| 1. |
- Hatami, Sepher, et al.
(författare)
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Fatigue Strength of 316 L Stainless Steel Manufactured by Selective Laser Melting
- 2020
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Ingår i: Journal of materials engineering and performance (Print). - : Springer. - 1059-9495 .- 1544-1024. ; 29, s. 3183-3194
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Tidskriftsartikel (refereegranskat)abstract
- In this study, the fatigue strength of 316 L stainless steel manufactured by selective laser melting (SLM) is evaluated. The effect of powder layer thickness and postmachining is investigated. Specimens were produced with 30 and 50 µm layer thickness and tested under high cycle fatigue in as-printed and postmachined conditions. Examination of the specimens reveals that in the as-printed condition, fatigue strength suffers from high roughness and surface tensile residual stresses as well as defects such as pores and lack of fusion voids. After machining, the fatigue strength was improved due to lower surface roughness, presence of compressive residual stresses, and removal of surface porosity. The results show that increasing the layer thickness (within the range tested) has a minor negative impact on fatigue strength; however, it has a major positive impact on the productivity of the SLM process. In addition, it is clear that the impact of postmachining on fatigue is far greater than that of the layer thickness. © 2020, The Author(s).
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| 2. |
- Hokka, Mikko, et al.
(författare)
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Effects of Microstructure on the Dynamic Strain Aging in Ferritic-Pearlitic Steels
- 2018
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Ingår i: Journal of Dynamic Behavior of Materials. - : Springer Science and Business Media LLC. - 2199-7446 .- 2199-7454. ; 4:4, s. 452-463
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Tidskriftsartikel (refereegranskat)abstract
- Effects of microstructure on the high strain rate high temperature mechanical response and dynamic strain aging of C45 and 27MnCr5 ferritic-pearlitic steels were studied using four different microstructural variants of the standard alloys. The high strain rate high temperature behavior of the steels was studied using a compression Split Hopkinson Pressure Bar device with high temperature testing capabilities. The steels were studied at strain rates up to 4500 s−1 and at temperatures from RT to 680 °C. Strong dynamic strain aging was observed for both steels in the studied temperature range. The results also show that the microstructure has a strong effect on the dynamic strain aging sensitivity of the steel. This is especially true at low plastic strains, where the effect of the microstructure is strongest. The effect of microstructure decreases as plastic strain increases. A coarse-grained microstructure showed the strongest dynamic strain aging sensitivity for both steels.
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| 3. |
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| 4. |
- Lundberg, Mattias, et al.
(författare)
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Fatigue Strength of Machined and Shot Peened Grey Cast Iron : Influence of Graphite Morphology
- 2013
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- A common opinion is that cast iron, especially grey cast iron, is not as notch sensitive as steel and has therefore not been treated by shot peening to suppress crack initiation. For a heterogeneous material that also is brittle, just like grey cast iron, the shot peening parameters needed to induce beneficial surface residual stresses can be problematic to identify. Fatigue testing under uniaxial loading with an R value of -1, on mechanically polished and shot peened specimens, has been performed to determine the fatigue strength at 107 cycles as well as complete Wöhler-curves. Two different types of specimen geometries were tested, one smooth and one notched specimen having kt equal to 1.05 resp. 1.33. With large shots and high peening intensity (heavy shot peening) the fatigue strength clearly decreased whereas small shots and low peening intensity (gentle shot peening) might have lowered the fatigue strength. A short annealing at 285° after gentle shot peening increased the fatigue strength. The results are discussed and explained based on x-ray diffraction measurements, i.e. residual stress and full width at half maximum profiles, as well as microstructural investigations using scanning electron microscope (SEM).
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| 5. |
- Lundberg, Mattias, et al.
(författare)
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Fatigue strength of Machined and Shot Peened Grey Cast Iron
- 2014
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Konferensbidrag (refereegranskat)abstract
- A common opinion is that cast iron, especially grey cast iron, is not as notch sensitive as steel therefore is not typically treated by shot peening to suppress crack initiation. For a heterogeneous material that also is brittle, just like grey cast iron, the shot peening parameters needed to induce beneficial surface residual stresses can be problematic to identify. Fatigue testing under uniaxial loading with an R value of -1, on mechanically polished and shot peened specimens, has been performed to determine the fatigue strength at 10(7) cycles as well as complete Wohler-curves. Two different types of specimen geometries were tested, one smooth and one notched specimen having k(t) equal to 1.05 and 1.33 respectively. With large shots and high peening intensity (heavy SP) the fatigue strength clearly decreased whereas small shots and low peening intensity (gentle SP) might have lowered the fatigue strength. A short annealing at 285 degrees after gentle SP increased the fatigue strength. The results are discussed and explained based on x-ray diffraction (XRD) measurements, i.e. residual stress and full width at half maximum profiles, as well as microstructural investigations using scanning electron microscope (SEM).
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| 6. |
- Lundberg, Mattias, et al.
(författare)
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Graphite Morphology's Influence on Shot Peening Results in Cast Irons
- 2014
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Konferensbidrag (refereegranskat)abstract
- The different shot peening responds of a grey cast iron (GI) with its flake graphite and a compacted cast iron (CGI) with its vermicular graphite was analyzed and compared in this paper. For peening using identical parameters, CGI showed a larger plastic deformation zone with higher subsurface compressive stresses than GI. Electron backscatter diffraction (EBSD) mapping and backscatter electron imaging revealed that plastic deformation of the matrix near graphite inclusions is affected by the size and geometry of the graphite. The different behaviors of graphite are explained by their capability to damp mechanical force but at the same time to cause stress concentration in the matrix. The better shot peening results for CGI may be attributed to a lower damping effect of its graphite inclusions and capability of the matrix for larger plastic deformation.
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| 7. |
- Lundberg, Mattias, et al.
(författare)
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Influence of Shot Peening Parameters on Residual Stresses in Flake and Vermicular Cast Irons
- 2014
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Konferensbidrag (refereegranskat)abstract
- Test samples of grey and compacted graphite cast irons with pearlitic matrix were shot-peened to different surface conditions using twelve different combinations of shot size, peening intensity and peening coverage percentage. Relatively high surface compressive residual stresses varying between 245 to 565 MPa were observed and the compressive residual stresses reached a depth between 280 µm and 770 µm. Within the range of peening parameters used, the compacted graphite cast iron with its vermicular graphite showed a somewhat better response to the same shot-peening treatment than the grey cast iron containing flake graphite, giving a larger peening affected zone with higher compressive residual stresses. For both the cast irons, an increase in peening coverage percentage, shot size or peening intensity led often to a lower surface compressive stress. However, peening using a higher intensity greatly increased the degree and extent of plastic deformation and therefore increased the magnitude and penetration depth of the subsurface compressive residual stresses, while the effect of increasing shot size also depends on the peening intensity. On the other hand, measurements on the grey cast iron samples showed that the peening coverage has little effect on the depth profile of residual stress.
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| 8. |
- Lundberg, Mattias
(författare)
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Residual Stresses and Fatigue of Shot Peened Cast Iron
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The complex geometry of cylinder head in heavy-duty diesel engine makes grey cast iron or compact graphite iron a perfect material choice due to its castability, thermal conductivity and damping capacity. To increase the efficiency of the engine, the fatigue property of the material needs to be improved. Shot peening is often used to increase the fatigue strength of components. The benefits are associated with the compressive stresses induced and with surface hardening. In this research project, these effects on grey and compact iron have been analyzed for different shot peening parameters using XRD, SEM and fatigue testing methods. The ultimate aim of the project is to increase the fatigue strength of cast irons by optimization of residual stresses.The XRD measurements and SEM examinations revealed that the shot peening parameters including shot size and peening intensity had significant influences on the resulted residual stresses and strain hardening while changing the coverage made little difference. Also differences in the peening results between the two materials were observed, which were ascribed to an effect of the different graphite morphology. Nevertheless, a residual stress profile similar to the one general considered to improve the fatigue strength in steels could be obtained in both grey and compact iron after shot peening.The axial fatigue testing with R=-1 on the grey iron showed that peening using large shot size and high peening intensity (heavy shot peening) resulted in a fatigue strength reduction of 15-20% in comparison with the mechanically polished surface. The negative effects are likely related to surface damage and relatively high tensile residual stresses in subsurface induced by the heavy peening. Grey cast iron has low ductility in tension and therefore tensile residual stresses may promote multiple cracking and crack networking during cyclic loading.Shot peening using much smaller shots and lower intensity (gentle shot peening) which resulted in a much smaller residual stress field gave no significant changes in fatigue strength. However, a short time annealing at 285°C of specimens being gently shot peened increased the fatigue strength roughly by 10%. The improvement could be an effect of precipitates formed due to the heat treatment, which lock the dislocation movement under cyclic loading.
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| 9. |
- Lundberg, Mattias, et al.
(författare)
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Residual Stresses in Shot Peened Grey and Compact Iron
- 2013
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- In this paper grey and compact cast irons with a pearlitic matrix are investigated after shot peening with twelve unique combinations of parameters, namely shot size, intensity and coverage, followed by residual stress measurements and evaluations. Cylindrical test samples were cut out from real cylinder heads and polished on the top flat surface to decrease effects from cutting. Residual stresses and the affected depth from the different peening conditions varied between -245 to -565 MPa and from 280 μm to 770 μm in depth. By an increase in shot size or intensity or coverage resultant surface compressive stresses will decrease whereas the affected depth increases with increasing intensity. The increased affected depth is as a result from the increased extent and magnitude of plastic deformation. The compacted cast iron was more affected than the grey cast iron, meaning that the same shot peening parameters resulted in both higher compressive stresses and larger deformation depth.
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| 10. |
- Lundberg, Mattias, et al.
(författare)
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Residual Stresses in Shot Peened Grey and Compact Iron
- 2014
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Ingår i: HTM Journal of Heat Treatment and Materials. - : Carl Hanser Verlag GmbH. - 1867-2493 .- 2194-1831. ; 69:1, s. 38-45
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Tidskriftsartikel (refereegranskat)abstract
- Grey cast iron and compacted graphite iron with a pearlitic matrix are investigated in this study after shot peening using twelve unique combinations of parameters, namely shot size, peening intensity and coverage, followed by residual stress measurements and evaluations. Cylindrical test samples were cut out from heavy truck cylinder heads and polished on the top flat surface to decrease effects from cutting. Residual stresses and the affected depth from the different peening conditions varied between −245 MPa to −565 MPa and from 280 μm to 770 μm in depth. Resultant surface compressive stresses decrease with increasing shot size, peening intensity or coverage whereas the affected depth increases with increasing intensity. The increased affected depth is a result from the increased extent and magnitude of plastic deformation. The compacted graphite iron was more affected by shot peening than the grey cast iron, meaning that the same shot peening parameters resulted in both higher compressive stresses and larger deformation depth.
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