@article{BrunoKachanovSevostianovetal.2018, author = {Bruno, Giovanni and Kachanov, Mark and Sevostianov, Igor and Shyam, Amit}, title = {Micromechanical modeling of non-linear stress-strain behavior of polycrystalline microcracked materials under tension}, series = {Acta materialia}, volume = {164}, journal = {Acta materialia}, publisher = {Elsevier}, address = {Oxford}, issn = {1359-6454}, doi = {10.1016/j.actamat.2018.10.024}, pages = {50 -- 59}, year = {2018}, abstract = {The stress-strain behavior of microcracked polycrystalline materials (such as ceramics or rocks) under conditions of tensile, displacement-controlled, loading is discussed. Micromechanical explanation and modeling of the basic features, such as non-linearity and hysteresis in stress-strain curves, is developed, with stable microcrack propagation and "roughness" of intergranular cracks playing critical roles. Experiments involving complex loading histories were done on large- and medium grain size beta-eucryptite ceramic. The model is shown to reproduce the basic features of the observed stress-strain curves. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.}, language = {en} } @article{BrunoVogel2017, author = {Bruno, Giovanni and Vogel, Sven C.}, title = {Simultaneous determination of high-temperature crystal structure and texture of synthetic porous cordierite}, series = {Journal of applied crystallography}, volume = {50}, journal = {Journal of applied crystallography}, publisher = {International Union of Crystallography}, address = {Chester}, issn = {1600-5767}, doi = {10.1107/S160057671700406X}, pages = {749 -- 762}, year = {2017}, abstract = {The evolution of the crystal structure and crystallographic texture of porous synthetic cordierite was studied by in situ high-temperature neutron diffraction up to 1373 K, providing the first in situ high-temperature texture measurement of this technologically important material. It was observed that the crystal texture slightly weakens with increasing temperature, concurrently with subtle changes in the crystal structure. These changes are in agreement with previous work, leading the authors to the conclusion that high-temperature neutron diffraction allows reliable crystallographic characterization of materials with moderate texture. It was also observed that structural changes occur at about the glass transition temperature of the cordierite glass (between 973 and 1073 K). Crystal structure refinements were conducted with and without quantitative texture analysis being part of the Rietveld refinement, and a critical comparison of the results is presented, contributing to the sparse body of literature on combined texture and crystal structure refinements.}, language = {en} } @article{BuljakBruno2018, author = {Buljak, Vladimir and Bruno, Giovanni}, title = {Numerical modeling of thermally induced microcracking in porous ceramics}, series = {Journal of the European Ceramic Society}, volume = {38}, journal = {Journal of the European Ceramic Society}, number = {11}, publisher = {Elsevier}, address = {Oxford}, issn = {0955-2219}, doi = {10.1016/j.jeurceramsoc.2018.03.041}, pages = {4099 -- 4108}, year = {2018}, abstract = {A numerical framework is developed to study the hysteresis of elastic properties of porous ceramics as a function of temperature. The developed numerical model is capable of employing experimentally measured crystallographic orientation distribution and coefficient of thermal expansion values. For realistic modeling of the microstructure, Voronoi polygons are used to generate polycrystalline grains. Some grains are considered as voids, to simulate the material porosity. To model intercrystalline cracking, cohesive elements are inserted along grain boundaries. Crack healing (recovery of the initial properties) upon closure is taken into account with special cohesive elements implemented in the commercial code ABAQUS. The numerical model can be used to estimate fracture properties governing the cohesive behavior through inverse analysis procedure. The model is applied to a porous cordierite ceramic. The obtained fracture properties are further used to successfully simulate general non-linear macroscopic stress-strain curves of cordierite, thereby validating the model.}, language = {en} } @article{BuljakOeschBruno2019, author = {Buljak, Vladimir and Oesch, Tyler and Bruno, Giovanni}, title = {Simulating Fiber-Reinforced Concrete Mechanical Performance Using CT-Based Fiber Orientation Data}, series = {Materials}, volume = {12}, journal = {Materials}, number = {5}, publisher = {MDPI}, address = {Basel}, issn = {1996-1944}, doi = {10.3390/ma12050717}, pages = {16}, year = {2019}, abstract = {The main hindrance to realistic models of fiber-reinforced concrete (FRC) is the local materials property variation, which does not yet reliably allow simulations at the structural level. The idea presented in this paper makes use of an existing constitutive model, but resolves the problem of localized material variation through X-ray computed tomography (CT)-based pre-processing. First, a three-point bending test of a notched beam is considered, where pre-test fiber orientations are measured using CT. A numerical model is then built with the zone subjected to progressive damage, modeled using an orthotropic damage model. To each of the finite elements within this zone, a local coordinate system is assigned, with its longitudinal direction defined by local fiber orientations. Second, the parameters of the constitutive damage model are determined through inverse analysis using load-displacement data obtained from the test. These parameters are considered to clearly explain the material behavior for any arbitrary external action and fiber orientation, for the same geometrical properties and volumetric ratio of fibers. Third, the effectiveness of the resulting model is demonstrated using a second, control experiment. The results of the control experiment analyzed in this research compare well with the model results. The ultimate strength was predicted with an error of about 6\%, while the work-of-load was predicted within 4\%. It demonstrates the potential of this method for accurately predicting the mechanical performance of FRC components.}, language = {en} } @article{CabezaMishurovaGarcesetal.2017, author = {Cabeza, Sandra and Mishurova, Tatiana and Garc{\´e}s, Gonzales and Sevostianov, Igor and Requena, Guillermo and Bruno, Giovanni}, title = {Stress-induced damage evolution in cast AlSi12CuMgNi alloy with one- and two-ceramic reinforcements}, series = {Journal of materials science}, volume = {52}, journal = {Journal of materials science}, publisher = {Springer}, address = {New York}, issn = {0022-2461}, doi = {10.1007/s10853-017-1182-7}, pages = {10198 -- 10216}, year = {2017}, abstract = {Two composites, consisting of an as-cast AlSi12CuMgNi alloy reinforced with 15 vol\% Al2O3 short fibres and with 7 vol\% Al2O3 short fibres + 15 vol\% SiC particles, were studied. Synchrotron computed tomography disclosed distribution, orientation, and volume fraction of the different phases. In-situ compression tests during neutron diffraction in direction parallel to the fibres plane revealed the load partition between phases. Internal damage (fragmentation) of the Si phase and Al2O3 fibres was directly observed in CT reconstructions. Significant debonding between Al matrix and SiC particles was also found. Finally, based on the Maxwell scheme, a micromechanical model was utilized for the new composite with two-ceramic reinforcements; it rationalizes the experimental data and predicts the evolution of all internal stress components in each phase.}, language = {en} } @article{CabezaMuellerPereyraetal.2018, author = {Cabeza, Sandra and M{\"u}ller, Bernd R. and Pereyra, Ricio and Fernandez, Ricardo and Gonzalez-Doncel, Gaspar and Bruno, Giovanni}, title = {Evidence of damage evolution during creep of Al-Mg alloy using synchrotron X-ray refraction}, series = {Journal of applied crystallography}, volume = {51}, journal = {Journal of applied crystallography}, publisher = {International Union of Crystallography}, address = {Chester}, issn = {1600-5767}, doi = {10.1107/S1600576718001449}, pages = {420 -- 427}, year = {2018}, abstract = {In order to provide further evidence of damage mechanisms predicted by the recent solid-state transformation creep (SSTC) model, direct observation of damage accumulation during creep of Al-3.85Mg was made using synchrotron X-ray refraction. X-ray refraction techniques detect the internal specific surface (i.e. surface per unit volume) on a length scale comparable to the specimen size, but with microscopic sensitivity. A significant rise in the internal specific surface with increasing creep time was observed, providing evidence for the creation of a fine grain substructure, as predicted by the SSTC model. This substructure was also observed by scanning electron microscopy.}, language = {en} } @article{ChenMuellerPrinzetal.2020, author = {Chen, Cong and M{\"u}ller, Bernd R. and Prinz, Carsten and Stroh, Julia and Feldmann, Ines and Bruno, Giovanni}, title = {The correlation between porosity characteristics and the crystallographic texture in extruded stabilized aluminium titanate for diesel particulate filter applications}, series = {Journal of the European Ceramic Society}, volume = {40}, journal = {Journal of the European Ceramic Society}, number = {4}, publisher = {Elsevier}, address = {Oxford}, issn = {0955-2219}, doi = {10.1016/j.jeurceramsoc.2019.11.076}, pages = {1592 -- 1601}, year = {2020}, abstract = {Porous ceramic diesel particulate filters (DPFs) are extruded products that possess macroscopic anisotropic mechanical and thermal properties. This anisotropy is caused by both morphological features (mostly the orientation of porosity) and crystallographic texture. We systematically studied those two aspects in two aluminum titanate ceramic materials of different porosity using mercury porosimetry, gas adsorption, electron microscopy, X-ray diffraction, and X-ray refraction radiography. We found that a lower porosity content implies a larger isotropy of both the crystal texture and the porosity orientation. We also found that, analogous to cordierite, crystallites do align with their axis of negative thermal expansion along the extrusion direction. However, unlike what found for cordierite, the aluminium titanate crystallite form is such that a more pronounced (0 0 2) texture along the extrusion direction implies porosity aligned perpendicular to it.}, language = {en} } @article{ChenMuellerLebedevetal.2019, author = {Chen, Cong and M{\"u}ller, Bernd Randolf and Lebedev, Oleg I. and Giovannelli, Fabien and Bruno, Giovanni and Delorme, Fabian}, title = {Effects of impurities on the stability of the low thermal conductivity in Fe2TiO5 ceramics}, series = {Materials characterization}, volume = {149}, journal = {Materials characterization}, publisher = {Elsevier}, address = {New York}, issn = {1044-5803}, doi = {10.1016/j.matchar.2019.01.021}, pages = {111 -- 117}, year = {2019}, abstract = {The stability of the low thermal conductivity in Fe2TiO5 pseudobrookite ceramics has been studied. An increase in thermal diffusivity is observed after only three cycles of measurement. X-ray refraction shows an increase in the mean value of specific surface after the thermal diffusivity measurements. By using scanning electron microscopy and high-angle annular dark-field scanning transmission electron microscope equipped with energy dispersive Xray spectroscopy, we observe a segregation of Ca- and F-rich nanocrystals at grain boundaries after three cycles of thermal diffusivity measurement. Therefore, impurities seem to be more efficient to scatter phonons as point defects in the pseudobrookite lattice rather than as nanocrystals at pseudobrookite grain boundaries. This emphasizes the importance of precursor purity and the influence of redistribution of impurities on thermoelectric properties: stability of micro-/nano-structures is a key point, and repeated thermoelectric measurements may allow detecting such metastable micro-/nano-structures and producing stable and reliable data.}, language = {en} } @article{CooperBrunoWheeleretal.2017, author = {Cooper, Ryan C. and Bruno, Giovanni and Wheeler, M. R. and Pandey, A. and Watkins, T. R. and Shyarn, A.}, title = {Effect of microcracking on the uniaxial tensile response of beta-eucryptite ceramics}, series = {Acta Materialia}, volume = {135}, journal = {Acta Materialia}, publisher = {Elsevier}, address = {Oxford}, issn = {1359-6454}, doi = {10.1016/j.actamat.2017.06.033}, pages = {361 -- 371}, year = {2017}, abstract = {A constitutive model for the nonlinear or "pseudoplastic" mechanical behavior in a linear-elastic solid with thermally induced microcracks is developed and applied to experimental results. The model is termed strain dependent microcrack density approximation (SDMDA) and is an extension of the modified differential scheme that describes the slope of the stress-strain curves of microcracked solids. SDMDA allows a continuous variation in the microcrack density with tensile loading. Experimental uniaxial tensile response of beta-eucryptite glass and ceramics with controlled levels of microcracking is reported. It is demonstrated that SDMDA can well describe the extent of non-linearity in the experimental uniaxial tensile response of beta-eucryptite with varying levels of microcracking. The advantages of the SDMDA are discussed in regard to tensile loading.}, language = {en} } @article{ErdmannKupschMuelleretal.2019, author = {Erdmann, Maren and Kupsch, Andreas and M{\"u}ller, Bernd Randolf and Hentschel, Manfred P. and Niebergall, Ute and B{\"o}hning, Martin and Bruno, Giovanni}, title = {Diesel-induced transparency of plastically deformed high-density polyethylene}, series = {Journal of materials science}, volume = {54}, journal = {Journal of materials science}, number = {17}, publisher = {Springer}, address = {New York}, issn = {0022-2461}, doi = {10.1007/s10853-019-03700-8}, pages = {11739 -- 11755}, year = {2019}, abstract = {High-density polyethylene becomes optically transparent during tensile drawing when previously saturated with diesel fuel. This unusual phenomenon is investigated as it might allow conclusions with respect to the material behavior. Microscopy, differential scanning calorimetry, density measurements are applied together with two scanning X-ray scattering techniques: wide angle X-ray scattering (WAXS) and X-ray refraction, able to extract the spatially resolved crystal orientation and internal surface, respectively. The sorbed diesel softens the material and significantly alters the yielding characteristics. Although the crystallinity among stretched regions is similar, a virgin reference sample exhibits strain whitening during stretching, while the diesel-saturated sample becomes transparent. The WAXS results reveal a pronounced fiber texture in the tensile direction in the stretched region and an isotropic orientation in the unstretched region. This texture implies the formation of fibrils in the stretched region, while spherulites remain intact in the unstretched parts of the specimens. X-ray refraction reveals a preferred orientation of internal surfaces along the tensile direction in the stretched region of virgin samples, while the sample stretched in the diesel-saturated state shows no internal surfaces at all. Besides from stretching saturated samples, optical transparency is also obtained from sorbing samples in diesel after stretching.}, language = {en} } @article{EvsevleevMishurovaCabezaetal.2018, author = {Evsevleev, Sergei and Mishurova, Tatiana and Cabeza, Sandra and Koos, R. and Sevostianov, Igor and Garc{\´e}s, Gonzales and Requena, Guillermo and Fernandez, R. and Bruno, Giovanni}, title = {The role of intermetallics in stress partitioning and damage evolution of AlSil2CuMgNi alloy}, series = {Materials Science and Engineering: A-Structural materials: properties, microstructure and processing}, volume = {736}, journal = {Materials Science and Engineering: A-Structural materials: properties, microstructure and processing}, publisher = {Elsevier}, address = {Lausanne}, issn = {0921-5093}, doi = {10.1016/j.msea.2018.08.070}, pages = {453 -- 464}, year = {2018}, abstract = {Load partitioning between phases in a cast AlSi12CuMgNi alloy was investigated by in-situ compression test during neutron diffraction experiments. Computed tomography (CT) was used to determine volume fractions of eutectic Si and intermetallic (IM) phases, and to assess internal damage after ex-situ compression tests. The CT reconstructed volumes showed the interconnectivity of IM phases, which build a 3D network together with eutectic Si. Large stresses were found in IMs, revealing their significant role as a reinforcement for the alloy. An existing micromechanical model based on Maxwell scheme was extended to the present case, assuming the alloy as a three-phase composite (Al matrix, eutectic Si, IM phases). The model agrees well with the experimental data. Moreover, it allows predicting the principal stresses in each phase, while experiments can only determine stress differences between the axial and radial sample directions. Finally, we showed that the addition of alloying elements not only allowed developing a 3D interconnected network, but also improved the strength of the Al matrix, and the ability of the alloy constituents to bear mechanical load.}, language = {en} } @article{EvsevleevPaciornikBruno2020, author = {Evsevleev, Sergei and Paciornik, Sidnei and Bruno, Giovanni}, title = {Advanced deep learning-based 3D microstructural characterization of multiphase metal matrix composites}, series = {Advanced engineering materials}, volume = {22}, journal = {Advanced engineering materials}, number = {4}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1438-1656}, doi = {10.1002/adem.201901197}, pages = {6}, year = {2020}, abstract = {The quantitative analysis of microstructural features is a key to understanding the micromechanical behavior of metal matrix composites (MMCs), which is a premise for their use in practice. Herein, a 3D microstructural characterization of a five-phase MMC is performed by synchrotron X-ray computed tomography (SXCT). A workflow for advanced deep learning-based segmentation of all individual phases in SXCT data is shown using a fully convolutional neural network with U-net architecture. High segmentation accuracy is achieved with a small amount of training data. This enables extracting unprecedently precise microstructural parameters (e.g., volume fractions and particle shapes) to be input, e.g., in micromechanical models.}, language = {en} } @article{FernandezCabezaMishurovaetal.2018, author = {Fernandez, R. and Cabeza, Sandra and Mishurova, Tatiana and Fernandez-Castrillo, P. and Gonzalez-Doncel, Gaspar and Bruno, Giovanni}, title = {Residual stress and yield strength evolution with annealing treatments in an age-hardenable aluminum alloy matrix composite}, series = {Materials Science and Engineering: A}, volume = {731}, journal = {Materials Science and Engineering: A}, publisher = {Elsevier}, address = {Lausanne}, issn = {0921-5093}, doi = {10.1016/j.msea.2018.06.031}, pages = {344 -- 350}, year = {2018}, abstract = {We investigated the possibility of minimizing tensile matrix residual stresses in age hardenable aluminum alloy metal matrix composites without detrimentally affect their mechanical properties (such as yield strength). Specifically, we performed thermal treatments at different temperatures and times in an age-hardenable aluminum matrix composite 2014Al-15vol\%Al2O3. Using X-ray synchrotron radiation diffraction and mechanical tests, we show that below a certain treatment temperature (250 degrees C) it is possible to identify an appropriate thermal treatment capable of relaxing residual stress in this composite while even increasing its yield strength, with respect to the as processed conditions.}, language = {en} } @article{FernandezBrunoGarcesetal.2020, author = {Fernandez, Ricardo and Bruno, Giovanni and Garces, Gerardo and Nieto-Luis, H. and Gonzalez-Doncel, Gaspar}, title = {Fractional brownian motion of dislocations during creep deformation of metals}, series = {Materials science \& engineering. A, Structural materials}, volume = {796}, journal = {Materials science \& engineering. A, Structural materials}, publisher = {Elsevier}, address = {Lausanne}, issn = {0921-5093}, doi = {10.1016/j.msea.2020.140013}, pages = {8}, year = {2020}, abstract = {The present work offers an explanation on how the long-range interaction of dislocations influences their movement, and therefore the strain, during creep of metals. It is proposed that collective motion of dislocations can be described as a fractional Brownian motion. This explains the noisy appearance of the creep strain signal as a function of time. Such signal is split into a deterministic and a stochastic part. These terms can be related to two kinds of dislocation motions: individual and collective, respectively. The description is consistent with the fractal nature of strain-induced dislocation structures predicated in previous works. Moreover, it encompasses the evolution of the strain rate during all stages of creep, including the tertiary one. Creep data from Al99.8\% and Al3.85\%Mg tested at different temperatures and stresses are used to validate the proposed ideas: it is found that different creep stages present different diffusion characters, and therefore different dislocation motion character.}, language = {en} } @article{FernandezGonzalezDoncelGarcesetal.2020, author = {Fernandez, Ricardo and Gonzalez-Doncel, Gaspar and Garces, Gerardo and Bruno, Giovanni}, title = {Towards a comprehensive understanding of creep}, series = {Materials science \& engineering. A, Structural materials: properties, microstructure and processing}, volume = {776}, journal = {Materials science \& engineering. A, Structural materials: properties, microstructure and processing}, publisher = {Elsevier}, address = {Lausanne}, issn = {0921-5093}, doi = {10.1016/j.msea.2020.139036}, pages = {7}, year = {2020}, abstract = {We show that the equation proposed by Takeuchi and Argon to explain the creep behavior of Al-Mg solid solution can be used to describe also the creep behavior of pure aluminum. In this frame, it is possible to avoid the use of the classic pre-exponential fitting parameter in the power law equation to predict the minimum creep strain rate. The effect of the fractal arrangement of dislocations, developed at the mesoscale, must be considered to fully explain the experimental data. These ideas allow improving the recently introduced SSTC model, fully describing the primary and secondary creep regimes of aluminum alloys without the need for fitting. Creep data from commercially pure A199.8\% and Al-Mg alloys tested at different temperatures and stresses are used to validate the proposed ideas.}, language = {en} } @article{FernandezSerranoBrunoGonzalezDoncel2018, author = {Fernandez Serrano, Ricardo and Bruno, Giovanni and Gonzalez-Doncel, Gaspar}, title = {Fractal nature of aluminum alloys substructures under creep and its implications}, series = {Journal of applied physics}, volume = {123}, journal = {Journal of applied physics}, number = {14}, publisher = {American Institute of Physics}, address = {Melville}, issn = {0021-8979}, doi = {10.1063/1.5012035}, pages = {8}, year = {2018}, abstract = {The present work offers an explanation for the variation of the power-law stress exponent, n, with the stress sigma normalized to the shear modulus G in aluminum alloys. The approach is based on the assumption that the dislocation structure generated with deformation has a fractal nature. It fully explains the evolution of n with sigma/G even beyond the so-called power law breakdown region. Creep data from commercially pure Al99.8\%, Al-3.85\%Mg, and ingot AA6061 alloy tested at different temperatures and stresses are used to validate the proposed ideas. Finally, it is also shown that the fractal description of the dislocation structure agrees well with current knowledge. Published by AIP Publishing.}, language = {en} } @article{KupschMuellerLangeetal.2017, author = {Kupsch, Andreas and Mueller, Bernd R. and Lange, Axel and Bruno, Giovanni}, title = {Microstructure characterisation of ceramics via 2D and 3D X-ray refraction techniques}, series = {Journal of the European Ceramic Society}, volume = {37}, journal = {Journal of the European Ceramic Society}, publisher = {Elsevier}, address = {Oxford}, issn = {0955-2219}, doi = {10.1016/j.jeurceramsoc.2016.12.031}, pages = {1879 -- 1889}, year = {2017}, abstract = {3D imaging techniques are very fashionable nowadays, and allow enormous progress in understanding ceramic microstructure, its evolution, and its link to mechanical, thermal, and transport properties. In this feature article, we report the use of a powerful, yet not so wide-spread, set of X-ray techniques based on refraction effects. X-ray refraction allows determining internal specific surface (surface per unit volume) in a non-destructive fashion, position and orientation sensitive, and with a nanometric detectability. While the techniques are limited by the X-ray absorption of the material under investigation, we demonstrate showcases of ceramics and composite materials, where understanding of process parameter influence or simply of microstructural parameters could be achieved in a way unrivalled even by high-resolution techniques such as electron microscopy or computed tomography. (C) 2016 Elsevier Ltd. All rights reserved.}, language = {en} } @article{LaquaiGouraudMuelleretal.2019, author = {Laquai, Rene and Gouraud, Fanny and M{\"u}ller, Bernd Randolf and Huger, Marc and Chotard, Thierry and Antou, Guy and Bruno, Giovanni}, title = {Evolution of Thermal Microcracking in Refractory ZrO2-SiO2 after Application of External Loads at High Temperatures}, series = {Materials}, volume = {12}, journal = {Materials}, number = {7}, publisher = {MDPI}, address = {Basel}, issn = {1996-1944}, doi = {10.3390/ma12071017}, pages = {15}, year = {2019}, abstract = {Zirconia-based cast refractories are widely used for glass furnace applications. Since they have to withstand harsh chemical as well as thermo-mechanical environments, internal stresses and microcracking are often present in such materials under operating conditions (sometimes in excess of 1700 °C). We studied the evolution of thermal (CTE) and mechanical (Young's modulus) properties as a function of temperature in a fused-cast refractory containing 94 wt.\% of monoclinic ZrO2 and 6 wt.\% of a silicate glassy phase. With the aid of X-ray refraction techniques (yielding the internal specific surface in materials), we also monitored the evolution of microcracking as a function of thermal cycles (crossing the martensitic phase transformation around 1000 °C) under externally applied stress. We found that external compressive stress leads to a strong decrease of the internal surface per unit volume, but a tensile load has a similar (though not so strong) effect. In agreement with existing literature on β-eucryptite microcracked ceramics, we could explain these phenomena by microcrack closure in the load direction in the compression case, and by microcrack propagation (rather than microcrack nucleation) under tensile conditions.}, language = {en} } @article{LaquaiMuellerSchneideretal.2020, author = {Laquai, Rene and M{\"u}ller, Bernd R. and Schneider, Judith Ann and Kupsch, Andreas and Bruno, Giovanni}, title = {Using SXRR to probe the nature of discontinuities in SLM additive manufactured inconel 718 specimens}, series = {Metallurgical and Materials Transactions A}, volume = {51}, journal = {Metallurgical and Materials Transactions A}, number = {8}, publisher = {Springer}, address = {New York}, issn = {1073-5623}, doi = {10.1007/s11661-020-05847-5}, pages = {4146 -- 4157}, year = {2020}, abstract = {The utilization of additive manufacturing (AM) to fabricate robust structural components relies on understanding the nature of internal anomalies or discontinuities, which can compromise the structural integrity. While some discontinuities in AM microstructures stem from similar mechanisms as observed in more traditional processes such as casting, others are unique to the AM process. Discontinuities in AM are challenging to detect, due to their submicron size and orientation dependency. Toward the goal of improving structural integrity, minimizing discontinuities in an AM build requires an understanding of the mechanisms of formation to mitigate their occurrence. This study utilizes various techniques to evaluate the shape, size, nature and distribution of discontinuities in AM Inconel 718, in a non-hot isostatic pressed (HIPed) as-built, non-HIPed and direct age, and HIPed with two step age samples. Non-destructive synchrotron radiation refraction and transmission radiography (SXRR) provides additional information beyond that obtained with destructive optical microscopy. SXRR was able to distinguish between voids, cracks and lack of melt in, due to its sensitivity to the orientation of the discontinuity.}, language = {en} } @article{LaquaiSchauppGriescheetal.2022, author = {Laquai, Ren{\´e} and Schaupp, Thomas and Griesche, Axel and M{\"u}ller, Bernd R. and Kupsch, Andreas and Hannemann, Andreas and Kannengiesser, Thomas and Bruno, Giovanni}, title = {Quantitative analysis of hydrogen-assisted microcracking in duplex stainless steel through X-ray refraction 3D imaging}, series = {Advanced engineering materials}, volume = {24}, journal = {Advanced engineering materials}, number = {6}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1527-2648}, doi = {10.1002/adem.202101287}, pages = {10}, year = {2022}, abstract = {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.}, language = {en} } @article{LeonardZhangKrebsetal.2020, author = {L{\´e}onard, Fabien and Zhang, Zhen and Krebs, Holger and Bruno, Giovanni}, title = {Structural and morphological quantitative 3D characterisation of ammonium nitrate prills by X-ray computed tomography}, series = {Materials}, volume = {13}, journal = {Materials}, number = {5}, publisher = {MDPI}, address = {Basel}, issn = {1996-1944}, doi = {10.3390/ma13051230}, pages = {16}, year = {2020}, abstract = {The mixture of ammonium nitrate (AN) prills and fuel oil (FO), usually referred to as ANFO, is extensively used in the mining industry as a bulk explosive. One of the major performance predictors of ANFO mixtures is the fuel oil retention, which is itself governed by the complex pore structure of the AN prills. In this study, we present how X-ray computed tomography (XCT), and the associated advanced data processing workflow, can be used to fully characterise the structure and morphology of AN prills. We show that structural parameters such as volume fraction of the different phases and morphological parameters such as specific surface area and shape factor can be reliably extracted from the XCT data, and that there is a good agreement with the measured oil retention values. Importantly, oil retention measurements (qualifying the efficiency of ANFO as explosives) correlate well with the specific surface area determined by XCT. XCT can therefore be employed non-destructively; it can accurately evaluate and characterise porosity in ammonium nitrate prills, and even predict their efficiency.}, language = {en} } @article{MackLaquaiMuelleretal.2019, author = {Mack, Daniel Emil and Laquai, Rene and Mueller, Bernd and Helle, Oliver and Sebold, Doris and Vassen, Robert and Bruno, Giovanni}, title = {Evolution of porosity, crack density, and CMAS penetration in thermal barrier coatings subjected to burner rig testing}, series = {Journal of the American Ceramic Society}, volume = {102}, journal = {Journal of the American Ceramic Society}, number = {10}, publisher = {Wiley}, address = {Hoboken}, issn = {0002-7820}, doi = {10.1111/jace.16465}, pages = {6163 -- 6175}, year = {2019}, abstract = {Degradation of thermal barrier coatings (TBCs) in gas-turbine engines due to calcium-magnesium-aluminosilicate (CMAS) glassy deposits from various sources has been a persistent issue since many years. In this study, state of the art electron microscopy was correlated with X-ray refraction techniques to elucidate the intrusion of CMAS into the porous structure of atmospheric plasma sprayed (APS) TBCs and the formation and growth of cracks under thermal cycling in a burner rig. Results indicate that the sparse nature of the infiltration as well as kinetics in the burner rig are majorly influenced by the wetting behavior of the CMAS. Despite the obvious attack of CMAS on grain boundaries, the interaction of yttria-stabilized zirconia (YSZ) with intruded CMAS has no immediate impact on structure and density of internal surfaces. At a later stage the formation of horizontal cracks is observed in a wider zone of the TBC layer.}, language = {en} } @article{MagkosKupschBruno2020, author = {Magkos, Sotirios and Kupsch, Andreas and Bruno, Giovanni}, title = {Direct iterative reconstruction of computed tomography trajectories reconstruction from limited number of projections with DIRECTT}, series = {Review of scientific instruments : a monthly journal devoted to scientific instruments, apparatus, and techniques}, volume = {91}, journal = {Review of scientific instruments : a monthly journal devoted to scientific instruments, apparatus, and techniques}, number = {10}, publisher = {American Institute of Physics}, address = {Melville}, issn = {0034-6748}, doi = {10.1063/5.0013111}, pages = {8}, year = {2020}, abstract = {X-ray computed tomography has many applications in materials science and non-destructive testing. While the standard filtered back-projection reconstruction of the radiographic datasets is fast and simple, it typically fails in returning accurate results from missing or inconsistent projections. Among the alternative techniques that have been proposed to handle such data is the Direct Iterative REconstruction of Computed Tomography Trajectories (DIRECTT) algorithm. We describe a new approach to the algorithm, which significantly decreases the computational time while achieving a better reconstruction quality than that of other established algorithms.}, language = {en} } @article{MagkosKupschBruno2021, author = {Magkos, Sotirios and Kupsch, Andreas and Bruno, Giovanni}, title = {Suppression of cone-beam artefacts with Direct Iterative Reconstruction Computed Tomography Trajectories (DIRECTT)}, series = {Journal of imaging : open access journal}, volume = {7}, journal = {Journal of imaging : open access journal}, number = {8}, publisher = {MDPI}, address = {Basel}, issn = {2313-433X}, doi = {10.3390/jimaging7080147}, pages = {9}, year = {2021}, abstract = {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.}, language = {en} } @article{ManiKupschMuelleretal.2022, author = {Mani, Deepak and Kupsch, Andreas and M{\"u}ller, Bernd R. and Bruno, Giovanni}, title = {Diffraction Enhanced Imaging Analysis with Pseudo-Voigt Fit Function}, series = {Journal of imaging : open access journal}, volume = {8}, journal = {Journal of imaging : open access journal}, number = {8}, publisher = {MDPI}, address = {Basel}, issn = {2313-433X}, doi = {10.3390/jimaging8080206}, pages = {13}, year = {2022}, abstract = {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.}, language = {en} } @article{MishurovaArtztHaubrichetal.2018, author = {Mishurova, Tatiana and Artzt, Katia and Haubrich, Jan and Requena, Guillermo and Bruno, Giovanni}, title = {New aspects about the search for the most relevant parameters optimizing SLM materials}, series = {Additive manufacturing}, volume = {25}, journal = {Additive manufacturing}, publisher = {Elsevier}, address = {Amsterdam}, issn = {2214-8604}, doi = {10.1016/j.addma.2018.11.023}, pages = {325 -- 334}, year = {2018}, abstract = {While the volumetric energy density is commonly used to qualify a process parameter set, and to quantify its influence on the microstructure and performance of additively manufactured (AM) materials and components, it has been already shown that this description is by no means exhaustive. In this work, new aspects of the optimization of the selective laser melting process are investigated for AM Ti-6Al-4V. We focus on the amount of near-surface residual stress (RS), often blamed for the failure of components, and on the porosity characteristics (amount and spatial distribution). First, using synchrotron x-ray diffraction we show that higher RS in the subsurface region is generated if a lower energy density is used. Second, we show that laser de-focusing and sample positioning inside the build chamber also play an eminent role, and we quantify this influence. In parallel, using X-ray Computed Tomography, we observe that porosity is mainly concentrated in the contour region, except in the case where the laser speed is small. The low values of porosity (less than 1\%) do not influence RS.}, language = {en} } @article{MishurovaArtztHaubrichetal.2019, author = {Mishurova, Tatiana and Artzt, Katia and Haubrich, Jan and Requena, Guillermo and Bruno, Giovanni}, title = {Exploring the correlation between subsurface residual stresses and manufacturing parameters in laser powder bed fused Ti-6Al-4V}, series = {Metals}, volume = {9}, journal = {Metals}, number = {2}, publisher = {MDPI}, address = {Basel}, issn = {2075-4701}, doi = {10.3390/met9020261}, pages = {13}, year = {2019}, abstract = {Subsurface residual stresses (RS) were investigated in Ti-6Al-4V cuboid samples by means of X-ray synchrotron diffraction. The samples were manufactured by laser powder bed fusion (LPBF) applying different processing parameters, not commonly considered in open literature, in order to assess their influence on RS state. While investigating the effect of process parameters used for the calculation of volumetric energy density (such as laser velocity, laser power and hatch distance), we observed that an increase of energy density led to a decrease of RS, although not to the same extent for every parameter variation. Additionally, the effect of support structure, sample roughness and LPBF machine effects potentially coming from Ar flow were studied. We observed no influence of support structure on subsurface RS while the orientation with respect to Ar flow showed to have an impact on RS. We conclude recommending monitoring such parameters to improve part reliability and reproducibility.}, language = {en} } @article{MishurovaBrunoEvsevleevetal.2020, author = {Mishurova, Tatiana and Bruno, Giovanni and Evsevleev, Sergei and Sevostianov, Igor}, title = {Determination of macroscopic stress from diffraction experiments}, series = {Journal of applied physics}, volume = {128}, journal = {Journal of applied physics}, number = {2}, publisher = {American Institute of Physics}, address = {Melville}, issn = {0021-8979}, doi = {10.1063/5.0009101}, pages = {14}, year = {2020}, abstract = {The paper is motivated by some inconsistencies and contradictions present in the literature on the calculation of the so-called diffraction elastic constants. In an attempt at unifying the views that the two communities of Materials Science and Mechanics of Materials have on the subject, we revisit and define the terminology used in the field. We also clarify the limitations of the commonly used approaches and show that a unified methodology is also applicable to textured materials with a nearly arbitrary grain shape. We finally compare the predictions based on this methodology with experimental data obtained by in situ synchrotron radiation diffraction on additively manufactured Ti-6Al-4V alloy. We show that (a) the transverse isotropy of the material yields good agreement between the best-fit isotropy approximation (equivalent to the classic Kroner's model) and the experimental data and (b) the use of a general framework allows the calculation of all components of the tensor of diffraction elastic constants, which are not easily measurable by diffraction methods. This allows us to extend the current state-of-the-art with a predictive tool.}, language = {en} } @article{MishurovaCabezaArtztetal.2017, author = {Mishurova, Tatiana and Cabeza, Sandra and Artzt, Katia and Haubrich, Jan and Klaus, Manuela and Genzel, Christoph and Requena, Guillermo and Bruno, Giovanni}, title = {An Assessment of Subsurface Residual Stress Analysis in SLM Ti-6Al-4V}, series = {Materials}, volume = {10}, journal = {Materials}, publisher = {MDPI}, address = {Basel}, issn = {1996-1944}, doi = {10.3390/ma10040348}, pages = {14}, year = {2017}, abstract = {Ti-6Al-4V bridges were additively fabricated by selective laser melting (SLM) under different scanning speed conditions, to compare the effect of process energy density on the residual stress state. Subsurface lattice strain characterization was conducted by means of synchrotron diffraction in energy dispersive mode. High tensile strain gradients were found at the frontal surface for samples in an as-built condition. The geometry of the samples promotes increasing strains towards the pillar of the bridges. We observed that the higher the laser energy density during fabrication, the lower the lattice strains. A relief of lattice strains takes place after heat treatment.}, language = {en} } @article{MishurovaCabezaThiedeetal.2018, author = {Mishurova, Tatiana and Cabeza, Sandra and Thiede, Tobias and Nadammal, Naresh and Kromm, Arne and Klaus, Manuela and Genzel, Christoph and Haberland, Christoph and Bruno, Giovanni}, title = {The Influence of the Support Structure on Residual Stress and Distortion in SLM Inconel 718 Parts}, series = {Metallurgical and Materials Transactions A}, volume = {49A}, journal = {Metallurgical and Materials Transactions A}, number = {7}, publisher = {Springer}, address = {New York}, issn = {1073-5623}, doi = {10.1007/s11661-018-4653-9}, pages = {3038 -- 3046}, year = {2018}, abstract = {The effect of support structure and of removal from the base plate on the residual stress state in selective laser melted IN718 parts was studied by means of synchrotron X-ray diffraction. The residual stresses in subsurface region of two elongated prisms in as-built condition and after removal from the base plate were determined. One sample was directly built on a base plate and another one on a support structure. Also, the distortion on the top surface due to stress release was measured by contact profilometry. High tensile residual stress values were found, with pronounced stress gradient along the hatching direction. In the sample on support, stress redistribution took place after removal from the base plate, as opposed to simple stress relaxation for the sample without support. The sample on support structure showed larger distortion compared to sample without support. We conclude that the use of a support decreases stress values but stress-relieving heat treatments are still needed.}, language = {en} } @article{MishurovaSydowThiedeetal.2020, author = {Mishurova, Tatiana and Sydow, Benjamin and Thiede, Tobias and Sizova, Irina and Ulbricht, Alexander and Bambach, Markus and Bruno, Giovanni}, title = {Residual stress and microstructure of a Ti-6Al-4V Wire Arc Additive Manufacturing hybrid demonstrator}, series = {Metals}, volume = {10}, journal = {Metals}, number = {6}, publisher = {MDPI}, address = {Basel}, issn = {2075-4701}, doi = {10.3390/met10060701}, pages = {15}, year = {2020}, abstract = {Wire Arc Additive Manufacturing (WAAM) features high deposition rates and, thus, allows production of large components that are relevant for aerospace applications. However, a lot of aerospace parts are currently produced by forging or machining alone to ensure fast production and to obtain good mechanical properties; the use of these conventional process routes causes high tooling and material costs. A hybrid approach (a combination of forging and WAAM) allows making production more efficient. In this fashion, further structural or functional features can be built in any direction without using additional tools for every part. By using a combination of forging basic geometries with one tool set and adding the functional features by means of WAAM, the tool costs and material waste can be reduced compared to either completely forged or machined parts. One of the factors influencing the structural integrity of additively manufactured parts are (high) residual stresses, generated during the build process. In this study, the triaxial residual stress profiles in a hybrid WAAM part are reported, as determined by neutron diffraction. The analysis is complemented by microstructural investigations, showing a gradient of microstructure (shape and size of grains) along the part height. The highest residual stresses were found in the transition zone (between WAAM and forged part). The total stress range showed to be lower than expected for WAAM components. This could be explained by the thermal history of the component.}, language = {en} } @article{MuellerCooperLangeetal.2017, author = {M{\"u}ller, B. R. and Cooper, R. C. and Lange, A. and Kupsch, Andreas and Wheeler, M. and Hentschel, M. P. and Staude, A. and Pandey, A. and Shyam, A. and Bruno, Giovanni}, title = {Stress-induced microcrack density evolution in beta-eucryptite ceramics}, series = {Acta materialia}, volume = {144}, journal = {Acta materialia}, publisher = {Elsevier}, address = {Oxford}, issn = {1359-6454}, doi = {10.1016/j.actamat.2017.10.030}, pages = {627 -- 641}, year = {2017}, abstract = {In order to investigate their microcracking behaviour, the microstructures of several beta-eucryptite ceramics, obtained from glass precursor and cerammed to yield different grain sizes and microcrack densities, were characterized by laboratory and synchrotron x-ray refraction and tomography. Results were compared with those obtained from scanning electron microscopy (SEM). In SEM images, the characterized materials appeared fully dense but computed tomography showed the presence of pore clusters. Uniaxial tensile testing was performed on specimens while strain maps were recorded and analyzed by Digital Image Correlation (DIC). X-ray refraction techniques were applied on specimens before and after tensile testing to measure the amount of the internal specific surface (i.e., area per unit volume). X-ray refraction revealed that (a) the small grain size (SGS) material contained a large specific surface, originating from the grain boundaries and the interfaces of TiO2 precipitates; (b) the medium (MGS) and large grain size (LGS) materials possessed higher amounts of specific surface compared to SGS material due to microcracks, which decreased after tensile loading; (c) the precursor glass had negligible internal surface. The unexpected decrease in the internal surface of MGS and LGS after tensile testing is explained by the presence of compressive regions in the DIC strain maps and further by theoretical arguments. It is suggested that while some microcracks merge via propagation, more close mechanically, thereby explaining the observed X-ray refraction results. The mechanisms proposed would allow the development of a strain hardening route in ceramics.}, language = {en} } @misc{MuellerKupschLaquaietal.2018, author = {M{\"u}ller, Bernd Randolf and Kupsch, Andreas and Laquai, Rene and Nellesen, Jens and Tillmann, Wolfgang and Kasperovich, Galina and Bruno, Giovanni}, title = {Microstructure Characterisation of Advanced Materials via 2D and 3D X-Ray Refraction Techniques}, series = {Materials Science Forum}, volume = {941}, journal = {Materials Science Forum}, publisher = {Trans Tech Publications Ltd}, address = {Zurich}, isbn = {978-3-0357-1208-7}, issn = {0255-5476}, doi = {10.4028/www.scientific.net/MSF.941.2401}, pages = {2401 -- 2406}, year = {2018}, abstract = {3D imaging techniques have an enormous potential to understand the microstructure, its evolution, and its link to mechanical, thermal, and transport properties. In this conference paper we report the use of a powerful, yet not so wide-spread, set of X-ray techniques based on refraction effects. X-ray refraction allows determining internal specific surface (surface per unit volume) in a non-destructive fashion, position and orientation sensitive, and with a nanometric detectability. We demonstrate showcases of ceramics and composite materials, where microstructural parameters could be achieved in a way unrivalled even by high-resolution techniques such as electron microscopy or computed tomography. We present in situ analysis of the damage evolution in an Al/Al2O3 metal matrix composite during tensile load and the identification of void formation (different kinds of defects, particularly unsintered powder hidden in pores, and small inhomogeneity's like cracks) in Ti64 parts produced by selective laser melting using synchrotron X-ray refraction radiography and tomography.}, language = {en} } @article{NadammalCabezaMishurovaetal.2017, author = {Nadammal, Naresh and Cabeza, Sandra and Mishurova, Tatiana and Thiede, Tobias and Kromm, Arne and Seyfert, Christoph and Farahbod, Lena and Haberland, Christoph and Schneider, Judith Ann and Portella, Pedro Dolabella and Bruno, Giovanni}, title = {Effect of hatch length on the development of microstructure, texture and residual stresses in selective laser melted superalloy Inconel 718}, series = {Materials \& Design}, volume = {134}, journal = {Materials \& Design}, publisher = {Elsevier}, address = {Oxford}, issn = {0264-1275}, doi = {10.1016/j.matdes.2017.08.049}, pages = {139 -- 150}, year = {2017}, abstract = {In the present study, samples fabricated by varying the deposition hatch length during selective laser melting of nickel based superalloy Inconel 718 were investigated. Microstructure and texture of these samples was characterized using scanning electron microscopy, combined with electron back-scattered diffraction, and residual stress assessment, using neutron diffraction method. Textured columnar grains oriented along the sample building direction were observed in the shorter hatch length processed sample. A ten-fold increase in the hatch length reduced the texture intensity by a factor of two attributed to the formation of finer grains in the longer hatch length sample. Larger gradients of transverse residual stress in the longer hatch length sample were also observed. Along the build direction, compressive stresses in the shorter hatch length and negligible stresses for the longer hatch length specimen were observed. Changes to the temperature gradient (G) in response to the hatch length variation, influenced the G to growth rate (R) ratio and the product GxR, in agreement with the microstructures and textures formed. For the residual stress development, geometry of the part also played an important role. In summary, tailored isotropy could be induced in Inconel 718 by a careful selection of parameters during selective laser melting.}, language = {en} } @article{NadammalMishurovaFritschetal.2021, author = {Nadammal, Naresh and Mishurova, Tatiana and Fritsch, Tobias and Serrano-Munoz, Itziar and Kromm, Arne and Haberland, Christoph and Portella, Pedro Dolabella and Bruno, Giovanni}, title = {Critical role of scan strategies on the development of microstructure, texture, and residual stresses during laser powder bed fusion additive manufacturing}, series = {Additive manufacturing}, volume = {38}, journal = {Additive manufacturing}, publisher = {Elsevier}, address = {Amsterdam}, issn = {2214-8604}, doi = {10.1016/j.addma.2020.101792}, pages = {13}, year = {2021}, abstract = {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.}, language = {en} } @article{NellesenLaquaiMuelleretal.2018, author = {Nellesen, Jens and Laquai, R. and M{\"u}ller, B. R. and Kupsch, Andreas and Hentschel, M. P. and Anar, N. B. and Soppa, E. and Tillmann, W. and Bruno, Giovanni}, title = {In situ analysis of damage evolution in an Al/ Al2O3 MMC under tensile load by synchrotron X-ray refraction imaging}, series = {Journal of materials science}, volume = {53}, journal = {Journal of materials science}, number = {8}, publisher = {Springer}, address = {New York}, issn = {0022-2461}, doi = {10.1007/s10853-017-1957-x}, pages = {6021 -- 6032}, year = {2018}, abstract = {The in situ analysis of the damage evolution in a metal matrix composite (MMC) using synchrotron X-ray refraction radiography (SXRR) is presented. The investigated material is an Al alloy (6061)/10 vol MMC after T6 heat treatment. In an interrupted tensile test the gauge section of dog bone-shaped specimens is imaged in different states of tensile loading. On the basis of the SXRR images, the relative change of the specific surface (proportional to the amount of damage) in the course of tensile loading was analyzed. It could be shown that the damage can be detected by SXRR already at a stage of tensile loading, in which no observation of damage is possible with radiographic absorption-based imaging methods. Moreover, the quantitative analysis of the SXRR images reveals that the amount of damage increases homogeneously by an average of 25\% with respect to the initial state. To corroborate the experimental findings, the damage distribution was imaged in 3D after the final tensile loading by synchrotron X-ray refraction computed tomography (SXRCT) and absorption-based synchrotron X-ray computed tomography (SXCT). It could be evidenced that defects and damages cause pronounced indications in the SXRCT images.}, language = {en} } @article{OeschWeiseBruno2020, author = {Oesch, Tyler and Weise, Frank and Bruno, Giovanni}, title = {Detection and quantification of cracking in concrete aggregate through virtual data fusion of X-ray computed tomography images}, series = {Materials}, volume = {13}, journal = {Materials}, number = {18}, publisher = {MDPI}, address = {Basel}, issn = {1996-1944}, doi = {10.3390/ma13183921}, pages = {27}, year = {2020}, abstract = {In this work, which is part of a larger research program, a framework called "virtual data fusion" was developed to provide an automated and consistent crack detection method that allows for the cross-comparison of results from large quantities of X-ray computed tomography (CT) data. A partial implementation of this method in a custom program was developed for use in research focused on crack quantification in alkali-silica reaction (ASR)-sensitive concrete aggregates. During the CT image processing, a series of image analyses tailored for detecting specific, individual crack-like characteristics were completed. The results of these analyses were then "fused" in order to identify crack-like objects within the images with much higher accuracy than that yielded by any individual image analysis procedure. The results of this strategy demonstrated the success of the program in effectively identifying crack-like structures and quantifying characteristics, such as surface area and volume. The results demonstrated that the source of aggregate has a very significant impact on the amount of internal cracking, even when the mineralogical characteristics remain very similar. River gravels, for instance, were found to contain significantly higher levels of internal cracking than quarried stone aggregates of the same mineralogical type.}, language = {en} } @article{OsterFritschUlbrichtetal.2022, author = {Oster, Simon and Fritsch, Tobias and Ulbricht, Alexander and Mohr, Gunther and Bruno, Giovanni and Maierhofer, Christiane and Altenburg, Simon}, title = {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}, series = {Metals : open access journal}, volume = {12}, journal = {Metals : open access journal}, number = {6}, publisher = {MDPI}, address = {Basel}, issn = {2075-4701}, doi = {10.3390/met12060947}, pages = {21}, year = {2022}, abstract = {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.}, language = {en} } @article{PauzonMishurovaEvsevleevetal.2021, author = {Pauzon, Camille and Mishurova, Tatiana and Evsevleev, Sergei and Dubiez-Le Goff, Sophie and Murugesan, Saravanakumar and Bruno, Giovanni and Hryha, Eduard}, title = {Residual stresses and porosity in Ti-6Al-4V produced by laser powder bed fusion as a function of process atmosphere and component design}, series = {Additive manufacturing}, volume = {47}, journal = {Additive manufacturing}, publisher = {Elsevier}, address = {Amsterdam}, issn = {2214-8604}, doi = {10.1016/j.addma.2021.102340}, pages = {10}, year = {2021}, abstract = {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.}, language = {en} } @article{PowierzaGollwitzerWolgastetal.2019, author = {Powierza, Bartosz and Gollwitzer, Christian and Wolgast, Dagmar and Staude, Andreas and Bruno, Giovanni}, title = {Fully experiment-based evaluation of few digital volume correlation techniques}, series = {Review of scientific instruments : a monthly journal devoted to scientific instruments, apparatus, and techniques}, volume = {90}, journal = {Review of scientific instruments : a monthly journal devoted to scientific instruments, apparatus, and techniques}, number = {11}, publisher = {American Institute of Physics}, address = {Melville}, issn = {0034-6748}, doi = {10.1063/1.5099572}, pages = {10}, year = {2019}, abstract = {Digital Volume Correlation (DVC) is a powerful set of techniques used to compute the local shifts of 3D images obtained, for instance, in tomographic experiments. It is utilized to analyze the geometric changes of the investigated object as well as to correct the corresponding image misalignments for further analysis. It can therefore be used to evaluate the local density changes of the same regions of the inspected specimens, which might be shifted between measurements. In recent years, various approaches and corresponding pieces of software were introduced. Accuracies for the computed shift vectors of up to about 1 parts per thousand of a single voxel size have been reported. These results, however, were based either on synthetic datasets or on an unrealistic setup. In this work, we propose two simple methods to evaluate the accuracy of DVC-techniques using more realistic input data and apply them to several DVC programs. We test these methods on three materials (tuff, sandstone, and concrete) that show different contrast and structural features. Published under license by AIP Publishing.}, language = {en} } @article{SchroederEvansMishurovaetal.2021, author = {Schr{\"o}der, Jakob and Evans, Alexander and Mishurova, Tatiana and Ulbricht, Alexander and Sprengel, Maximilian and Serrano-Munoz, Itziar and Fritsch, Tobias and Kromm, Arne and Kannengießer, Thomas and Bruno, Giovanni}, title = {Diffraction-based residual stress characterization in laser additive manufacturing of metals}, series = {Metals : open access journal}, volume = {11}, journal = {Metals : open access journal}, number = {11}, publisher = {MDPI}, address = {Basel}, issn = {2075-4701}, doi = {10.3390/met11111830}, pages = {34}, year = {2021}, abstract = {Laser-based additive manufacturing methods allow the production of complex metal structures within a single manufacturing step. However, the localized heat input and the layer-wise manufacturing manner give rise to large thermal gradients. Therefore, large internal stress (IS) during the process (and consequently residual stress (RS) at the end of production) is generated within the parts. This IS or RS can either lead to distortion or cracking during fabrication or in-service part failure, respectively. With this in view, the knowledge on the magnitude and spatial distribution of RS is important to develop strategies for its mitigation. Specifically, diffraction-based methods allow the spatial resolved determination of RS in a non-destructive fashion. In this review, common diffraction-based methods to determine RS in laser-based additive manufactured parts are presented. In fact, the unique microstructures and textures associated to laser-based additive manufacturing processes pose metrological challenges. Based on the literature review, it is recommended to (a) use mechanically relaxed samples measured in several orientations as appropriate strain-free lattice spacing, instead of powder, (b) consider that an appropriate grain-interaction model to calculate diffraction-elastic constants is both material- and texture-dependent and may differ from the conventionally manufactured variant. Further metrological challenges are critically reviewed and future demands in this research field are discussed.}, language = {en} } @article{SchroederEvansPolatidisetal.2022, author = {Schr{\"o}der, Jakob and Evans, Alexander and Polatidis, Efthymios and Mohr, Gunther and Serrano-Munoz, Itziar and Bruno, Giovanni and Čapek, Jan}, title = {Understanding the impact of texture on the micromechanical anisotropy of laser powder bed fused Inconel 718}, series = {Journal of materials science}, volume = {57}, journal = {Journal of materials science}, number = {31}, publisher = {Springer}, address = {New York}, issn = {0022-2461}, doi = {10.1007/s10853-022-07499-9}, pages = {15036 -- 15058}, year = {2022}, abstract = {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.}, language = {en} } @article{SerranoMunozFernandezSerranoSaliwanNeumannetal.2022, author = {Serrano-Munoz, Itziar and Fern{\´a}ndez Serrano, Ricardo and Saliwan-Neumann, Romeo and Gonzalez-Doncel, Gaspar and Bruno, Giovanni}, title = {Dislocation substructures in pure aluminium after creep deformation as studied by electron backscatter diffraction}, series = {Journal of applied crystallography / International Union of Crystallography}, volume = {55}, journal = {Journal of applied crystallography / International Union of Crystallography}, publisher = {Munksgaard}, address = {Copenhagen}, issn = {1600-5767}, doi = {10.1107/S1600576722005209}, pages = {860 -- 869}, year = {2022}, abstract = {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.}, language = {en} } @article{SerranoMunozFritschMishurovaetal.2020, author = {Serrano-Munoz, Itziar and Fritsch, Tobias and Mishurova, Tatiana and Trofimov, Anton and Apel, Daniel and Ulbricht, Alexander and Kromm, Arne and Hesse, Rene and Evans, Alexander and Bruno, Giovanni}, title = {On the interplay of microstructure and residual stress in LPBF IN718}, series = {Journal of materials science}, volume = {56}, journal = {Journal of materials science}, number = {9}, publisher = {Springer}, address = {New York}, issn = {0022-2461}, doi = {10.1007/s10853-020-05553-y}, pages = {5845 -- 5867}, year = {2020}, abstract = {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.}, language = {en} } @article{SerranoMunozMishurovaThiedeetal.2020, author = {Serrano-Munoz, Itziar and Mishurova, Tatiana and Thiede, Tobias and Sprengel, Maximilian and Kromm, Arne and Nadammal, Naresh and Nolze, Gert and Saliwan-Neumann, Romeo and Evans, Alexander and Bruno, Giovanni}, title = {The residual stress in as-built laser powder bed fusion IN718 alloy as a consequence of the scanning strategy induced microstructure}, series = {Scientific reports}, volume = {10}, journal = {Scientific reports}, number = {1}, publisher = {Macmillan Publishers Limited, part of Springer Nature}, address = {London}, issn = {2045-2322}, doi = {10.1038/s41598-020-71112-9}, pages = {15}, year = {2020}, abstract = {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.}, language = {en} } @article{SevostianovBruno2018, author = {Sevostianov, Igor and Bruno, Giovanni}, title = {Maxwell scheme for internal stresses in multiphase composites}, series = {Mechanics of Materials}, volume = {129}, journal = {Mechanics of Materials}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0167-6636}, doi = {10.1016/j.mechmat.2018.12.005}, pages = {320 -- 331}, year = {2018}, abstract = {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.}, language = {en} } @article{ShashevKupschLangeetal.2017, author = {Shashev, Yury and Kupsch, Andreas and Lange, Axel and Evsevleev, Sergei and M{\"u}ller, Bernd R. and Osenberg, Markus and Manke, Ingo and Hentschel, Manfred P. and Bruno, Giovanni}, title = {Optimizing the visibility of X-ray phase grating interferometry}, series = {Materials testing : Materialpr{\"u}fung ; materials and components, technology and application}, volume = {59}, journal = {Materials testing : Materialpr{\"u}fung ; materials and components, technology and application}, publisher = {Hanser}, address = {M{\"u}nchen}, issn = {0025-5300}, doi = {10.3139/120.111097}, pages = {974 -- 980}, year = {2017}, abstract = {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.}, language = {en} } @article{ShashevKupschLangeetal.2016, author = {Shashev, Yury and Kupsch, Andreas and Lange, Axel and M{\"u}ller, Bernd R. and Bruno, Giovanni}, title = {Improving the visibility of phase gratings for Talbot-Lau X-ray imaging}, series = {Materials testing : Materialpr{\~A}¼fung ; materials and components, technology and application}, volume = {58}, journal = {Materials testing : Materialpr{\~A}¼fung ; materials and components, technology and application}, publisher = {Hanser}, address = {M{\"u}nchen}, issn = {0025-5300}, doi = {10.3139/120.110948}, pages = {970 -- 974}, year = {2016}, abstract = {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.}, language = {en} } @article{SprengelMohrAltenburgetal.2021, author = {Sprengel, Maximilian and Mohr, Gunther and Altenburg, Simon J. and Evans, Alexander and Serrano-Munoz, Itziar and Kromm, Arne and Pirling, Thilo and Bruno, Giovanni and Kannengießer, Thomas}, title = {Triaxial residual stress in Laser Powder Bed Fused 316L}, series = {Advanced engineering materials}, volume = {24}, journal = {Advanced engineering materials}, number = {6}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1438-1656}, doi = {10.1002/adem.202101330}, pages = {13}, year = {2021}, abstract = {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.}, language = {en} } @article{StegemannCabezaPelkneretal.2018, author = {Stegemann, Robert and Cabeza, Sandra and Pelkner, Matthias and Lyamkin, Viktor and Pittner, Andreas and Werner, Daniel and Wimpory, Robert and Boin, Mirko and Kreutzbruck, Marc and Bruno, Giovanni}, title = {Influence of the microstructure on magnetic stray fields of low-carbon steel welds}, series = {Journal of Nondestructive Evaluation}, volume = {37}, journal = {Journal of Nondestructive Evaluation}, number = {3}, publisher = {Springer}, address = {New York}, issn = {0195-9298}, doi = {10.1007/s10921-018-0522-0}, pages = {18}, year = {2018}, abstract = {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.}, language = {en} } @article{ThiedeCabezaMishurovaetal.2018, author = {Thiede, Tobias and Cabeza, Sandra and Mishurova, Tatiana and Nadammal, Naresh and Kromm, Arne and Bode, Johannes and Haberland, Christoph and Bruno, Giovanni}, title = {Residual Stress in Selective Laser Melted Inconel 718}, series = {Materials performance and characterization}, volume = {7}, journal = {Materials performance and characterization}, number = {4}, publisher = {American Society for Testing and Materials}, address = {West Conshohocken}, issn = {2379-1365}, doi = {10.1520/MPC20170119}, pages = {717 -- 735}, year = {2018}, abstract = {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.}, language = {en} } @article{UlbrichtMohrAltenburgetal.2021, author = {Ulbricht, Alexander and Mohr, Gunther and Altenburg, Simon J. and Oster, Simon and Maierhofer, Christiane and Bruno, Giovanni}, title = {Can potential defects in LPBF be healed from the laser exposure of subsequent layers?}, series = {Metals : open access journal}, volume = {11}, journal = {Metals : open access journal}, number = {7}, publisher = {MDPI}, address = {Basel}, issn = {2075-4701}, doi = {10.3390/met11071012}, pages = {14}, year = {2021}, abstract = {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.}, language = {en} }