Caratterizzazione di campioni metallici stampati in Laser Powder Bed Fusion mediante Bragg Edge Neutron Transmission analysis

Bragg Edge Neutron Transmission analysis (BENT) is a non-destructive technique for investigation of some properties of crystalline solids. The idea behind this method is to irradiate the sample with a neutron beam and acquire different monochromatic radiograms at a selected wavelength around the main Bragg Edges of the sample, studying then the transmission spectrum. By interacting with the crystalline lattice of the materials, neutrons will give coherent elastic scattering until the Bragg condition is verified: from the well-known Bragg’s law it can be seen that there is a maximum wavelength above which the Bragg scattering is no longer possible, and at this value of λ an edge occurs in the transmission spectrum. By taking advantage of the abrupt collapse of the cross-section it is then possible to integrate image information over the wavelength stack; this allows to study sample’s properties such as texture, microstructure, strain or defects. The novelty presented in this thesis is the application of BENT as a diagnostic technique for the characterization of Additive Manufacturing samples. In particular the focus is on samples printed with Laser Powder Bed Fusion (LPBF); despite its undisputed advantages, this printing process produces various microstructural defects that can affect the overall functionality of the manufactured part, and therefore there are still open questions about the structural strength of the artefacts. The lack of a non-invasive technique allowing to study in depth the microstructural structure of AM samples leads to the proposal to use BENT analysis to provide detailed quantitative feedbacks to the manufacturing process, such as mapping of strain, phases and texture. This work deals with the characterization of three different samples printed in LPBF, two of them in steel (316L and 16MnCr5) and the other one in pure copper. Some artificial defects were inserted inside these samples during the printing process, so that it is possible to study the ability of BENT analysis to identify them, obtaining a qualification of the technique itself. This image acquisition campaign was carried out in November 2023 at BOA, a neutron facility at the Paul Scherrer Institute in Switzerland.