| 1. |
- Balachandramurthi, Arun Ramanathan, 1989-
(author)
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Towards understanding the fatigue behaviour of Alloy 718 manufactured by Powder Bed Fusion processes
- 2020
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Doctoral thesis (other academic/artistic)abstract
- Additive Manufacturing (AM) is a disruptive modern manufacturing process in which parts are manufactured in a layer-wise fashion. Among the metal AM processes, Powder Bed Fusion (PBF) technology — comprised of Electron Beam Powder Bed Fusion (EB-PBF) and Laser Beam Powder Bed Fusion (LB-PBF) —has opened up a design space that was formerly unavailable with conventionalmanufacturing processes. PBF processes offer several advantages; however, thesuitability of these processes to replace the conventional processes must be investigatedin detail. Therefore, understanding the AM process – post-processing –microstructure – property relationships is crucial for the manufacturing of high performance components. In this regard, only limited work has been done towards understanding the fatigue behaviour of PBF Alloy 718. The aim of this work, therefore, is to understand how the fatigue behaviour of PBF Alloy 718 is affected by its microstructure. Besides, the influence of the rough as-built surface is also investigated. In general, the <100> fibre texture along the build direction that resulted from PBF processing of Alloy 718 led to anisotropy in Young's modulus. Consequently,the fatigue performance under controlled amplitudes of strain was anisotropic such that the low-modulus direction had longer fatigue life and vice versa. This texture-induced elasticity-dependent anisotropic strain-life behaviour couldbe normalized by the pseudo-elastic stress vs fatigue life approach.Inclusions and defects had a detrimental effect on fatigue performance. Numerousfactors, such as their geometry, volume fraction, and distribution, determinedthe effect on fatigue behaviour. Hot Isostatic Pressing (HIP) eliminated most defect sand led to an improvement in fatigue performance. However, HIP did not alter the inclusions, which acted as crack initiation sites and reduced fatigue life. The rough as-built surface, which had numerous notch-like crack initiation sites, deteriorated fatigue performance; however, it lowered the scatter in fatigue life. Machining off the as-built surface improved fatigue life but increased the scatter.
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| 2. |
- Azeez, Ahmed, 1991-, et al.
(author)
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Out-of-phase thermomechanical fatigue crack propagation in a steam turbine steel — modelling of crack closure
- 2021
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In: International Journal of Fatigue. - : Elsevier. - 0142-1123 .- 1879-3452. ; 149
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Journal article (peer-reviewed)abstract
- Understanding of crack growth behaviour is necessary to predict accurate fatigue lives. Out-of-phase thermomechanical fatigue crack propagation tests were performed on FB2 steel used in high-temperature steam turbine sections. Testing results showed crack closure where the compressive part of the fatigue cycle affected crack growth rate. Crack closing stress was observed to be different, and had more influence on the growth rate, than crack opening stress. Crack growth rate was largely controlled by the minimum temperature of the cycle, which agreed with an isothermal crack propagation test. Finite element models with stationary sharp cracks captured the crack closure behaviour.
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| 3. |
- Azeez, Ahmed, 1991-, et al.
(author)
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The effect of dwell times and minimum temperature on out-of-phase thermomechanical fatigue crack propagation in a steam turbine steel - Crack closure prediction
- 2022
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In: International Journal of Fatigue. - : Elsevier Science Ltd. - 0142-1123 .- 1879-3452. ; 162
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Journal article (peer-reviewed)abstract
- Exploring crack growth behaviour is needed to establish accurate fatigue life predictions. Cracked specimens were tested under strain-controlled out-of-phase thermomechanical fatigue conditions. The tests included dwell times and three different minimum temperatures. Higher minimum temperature gave faster crack growth rates while the additions of dwell times showed no effects. Crack closure was observed in all the tests where the addition of dwell times and change in minimum temperature displayed little to no effect on crack closure stresses. Finite element models with a sharp stationary crack and material parameters switching provided acceptable predictions for the maximum, minimum, and crack closure stresses.
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| 4. |
- Balachandramurthi, Arun Ramanathan, 1989-, et al.
(author)
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Anisotropic fatigue properties of Alloy 718 manufactured by Electron Beam Powder Bed Fusion
- 2020
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In: International Journal of Fatigue. - : Elsevier. - 0142-1123 .- 1879-3452. ; 141
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Journal article (peer-reviewed)abstract
- In this study, Alloy 718 specimens manufactured by Electron Beam Powder Bed Fusion process are subjected to two different post-treatments to have different microstructural features. Low cycle fatigue testing has been performed both parallel and transverse to the build direction. EB-PBF Alloy 718 exhibits anisotropic fatigue behaviour; the fatigue life is better along the parallel direction compared to the transverse direction. The anisotropy in fatigue life is related to the anisotropy in the Young’s modulus. The pseudo-elastic stress vs. fatigue life approach is presented as a potential solution to handle anisotropy in fatigue life assessment of additively manufactured engineering components. © 2020 The Authors
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| 5. |
- Balachandramurthi, Arun Ramanathan, 1989-, et al.
(author)
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On the microstructure of laser beam powder bed fusion alloy 718 and its influence on the low cycle fatigue behaviour
- 2020
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In: Materials. - : MDPI AG. - 1996-1944. ; 13:22
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Journal article (peer-reviewed)abstract
- Additive manufacturing of Alloy 718 has become a popular subject of research in recent years. Understanding the process-microstructure-property relationship of additively manufactured Alloy 718 is crucial for maturing the technology to manufacture critical components. Fatigue behaviour is a key mechanical property that is required in applications such as gas turbines. Therefore, in the present work, low cycle fatigue behaviour of Alloy 718 manufactured by laser beam powder bed fusion process has been investigated. The material was tested in as-built condition as well as after two different thermal post-treatments. Three orientations with respect to the building direction were tested to evaluate the anisotropy. Testing was performed at room temperature under controlled amplitudes of strain. It was found that defects, inclusions, strengthening precipitates, and Young’s modulus influence the fatigue behaviour under strain-controlled conditions. The strengthening precipitates affected the deformation mechanism as well as the cycle-dependent hardening/softening behaviour. The defects and the inclusions had a detrimental effect on fatigue life. The presence of Laves phase in LB-PBF Alloy 718 did not have a detrimental effect on fatigue life. Young’s modulus was anisotropic and it contributed to the anisotropy in strain-life relationship. Pseudo-elastic stress vs. fatigue life approach could be used to handle the modulus-induced anisotropy in the strain-life relationship. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
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| 6. |
- Cui, Luqing, et al.
(author)
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A new approach for determining GND and SSD densities based on indentation size effect : An application to additive-manufactured Hastelloy X
- 2022
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In: Journal of Materials Science & Technology. - : Elsevier. - 1005-0302. ; 96, s. 295-307
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Journal article (peer-reviewed)abstract
- Dislocation plays a crucial role in controlling the strength and plasticity of bulk materials. However, determining the densities of geometrically necessary dislocations (GNDs) and statistically stored dislocations (SSDs) is one of the classical problems in material research for several decades. Here, we proposed a new approach based on indentation size effect (ISE) and strengthening theories. This approach was performed on a laser powder bed fused (L-PBF) Hastelloy X (HX), and the results were verified by the Hough-based EBSD and modified Williamson–Hall (m-WH) methods. Furthermore, to better understand the new approach and essential mechanisms, an in-depth investigation of the microstructure was conducted. The distribution of dislocations shows a clear grain orientation-dependent: low density in large <101> preferentially orientated grains while high density in fine <001> orientated grains. The increment of strengthening in L-PBF HX is attributed to a huge amount of edge-GNDs. Planar slip is the main operative deformation mechanism during indentation tests, and the slip step patterns depend mostly on grain orientations and stacking fault energy. This study provides quantitative results of GND and SSD density for L-PBF HX, which constructs a firm basis for future quantitative work on other metals with different crystal structures.
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| 7. |
- Cui, Luqing, et al.
(author)
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Cyclic Response of Additive Manufactured 316L Stainless Steel : The Role of Cell Structures
- 2021
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In: Scripta Materialia. - : Elsevier. - 1359-6462 .- 1872-8456. ; 205
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Journal article (peer-reviewed)abstract
- We report the effect of cell structures on the fatigue behavior of additively manufactured (AM) 316L stainless steel (316LSS). Compared with the cell-free samples, the fatigue process of fully cellular samples only consists of steady and overload stages, without an initial softening stage. Moreover, the fully cellular sample possesses higher strength, lower cyclic softening rate and longer lifetime. Microscopic analyses show no difference in grain orientations, dimensions, and shapes. However, the fully cellular samples show planar dislocation structures, whereas the cell-free samples display wavy dislocation structures. The existence of cell structures promotes the activation of planar slip, delays strain localization, and ultimately enhances the fatigue performance of AM 316LSS.
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| 8. |
- Cui, Luqing, et al.
(author)
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Dependence of microstructures on fatigue performance of polycrystals : A comparative study of conventional and additively manufactured 316L stainless steel
- 2022
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In: International journal of plasticity. - : Elsevier. - 0749-6419 .- 1879-2154. ; 149
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Journal article (peer-reviewed)abstract
- The fatigue properties and microstructural evolution of 316 L stainless steel (316LSS) manufactured by laser powder bed fusion (L-PBF) were systematically studied and compared with its wrought counterpart. The as-built L-PBF 316LSS shows a pronounced heterogeneity, not only structurally but also chemically, with a unique microstructure of highly serrated grain boundaries, bimodal grain structure, nano-precipitates, solidification cell structures, and chemical segregations. The microindentation test showed that the hardness of the as-built L-PBF 316LSS reached 2.589 GPa, which was about 1.6 times higher than that of the wrought solution annealed counterpart, and the sparser slip steps around indentations revealed its greater dislocation storage capability. The S-N curves indicated that the fatigue resistance of the as-built L-PBF 316LSS was significantly better than that of the wrought solution annealed samples, and this was ascribed to its unique microstructural characteristics, especially the pre-existing high-density dislocations and chemical microsegregation within cellular solidification features. Furthermore, the enhanced planar slip in L-PBF 316LSS by its unique microstructure, especially the formation of deformation twins, delays the strain localization and restrains slip band generation, thereby significantly inhibiting crack initiation, and contributing greatly to the fatigue performance. The unique cell structure appears to be more effective in improving the low-cycle fatigue performance of L-PBF 316LSS due to the enhanced ductility.
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| 9. |
- Cui, Luqing, et al.
(author)
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Low Cycle Fatigue Behavior and Microstructural Evolution of Nickel-based Superalloy M951G at Elevated Temperatures
- 2020
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In: Materials Characterization. - : Elsevier. - 1044-5803 .- 1873-4189. ; 163
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Journal article (peer-reviewed)abstract
- Low cycle fatigue (LCF) tests of the newly developed nickel-based superalloy M951G have been conducted at 900 and 1000 °C under different total strain amplitudes. Results show that the fatigue properties, fracture mechanisms as well as coarsening of γ′ precipitates are dependent on testing temperatures and strain amplitudes. Fatigue life and cyclic stress response under the same total strain amplitude at 1000 °C are lower than that at 900 °C, which is due to the degradation of microstructures, shearing of γ′ precipitates by dislocations and serious oxidation. Fracture modes change from intergranular cracking to the mixed mode cracking as the strain amplitude increases. At low strain amplitudes, M951G alloy fails in the form of intergranular cracking owing to the oxidation of surface carbides and the relatively low deformation rate. At higher strain amplitudes, the strain localization in grain interior, the distribution of broken carbides and eutectics as well as the relatively higher strain rate are the main reasons for the formation of transgranular microcracks. Ultimately, the effects of fatigue conditions on coarsening of cubic γ′ precipitates are also analyzed from the aspect of γ′ volume fraction, fatigue life and flow stress difference between the γ/γ′ interfaces.
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| 10. |
- Cui, Luqing, et al.
(author)
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Revealing Relationships between Microstructure and Hardening Nature of Additively Manufactured 316L Stainless Steel
- 2021
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In: Materials & Design. - : Elsevier. - 0261-3069 .- 0264-1275. ; 198
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Journal article (peer-reviewed)abstract
- Relationships between microstructures and hardening nature of laser powder bed fused (L-PBF) 316 L stainless steel have been studied. Using integrated experimental efforts and calculations, the evolution of microstructure entities such as dislocation density, organization, cellular structure and recrystallization behaviors were characterized as a function of heat treatments. Furthermore, the evolution of dislocation-type, namely the geometrically necessary dislocations (GNDs) and statistically stored dislocations (SSDs), and their impacts on the hardness variation during annealing treatments for L-PBF alloy were experimentally investigated. The GND and SSD densities were statistically measured utilizing the Hough-based EBSD method and Taylor's hardening model. With the progress of recovery, the GNDs migrate from cellular walls to more energetically-favourable regions, resulting in the higher concentration of GNDs along subgrain boundaries. The SSD density decreases faster than the GND density during heat treatments, because the SSD density is more sensitive to the release of thermal distortions formed in printing. In all annealing conditions, the dislocations contribute to more than 50% of the hardness, and over 85.8% of the total dislocations are GNDs, while changes of other strengthening mechanism contributions are negligible, which draws a conclusion that the hardness of the present L-PBF alloy is governed predominantly by GNDs.
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