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- Leidermark, Daniel, 1980-
(författare)
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Modelling of constitutive and fatigue behaviour of a single-crystal nickel-base superalloy
- 2010
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this licentiate thesis the work done in the project KME410 will be presented. The overall objective of this project is to evaluate and develop tools for designing against fatigue in single-crystal nickel-base superalloys in gas turbines. Experiments have been done on single-crystal nickel-base superalloy specimens in order to investigate the mechanical behaviour of the material. The constitutive behaviour has been modelled and verified by simulations of the experiments. Furthermore, the microstructural degradation during long-time ageing has been investigated with respect to the component’s yield limit. The effect has been included in the constitutive model by lowering the resulting yield limit. Finally, the fatigue crack initiation of a component has been analysed and modelled by using a critical plane approach.This thesis is divided into three parts. In the first part the theoretical framework, based upon continuum mechanics, crystal plasticity and the critical plane approach, is derived. This framework is then used in the second part, which consists of three included papers. Finally, in the third part, details are presented of the used numerical procedures.
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| 2. |
- Hallström, Samuel
(författare)
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On high temperature oxidation resistance : Towards the materials genome of high temperature alloys
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The efficiency of a heat engine increases with increasing service temperature. This leads to a demand for material that can withstand aggressive environments and sometimes also high load at higher and higher temperature. That is the rationale for the work of this thesis. It is divided into two parts, both addressing model alloy systems for components used in oxidizing high temperature environments. In the first part the phase equilibria and phase diagrams of Ni-Ru and Al-Ni-Ru are investigated from a thermodynamic point of view using the well known Calphad method. In particular, the debated existance of a miscibility gap between the aluminides NiAl and RuAl is considered. This led to a combined ab initio/Calphad approach and it is suggested that there is a miscibility gapat low temperature. In the second part, first of its kind diffusion simulations in oxides are performed in the technologically important Fe-O and Cr-O systems. In the simulations, the moving phase boundary problem is solved with use of temperature and composition dependent diffusion coefficients, that are evaluated for complex oxide phases modeled with up to four sublattices. This type of simulations attracts a lot of interest and it looks very promising for future extension to higher order systems.
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| 3. |
- Segersäll, Mikael
(författare)
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Nickel-Based Single-Crystal Superalloys : the crystal orientation influence on high temperature properties
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Superalloys are a group of materials that are used in high temperature applications, for example gas turbines and aero engines. Gas turbines are most commonly used for power generation, and it is only the very critical components which are exposed to the most severe conditions within the turbine, which are made from superalloy material.Today, energy consumption in many parts of the world is very high and is tending to increase. This implies that all power generating sources, including gas turbines, must aim for higher efficiency. For the gas turbine industry, it is a continuous challenge to develop more energy-efficient turbines. One way to do this is to increase the temperature within the hot stage of the turbine. However, increased temperature in the hot stage also challenges the materials that are used there. Today’s materials are already pushed to the limit, i.e. they cannot be exposed to the temperatures which are required to further increase the turbine efficiency. To solve this problem, research which later can lead to better superalloys that can withstand even higher temperatures, has to be conducted within the area of superalloys.The aim of this licentiate thesis is to increase our knowledge about deformation and damage mechanisms that occur in the microstructure in superalloys when they are subjected to high temperatures and loads. This knowledge can later be used when developing new superalloys. In addition, increased knowledge of what is happening within the material when it is exposed to those severe conditions, will facilitate the development of material models. Material models are used for FEM simulations, when trying to predict life times in gas turbine components during the design process.This licentiate thesis is based on results from thermomechanical fatigue (TMF) testing of Ni-based single-crystal superalloys. Results show that the deformation within the microstructure during TMF is localized to several deformation bands. In addition, the deformation mechanisms are mainly twinning and shearing of the microstructure. Results also indicate that TMF cycling seems to influence the creep rate of single-crystal superalloys.
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| 4. |
- Odenberger, Eva-Lis
(författare)
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Concepts for hot sheet metal forming of titanium alloys
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- To increase the competitiveness of the Swedish aero engine industry alternative manufacturing methods for load carrying aero engine structures are desired, in order to reduce product cost and enable weight reduction and thereby fuel consumption. Traditionally, these structures mainly consist of large-scaled single castings of e.g. titanium- and nickel based super alloys. By fabrication, the structures are instead built from sheet metal parts, small ingots and simple forgings which are welded together and heat treated. The alternative approach implies the need for time and cost efficient evaluations of candidate manufacturing techniques, early in the product development process. One challenge in producing complete structures within shape tolerance lies in accurate predictions of springback and compensation for shape deviation which occurs in the different processes of the manufacturing chain. Finite Element (FE) simulations are used extensively in e.g. the sheet metal stamping industry where the technology has contributed to a better understanding of chosen forming processes and where the prediction capabilities has significantly reduced the time consuming, inexact and costly die tryouts. However, the reliability of the numerical simulations depends not only on the models and methods used but also on the accuracy and applicability of the input data. The material model and related property data must be consistent with the conditions of the material in the process of interest. In addition, creating as little deviance as possible between the FE model and the experimental setup is a prerequisite for the correlation between predicted and measured values. Naturally, difficulties regarding e.g. modelling and estimations of friction arise, among others.The objective of this thesis is to suggest possible hot and cold forming concepts based on FE analyses for the production of sheet metal prototype components in the titanium alloys Ti-6Al-4V and Ti-6242 together with the nickel based super alloy Inconel 718, respectively. The research activities are focused on material characterisation, evaluation of suitable constitutive models and its calibration, virtual tool design and manufacturing of sheet metal forming tools together with production of prototype components. The aim is to perform a direct-hit research and development work in which the lead time is short and the need for the manual die tryout can be kept at a minimum. The forming tests functions as validation tests in which predicted responses of global forming force, draw-in, temperature, strain localisation and shape deviation are correlated with predicted responses. Different yield criteria which include the anisotropy and strength differential (asymmetry in yielding between tension and compression) of the titanium alloy Ti-6Al-4V are compared with an isotropic assumption. Special emphasises are made to models and methods suitable for analyses in the medium temperature range, for evaluations early in the product development process.In paper A, compression tests on Ti-6Al-4V were performed at different temperatures in order to study the mechanical behaviour and create experimental reference data for identification of material model parameters of constitutive equations. Inverse modelling was used as a method for the parameter identification, in which three different equations were studied. At a temperature of 500°C, none of the studied constitutive equations were found able to satisfactory describe the flow behaviour. However, the method was found suitable for the purpose of identifying model parameters. Later on, the physically based constitutive equation developed by Nemat-Nasser et al. (2001) was found able to describe the flow behaviour of Ti-6242 [1]. In the work by Nemat-Nasser et al. the model has been shown to be able to describe the flow behaviour of Ti-6Al-4V at different temperatures and strain rates accurately. The equation was applied in FE analyses of a hot forming test, a U-bend test, of Ti-6242. The experimental study of Ti-6242, including U-bend tests, at different thermo-mechanical conditions performed in Paper B, revealed that the formability is increased and that the springback can be decreased with increasing temperature. However, it was also found that an increased temperature alone does not necessary imply a reduced shape deviation. In paper C, a short lead time methodology for the design, compensation and manufacturing of deep drawing tools in the nickel based alloy Inconel 718 is suggested. Rather than stating a new methodology, the work contributes to the idea that it is possible to perform a virtual direct-hit development work for the production of five different double-curved components within tolerance at an extremely short lead time. Compensation for the predicted shape deviation was performed in which the tool surfaces were over compensated by means of FE analyses. In paper D and F, the short lead time methodology was applied for the development of hot forming concepts to produce two different Ti-6Al-4V sheet metal components. The material characterisation, presented in paper E, provides with experimental reference data for calibration of three different yield criteria. Predicted responses such as punch force, draw-in and shape deviation show promising agreement with experimental observations when applying anisotropic yield formulations. The shape of the yield surface was found important for the prediction of shape deviation and the occurrence of strain localisation. Some issues of the FE-model suggest areas for further development. An interesting extension to the present work would be to include models for phase transformation and creep or stress relaxation and include the effect of strain rate for sheet metal forming in the higher temperature range. Further on, it is of interest to extent anisotropic yield criteria to function in coupled thermo-mechanical analyses and to include orthotropic elasticity. This, in order to increase the possibility to perform detailed studies of the temperature as an important process parameter for the prediction of shape deviation and studies of strain localisation limits.
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