| 1. |
- Gruber, Hans, 1983, et al.
(författare)
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Effect of Powder Recycling in Electron Beam Melting on the Surface Chemistry of Alloy 718 Powder
- 2019
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Ingår i: Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science. - : Springer Science and Business Media LLC. - 1073-5623. ; 50:9, s. 4410-4422
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Tidskriftsartikel (refereegranskat)abstract
- Process-induced degradation of the powder feedstock in additive manufacturing may have a negative influence on the final properties of built components. Consequently, it may lower the cost-effectiveness of powder bed additive manufacturing, which relies on recycling of the nonconsumed powder. This is especially the case for production of high-performance aero engine components where high material and process reliability is required. This study comprises a detailed investigation on the degradation of Alloy 718 powder during multicycle electron beam melting (EBM). The surface-sensitive analysis methods, X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES), were combined with scanning electron microscopy (SEM) to depict the differences in surface morphology, and surface composition of powder samples exposed to varying numbers of re-use cycles. The results show a significant change in surface characteristics after exposing the powder to the process and the environment in the build chamber. The virgin powder is covered mainly by a relatively thin and homogeneous oxide layer. The re-used powder, however, has undergone transformation to a heterogeneous oxide layer, rich in thermodynamically stable Al-rich oxide particulates, which started already during the first build cycle. Significant growth of the Al-rich oxide occurs via selective oxidation of Al under the conditions in the build chamber, including both pick-up of oxygen from the process atmosphere and redistribution of initial surface-bound oxygen from less-stable products like Ni-oxide and/or hydroxide.
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| 2. |
- Gruber, Hans, 1983
(författare)
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ELECTRON BEAM MELTING OF ALLOY 718 - POWDER RECYCLING AND ITS EFFECT ON DEFECT FORMATION
- 2019
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- As for any production process, the performance of additively manufactured components is ultimately dependent on the quality of the feedstock material. Consequently, for critical components, the feedstock needs to be carefully controlled to assure a stable and reliable quality. At the same time, the materials efficiency of additive manufacturing is closely related to powder recycling, which may affect both physical and chemical properties of the powder. This is especially the case for electron beam melting (EBM) where the recycled powder may change significantly from exposure at the high temperature in the build chamber. The aim of this study is to investigate the connection between powder recycling, powder chemistry and presence of defects in EBM processed Alloy 718. For this purpose, recycled powder was studied with reference to its virgin counterpart to detect differences in surface morphology, surface chemical composition as well as bulk chemistry as a consequence of powder recycling. The amount of defects and their distribution in samples produced from virgin and recycled powder was studied by means of image analysis and oxygen measurements. Morphological analysis using scanning electron microscopy was performed to understand their origin and formation mechanism. The results show a significant change in surface characteristics after exposing the powder to the process and the environment in the build chamber. While the virgin powder is covered by a relatively thin and homogeneous oxide layer, the recycled powder has undergone transformation to a heterogeneous oxide layer rich in thermodynamically stable Al-rich oxide particulates. Significant growth of the Al-rich oxide occurs via selective oxidation of Al at the conditions in the build chamber, including both pick-up of oxygen from the process atmosphere and re-distribution of oxygen from less stable oxide products. The increasing amount of oxide is confirmed by an increase in total oxygen level with progressive recycling. Furthermore, a clear correlation between the powder oxygen level and the amount of oxide inclusions in the EBM fabricated samples was observed. Hot isostatic pressing can be used to reach a near-full densify in samples produced from virgin powder. The samples produced from recycled powder, however, have a higher amount of aluminium-rich oxide inclusions which remain after HIP treatment. A variety of oxide defects was observed, ranging from finely dispersed oxide particulates inside lack of fusion defects to large oxide agglomerates in the bulk metal. Based on their morphology, it is shown that most of them originate from aluminium-rich oxide particulates on the surface of the recycled powder. It is suggested that the quality of EBM processed Alloy 718 is at present dependent on the oxygen level in the powder in general and the surface chemistry of the powder in particular, which needs to be controlled to maintain a low amount of inclusion defects.
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| 3. |
- Xu, Jinghao, et al.
(författare)
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Short-term creep behavior of an additive manufactured non-weldable Nickel-base superalloy evaluated by slow strain rate testing
- 2019
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 179, s. 142-157
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Tidskriftsartikel (refereegranskat)abstract
- Additive manufacturing (AM) of high γ′ strengthened Nickel-base superalloys, such as IN738LC, is of high interest for applications in hot section components for gas turbines. The creep property acts as the critical indicator of component performance under load at elevated temperature. However, it has been widely suggested that the suitable service condition of AM processed IN738LC is not yet fully clear. In order to evaluate the short-term creep behavior, slow strain rate tensile (SSRT) tests were performed. IN738LC bars were built by laser powder-bed-fusion (L-PBF) and then subjected to hot isostatic pressing (HIP) followed by the standard two-step heat treatment. The samples were subjected to SSRT testing at 850 °C under strain rates of 1 × 10−5/s, 1 × 10−6/s, and 1 × 10−7/s. In this research, the underlying creep deformation mechanism of AM processed IN738LC is investigated using the serial sectioning technique, electron backscatter diffraction (EBSD), transmission electron microscopy (TEM). On the creep mechanism of AM polycrystalline IN738LC, grain boundary sliding is predominant. However, due to the interlock feature of grain boundaries in AM processed IN738LC, the grain structure retains its integrity after deformation. The dislocation motion acts as the major accommodation process of grain boundary sliding. Dislocations bypass the γ′ precipitates by Orowan looping and wavy slip. The rearrangement of screw dislocations is responsible for the formation of subgrains within the grain interior. This research elucidates the short-creep behavior of AM processed IN738LC. It also shed new light on the creep deformation mechanism of additive manufactured γ′ strengthened polycrystalline Nickel-base superalloys.
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