| 1. |
- Bjurenstedt, Anton, 1979-
(författare)
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On the influence of imperfections on microstructure and properties of recycled Al-Si casting alloys
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- There are great energy savings to be made by recycling aluminium; as little as 5% of the energy needed for primary aluminium production may be required. Striving to produce high quality aluminium castings requires knowledge of microstructural imperfections, which is extra important when casting recycled aluminium that generally contains higher levels of imperfections compared to primary aluminium. Imperfections include amongst others Si, Fe, and Mn as well as oxides. Si is needed for castability, but it may also initiate fracture. There are different types of Fe-rich intermetallics influencing properties of castings, generally in a negative direction. Oxides constitute cracks and they are elusive because they are difficult to quantify.This thesis aims to increase knowledge about imperfections in recycled aluminium castings originating from alloying elements and the melt. Experiments were performed in advanced laboratory equipment, including X-radiographic imaging during solidification and in-situ tensile testing in a scanning electron microscope. Experiments were also performed at industrial foundry facilities.The experiments showed that the nucleation temperature of primary α-Fe intermetallics increased with higher Fe, Mn, and Cr contents. Primary α-Fe are strongly suggested to nucleate on oxides and to grow in four basic morphologies. Lower nucleation frequency of α-Fe promoted faster growth and hopper crystals while higher nucleation frequency promoted slower growth rates and massive crystals. Results also showed that a decrease in the size of the eutectic Si and plate-like β-Fe intermetallics improved tensile properties, foremost the elongation to fracture. In β-Fe containing alloys the transversely oriented intermetallics initiated macrocracks that are potential fracture initiation sites. In alloys with primary α-Fe foremost clusters of intermetallics promoted macrocracks. In fatigue testing, a transition from β-Fe to α-Fe shifted the initiation sites from oxides and pores to the α-Fe, resulting in a decrease of fatigue strength. Oxides in Al-Si alloys continue to be elusive; no correlations between efforts to quantify the oxides and tensile properties could be observed.
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| 2. |
- Bogdanoff, Toni
(författare)
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The influence of microstructure on the crack initiation and propagation in Al-Si casting alloys
- 2021
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- For reducing the CO2 footprint in many industrial fields, the goal is to produce lighter components. The aluminium-silicon (Al-Si) cast alloys are promising candidates to fulfill these goals with a high weight-to-strength ratio, good corrosion properties, excellent castability, and recyclable material. However, the variations within these components need to be understood to produce high-performance components for critical applications. The main reason for the rejection in these applications is defects and microstructural features that reduce the mechanical properties. The addition of copper (Cu) is one way of increasing the mechanical properties in Al-Si alloys and is commonly used in the automotive industry. Casting defects harm the mechanical properties, and these defects can be reduced by improving the melt quality, the correct design of the component, and the gating system.The study aims to investigate the static mechanical properties and the crack initiation and propagation under cyclic loading in an Al-7Si-Mg cast alloy with state-of-the-art experiments. The main focuses were on the effect of the HIP process and the role of Cu addition. In-situ cyclic testing using a scanning electron microscope coupled with electron back-scattered diffraction, digital image correlation, focused ion beam (FIB) slicing, and computed tomography scanning was used to evaluate the complex interaction between the crack path and the microstructural features.The amount of Cu retained in the α-Al matrix in as-cast and heat-treated conditions significantly influenced the static mechanical properties by increasing yield strength and ultimate tensile strength with a decrease in elongation. The three-nearest-neighbor distance of eutectic Si and Cu-rich particles and crack tortuosity were new tools to describe the crack propagation in the alloys, showing that a reduced distance between the Cu-rich phases is detrimental for the mechanical properties. Three dimensional tomography using a FIB revealed that the alloy with 3.2 wt.% Cu had a significantly increased quantity of cracked Si particles and intermetallic phases ahead of the crack tip than the Cu-free alloy. The effect of Cu and HIP process in this work shows the complex interaction between the microstructural features and the mechanical properties, and this needs to be considered to produce high-performance components.
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| 3. |
- Chen, Zhe
(författare)
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Surface Integrity and Fatigue Performance of Nickel-based Superalloys
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Due to global warming, the demand for more efficient gas turbines has increased. A way to achieve this, is by increasing the operating temperature of the gas turbine. Therefore, nickel-based superalloys have been developed to withstand these extreme temperatures and loads, especially in the hot sections. Today, the way of operating land-based gas turbines is changing. Instead of running for long periods of time, the operation is becoming more flexible, with ever-increasing cyclic loads and number of start and stop cycles. To handle the increased stress and cycles, component resistance to fatigue failures need to be improved.Surface integrity is critical to fatigue performance, since fatigue cracks normally initiate at a surface. Machining changes the surface integrity which can result in worse fatigue resistance. The work presented in this Ph.D. thesis was conducted in collaboration with Siemens Industrial Turbomachinery AB in Finspång, Sweden. Surface integrity changes which are induced during machining and their effects on fatigue performance have been studied on the alloy Inconel 718. Inconel 718 is a widely-used nickel-based superalloy for high temperature applications in modern gas turbines.Broaching, milling, and wire electrical discharge machining, related to component manufacturing in turbo machinery industries, were included in this study. Machining induced surface defects provide preferential sites for fatigue crack initiation which influence the fatigue performance of the alloy. If compressive residual stresses are induced during machining, they benefit the fatigue life by retarding fatigue crack initiation away from surface regions. Shot peening was performed on machined Inconel 718, by which high compressive residual stresses are deliberately induced. It results in enhanced fatigue performance.The high temperatures in gas turbines generally deteriorate the surface integrity. Recrystallization often occurs in the highly deformed surface layer. Microstructural degradation, in the form of α-Cr precipitates, have also been frequently observed in the deformed surface and sub-surface microstructure. Oxidation at elevated temperatures degrades the surface integrity and thereby also the fatigue performance. Fatigue cracks are preferably initiated at oxidized surface carbides, if thermal exposure has been made prior to the test. It is even worse when high temperatures relax the beneficial surface compression induced by shot-peening and thereby lowering the fatigue resistance.Machinability of a newly developed nickel-based superalloy, AD 730TM, and the surface integrity induced during turning have also been studied in this thesis project. AD 730TM is a candidate for turbine disc applications with an operating temperature above 650 °C. At such high temperatures, Inconel 718 is no longer stable and its mechanical properties start to degrade.To summarize, the results from this thesis work show the importance of understanding surface integrity effects for fatigue applications, especially in harsh environments. Moreover, the knowledge gained through this work could be used for surface enhancement of turbine components which are subjected to a high risk of fatigue failure. These will contribute to more efficient and flexible power generation by gas turbines.
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| 4. |
- 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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| 5. |
- Gustafsson, David, 1983-
(författare)
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Constitutive and fatigue crack propagation behaviour of Inconel 718
- 2010
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this licentiate thesis the work done in the TURBO POWER project Influence of high temperature hold times on the fatigue life of nickel-based superalloys will be presented. The overall objective of this project is to develop and evaluate tools for designing against fatigue in gas turbine applications, with special focus on the nickel-based superalloy Inconel 718. Firstly, the constitutive behaviour of the material has been been studied, where focus has been placed on trying to describe the mean stress relaxation and initial softening of the material under intermediate temperatures. Secondly, the fatigue crack propagation behaviour under high temperature hold times has been studied. Focus has here been placed on investigating the main fatigue crack propagation phenomena with the aim of setting up a basis for fatigue crack propagation modelling.This thesis is divided into two parts. The first part describes the general framework, including basic constitutive and fatigue crack propagation behaviour as well as a theoretical background for the constitutive modelling of mean stress relaxation. This framework is then used in the second part, which consists of the four included papers.
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| 6. |
- Nordström, Joakim, 1971-
(författare)
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Deformation twinning in corrosion-resistant nickel alloys : with a rising nickel content
- 2024
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Sanicro 28 and Alloy 625 are corrosion-resistant nickel alloys with a fully austenitic structure and a very low carbon content, which means they are both well suited for cold working. Since the millennium shift deformation twinning has been a live research issue as it enhances strength and ductility simultaneously. As nickel has been pointed out as a high stacking fault energy element and deformation twinning should be promoted by a low stacking fault energy level they have been considered as opposite poles. Nonetheless, it is known since long that deformation twins can emerge in high stacking fault face centred cubic elements at low temperatures.In this thesis, we have investigated deformation twinning behaviour in corrosion-resistant nickel alloys. The objective is trying to distinguish between deformation twinning in TWIP steel and corrosion resistant nickel alloys regarding for instance size and bundles.Interrupted uniaxial tensile tests have been performed at several cold working temperatures for the alloys: Sanicro 28 (31% nickel) and Alloy 625 (61% nickel). The microstructure has been characterized in homogeneous deformation volume, by scanning electron microscopy electron backscattering diffraction and electron channelling contrast imaging, transmission electron microscopy and X-ray diffraction. In one investigation fracture behaviour has also been studied with secondary electrons. Ab initio calculations, crystal plasticity modelling and DAMASK simulations have been performed to support emphasizing active deformation mechanisms.It has been revealed that deformation twinning can occur in high Ni alloys. With increasing deformation twinning levels, the diffuse necking decreases. Ab initio calculations indicates that the initiation of deformation twins cannot be determined solely by the stacking fault energy. Distinct features were discovered at low strains that could be rejected from being neither deformation twins nor stacking faults. Level of texture increases with increasing strain and decreasing temperature and the texture modes are changed with decreasing temperature.
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| 7. |
- Palmert, Frans, 1986-
(författare)
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Crack growth in single crystal gas turbine blade alloys under service-like conditions
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This work concerns the fatigue crack growth behaviour of nickel base single crystal superalloys. The main industrial application of this class of materials is in gas turbine blades, where the ability to withstand severe mechanical loading in combination with high temperatures is required. In order to ensure the structural integrity of gas turbine blades, knowledge of the fatigue crack growth behaviour under service-like conditions is of utmost importance. The aim of the present work is both to improve the understanding of the crack growth behaviour of single crystal superalloys and to improve the testing and evaluation methodology for crack propagation under thermomechanical fatigue loading conditions. Single crystal superalloys have anisotropic mechanical properties and are prone to localization of inelastic deformation along the close packed planes of the crystal lattice. Under some conditions, crystallographic crack growth occurs along these planes, and this is a complicating factor throughout the whole chain of crack propagation life simulation; from material data generation to component calculation. Crack growth testing has been performed, both using conventional isothermal testing methods and using thermomechanical fatigue crack growth testing. Experimental observations regarding crystallographic crack growth have been made and its dependence on crystal orientation and testing temperature has been investigated. Quantitative crack growth data are presented for the case of Mode I crack growth under isothermal as well as thermomechanical fatigue conditions. Microstructural investigations have been undertaken to investigate the deformation mechanisms governing the crack growth behaviour. A compliance-based method for the evaluation of crack opening force under thermomechanical fatigue conditions was developed, to enable a detailed analysis of the test data. The crack opening force evaluation proved to be of key importance for the understanding of the crack driving force under different testing conditions. The influence of hold time on crack growth behaviour was analysed, both in terms of creep crack growth and in terms of creep effects on the crack opening force. The transition between non-crystallographic and crystallographic crack growth was studied in detail and a criterion was developed to enable accurate predictions of this transition under a wide range of loading conditions representative for gas turbine blades.
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| 8. |
- Pant, Prabhat, 1990-
(författare)
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Residual Stress in Additive Manufacturing : Control using orientation and scan strategies
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Components with complex features that are designed with their function as a core aspect often are not viable to be manufactured with traditional methods. This has been a bottleneck in the past, leading to heavier parts with various sub-assemblies and a significant waste of material. With the emergence of additive manufacturing (AM) technology manufacturing of complex components has now turned into reality. Within AM, the laser-based powder-based fusion (LPBF) method is one of the most widely adopted methods to manufacture near net shape complex metal components. However, to be implemented on a larger scale various hurdles must be mitigated first.One of the main persistent issues in LPBF is of residual stresses (RS), which are formed due to repeated sequences of heating and cooling, creating a high thermal gradient between the layers. These RS can play a significant role in the component’s functionality during service, but also can affect the manufacturing process. Therefore, a detailed investigation into the formation and control of RS is of foremost importance. This thesis aims at shedding light on various aspects of the RS formation especially, the effect of build orientations and different scan strategies. For this purpose, Inconel 718 (IN718) was selected as a material for investigation due to its wide use in gas turbine components and good weldability making it a good material for additive manufacturing processes.L-shaped components and test cubes were prepared for residual stress mapping and microstructure study. The RS were measured using neutron and X-ray diffraction methods where applicable. From the investigations, it was revealed that the L-shape components built in different orientations showed significant variation in RS magnitude, but a general trend of RS distribution with tensile stresses at the surface and compressive at the bulk for all the components. A simplified finite element model for RS prediction was established and validated based on the experimental results. Similarly, the use of different scan strategies can lead to a different magnitude of RS for the L-shape components. The remelting strategy with remelting done after every 3rd printed layer seems to decrease the RS magnitude in comparison to the counterparts printed without remelting. This has also been verified with a simplified finite element simulation. The microstructure study showed that crystallographic texture can also vary with the different scan strategies and no significant preferred orientations of the grains were found with the remelting done at every 3rd printed layer. However, with the total fill strategy, strong crystallographic texture was observed in the scan direction. Further investigations into the remelting scan strategies with different variables of remelting such as power, speed, and number of layers between the remelting scan revealed an effect of the laser power in the increment of texture intensity along the building direction. A combination of chess pattern and remelting every 3rd layer decreased the RS magnitude in comparison with other samples, where parameters for remelting strategies were changed. In addition, the crystallographic texture varied with different process parameters used for the remelting. For further reduction of RS without employing post-processing, investigations into novel scan strategies need to be undertaken and at the same time texture formation also needs to be investigated.
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| 9. |
- Calmunger, Mattias
(författare)
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High-Temperature Behaviour of Austenitic Alloys : Influence of Temperature and Strain Rate on Mechanical Properties and Microstructural Development
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The global increase in energy consumption and the global warming from greenhouse gas emission creates the need for more environmental friendly energy production processes. Biomass power plants with higher efficiency could generate more energy but also reduce the emission of greenhouse gases, e.g. CO2. Biomass is the largest global contributor to renewable energy and offers no net contribution of CO2 to the atmosphere. One way to increase the efficiency of the power plants is to increase temperature and pressure in the boiler parts of the power plant.The materials used for the future biomass power plants, with higher temperature and pressure, require improved properties, such as higher yield strength, creep strength and high-temperature corrosion resistance. Austenitic stainless steels and nickel-base alloys have shown good mechanical and chemical properties at the operation temperatures of today’s biomass power plants. However, the performance of austenitic stainless steels at the future elevated temperatures is not fully understood.The aim of this licentiate thesis is to increase our knowledge about the mechanical performance of austenitic stainless steels at the demanding conditions of the new generation power plants. This is done by using slow strain rate tensile deformation at elevated temperature and long term hightemperature ageing together with impact toughness testing. Microscopy is used to investigate deformation, damage and fracture behaviours during slow deformation and the long term influence of temperature on toughness in the microstructure of these austenitic alloys. Results show that the main deformation mechanisms are planar dislocation deformations, such as planar slip and slip bands. Intergranular fracture may occur due to precipitation in grain boundaries both in tensile deformed and impact toughness tested alloys. The shape and amount of σ-phase precipitates have been found to strongly influence the fracture behaviour of some of the austenitic stainless steels. In addition, ductility is affected differently by temperature depending on alloy tested and dynamic strain ageing may not always lead to a lower ductility.
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| 10. |
- Calmunger, Mattias, 1986-
(författare)
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On High-Temperature Behaviours of Heat Resistant Austenitic Alloys
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Advanced heat resistant materials are important to achieve the transition to long term sustainable power generation. The global increase in energy consumption and the global warming from greenhouse gas emissions create the need for more sustainable power generation processes. Biomass-fired power plants with higher efficiency could generate more power but also reduce the emission of greenhouse gases, e.g. CO2. Biomass offers no net contribution of CO2 to the atmosphere. To obtain greater efficiency of power plants, one option is to increase the temperature and the pressure in the boiler section of the power plant. This requires improved material properties, such as higher yield strength, creep strength and high-temperature corrosion resistance, as well as structural integrity and safety.Today, some austenitic stainless steels are design to withstand temperatures up to 650 °C in tough environments. Nickel-based alloys are designed to withstand even higher temperatures. Austenitic stainless steels are more cost effective than nickel-based alloys due to a lower amount of expensive alloying elements. However, the performance of austenitic stainless steels at the elevated temperatures of future operation conditions in biomass-red power plants is not yet fully understood.This thesis presents research on the influence of long term high-temperature ageing on mechanical properties, the influence of very slow deformation rates at high-temperature on deformation, damage and fracture, and the influence of high-temperature environment and cyclic operation conditions on the material behaviour. Mechanical and thermal testing have been performed followed by subsequent studies of the microstructure, using scanning electron microscopy, to investigate the material behaviours.Results shows that long term ageing at high temperatures leads to the precipitation of intermetallic phases. These intermetallic phases are brittle at room temperature and become detrimental for the impact toughness of some of the austenitic stainless steels. During slow strain rate tensile deformation at elevated temperature time dependent deformation and recovery mechanisms are pronounced. The creep-fatigue interaction behaviour of an austenitic stainless steel show that dwell time gives shorter life at a lower strain range, but has none or small effect on the life at a higher strain range.Finally, this research results in an increased knowledge of the structural, mechanical and chemical behaviour as well as a deeper understanding of the deformation, damage and fracture mechanisms that occur in heat resistant austenitic alloys at high-temperature environments. It is believed that in the long term, this can contribute to material development achieving the transition to more sustainable power generation in biomass-red power plants.
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