| 1. |
- Chen, Zhe, et al.
(författare)
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Damage analysis of a retired gas turbine disc
- 2014
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Ingår i: Proceedings of the 2014 Energy Materials Conference. - : John Wiley & Sons. - 9781119027942 ; , s. 405-410
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Konferensbidrag (refereegranskat)abstract
- Gas turbine discs operate mostly at high temperature gradients and are subjected to mechanical loads simultaneously. The high thermal and mechanical loads eventually could result in degradation and damages in disc material, thereby increasing the risk of disc failure. In this study, a damage analysis was performed in a retired gas turbine disc made of Inconel 718. Oxidation attack and microstructural degradation as the consequence of the high service temperature have been found to be the main damages that take place in the non-contact area of the retired disc. In the blade/disc contact area, fretting fatigue occurs, with a result that cracks initiate from the oxide/metal interface and propagate in the disc alloy parallel to the sliding direction of fretting, consequently reducing the stability and safety of the disc. Meantime, oxygen diffuses into the fretting fatigue cracks, thereby exacerbating the oxidation attack. A multi-layered scale with periodic formation of the Fe-oxide/spinel layer and the metallic layer is formed on the contact surface. In both contact and non-contact area, recrystallization and α-Cr precipitation take place in the surface layer of the disc alloy. The locations where α-Cr precipitates are commonly considered to be the natural sites for mechanical weakness.
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| 2. |
- Chen, Zhe, et al.
(författare)
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ECCI and EBSD study of subsurface damages in high speed turning of inconel 718 under different tools and machining parameters
- 2013
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Ingår i: 13th International Conference on Fracture 2013, ICF 2013. - 9781629933696 ; 6, s. 4697-4704
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Konferensbidrag (refereegranskat)abstract
- Inconel 718 is a Ni-based superalloy that can perform excellently at elevated temperatures. However, surface and subsurface damages in the form of microstructural and property changes and tensile residual stresses are common in a machined Inconel 718 component because of its poor machinability. Such damages have a significant influence on performance and the life time of the part. To characterise microstructural damages and understand how they are correlated to machining conditions are not only important for the evaluation of surface integrity but also for the optimization of machining operations to minimise effects from the machining process. This paper uses the ECCI (electron channelling contrast imaging) and EBSD (electron back scatter diffraction) methods to study the effect of cutting tools and cutting speeds on subsurface plastic deformation of machined Inconel 718. When turning at 200 m/min, a comparable level of plastic deformation was found under the surface machined with uncoated cubic boron nitride (CBN), titanium nitride coated CBN and whisker reinforced alumina (WRA). With an increase in cutting speed, the plastic deformation depth increased, and uncoated CBN tools showed superior performance in term of subsurface microstructure alterations compared to the other tool materials.
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| 3. |
- Chen, Zhe, 1987-
(författare)
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Surface Integrity of Broached Inconel 718 and Influence of Thermal Exposure
- 2014
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Inconel 718 is a nickel-based superalloy that is extensively used as a disc material in gas turbine engines. The service life of gas turbine discs is normally governed by the modes of material degradation and fatigue failure since they work mostly at high temperatures and are subjected to cyclic mechanical loadings. Fatigue failures often start with the initiation of cracks at the surface and the precise details of the failure process significantly depend on the surface conditions. In turbine disc production, one of the last manufacturing steps is to broach root fixings, commonly of fir-tree design, for blade mounting. It has always been a challenge when machining Inconel 718 due to its high strength retention at elevated temperatures, rapid work hardening, as well as low thermal conductivity. This usually leads to rapid tool wear, and consequently shorter tool life, and at the end to the deterioration of the surface integrity of the machined components.The aim of this licentiate thesis is to increase our knowledge about the surface integrity, especially microstructure and residual stresses, of broached Inconel 718 and its stability under thermal exposure. This knowledge can later be used for analyzing the initiation and propagation of fatigue cracks in broached Inconel 718, particularly in the case of high temperature fatigue, thereby giving a better understanding of the failure mechanism of gas turbine discs from a fatigue point of view.A broaching operation has been performed using similar cutting conditions as that used in turbo machinery industries for producing fir-tree root fixings. In addition, service damages were analyzed in a retired dis of Inconel 718. Surface defects, severe plastic deformation and generation of high tensile residual stresses have been found to be the main damages to the surface integrity caused by the broaching operation. The machining induced plastic deformation was found to accelerate the microstructural degradation beneath the broached surface when subjected to thermal exposure. The surface tensile residual stresses can be completely removed after short thermal exposure, the tensile layer in the sub-surface region, however, exhibited a high resistance to stress relief at high temperatures. The damage analysis on the retired disc indicated that presence of the highly deformed layer on the machined surface is negative for preventing the occurrence of fretting fatigue in turbine discs.
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| 4. |
- 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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| 5. |
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| 6. |
- Sadrossadat, Mohsen, 1963-, et al.
(författare)
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EBSD investigation of the effect of the solidification rate on the nucleation behavior of eutectic components in a hypoeutectic Al-Si-Cu alloy
- 2012
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Ingår i: Metals and Materials International. - : Springer Science and Business Media LLC. - 1598-9623 .- 2005-4149. ; 18:3, s. 405-411
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Tidskriftsartikel (refereegranskat)abstract
- This article is devoted to a study of the influence of solidification rate on the crystallographic orientation of eutectic components with respect to primary α-Al in the tested hypoeutectic alloy. Electron backscattering diffraction (EBSD) patterns were produced from Al-Si cast specimens, solidified with different cooling rates, and prepared by ion etch polishing as a complementary method after mechanical polishing. Results indicated a strong orientation relationship between the primary α-Al and eutectic aluminum phase at all cooling rates. It is also found that the silicon eutectic flakes are nucleated heterogeneously in the interdendritic eutectic liquid. Increasing the cooling rate from 2 to 80 mm/min is observed to be effective in lowering the intensity of relationship between the primary α-Al and eutectic aluminum phase and changing the misorientation angle clustering between primary α-Al and eutectic silicon phase in the interval 41-60 to lower angle intervals.
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| 7. |
- Sadrossadat, Mohsen, 1963-
(författare)
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On the Residual Stresses and Microstructure Formation of Aluminum-Silicon Cast Alloys
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Cast aluminum-silicon alloys are being increasingly used in automotive and aerospace industries for critical structure applications because of their excellent castability, low density, acceptable mechanical properties and low cost. Different casting and heat treatment parameters largely affect the microstructure and residual stress of the components, which in turn, has a great impact on their mechanical properties. In cast components, residual stresses are those stresses which may remain in the casting after it has been removed from the mould. The magnitude and distribution of the residual stresses can be critical to performance and should be considered in the design of a component. This work has been devoted to study the microstructure formation, residual stresses, and mechanical properties of aluminum-silicon castings. The effects of casting parameters on the microstructure of the selected alloys were investigated by means of optical and scanning electron microscopes. With the help of EDS, WDS, EBSD, and quantitative analysis techniques, it was found that solidification rate, modification, superheat, casting temperature can significantly affect the nucleation and growth, morphology and chemical composition of different phases. Based on the performed microsegregation analysis, a clear correlation between the concentration of silicon in the primary phase and cooling rate was found. Tensile tests at room and elevated temperatures in addition to microhardness experiments were performed to analyse the behaviour of the alloys under mechanical loads. The results showed that elongation to fracture, modulus of elasticity, and ultimate tensile strength of the tested alloys are temperature dependent. Moreover, the obtained information was used to establish a good model for simulating the behaviour of the cast alloys, as well as the mechanical properties and residual stresses. The type and magnitude of residual stresses were mainly evaluated by strain gauge, sectioning, thermal analysis methods, and was also simulated by finite element analysis using Abaqus software. It was found that casting parameters such as superheat, mould hardness, casting temperature, modification, and the casting geometry, can influence the accumulated residual stress in the component. The thermal treatment experiments also indicated that the base temperature of the cast part before fast cooling, maximum temperature difference within the component, and cooling water flow can influence the residual stress. Extensive simulation work done by Abaqus showed that the results obtained by simulation are in a reasonable relationship with the experimental measurements, considering the linearly elastic/linearly isotropically hardening plastic model.
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| 8. |
- Yuan, Kang, 1987-
(författare)
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Oxidation and Corrosion of New MCrAlX Coatings : Modelling and Experiments
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- MCrAlY coatings (“M” for Ni and/or Co) are widely used for the protection of superalloy components operated at high temperatures such as in the hot sections of gas turbines. The exposure to high temperature can cause coating degradation due to oxidation or hot corrosion at the coating surface. Microstructures in the coating and the coating life are affected also by the diffusion of alloying elements through the coating-superalloy interface. This PhD project, by applying thermodynamic modelling and experimental tests, investigates the oxidation and hot corrosion behavior of new MCrAlX coatings, in which X, referring to minor elements, is used to highlight the functions of such elements.In order to understand and predict the coating degradation progress during thermal exposure, an oxidation-diffusion model has been established for MCrAlX coating-superalloy systems, which integrates the oxidation of aluminum at coating surface, diffusion of alloying elements, and the diffusion-blocking effect in the materials. The predicted chemical composition profile and microstructure agreed well with experimental results in a CoNiCrAlYSiTa-Inconel 792 system. The model was further applied in several coating-superalloy systems to study the influence of coating composition, superalloy composition and temperature on the evolution of microstructure in the coating and the coating life. The results have demonstrated the potential of the model in designing new durable MCrAlX coatings. In addition to the applications in coating-superalloy systems, the model was also adapted for studying the microstructural development in a superalloy in which internal oxidation and nitridation occurred in an oxidation process.The oxidation behavior of some HVOF MCrAlX coatings was studied by thermal exposure at different temperatures (900, 1000, 1100 °C). Different spinels formed above the alumina scale, depending on the oxidation temperature. The minor alloying elements, Ru and Ir, had no direct influence on the oxidation behavior but may affect the phase stability in the coating.MCrAlX coatings were also tested in 48-hour cycles at 900 °C in different hot corrosion environments containing sulphates and/or SO2. The results showed that the coating performance was dependent on coating quality, concentration of Al and Cr in the coating, and the hot corrosion condition. It was also found that the addition of SO2 in the environment may not necessarily be bad for hot corrosion resistance of some MCrAlY coatings.
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| 9. |
- Yuan, Kang
(författare)
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Thermal and Mechanical Behaviors of High Temperature Coatings
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- With superior oxidation and corrosion resistance, metallic coatings (i.e. diffusion coatings and MCrAlX coatings) are widely used to protect hot components made of superalloys in turbine engines. Two issues are critically important for the coating at high temperatures: thermal property related to oxidation/corrosion behavior and microstructure stability, and mechanical properties (e.g. creep and fatigue). The aim of this project is to develop better understanding of the thermal and mechanical behaviors of metallic coatings on superalloys and to improve the accuracy of prediction of their lifetime by thermodynamic modeling. The present work includes an investigation on the oxidation behavior of MCrAlX coating with a new lifetimeprediction model and a study on the influence of diffusion coatings on creep and fatigue behaviors of the superalloy IN792.Experiments on isothermal and thermal cycling oxidation were designed to investigate the oxidation behavior of a HVOF CoNiCrAlYSi coating on superalloy IN792. It is found that the oxidation behaviors of the coating are related to its thermodynamic property. A diffusion model has been established using the homogenization models in the DICTRA software and taking into consideration of the influence of surface oxidation, coating-substrate interdiffusion and diffusion blocking effect caused by internal voids and oxides. The simulation results show an improved accuracy of lifetime prediction by introducing the diffusion blocking effect.Microstructural evolution during creep process at high temperatures was studied in different diffusion coatings (NiAl and PtAl). It is found that the inward diffusion of aluminum controls the thickening rate of the diffusion coatings. The developed coatings displayed two types of mechanical behavior - being easily plasticized or cracked - dependent on temperature and type of coating, and therefore could be considered as non-load carrying material during creep test. The influence of cracking of PtAl coating on the high-cycle fatigue (HCF) behavior of the IN792 was also investigated. The results show that precracking of the coating prior to the fatigue test has little influence on the fatigue limit of specimens with thin coating (50 μm) but lowers the fatigue limit of specimens with thick coating (70 μm). The through-coating crack has enough mobility to penetrate into the substrate and causes fatigue failure only when the driving force for crack propagation is increased above a critical value due to a higher applied stress or a larger crack length (thicker coating).
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| 10. |
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- 2011
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swepub:Mat__t (refereegranskat)
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