| 1. |
- Agca, Can, et al.
(författare)
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Experimental and computational studies of melting of the spinel phase in the Fe-Al-O ternary system
- 2020
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Ingår i: Calphad. - : Elsevier. - 0364-5916 .- 1873-2984. ; 70
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Tidskriftsartikel (refereegranskat)abstract
- The melting behavior of spinel in the Fe-Al-O system at high temperatures (1500-1800 degrees C) was studied by a combination of experimental and computational investigations. Differential thermal analysis (DTA) at ultra-high temperatures coupled with cooling traces on CO2 laser-heated levitated samples provided melting temperatures and the heats of fusion of (Fe,Al)(3)O-4 spinel phases. The experimental results are in fair agreement with the predictions using a published CALPHAD description and areas for modeling improvement are identified. New insights into the melting of defect spinel are provided.
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| 2. |
- Ananthanarayanan, Durga, et al.
(författare)
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Grain refinement in additively manufactured ferritic stainless steel by in situ inoculation using pre-alloyed powder
- 2021
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Ingår i: Scripta Materialia. - : Elsevier BV. - 1359-6462 .- 1872-8456. ; 194
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Tidskriftsartikel (refereegranskat)abstract
- For ferritic stainless steels, TiN has effectively been used as an inoculant to produce equiaxed grain structures in casting and welding. However, it is not established whether TiN would be an effective inoculant in additive manufacturing. In this study, the effectiveness of TiN as an inoculant in a ferritic stainless steel processed by laser powder-bed fusion is studied. An alloy without Ti is fabricated and compared to an alloy designed to form a high amount of TiN early during solidification. The work shows that the presence of TiN provides general grain refinement and that TiN-covered oxide particles are effective in enabling columnar-to-equiaxed transition in certain regions of the meltpool. The applied approach of pre-alloying powders with inoculant-forming elements offers a straightforward route to achieving fine, equiaxed grain structures in additively manufactured metallic materials. It also shows how oxygen present during the process can be utilized to nucleate effective inoculating phases.
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| 3. |
- Ananthanarayanan, Durga, et al.
(författare)
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Modelling columnar-to-equiaxed transition during fusion-based metal additive manufacturing
- 2023
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Ingår i: Additive Manufacturing. - : Elsevier BV. - 2214-8604 .- 2214-7810. ; 78
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Tidskriftsartikel (refereegranskat)abstract
- During fusion-based metal additive manufacturing, there is an inherent directionality in heat transfer, which leads to columnar grain growth. This may result in cracking and anisotropic mechanical properties in many alloy systems. Therefore, it is important to study the conditions under which columnar-to-equiaxed transition in grain structure occurs. The grain morphology is determined by several factors such as process conditions, local alloy composition, and number density of nucleating sites. In the present work, a model for simulating columnar-to-equiaxed transition is formulated, considering nucleating site size distribution, rapid solidification and constitutional undercooling in multicomponent alloys. Furthermore, the model is coupled with multicomponent Calphad-based thermodynamic and diffusion mobility descriptions. It is demonstrated that including the above aspects is important in accurately predicting the columnar-to-equiaxed transition by comparing with experimental data for an additively manufactured TiB2-reinforced AlSi10Mg alloy. The framework developed in this work may be used to predict columnar-to-equiaxed transition in additively manufactured technical alloys consisting of multiple elements.
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| 4. |
- Bidola, Pidassa M., et al.
(författare)
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A high-speed X-ray Radiography Setup for in-situ Electron Beam Powder Bed Fusion at PETRA III
- 2023
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Ingår i: Advances in X-Ray/EUV Optics and Components XVIII. - : SPIE-Intl Soc Optical Eng.
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Konferensbidrag (refereegranskat)abstract
- A high-energy white synchrotron X-ray beam enables penetration of relatively thick and highly absorbing samples. At the P61A White Beam Engineering Materials Science Beamline, operated by Helmholtz-Zentrum Hereon at the PETRA III ring of the Deutsches Elektronen-Synchrotron (DESY), a tailored X-ray radiography system has been developed to perform in-situ X-ray imaging experiments at high temporal resolution, taking advantage of the unprecedented X-ray beam flux delivered by ten successive damping wigglers. The imaging system is equipped with an ultrahigh-speed camera (Phantom v2640) enabling acquisition rates up to 25 kHz at maximal resolution and binned mode. The camera is coupled with optical magnification (5x, 10x) and focusing lenses to enable imaging with a pixel size of 1,35 micrometre. The scintillator screens are housed in a special nitrogen gas cooling environment to withstand the heat load induced by the beam, allowing spatial resolution to be optimized down to few micrometres. We present the current state of the system development, implementation and first results of in situ investigations, especially of the electron beam powder bed fusion (PBF-EB) process, where the details of the mechanism of crack and pore formation during processing of different powder materials, e.g. steels and Ni-based alloys, is not yet known.
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| 5. |
- Chou, Chia-Ying, et al.
(författare)
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Influence of solidification structure on austenite to martensite transformation in additively manufactured hot-work tool steels
- 2021
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 215
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Tidskriftsartikel (refereegranskat)abstract
- The microstructure of a hot-work tool steel additively manufactured using laser powder-bed fusion (L-PBF), and its response to post heat treatment, is studied in detail by microstructure characterization and computational thermodynamics and kinetics. The high solidification and cooling rates during the L-PBF process lead to suppression of delta-ferrite and instead solidification of an austenite phase directly containing a cellular substructure where the alloying elements have segregated to the inter-cellular regions and where solidification carbides have formed in the cell junctions. The austenite is then partly decomposed into martensite at lower temperatures. The micro-segregation can be predicted by reducing the complex solidification behavior to a diffusion problem in one dimension enabling detailed comparisons with the measured segregation profiles quantified at a nanometer scale. Martensite start temperature (M-s) calculations along the spatially varying composition show that the M-s temperature decreases in the inter-cellular regions where austenite is observed. The network of austenite in the as-built microstructure can be understood from the combined influence of the composition dependence of the M-s temperature in relation to the build plate temperature and the mechanical stabilization of the small-sized austenite regions. This work demonstrates the power of computational tools based on computational thermodynamics and kinetics for designing tool steels for additive manufacturing by predictions of the steel's response to the L-PBF process and post heat treatments.
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| 6. |
- Chou, Chia-Ying, 1990-
(författare)
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Integrated Computational and Experimental Study of Additively Manufactured Steels
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The design freedom Additive Manufacturing (AM) offers provides new solutions for improving functionality in industrial applications. It also offers unique opportunities when it comes to materials design.Powder Bed Fusion – Laser Beam (PBF-LB) is currently one of the most popular commercial AM techniques for metallic materials partly due to the relatively low surface roughness and the large design flexibility. However, the number of materials suitable for the PBF-LB process is still rather low and to accelerate the development of grades tailored for this AM process, dedicated computational tools for alloy design are needed. Of importance for materials design, is computational thermodynamics and kinetics coupled with CALPHAD materials descriptions since it enables calculations for multicomponent materials making it possible to predict the effect of varying composition.In this thesis, computational thermodynamic and kinetics coupled with materials characterization are applied to study the microstructure evolution during PBF-LB. Two material classes are in focus – hot-work tool steels and ferritic stainless steels. For the hot-work tool steel, the cooling rates during PBF-LB processing are high enough to induce martensite transformation and in the as-built microstructure a martensitic matrix is observed and some fraction of retained austenite. A solidification sub-structure due to micro-segregation during printing is also observed. Solidification calculations are performed to predict the micro-segregation showing agreement with experimental measurements. The segregation results are then used as input to a semi-empirical martensite start temperature model making it possible to explain the location and amount of retained austenite.In addition to compositional optimization, a computational framework for AM alloy design needs to include the possibility to tailor the AM post heat treatments. An alternative to the conventional hardening treatment is thus studied in the current work. A model for precipitation kinetics is combined with experimental characterization to explore the effect of tempering on the as-built microstructure in comparison to the tempering effect on an austenitized microstructure. The results show that the precipitation kinetics is strongly dependent on the starting structure and that direct tempering of the as-built microstructure changes the precipitation sequence compared to the conventional heat treatment route. The calculations reproduce this result suggesting that it is a thermodynamic effect stemming from different matrix compositions.The other material class, the ferritic stainless steels, is studied in terms of its response to post-heat treatments. The as-built microstructure is characterized by high dislocation density and a fine grain structure in some cases as well as a solidification sub-structure. The mechanical properties of the as-built material are in general good for these steels, however, stress relieving is most often a required post process for the as-built components which may decrease the mechanical properties. To maximize the gained benefits from the unique process condition of PBF-LB, simulations are applied to study the possibility of post heat treatment optimization.To construct a computational framework for AM materials design, multiscale modeling capabilities are needed. This work shows the value of computational thermodynamics and kinetics for understanding the materials behavior on the microscale and hence, contributes to the construction of such a framework. By understanding the physical metallurgy, and enable modeling of the AM processes, the industrialization of AM can be accelerated.
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| 7. |
- Chou, Chia-Ying, et al.
(författare)
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Precipitation kinetics during post-heat treatment of a L-PBF processed SS446 ferritic stainless steel
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The microstructure response to post-heat treatments of SS446 ferritic stainless steel processed by laser powder bed fusion is investigated, with focus on the precipitation kinetics of Cr2N. Precipitation simulations are conducted using the precipitation module (TC-PRISMA) within the Thermo-Calc Software Package. The annealed microstructure is characterized by scanning electron microscopy and the results are used for model validation and calibration. The microstructure and hardness responses to post heat treatments is discussed in terms of optimal solid-solution strengthening in SS446 ferritic stainless steel alloys processed by laser-powder bed fusion.
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| 8. |
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| 9. |
- Chou, Chia-Ying, et al.
(författare)
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Precipitation Kinetics During Post-heat Treatment of an Additively Manufactured Ferritic Stainless Steel
- 2022
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Ingår i: Metallurgical and Materials Transactions. A. - : Springer Nature. - 1073-5623 .- 1543-1940. ; 53:8, s. 3073-3082
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Tidskriftsartikel (refereegranskat)abstract
- The microstructure response of laser-powder bed fusion (L-PBF)-processed ferritic stainless steel (AISI 441) during post-heat treatments is studied in detail. Focus is on the precipitation kinetics of the Nb-rich phases: Laves (Fe2Nb) and the cubic carbo-nitride (NbC), as well as the grain structure evolution. The evolution of the precipitates is characterized using scanning and transmission electron microscopy (SEM and TEM) and the experimental results are used to calibrate precipitation kinetics simulations using the precipitation module (TC-PRISMA) within the Thermo-Calc Software package. The calculations reproduce the main trend for both the mean radii for the Laves phase and the NbC, and the amount of Laves phase, as a function of temperature. The calibrated model can be used to optimize the post-heat treatment of additively manufactured ferritic stainless steel components and offer a creator tool for process and structure linkages in an integrated computational materials engineering (ICME) framework for alloy and process development of additively manufactured ferritic steels.
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| 10. |
- Deirmina, Faraz, et al.
(författare)
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Heat Treatment And Mechanical Properties Of A Novel Ultrahigh Strength Co-free Maraging Steel Fabricated By Additive Manufacturing
- 2021
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Ingår i: Euro PM2021 Congress Proceedings. - : European Powder Metallurgy Association (EPMA).
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Konferensbidrag (refereegranskat)abstract
- We report on the fabrication of a Co-free ultra-high strength maraging steel designed for the laser-powder bed fusion (L-PBF) process using computational approaches. The aim was to obtain an essentially martensitic microstructure after the L-PBF process, and to achieve an ultimate tensile strength (UTS) of minimum 1700 MPa after direct ageing of the as-built microstructure. Microstructural characterization, dilatometry, and tensile and impact toughness tests were used to evaluate the employed approach.
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