A novel tomographic characterisation approach for sag and dross defects in metal additively manufactured channels

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.