| 1. |
- Huang, Jing-Jia, 1990-, et al.
(författare)
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Growth of silicon carbide multilayers with varying preferred growth orientation
- 2022
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Ingår i: Surface & Coatings Technology. - : Elsevier. - 0257-8972 .- 1879-3347. ; 447
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Tidskriftsartikel (refereegranskat)abstract
- SiC multilayer coatings were deposited via thermal chemical vapor deposition (CVD) using silicon tetrachloride (SiCl4) and various hydrocarbons under identical growth conditions, i.e. at 1100 °C and 10 kPa. The coatings consisted of layers whose preferred growth orientation alternated between random and highly 〈111〉-oriented. The randomly oriented layers were prepared with either methane (CH4) or ethylene (C2H4) as carbon precursor, whereas the highly 〈111〉-oriented layers were grown utilizing toluene (C7H8) as carbon precursor. In this work, we demonstrated how to fabricate multilayer coatings with different growth orientations by merely switching between hydrocarbons. Moreover, the success in depositing multilayer coatings on both flat and structured graphite substrates has strengthened the assumption proposed in our previous study that the growth of highly 〈111〉-oriented SiC coatings using C7H8 was primarily driven by chemical surface reactions.
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| 2. |
- Wennersten, Karin, et al.
(författare)
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Feasibility of Melting NbC Using Electron Beam Powder Bed Fusion
- 2024
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Ingår i: Advanced Engineering Materials. - : WILEY-V C H VERLAG GMBH. - 1438-1656 .- 1527-2648.
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Tidskriftsartikel (refereegranskat)abstract
- High melting point materials such as ceramics and metal carbides are in general difficult to manufacture due to their physical properties, which imposes the need for new manufacturing methods where electron beam powder bed fusion (EB-PBF) seems promising. Most materials that have been successfully printed with EB-PBF are metals and metal alloys with good electrical conductivity, whereas dielectric materials such as ceramics are generally difficult to print. Catastrophic problems such as smoking and spattering can occur during the EB-PBF processing owing to inappropriate physical properties such as lack of electrical, and thermal conductivity and high melting point, which are challenging to overcome by process optimization. Due to these difficulties, a limited level of understanding has been achieved regarding melting ceramics and refractory alloys. Herein, three different substrates of niobium carbide (NbC) are melted using EB-PBF. The established process parameter window shows a good correlation between EB-PBF process parameters, surface, and melt characteristics, which can be used as a baseline for a printing process. Melting NbC is proven feasible using EB-PBF; the work also points out challenges related to arc trips and spattering, as well as future investigations necessary to create a stable printing process. Additive manufacturing offer new ways of manufacturing ceramics and metal carbides otherwise hard to produce. This study presents one of the first attempts at melting niobium carbide using electron beam powder bed fusion by identifying process window and investigating how the different process parameters affect the melt characteristics, as well as identifying potential issues regarding printing metal carbides.image (c) 2024 WILEY-VCH GmbH
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| 3. |
- Xu, Jinghao, 1992-
(författare)
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Alloy Design and Characterization of γ′ Strengthened Nickel-based Superalloys for Additive Manufacturing
- 2021
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Nickel-based superalloys, an alloy system bases on nickel as the matrix element with the addition of up to 10 more alloying elements including chromium, aluminum, cobalt, tungsten, molybdenum, titanium, and so on. Through the development and improvement of nickel-based superalloys in the past century, they are well proved to show excellent performance at the elevated service temperature. Owing to the combination of extraordinary high-temperature mechanical properties, such as monotonic and cyclic deformation resistance, fatigue crack propagation resistance; and high-temperature chemical properties, such as corrosion and oxidation resistance, phase stability, nickel-based superalloys are widely used in the critical hot-section components in aerospace and energy generation industries.The success of nickel-based superalloy systems attributes to both the well-tailored microstructures with the assistance of carefully doped alloying elements, and the intently developed manufacturing processes. The microstructure of the modern nickel-based superalloys consists of a two-phase configuration: the intermetallic precipitates (Ni,Co)3(Al,Ti,Ta) known as γ′ phase dispersed into the austenite γ matrix, which is firstly introduced in the 1940s. The recently developed additive manufacturing (AM) techniques, acting as the disruptive manufacturing process, offers a new avenue for producing the nickel-based superalloy components with complicated geometries. However, γ′ strengthened nickel-based superalloys always suffer from the micro-cracking during the AM process, which is barely eliminated by the process optimization.On this basis, the new compositions of γ′ strengthened nickel-based superalloy adapted to the AM process are of great interest and significance. This study sought to design novel γ′ strengthened nickel-based superalloys readily for AM process with limited cracking susceptibility, based on the understanding of the cracking mechanisms. A two-parameter model is developed to predict the additive manufacturability for any given composition of a nickel-based superalloy. One materials index is derived from the comparison of the deformation-resistant capacity between dendritic and interdendritic regions, while another index is derived from the difference of heat resistant capacity of these two spaces. By plotting the additive manufacturability diagram, the superalloys family can be categorized into the easy-to-weld, fairly-weldable, and non-weldable regime with the good agreement of the existed knowledge. To design a novel superalloy, a Cr-Co-Mo-W-Al-Ti-Ta-Nb-Fe-Ni alloy family is proposed containing 921,600 composition recipes in total. Through the examination of additive manufacturability, undesired phase formation propensity, and the precipitation fraction, one composition of superalloy, MAD542, out of the 921,600 candidates is selected.Validation of additive manufacturability of MAD542 is carried out by laser powder bed fusion (LPBF). By optimizing the LPBF process parameters, the crack-free MAD542 part is achieved. In addition, the MAD542 superalloy shows great resistance to the post-processing treatment-induced cracking. During the post-processing treatment, extensive annealing twins are promoted to achieve the recrystallization microstructure, ensuring the rapid reduction of stored energy. After ageing treatment, up to 60-65% volume fraction of γ′ precipitates are developed, indicating the huge potential of γ′ formation. Examined by the high-temperature slow strain rate tensile and constant loading creep testing, the MAD542 superalloy shows superior strength than the LPBF processed and hot isostatic pressed plus heat-treated IN738LC superalloy. While the low ductility of MAD542 is existed, which is expected to be improved by modifying the post-processing treatment scenarios and by the adjusting building direction in the following stages of the Ph.D. research.MAD542 superalloy so far shows both good additive manufacturability and mechanical potentials. Additionally, the results in this study will contribute to a novel paradigm for alloy design and encourage more γ′-strengthened nickel-based superalloys tailored for AM processes in the future.
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| 4. |
- Xu, Jinghao, 1992-, et al.
(författare)
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Effect of heat treatment on the microstructure characteristics and microhardness of a novel γ′ nickel-based superalloy by laser powder bed fusion
- 2021
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Ingår i: Results in Materials. - : Elsevier BV. - 2590-048X. ; 12
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Tidskriftsartikel (refereegranskat)abstract
- The fabrication of gamma prime (γ′) strengthened nickel-based superalloys by additive manufacturing (AM) techniques is of huge interest from the industrial and research community owing to their excellent high-temperature properties. The effect of post-AM-processing heat treatment on the microstructural characteristics and microhardness response of a laser powder bed fused (LPBF) γ′ strengthened nickel-based superalloy, MAD542, is systematically investigated. Post-processing heat treatment shows the significant importance of tailoring the γ′ morphology. With insufficient solutioning duration time, coarse γ′ formed in the interdendritic region heterogeneously, due to the lack of chemical composition homogenization. The cooling rate from the super-solvus solutioning plays an important role in controlling the γ′ size and morphology. Spherical γ′ is formed during the air cooling while irregularly shaped γ′ formed during the furnace cooling. The following aging heat treatment further tunes the γ′ morphology and γ channel width. After two-step aging, cuboidal γ′ is developed in the air-cooled sample, while in contrast, bi-modally distributed γ′ is developed in the furnace cooled sample with fine spherical γ′ embedded in the wide γ channel between coarse irregular shaped secondary γ′. More than 90% of the grains recrystallized during solutioning treatment at the super-solvus temperature for 30 min. The rapid recrystallization kinetics are attributed to the formation of annealing twins which significantly reduced the stored energy. Microhardness responses from different heat-treated conditions were examined.
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| 5. |
- Xu, Jinghao, 1992-, et al.
(författare)
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Effect of post-processes on the microstructure and mechanical properties of laser powder bed fused IN718 superalloy
- 2021
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Ingår i: Additive Manufacturing. - : Elsevier B.V.. - 2214-8604 .- 2214-7810. ; 48
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Tidskriftsartikel (refereegranskat)abstract
- The post-processing on the additively manufactured component is of huge interest as the key to tailor the microstructure to obtain certain mechanical properties. In this present study, the effects of hot isostatic pressing, as well as heat treatment on the microstructure, phase configuration and mechanical properties of laser powder bed fused (LPBF) IN718 superalloy were systematically investigated. Three different post-processes were studied such as hot isostatic pressing (HIP), heat treatment (HT), and HIP followed by HT (HIP+HT). The HIP process effectively eliminated the Laves phase remained in the as-built microstructure and brought uniformly distributed super fine γ″ precipitates in nano-meter size. In the heat-treated microstructure, larger γ″ precipitates were promoted directly from the as-built material. In comparison the HIP+HT process caused a moderate growth of γ″. In the latter two cases, the developed γ″ significantly strengthened the material. Yield strength of IN718 was increased from 738 MPa in as-built condition to 1015 MPa and 1184 MPa after HT and HIP+HT, respectively. On the contrary the ductility in the as-built IN718 condition was reduced by more than 40% after HT and HIP+HT. This can be compared to an increase in the ductility by almost 30% when subjected the as-built specimens to only HIPping. Finally, the correlation between microstructure evolution and mechanical properties is discussed in detail. © 2021 The Authors
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| 6. |
- Xu, Jinghao, 1992-
(författare)
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High-performance Nickel-based Superalloys for Additive Manufacturing
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Additive manufacturing (AM), e.g., laser powder bed fusion (LPBF) technique, has become a powerful manufacturing process for producing metallic components with the advantages of design freedom, net-shape-forming flexibility, product customization, and reduced lead time to market. Nickel-based superalloys is one of the most significant alloy families used at elevated temperatures. Nickel-based superalloys commonly contain up to 10 more alloying elements like chromium, aluminum, cobalt, tungsten, molybdenum, titanium, and so on. The great capacity of the nickel-based superalloys for high-temperature operation is ensured by the well-tailored microstructures with the assistance of carefully doped alloying elements, and the intently developed corresponding manufacturing processes. However, high-performance nickel-based superalloys generally suffer from structural integrity issues during AM process, i.e., this class of superalloys is highly susceptible to crack. Therefore, new nickel-based superalloys adapted to AM process with tailored chemical composition are under the urgent call. Meanwhile, high-temperature performance is another prioritized target for the new superalloys.The first topic is the chemical composition-dependent cracking mechanisms. The interdendritic region formed at the last-stage solidification has been found as the cracked spaces. Owing to the suppression of precipitate formation, the cracking mechanism is generalized as (1) the large mismatch of the solidification steps accounting for the crack initiation, and (2) the large mismatch of load-bearing capacity accounting for the crack propagation, between the dendritic and interdendritic regions. To quantitatively formulate the additive manufacturability of nickel-based superalloys, herein a two-parameter-based, heat resistance, and deformation resistance (HR-DR) model, has been successfully proposed to predict the printability on accounting for the relation between chemical composition (both major and minor elements) and cracking susceptibility. The concept of this model is formulated as that if the interdendritic region obtains both higher heat and deformation resistances than the rest dendritic region, this alloy is expected to be crack resistant. Validated by the experimental results and hitherto reported literature data, the HR-DR model provides an excellent sound prediction on the crack susceptibility of nickel-based superalloy during AM process. By considering the combination of additive manufacturability and high-temperature performance, a novel high-strength nickel-based superalloy, MAD542 has been developed based on the materials selection procedure from 921,600 candidate compositions. In addition, another precipitation-strengthened nickel-based superalloy, Alloy738+ has been developed based on the modification of the composition of heritage superalloy IN738LC, aiming for improving the additive manufacturability, creep, and oxidation resistance.The second topic is the post-processing treatments related to microstructural evolution and mechanical properties. Owing to the thermal history during the LPBF process, the as-built microstructure commonly consists of columnar grains nearly parallel to the building direction with strong crystallographic texture. Subjected to the post-processing treatments, the solution treatment is the key to controlling the grain evolution. It has been shown for both LPBF MAD542 and heritage LPBF CM247 superalloys, the high crystallographic texture is maintained at the sub-γ′-solvus temperatures because of the grain boundary pinning effect from grain boundary precipitates. Whilst the crystal anisotropy is highly reduced by the treatment at super-γ′-solvus temperatures driven by the means of recrystallization. However, fully recrystallized microstructure with low texture largely reduced the mechanical properties by the embrittlement manner at elevated temperatures accordingly.The third topic is the examination of creep and oxidation performance of various LPBF superalloys. A strong building direction-dependent creep performance is found for an LPBF IN738LC superalloy fabricated by the vertical and horizontal build. Vertically built samples show 7-40 times longer rupture life and approximately 2 times longer elongation at fracture than the horizontally produced samples, for the creep at 150-300 MPa at 850 °C. To evaluate the short-term creep performance, constant displacement rate-controlled slow strain rate tensile (SSRT) testing was carried out. The constant load-controlled creep and SSRT are correlated by deformation rate-based power-law type analysis. The new superalloy LPBF MAD542 generally displays a 5 times slower deformation rate than the LPBF IN738LC superalloy at 850 °C. The new superalloy Alloy738+ shows a comparable creep performance to LPBF IN738LC. Oxidation tests were conducted at 850/950/1050 °C. The new superalloy Alloy738+ presents an excellent oxidation resistance at 850 and 950 °C. By comparison, for example, Alloy738+ has 3 times slower oxidation kinetics than IN738LC at 950 °C.The several investigations associated with the composition/processing/property in multiple precipitation-strengthened nickel-based superalloys fabricated by AM in this thesis have proven that the materials development requires comprehensive in-depth considerations. The presented results can contribute to the fundamental understanding and/or serve as the reference data for other superalloys by AM from the properties perspective.
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