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 - Thiede, Tobias A1 - Cabeza, Sandra A1 - Mishurova, Tatiana A1 - Nadammal, Naresh A1 - Kromm, Arne A1 - Bode, Johannes A1 - Haberland, Christoph A1 - Bruno, Giovanni T1 - Residual Stress in Selective Laser Melted Inconel 718 BT - Influence of the Removal from Base Plate and Deposition Hatch Length JF - Materials performance and characterization N2 - The residual stress distribution in IN718 elongated prisms produced by selective laser melting was studied by means of neutron (bulk) and laboratory X-ray (surface) diffraction. Two deposition hatch lengths were considered. A horizontal plane near the top surface (perpendicular to the building direction) and a vertical plane near the lateral surface (parallel to the building direction) were investigated. Samples both in as-built (AB) condition and removed from the base plate (RE) were characterized. While surface stress fields seem constant for the AB condition, X-ray diffraction shows stress gradients along the hatch direction in the RE condition. The stress profiles correlate with the distortion maps obtained by tactile probe measurements. Neutron diffraction shows bulk stress gradients for all principal components along the main sample directions. We correlate the observed stress patterns with the hatch length, i.e., with its effect on temperature gradients and heat flow. The bulk stress gradients partially disappear after removal from the base plate. KW - residual stress KW - IN718 KW - neutron diffraction KW - laboratory x-ray diffraction KW - additive manufacturing KW - selective laser melting KW - coordinate measurement machine Y1 - 2018 U6 - https://doi.org/10.1520/MPC20170119 SN - 2379-1365 SN - 2165-3992 VL - 7 IS - 4 SP - 717 EP - 735 PB - American Society for Testing and Materials CY - West Conshohocken ER - TY - JOUR A1 - Stegemann, Robert A1 - Cabeza, Sandra A1 - Pelkner, Matthias A1 - Lyamkin, Viktor A1 - Pittner, Andreas A1 - Werner, Daniel A1 - Wimpory, Robert A1 - Boin, Mirko A1 - Kreutzbruck, Marc A1 - Bruno, Giovanni T1 - Influence of the microstructure on magnetic stray fields of low-carbon steel welds JF - Journal of Nondestructive Evaluation N2 - This study examines the relationship between the magnetic mesostructure with the microstructure of low carbon steel tungsten inert gas welds. Optical microscopy revealed variation in the microstructure of the parent material, in the heat affected and fusion zones, correlating with distinctive changes in the local magnetic stray fields measured with high spatial resolution giant magneto resistance sensors. In the vicinity of the heat affected zone high residual stresses were found using neutron diffraction. Notably, the gradients of von Mises stress and triaxial magnetic stray field modulus follow the same tendency transverse to the weld. In contrast, micro-X-ray fluorescence characterization indicated that local changes in element composition had no independent effect on magnetic stray fields. KW - TIG-welding KW - GMR sensors KW - Magnetic stray field KW - Neutron diffraction KW - Residual stress KW - Microstructure KW - Low carbon steel Y1 - 2018 U6 - https://doi.org/10.1007/s10921-018-0522-0 SN - 0195-9298 SN - 1573-4862 VL - 37 IS - 3 PB - Springer CY - New York ER - TY - JOUR A1 - Sprengel, Maximilian A1 - Mohr, Gunther A1 - Altenburg, Simon J. A1 - Evans, Alexander A1 - Serrano-Munoz, Itziar A1 - Kromm, Arne A1 - Pirling, Thilo A1 - Bruno, Giovanni A1 - Kannengießer, Thomas T1 - Triaxial residual stress in Laser Powder Bed Fused 316L BT - effects of interlayer time and scanning velocity JF - Advanced engineering materials N2 - The control of residual stress (RS) remains a challenge in the manufacturing of metallic parts using the laser powder bed fusion process (LPBF). This layer-by-layer manufacturing approach gives rise to complex triaxial RS distributions, which require extensive characterization effort for a broader acceptance of LPBF in industry. This study focuses on the distribution of bulk triaxial RS and surface RS in LPBF austenitic steel 316L. The RS are determined by X-ray and neutron diffraction to characterize the RS distribution. Variations in the LPBF parameters interlayer time (ILT) and scanning velocity and their influence on the temperature distribution and resulting RS is investigated using thermographic data from in situ process monitoring. The RS in the LPBF 316L is tensile at the surface and compressive in the bulk. The RS is directly related to the thermal history of the part as shown by the in situ thermography data. Shorter ILT leads to higher temperatures of the part during the manufacturing, which decrease the RS and RS formation mechanisms. Interestingly, the surface RS does not agree with this observation. This study highlights the benefit of using multiple RS determination methods and in situ thermography monitoring to characterize the RS in LPBF processed parts. KW - in situ thermography KW - interlayer time KW - laser powder bed fusions KW - triaxial residual stresses KW - X-ray and neutron diffractions Y1 - 2021 U6 - https://doi.org/10.1002/adem.202101330 SN - 1438-1656 SN - 1527-2648 VL - 24 IS - 6 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Shashev, Yury A1 - Kupsch, Andreas A1 - Lange, Axel A1 - Müller, Bernd R. A1 - Bruno, Giovanni T1 - Improving the visibility of phase gratings for Talbot-Lau X-ray imaging JF - Materials testing : Materialprüfung ; materials and components, technology and application N2 - Talbot-Lau interferometry provides X-ray imaging techniques with significant enhancement of the radiographic contrast of weakly absorbing objects. The grating based technique allows separation of absorption, refraction and small angle scattering effects. The different efficiency of rectangular and triangular shaped phase gratings at varying detector distances is investigated. The interference patterns (Talbot carpets) are modeled for parallel monochromatic radiation and measured by synchrotron radiation. In comparison to rectangular shapes of phase gratings much higher visibility is obtained for triangular shapes which yield enhanced contrast of a glass capillary test specimen. KW - Talbot-Lau interferometry KW - phase grating KW - synchrotron imaging KW - visibility KW - X-ray refraction Y1 - 2016 U6 - https://doi.org/10.3139/120.110948 SN - 0025-5300 VL - 58 SP - 970 EP - 974 PB - Hanser CY - München ER - TY - JOUR A1 - Shashev, Yury A1 - Kupsch, Andreas A1 - Lange, Axel A1 - Evsevleev, Sergei A1 - Müller, Bernd R. A1 - Osenberg, Markus A1 - Manke, Ingo A1 - Hentschel, Manfred P. A1 - Bruno, Giovanni T1 - Optimizing the visibility of X-ray phase grating interferometry JF - Materials testing : Materialprüfung ; materials and components, technology and application N2 - The performance of grating interferometers coming up now for imaging interfaces within materials depends on the efficiency (visibility) of their main component, namely the phase grating. Therefore, experiments with monochromatic synchrotron radiation and corresponding simulations are carried out. The visibility of a phase grating is optimized by different photon energies, varying detector to grating distances and continuous rotation of the phase grating about the grid lines. Such kind of rotation changes the projected grating shapes, and thereby the distribution profiles of phase shifts. This yields higher visibilities than derived from ideal rectangular shapes. By continuous grating rotation and variation of the propagation distance, we achieve 2D visibility maps. Such maps provide the visibility for a certain combination of grating orientation and detector position. Optimum visibilities occur at considerably smaller distances than in the standard setup. KW - X-ray imaging KW - grating interferometry KW - Talbot-Lau interferometer KW - X-ray refraction KW - X-ray phase contrast Y1 - 2017 U6 - https://doi.org/10.3139/120.111097 SN - 0025-5300 VL - 59 SP - 974 EP - 980 PB - Hanser CY - München ER - TY - JOUR A1 - Sevostianov, Igor A1 - Bruno, Giovanni T1 - Maxwell scheme for internal stresses in multiphase composites JF - Mechanics of Materials N2 - The paper focuses on the reformulation of classic Maxwell's (1873) homogenization method for calculation of the residual stresses in matrix composites. For this goal, we equate the far fields produced by a set of inhomogeneities subjected to known eigenstrains and by a fictitious domain with unknown eigenstrain. The effect of interaction between the inhomogeneities is reduced to the calculation of the additional field acting on an inhomogeneity due to the eigenstrains in its neighbors. An explicit formula for residual stresses is derived for the general case of a multiphase composite. The method is illustrated by several examples. The results are compared with available experimental data as well as with predictions provided by the non-interaction approximation (Eshelby solution). It is shown that accounting for interaction can explain many experimentally observed phenomena and is required for adequate quantitative analytical modeling of the residual stresses in matrix composites. KW - Residual stress KW - Multiphase composites KW - Interaction KW - Micromechanical schemes KW - Anisotropy Y1 - 2018 U6 - https://doi.org/10.1016/j.mechmat.2018.12.005 SN - 0167-6636 SN - 1872-7743 VL - 129 SP - 320 EP - 331 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Serrano-Munoz, Itziar A1 - Mishurova, Tatiana A1 - Thiede, Tobias A1 - Sprengel, Maximilian A1 - Kromm, Arne A1 - Nadammal, Naresh A1 - Nolze, Gert A1 - Saliwan-Neumann, Romeo A1 - Evans, Alexander A1 - Bruno, Giovanni T1 - The residual stress in as-built laser powder bed fusion IN718 alloy as a consequence of the scanning strategy induced microstructure JF - Scientific reports N2 - The effect of two types of scanning strategies on the grain structure and build-up of Residual Stress (RS) has been investigated in an as-built IN718 alloy produced by Laser Powder Bed Fusion (LPBF). The RS state has been investigated by X-ray diffraction techniques. The microstructural characterization was performed principally by Electron Backscatter Diffraction (EBSD), where the application of a post-measurement refinement technique enables small misorientations (< 2 degrees) to be resolved. Kernel average misorientation (KAM) distributions indicate that preferably oriented columnar grains contain higher levels of misorientation, when compared to elongated grains with lower texture. The KAM distributions combined with X-ray diffraction stress maps infer that the increased misorientation is induced via plastic deformation driven by the thermal stresses, acting to self-relieve stress. The possibility of obtaining lower RS states in the build direction as a consequence of the influence of the microstructure should be considered when envisaging scanning strategies aimed at the mitigation of RS. KW - EBSD KW - components KW - deposition KW - diffraction KW - distortion KW - heat-treatment KW - mechanical properties KW - melting slm KW - superalloys KW - texture Y1 - 2020 U6 - https://doi.org/10.1038/s41598-020-71112-9 SN - 2045-2322 VL - 10 IS - 1 PB - Macmillan Publishers Limited, part of Springer Nature CY - London ER - TY - JOUR A1 - Serrano-Munoz, Itziar A1 - Fritsch, Tobias A1 - Mishurova, Tatiana A1 - Trofimov, Anton A1 - Apel, Daniel A1 - Ulbricht, Alexander A1 - Kromm, Arne A1 - Hesse, Rene A1 - Evans, Alexander A1 - Bruno, Giovanni T1 - On the interplay of microstructure and residual stress in LPBF IN718 JF - Journal of materials science N2 - The relationship between residual stresses and microstructure associated with a laser powder bed fusion (LPBF) IN718 alloy has been investigated on specimens produced with three different scanning strategies (unidirectional Y-scan, 90 degrees XY-scan, and 67 degrees Rot-scan). Synchrotron X-ray energy-dispersive diffraction (EDXRD) combined with optical profilometry was used to study residual stress (RS) distribution and distortion upon removal of the specimens from the baseplate. The microstructural characterization of both the bulk and the near-surface regions was conducted using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). On the top surfaces of the specimens, the highest RS values are observed in the Y-scan specimen and the lowest in the Rot-scan specimen, while the tendency is inversed on the side lateral surfaces. A considerable amount of RS remains in the specimens after their removal from the baseplate, especially in the Y- and Z-direction (short specimen dimension and building direction (BD), respectively). The distortion measured on the top surface following baseplate thinning and subsequent removal is mainly attributed to the amount of RS released in the build direction. Importantly, it is observed that the additive manufacturing microstructures challenge the use of classic theoretical models for the calculation of diffraction elastic constants (DEC) required for diffraction-based RS analysis. It is found that when the Reuss model is used for the calculation of RS for different crystal planes, as opposed to the conventionally used Kroner model, the results exhibit lower scatter. This is discussed in context of experimental measurements of DEC available in the literature for conventional and additively manufactured Ni-base alloys. Y1 - 2020 U6 - https://doi.org/10.1007/s10853-020-05553-y SN - 0022-2461 SN - 1573-4803 VL - 56 IS - 9 SP - 5845 EP - 5867 PB - Springer CY - New York 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 -