| 1. |
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| 2. |
- Deng, Dunyong, et al.
(författare)
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Microstructural Characterization of AS-Manufactured and Heat Treated Electron Beam Melted Inconel 718
- 2016
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Ingår i: Materials Science and Technology 2016 (MS&T16). - Warrendale, PA : Association for Iron and Steel Technology, AISTECH. - 9781510833142 ; , s. 105-112
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Konferensbidrag (refereegranskat)abstract
- As manufactured and heat treated electron beam melted Inconel 718 exhibit inhomogeneous grain morphologies, mainly equiaxed close to the surface and more elongated columnar grains in the bulk. Different processing settings i.e., contouring in the surface region and hatching in the bulk region are responsible for differences in grain morphologies and textures, which are characterized using the SEM techniques electron contrast channeling imagine (ECCI) and electron backscattering diffraction (EBSD). Niobium carbides prefer to nucleate at TiN and are found to be in the grain growth direction. Regarding processing conditions, defects, such as porosities and segregations are discussed, as well as differences in microstructure emphasizing on grain morphologies and precipitates with regards to heat treatment and hardness indentation.
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| 3. |
- Lundberg, Mattias, 1985-, et al.
(författare)
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Effective X-ray Elastic Constant of Cast Iron
- 2018
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Ingår i: Journal of Materials Science. - : Springer Science and Business Media LLC. - 0022-2461 .- 1573-4803. ; 53:4, s. 2766-2773
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Tidskriftsartikel (refereegranskat)abstract
- X-ray diffraction is a non-destructive method used for strain measurements in crystalline materials. Conversion of strain to stress can be achieved using the X-ray elastic constants (XEC), s1 and ½s2. The sin2ψ method was used during in situ loading to determine XEC for flake, vermicular, and spherical graphite iron. A fully pearlitic steel was used as reference. Uniaxial testing was conducted on the cast iron to create a homogeneous strain field, as well as four-point bending in both tension and compression due to the tension/compression asymmetry. The commonly used XEC value ½s2 = 5.81 × 10−6 MPa−1 is theoretically derived from an α-Fe single crystal. When investigating materials that contain ferrite, such as polycrystalline cast iron, this value is not accurate. Determination of an effective XEC for polycrystalline cast iron yields a better correlation between the measured microstrains and the properties observed on a macroscopic scale. The need for an effective XEC is evident, especially when it comes to model validation of, for example, casting simulations. Effective XEC values have been determined for flake, vermicular, and spherical graphite iron. The determined value is lower than the theoretical value.
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| 4. |
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| 5. |
- Lundberg, Mattias, et al.
(författare)
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Surface Integrity and Fatigue Behaviour of Electric Discharged Machined and Milled Austenitic Stainless Steel
- 2017
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Ingår i: Materials Characterization. - : Elsevier. - 1044-5803 .- 1873-4189. ; 124, s. 215-222
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Tidskriftsartikel (refereegranskat)abstract
- Machining of austenitic stainless steels can result in different surface integrities and different machining process parameters will have a great impact on the component fatigue life. Understanding how machining processes affect the cyclic behaviour and microstructure are of outmost importance in order to improve existing and new life estimation models. Milling and electrical discharge machining (EDM) have been used to manufacture rectangular four-point bend fatigue test samples; subjected to high cycle fatigue. Before fatigue testing, surface integrity characterisation of the two surface conditions was conducted using scanning electron microscopy, surface roughness, residual stress profiles, and hardness profiles. Differences in cyclic behaviour were observed between the two surface conditions by the fatigue testing. The milled samples exhibited a fatigue limit. EDM samples did not show the same behaviour due to ratcheting. Recrystallized nano sized grains were identified at the severely plastically deformed surface of the milled samples. Large amounts of bent mechanical twins were observed ~ 5 μm below the surface. Grain shearing and subsequent grain rotation from milling bent the mechanical twins. EDM samples showed much less plastic deformation at the surface. Surface tensile residual stresses of ~ 500 MPa and ~ 200 MPa for the milled and EDM samples respectively were measured.
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| 6. |
- Saarimäki, Jonas, et al.
(författare)
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3D Residual Stresses in Selective Laser Melted Hastelloy X
- 2017
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Ingår i: Residual Stresses 2016: ICRS-10, Materials Research Proceedings 2 (2016). - : Materials Research Forum LLC. ; , s. 73-78
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Konferensbidrag (refereegranskat)abstract
- 3D residual stresses in as manufactured EOS NickelAlloy HX, produced by laser powder bed additive manufacturing, are analysed on the surface closest to the build-plate. Due to the severe thermal gradient produced during the melting and solidification process, profound amounts of thermal strains are generated. Which can result in unwanted geometrical distortion and effect the mechanical properties of the manufactured component. Measurements were performed using a four-circle goniometer Seifert X-ray machine, equipped with a linear sensitive detector and a Cr-tube. Evaluation of the residual stresses was conducted using sin2ψ method of the Ni {220} diffraction peak, together with material removal technique to obtain in-depth profiles. An analysis of the material is reported. The analysis reveals unwanted residual stresses, and a complicated non-uniform grain structure containing large grains with multiple low angle grain boundaries together with nano-sized grains. Grains are to a large extent, not equiaxed, but rather elongated.
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| 7. |
- Saarimäki, Jonas, et al.
(författare)
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Anisotropy effects during dwell-fatigue caused by δ-phase orientation in forged Inconel 718
- 2017
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Ingår i: Materials Science & Engineering A: Structural Materials: Properties, Microstructure and Processing. - : Elsevier BV. - 0921-5093 .- 1873-4936. ; 692, s. 174-181
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Tidskriftsartikel (refereegranskat)abstract
- Inconel 718 is a commonly used superalloy for turbine discs in the gas turbine industry. Turbine discs are often subjected to dwell-fatigue as a result of long constant load cycles. The effect of anisotropy on dwell-fatigue cracking in forged turbine discs have not yet been thoroughly investigated. Crack propagation behaviour was characterised using compact tension (CT) samples cut in different orientations from a real turbine disc forging. Samples were also cut in two different thicknesses in order to investigate the influence of plane strain and plane stress condition on the crack propagation rates. The samples were subjected to dwell-fatigue tests at 550 °C with 90 s or 2160 s dwell-times at maximum load. Microstructure characterisation was done using scanning electron microscopy (SEM) techniques such as electron channelling contrast imaging (ECCI), electron backscatter diffraction (EBSD), and light optical microscopy (LOM). The forged alloy exhibits strong anisotropic behaviour caused by the non-random δ-phase orientation. When δ-phases were oriented perpendicular compared to parallel to the loading direction, the crack growth rates were approximately ten times faster. Crack growth occurred preferably in the interface between the γ-matrix and the δ-phase.
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| 8. |
- Saarimäki, Jonas, et al.
(författare)
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Characterization of Hastelloy X Produced by Laser Powder Bed Additive Manufacturing
- 2016
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Ingår i: World PM2016 Proceedings. - : European Powder Metallurgy Association (EPMA). - 9781899072484 ; , s. 1-6
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Konferensbidrag (refereegranskat)abstract
- The grain structure of the selective laser melting (SLM) material is complicated and cannot easily be resolved by etching of the as-manufactured surface. One method to reveal the true morphology is by performing an electron backscatter diffraction (EBSD) analysis of the material. An analysis of the SLM material is reported. The analysis reveals that the grain structure is very complicated and the grain size is non-uniform. The microstructure contains large grains with multiple low angle grain boundaries together with small grains that approach nanometre size in the smallest size range. Grains are to a large extent, not equiaxed, but rather elongated.Varying degrees of texture in different regions of the material have been observed. The degree of texture coincides with material volumes manufactured with different process parameter settings. Also areas with and without indications of texture have shown to exhibit different hardness. This is an indication of variation in mechanical properties in the different material volumes. Thin-walled areas, surface regions of solid material and lattices constitutes one group with similar microstructure indicating one class of material and secondly the bulk material with another microstructure would constitute the other material class
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| 9. |
- Saarimäki, Jonas, 1985-
(författare)
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Cracks in superalloys
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Gas turbines are widely used in industry for power generation and as a power source at hard to reach locations where other possibilities for electrical power supplies are insufficient. New ways of producing greener energy is needed to reduce emission levels. This can be achieved by increasing the combustion temperature of gas turbines. High combustion temperatures can be detrimental and degrade critical components. This raises the demands on the high temperature performance of the superalloys used in gas turbine components. These components are frequently subjected to different cyclic loads combined with for example dwell-times and overloads at elevated temperatures, which can influence the crack growth. Dwell-times have been shown to accelerate crack growth and change cracking behaviour in both Inconel 718, Haynes 282 and Hastelloy X. On the other hand, overloads at the beginning of a dwell-time cycle have been shown to retard the dwell-time effect on crack growth in Inconel 718. More experiments and microstructural investigations are needed to better understand these effects.The work presented in this thesis was conducted under the umbrella of the research program Turbo Power; "High temperature fatigue crack propagation in nickel-based superalloys", where I have mainly looked at fatigue crack growth mechanisms in superalloys subjected to dwell-fatigue, which can have a devastating effect on crack propagation behaviour. Mechanical testing was performed under operation-like cycles in order to achieve representative microstructures and material data for the subsequent microstructural work. Microstructures were investigated using light optical microscopy and scanning electron microscopy (SEM) techniques such as electron channeling contrast imaging (ECCI) and electron backscatter diffraction (EBSD). The outcome of this work has shown that there is a significant increase in crack growth rate when dwell-times are introduced at maximum load (0 % overload) in the fatigue cycle. With the introduction of a dwell-time there is also a shift from transgranular to intergranular crack growth for both Inconel 718 and Haynes 282. The crack growth rate decreases with increasing overload levels in Inconel 718 when an overload is applied prior to the dwell-time. At high temperature, intergranular crack growth was observed in Inconel 718 as a result of oxidation and the creation of nanometric voids. Another observed growth mechanism was crack advance along δ-phase boundaries with subsequent oxidation of the δ-phase. This thesis comprises two parts. Part I gives an introduction to the field of superalloys and the acting microstructural mechanisms related to fatigue and crack propagation. Part II consists of five appended papers, which report the work completed as part of the project.
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| 10. |
- Saarimäki, Jonas, et al.
(författare)
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Damage Development in Thin Walled Selective Laser Melted Structures
- 2016
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Ingår i: Materials Science and Technology 2016 (MS&T16). - Warrendale, PA : Association for Iron and Steel Technology, AISTECH. - 9781510833142 ; , s. 79-86
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Konferensbidrag (refereegranskat)abstract
- Additive manufacturing has gained vast interest in e.g., aeronautical industry. The large number of superalloys available in powder form has enabled the utilization of the technology for hot gas turbine parts, leading to ground breaking design changes, in fuselage parts of large aircraft, and turbines.Different lattice truss structures were manufactured with the SLM process in parallel with solid SLM bars were produced. Hollow rectangular tubes and composites of tubes with an interior of lattice truss structures were also manufactured. Hot rolled material of Hastelloy X was included for reference. Mechanical testing was performed in tension showing that the SLM material is highly anisotropic and that the material has many advantages compared to the traditionally manufactured Hastelloy X alloy. Tests also show that fracture is promoted along certain planes in the lattice truss structure.lattice truss structures were analysed using digital imaging. At small deformations, the deformation in both the lattice and hybrid specimen is homogeneous. The deformation at onset of failure in the hybrid test bar is mainly a shearing of the lattice struts along a preferred plane, while the deformation map for the hybrid material still shows a planar deformation front perpendicular to the loading direction.
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