increased significantly with reuse. In fact, it exceeded the concentration limit (0.2%) in just 11 build cycles. Overall, powder reuse should be considered carefully in the development of titanium parts for performance critical applications by EBM AM.

Metal additive manufacturing (AM) is being rapidly adopted in the aerospace and biomedical industries. Powder bed fusion AM processes are leading this trend. To maximize process economy, excess “unmelted” powder retrieved from the build chamber is used in subsequent build cycles. The metal properties and component reliability could undergo degradation with powder reuse. This study investigates the effects of powder reuse on fracture surface characteristics of Ti6Al4V specimens fabricated by electron beam melting AM over 30 sequential build cycles. Optical microscopy and scanning electron microscopy were used to evaluate the changes in fracture surface features of tensile failures with powder reuse. Macroscopically, slant fractures were most common in early builds, which transitioned to orthogonal fracture surfaces with poorly defined shear lips with increasing reuse. Regardless of the build number, the fracture origins were consistently from the as-built surfaces. Microscopically, ductile features such as micro-void coalescence were evident throughout the 30 build cycles. However, increasing flute content with reuse suggests that rising oxygen levels causes solution strengthening and limits the participation of active slip systems. These results highlight the importance of surface roughness and powder oxidation to metal performance in AM, and the evolution of fractographic features with powder reuse.