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Channels and bores in metal components produced by laser powder bed fusion (LPBF) are internal features that are typically affected by defects such as dross and sag formation, dimensional errors and global deformations in different proportions. Such deviations from the ideal geometry may strongly limit the functionality of the channels, but are difficult to prevent, due to complex multi-physical production aspects. Different destructive and non-destructive approaches are available to investigate the geometry of the internal features and possibly correlate their results to the LPBF process parameters; however, such approaches do not offer a systematic method to derive key characteristics of the main contributors for channel deviations. Hence, this work proposes a novel tomographic non-destructive analysis of LPBF channels and bores, focusing on the derivation of sag and dross key parameters. The methodology works on polar-transformed profiles obtained from image stacks which are extracted perpendicularly to the channel axis from the X-ray computed tomography (CT) reconstructed volume. The method allows for the clear determination of surface characteristics and includes the quantitative evaluation of descriptors through an algorithm specifically developed for the purpose. In particular, general form deviations are addressed by fitting sinusoidals on the unwrapped mean surface profile, to tackle deviations induced by thermal residual stresses. Proposed descriptors of sag and dross are the onset angle of protrusions, separation criteria between sag and dross effects, and the peak analysis of the mean profile after approximation with a least squares spline. The developed algorithm is tested in the case study of a LPBF AlSi7Mg0.6 benchmark part comprising hollow cylinders and inter-connecting frusta with different diameters. The resulting evaluation of the benchmark part also corroborates how the proposed methodology can help to obtain more precise information regarding the correlation of LPBF fabrication conditions and obtained channels geometrical deviations. Furthermore, the results show possible routes to enable an a-priori compensation of the nominal channel design for first-time right LPBF manufacturing.
Additive metal manufacturing processes, such as laser powder bed fusion, still show difficulties when producing overhang features or internal structures such as channels or bores. Channels are often mutilated by sag defects and dross formation at their upper part, when the channel-axis is close to parallel to the base plate and in the particular case when support structures cannot be used as it would be impossible to remove them after the build. The problem is still not completely solved, although various design guidelines have been developed for various processes and materials in use. So far, a general approach is to tweak the processing parameters or to orient the design on the build plate to reduce downfacing regions at the most critical features of the parts. This work proposes to use feedback from X-ray computed tomography measurements and a new evaluation approach for the additive manufacturing process-chain to obtain improved geometrical accuracy of internal channels. Preliminary results on the evaluation are presented, with the future scope of reducing sag and dross defects by adapting the channels and bores during the design stage.
PAM^2, which stands for Precision Additive Metal Manufacturing, is a European MSCA project in which 10 beneficiaries and 2 partners collaborate on improving the precision of metal Additive Manufacturing. Within this project, research is done for each process stage of AM, going from the design stage to modelling, fabricating, measuring and assessment. For each step we aim to progress the state of the art with a view on improving the final AM part precision and quality by implementing good precision engineering practice. In this article we will list PAM^2’s detailed objectives, the envisioned approach and the results achieved after 1,5 years of research.
State-of-the-art micro-focus X-ray sources are extensively used in industrial computed tomography systems to achieve micrometric resolution, which is fundamental when measuring for example micro-components or parts characterized by micro-features and structures. Resolution limitations are mainly connected to the size of the interaction cross-section between the electron beam and the target material (i.e. focal spot). Therefore, much effort has been put into research reducing the size of the focal spot of X-ray sources. However, the accurate measurement of the effective focal spot size is still challenging. Within this scope, an evaluation method to determine the spot size and shape of the source of a computed tomography system is explained in detail. The concepts of modulation transfer function in the Fourier regime and the full width at half maximum in the real space are used for the evaluation. Different voltages and powers are investigated, as the spot size strongly depends on these parameters. These measurements are compared to computer simulated reference values obtained with a simulated resolution chart.
Up to date, several different approaches are proposed in standards and guidelines to determine the spatial resolution and the focal spot size of micro X-ray sources. From a metrological point of view, the precise measurement of the focal spot is essential, as this is one of the main factors determining the uncertainty of computed tomography dimensional measurements. Besides the fact that the available standards and guidelines are only specified for spot sizes down to 5 µm, there is no consistency among the results of the different approaches. In this work, two commonly adopted approaches, one using a knife edge and the other one using a resolution test chart with line and radial features, are used to evaluate the focal spot size of a micro X-ray computed tomography system. With the combination of these two methods, it is possible to characterize the focal spot of micro X-ray sources over a broad range of the input parameters consistently. The obtained results show the evolution of the focal spot for different source parameters and can be used to determine the optimal source parameters to be set for a desired resolution. X-ray computed tomography, Spatial Resolution, Spot Size.