| 1. |
- Kilbrink, Nina, 1974-
(författare)
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Lära för framtiden : Transfer i teknisk yrkesutbildning
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Den här avhandlingen handlar om transfer i relation till tekniklärande och teknisk yrkesutbildning. Transfer är centralt för all utbildning och att kunna använda och bygga vidare på tidigare kunskaper och erfarenheter i nya situationer är nödvändigt i ett föränderligt samhälle med en snabb teknisk utveckling.Syftet med avhandlingen är att studera transfer ur ett erfarenhetsperspektiv. Avhandlingen grundar sig i ett livsvärldsfenomenologiskt perspektiv. Elevers, lärares och handledares erfarenheter av undervisning och lärande är en central utgångspunkt. I tre av avhandlingens delstudier fokuseras berättelser om teknisk gymnasial yrkesutbildning som lärlingsutbildning och i den fjärde delstudien fokuseras hur uppgifter av teoretisk och praktisk karaktär erfars och hanteras i teknikundervisningen i grundskolan. Resultaten visar på behovet av lärande i olika arenor och behovet av ett holistiskt lärande där teori och praktik hanteras integrerat. Eleverna behöver erbjudas en variation av erfarenheter från olika arenor och flera arbetsplatser, där lärare och handledare skapar förutsättningar för reflektion i relation till den praktiska erfarenheten. Något som också starkt betonas är ett fungerande samarbete mellan skola och arbetsplatser i yrkesutbildning.
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| 2. |
- Medvedeva, Anna, 1981-
(författare)
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Performance of advanced tool steels for cutting tool bodies
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Performance of indexable insert cutting tools is not only about the performance of cutting inserts. It is also about the cutting tool body, which has to provide a secure and accurate insert positioning as well as its quick and easy handling under severe working conditions. The common damage mechanisms of cutting tool bodies are fatigue and plastic deformation. Cutting tools undergo high dynamic stresses going in and out cutting engagement; as a result, an adequate level of fatigue strength is the essential steel property. Working temperatures of tool bodies in the insert pocket can reach up to 600°C, why the tool steel requires high softening resistance to avoid plastic deformation. Machinability is also essential, as machining of the steel represents a large fraction of the production cost of a cutting tool. The overall aim of the study is to improve the tool body performance by use of an advanced steel grade with an optimized combination of all the demanding properties. Due to the high-temperature conditions, the thesis concerns mostly hot-work tool steels increasing also the general knowledge of their microstructure, mechanical properties and machinability. Knowing the positive effect of sulphur on machinability of steels, the first step was to indentify a certain limit of the sulphur addition, which would not reduce the fatigue strength of the tool body below an acceptable level. In tool bodies, where the demand on surface roughness was low and a geometrical stress concentrator was present, the addition of sulphur could be up to 0.09 wt%. Fatigue performance of the cutting tools to a large extent depended on the steel resistance to stress relaxation under high dynamic loading and elevated temperatures. The stress relaxation behaviour, material substructure and dislocation characteristics in low-alloyed and hot-work tool steels were studied using X-ray diffraction under thermal and mechanical loading. Different tool steels exhibited different stress relaxation resistance depending on their microstructure, temper resistance and working temperature. Hot-work tool steels showed to be more preferable to low-alloyed tool steels because of their ability to inhibit the rearrangement and annihilation of induced dislocations. High-temperature softening resistance of the hot-work tool steels was investigated during high-temperature hold-times and isothermal fatigue and discussed with respect to their microstructure. Carbide morphology and precipitation were determined using scanning and transmission electron microscopy. Machinability of a prehardened hot-work tool steel of varying nickel content from 1 to 5 wt% was investigated in end milling and drilling operations. Machining the higher nickel containing steels resulted in longer tool life and generated lower cutting forces and tool/workpiece interface temperature. The difference in machinability of the steels was discussed in terms of their microstructure and mechanical properties.
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| 3. |
- W. Lindvall, Fredrik, 1980-
(författare)
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Adhesive wear testing and modelling of tool steels sliding against sheet metals
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Sheet metal forming is a manufacturing method used because of its versatility. Sheets are plastically deformed by a tool to create a product. A tool is expected to last for several 100,000 forming operations and efforts are made to optimize the tools. A common type of wear referred to as galling is the adhesion of sheet material to the tools. This problem has become more prevalent as new high strength sheet materials have been developed at the same time as lubricants have become heavily regulated. This has forced the development of new tool steels with improved resistance to galling. There are many parameters influencing the response to galling. In this work the influence of surface preparation, contact geometry, material selection and lubrication has been investigated. The surface of the forming tools has a large influence on the tools effective life. To refurbish a forming tool is expensive and often requires special shops and hand polishing. The influence on galling of different surface preparations suited for sheet metal forming was investigated using a strip-reduction equipment. The contact conditions of a tool sliding against metal sheets were investigated using FE models. The contact conditions were calculated for a U-bending test and for a sliding-on-flat-surface wear tester. The results were compared to those found in literature. One model incorporated the surface roughness of a sheet as measured by optical profilometry. The strength of the interface between the tool and the sheet material determine if material can be transferred onto the tool. The interface between the tool and adhered sheet material was closely studied using transmission electron microscopy of thin lamellas produced by focused ion beam milling. This showed sheet material adhering to the tool without the formation of an interlayer. Finally, several different combinations of tool steels and sheet materials were tested with regards to their ability to withstand galling.
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| 4. |
- Zhang, Qing
(författare)
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Mechanical Properties of Semi-Solid Al Castings : Role of Stirring
- 2022
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Semi-solid metal (SSM) casting has been widely used in automotive industries to reduce the weight. In RheoMetalTM process which is one of the variations of SSM, the slurry fabrication can be finished within 30 second and can the slurry making process can be integrated into a high pressure die casting (HPDC) route without significant adjustments, making the process a promising alternative for industrial application. However, the application of SSM is still limited due to the semi-solid deformation-induced casting defects, such as macrosegregation and large pores. Due to the short stirring duration, the inefficient stirring is the leading cause of defects formation. Another critical issue in the RheoMetalTM process is the oxidation during the stirring process, which results in the increase of oxides in the castings, reducing the mechanical properties.This study aims to investigate the ductility and the fatigue performance of SSM castings. The main focuses were on the role of the stirring and oxides. The quenched slurry was analyzed to evaluate the effect of the stirring on the particle distribution in the slurry, and its effect on the formation of pores and segregation was discussed. To investigate the oxidation during the slurry making process, two alloys with different Mg content were cast. Scanning electron microscopy (JEOL JSM-7001F SEM) equipped with Energy-dispersive X-ray spectroscopy (EDS) was used to identify the oxides on the fracture surface. Tensile test and fatigue test accompanied with direct current potential drop (DCPD) were performed to investigate the ductility and fatigue performance, respectively.The results suggest that the intensive stirring can avoid the formation of the large clusters, making the particle distribution homogeneous in the slurry. The Mg contents determined the types of the oxides formed in the slurry making process. For alloys with sufficient Mg, the oxides would be MgAl2O4, which exist as small films with numerous cracks, while a large oxides film will be formed in the case of low Mg content.The combined influence of porosity and oxides was concluded. In the 42000 alloy, because of the low Mg content, the ductility was dominated by the large oxide films. In contrast, in the Magsimal 59 alloy, the presence of small oxides (less than 0.2 mm in majority) leads to the influence of oxides on the elongation negligible. However, a good correlation was obtained between the largest pores and ductility.The fatigue test shows that the surface liquid segregation (SLS) determined the fatigue strength under cyclic bend loading, due to its higher hardness. The effect of the inner pores on the fatigue performance was negligible, as the maximum stress was applied on the surface.
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| 5. |
- Ekengren, Jens, 1976-
(författare)
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Estimating inclusion content in high performance steels
- 2008
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Non-metallic inclusions in steel pose a major problem for the fatigue resistance, especially regarding fatigue at very long lives corresponding to low cyclic stress levels, as well as being detrimental to material toughness and polishability.The largest inclusions are quite rare, which makes conventional detection methods timeconsuming if reliable results are to be obtained. Based on surface scanning using light or electron microscopes, these methods provide results that have to be converted to reflect the statistical volume distribution of inclusions.Very high cycle fatigue (in the order of 109 cycles or more) using ultrasonic fatigue at 20 kHz has been found efficient at finding the largest inclusions in volumes of about 300 mm3 per specimen. The inclusions found at the fatigue initiation site can then been used to estimate the distribution of large inclusions using extreme value statistics.In this work, a new method for estimating the volume distribution of large inclusions is presented as well as a suggested ranking variable based on the volume distribution.Results from fatigue fractography and area scanning methods are compared to the endurance limit at 109 cycles for a number of batches from two high performance steels.In addition, the extreme value distributions of fatigue initiating inclusions in six high performace steels, produced by different routes, are presented. It is shown that all modes of the Generalized Extreme Values distribution can be found in different materials. This result shows that the assumption of mode I distribution, also known as Gumbel or Largest Extreme Value distribution, must be substantiated.
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| 6. |
- Gåård, Anders, 1977-
(författare)
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Wear mechanisms in sheet metal forming : Effects of tool microstructure, adhesion and temperature
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The general trend in the car body manufacturing industry is towards low-series production and reduction of press lubricants and car weight. The limited use of lubricants, in combination with the introduction of high and ultrahigh-strength sheet materials, continuously increases the demands on the forming tools. The major cause for tool failure during the forming process is transfer and accumulation of sheet material on the tool surfaces, generally referred to as galling. The adhered material creates unstable frictional conditions and scratching of the tool/sheet interface. To provide the means of forming new generations of sheet materials, development of new tool materialswith improved galling resistance is required, which may include tailored microstructures introducing specific carbides and nitrides, coatings and improved surface finish. In the present work, the galling wear mechanisms in real forming operations have been studied and emulated at a laboratory scale by developing a test equipment. The wear mechanisms, identified in the real forming process, were distinguished into a sequence of events. At the initial stage, local adhesive wear of the sheets led to transfer of sheet material to the tool surfaces. Successive forming operations resulted in growth of the transfer layer with initiation of scratching of the sheets. Finally, scratching changed into severe adhesive wear, associated withgross macroscopic damage. The wear process was successfully repeated in the laboratory test equipment in sliding between several tool materials, ranging from cast iron and conventional ingot cast tool steels, to advanced powder metallurgy tool steel, sliding against medium and high-strength steel sheets. By use of the test equipment, selected tool materials were ranked regarding galling resistance. The controlling mechanism for galling in sheet metal forming is adhesion. The initial sheet material transfer was found to occur, preferably, to the metallic matrix of the tool steels. Hence, the carbides in the particular steels appeared less prone to adhesion as compared to the metallic matrix. Therefore, an improved galling resistance was observed for a tool steel comprising a high amount of small homogeneously distributed carbides offering a low-strength interface to the transferred sheet material.Further, atomic force microscopy showed that nanoscale adhesion was influenced by temperature, with increasing adhesion as temperature increases. A similar dependence was observed at the macroscale where increasing surface temperature led to initiation of severe adhesive wear. The results were in good agreement to the nano scale observations and temperature-induced high adhesion was suggested as a possible mechanism.
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| 7. |
- Kasvayee, Keivan Amiri, 1986-
(författare)
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On the deformation behavior and cracking of ductile iron; effect of microstructure
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis focuses on the effect of microstructural variation on the mechanical properties and deformation behavior of ductile iron. To research and determine these effects, two grades of ductile iron, (i) GJS-500-7 and (ii) high silicon GJS-500-14, were cast in a geometry containing several plates with different section thicknesses in order to produce microstructural variation. Microstructural investigations as well as tensile and hardness tests were performed on the casting plates. The results revealed higher ferrite fraction, graphite particle count, and yield strength in the high silicon GJS-500-14 grade compared to the GJS-500-7 grade.To study the relationship between the microstructural variation and tensile behavior on macroscale, tensile stress-strain response was characterized using the Ludwigson equation. The obtained tensile properties were modeled, based on the microstructural characteristics, using multiple linear regression and analysis of variance (ANOVA). The models showed that silicon content, graphite particle count, ferrite fraction, and fraction of porosity are the major contributing factors that influence tensile behavior. The models were entered into a casting process simulation software, and the simulated microstructure and tensile properties were validated using the experimental data. This enabled the opportunity to predict tensile properties of cast components with similar microstructural characteristics.To investigate deformation behavior on micro-scale, a method was developed to quantitatively measure strain in the microstructure, utilizing the digital image correlation (DIC) technique together with in-situ tensile testing. In this method, a pit-etching procedure was developed to generate a random speckle pattern, enabling DIC strain measurement to be conducted in the matrix and the area between the graphite particles. The method was validated by benchmarking the measured yield strength with the material’s standard yield strength.The microstructural deformation behavior under tensile loading was characterized. During elastic deformation, strain mapping revealed a heterogeneous strain distribution in the microstructure, as well as shear bands that formed between graphite particles. The crack was initiated at the stress ranges in which a kink occurred in the tensile curve, indicating the dissipation of energy during both plastic deformation and crack initiation. A large amount of strain localization was measured at the onset of the micro-cracks on the strain maps. The micro-cracks were initiated at local strain levels higher than 2%, suggesting a threshold level of strain required for micro-crack initiation.A continuum Finite Element (FE) model containing a physical length scale was developed to predict strain on the microstructure of ductile iron. The material parameters for this model were calculated by optimization, utilizing the Ramberg-Osgood equation. The predicted strain maps were compared to the strain maps measured by DIC, both qualitatively and quantitatively. To a large extent, the strain maps were in agreement, resulting in the validation of the model on micro-scale.In order to perform a micro-scale characterization of dynamic deformation behavior, local strain distribution on the microstructure was studied by performing in-situ cyclic tests using a scanning electron microscope (SEM). A novel method, based on the focused ion beam (FIB) milling, was developed to generate a speckle pattern on the microstructure of the ferritic ductile iron (GJS-500-14 grade) to enable quantitative DIC strain measurement to be performed. The results showed that the maximum strain concentration occurred in the vicinity of the micro-cracks, particularly ahead of the micro-crack tip.
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| 8. |
- Kazymyrovych, Vitaliy, 1978-
(författare)
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Very high cycle fatigue of tool steels
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- An increasing number of engineering components are expected to have fatigue life in the range of 107 - 1010 load cycles. Some examples of such components are found in airplanes, automobiles and high speed trains. For many materials fatigue failures have lately been reported to occur well after 107 load cycles, namely in the Very High Cycle Fatigue (VHCF) range. This finding contradicts the established concept of a fatigue limit, which postulates that having sustained around 107 load cycles the material is capable of enduring an infinite number of cycles provided that the service conditions are unchanged. With the development of modern ultrasonic fatigue testing equipment it became possible to experimentally establish VHCF behaviour of various materials. For many of them the existence of the fatigue limit at 107 load cycles has been proved wrong and their fatigue strength continues to decrease with increasing number of load cycles. High performance steels is an important group of materials used for the components subjected to VHCF. This study explores the VHCF phenomenon using experimental data generated by ultrasonic fatigue testing of selected tool steels. The overall aim is to gain knowledge of VHCF behaviour of some common tool steel grades, while establishing a fundamental understanding of mechanisms for crack development in the very long life regime. The study demonstrates that VHCF cracks in tested steels initiate from microstructural defects like slag inclusions, large carbides or voids. It is established that VHCF life is almost exclusively spent during crack formation at below threshold stress intensity values which results in a unique for VHCF morphology on the fracture surface. Significant attention is devoted in the thesis to the ultrasonic fatigue testing technique, i.e. the validity and applicability of its results. FEM is employed to give an additional perspective to the study. It was used to calculate local stresses at fatigue initiating defects; examine the effect of material damping on ultrasonic stresses; and to evaluate various specimen geometries with respect to resulting stress gradient and maximum stressed material volume.
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| 9. |
- Lundberg, Mattias
(författare)
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Residual Stresses and Fatigue of Shot Peened Cast Iron
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The complex geometry of cylinder head in heavy-duty diesel engine makes grey cast iron or compact graphite iron a perfect material choice due to its castability, thermal conductivity and damping capacity. To increase the efficiency of the engine, the fatigue property of the material needs to be improved. Shot peening is often used to increase the fatigue strength of components. The benefits are associated with the compressive stresses induced and with surface hardening. In this research project, these effects on grey and compact iron have been analyzed for different shot peening parameters using XRD, SEM and fatigue testing methods. The ultimate aim of the project is to increase the fatigue strength of cast irons by optimization of residual stresses.The XRD measurements and SEM examinations revealed that the shot peening parameters including shot size and peening intensity had significant influences on the resulted residual stresses and strain hardening while changing the coverage made little difference. Also differences in the peening results between the two materials were observed, which were ascribed to an effect of the different graphite morphology. Nevertheless, a residual stress profile similar to the one general considered to improve the fatigue strength in steels could be obtained in both grey and compact iron after shot peening.The axial fatigue testing with R=-1 on the grey iron showed that peening using large shot size and high peening intensity (heavy shot peening) resulted in a fatigue strength reduction of 15-20% in comparison with the mechanically polished surface. The negative effects are likely related to surface damage and relatively high tensile residual stresses in subsurface induced by the heavy peening. Grey cast iron has low ductility in tension and therefore tensile residual stresses may promote multiple cracking and crack networking during cyclic loading.Shot peening using much smaller shots and lower intensity (gentle shot peening) which resulted in a much smaller residual stress field gave no significant changes in fatigue strength. However, a short time annealing at 285°C of specimens being gently shot peened increased the fatigue strength roughly by 10%. The improvement could be an effect of precipitates formed due to the heat treatment, which lock the dislocation movement under cyclic loading.
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| 10. |
- Medvedeva, Anna, 1981-
(författare)
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Tool steel for tool holder applications : microstructure and mechanical properties
- 2008
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Large improvements in cutting tool design and technology, including the application of advanced surface engineering treatments on the cemented carbide insert, have been achieved in the last decades to enhance tool performance. However, the problem of improving the tool body material is not adequately studied. Fatigue is the most common failure mechanism in cutting tool bodies. Rotating tools, tool going in and out of cutting engagement, impose dynamic stresses and require adequate fatigue strength of the tool. Working temperatures of milling cutter bodies in the insert pocket can reach up to 600°C depending on the cutting conditions and material of the workpiece. As a result, steel for this application shall have good hot properties such as high temper resistance and high hot hardness values to avoid plastic deformation in the insert pocket of the cutting tool. Machinability of the steel is also essential, as machining of steel represents a large fraction of the production cost of a milling cutter. This thesis focus on the improvement of the cutting tool performance by the use of steel grades for tool bodies with optimized combination of fatigue strength, machinability and properties at elevated temperatures. The first step was to indentify the certain limit of the sulphur addition for improved machinability which is allowable without reducing the fatigue strength of the milling cutter body below an acceptable level. The combined effect of inclusions, surface condition and geometrical stress concentrator on the fatigue life of the tool steel in smooth specimens and in tool components were studied in bending fatigue. As the fatigue performance of the tools to a large extent depends on the stress relaxation resistance at elevated temperature use, the second step in this research was to investigate the stress relaxation of the commonly used milling cutter body materials and a newly steel developed within the project. Compressive residual stresses were induced by shot peening and their response to mechanical and thermal loading as well as the material substructures and their dislocation characteristics were studied using X-ray diffraction. Softening resistance of two hot work tool steels and a newly developed steel was investigated during high temperature hold times and isothermal fatigue and discussed of with respect to their microstructure. Carbide morphology and precipitation as well as dislocation structure were determined using transmission electron microscopy and X-ray line broadening analysis.
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