| 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
-
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).
-
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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| 11. |
- Garcia, J., et al.
(författare)
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Kinetics of formation of graded layers on cemented carbides : Experimental investigations and DICTRA simulations
- 2011
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Ingår i: International Journal of Refractory Metals and Hard Materials. - : Elsevier BV. - 0263-4368. ; 29:2, s. 256-259
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Tidskriftsartikel (refereegranskat)abstract
- Kinetics of formation of fcc-free layers on Co-W-Ti-Ta-Nb-C-N cemented carbides was investigated by experimental methods and DICTRA simulations. The layer formation obeys a parabolic law, indicating a diffusion-controlled process. For DICTRA simulations, the influence of the mobilities for all diffusing elements in the liquid binder phase at the sintering temperature was investigated. The best agreement between experimental and simulations was obtained considering that the mobility of all metallic elements is two times slower compared with the mobility of the non-metallic elements. © 2010 Elsevier Ltd. All rights reserved.
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| 12. |
- Gheno, Thomas, et al.
(författare)
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On the Simulation of Composition Profiles in NiCoCrAl Alloys During Al2O3 Scale Growth in Oxidation and Oxidation-Dissolution Regimes
- 2019
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Ingår i: Oxidation of Metals. - : SPRINGER/PLENUM PUBLISHERS. - 0030-770X .- 1573-4889. ; 91:3-4, s. 243-257
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Tidskriftsartikel (refereegranskat)abstract
- The ability to simulate the oxidation behavior of multicomponent alloys is a powerful tool for alloy development and oxidation research. The present work shows how the DICTRA module of Thermo-Calc can be applied to reproduce composition profiles in NiCoCrAl alloys during Al2O3 scale growth when used in conjunction with appropriate Calphad thermodynamic and diffusion mobility databases. Profiles were calculated in pure oxidation and oxidation-dissolution regimes to simulate reaction in air and in a molten silicate. For each regime, a simple analytical expression was used to set the outward Al flux at the alloy surface as a boundary condition. The simulations were performed using different combinations of thermodynamic and diffusion mobility databases, which demonstrated the relative importance of the thermodynamic and kinetic contributions to the interdiffusion coefficients, and in turn to the concentration profiles. The simulations done with the developmental NISTCoNi-mob mobility database were found to be in good agreement with experimental data, while those done with MOBNI4 significantly underestimated Al depletion (and Co, Cr and Ni enrichment), due in part to an incomplete description of diffusivity in the Co-Ni binary system. All the tested thermodynamic databases yielded similar results. This work provides a quantitative illustration of the importance of critically assessed diffusion mobility descriptions in designing oxidation-resistant materials.
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| 13. |
- Ioannidou, Chrysoula, et al.
(författare)
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In-situ synchrotron X-ray analysis of metal Additive Manufacturing : Current state, opportunities and challenges
- 2022
-
Ingår i: Materials & design. - : Elsevier BV. - 0264-1275 .- 1873-4197. ; 219, s. 110790-
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Tidskriftsartikel (refereegranskat)abstract
- Additive Manufacturing (AM) is becoming an important technology for manufacturing of metallic materials. Laser-Powder Bed Fusion (L-PBF), Electron beam-Powder Bed Fusion (E-PBF) and Directed Energy Deposition (DED) have attracted significant interest from both the scientific community and the industry since these technologies offer great manufacturing opportunities for niche applications and complex geometries. Understanding the physics behind the complex and dynamic phenomena occurring during these processes is essential for overcoming the barriers that constrain the metal AM development. Insitu synchrotron X-ray characterization is suitable for investigating the microstructure evolution during processing and provides new profound insights. Here, we provide an overview of the research on metal PBF and DED using in-situ synchrotron X-ray imaging, diffraction and small-angle scattering, highlighting the state of the art, the instrumentation, the challenges and the gaps in knowledge that need to be filled. We aim at presenting a scientific roadmap for in-situ synchrotron analysis of metal PBF and DED where future challenges in instrumentation such as the development of experimental stations, sample environments and detectors as well as the need for further application oriented research are included.
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| 14. |
- Karlsson, Dennis, et al.
(författare)
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Additive manufacturing of the ferritic stainless steel SS441
- 2020
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Ingår i: Additive Manufacturing. - AMSTERDAM, NETHERLANDS : Elsevier BV. - 2214-8604 .- 2214-7810. ; 36
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Tidskriftsartikel (refereegranskat)abstract
- In this study, the ferritic stainless steel SS441 was produced with excellent mechanical properties using laser powder bed fusion (L-PBF) compared to samples produced by conventional casting and hot-rolling. In addition, thermodynamic calculations were utilized to study the phase stability at elevated temperatures and to understand the solidification behavior. The hot-rolled sample showed a grain size up to several hundred mu m with additional precipitates of TiN and Nb(C,N). In contrast, the as-built L-PBF samples displayed a grain size in the mu m range. Spherical precipitates with a size of around 50 nm could be observed and were attributed to a corundum phase from the thermodynamic calculations. The printed material shows superior mechanical properties, with more than 30 times higher impact energy compared to the hot-rolled alloy (217 +/- 5 J vs. 7 +/- 0.5 J). Furthermore, the properties are anisotropic for the L-PBF produced alloy, with the highest tensile strength vertical to the build direction. The superior mechanical properties of the L-PBF produced sample can be attributed to a smaller grain size, giving a higher strength according to the Hall-Petch relationship. The anisotropy of the material can be eliminated by heat treatments at 900 degrees C followed by water quenching, but the absolute strength decreases slightly due to formation of intermetallic phases such as Nb(C,N) and the Fe2Nb Laves phase. The results clearly illustrates that L-PBF provides a promising manufacturing mute for enhanced strength of ferritic stainless steels.
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| 15. |
- Karlsson, Dennis, et al.
(författare)
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Binder jetting of the AlCoCrFeNi alloy
- 2019
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Ingår i: Additive Manufacturing. - : Elsevier. - 2214-8604 .- 2214-7810. ; 27, s. 72-79
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Tidskriftsartikel (refereegranskat)abstract
- High density components of an AlCoCrFeNi alloy, often described as a high-entropy alloy, were manufactured by binder jetting followed by sintering. Thermodynamic calculations using the CALPHAD approach show that the high-entropy alloy is only stable as a single phase in a narrow temperature range below the melting point. At all other temperatures, the alloy will form a mixture of phases, including a sigma phase, which can strongly influence the mechanical properties. The phase stabilities in built AlCoCrFeNi components were investigated by comparing the as-sintered samples with the post-sintering annealed samples at temperatures between 900 °C and 1300 °C. The as-sintered material shows a dominant B2/bcc structure with additional fcc phase in the grain boundaries and sigma phase precipitating in the grain interior. Annealing experiments between 1000 °C and 1100 °C inhibit the sigma phase and only a B2/bcc phase with a fcc phase is observed. Increasing the temperature further suppresses the fcc phase in favor for the B2/bcc phases. The mechanical properties are, as expected, dependent on the annealing temperature, with the higher annealing temperature giving an increase in yield strength from 1203 MPa to 1461 MPa and fracture strength from 1996 MPa to 2272 MPa. This can be explained by a hierarchical microstructure with nano-sized precipitates at higher annealing temperatures. The results enlighten the importance of microstructure control, which can be utilized in order to tune the mechanical properties of these alloys. Furthermore, an excellent oxidation resistance was observed with oxide layers with a thickness of less than 5 μm after 20 h annealing at 1200 °C, which would be of great importance for industrial applications.
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| 16. |
- König, Hans-Henrik, et al.
(författare)
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MiniMelt : An instrument for real-time tracking of electron beam additive manufacturing using synchrotron x-ray techniques
- 2023
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Ingår i: Review of Scientific Instruments. - : AIP Publishing. - 0034-6748 .- 1089-7623. ; 94:12
-
Tidskriftsartikel (refereegranskat)abstract
- The development of a sample environment for in situ x-ray characterization during metal Electron Beam Powder Bed Fusion (PBF-EB), called MiniMelt, is presented. The design considerations, the features of the equipment, and its implementation at the synchrotron facility PETRA III at Deutsches Elektronen-Synchrotron, Hamburg, Germany, are described. The equipment is based on the commercially available Freemelt ONE PBF-EB system but has been customized with a unique process chamber to enable real-time synchrotron measurements during the additive manufacturing process. Furthermore, a new unconfined powder bed design to replicate the conditions of the full-scale PBF-EB process is introduced. The first radiography (15 kHz) and diffraction (1 kHz) measurements of PBF-EB with a hot-work tool steel and a Ni-base superalloy, as well as bulk metal melting with the CMSX-4 alloy, using the sample environment are presented. MiniMelt enables time-resolved investigations of the dynamic phenomena taking place during multi-layer PBF-EB, facilitating process understanding and development of advanced process strategies and materials for PBF-EB.
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| 17. |
- König, Hans-Henrik, et al.
(författare)
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Solidification modes during additive manufacturing of steel revealed by high-speed X-ray diffraction
- 2023
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 246, s. 118713-
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Tidskriftsartikel (refereegranskat)abstract
- Solidification during fusion-based additive manufacturing (AM) is characterized by high solidification velocities and large thermal gradients, two factors that control the solidification mode of metals and alloys. Using two synchrotron-based, in situ setups, we perform high-speed X-ray diffraction measurements to investigate the impact of the solidification velocities and thermal gradients on the solidification mode of a hot-work tool steel over a wide range of thermal conditions of relevance to AM of metals. The solidification mode of primary delta-ferrite is observed at a cooling rate of 2.12 x 104 K/s, and at a higher cooling rate of 1.5 x 106 K/s, delta-ferrite is sup-pressed, and primary austenite is observed. The experimental thermal conditions are evaluated and linked to a Kurz-Giovanola-Trivedi (KGT) based solidification model. The modelling results show that the predictions from the multicomponent KGT model agree with the experimental observations. This work highlights the role of in situ XRD measurements for a fundamental understanding of the microstructure evolution during AM and for vali-dation of computational thermodynamics and kinetics models, facilitating parameter and alloy development for AM processes.
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| 18. |
- Lindwall, Greta, et al.
(författare)
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Assessment and evaluation of mobilities for diffusion in the bcc Cr-Mo-Fe system
- 2012
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Ingår i: Journal of Phase Equilibria and Diffusion. - : ASM International. - 1547-7037 .- 1863-7345. ; 33:5, s. 375-389
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Tidskriftsartikel (refereegranskat)abstract
- An assessment of the diffusion mobility functions for diffusion in the bcc Cr-Fe-Mo system ispresented. The optimization of the mobility parameters is performed by utilizing experimentaldiffusion data available in literature. New diffusion data for the Mo diffusion is produced by adiffusion couple experiment and is accounted for in the optimization. Agreement between calculatedand measured diffusion coefficients is found. An M6C coarsening experiment is performedand the measured coarsening rate can be reproduced by diffusion calculation using the DICTRA software and the developed kinetic description. The mobility parameters are shown tohave a strong influence on the calculated coarsening rate.
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| 19. |
- Lindwall, Greta, et al.
(författare)
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Assessment and Evaluation of Mobilities for Diffusion in the bcc Cr- Mo-Fe System
- 2011
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Rapport (refereegranskat)abstract
- Assessments of diffusion mobility parameters are performed for diffusion in the bcc Mo-Cr-Fe system by taking available literature data into account. The Mo mobility is in main focus where, in addition to all binaries, ternary interaction parameters are assessed. A diffusion couple is produced and heat treated to cause elemental interdiffusion which is measured experimentally and compared with DICTRA calculations. The obtained experimental information is used for optimization of a mobility parameter. An experiment is also performed in order to study the coarsening of Mo rich M6C carbides embedded in a ferrite matrix, where the diffusion of Mo is believed to be of major importance. The measured coarsening rate is compared with the, by DICTRA, calculated rate showing improved agreement when the assessed kinetic description is applied. We judge the assessment of the bcc Mo- Fe-Cr system to be an important step towards a confident use of kinetic calculations as a tool for development of tool steel materials where Mo diffusion is of importance; for instance, when calculating secondary carbide precipitation during tempering of tool steels.
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| 20. |
- Lindwall, Greta, 1981-, et al.
(författare)
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Assessment and evaluation of mobilities for diffusion in the bcc Cr-V-Fe system
- 2009
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Ingår i: Journal of phase equilibria and diffusion. - : ASM International. - 1547-7037 .- 1863-7345. ; 30:4, s. 323-333
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Tidskriftsartikel (refereegranskat)abstract
- Assessments of diffusion mobility parameters are performed for the bcc Cr-V-Fe alloy systemby taking available literature data into account. The main focus is on the diffusion of V in thebcc phase where, in addition to all binaries, ternary interaction parameters are assessed. Anexperiment is performed in order to study the coarsening of V-rich MC carbide, where thediffusion of V is believed to be of major importance. The measured coarsening rate is comparedwith the rate calculated using DICTRA, and found to be in satisfactory agreement. Theaspects of coarsening experiments as a method to evaluate diffusion mobility data arediscussed.
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| 21. |
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| 22. |
- Lindwall, Greta, 1981-, et al.
(författare)
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Coarsening investigations of precipitates in PM tool steel grades - an effect of nitrogen
- 2012
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Ingår i: Berg- und Huttenmännische Monatshefte (BHM). - : Springer Science and Business Media LLC. - 0005-8912 .- 1613-7531. ; 157:11, s. 406-413
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Tidskriftsartikel (refereegranskat)abstract
- We present experimental and theoretical comparisons of the coarsening rate for the MX(X=C, N or C+N) precipitate in five different tool steel grades produced by PowderMetallurgy (PM). The alloys contain different amounts of nitrogen and carbon; ranging fromhigh carbon/low nitrogen to high nitrogen/low carbon. Common for the grades is a highamount of precipitate forming alloying elements resulting in a high fraction of hard phase.Experimentally we have found out that the nitrogen rich precipitates coarsen slower, both forhigher and lower carbon content, than the carbon rich precipitates. Coarsening simulations areperformed utilizing the DICTRA software and an agreement between calculations andmeasurements can be concluded.
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| 23. |
- Lindwall, Greta, et al.
(författare)
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Development of a Diffusion Mobility Database for Co-Based Superalloys
- 2022
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Ingår i: Journal of Phase Equilibria and Diffusion. - : Springer. - 1547-7037 .- 1863-7345. ; 43:6, s. 931-952
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Tidskriftsartikel (refereegranskat)abstract
- To facilitate the development of high-temperature Co-based γ-γ′ superalloys, a Co-Ni based diffusion mobility database is developed for the eight component FCC (Face Centered Cubic) system of Co-Al-W-Ni-Cr-Ti-Ta-Re. A CALPHAD approach is used to represent the temperature and composition dependency of the multicomponent system. The mobility descriptions were based on previous assessment work for the Ni-based superalloys, published experimental and computational data, and established diffusion correlations. The initial mobility descriptions were then refined using additional diffusion couple experimental data, particularly for the Co-Cr, Co-Ta, and Ni-Ta systems. After re-optimizing the descriptions with the new experimental data, the mobility descriptions were validated using a collection of published diffusion couple composition profiles, which were not included in the initial assessment process.
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| 24. |
- Lindwall, Greta, 1981-, et al.
(författare)
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Diffusion calculations as a tool for tool steel design
- 2009
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Ingår i: Proceedings of the 8th International Tooling Conference, RWTH Aachen, Aachen, Germany. ; , s. 1063-1070
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Konferensbidrag (refereegranskat)abstract
- Thermodynamic calculations have shown to be a powerful tool for predications of phasecompositions and phase fractions in steels. Coupling of thermodynamic and diffusion mobilitydatabases make diffusion calculations possible, and along with the accessibility of accurate anduser-friendly software on the market, the quality demands on kinetic data have increased.This work concerns development of multicomponent diffusion data for calculations applied todiffusion dependent processes, and focus is on alloy systems alloyed with Co. The possibility toevaluate diffusion data with the help of coarsening experiments and diffusion couples isinvestigated.
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| 25. |
- Lindwall, Greta, et al.
(författare)
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Diffusion in the Ti-Al-V System
- 2018
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Ingår i: JOURNAL OF PHASE EQUILIBRIA AND DIFFUSION. - : Springer. - 1547-7037 .- 1863-7345. ; 39:5, s. 731-746
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Tidskriftsartikel (refereegranskat)abstract
- Diffusion in the Ti-Al-V system is studied and a CALculation of Phase Diagrams (CALPHAD) diffusion mobility description is developed. Diffusion couple experiments are performed to obtain diffusion paths in the hcp phase at 923 K, 1023 K and 1123 K. The diffusion coefficient of V in the hcp-Ti phase is found to decrease with increasing Al alloying. A forward-simulation analysis is used to evaluate the impurity diffusion coefficient for Al and V diffusion in the hcp Ti-V and the Ti-Al systems which are used as input in the mobility modeling. The composition dependency for the diffusion in the hcp phase in the ternary system is accounted for and a CALPHAD diffusion mobility description is obtained by directly optimizing the mobility parameters as a function of the experimental composition profiles from the diffusion couples. Both experimental data and previous diffusion mobility descriptions in the literature for the bcc Ti-Al-V phase are adopted. A complete description of diffusion in both the hcp and bcc phases for the Ti-Al-V system is presented with the aim to be used for design of Ti alloys and processes.
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| 26. |
- Lindwall, Greta, 1981-, et al.
(författare)
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Experimental and theoretical investigations of hot isostatically pressed-produced stainless steel/high alloy tool steel compound materials
- 2010
-
Ingår i: Metallurgical and Materials Transactions. A. - : The Minerals, Metals, and Materials Society. - 1073-5623 .- 1543-1940. ; 42:5, s. 1165-1172
-
Tidskriftsartikel (refereegranskat)abstract
- Consolidation of tool steel powders and simultaneous joining to a stainless 316L steel areperformed by hot isostatic pressing (HIP). Two tool steel grades are considered: a high vanadiumalloyed carbon tool steel, and a high vanadium and chromium alloyed nitrogen tool steel.The boundary layer arising during diffusion bonding is in focus and, in particular, the diffusionof carbon and nitrogen over the joint.Measurements of the elemental concentration profiles andcorrosion tests by the double loop–electrochemical potentiokinetic reactivation (DL-EPR)method are performed. Comparative calculations with the DICTRA software are performedand are found to be in agreement with the experimental results. It is found that the carbon toolsteel grade has a more critical influence on the corrosion resistance of the stainless 316L steel incomparison to the nitrogen tool steel grade.
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| 27. |
- Lindwall, Greta
(författare)
-
Multicomponent diffusional reactions in tool steels : Experiment and Theory
- 2012
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Many phenomena determining the microstructure of a tool steel and consequently the properties of the material, are governed by multicomponent diffusion. The diffusion driven reactions that take place during, for example, tempering of a hot-work tool steel or when the microstructure develops during hot isostatic pressing of cold-work tool steel, are dependent on the types and amounts of alloying elements. In order for computational methods to be usable, these alloying effects need to be understood and incorporated in the models.In this work the influence of some typical tool steel alloying elements on the coarsening behavior of precipitates is investigated. Experimental coarsening studies are performed and the impact of the diffusion mobility descriptions and the thermodynamic descriptions are investigated by means of DICTRA coarsening calculations. The kinetic descriptions for diffusion in the body centered-cubic phase in the case of the chromium-iron-vanadium system and the chromium-iron-molybdenum system are improved by assessments of diffusion mobility parameters, and are shown to have a large impact on the calculated coarsening rate for vanadium-rich and molybdenum-rich precipitates.The effect of cobalt is examined by a coarsening experiment for vanandiumrich carbides and by a diffusion couple experiment for the investigation of the vanadium interdiffusion. The presence of cobalt is experimentally shown to have retarding effect on the coarsening rate of the carbides, but not on the vanadium diffusion.The coarsening rate of nitrogen-rich precipitates is compared to the coarsening rate of carbon-rich precipitates, and a lower coarsening rate for nitrides compared to carbides can be confirmed. Correlation between coarsening calculations and experiments is obtained suggesting that the thermodynamic description of the two systems is the underlaying reason for the different coarsening rates.Further, calculations utilizing the DICTRA software are combined with experimental investigations in order to study the possibility to apply computational methods for compound material development and explore application areas for high nitrogen alloyed tool steels produced by powder metallurgy.
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| 28. |
- Lindwall, Greta, et al.
(författare)
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Simulation of TTT Curves for Additively Manufactured Inconel 625
- 2019
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Ingår i: Metallurgical and Materials Transactions. A. - : SPRINGER. - 1073-5623 .- 1543-1940. ; 50A:1, s. 457-467
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Tidskriftsartikel (refereegranskat)abstract
- The ability to use common computational thermodynamic and kinetic tools to study the microstructure evolution in Inconel 625 (IN625) manufactured using the additive manufacturing (AM) technique of laser powder-bed fusion is evaluated. Solidification simulations indicate that laser melting and re-melting during printing produce highly segregated interdendritic regions. Precipitation simulations for different degrees of segregation show that the larger the segregation, i.e., the richer the interdendritic regions are in Nb and Mo, the faster the -phase (Ni3Nb) precipitation. This is in accordance with the accelerated precipitation observed experimentally during post-build heat treatments of AM IN625 compared to wrought IN625. The -phase may be undesirable since it can lead to detrimental effects on the mechanical properties. The results are presented in the form of a TTT diagram and agreement between the simulated diagram and the experimental TTT diagram demonstrate how these computational tools can be used to guide and optimize post-build treatments of AM materials.
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| 29. |
- Lindwall, Greta, 1981-, et al.
(författare)
-
The effect of nitrogen on the coarsening rate of precipitate phases in iron-based alloys with chromium and vanadium: experimental and theoretical investigations
- 2013
-
Ingår i: International Journal of Materials Research - Zeitschrift für Metallkunde. - : Carl Hanser Verlag. - 1862-5282 .- 2195-8556. ; 105:5, s. 442-451
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Tidskriftsartikel (refereegranskat)abstract
- A comparison of the coarsening of nitrogen-rich vanadium precipitates and the coarsening ofcarbon-rich vanadium precipitates is presented. The precipitate phases are studied experimentally,via fabrication of model alloys, and theoretically, via simulations utilizing the DICTRA software.The experimental investigations indicate that the nitrogen-rich precipitates exhibit a slowercoarsening behaviour than the carbon-rich precipitates. Analysis using thermodynamic andkinetic modelling shows that this can be explained by the higher thermodynamic stability of thenitrogen-rich precipitate compared to the carbon-rich precipitate. The calculated coarsening ratesare compared with the measured rates, and found to be in satisfactory agreement using reasonablevalues for the interfacial energies. The investigations are motivated by the ne precipitate sizedistribution of nitrides and carbonitrides characteristic for high nitrogen alloyed tool steels producedby means of powder metallurgy.
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| 30. |
- Lindwall, Greta, et al.
(författare)
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The Effect of Oxygen on Phase Equilibria in the Ti-V System : Impacts on the AM Processing of Ti Alloys
- 2018
-
Ingår i: JOM. - : Minerals, Metals and Materials Society. - 1047-4838 .- 1543-1851. ; 70:9, s. 1692-1705
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Tidskriftsartikel (refereegranskat)abstract
- Oxygen is always a constituent in “real” titanium alloys including titanium alloy powders used for powder-based additive manufacturing (AM). In addition, oxygen uptake during powder handling and printing is hard to control and, hence, it is important to understand and predict how oxygen is affecting the microstructure. Therefore, oxygen is included in the evaluation of the thermodynamic properties of the titanium-vanadium system employing the CALculation of PHAse Diagrams method and a complete model of the O-Ti-V system is presented. The β-transus temperature is calculated to increase with increasing oxygen content whereas the extension of the α-Ti phase field into the binary is calculated to decrease, which explains the low vanadium solubilities measured in some experimental works. In addition, the critical temperature of the metastable miscibility gap of the β-phase is calculated to increase to above room temperature when oxygen is added. The effects of oxygen additions on phase fractions, martensite and ω formation temperatures are discussed, along with the impacts these changes may have on AM of titanium alloys.
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| 31. |
- Lindwall, Greta, et al.
(författare)
-
Zero-phonon linewidth and phonon satellites in the optical absorption of nanowire-based quantum dots
- 2007
-
Ingår i: Physical Review Letters. - 1079-7114. ; 99:8, s. 1-087401
-
Tidskriftsartikel (refereegranskat)abstract
- The optical properties of quantum dots embedded in a catalytically grown semiconductor nanowire are studied theoretically. In comparison to dots in a bulk environment, the excitonic absorption is strongly modified by the one-dimensional character of the nanowire phonon spectrum. In addition to pronounced satellite peaks due to phonon-assisted absorption, we find a finite width of the zero-phonon line already in the lowest-order calculation.
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| 32. |
- Liu, Xuan L., et al.
(författare)
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First-Principles Calculations, Experimental Study, and Thermodynamic Modeling of the Al-Co-Cr System
- 2015
-
Ingår i: PLOS ONE. - : Public Library of Science (PLoS). - 1932-6203. ; 10:4
-
Tidskriftsartikel (refereegranskat)abstract
- The phase relations and thermodynamic properties of the condensed Al-Co-Cr ternary alloy system are investigated using first-principles calculations based on density functional theory (DFT) and phase-equilibria experiments that led to X-ray diffraction (XRD) and electron probe micro-analysis (EPMA) measurements. A thermodynamic description is developed by means of the calculations of phase diagrams (CALPHAD) method using experimental and computational data from the present work and the literature. Emphasis is placed on modeling the bcc-A2, B2, fcc-gamma, and tetragonal-sigma phases in the temperature range of 1173 to 1623 K. Liquid, bcc-A2 and fcc-gamma phases are modeled using substitutional solution descriptions. First-principles special quasirandom structures (SQS) calculations predict a large bcc-A2 (disordered)/B2 (ordered) miscibility gap, in agreement with experiments. A partitioning model is then used for the A2/B2 phase to effectively describe the order-disorder transitions. The critically assessed thermodynamic description describes all phase equilibria data well. A2/B2 transitions are also shown to agree well with previous experimental findings.
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| 33. |
- Malladi, Bala, 1993, et al.
(författare)
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Single track versus bulk samples : Understanding the grain refinement in inoculated ferritic stainless steels manufactured by powder bed fusion-laser beam
- 2023
-
Ingår i: Materialia. - : Elsevier BV. - 2589-1529. ; 32
-
Tidskriftsartikel (refereegranskat)abstract
- One generic challenge in powder bed fusion - laser beam (PBF-LB) is the formation of epitaxially grown columnar grains, which lead to the undesirable anisotropy of mechanical properties. This anisotropy could be rectified by ex-situ or in-situ inoculation in some particular alloy systems. Understanding the grain refinement mechanism caused by in-situ inoculation is, however, complicated by remelting caused by the overlapping between neighboring scan tracks, when printing bulk samples using multiple tracks. Here in this work, a series of single tracks using ferritic stainless steels feedstock powder with and without pre-alloyed inoculant-forming elements, were printed at different scanning speeds to gain refreshed understanding on the mechanism of the observed grain refinement. Interestingly, the grain refinement in single tracks and bulk samples printed from the powder with and without inoculant-forming elements showed an opposite tendency. When using the powder without inoculant-forming elements, the single tracks showed large columnar grains, while the bulk samples showed even larger grain sizes; when using the powder with pre-alloyed inoculant-forming elements, fine equiaxed grains are found at the centers of the melt pools, surrounded by slightly coarser columnar grains at melt pool boundaries, in both single tracks and bulk samples. Noticeably, the mean grain sizes in the bulk samples are however smaller compared to those for single tracks because of remelting. Our work provides new insights on the grain refinement via in-situ inoculation during the PBF-LB process and highlights the importance of studying single tracks to better understand the melting and solidification behavior.
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| 34. |
- Moustafa, Abdel R., et al.
(författare)
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Scheil ternary projection (STeP) diagrams for designing additively manufactured functionally graded metals
- 2020
-
Ingår i: Additive Manufacturing. - : Elsevier BV. - 2214-8604 .- 2214-7810. ; 32
-
Tidskriftsartikel (refereegranskat)abstract
- Functionally graded metals fabricated using high-temperature additive manufacturing can form intermetallics that fracture during printing due to thermal stresses generated by the heat source. To address this problem, we introduce a new class of non-equilibrium phase diagrams, termed Scheil Ternary Projection (STeP) diagrams, for designing optimal composition gradients that avoid brittle phases. Using the Fe-Cr-Al ternary as a model system, we compare the phase fields in equilibrium and STeP diagrams to show that intermetallic phase fields are dramatically expanded under the rapid solidification conditions in melt-based additive manufacturing, an important effect that must be accounted for when designing composition gradients.
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| 35. |
- Nilsson Åhman, Hanna, et al.
(författare)
-
An Enhanced Understanding of the Powder Bed Fusion-Laser Beam Processing of Mg-Y-3.9wt%-Nd-3wt%-Zr-0.5wt% (WE43) Alloy through Thermodynamic Modeling and Experimental Characterization
- 2022
-
Ingår i: Materials. - : MDPI AG. - 1996-1944. ; 15:2
-
Tidskriftsartikel (refereegranskat)abstract
- Powder Bed Fusion-Laser Beam (PBF-LB) processing of magnesium (Mg) alloys is gaining increasing attention due to the possibility of producing complex biodegradable implants for improved healing of large bone defects. However, the understanding of the correlation between the PBF-LB process parameters and the microstructure formed in Mg alloys remains limited. Thus, the purpose of this study was to enhance the understanding of the effect of the PBF-LB process parameters on the microstructure of Mg alloys by investigating the applicability of computational thermodynamic modelling and verifying the results experimentally. Thus, PBF-LB process parameters were optimized for a Mg WE43 alloy (Mg-Y-3.(9wt%)-Nd-3wt%-Zr-0.5wt%) on a commercially available machine. Two sets of process parameters successfully produced sample densities >99.4%. Thermodynamic computations based on the Calphad method were employed to predict the phases present in the processed material. Phases experimentally established for both processing parameters included alpha-Mg, Y2O3, Mg3Nd, Mg24Y5 and hcp-Zr. Phases alpha-Mg, Mg24Y5 and hcp-Zr were also predicted by the calculations. In conclusion, the extent of the applicability of thermodynamic modeling was shown, and the understanding of the correlation between the PBF-LB process parameters and the formed microstructure was enhanced, thus increasing the viability of the PBF-LB process for Mg alloys.
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| 36. |
- Nilsson Åhman, Hanna, et al.
(författare)
-
Computational and Experimental Microstructural Characterization of A Magnesium WE43 Alloy Processed on A Commercially Available PBF-LB Machine
- 2020
-
Ingår i: Proceedings - Euro PM2020 Congress and Exhibition. - : European Powder Metallurgy Association (EPMA).
-
Konferensbidrag (refereegranskat)abstract
- Magnesium (Mg) has gained a lot of attention for its biodegradable properties. Additive manufacturing of Mg would further provide an opportunity to manufacture optimized structures for implants. However, much work remains to ensure stable processes and to understand the correlation between processing parameters, microstructure and related properties. The aim of this study was to process gas-atomized WE43 powder by Powder Bed Fusion-Laser Beam (PBF-LB) in a commercially available machine (EOS M290). The built material was characterized in terms of microstructure and composition and compared with the virgin powder. Thermodynamic computations based on the Calphad-method were for the first time employed to an additively manufactured Mg-alloy to predict phases present. Samples with above 99% density were successfully manufactured. The phases predicted using computational thermodynamics partly overlapped with phases observed experimentally. The study confirms the feasibility of using commercial PBF-LB systems for processing Mg-alloys, and the applicability of thermodynamic modelling to this system.
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| 37. |
- Pacheco, Victor, et al.
(författare)
-
Thermal Stability of the HfNbTiVZr High-Entropy Alloy
- 2019
-
Ingår i: Inorganic Chemistry. - : AMER CHEMICAL SOC. - 0020-1669 .- 1520-510X. ; 58:1, s. 811-820
-
Tidskriftsartikel (refereegranskat)abstract
- The multicomponent alloy HfNbTiVZr has been described as a single-phase high-entropy alloy (HEA) in the literature, although some authors have reported that additional phases can form during annealing. The thermal stability of this alloy has therefore been investigated with a combination of experimental annealing studies and thermodynamic calculations using the CALPHAD approach. The thermodynamic calculations show that a single-phase HEA is stable above about 830 degrees C. At lower temperatures, the most stable state is a phase mixture of bcc, hcp, and a cubic C15 Laves phase. Annealing experiments followed by quenching confirm the results from thermodynamic calculations with the exception of the Laves phase structure, which was identified as a hexagonal C14 type instead of the cubic C15 type. Limitations of the applied CALPHAD thermodynamic description of the system could be an explanation for this discrepancy. As-synthesized HfNbTiVZr alloys prepared by arc-melting form a single-phase bcc HEA at room temperature. In situ annealing studies of this alloy show that additional phases start to form above 600 degrees C. This indicates that the observed HEA is metastable at room temperature and stabilized by a slow kinetics during cooling. X-ray diffraction analyses using different cooling rates and annealing times show that the phase transformations in this HEA are slow and that completely different phase compositions can be obtained depending on the annealing procedure. In addition, it has been shown that the sample preparation method (mortar grinding, heat treatment, etc.) has a significant influence on the collected diffraction patterns and therefore on the phase identification and analysis.
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| 38. |
- Reddy, Vasudeva Reddy Minnam, et al.
(författare)
-
alpha-SnSe thin film solar cells produced by selenization of magnetron sputtered tin precursors
- 2018
-
Ingår i: Solar Energy Materials and Solar Cells. - : Elsevier. - 0927-0248 .- 1879-3398. ; 176, s. 251-258
-
Tidskriftsartikel (refereegranskat)abstract
- The temperature-pressure-composition phase diagrams of Sn-Se system were calculated using the CALPHAD (CALculation of PHase Diagram) models. The phase diagrams showed the formation of alpha-SnSe phase at selenium-rich side with pressures lower than atmospheric pressure and in the temperature range of 300-500 degrees C. As a first step, the effect of Sn/Se ratio on the phase formation was studied experimentally by selenization of tin metal precursor films using effusion cell evaporation. The Sn/Se ratio was varied by changing the selenium weight in the range of 0.5-1.5 g. The physical properties of the films were studied with suitable characterization techniques and the obtained results showed the formation of single phase alpha-SnSe at 1.0 g of selenium. Further, alpha-SnSe/CdS interface was studied by photoelectron yield spectroscopy (PYS), which showed a 'type-I' band alignment with a valence-band offset (Delta E-v) of 1.3 eV and a conduction-band offset (Delta E-c) of 0.2 eV. Finally, alpha-SnSe solar cells with a device structure of soda-lime glass (SLG)/Mo/alpha-SnSe/CdS/i-ZnO/Al:ZnO/Ni/Ag were fabricated and a power conversation efficiency of 1.42% was achieved at 1.0 g of selenium.
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| 39. |
- Ross, A. J., et al.
(författare)
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A first-principles based description of the Hf-Ni system supported by high-temperature synchrotron experiments
- 2018
-
Ingår i: Thermochimica Acta. - : Elsevier B.V.. - 0040-6031 .- 1872-762X. ; 668, s. 142-151
-
Tidskriftsartikel (refereegranskat)abstract
- Hf-Ni is an important binary system for high temperature alloys and shape memory alloys which has been investigated several times in the literature but often using samples of Hf contaminated by Zr. The thermodynamics of this system are remodeled in this work based on first-principles calculations and additional experiments using Hf with relatively low Zr contamination (0.25 wt. %). Diffusion couples in the Ni-rich portion of the Hf-Ni system heat treated at 1173, 1273 and 1373 K are used to measure phase stability and Hf solubility in the fcc phase. The solubility observed in fcc Ni from Ni/Ni50Hf50 (at.%) diffusion couples is larger than that observed in previous experiments. These results are the only source fit to during modeling of the fcc solubility to mitigate effects from Zr contamination. Data in the literature suggests that the high temperature crystal structure of the B33 NiHf phase is, in fact, the B2 structure. High temperature synchrotron measurements provide confirmation of this crystal structure. Modeling of the B2 phase was aided by first-principles calculations using special quasi-random structures (SQS). The present CALPHAD model will prove useful when designing shape memory alloys containing Hf and when modeling the Hf activity in Ni-base high temperature alloys.
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| 40. |
- Ross, A., et al.
(författare)
-
Tailoring critical Al concentration to form external Al2O3 scale on Ni–Al alloys by computational approach
- 2022
-
Ingår i: Journal of The American Ceramic Society. - : Wiley. - 0002-7820 .- 1551-2916. ; 105:12, s. 7770-7777
-
Tidskriftsartikel (refereegranskat)abstract
- Nickel (Ni)-based superalloys for high-temperature applications are often designed to form a continuous and slow-growing oxide scale by adding Al and Cr and other beneficial elements. In the present work, the critical Al concentration in Ni–Al alloys needed to establish an α-Al2O3 scale in contrast to internal oxide formation is predicted as a function of temperature by means of the CALPHAD approach coupled with models in the literature, which account for the thermodynamics and kinetics of oxidation. The present thermodynamic remodeling of the Ni–O system results in a better agreement with experimental data of oxygen solubility in Ni at high temperatures. The oxygen solubility is combined with kinetic parameters to determine oxygen permeability in Ni, and the critical Al concentration needed to establish an α-Al2O3 scale at a given exposure temperature. Good agreement is found with available experimental data for both oxygen permeability and critical Al concentration, indicating the capacity of the CALPHAD approach to tailor oxidation resistance for materials of interest using thermodynamic and kinetic knowledge.
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| 41. |
- Sjöstrom, Julia, et al.
(författare)
-
Linkage of Macro- and Microscale Modeling Tools for Additive Manufacturing of Steels
- 2022
-
Ingår i: FRONTIERS IN MATERIALS. - : Frontiers Media SA. - 2296-8016. ; 9
-
Tidskriftsartikel (refereegranskat)abstract
- Additive manufacturing (AM) offers several benefits including the capability to produce unique microstructures, geometrical freedom allowing for material and energy savings, and easy production lines with fewer post-processing steps. However, AM processes are complex and phenomena occurring at different length and time scales need to be understood and controlled to avoid challenges with, for example, defects, residual stresses, distortions, and alloy restrictions. To overcome some of these challenges and to have more control over the final product, computational tools for different length scales need to be combined. In this work, an 18Ni300 maraging steel part is studied to understand the link between the process parameters and the as-built microstructure. The temperature evolution during laser powder bed fusion is simulated using the MSC simulation software Simufact Additive. This result is then linked to microscale models within the Thermo-Calc software package to predict the elemental micro-segregation, martensite start (M-s) temperature, and martensite fraction. The different values of the key process parameters such as laser speed, laser power, heating efficiency, and baseplate temperature are considered, leading to different thermal histories. The thermal histories affect the elemental segregation across the solidification structure, which in turn results in different M-s temperatures at different locations of the built part. It is found that higher laser energy generally causes higher temperatures and higher cooling rates, which results in a larger degree of elemental segregation and lower M-s temperatures in segregated regions. Furthermore, the segregated regions are predicted to have M-s temperatures below 200 degrees C, which would result in retained austenite when using a baseplate temperature of 200 degrees C. On the other hand, by using a baseplate temperature of 100 degrees C, all regions would reach temperatures below the M-s temperature, and an almost fully martensitic structure would be possible. In summary, it is demonstrated how the linkage of macro- and microscale modeling tools for AM can be used to optimize the process and produce the desired microstructure, thereby achieving the desired mechanical properties.
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| 42. |
- Stoudt, M. R., et al.
(författare)
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The Influence of Annealing Temperature and Time on the Formation of delta-Phase in Additively-Manufactured Inconel 625
- 2018
-
Ingår i: Metallurgical and Materials Transactions. A. - : SPRINGER. - 1073-5623 .- 1543-1940. ; 49A:7, s. 3028-3037
-
Tidskriftsartikel (refereegranskat)abstract
- This research evaluated the kinetics of delta-phase growth in laser powder bed additively-manufactured (AM) Inconel 625 during post-build stress-relief heat treatments. The temperatures ranged between 650 A degrees C and 1050 A degrees C, and the times from 0.25 to 168 hours. The presence of delta-phase was verified for each temperature/time combination through multiple techniques. A conventional time-temperature-transformation diagram was constructed from the time-temperature data. Comparison to the growth in wrought IN625 with a similar nominal composition revealed that delta-phase formation occurred at least two orders of magnitude faster in the AM IN625. The results of this study also revealed that the segregated microstructure in the as-built condition has a strong influence on the kinetics of delta-phase formation in AM IN625 as compared to a homogenized material. Since control of the delta-phase growth is essential for reliable prediction of the performance of IN625 components in service, avoiding heat treatments that promote the formation of delta-phase in AM components that are not homogenized is highly recommended. This will be particularly true at elevated temperatures where the microstructural stability and the consistency of mechanical properties are more likely to be affected by the presence of delta-phase.
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| 43. |
- Sullivan, Ethan M., et al.
(författare)
-
Effect of powder particle size distribution and contouring on build quality in electron beam powder bed fusion of a medium-C hot-work tool steel
- 2023
-
Ingår i: The International Journal of Advanced Manufacturing Technology. - : Springer Nature. - 0268-3768 .- 1433-3015. ; 128:7-8, s. 2953-2967
-
Tidskriftsartikel (refereegranskat)abstract
- An electron beam powder bed fusion (EPBF) printability study of a medium-C hot-work tool steel with focus on part density and surface roughness was performed using three different powder particle size distributions (PSDs) of 45–105 μm (typical for EPBF), 20–60 μm (typical for laser beam powder bed fusion), and a 50–50 wt.% mixture of these two powders. First, acceptable process parameter windows were generated based on as-printed density for each PSD. Full density parts (at least 99.5% dense according to NIST) were produced using the 20–60 μm PSD and the mix PSD. Fifteen different contouring strategies were also tested for potential improvement of the as-printed side surface roughnesses, which ranged from 23.3 to 25.7 μm among the three PSDs. Side surface roughness as low as 13.8 μm was attained by using contouring strategies employing two contouring lines, which were typically observed to be more effective than one-line strategies. Overall, the 20–60 μm PSD was determined to convey a better as-printed build quality over a wider range of parameters without sacrificing process productivity.
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| 44. |
- Sun, Xiaoyu
(författare)
-
Performance of High-temperature Coatings : Oxidation and Interdiffusion
- 2023
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The use of aluminiferous coatings profoundly improves the service life of superalloys but leads to the microstructural degradation of superalloys and thus the loss of mechanical properties. To solve this trade-off, two strategies were employed in this research.At first, we modified MCrAlY coatings by inducing Ta to reduce the interdiffusion effect on substrate alloys. This strategy was verified by 2000 h/1100 °C oxidation tests in two Ta-containing MCrAlY-IN792 systems. The system with 3.3 wt.% Ta MCrAlY displays an outstanding resistance to γ′ depletion in the substrate and comparable oxidation property in comparison with a reference system of Ta-free MCrAlY-IN792. Increasing Ta to 7.4 wt.% results in reduced oxidation resistance. Thermodynamic simulations revealed the phase-transformation mechanism induced by initial interdiffusion, uncovering the cause of γ′ depletion in the substrate and the mechanism behind improving resistance to γ′ depletion by Ta addition.In addition, we developed novel Y-doped AlCoCrFeNi high-entropy alloys by tuning Al/Cr ratio ACR. After a long-term isothermal test in air at 1100 °C up to 1000 h, the higher ACR alloy displayed a stronger oxidation resistance at the early oxidation stage, whereas a contrary result could be detected in the later stage. The microstructural analysis confirmed that the fast growth of spinel dominated the early oxidation process, leading to higher oxidation rate of the lower ACR alloys. The later stage was governed by the growth of Al2O3. Lager size Al2O3 gain formed on the lower ACR alloy impeded the inward diffusion of oxygen and thus reduced the oxidation rate, which was further verified by our thermodynamic calculations.
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| 45. |
- Tan, Qiyang, et al.
(författare)
-
Unravelling the roles of TiN-nanoparticle inoculant in additively manufactured 316 stainless steel
- 2024
-
Ingår i: Journal of Materials Science & Technology. - : Elsevier BV. - 1005-0302. ; 175, s. 153-169
-
Tidskriftsartikel (refereegranskat)abstract
- As a potent grain refiner for steel casting, TiN is now widely used to refine γ-austenite in steel additive manufacturing (AM). However, the refining mechanism of TiN during AM remains unclear despite intensive research in recent years. This work aims to boost our understanding on the mechanism of TiN in refining the γ-austenite in AM-fabricated 316 stainless steel and its corresponding effect on the mechanical behaviour. Experimental results show that addition of 1 wt.% TiN nanoparticles led to complete columnar-to-equiaxed transition and significant refinement of the austenite grains to ∼2 µm in the 316 steel. Thermodynamic and kinetic simulations confirmed that, despite the rapid AM solidification, δ-ferrite is the primary solid phase during AM of the 316 steel and γ-austenite forms through subsequent peritectic reaction or direct transformation from the δ-ferrite. This implies that the TiN nanoparticles actually refined the δ-ferrite through promoting its heterogenous nucleation, which in turn refined the γ-austenite. This assumption is verified by the high grain refining efficiency of TiN nanoparticles in an AM-fabricated Fe-4 wt.%Si δ-ferrite alloy, in which δ-ferrite forms directly from the melt and is retained at room temperature. The grain refinement is attributed to the good atomic matching between δ-ferrite and TiN. Grain refinement in the 316 steel through 1 wt.% TiN inoculation not only eliminated the property anisotropy but also led to a high strain-hardening rate upon plastic deformation and thereby a superior strength-ductility synergy with yield strength of 561 MPa, tensile strength of 860 MPa and elongation of 48%.
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| 46. |
- Ward, Austin A., et al.
(författare)
-
Ultrasonic additive manufacturing of nanocrystalline laminated composites
- 2022
-
Ingår i: Journal of Materials Research. - : Springer Nature. - 0884-2914 .- 2044-5326. ; 37:1, s. 247-258
-
Tidskriftsartikel (refereegranskat)abstract
- Ultrasonic additive manufacturing has been used to fabricate laminated composites of commercially pure aluminum and a nanocrystalline nickel-cobalt (nc-NiCo) alloy. The nc-NiCo alloy would not weld to itself but readily welded to aluminum. Thus, by alternating between foils of nc-NiCo and Al, we achieved multi-material laminates with strong interlayer bonding. Electron microscopy showed that the nanoscale grain structure of the nc-NiCo was preserved during deposition and that the nc-NiCo/Al weld interface was decorated with comminuted surface oxides as well as Al-Ni-Co intermetallics.These findings are considered in light of process models of junction growth, interdiffusion, and grain growth, which together reveal how the different pressure- and temperature dependences of these phenomena give rise to a range of processing conditions that maximize bonding while minimizing coarsening and intermetallic formation. This analysis quantitatively demonstrates that using a soft, low melting point interlayer material decouples junction growth at the weld interface from grain growth in the nc-NiCo, expanding the range of optimal processing conditions.
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| 47. |
- Weber, Carsten, et al.
(författare)
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Probing Confined Phonon Modes by Transport through a Nanowire Double Quantum Dot
- 2010
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Ingår i: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 104:3, s. 1-036801
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Tidskriftsartikel (refereegranskat)abstract
- Strong radial confinement in semiconductor nanowires leads to modified electronic and phononic energy spectra. We analyze the current response to the interplay between quantum confinement effects of the electron and phonon systems in a gate-defined double quantum dot in a semiconductor nanowire. We show that current spectroscopy of inelastic transitions between the two quantum dots can be used as an experimental probe of the confined phonon environment. The resulting discrete peak structure in the measurements is explained by theoretical modeling of the confined phonon mode spectrum, where the piezoelectric coupling is of crucial importance.
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| 48. |
- Weber, Carsten, et al.
(författare)
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Zero-phonon line broadening and satellite peaks in nanowire quantum dots: The role of piezoelectric coupling
- 2009
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Ingår i: Physica Status Solidi. B: Basic Research. - : Wiley. - 0370-1972 .- 1521-3951. ; 246:2, s. 337-341
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the influence of the one-dimensional character of the phonon modes in a catalytically grown GaAs nanowire on the absorption spectrum of an embedded quantum dot, focusing on the on the contribution from the piezoelectric coupling. In general, the reduced dimensionally of the phonons leads to spectral side peaks and a zero-phonon line broadening due to the energetically lowest (acoustic) phonon mode. While the deformation potential predominantly couples to radial modes, the piezoelectric interaction can also couple strongly to modes of axial character, leading to additional absorption features. The contribution of the piezoelectric coupling to the zero-phonon line is negligible. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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| 49. |
- Ye, Jihui, et al.
(författare)
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Revealing the Mechanisms of Smoke during Electron Beam-Powder Bed Fusion by High-Speed Synchrotron Radiography
- 2024
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Ingår i: Journal of Manufacturing and Materials Processing. - : MDPI AG. - 2504-4494. ; 8:3
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Tidskriftsartikel (refereegranskat)abstract
- Electron beam-powder bed fusion (PBF-EB) is an additive manufacturing process that utilizes an electron beam as the heat source to enable material fusion. However, the use of a charge-carrying heat source can sometimes result in sudden powder explosions, usually referred to as "Smoke", which can lead to process instability or termination. This experimental study investigated the initiation and propagation of Smoke using in situ high-speed synchrotron radiography. The results reveal two key mechanisms for Smoke evolution. In the first step, the beam-powder bed interaction creates electrically isolated particles in the atmosphere. Subsequently, these isolated particles get charged either by direct irradiation by the beam or indirectly by back-scattered electrons. These particles are accelerated by electric repulsion, and new particles in the atmosphere are produced when they impinge on the powder bed. This is the onset of the avalanche process known as Smoke. Based on this understanding, the dependence of Smoke on process parameters such as beam returning time, beam diameter, etc., can be rationalized.
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| 50. |
- Zendejas Medina, León, et al.
(författare)
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Enhancing corrosion resistance, hardness, and crack resistance in magnetron sputtered high entropy CoCrFeMnNi coatings by adding carbon
- 2021
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Ingår i: Materials & design. - : Elsevier BV. - 0264-1275 .- 1873-4197. ; 205
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Tidskriftsartikel (refereegranskat)abstract
- This study explores carbon addition as a materials design approach for simultaneously improving the hardness, crack resistance, and corrosion resistance of high entropy thin films. CoCrFeMnNi was selected as a starting point, due to its high concentration of weak carbide formers. The suppression of carbides is crucial to the approach, as carbide formation can decrease both ductility and corrosion resistance. Films with 0, 6, and 11 at.% C were deposited by magnetron co-sputtering, using a graphite target and a sintered compound target. The samples with 0 at.% C crystallized with a mixture of a cubic closed packed (ccp) phase and the intermetallic chi-phase. With 6 and 11 at.% C, the films were amorphous and homogenous down to the nm-scale. The hardness of the films increased from 8 GPa in the carbon-free film to 16 GPa in the film with 11 at.% C. Furthermore, the carbon significantly improved the crack resistance as shown in fragmentation tests, where the crack density was strongly reduced. The changes in mechanical properties were primarily attributed to the shift from crystalline to amorphous. Lastly, the carbon improved the corrosion resistance by a progressive lowering of the corrosion current and the passive current with increasing carbon concentration.
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