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Subasic, M., Olsson, M., Dadbakhsh, S., Zhao, X., Krakhmalev, P. & Mansour, R. (2024). Fatigue strength improvement of additively manufactured 316L stainless steel with high porosity through preloading. International Journal of Fatigue, 180, Article ID 108077.
Open this publication in new window or tab >>Fatigue strength improvement of additively manufactured 316L stainless steel with high porosity through preloading
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2024 (English)In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 180, article id 108077Article in journal (Refereed) Published
Abstract [en]

This work investigates the influence of a single tensile preload, applied prior to fatigue testing, on the fatigue strength of 316L stainless steel parts manufactured using laser-based powder bed fusion (PBF-LB) with a porosity of up to 4 %. The specimens were produced in both the horizontal and vertical build directions and were optionally preloaded to 85 % and 110 % of the yield strength before conducting the fatigue tests. The results indicate a clear tendency of improved fatigue life and fatigue limit with increasing overload in both cases. The fatigue limits increased by 25.8 % and 24.6 % for the horizontally and vertically built specimens, respectively. Extensive modelling and experiments confirmed that there was no significant alteration in the shape and size of the porosity before and after preloading. Therefore, the observed enhancement in fatigue performance was primarily attributed to the imposed local compressive residual stresses around the defects.

Place, publisher, year, edition, pages
Elsevier BV, 2024
Keywords
316L stainless steel, Defects, Fatigue strength, Overload, PBF-LB, Porosity, Preload
National Category
Applied Mechanics
Identifiers
urn:nbn:se:kth:diva-342189 (URN)10.1016/j.ijfatigue.2023.108077 (DOI)001174246000001 ()2-s2.0-85181121906 (Scopus ID)
Note

QC 20240503

Available from: 2024-01-15 Created: 2024-01-15 Last updated: 2024-05-03Bibliographically approved
Zhao, X., Wei, Y., Mansour, R., Dadbakhsh, S. & Rashid, A. (2023). Effect of Scanning Strategy on Thermal Stresses and Strains during Electron Beam Melting of Inconel 625: Experiment and Simulation. Materials, 16(1), Article ID 443.
Open this publication in new window or tab >>Effect of Scanning Strategy on Thermal Stresses and Strains during Electron Beam Melting of Inconel 625: Experiment and Simulation
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2023 (English)In: Materials, E-ISSN 1996-1944, Vol. 16, no 1, article id 443Article in journal (Refereed) Published
Abstract [en]

This paper develops a hybrid experimental/simulation method for the first time to assess the thermal stresses generated during electron beam melting (EBM) at high temperatures. The bending and rupture of trusses supporting Inconel 625 alloy panels at similar to 1050 degrees C are experimentally measured for various scanning strategies. The generated thermal stresses and strains are thereafter simulated using the Finite-Element Method (FEM). It is shown that the thermal stresses on the trusses may reach the material UTS without causing failure. Failure is only reached after the part experiences a certain magnitude of plastic strain (similar to 0.33 +/- 0.01 here). As the most influential factor, the plastic strain increases with the scanning length. In addition, it is shown that continuous scanning is necessary since the interrupted chessboard strategy induces cracking at the overlapping regions. Therefore, the associated thermal deformation is to be minimized using a proper layer rotation according to the part length. Although this is similar to the literature reported for selective laser melting (SLM), the effect of scanning pattern is found to differ, as no significant difference in thermal stresses/strains is observed between bidirectional and unidirectional patterns from EBM.

Place, publisher, year, edition, pages
MDPI AG, 2023
Keywords
thermal distortion, scanning strategy, electron beam melting (EBM), additive manufacturing simulation
National Category
Manufacturing, Surface and Joining Technology
Identifiers
urn:nbn:se:kth:diva-323428 (URN)10.3390/ma16010443 (DOI)000908817600001 ()36614787 (PubMedID)2-s2.0-85145774044 (Scopus ID)
Note

QC 20230201

Available from: 2023-02-01 Created: 2023-02-01 Last updated: 2024-03-18Bibliographically approved
Zeyu, L., Dadbakhsh, S. & Rashid, A. (2023). Increasing precision towards NiTi lattice structure using PBF-EB. In: : . Paper presented at 34th Annual Symposium: Solid Freeform Fabrication, August 14-16, 2023, Austin, Texas, United States of America.
Open this publication in new window or tab >>Increasing precision towards NiTi lattice structure using PBF-EB
2023 (English)Conference paper, Poster (with or without abstract) (Refereed)
Abstract [en]

The electron beam powder bed fusion (PBF-EB) is limitedly used to manufacture complex structures such as delicate lattices. Nickel titanium (NiTi) has been chosen for fabricating the lattice structure due to its widely utilization in the biomedical sector. However, issues may arise when manufacturing angled trusses while the dimensional inaccuracy increased with the increasing of the angle between the truss member and the vertical build direction. Therefore, two different scan strategies: spot melting and linear melting were used to manufacture the lattice structures respectively to compare the dimensional accuracy of different structures. This investigation highlights that linear melting is prone to maintain the geometrical accuracy of line-based structure with a limited influence from the scan speed while the spot melting is more capable of manufacturing the point-based structure with a higher geometrical resolution.  

National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-336584 (URN)
Conference
34th Annual Symposium: Solid Freeform Fabrication, August 14-16, 2023, Austin, Texas, United States of America
Note

QC 20230915

Available from: 2023-09-13 Created: 2023-09-13 Last updated: 2023-09-15Bibliographically approved
Lin, Z., Surreddi, K. B., Hulme-Smith, C., Dadbakhsh, S. & Rashid, A. (2023). Influence of Electron Beam Powder Bed Fusion Process Parameters on Transformation Temperatures and Pseudoelasticity of Shape Memory Nickel Titanium. Advanced Engineering Materials
Open this publication in new window or tab >>Influence of Electron Beam Powder Bed Fusion Process Parameters on Transformation Temperatures and Pseudoelasticity of Shape Memory Nickel Titanium
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2023 (English)In: Advanced Engineering Materials, ISSN 1438-1656, E-ISSN 1527-2648Article in journal, Editorial material (Refereed) Published
Abstract [en]

Electron beam powder bed fusion (PBF-EB) is used to manufacture dense nickel titanium parts using various parameter sets, including the beam current, scan speed and post cooling condition. The density of manufactured NiTi parts are investigated with relation to the linear energy input. The results implies the part density increases with increasing linear energy density to over 98% of the bulk density. With a constant energy input, a combination of low power and low scan speed leads to denser parts. This is attributed to lower electrostatic repulsive forces from lower number density of the impacting electrons. After manufacturing, densest parts with distinct parameter sets are categorized into three groups: i) high power with high scan speed and vacuum slow cooling, ii) low power with low scan speed and vacuum slow cooling and iii) low power with low scan speed and medium cooling rate in helium gas. Among these, a faster cooling rate suppresses phase transformation temperatures, while vacuum cooling combinations do not affect the phase transformation temperatures significantly. All the printed parts in this study exhibit almost 8% pseudoelasticity regardless of the process parameters, while the parts cooled in helium have a higher energy dissipation efficiency ( ), which implies faster damping of oscillations. 

Place, publisher, year, edition, pages
John Wiley & Sons, 2023
Keywords
PBF-EB, cooling rate, NiTi, AM, process paremeters
National Category
Materials Engineering
Research subject
Industrial Engineering and Management
Identifiers
urn:nbn:se:kth:diva-326103 (URN)10.1002/adem.202201818 (DOI)000975548500001 ()2-s2.0-85154049056 (Scopus ID)
Note

QC 20230426

Available from: 2023-04-24 Created: 2023-04-24 Last updated: 2023-11-30Bibliographically approved
Holmberg, J., Berglund, J., Brohede, U., Åkerfeldt, P., Sandell, V., Rashid, A., . . . Hosseini, S. (2023). Machining of additively manufactured alloy 718 in as-built and heat-treated condition: surface integrity and cutting tool wear. The International Journal of Advanced Manufacturing Technology, 130(3-4), 1823-1842
Open this publication in new window or tab >>Machining of additively manufactured alloy 718 in as-built and heat-treated condition: surface integrity and cutting tool wear
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2023 (English)In: The International Journal of Advanced Manufacturing Technology, ISSN 0268-3768, E-ISSN 1433-3015, Vol. 130, no 3-4, p. 1823-1842Article in journal (Refereed) Published
Abstract [en]

Additive manufacturing (AM) using powder bed fusion is becoming a mature technology that offers great possibilities and design freedom for manufacturing of near net shape components. However, for many gas turbine and aerospace applications, machining is still required, which motivates further research on the machinability and work piece integrity of additive-manufactured superalloys. In this work, turning tests have been performed on components made with both Powder Bed Fusion for Laser Beam (PBF-LB) and Electron Beam (PBF-EB) in as-built and heat-treated conditions. The two AM processes and the respective heat-treatments have generated different microstructural features that have a great impact on both the tool wear and the work piece surface integrity. The results show that the PBF-EB components have relatively lower geometrical accuracy, a rough surface topography, a coarse microstructure with hard precipitates and low residual stresses after printing. Turning of the PBF-EB material results in high cutting tool wear, which induces moderate tensile surface stresses that are balanced by deep compressive stresses and a superficial deformed surface that is greater for the heat-treated material. In comparison, the PBF-LB components have a higher geometrical accuracy, a relatively smooth topography and a fine microstructure, but with high tensile stresses after printing. Machining of PBF-LB material resulted in higher tool wear for the heat-treated material, increase of 49%, and significantly higher tensile surface stresses followed by shallower compressive stresses below the surface compared to the PBF-EB materials, but with no superficially deformed surface. It is further observed an 87% higher tool wear for PBF-EB in as-built condition and 43% in the heat-treated condition compared to the PBF-LB material. These results show that the selection of cutting tools and cutting settings are critical, which requires the development of suitable machining parameters that are designed for the microstructure of the material.

Place, publisher, year, edition, pages
Springer Nature, 2023
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-342428 (URN)10.1007/s00170-023-12727-w (DOI)001122504100001 ()2-s2.0-85179663025 (Scopus ID)
Funder
Vinnova, 2016-05175Swedish Foundation for Strategic Research, GMT14-048Swedish Research Council, 2016-05460
Note

QC 20240122

Available from: 2024-01-18 Created: 2024-01-18 Last updated: 2024-01-22Bibliographically approved
Zeyu, L. & Dadbakhsh, S. (2023). The processing windows for NiTi alloy manufactured by PBF-EBessing windows for NiTi alloy manufactured by PBF-EB. In: The processing windows for NiTi alloy manufactured by PBF-EB: . Paper presented at Swedish Arena for additive manufacturing of metals 2023, Stockholm, Sweden, Januari 11–12, 2023.
Open this publication in new window or tab >>The processing windows for NiTi alloy manufactured by PBF-EBessing windows for NiTi alloy manufactured by PBF-EB
2023 (English)In: The processing windows for NiTi alloy manufactured by PBF-EB, 2023Conference paper, Oral presentation with published abstract (Other academic)
Abstract [en]

Nickel titanium (NiTi) as one of the most utilized shape memory alloy has drawn significant interest due to its unique characteristics. However, NiTi is also considered a susceptible material to smoke during electron beam powder bed fusion (PBF-EB) process, which restricts the manufacturing possibility of the components. This work investigates processing windows for pre-heating and melting of NiTi powder to allow fabricating healthy parts. The smoke tests were carried out at different focus offsets and beam currents in relation to beam speeds. It is noted that a smaller EB spot can effectively prevents smoking while it may cause the strong powder bonding which can affect powder recycling negatively. Thus, a less focused beam (or larger EB spot) was selected to reach medium but efficient sintering. Moreover, it was observed that a negative defocused EB mitigates the smoke phenomenon compared to the positive defocused EB with a similar spot size. After that, parts having a relative density over 99% were successfully manufactured with PBF-EB. It is also found that with the same level of energy input, a set of low power with low scan speed leads to denser parts compared to a set of high power with high scan speed. This is attributed to less complexities in the melt dynamic which is related to lower density of impacting electrons. Besides, the combination of low power with low scan speed also improves geometrical accuracy of the parts attributed to the smaller spot size and smaller melt pool sizes. 

National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-326942 (URN)
Conference
Swedish Arena for additive manufacturing of metals 2023, Stockholm, Sweden, Januari 11–12, 2023
Note

QC 20230807

Available from: 2023-05-15 Created: 2023-05-15 Last updated: 2023-08-07Bibliographically approved
Matija, M., Mao, H., Tibert, G. & Dadbakhsh, S. (2022). Design and Development of Damping SandwichPanels for Satellite Housing Using AdditiveManufacturing. In: : . Paper presented at International Conference on Design for 3D Printing.
Open this publication in new window or tab >>Design and Development of Damping SandwichPanels for Satellite Housing Using AdditiveManufacturing
2022 (English)Conference paper, Published paper (Refereed)
Abstract [en]

The present work investigates the performance of additively-manufactured sandwich structures with the goal of reducing the effect of vibrations on a spacecraft during launch, whilst minimizing mass. Additive manufacturing allows designers to implement custom and complex geometries, such as the sheet gyroid structures, inside sandwich panels. Accordingly, this work details the development of gyroid-based sandwich structures for damping. Several test specimens are designed, additively manufactured using ABS plastic, and their damping performances are evaluated based on both simulation and experiments. Damping values are identified using frequency response transfer functions. The results show that as theory predicts, adding more mass, through the added thickness of the gyroid reduces the amplitude of vibrations. However, on a damping-per-unit-mass basis, the experimental results are inconclusive mainly due to the measurements of vibrations in the center of the sandwich panels instead of the sides where the vibrations can be maximum. Therefore, simulations better illustrate the changes of the damping behavior at different applied frequencies. Lessons and experiences are summarized for future work, particularly in exploring the effects of varying other 3D printed composite meta-lattice sandwich structures for satellites. 

National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-323991 (URN)
Conference
International Conference on Design for 3D Printing
Note

QC 20230220

Available from: 2023-02-17 Created: 2023-02-17 Last updated: 2023-02-27Bibliographically approved
Zeyu, L., Dadbakhsh, S. & Rashid, A. (2022). Developing processing windows for powder pre-heating in electron beam melting. Journal of Manufacturing Processes, 83, 180-191
Open this publication in new window or tab >>Developing processing windows for powder pre-heating in electron beam melting
2022 (English)In: Journal of Manufacturing Processes, ISSN 1526-6125, Vol. 83, p. 180-191Article in journal (Refereed) Published
Abstract [en]

Powder pre-heating is a critical step in electron beam melting (EBM), while there has been no systematic work tostudy the corresponding processing windows so far. Accordingly, this work investigates the relation between thesintering and the issues appearing during pre-heating (e.g., smoking or excessive sintering) in EBM of highlysusceptible-to-smoke Nickel-Titanium (NiTi) powder. First, the EB spot size was assessed depending on differentfocus offsets and beam currents from beam tracking experiments on a ceramic-coated stainless steel plate. Af-terwards, the smoke tests were carried out at different focus offsets and beam currents in terms of beam speeds. Itis shown that a smaller EB spot can effectively prevents smoking by enhancing the sintering degree. However,since this high sintering degree can cause strong powder bonding preventing the powder recycling, less focusedbeam (or larger EB spot) was selected to reach medium but efficient sintering in the level of around 30 %.Moreover, due to the influence of the diverging angle on the EB-material interaction, it is found that the negativedefocused EB mitigates the smoke phenomenon compared to the positive defocused EB with a similar spot size.Based on the smoke test results, linked to the sintering degree, the processing windows for pre-heating NiTipowder are developed demonstrating three different modes: smoke-heating, melting-heating and healthy-heating. 

Place, publisher, year, edition, pages
Elsevier BV, 2022
National Category
Engineering and Technology Production Engineering, Human Work Science and Ergonomics
Research subject
Production Engineering
Identifiers
urn:nbn:se:kth:diva-317459 (URN)10.1016/j.jmapro.2022.08.063 (DOI)000870827000003 ()2-s2.0-85137722020 (Scopus ID)
Note

QC 20220930

Available from: 2022-09-12 Created: 2022-09-12 Last updated: 2023-11-30Bibliographically approved
Zhao, X., Yuan, W., Mansour, R., Dadbakhsh, S. & Rashid, A. (2022). Effect of scanning strategy on thermal stresses and strains during electron beam melting of Inconel 625: experiment and simulation.
Open this publication in new window or tab >>Effect of scanning strategy on thermal stresses and strains during electron beam melting of Inconel 625: experiment and simulation
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2022 (English)In: Article in journal (Other academic) Submitted
National Category
Manufacturing, Surface and Joining Technology
Identifiers
urn:nbn:se:kth:diva-312412 (URN)
Note

QCR 20220523

Available from: 2022-05-18 Created: 2022-05-18 Last updated: 2024-01-17Bibliographically approved
Shi, Q., Mertens, R., Dadbakhsh, S., Li, G. & Yang, S. (2022). In-situ formation of particle reinforced Aluminium matrix composites by laser powder bed fusion of Fe2O3/AlSi12 powder mixture using laser melting/remelting strategy. Journal of Materials Processing Technology, 299, Article ID 117357.
Open this publication in new window or tab >>In-situ formation of particle reinforced Aluminium matrix composites by laser powder bed fusion of Fe2O3/AlSi12 powder mixture using laser melting/remelting strategy
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2022 (English)In: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 299, article id 117357Article in journal (Refereed) Published
Abstract [en]

In-situ preparation of particle reinforced Al matrix composites (PRAMCs) by laser powder bed fusion (LPBF) is a promising strategy to strengthen Al-based alloys. The laser-driven thermite reaction can be a practical mechanism to in-situ synthesise PRAMCs. However, the introduction of elements oxygen by adding Fe2O3 makes the powder mixture highly sensitive to form porosity and Al2O3 film during LPBF, bringing challenges to prepare dense materials. This work develops an LPBF processing strategy combined with consecutive high-energy laser melting scanning and low-energy laser remelting scanning to prepare dense PRAMCs from Fe2O3/AlSi12 powder mixture. A high relative density (98.2 ± 0.55 %) was successfully obtained by optimising laser melting (Emelting) and remelting energy density (Eremelting) to Emelting = 35 J/mm2 and Eremelting = 5 J/mm2. Results reveal the necessity to increase Emelting to improve metal liquid’s spreading/wetting by breaking up Al2O3 films surrounding molten pools; however, the high-energy laser melting produced much porosity. Low-energy laser remelting could close the resulting internal pores, backfill open gaps and smoothen solidified surfaces. Although with two-times laser scanning, the microstructure still shows fine cellular Si networks with Al grains inside (grain size 370 nm) and in-situ nano-precipitates (Al2O3, Si and Al-Fe(-Si) intermetallics). Finally, the fine microstructure, nano-structured dispersion strengthening and high-level densification strengthen the prepared in-situ PRAMCs, reaching yield strength of 426 ± 4 MPa and tensile strength of 473 ± 6 MPa. Furthermore, the results can provide valuable information to process other powder mixtures with severe porosity/oxide-film formation potential considering the evidenced contribution of laser melting/remelting strategy to densify material and obtain good mechanical properties during LPBF.

Keywords
Laser powder bed fusion (LPBF), Metal matrix composites (MMCs), Densification, Microstructures, Mechanical properties
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-302614 (URN)10.1016/j.jmatprotec.2021.117357 (DOI)000706415300002 ()2-s2.0-85114729019 (Scopus ID)
Note

QC 20211101

Available from: 2021-09-28 Created: 2021-09-28 Last updated: 2024-03-18Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0003-4120-4790

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