| 21. |
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| 22. |
- Fahlström, Karl
(författare)
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Laser welding of boron steels for light-weight vehicle applications
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Laser beam welding has gained a significant interest during the last two decades. The suitability of the process for high volume production has the possibility to give a strong advantage compared to several other welding methods. However, it is important to have the process in full control since various quality issues may otherwise occur. During laser welding of boron steels quality issues such as imperfections, changes in local and global geometry as well as strength reduction can occur. The aspects that need to be considered are strongly depending on alloy content, process parameters etc. These problems that can occur could be fatal for the construction and the lowest level of occurrence is wanted, independent of industry.The focus of this study has been to investigate the properties of laser welded boron steel. The study includes laser welding of boron alloyed steels with strengths of 1500 MPa and a recently introduced 1900 MPa grade. Focus has been to investigate weldability and the occurrence of cracks, porosity and strength reducing microstructure that can occur during laser welding, as well as distortion studies for tolerances in geometry. The results show that both conventional and 1900 MPa boron alloyed steel are suitable for laser welding.Due to the martensitic structure of welds the material tends to behave brittle. Cracking and porosity do not seem to be an issue limiting the use of these steels. For tolerances in geometry for larger structures tests has been done simulating laser welding of A-pillars and B-pillars. Measurements have been done with Vernier caliper as well as a more advanced optical method capturing the movements during the welding sequence. Results from the tests done on Ushaped beams indicates that depending on the geometry of the structure and heat input distortions can be controlled to give distortions from 1 to 8 mm, at a welding length of 700 mm. This means that important geometry points can be distorted several millimeters if the laser welding process not is controlled.
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| 23. |
- Fuentes Martinez, Ana
(författare)
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Teachers’ tactics when programming and mathematics converge
- 2021
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Teachers’ everyday practices are embedded in school contexts in which their teaching autonomy is constrained by rules, moral obligations, physical settings,and official directives. When a curricular revision mandated that programming was to be a part of mathematics in upper secondary education, teachers’ conditions changed. How teachers adapted to the new curriculum and how they navigated the tensions and contradictions that they encountered is in this thesis analyzed in terms of teachers’ tactics and policy strategies. The overall goal of the investigation is to contribute to a critical understanding of how mathematics teachers integrate programming in their professional practice and how this integration aligns and diverges from the intentions behind the reform. The empirical material is drawn from nine individual interviews with mathematics teachers that were already proficient in programming. The teachers’ unit plans and other lesson materials featuring programming activities served as a trigger point to delve into further reflections upon their own professional practices. To complete the scene, the policy documents were also examined. These included the mathematics curriculum, as well as related official documents and a collection of institutionally sanctioned programming exercises and demonstrations.Two tactical approaches were made apparent when mathematics teachers began to integrate computer programming in their subject: Dual teaching and Interspersed programming. The teacher’s use of dual teaching practices or interspersed programming are tactics shaped by and in response to the conditions of the new curriculum and their own preferences and views on student learning. These two tactics disclose different ontological commitments in relation to the strategies dictated by the curriculum and reflect a cardinal distinction between planning mathematics activities with elements of programming and planning programming activities with elements of mathematics. Of relevance for teachers and curriculum designers is the understanding of (a) how the notion of programming and mathematics as separate subjects oversimplifies teachers’ actual integration practices, and (b) how the curricular choices made by policy can shape the teaching tactics adopted by educators.
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| 24. |
- Ganvir, Ashish, 1991-
(författare)
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Microstructure and Thermal Conductivity of Liquid Feedstock Plasma Sprayed Thermal Barrier Coatings
- 2016
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Thermal barrier coating (TBC) systems are widely used on gas turbine components to provide thermal insulation and oxidation protection. TBCs, incombination with advanced cooling, can enable the gas turbine to operate at significantly higher temperatures even above the melting temperature of the metallic materials. There is a permanent need mainly of environmental reasons to increase the combustion turbine temperature, hence new TBC solutions are needed.By using a liquid feedstock in thermal spraying, new types of TBCs can be produced. Suspension plasma/flame or solution precursor plasma spraying are examples of techniques that can be utilized for liquid feedstock thermal spraying.This approach of using suspension and solution feedstock, which is an alternative to the conventional solid powder feed stock spraying, is gaining increasing research interest, since it has been shown to be capable of producing coatings with superior coating performance.The objective of this research work was to explore relationships between process parameters, coating microstructure, thermal diffusivity and thermal conductivity in liquid feedstock thermal sprayed TBCs. A further aim was to utilize this knowledge to produce a TBC with lower thermal diffusivity and lower thermal conductivity compared to state-of-the-art in industry today, i.e. solid feed stock plasma spraying. Different spraying techniques, suspension high velocity oxy fuel,solution precursor plasma and suspension plasma spraying (with axial and radialfeeding) were explored and compared with solid feedstock plasma spraying.A variety of microstructures, such as highly porous, vertically cracked and columnar, were obtained. It was shown that there are strong relationships between the microstructures and the thermal properties of the coatings.Specifically axial suspension plasma spraying was shown as a very promising technique to produce various microstructures as well as low thermal diffusivity and low thermal conductivity coatings.
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| 25. |
- Glorieux, Emile
(författare)
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Constructive cooperative coevolution for optimising interacting production stations
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Engineering problems have characteristics such as a large number of variables, non-linear, computationally expensive, complex and black-box (i.e. unknown internal structure). These characteristics prompt difficulties for existing optimisation techniques. A consequence of this is that the required optimisation time rapidly increases beyond what is practical. There is a needfor dedicated techniques to exploit the power of mathematical optimisation tosolve engineering problems. The objective of this thesis is to investigate thisneed within the field of automation, specifically for control optimisation ofautomated systems.The thesis proposes an optimisation algorithm for optimising the controlof automated interacting production stations (i.e. independent stations thatinteract by for example material handling robots). The objective of the optimisation is to increase the production rate of such systems. The non-separable nature of these problems due to the interactions, makes them hard to optimise.The proposed algorithm is called the Constructive Cooperative CoevolutionAlgorithm (C3). The thesis presents the experimental evaluation of C3, bothon theoretical and real-world problems. For the theoretical problems, C3 istested on a set of standard benchmark functions. The performance, robustness and convergence speed of C3 is compared with the algorithms. This shows that C3 is a competitive optimisation algorithm for large-scale non-separable problems.C3 is also evaluated on real-world industrial problems, concerning thecontrol of interacting production stations, and compared with other optimisation algorithms on these problems. This shows that C3 is very well-suited for these problems. The importance of considering the energy consumption and equipment wear, next to the production rate, in the objective function is also investigated. This shows that it is crucial that these are considered to optimise the overall performance of interacting production stations.
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| 26. |
- Goel, Sneha, 1993-
(författare)
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Post-treatment of Alloy 718 produced by electron beam melting
- 2019
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Electron beam melting (EBM), a metal additive manufacturing (AM) process, has received considerable industrial attention for near net shape manufacture of complex geometries with traditionally difficult-to-machine materials. This has fuelled considerable academic interest in investigating EBM of Alloy 718, a nickel ironbased superalloy possessing an exciting combination of good mechanical behaviour and cost effectiveness. EBM production of Alloy 718 is particularly promising for aerospace and other sectors which value rapid production of components with large scope for design flexibility. The EBM builds are characterized by presence of inevitable defects and, anisotropy within a build is also a concern. Consequently, as-built Alloy 718 has to be subjected to post-build thermal-treatments (post-treatments) to ensure that the parts eventually meet the critical service requirements. Not withstanding the above, limited knowledge is available about optimal post-treatments for EBM-built Alloy 718. Therefore, the main focus of the work presented in this thesis was to systematically investigate the response of EBM-built material to post-treatments, which include hotisostatic pressing (HIPing), solution treatment (ST), and aging.HIPing of EBM-built Alloy 718 led to more than an order of magnitude reduction in defect content, which was reduced from as high as 17% to < 0.2% in samples built with intentionally introduced porosity to investigate limits of defect closure achievable through HIPing. In addition, HIPing also caused complete dissolution of δ and γ" phases present in the as-built condition, with the latter causing dropin hardness of the material. HIPing had no effect on the carbides and inclusions such as TiN, Al2O3 present in the built material. The evolution of microstructure during ST and aging was systematically investigated. Growth of potentially beneficial grain boundary δ phase precipitates was found to cease after a certain duration of ST, with samples subjected to prior-HIPing exhibiting lesser precipitation of the δ phase during ST. While the specimen hardness increased onaging, it was observed to plateau after a duration significantly shorted than the specified ASTM 'standard' aging cycle. Therefore, prima facie there are promising prospects for shortening the overall heat treatment duration. A combination of HIPing, ST, and aging treatments in a single uninterrupted cycle was also explored. Future work involving incorporation of a shortened heat treatment schedule in a combined cycle can have significant industrial implications.
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| 27. |
- Gupta, Mohit Kumar, 1986-
(författare)
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Design of Microstructures in Thermal Barrier Coatings : A Modelling Approach
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Plasma sprayed Thermal Barrier Coating systems (TBCs) are commonly used for thermal protection of components in modern gas turbine application such as power generation, marine and aero engines. The material that is most commonly used in these applications is Yttria Stabilized Zirconia (YSZ) because of this ceramic’s favourable properties, such as low thermal conductivity, phase stability to high temperature, and good erosion resistance. The coating microstructures in YSZ coatings are highly heterogeneous, consisting of defects such as pores and cracks of different sizes which determine the coating’s final thermal and mechanical properties, and the service lives of the coatings. Determination of quantitative microstructure–property correlations is of great interest as experimental procedures are time consuming and expensive.This objective of this thesis work was to investigate the relationships between coating microstructure and thermal-mechanical properties of TBCs, and to utilise these relationships to design an optimised microstructure to be used for next generation TBCs. Simulation technique was used to achieve this goal. Important microstructural parameters influencing the performance of TBCs were identified and coatings with the identified microstructural parameters were designed, modelled and experimentally verified. TBCs comprising of large globular pores with connected cracks inherited within the coating microstructure were shown to have significantly enhanced performance. Low thermal conductivity, low Young‘s modulus and high lifetime were exhibited by these coatings. The modelling approach described in this work can be used as a powerful tool to design new coatings as well as to achieve optimised microstructures.
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| 28. |
- Gurdal, Sevtap, 1976-
(författare)
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Parenting Across Cultures : Parental attributions, attitudes and behaviour
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Previously studies in parenting have mainly been conducted in Western countries. Not uncommonly results from such studies are used to describe general, worldwide trends. In an attempt to make the field of parenting research more culturally heterogeneous, an international research project, Parenting Across Cultures, was started. The project includes nine participant countries (China, Colombia, Italy, Jordan, Kenya, Philippines, Sweden, Thailand, and USA) and its purpose is to examine parenting across cultures. This thesis is based on reports from parent participants. The aim of Study I was to investigate mothers’ and fathers’ (77 participants from each group) attributions and attitudes in Sweden. The results revealed that Swedish parents are more polarized in their attitudes than in their attributions, they think more alike for parenting attitudes and there was greater variability for parenting attributions, particularly regarding uncontrollable success, as opposed to attributions regarding adult- or child-controlled failure. Regarding attitudes, mothers and fathers reported more progressive than authoritarian attitudes. Fathers reported higher adult-controlled failure and child-controlled failure attributions than mothers. In Study II the aim was to assess whether mothers’ and fathers’ self-reports of acceptance-rejection, warmth, and hostility/rejection/neglect of their children differ in the nine countries. A total of 1996 parents (998 mothers and 998 fathers) participated in the study. Mothers and fathers reported high acceptance and warmth and low rejection and hostility/rejection/neglect (HRN) of their children in all nine countries. Despite the overwhelmingly high levels of acceptance and low levels of rejection across all countries, and despite our use of statistical controls for parental age, education, social desirability, and child age, some systematic differences between countries emerged. In summary, parents in the studies report higher similarity about parenting in some cases, for example concerning acceptance and warmth and hostility/rejection/neglect, but lower in others, such as the Swedish parents’ reports about attributions.
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| 29. |
- Hagqvist, Petter, 1986-
(författare)
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Instrumentation and estimation for high temperature control
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Within a variety of industrially relevant high temperature production processes such as welding, heat treatment and metal deposition, the quality of the manufactured component is largely affected by how well parameters can be controlled during processing. These parameters might be, in the case of metal deposition, power input, material feed, or a parameter which is common for all of the aforementioned processes: material temperature. The ability to correctly measure, or in other ways estimate process parameters is vital in order to successfully control high temperature processes such as above 700 degrees Celsius. In this work, instrumentation and estimation solutions adapted to high temperature control are proposed and implemented with a focus on the laser metal wire deposition process. Special attention is given to temperature measurements on specimens with varying emissivity as commonly found in high temperature processes. A calibration procedure for a single-wavelength pyrometer is also presented together with a general discussion on limitations of such a system for measurands with varying emissivity. A new method for non-contact emissivity compensated temperature estimations using a spectrometer is presented. Simulations and industrially relevant experiments have been carried out validating the method. The theoretical framework for the developed method will be further investigated in the future together with additional experimental validation. In addition to temperature measurements, a method for real-time process control of laser metal wire deposition has been developed and implemented with good results. This control scheme estimates and controls the tool-to-workpiece distance based on resistance measurements. Such measurements allow for placement of instruments outside of the processing chamber and easy integration into existing equipment. Future work will be directed towards incorporation of resistance measurements into an iterative learning control scheme. Also, improvement on the resistance-distance model and further investigation into suitable signal processing methods for the resistance signal will be pursued.
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| 30. |
- Harati, Ebrahim
(författare)
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Fatigue strength of welds in 800 MPa yield strength steels : Effects of weld toe geometry and residual stress
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Nowadays there is a strong demand for lighter vehicles in order to increase the pay load. Through this the specific fuel consumption is decreased, the amount of greenhouse gases is lowered and the transport economy improved. One possibility to optimize the weight is to make the components from high strength steels and join them by welding. Welding is the main joining method for fabrication of a large proportion of all engineering structures. Many components experience fatigue loading during all or part of their life time and welded connections are often the prime location of fatigue failure.Fatigue fracture in welded structures often initiates at the weld toe as aconsequence of large residual stresses and changes in geometry acting as stress concentrators. The objective of this research is to increase the understanding of the factors that control fatigue life in welded components made from very high strength steels with a yield strength of more than 800 MPa. In particular the influences of the local weld toe geometry (weld toe radius and angle) and residual stress on fatigue life have been studied. Residual stresses have been varied by welding with conventional as well as Low Transformation Temperature (LTT) filler materials. The three non-destructive techniques Weld Impression Analysis (WIA), Laser Scanning Profiling (LSP) and Structured Light Projection (SLP) have been applied to evaluate the weld toe geometry.Results suggest that all three methods could be used successfully to measure the weld toe radius and angle, but the obtained data are dependent on the evaluation procedure. WIA seems to be a suitable and economical choice when the aim is just finding the radius. However, SLP is a good method to fast obtain a threedimensional image of the weld profile, which also makes it more suitable for quality control in production. It was also found that the use of LTTconsumables increased fatigue life and that residual stress has a relatively larger influence than the weld toe geometry on fatigue strength of welded parts.
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