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| 2. |
- Hassila Karlsson, Carl Johan, et al.
(författare)
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Influence of scanning strategy on residual stresses in laser powder bed fusion manufactured alloy 718: Modeling and experiments
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Residual stresses are a known phenomenon in additively manufactured materials. The residual stresses increase the risk of cracks, limit in-service performance, and distort printed parts. In this work, thermo-mechanical finite element models using the hatch-by-hatch and layer-by-layer approach, and the inherent strain method has been developed and applied to predict the effects of different scanning strategies on the deflection and the residual stresses of two PBF-LB processed geometries. To account for viscoplasticty and relaxation effects, a mechanism-based material model have been implemented and used. It is shown that the hatch-by-hatch approach and inherent strain method both successfully predicted the experimentally measured deflections of the first geometry, which was printed using different scanning directions. To predict the stress field experimentally, high-energy synchrotron measurements have been used to. The thermo-mechanical models and the inherent strain method both captures the trend of experimentally measured residual stress fields, although with an overall underprediction. The predictions of the models were evaluated, and their accuracy discussed in terms of physical aspects of the Powder Bed Fusion – Laser Beam process.
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| 3. |
- Hassila, Carl Johan
(författare)
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Additive Manufacturing of Ni-Fe Superalloys : Exploring the Alloying Envelope and the Impact of Process on Mechanical Properties
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Additive manufacturing of metals has received a lot of attention in the last decade as this family of manufacturing processes allows the manufacturing of complicated geometries which would be difficult to produce using conventional manufacturing techniques. Additive manufacturing of the Ni-Fe based superalloys 625 and 718 using the Powder Bed Fusion – Laser Beam (PBF-LB) process is facilitated by the fact that these alloys were developed as weldable alternatives to other high-strength, high-temperature Ni-based superalloys. However, given that these alloys were developed with casting and forging as the main manufacturing route, the alloying composition of these alloys may possibly be tuned to better suit the PBF-LB process. In this thesis, small changes to the alloy 625 and 718 alloy compositions were made, with the goal of either improving material properties or reducing the environmental footprint of the produced materials. For alloy 718, the influence of carbon content on the resulting microstructure and mechanical properties was investigated both in the as-built and heat-treated conditions using tensile and impact testing. A similar study, but also including corrosion experiments, was performed on an alloy 625 composition which had been tuned to allow it to be atomized using nitrogen instead of argon, a transition that results in environmental benefits as argon gas carries with it a larger environmental footprint compared to nitrogen gas. In addition to the above, as the process conditions in the PBF-LB process have a strong influence on the developing microstructure, their influence on rolling contact fatigue and residual stresses in printed alloys 625 and 718 were investigated. Rolling contact fatigue experiments were performed on alloy 625 and were complemented by a fractographic study which showed that the different grain structures achieved depending on the used process condition affected the pitting damage development. Meanwhile, the residual stress experiments were performed on PBF-LB processed alloy 625 and 718. The residual stresses in the materials were first calculated using experimental data attained from high energy synchrotron diffraction experiments. These results were then compared to the predicted stresses from a thermo-mechanical model. The thermomechanical model included a built-in mechanism-based material model which was shown to successfully simulate relaxation effects stemming from the cyclic heating of the material during the PBF-LB process. Lastly, a modelling approach using the thermo-mechanical model was developed which allowed the model to successfully predict the stresses also when using different scanning strategies.
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| 4. |
- Holmberg, Jonas, 1976-, et al.
(författare)
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Machining of additively manufactured alloy 718 in as-built and heat-treated condition: surface integrity and cutting tool wear
- 2024
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Ingår i: The International Journal of Advanced Manufacturing Technology. - : Springer Nature. - 0268-3768 .- 1433-3015. ; 130:3-4, s. 1823-1842
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 5. |
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| 6. |
- Lindwall, Johan, et al.
(författare)
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Simulation of phase evolution in a Zr-based glass forming alloy during multiple laser remelting
- 2022
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Ingår i: Journal of Materials Research and Technology. - : Elsevier. - 2238-7854 .- 2214-0697. ; 16, s. 1165-1178
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Tidskriftsartikel (refereegranskat)abstract
- Additive manufacturing by laser-based powder bed fusion is a promising technique for bulk metallic glass production. But, reheating by deposition of subsequent layers may cause local crystallisation of the alloy. To investigate the crystalline phase evolution during laser scanning of a Zr-based metallic glass-forming alloy, a simulation strategy based on the finite element method and the classical nucleation theory has been developed and compared with experimental results from multiple laser remelting of a single-track. Multiple laser remelting of a single-track demonstrates the crystallisation behaviour by the influence of thermal history in the reheated material. Scanning electron microscopy and transmission electron microscopy reveals the crystalline phase evolution in the heat affected zone after each laser scan. A trend can be observed where repeated remelting results in an increased crystalline volume fraction with larger crystals in the heat affected zone, both in simulation and experiment. A gradient of cluster number density and mean radius can also be predicted by the model, with good correlation to the experiments. Prediction of crystallisation, as presented in this work, can be a useful tool to aid the development of process parameters during additive manufacturing for bulk metallic glass formation.
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| 7. |
- Hassila, Carl Johan, et al.
(författare)
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Rolling contact fatigue crack propagation relative to anisotropies in additive manufactured Inconel 625
- 2019
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Ingår i: Wear. - : ELSEVIER SCIENCE SA. - 0043-1648 .- 1873-2577. ; 426-427:Part B, s. 1837-1845
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Tidskriftsartikel (refereegranskat)abstract
- Additive manufacturing is steadily gaining acceptance in certain industry segments as a process for the manufacturing of dense metallic components. The Ni-based superalloys belonging to the Inconel family have for many years been in focus for AM research and AM produced components are now becoming commercially available. However, it is still unclear how the microstructural anisotropy, inherent to most AM materials, affects the material performance in a given application. The anisotropy may e.g. influence the mechanical properties and the performance in certain tribological situations, such as when subjected to rolling contact fatigue. Like most AM methods, the powder bed fusion - laser beam process gives the produced components a relatively rough surface. To perform well in demanding tribological situations, the components are commonly machined to a smooth finish. In this work, Inconel 625 produced using PBF-LB is evaluated in a rolling contact fatigue test. Test cylinders (empty set 10 mm) have been produced using different build directions and scan strategies, resulting in varying microstructures and textures. In the rolling contact fatigue test, a cylindrical sample is mounted between two empty set 140 mm metal rollers, pulled together via a spring. After testing, the contact tracks are studied using SEM and EBSD to reveal cracks. Cracks were analysed with respect to the microstructure and anisotropies. It was found that the anisotropy influences both the nucleation and growth of cracks. The AM produced specimens were also found to be more prone to transgranular cracking than conventional Inconel 625, which predominantly displayed intergranular cracks.
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| 8. |
- Hassila Karlsson, Carl Johan, et al.
(författare)
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Influence of carbon content on microstructure and mechanical properties of Inconel 718 processed with Powder Bed Fusion – Laser Beam
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Additive manufacturing of alloys belonging to the Inconel family is today a hot research topic. Given that these alloys were developed with casting and forging as the main manufacturing route, the possibilities of adjusting their composition to make them better suited for the additive manufacturing processes should be investigated. In this work we study two different versions of Alloy 718 with different carbon content; one having a normal carbon content and one having a very low carbon content. Test pieces from these alloys were produced using the Powder Bed Fusion – Laser Beam process. TEM and SEM with EDS/EBSD was used to study the resulting microstructures both in the as-built and after heat treatment. Mechanical properties were evaluated for samples printed in different build directions using tensile and impact testing. The achieved materials in the as-built condition were very similar to one another in both the microstructure and the displayed mechanical properties. After heat treatment, differences in the microstructures could be identified as secondary carbides were found to precipitate exclusively in the alloy with a normal carbon content. Additionally, the different carbon contents affected the number of annealing twins that formed in the respective alloys, where more twins formed in the alloy with a low carbon content. This was attributed to the alloy’s lower stacking-fault energy. As annealing twins facilitate the transformation of the anisotropic as-built grain structure, effectively making the materials more isotropic, only the low carbon content alloy showed isotropic material properties after heat treatment.
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| 9. |
- Hassila Karlsson, Carl Johan, et al.
(författare)
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Rolling contact fatigue test performed on additively manufactured inconel 718 produced by selective laser melting
- 2018
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Ingår i: 18th Nordic Symposium on Tribology - NORDTRIB 2018, Uppsala, Sweden, June 18-21, 2018. - Uppsala.
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Konferensbidrag (refereegranskat)abstract
- Powder bed additive manufacturing (AM) is a method for producing near-net-shape components by selectively melting metal powder where material is desired, layer by later. This is directly contrary to conventional machining being a solely subtractive process where material is removed from a large work piece. It is well known that AM often results in microstructural anisotropy related to the build direction. In the case of Inconel 718, columnar grains are formed, oriented along the build direction. Furthermore, utilizing different scan strategies such as contouring will result in different grain structures in the near surface region compared to the bulk of the material. The aim of this work was to study the effects of these microstructural anisotropies on the mechanical and tribological properties. Cylindrical test specimens with were built with different build directions and scan strategies and mechanically characterized and tested in a rolling contact fatigue test rig where a specimen was placed between two hard rollers. As the very surface presents a rough topography comprised of partly melted powder particles which hinders any tribological function of the component the samples were grinded prior to the rolling contact fatigue tests. After testing, the contact tracks were analyzed using, SEM and EBSD.
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| 10. |
- Lindwall, J., et al.
(författare)
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Boundary conditions for simulation of powder bed fusion for metallic glass formation : measurements and calibrations
- 2019
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Ingår i: Second International Conference on Simulation for Additive Manufacturing (Sim-AM 2019). - : International Centre for Numerical Methods in Engineering. - 9788494919480 ; , s. 51-59
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Konferensbidrag (refereegranskat)abstract
- A finite element model for prediction of the temperature field in the powder bed fusion process is presented and compared to measurements. Accurate temperature predictions at the base plate are essential to accurately predict the formation of crystals in a metallic glass forming material. The temperature measurements were performed by equipping the base plate with thermocouples during manufacturing of a cylinder with the glass forming alloy AMZ4. Boundary conditions for heat losses through the base plate/machine contact interfaces was calibrated to fit the measurements. Additional heat losses was used to account for radiation at the top surface and conduction through the powder bed. An interface boundary condition based on conservation of heat flux was examined to match the heat flow to the machine structure and the temperature predictions was satisfying. Still, temperature predictions with a constant heat transfer coefficient matched the measurements within 1.5 degrees C during the entire building process of about 9 hours.
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