| 1. |
- Zhang, Pimin, 1990-, et al.
(författare)
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Effects of surface finish on the initial oxidation of HVAF-sprayed NiCoCrAlY coatings
- 2019
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Ingår i: Surface & Coatings Technology. - Elsevier : Elsevier BV. - 0257-8972 .- 1879-3347. ; 364, s. 43-56
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Tidskriftsartikel (refereegranskat)abstract
- Oxide scale formed on HVAF-sprayed NiCoCrAlY coatings and the effect of surface treatment were investigated by a multi-approach study combining photo-stimulated luminescence, microstructural observation and mass gain. The initial oxidationbehaviour of as-sprayed, polished and shot-peened coatings at 1000 °C is studied. Both polished and shot-peened coatings exhibited superior performance due to rapid formation of α-Al2O3 fully covering the coating and suppressing the growth of transient alumina, assisted by a high density of α-Al2O3 nuclei on surface treatment induced defects. Moreover, the fast development of a two-layer alumina scale consisting of an inward-grown inner α-Al2O3 layer and an outer layer transformed from outward-grown transient alumina resulted in a higher oxide growth rate of the as-sprayed coating.
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| 2. |
- Zhang, Pimin, 1990-, et al.
(författare)
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Isothermal oxidation behavior of HVAF-sprayed NiCoCrAlY coatings : Effect of surface treatment
- 2017
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Ingår i: Proceedings of the International Thermal Spray Conference & Exposition (ITSC 2017). - New York : Curran Associates, Inc. - 9781510858220 ; , s. 456-461
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Konferensbidrag (refereegranskat)abstract
- NiCoCrAlY coatings are widely used as bond coats for ceramic thermal barrier coatings (TBCs) andoxidation and corrosion protective overlay coatings in industrial gas turbines. High temperature oxidation behaviour of NiCoCrAlYs has a great influence on the coating performance and lifetime of TBCs. A promising route to decrease the oxidation rate of such coatings is post-coating surface modification which can facilitate formation of a uniform alumina scale with a considerably slower growth rate compared to the as-sprayed coatings. In this work, the effect of surface treatment by means of shot peening and laser surface melting (LSM) on the oxidation resistance of high velocity air-fuel (HVAF) sprayed NiCoCrAlY coatings was studied. Isothermal oxidation was carried out at 1000⁰C for 1000h. Results showed that the rough surface of as-sprayed HVAF sprayed coatings was significantly changed after shot peening and LSM treatment, with a compact and smooth appearance. After the exposure, the oxide scales formed on surface-treated NiCoCrAlY coatings showed different morphology and growth rate compared to those formed on as-sprayed coating surface. The oxidation behaviour of surface treated HVAF-sprayed NiCoCrAlY coatings were revealed and discussed.
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| 3. |
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| 4. |
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| 5. |
- Chen, Zhe, 1987-, et al.
(författare)
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Effect of Machining Parameters on Cutting Force and Surface Integrity when High-Speed Turning AD730™ with PCBN Tools
- 2019
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Ingår i: The International Journal of Advanced Manufacturing Technology. - : Springer London. - 0268-3768 .- 1433-3015. ; 100:9-12, s. 2601-2615
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Tidskriftsartikel (refereegranskat)abstract
- The novel wrought nickel-based superalloy, AD 730™, is a good candidate material for turbine disc applications at high temperatures beyond 650 °C. The present study focuses on the machining performance of this newly developed alloy under high-speed turning conditions with advanced PCBN tools. Meanwhile, the machined surface integrity as influenced by cutting speed and feed rate was also investigated. The surface integrity was thoroughly characterized in terms of surface roughness and morphology, machining-induced plastic deformation, white layer formation, and residual stresses. It has been found that the cutting speed and feed rate had a strong effect on the cutting forces and resultant surface integrity. The cutting forces required when machining the alloy were gradually reduced with increasing cutting speed, while at 250 m/min and above, the flank tool wear became stronger which led to increased thrust force and feed force. A higher feed rate, on the other hand, always resulted in higher cutting forces. Increasing the cutting speed and feed rate in general deteriorated the surface integrity. High cutting speeds within the range of 200–250 m/min and a low feed rate of 0.1 mm/rev are preferable in order to implement more cost-effective machining without largely reducing the surface quality achieved. The formation of tensile residual stresses on the machined AD 730™, however, could be of a concern where good fatigue resistance is critical.
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| 6. |
- Chen, Z., et al.
(författare)
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Plastic Deformation and Residual Stress in High Speed Turning of AD730™ Nickel-based Superalloy with PCBN and WC Tools
- 2018
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Ingår i: Procedia CIRP. - : Elsevier BV. - 2212-8271. ; 71, s. 440-445, s. 440-445
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Konferensbidrag (refereegranskat)abstract
- A higher gas turbine efficiency can be achieved by increasing the operating temperature in hot sections. AD730™ is a recently-developed wrought/cast nickel-based superalloy which can maintain excellent mechanical properties above 700. However, machining of AD730™ could be a difficult task like other nickel-based superalloys. Therefore, studies are needed with respect to the machinability of this new alloy. In this paper, high-speed turning was performed on AD730™ using polycrystalline cubic boron nitride (PCBN) tools and coated tungsten carbide (WC) tools at varied cutting speeds. The surface integrity was assessed in two important aspects, i.e., surface and sub-surface plastic deformation and residual stresses. The PCBN tools generally showed better performance compared with the WC tools since it led to reduced machining time without largely compromising the surface integrity achieved. The optimal cutting speed was identified in the range of 200-250 m/min when using the PCBN tools, which gives rise to a good combination of machining efficiency and surface integrity. The further increase of the cutting speed to 300 m/min resulted in severe and deep plastic deformation. Meanwhile, a continuous white layer was formed at the machined surface. When turning with the WC tools, the increased cutting speed from 80 m/min to 100 m/min showed very little effect with respect to the plastic deformation on the machined surface. It was found that tensile residual stresses were developed on all machined surfaces no matter when the PCBN or WC tools were used, and the surface tension was generally increased with increasing cutting speed. The tensile layer might need to be modified by e.g., post-machining surface treatments such as shot peening, if taking good fatigue performance into consideration.
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| 7. |
- Cui, Luqing, et al.
(författare)
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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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Ingår i: Journal of Materials Science & Technology. - : Elsevier. - 1005-0302. ; 96, s. 295-307
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Tidskriftsartikel (refereegranskat)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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| 8. |
- Cui, Luqing, et al.
(författare)
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Cyclic Response of Additive Manufactured 316L Stainless Steel : The Role of Cell Structures
- 2021
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Ingår i: Scripta Materialia. - : Elsevier. - 1359-6462 .- 1872-8456. ; 205
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Tidskriftsartikel (refereegranskat)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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| 9. |
- Cui, Luqing, et al.
(författare)
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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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Ingår i: International journal of plasticity. - : Elsevier. - 0749-6419 .- 1879-2154. ; 149
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Tidskriftsartikel (refereegranskat)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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| 10. |
- Cui, Luqing, et al.
(författare)
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Low Cycle Fatigue Behavior and Microstructural Evolution of Nickel-based Superalloy M951G at Elevated Temperatures
- 2020
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Ingår i: Materials Characterization. - : Elsevier. - 1044-5803 .- 1873-4189. ; 163
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Tidskriftsartikel (refereegranskat)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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