TY - JOUR A1 - Oster, Simon A1 - Fritsch, Tobias A1 - Ulbricht, Alexander A1 - Mohr, Gunther A1 - Bruno, Giovanni A1 - Maierhofer, Christiane A1 - Altenburg, Simon T1 - On the registration of thermographic in situ monitoring data and computed tomography reference data in the scope of defect prediction in laser powder bed fusion JF - Metals : open access journal N2 - The detection of internal irregularities is crucial for quality assessment in metal-based additive manufacturing (AM) technologies such as laser powder bed fusion (L-PBF). The utilization of in-process thermography as an in situ monitoring tool in combination with post-process X-ray micro computed tomography (XCT) as a reference technique has shown great potential for this aim. Due to the small irregularity dimensions, a precise registration of the datasets is necessary as a requirement for correlation. In this study, the registration of thermography and XCT reference datasets of a cylindric specimen containing keyhole pores is carried out for the development of a porosity prediction model. The considered datasets show variations in shape, data type and dimensionality, especially due to shrinkage and material elevation effects present in the manufactured part. Since the resulting deformations are challenging for registration, a novel preprocessing methodology is introduced that involves an adaptive volume adjustment algorithm which is based on the porosity distribution in the specimen. Thus, the implementation of a simple three-dimensional image-to-image registration is enabled. The results demonstrate the influence of the part deformation on the resulting porosity location and the importance of registration in terms of irregularity prediction. KW - selective laser melting (SLM) KW - laser powder bed fusion (L-PBF) KW - additive KW - manufacturing (AM) KW - process monitoring KW - infrared thermography KW - X-ray KW - micro computed tomography (XCT) KW - defect detection KW - image registration Y1 - 2022 U6 - https://doi.org/10.3390/met12060947 SN - 2075-4701 VL - 12 IS - 6 PB - MDPI CY - Basel ER - TY - JOUR A1 - Schröder, Jakob A1 - Evans, Alexander A1 - Polatidis, Efthymios A1 - Mohr, Gunther A1 - Serrano-Munoz, Itziar A1 - Bruno, Giovanni A1 - Čapek, Jan T1 - Understanding the impact of texture on the micromechanical anisotropy of laser powder bed fused Inconel 718 JF - Journal of materials science N2 - The manufacturability of metallic alloys using laser-based additive manufacturing methods such as laser powder bed fusion has substantially improved within the last decade. However, local melting and solidification cause hierarchically structured and crystallographically textured microstructures possessing large residual stress. Such microstructures are not only the origin of mechanical anisotropy but also pose metrological challenges for the diffraction-based residual stress determination. Here we demonstrate the influence of the build orientation and the texture on the microstructure and consequently the mechanical anisotropy of as-built Inconel 718. For this purpose, we manufactured specimens with [001]/[011]-, [001]- and [011]/[11 (1) over bar]-type textures along their loading direction. In addition to changes in the Young's moduli, the differences in the crystallographic textures result in variations of the yield and ultimate tensile strengths. With this in mind, we studied the anisotropy on the micromechanical scale by subjecting the specimens to tensile loads along the different texture directions during in situ neutron diffraction experiments. In this context, the response of multiple lattice planes up to a tensile strain of 10% displayed differences in the load partitioning and the residual strain accumulation for the specimen with [011]/[(1) over bar 11]-type texture. However, the relative behavior of the specimens possessing an [001] /[011]- and [001]-type texture remained qualitatively similar. The consequences on the metrology of residual stress determination methods are discussed. Y1 - 2022 U6 - https://doi.org/10.1007/s10853-022-07499-9 SN - 0022-2461 SN - 1573-4803 VL - 57 IS - 31 SP - 15036 EP - 15058 PB - Springer CY - New York ER - TY - JOUR A1 - Laquai, René A1 - Schaupp, Thomas A1 - Griesche, Axel A1 - Müller, Bernd R. A1 - Kupsch, Andreas A1 - Hannemann, Andreas A1 - Kannengiesser, Thomas A1 - Bruno, Giovanni T1 - Quantitative analysis of hydrogen-assisted microcracking in duplex stainless steel through X-ray refraction 3D imaging JF - Advanced engineering materials N2 - While the problem of the identification of mechanisms of hydrogen-assisted damage has and is being thoroughly studied, the quantitative analysis of such damage still lacks suitable tools. In fact, while, for instance, electron microscopy yields excellent characterization, the quantitative analysis of damage requires at the same time large field-of-views and high spatial resolution. Synchrotron X-ray refraction techniques do possess both features. Herein, it is shown how synchrotron X-ray refraction computed tomography (SXRCT) can quantify damage induced by hydrogen embrittlement in a lean duplex steel, yielding results that overperform even those achievable by synchrotron X-ray absorption computed tomography. As already reported in the literature, but this time using a nondestructive technique, it is shown that the hydrogen charge does not penetrate to the center of tensile specimens. By the comparison between virgin and hydrogen-charged specimens, it is deduced that cracks in the specimen bulk are due to the rolling process rather than hydrogen-assisted. It is shown that (micro)cracks propagate from the surface of tensile specimens to the interior with increasing applied strain, and it is deduced that a significant crack propagation can only be observed short before rupture. KW - 2101 duplex stainless steel KW - computed tomography KW - fractography KW - hydrogen KW - embrittlement KW - microcracking KW - synchrotron radiation KW - X-ray refraction Y1 - 2022 U6 - https://doi.org/10.1002/adem.202101287 SN - 1527-2648 VL - 24 IS - 6 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Serrano-Munoz, Itziar A1 - Fernández Serrano, Ricardo A1 - Saliwan-Neumann, Romeo A1 - Gonzalez-Doncel, Gaspar A1 - Bruno, Giovanni T1 - Dislocation substructures in pure aluminium after creep deformation as studied by electron backscatter diffraction JF - Journal of applied crystallography / International Union of Crystallography N2 - In the present work, electron backscatter diffraction was used to determine the microscopic dislocation structures generated during creep (with tests interrupted at the steady state) in pure 99.8% aluminium. This material was investigated at two different stress levels, corresponding to the power-law and power-law breakdown regimes. The results show that the formation of subgrain cellular structures occurs independently of the crystallographic orientation. However, the density of these cellular structures strongly depends on the grain crystallographic orientation with respect to the tensile axis direction, with (111) grains exhibiting the highest densities at both stress levels. It is proposed that this behaviour is due to the influence of intergranular stresses, which is different in (111) and (001) grains. KW - creep KW - pure aluminium KW - electron backscatter diffraction (EBSD) KW - cellular KW - structures KW - power law and power-law breakdown Y1 - 2022 U6 - https://doi.org/10.1107/S1600576722005209 SN - 1600-5767 VL - 55 SP - 860 EP - 869 PB - Munksgaard CY - Copenhagen ER - TY - JOUR A1 - Mani, Deepak A1 - Kupsch, Andreas A1 - Müller, Bernd R. A1 - Bruno, Giovanni T1 - Diffraction Enhanced Imaging Analysis with Pseudo-Voigt Fit Function JF - Journal of imaging : open access journal N2 - Diffraction enhanced imaging (DEI) is an advanced digital radiographic imaging technique employing the refraction of X-rays to contrast internal interfaces. This study aims to qualitatively and quantitatively evaluate images acquired using this technique and to assess how different fitting functions to the typical rocking curves (RCs) influence the quality of the images. RCs are obtained for every image pixel. This allows the separate determination of the absorption and the refraction properties of the material in a position-sensitive manner. Comparison of various types of fitting functions reveals that the Pseudo-Voigt (PsdV) function is best suited to fit typical RCs. A robust algorithm was developed in the Python programming language, which reliably extracts the physically meaningful information from each pixel of the image. We demonstrate the potential of the algorithm with two specimens: a silicone gel specimen that has well-defined interfaces, and an additively manufactured polycarbonate specimen. KW - diffraction enhanced imaging KW - analyzer-based imaging KW - X-ray refraction; KW - non-destructive evaluation KW - Pseudo-Voigt fit function KW - Python Y1 - 2022 U6 - https://doi.org/10.3390/jimaging8080206 SN - 2313-433X VL - 8 IS - 8 PB - MDPI CY - Basel ER - TY - JOUR A1 - Mishurova, Tatiana A1 - Stegemann, Robert A1 - Lyamkin, Viktor A1 - Cabeza, Sandra A1 - Evsevleev, Sergei A1 - Pelkner, Matthias A1 - Bruno, Giovanni T1 - Subsurface and bulk residual stress analysis of S235JRC+C Steel TIG weld by diffraction and magnetic stray field measurements JF - Experimental mechanics : an international journal of the Society for Experimental Mechanics N2 - Background Due to physical coupling between mechanical stress and magnetization in ferromagnetic materials, it is assumed in the literature that the distribution of the magnetic stray field corresponds to the internal (residual) stress of the specimen. The correlation is, however, not trivial, since the magnetic stray field is also influenced by the microstructure and the geometry of component. The understanding of the correlation between residual stress and magnetic stray field could help to evaluate the integrity of welded components. Objective This study aims at understanding the possible correlation of subsurface and bulk residual stress with magnetic stray field in a low carbon steel weld. Methods The residual stress was determined by synchrotron X-ray diffraction (SXRD, subsurface region) and by neutron diffraction (ND, bulk region). SXRD possesses a higher spatial resolution than ND. Magnetic stray fields were mapped by utilizing high-spatial-resolution giant magneto resistance (GMR) sensors. Results The subsurface residual stress overall correlates better with the magnetic stray field distribution than the bulk stress. This correlation is especially visible in the regions outside the heat affected zone, where the influence of the microstructural features is less pronounced but steep residual stress gradients are present. Conclusions It was demonstrated that the localized stray field sources without any obvious microstructural variations are associated with steep stress gradients. The good correlation between subsurface residual stress and magnetic signal indicates that the source of the magnetic stray fields is to be found in the range of the penetration depth of the SXRD measurements. KW - residual stress KW - magnetic stray field KW - synchrotron X-ray diffraction KW - neutron diffraction KW - TIG-welding Y1 - 2022 U6 - https://doi.org/10.1007/s11340-022-00841-x SN - 0014-4851 SN - 1741-2765 VL - 62 IS - 6 SP - 1017 EP - 1025 PB - Springer CY - New York ER - TY - JOUR A1 - Magkos, Sotirios A1 - Kupsch, Andreas A1 - Bruno, Giovanni T1 - Suppression of cone-beam artefacts with Direct Iterative Reconstruction Computed Tomography Trajectories (DIRECTT) JF - Journal of imaging : open access journal N2 - The reconstruction of cone-beam computed tomography data using filtered back-projection algorithms unavoidably results in severe artefacts. We describe how the Direct Iterative Reconstruction of Computed Tomography Trajectories (DIRECTT) algorithm can be combined with a model of the artefacts for the reconstruction of such data. The implementation of DIRECTT results in reconstructed volumes of superior quality compared to the conventional algorithms. KW - iteration method KW - signal processing KW - X-ray imaging KW - computed tomography Y1 - 2021 U6 - https://doi.org/10.3390/jimaging7080147 SN - 2313-433X VL - 7 IS - 8 PB - MDPI CY - Basel ER - TY - JOUR A1 - Nadammal, Naresh A1 - Mishurova, Tatiana A1 - Fritsch, Tobias A1 - Serrano-Munoz, Itziar A1 - Kromm, Arne A1 - Haberland, Christoph A1 - Portella, Pedro Dolabella A1 - Bruno, Giovanni T1 - Critical role of scan strategies on the development of microstructure, texture, and residual stresses during laser powder bed fusion additive manufacturing JF - Additive manufacturing N2 - Laser based powder bed fusion additive manufacturing offers the flexibility to incorporate standard and user-defined scan strategies in a layer or in between the layers for the customized fabrication of metallic components. In the present study, four different scan strategies and their impact on the development of microstructure, texture, and residual stresses in laser powder bed fusion additive manufacturing of a nickel-based superalloy Inconel 718 was investigated. Light microscopy, scanning electron microscopy combined with electron back-scatter diffraction, and neutron diffraction were used as the characterization tools. Strong textures with epitaxially grown columnar grains were observed along the build direction for the two individual scan strategies. Patterns depicting the respective scan strategies were visible in the build plane, which dictated the microstructure development in the other planes. An alternating strategy combining the individual strategies in the successive layers and a 67 degrees rotational strategy weakened the texture by forming finer micro-structural features. Von Mises equivalent stress plots revealed lower stress values and gradients, which translates as lower distortions for the alternating and rotational strategies. Overall results confirmed the scope for manipulating the microstructure, texture, and residual stresses during laser powder bed fusion additive manufacturing by effectively controlling the scan strategies. KW - Additive manufacturing KW - Laser powder bed fusion KW - Nickel-based KW - superalloys KW - Scan strategies KW - Residual stresses KW - Microstructure and KW - texture Y1 - 2021 U6 - https://doi.org/10.1016/j.addma.2020.101792 SN - 2214-8604 VL - 38 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Ulbricht, Alexander A1 - Mohr, Gunther A1 - Altenburg, Simon J. A1 - Oster, Simon A1 - Maierhofer, Christiane A1 - Bruno, Giovanni T1 - Can potential defects in LPBF be healed from the laser exposure of subsequent layers? BT - A quantitative study JF - Metals : open access journal N2 - Additive manufacturing (AM) of metals and in particular laser powder bed fusion (LPBF) enables a degree of freedom in design unparalleled by conventional subtractive methods. To ensure that the designed precision is matched by the produced LPBF parts, a full understanding of the interaction between the laser and the feedstock powder is needed. It has been shown that the laser also melts subjacent layers of material underneath. This effect plays a key role when designing small cavities or overhanging structures, because, in these cases, the material underneath is feed-stock powder. In this study, we quantify the extension of the melt pool during laser illumination of powder layers and the defect spatial distribution in a cylindrical specimen. During the LPBF process, several layers were intentionally not exposed to the laser beam at various locations, while the build process was monitored by thermography and optical tomography. The cylinder was finally scanned by X-ray computed tomography (XCT). To correlate the positions of the unmolten layers in the part, a staircase was manufactured around the cylinder for easier registration. The results show that healing among layers occurs if a scan strategy is applied, where the orientation of the hatches is changed for each subsequent layer. They also show that small pores and surface roughness of solidified material below a thick layer of unmolten material (>200 mu m) serve as seeding points for larger voids. The orientation of the first two layers fully exposed after a thick layer of unmolten powder shapes the orientation of these voids, created by a lack of fusion. KW - selective laser melting (SLM) KW - additive manufacturing (AM) KW - process KW - monitoring KW - infrared thermography KW - optical tomography KW - X-ray computed KW - tomography (XCT) KW - healing KW - in situ monitoring Y1 - 2021 U6 - https://doi.org/10.3390/met11071012 SN - 2075-4701 VL - 11 IS - 7 PB - MDPI CY - Basel ER - TY - JOUR A1 - Pauzon, Camille A1 - Mishurova, Tatiana A1 - Evsevleev, Sergei A1 - Dubiez-Le Goff, Sophie A1 - Murugesan, Saravanakumar A1 - Bruno, Giovanni A1 - Hryha, Eduard T1 - Residual stresses and porosity in Ti-6Al-4V produced by laser powder bed fusion as a function of process atmosphere and component design JF - Additive manufacturing N2 - The influence of the process gas, laser scan speed, and sample thickness on the build-up of residual stresses and porosity in Ti-6Al-4V produced by laser powder bed fusion was studied. Pure argon and helium, as well as a mixture of those (30% helium), were employed to establish process atmospheres with a low residual oxygen content of 100 ppm O-2. The results highlight that the subsurface residual stresses measured by X-ray diffraction were significantly lower in the thin samples (220 MPa) than in the cuboid samples (645 MPa). This difference was attributed to the shorter laser vector length, resulting in heat accumulation and thus in-situ stress relief. The addition of helium to the process gas did not introduce additional subsurface residual stresses in the simple geometries, even for the increased scanning speed. Finally, larger deflection was found in the cantilever built under helium (after removal from the baseplate), than in those produced under argon and an argon-helium mixture. This result demonstrates that complex designs involving large scanned areas could be subjected to higher residual stress when manufactured under helium due to the gas's high thermal conductivity, heat capacity, and thermal diffusivity. KW - Residual stresses KW - Laser powder bed fusion KW - Process atmosphere KW - Helium KW - Ti-6Al-4V Y1 - 2021 U6 - https://doi.org/10.1016/j.addma.2021.102340 SN - 2214-8604 VL - 47 PB - Elsevier CY - Amsterdam ER -