Optimization of surface determination strategies to enhance detection of unfused powder in metal additive manufactured components

The evolution of industrial computer tomography in recent years has enabled the inspection of the integrity of mechanical components without sectioning or destroying them. For many valuable components and prototypes nondestructive inspection (NDI) can save precious time and costs allowing for pre-failure detection and accurate finite element analysis. The mechanical properties of cast components are well documented and nondestructive inspection is well established, but with additive manufacturing (AM) quickly being recognized as a core technology for producing complex and individual components the need for well understood NDI is more urgent than ever. The challenges in inspecting (AM) components are quite different than cast ones and unfused powder detection is one of the biggest issue, as the powder size used in sintering the (AM) component can be smaller than 20 μm. Detecting small pores/defects of such a size requires the use of high magnification and bespoke XCT settings. This paper investigates the impact of surface determination on the detection of unfused powder in a EBM process and assesses the ability of utilizing the standard ISO 50 surface determination protocol to visualize this. The powder used was Ti6AL4V 45-100-micron grain, chosen as it is the powder commonly used in an Arcam electron beam melting (EBM) machine. A Nikon XTH 225 (Nikon Metrology, Tring) industrial CT was used to analyze the pores/defects' location and volume. Defects of between 50 and 1400 microns in diameter were machined into the surface of the designed artefact using a CNC machine equipped with micro-drills. Once this was achieved, the defects were characterized using a Alicona G4 (Alicona, Graz) focus variation instrument. Virgin Ti6AL4V powder was then added to fill the 1400μm and 500μm diameter defects. Data processing, surface determination process and defect analysis was carried out using VG Studio Max 3.1 (Volume Graphics, Heidelberg). The focus of the study is on providing best practice guidance regarding the selection of inspection parameters and identifying the capability of ISO 50 surface determination in detecting unfused powder.