Abstract
X-ray tomography has emerged as a uniquely powerful and non-destructive tool to analyze defects in additive manufacturing. Defects include unintended porosity, rough surfaces and deviations from design, which can have different root causes and can vary significantly among samples. Powder material properties, non-uniform delivery of the powder layer, deformation during manufacturing, deviations from optimal process-parameters caused by changes in the laser beam, the optical components and the scanning system operation, may result in lack of fusion pores, metallurgical pores, keyhole pores, etc. These different types of pores have different typical sizes, shapes and 3D distributions. All types of defects have effects on the mechanical properties of a final part. The use of X-ray tomography to visualize pores in parts (non-destructively) prior to mechanical testing has allowed us to improve our understanding of the effect of this porosity on the mechanical properties of the part (also referred to as “effect of defect”). This can provide the possibility to discriminate critical defects from harmless ones, and thereby build confidence in additive manufacturing processes. This paper reviews the current state of knowledge with regard to the “effect of defect” in metal additive manufacturing, and highlights some relevant examples from our recent work.
Highlights
X-ray tomographyX-ray micro computed tomography (microCT or X-ray tomography or CT scanning) is an emerging technology used to nondestructively investigate the structural integrity and internal details of samples in various research application fields including materials sciences [26], geosciences [66], concrete and asphalt building materials [67], biological materials [68,69,70,71] and in industrial applications [25,72]
Effects of defects on mechanical properties in metal additive manufacturing: A review focusing on X-ray tomography insights
Evaluating the effect of defects on standard test geometries is important, but how does this relate to complex geometries as realized by laser powder bed fusion? For example it is possible with L-PBF to realize parts with biomimetic designs with organic shapes, cellular or lattice structures or combinations of complex design approaches [7]
Summary
X-ray micro computed tomography (microCT or X-ray tomography or CT scanning) is an emerging technology used to nondestructively investigate the structural integrity and internal details of samples in various research application fields including materials sciences [26], geosciences [66], concrete and asphalt building materials [67], biological materials [68,69,70,71] and in industrial applications [25,72]. A recent round robin study where parts were produced at various metal L-PBF production facilities and subsequently analyzed by X-ray tomography under identical conditions, showed the presence of a variety of different defect types and distributions, even while all parts had a density over 99.87% [37] The third example (sample C) shown contains spherical pores under the top surface of the sample mainly, which is suspected to be keyhole mode porosity that was generated with up-skin scanning of top layers These examples illustrate a useful insight into process conditions and provide a tool to optimize these conditions to minimize the porosity distributions in the first place. The transfer of these porosity distributions to complex parts has been demonstrated, but in complex parts additional porosity causes may be present which must be considered
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