| 1. |
- Gruber, Hans, 1983, et al.
(författare)
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Effect of Powder Recycling on Defect Formation in Electron Beam Melted Alloy 718
- 2020
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Ingår i: Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science. - : Springer Science and Business Media LLC. - 1073-5623. ; 51:5, s. 2430-2443
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Tidskriftsartikel (refereegranskat)abstract
- The extent to which powder recycling can be permitted before risking a loss in performance of critical components is a major aspect for the viability of electron beam melting (EBM). In this study, the influence of powder oxidation during multi-cycle EBM processing on the formation of oxide-related defects in Alloy 718 is investigated. The amount of defects and their distribution in samples produced from virgin and re-used powder is studied by means of image analysis and oxygen measurements. Morphological analysis using scanning electron microscopy is performed to understand their origin and formation mechanism. The results indicate a clear correlation between the powder oxygen content and the amount of oxide inclusions present in the investigated samples. The inclusions consist of both molten and unmolten Al-rich oxide which originates from the surface of the recycled powder. Upon interaction with the electron beam, the oxide tends to cluster in the liquid metal and form critical sized defects. Hot isostatic pressing can be successfully used to densify samples produced from virgin powder. However, in the material fabricated from recycled powder, a considerable amount of damage relevant oxide inclusion defects remain after HIP treatment, especially in the contour region. 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 on the surface chemistry of the power in particular, which needs to be controlled to maintain a low amount of inclusions.
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| 2. |
- Gruber, Hans, 1983
(författare)
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Powder bed fusion processing of Ni-base superalloys - Defect formation and its mitigation
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Powder bed fusion of Ni- and Ni-Fe-base superalloys is actively considered a promising manufacturing technology for critical components for the aerospace and industrial gas turbine industries. Such components often operate under harsh conditions, and hence, high demands are placed on both process and feedstock material to meet the strict safety and long-term reliability requirements. The aim of this thesis is to provide knowledge regarding the formation of damage-relevant defects in Ni- and Ni-Fe-base superalloys fabricated by powder bed fusion as well as how they can be mitigated. The first part of the thesis presents the connection between the surface oxidation of Alloy 718 powder for EBM, as a consequence of powder re-use, and the presence of oxide-related defects in the EBM fabricated material. The results indicate a clear connection between powder re-use and surface oxidation of the powder. Surface analysis of the progressively re-used powder by means of SEM, XPS and AES reveals significant growth of Al-rich oxide, which occurs via selective oxidation of Al due to the environment in the build chamber. Furthermore, the increased amount of oxide on the surface of the re-used powder results in an increased amount of oxide inclusions and lack of fusion defects in the EBM fabricated material. The morphology of the defects reveals that they originate from Al-rich oxide particulates on the surface of the re-used powder. The second part of the thesis presents a study on the cracking of IN-738LC fabricated by means of LPBF. Implementation of custom designed powder grades with varying content of B and Zr indicates that both elements have a strong negative effect on the susceptibility to grain boundary microcracking of the alloy during LPBF. The XPS, AES and APT analyses show the enrichment of B and Zr at the cracked grain boundaries. Moreover, a significant portion of both elements are found to be connected to oxide. Hence, it is suggested that the increased microcracking susceptibility of IN-738LC is connected to the embrittlement of high-angle grain boundaries due to the formation of B- and Zr-containing oxide. In addition, post-LPBF hot isostatic pressing (HIP) is evaluated as a concept for microcrack healing. A HIP strategy that suppresses formation of macrocracks during the HIP treatment is developed by tailoring the temperature and pressure profiles during the heating stage. However, when applying the developed HIP strategy to the material grade with high levels of B and Zr, brittleness-inducing secondary phase particles at the grain boundaries appear after HIP at 1210°C, leading to a significant reduction of the impact toughness. Formation of the secondary phase is suppressed by lowering the HIP temperature to 1120°C. Results from microscopy and Charpy impact testing suggest that significant healing of the microcracks is accomplished when applying the developed HIP strategy.
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| 3. |
- Xu, Jinghao, et al.
(författare)
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A Novel gamma y'-Strengthened Nickel-Based Superalloy for Laser Powder Bed Fusion
- 2020
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Ingår i: Materials. - : MDPI AG. - 1996-1944. ; 13:21, s. 1-12
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Tidskriftsartikel (refereegranskat)abstract
- An experimental printable gamma '-strengthened nickel-based superalloy, MAD542, is proposed. By process optimization, a crack-free component with less than 0.06% defect was achieved by laser powder bed fusion (LPBF). After post-processing by solution heat treatment, a recrystallized structure was revealed, which was also associated with the formation of annealing twins. After the aging treatment, 60-65% gamma ' precipitates were obtained with a cuboidal morphology. The success of printing and post-processing the new MAD542 superalloy may give new insights into alloy design approaches for additive manufacturing.
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| 4. |
- Xu, Jinghao, et al.
(författare)
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On the strengthening and embrittlement mechanisms of an additively manufactured Nickel-base superalloy
- 2020
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Ingår i: Materialia. - : Elsevier BV. - 2589-1529. ; 10
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Tidskriftsartikel (refereegranskat)abstract
- The γ′ phase strengthened Nickel-base superalloy is one of the most significant dual-phase alloy systems for high-temperature engineering applications. The tensile properties of laser powder-bed-fused IN738LC superalloy in the as-built state have been shown to have both good strength and ductility compared with its post-thermal treated state. A microstructural hierarchy composed of weak texture, sub-micron cellular structures and dislocation cellular walls was promoted in the as-built sample. After post-thermal treatment, the secondary phase γ′ precipitated with various size and fraction depending on heat treatment process. For room-temperature tensile tests, the dominated deformation mechanism is planar slip of dislocations in the as-built sample while dislocations bypassing the precipitates via Orowan looping in the γ′ strengthened samples. The extraordinary strengthening effect due to the dislocation substructure in the as-built sample provides an addition of 372 MPa in yield strength. The results of our calculation are in agreement with experimental yield strength for all the three different conditions investigated. Strikingly, the γ′ strengthened samples have higher work hardening rate than as-built sample but encounter premature failure. Experimental evidence shows that the embrittlement mechanism in the γ′ strengthened samples is caused by the high dislocation hardening of the grain interior region, which reduces the ability to accommodate further plastic strain and leads to premature intergranular cracking. On the basis of these results, the strengthening micromechanism and double-edge effect of strength and ductility of Nickel-base superalloy is discussed in detail.
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