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 - Schröder, Jakob A1 - Evans, Alexander A1 - Mishurova, Tatiana A1 - Ulbricht, Alexander A1 - Sprengel, Maximilian A1 - Serrano-Munoz, Itziar A1 - Fritsch, Tobias A1 - Kromm, Arne A1 - Kannengießer, Thomas A1 - Bruno, Giovanni T1 - Diffraction-based residual stress characterization in laser additive manufacturing of metals JF - Metals : open access journal N2 - 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. KW - laser-based additive manufacturing KW - residual stress analysis KW - X-ray and KW - neutron diffraction KW - diffraction-elastic constants KW - strain-free lattice KW - spacing Y1 - 2021 U6 - https://doi.org/10.3390/met11111830 SN - 2075-4701 VL - 11 IS - 11 PB - MDPI CY - Basel ER - TY - JOUR A1 - Mishurova, Tatiana A1 - Sydow, Benjamin A1 - Thiede, Tobias A1 - Sizova, Irina A1 - Ulbricht, Alexander A1 - Bambach, Markus A1 - Bruno, Giovanni T1 - Residual stress and microstructure of a Ti-6Al-4V Wire Arc Additive Manufacturing hybrid demonstrator JF - Metals N2 - 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. KW - residual stress KW - WAAM KW - Ti-6Al-4V KW - additive manufacturing KW - neutron KW - diffraction KW - hybrid manufacturing Y1 - 2020 U6 - https://doi.org/10.3390/met10060701 SN - 2075-4701 VL - 10 IS - 6 PB - MDPI CY - Basel ER - TY - JOUR A1 - Oster, Simon A1 - Fritsch, Tobias A1 - Ulbricht, Alexander A1 - Mohr, Gunther A1 - Bruno, Giovanni A1 - Maierhofer, Christiane A1 - Altenburg, Simon T1 - On the registration of thermographic in situ monitoring data and computed tomography reference data in the scope of defect prediction in laser powder bed fusion JF - Metals : open access journal N2 - The detection of internal irregularities is crucial for quality assessment in metal-based additive manufacturing (AM) technologies such as laser powder bed fusion (L-PBF). The utilization of in-process thermography as an in situ monitoring tool in combination with post-process X-ray micro computed tomography (XCT) as a reference technique has shown great potential for this aim. Due to the small irregularity dimensions, a precise registration of the datasets is necessary as a requirement for correlation. In this study, the registration of thermography and XCT reference datasets of a cylindric specimen containing keyhole pores is carried out for the development of a porosity prediction model. The considered datasets show variations in shape, data type and dimensionality, especially due to shrinkage and material elevation effects present in the manufactured part. Since the resulting deformations are challenging for registration, a novel preprocessing methodology is introduced that involves an adaptive volume adjustment algorithm which is based on the porosity distribution in the specimen. Thus, the implementation of a simple three-dimensional image-to-image registration is enabled. The results demonstrate the influence of the part deformation on the resulting porosity location and the importance of registration in terms of irregularity prediction. KW - selective laser melting (SLM) KW - laser powder bed fusion (L-PBF) KW - additive KW - manufacturing (AM) KW - process monitoring KW - infrared thermography KW - X-ray KW - micro computed tomography (XCT) KW - defect detection KW - image registration Y1 - 2022 U6 - https://doi.org/10.3390/met12060947 SN - 2075-4701 VL - 12 IS - 6 PB - MDPI CY - Basel ER - TY - JOUR A1 - 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 -