| 61. |
- Thiery, Florian
(författare)
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Simplified models to evaluate nonlinear dynamics in hydropower rotors
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In hydropower systems, it is essential to avoid catastrophic failures that lead to human and economic losses. Unfortunately, a hydropower rotor can behave abnormally since several nonlinear effects occur during start-up, shut-downs or when running at nominal speed. Weak nonlinear interaction in the bearings, electromagnetic interaction between the generator and rotor or fluid-structure interaction in turbines are typical nonlinear effects that may appear. Moreover, strong nonlinearities can also occur due to blade contacts and assembly errors. These types of nonlinearities can be dangerous in case of bad design of the rotor, and it could even lead to catastrophes in the worst case. In this thesis, simple models are used to describe the different types of nonlinearities, with focus on blade rubbing, bearings and electromagnetic interaction. These interactions are usually investigated on Jeffcott rotors or rigid rotors to reduce the dimension of the design space to a few important parameters only. The dynamics of the system are evaluated by using common tools such as Poincare sections, bifurcation diagrams, Maximum Lyapunov Exponent (MLE), Lyapunov spectrum and waterfall plots of the Fast Fourier Transform (FFT). The numerical results have been compared with experimental results to ensure that these models are satisfying in our range of study. Once these simple models have been verified, they can be used to simulate the full hydropower rotor by including all interaction types. A focus is made on the numerical methods to employ and reduction methods to gain computation time, as well as to know under which circumstances the nonlinear interactions have to be included in comparison with the linear analysis. As a result, this work intends to provide guidelines about the models to use to perform dynamic simulations on fullscale turbines and to know when a linear model can be sufficient to evaluate a machine at design stage or when changing any mechanical component in the hydropower rotor.
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| 62. |
- Tsandzana, Afonso Fernando, 1969-
(författare)
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Homogenization of some new mathematical models in lubrication theory
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- We consider mathematical modeling of thin film flow between two rough surfaces which are in relative motion. For example such flows take place in different kinds of bearings and gears when a lubricant is used to reduce friction and wear between the surfaces. The mathematical foundations of lubrication theory is given by the Navier--Stokes equation, which describes the motion of viscous fluids. In thin domains several approximations are possible which lead to the so called Reynolds equation. This equation is crucial to describe the pressure in the lubricant film. When the pressure is found it is possible to predict vorous important physical quantities such as friction (stresses on the bounding surfaces), load carrying capacity and velocity field.In hydrodynamic lubrication the effect of surface roughness is not negligible, because in practical situations the amplitude of the surface roughness are of the same order as the film thickness. Moreover, a perfectly smooth surface does not exist in reality due to imperfections in the manufacturing process. Therefore, any realistic lubrication model should account for the effects of surface roughness. This implies that the mathematical modeling leads to partial differential equations with coefficients that will oscillate rapidly in space and time. A direct numerical computation is therefore very difficult, since an extremely dense mesh is needed to resolve the oscillations due to the surface roughness. A natural approach is to do some type of averaging.In this PhD thesis we use and develop modern homogenization theory to be able to handle the questions above. Especially, we use, develop and apply the method based on the multiple scale expansions and two-scale convergence. The thesis is based on five papers (A-E), with an appendix to paper A, and an extensive introduction, which puts these publications in a larger context.In Paper A the connection between the Stokes equation and the Reynolds equation is investigated. More precisely, the asymptotic behavior as both the film thickness and wavelength of the roughness tend to zero is analyzed and described. Three different limit equations are derived. Time-dependent equations of Reynolds type are obtained in all three cases (Stokes roughness, Reynolds roughness and high frequency roughness regime). In paper C we extend the work done in Paper A where we compare the roughness regimes by numeric computations for the stationary case.In paper B we present a mathematical model that takes into account cavitation, surfaces roughness and compressibility of the fluid. We compute the homogenized coefficients in the case of unidirectional roughness.In the paper D we derive a mathematical model of thin film flow between two close rough surfaces, which takes into account cavitation, surface roughness and pressure dependent density. Moreover, we use two-scale convergence to homogenize the model. Finally, in paper E we prove the existence of solutions to a frequently used mathematical model of thin film flow, which takes cavitation into account.
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| 63. |
- Wallström, Peter
(författare)
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On Value and Waste
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Value and waste are concepts that are used in improvement projects. In lean the concepts are fairly simple. Reduce the waste and the value has increased. However, value is both multidimensional and differs over time. If the concepts value and waste are to be used, the concepts must be clearly defined and measured. Otherwise, value can be reduced for the customer/user and the cost increased for the producer/seller. The purpose in this thesis is to investigate how value and waste are perceived by different stakeholders, how value and waste are related, and how value and waste are measured. The focus of the study is the improvement of production and services. The study does not investigate the product/service development. The conclusions are based on a number of cases and research from different fields such as resource-based view and marketing. The study use mix of qualitative and quantitative methods. Measures of forecasting accuracy and their relations where explored with different statistical tools in order to understand the influence of measures and dimensions. The view of value concerning energy efficiency was examined in a statistical analysis of a survey concerning stakeholders’ view of a specific value, energy efficiency, as well as their influence on the value creation process. A multiple qualitative case study explores the relation between value and waste in different settings and the consequences of waste focus. The findings in the multiple case study are confirmed and elaborated further by an additional case study, both qualitative and quantitative, of value stream mapping.Value and waste are analysed with the use of order winners and qualifiers. Also, a model to clarify the consequences of mixing value creation and value exchange for customer/user and producer/seller have been defined and used in the analysis.Depending on the stakeholder there is a difference between whether value can be regarded as a use value, exchange value or both. Even if exchange value is related to a specific moment in time, use value is not. The view of value differs among stakeholders which increase the risk of sub-optimisation in production.Value and waste have multi-dimensional properties and there are links between the different dimensions. The relationships depend upon the situation in question. The lean seven types of waste are not independent dimensions. Also, the concept of waste as anti-value is too simplistic. In all cases studied the focus is on waste, not value. Also, it is often the symptoms of waste that are of interest in measures taken not the root causes. Reduction of waste without considering the value can create new waste. Since waste is a dependent variable, it should not be measured without considering value. Another complication is that value and waste often occurs at different points in time and in different settings.Single measures are sensitive to its environment. Several measures are more robust. Measures distort and influence the perception and thereby the decision of the studied phenomena. Also, the notion of value and waste becomes harder to define and trace as the resolution and detailing of the studied process increases.
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| 64. |
- Wenngren, Johan
(författare)
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Team activities in concept development : Addressing open-ended problems
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The concept development stage is a fundamental part of the development process; in this early stage, teams define what constitutes the problem and find direction for the subsequent problem-solving activities. Concept development that searches for innovative results starts from a problem formulation, or design brief, that should not be too precise; keeping parts open for interpretation and alternatives is important for allowing for the possibility to create new solutions. However, handling such open-ended development problems is a challenge as they do not point to a specific solution from the start. The difficulties that the development team encounters in order to find a way forward are the focus of this thesis. Thus, the purpose of this thesis is to identify, describe and discuss how teams explore and define open-ended problems in concept development. Fundamentally, teams have to explore and define open-ended problems in order to retrieve answers needed to undertake purposeful actions. The exploration and definition procedure creates stepwise understanding of sub-problems as open-ended problems typically have interdependencies. This implies that the team must suggest a number of representations before they can propose corresponding alternative solutions. The failure to adopt such a procedure leads to a risk of fixation and prematurely closing the definition of what constitutes the core issues of the design problem. This study specifically shows that confronting the design brief or task is an essential activity in the resolution of open-ended problems. Moreover, a number of practical steps for iteratively managing open-ended problem resolution are proposed. The study in this thesis is based on an explorative approach utilising experiments and observations as methods for data collection. The empirical data come from a workshop format (i.e., an innovation contest), teams on student projects and, to some extent, companies. The contribution from this thesis to engineering design relates mainly to the social and participatory aspects of innovation in concept development. The contribution to practice is mainly the formulation of iterative steps; these could be applicable to different types of open-ended problem resolution. One additional area of application may be a more modern type of product development in the manufacturing industry — namely, where product logics and service logics are interlinked.
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| 65. |
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| 66. |
- Ye, Pengcheng
(författare)
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Zeolite Membrane Separation at Low Temperature
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The energy consumption of separation processes accounts for a large part of the total energy consumption in chemical industry. Membrane separation processes require much less energy than the currently used thermally driven separation processes and could therefore reduce energy consumption in industry considerably. Today, most commercially available membranes are organic polymeric membranes. Inorganic zeolite membranes have several superiorities over polymeric membranes, e.g., higher flux and selectivity, higher chemical and thermal stability, and thus have great potential for a variety of gas and liquid separations. Whereas there have been extensive studies on zeolite membrane separation at high temperature during the past decades, scientific reports on the low temperature applications of zeolite membranes is extremely scarce and there are no reports at cryogenic temperature. This work is pioneering research on investigation of the performance of zeolite membranes for separation of various gas mixtures at unprecedentedly low temperature, down to cryogenic temperature. In the present work, zeolite membranes were, for the first time, evaluated for gas separation at cryogenic temperature. Air separation by ultra-thin MFI membranes was carried out at a feed pressure ranging from 100 mbar to 5 bar over the temperature range of 62–110 K. The membranes were found to be oxygen selective at all the conditions investigated. The observed results were well above the upper bound in the 2008 Robeson selectivity-permeability plot when the feed pressure was less than or equal to 1 bar. The O2/N2 separation factor reached 5.0 at 67 K and 100 mbar, with a high O2 permeance of 8.6 × 10-7 mol m-2 s-1 Pa-1. The performance of our membranes (in terms of selectivity) was comparable to that recently reported for promising polymeric membranes, but 100 times higher in terms of permeance and flux. The membrane selectivity was found to increase with decreasing temperature and feed pressure. The present work has therefore indicated the optimum conditions for air separation using MFI membranes, namely low feed pressures and cryogenic temperatures. A mathematical model showed that the selectivity to O2 emanated from O2/N2 adsorption selectivity. N2/He separation is essential for helium recovery from natural gas and helium reclamation for airships and submarines. Zeolite membranes were evaluated for this separation over the temperature range of 85–260 K, possessing high N2-selectivity at all the conditions investigated. When the feed pressure was 5 bar and the permeate pressure was 0.5 bar, a highest N2/He separation factor of 62 was observed at 124 K. The N2 permeance was rather high, up to 39 ×10−7 mol m−2 s−1 Pa−1. The separation was attributed to adsorption selectivity of the membranes to N2, effectively suppressing the transport of He in the zeolite pores and this effect was more significant at cryogenic temperature. A mathematical model showed that the largest difference of adsorbed loading over the film at ca. 120 K was probably the main reason for the observed maximum selectivity at this temperature. The model also indicated that the selectivity could even be increased by 2–3 times if the membrane was totally defect-free. This work demonstrates that a zeolite membrane process could be rather competitive for N2/He separation. Synthesis gas generated from biomass is a valuable, renewable resource that can be used for production of clean energy and various chemicals. It is mainly a mixture of CO, CO2, and H2. CO2 is an undesired component in the syngas and should, therefore, be removed. In this work, CO2 separation from H2 and CO using zeolite membranes was studied for at low temperatures, down to 235 K and at a feed pressure of 9 bar. The membrane performance in terms of both selectivity and flux was superior to that reported for the state-of-the-art polymeric and inorganic membranes. The highest separation factor was 202 for CO2/H2 separation at 235 K and 21 for CO2/CO separation at 258 K, significantly higher than that at room temperature. The observed CO2 flux was very high, i.e., 300-420 kg m-2 h-1, in the entire temperature range of 235–310 K. Initial cost estimation revealed that high flux zeolite membranes were economically competitive with the present commercial polymeric membranes. Moreover, the process relying on our zeolite membranes was shown to be appreciably more space-efficient. Efficient light olefins/N2 separation technologies are of great interest to recover monomers from N2 purge gas in polymer plants. C3H6/N2 and C2H4/ N2 separation were investigated using zeolite membranes in a temperature range of 258–356 K. The membranes were rather selective towards the hydrocarbons. For C3H6/N2 separation, a maximum separation factor of 43 was observed at room temperature with a C3H6 permeance of 22×10-7 mol m-2 s-1 Pa-1. For C2H4/N2 separation, the maximum separation factor was 6 at 277 K with a C2H4 permeance of 57×10-7 mol m-2 s-1 Pa-1. The findings reveal that zeolite membranes are promising candidates for light olefins/N2 separation in petrochemical processes. The adsorption properties dominate separation performance for systems studied in the present work. The high selectivity emanates from competitive adsorption, e.g., the strongly adsorbing components hinder the permeances of the weakly adsorbing ones and the effect was stronger at low temperature. In addition, gas permeances through zeolite membranes tend to decrease at low temperature most likely due to decreasing diffusivity, especially at cryogenic temperature. However, the permeances of our membranes even at low temperature were still one to two orders of magnitude higher than those reported for inorganic and polymeric membranes. Thus, the high-flux membranes have great superiority in this case. The fairly high permeance even at low temperatures was ascribed to the ultra-thin (< 1µm) film and highly permeable support used. We provide here a promising candidate, ultra-thin zeolite membranes, with high permeance and excellent selectivity for gas separation application at low temperature.
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| 67. |
- Zhang, Liangwei
(författare)
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Big Data Analytics for Fault Detection and its Application in Maintenance
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Big Data analytics has attracted intense interest recently for its attempt to extract information, knowledge and wisdom from Big Data. In industry, with the development of sensor technology and Information & Communication Technologies (ICT), reams of high-dimensional, streaming, and nonlinear data are being collected and curated to support decision-making. The detection of faults in these data is an important application in eMaintenance solutions, as it can facilitate maintenance decision-making. Early discovery of system faults may ensure the reliability and safety of industrial systems and reduce the risk of unplanned breakdowns.Complexities in the data, including high dimensionality, fast-flowing data streams, and high nonlinearity, impose stringent challenges on fault detection applications. From the data modelling perspective, high dimensionality may cause the notorious “curse of dimensionality” and lead to deterioration in the accuracy of fault detection algorithms. Fast-flowing data streams require algorithms to give real-time or near real-time responses upon the arrival of new samples. High nonlinearity requires fault detection approaches to have sufficiently expressive power and to avoid overfitting or underfitting problems.Most existing fault detection approaches work in relatively low-dimensional spaces. Theoretical studies on high-dimensional fault detection mainly focus on detecting anomalies on subspace projections. However, these models are either arbitrary in selecting subspaces or computationally intensive. To meet the requirements of fast-flowing data streams, several strategies have been proposed to adapt existing models to an online mode to make them applicable in stream data mining. But few studies have simultaneously tackled the challenges associated with high dimensionality and data streams. Existing nonlinear fault detection approaches cannot provide satisfactory performance in terms of smoothness, effectiveness, robustness and interpretability. New approaches are needed to address this issue.This research develops an Angle-based Subspace Anomaly Detection (ABSAD) approach to fault detection in high-dimensional data. The efficacy of the approach is demonstrated in analytical studies and numerical illustrations. Based on the sliding window strategy, the approach is extended to an online mode to detect faults in high-dimensional data streams. Experiments on synthetic datasets show the online extension can adapt to the time-varying behaviour of the monitored system and, hence, is applicable to dynamic fault detection. To deal with highly nonlinear data, the research proposes an Adaptive Kernel Density-based (Adaptive-KD) anomaly detection approach. Numerical illustrations show the approach’s superiority in terms of smoothness, effectiveness and robustness.
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| 68. |
- Zhang, Yingying
(författare)
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Thermodynamic Analysis and Screening ILs/DESs-based Absorbents for CO2 Separation
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- CO2 separation plays an important role in both biofuel production, and CO2 capture and storage (CCS) implementation to deal with global warming. The available CO2 separation technologies are either energy-intensive or require large-scale operations, and it is crucial to develop novel CO2 separation technology in order to optimize the energy uses and the amounts of CO2-absorbents/adsorbents.Recently, ionic liquids (ILs) have been proposed as potential liquid absorbents for CO2 separation with remarkable properties. A lot of ILs have been synthesized for this purpose. The CO2 absorption capacity/selectivity and the energy use have been considered in screening ILs, while the amounts of ILs needed have seldom been considered in the screening process. Meanwhile, the high-cost, toxicity and poor biodegradability of the conventional ILs limit their applications in large-scale. Deep eutectic solvents (DESs) have emerged as a new type of ILs, and in particular, those based on choline salts (i.e. choline-based DESs) show additional advantages in cost, environmental impact and synthesis. Choline-based DESs have been synthesized and the research work related to CO2 separation with this series of DESs and their aqueous solutions has been carried out. However, it is still unclear which absorbent can achieve a better performance for CO2 separation.The choice of absorbents for CO2 separation depends on gas streams, and the performances of absorbents for CO2 separation relate to the energy uses and the amounts of absorbents needed. In this thesis work, four gas streams (i.e. flue gas and lime kiln gas from the combustion of fossil-fuels, biogas from the anaerobic digestion of biomass as well as bio-syngas from the gasification of biomass) with different temperature, pressure, CO2 concentration and gaseous components were considered, and CO2 separation from four gas streams was analyzed thermodynamically based on Gibbs free energy change. The analysis shows that biogas is the CO2 stream with the lowest theoretical energy penalty. Therefore, biogas was chosen as a specific CO2 stream for further evaluating the performances of CO2 absorbents.In evaluation, the conventional ILs were first analyzed and screened for CO2 separation from biogas with three options (i.e. option 1: the CO2 dissolution enthalpy and CO2 working capacity, option 2: the energy use, and option 3: the energy use and the amount of IL needed). The investigation shows that the screen of ILs is strongly related to the operational condition and the screening criteria. In the option of “the energy use and the amount of IL needed”, the operational condition was optimized based on the minimum Gibbs free energy change, and the energy use and the amount of IL needed were considered in screening. While in other screening options, the operational conditions were presumed and the amounts of ILs needed were not considered. Therefore, the option of “the energy use and the amount of IL needed” is more reasonable compared to the other two options. The performances of these screened conventional ILs were further compared with those of the commercial CO2 absorbents. It shows that the conventional ILs are promising CO2 absorbents due to lower energy uses or lower amounts of ILs needed combined with the advantage of non-volatility.The research work on choline-based DESs and their aqueous solutions for CO2 separation was surveyed and reviewed. Generally, the properties of choline-based DESs are similar to those of conventional ILs. Considering the additional advantages of low-cost, non-toxicity and biodegradability, choline-based DESs are more promising for CO2 separation. However, due to the limited available research work, further studies need to be carried out from experimental measurements to model developments. The performances of choline-based-DESs for CO2 separation from biogas were analyzed. Based on the option of “the energy use and the amount of absorbent needed”, the choline-based-DESs were screened and then compared with the conventional ILs and the commercial CO2 absorbents. The comparison results show that the choline-based-DESs are more promising for CO2 separation from biogas due to the non-volatility, lower energy uses or lower amounts of absorbents needed. In addition, CO2 separation from other CO2 streams was further investigated. It shows that the physical absorbents are more suitable for the CO2 streams with high CO2 concentration (i.e. biogas, lime kiln gas and bio-syngas), while the chemical CO2 absorbents are more suitable for that with low CO2 concentration and high temperature (i.e. flue gas). Considering the high amounts of physical absorbents, further study needs to be carried out with techno-economic analysis.
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| 69. |
- Zrida, Hana
(författare)
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Composites with bundle mesostructure: Elastic properties and Damage
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Many types of composite materials are today used in various types of load carrying structures, due to their excellent strength and stiffness to weight ratio. Simplicity, reliability and low cost of the material processing are important factors affecting the final selection.With the textile reinforced composites, the cost-efficiency is reached by using dry preforms which are impregnated by resin infusion, resin transfer molding etc.; this have made a break-through and have been widely used. Textile composites with bundle meso-structure have been studied in this thesis for elastic properties and damage investigations. The first part of this thesis deals with elastic properties modeling for Non-crimp fabric (NCF) based composites for investigating the effect of meso-structure defects on mechanical properties degradation. The objective of the work is to formulate a model for the NCF composite mesostructure in an attempt to investigate the effect of the waviness on stiffness reduction. Moreover, the stiffness calculation methods for the complex geometry are explained and justified and finally, the different geometrical parameters changes are taken into consideration and included in the calculation.The damage initiation and development is presented is the second part, where woven fabric composites designated for high temperature application were investigated under severe thermal conditions to study their thermal stability and their resistance to thermal damage. The mechanical performance of the same composites was studied. The effect of aging was also investigated. 3D models were realized with Finite elements in order to explain the edge effect on the evolution of the cracks observed during the tensile tests. In addition, the differences and similarities in cracking in different layers were analysed using probabilistic approaches (a simple one as well as Monte Carlo simulations with Hashin’s and also shear lag model) and fracture mechanics arguments.
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| 70. |
- Åkerfeldt, Pia
(författare)
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Additive Manufacturing of Ti-6Al-4V: Relationship between Microstructure, Defects and Mechanical Properties
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Additive manufacturing (AM) is a relatively new technology that is labelled to be innovative, disruptive, near-net shaping, enabling manufacturing of complex and customised products, for limitless number of applications, directly from the CAD model into real physical parts. For titanium alloys in aerospace applications, AM moreover stands for a reduced material cost, but also for large challenges when considering consistency and qualification of material properties and components in serial production. In the AM process the feedstock material is melted by a heat source that moves according to a building sequence defined by the CAD model. Layer-by-layer the material solidifies into the wanted shape and accordingly the microstructure forms,which determines the average mechanical properties of the manufactured component. However, even if the AM process seems to be very straight forward, the prediction of mechanical and metallurgical properties is complex, partly because of its building in layer nature which generates a complex thermal history dictating the mechanical properties, and partly because of the number of parameters involved during the AM process itself. The objective of the present work was to increase the fundamental understanding of the relationship between microstructure, defects and mechanicalproperties of AM:ed Ti-6Al-4V. Three AM techniques were investigated, namely laser metal-wire deposition (LMwD), electron beam melting (EBM), and gas tungsten arc welding (GTAW) wire feed AM, with the main focus on LMwD. The different techniques were evaluated with regard to microstructure and tensile and fatigue properties. In addition, the EBM Ti-6Al-4V was tested in a hydrogen atmosphere to simulate the working environment for a certain engine application. One of the core findings in the present work was that AM:ed Ti-6Al-4V exhibited a columnar microstructure with elongated prior beta grains growing through several layers following the temperature gradient direction in the built material. To cover the different characteristics of the columnar microstructure, the mechanical properties were evaluated in two orientations of the built Ti-6Al-4V. The mechanical properties, both static and dynamic, were found to be anisotropic, which was further evaluated indetail with respect to the microstructure evolution and defects generated by the AM process. Among the results, when different process conditions were tested, it was concluded that the thickness of the grain boundary alpha along the prior beta grain boundary did not influence the level of anisotropy. However, the prior beta grain boundary was observed to be the weakest microconstituent when the load was applied perpendicular to its prevalence in both tensile and LCF testing. In order to get a better understanding of how the columnar microstructure influences the fatigue properties, the fatigue crack propagation characteristics were investigated with respect to the columnar prior beta grains and crystal orientation. An extensive fractographic study was carried out on all tested specimens. Lack of fusion (LoF) defects were concluded to be the individually most detrimental type of defect to the material properties. The influence of the LoF defects was further concluded to be very dependent on its prevalence in relation to the loading direction; the largest impact on the fatigue life was observed when the LoF defect wasperpendicular to the loading direction. Finally, a part of the aim of the present work was to support the development of a microstructure model that will be implemented in a thermo-mechanical model when simulating AM of Ti-6Al-4V. In order to validate the material model developed, the alpha lath thickness and the fraction of grain boundary alpha were quantified atspecific locations in single and multiple bead walls of GTAW wire feed AM:ed Ti-6Al-4V and compared with the results of the simulated AM process of Ti-6Al-4V.
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