Modulräume supersingulärer Enriquesflächen

In the first part of this thesis we use Ogus’ crystalline Torelli theorem and lattice theory to investigate the number of Enriques quotients of supersingular K3 surfaces over fields of odd characteristic. In the second part we discuss fibrations between the strata of the period space of supersingular K3 surfaces. In the last part we construct a moduli space for Enriques surface with supersingular K3 cover over fields of odd characteristic and we prove a Torelli theorem for such Enriques surfacesIm ersten Teil dieser Arbeit benutzen wir Ogus' kristallinen Torelli-Satz und Gittertheorie, um die Zahl der Enriquesquotienten supersingulärer K3-Flächen über Körpern ungerader Charakteristik zu studieren. Im zweiten Teil diskutieren wir Faserungen zwischen den Strata des Periodenraumes der supersingulären K3-Flächen. Im letzten Teil konstruieren wir einen Modulraum für Enriquesflächen mit supersingulärer K3-Überlagerung und beweisen einen Torelli-Satz für solche Enriquesflächen

Investigations on the consolidation of TNM powder by admixing different elemental powders

In the conventional powder metallurgy (PM) process route, finished components are produced from metallic powder materials by pressing at room temperature followed by pressureless sintering in a furnace. This simple method for fast and cost-effective processing is not suitable for y-based titanium aluminides. Due to their brittleness, these cannot be compacted in the conventional way. In order to qualify this group of alloys for the PM route, a possible solution is the addition of elemental titanium or aluminum powder. In this study, the basis is commercially available pre-alloyed TiAI44-4Nb-0.7Mo-0.1B (TNM) powder with high application potential for example in the aerospace industry. Investigations are carried out to determine the influence of different admixtures of elemental powders on the compaction result and the resulting mechanical properties. The TNM powder is mixed with pure titanium and aluminum powders and pressed to a compact using graphite as tool lubricant. The results show that the admixture of elemental powders enables the consolidation of TNM powder. However, depending on the load applied, a certain minimum proportion of the respective elemental powder is necessary to produce a compact. Furthermore, a significant influence on the relative density as well as the strength of the pressed product can be observed

Influence of the connection between forming die and heatpipe on the heat transfer

Hot forming tools are exposed to cyclically changing thermal loads. These conditions are caused by the heat exchange between tool and workpiece during forming followed by spray cooling. This can lead to crack initiation and tool failure. A continuous cooling with heatpipes (HP) inside the active tool components could prevent this. HP use a circular flow of a cooling fluid inside a closed tube, often made of copper. Previous studies showed an influence of the connection by thermal paste between the forming die and the HP, its orientation, as well as its inner surface structure. The use of paste proved essential for closing the contact by filling the microscopic air pockets between the surfaces. Only sintered inner structures can be used for force fit, since others are damaged by deformation and thus lose their efficiency. This research paper deals with the influence of the form and force fit between die and HP. To test the impact, HP were connected with heated model dies on one side and an aluminium block (AB) on the other. Thermocouples were used to monitor the temperature of both, the AB and the model dies. The measured temperature and time difference, the weight and the thermal capacity of the AB were used to calculate the heat flow. Different inner surface structures of HP were varied in addition to their fitting type with the model die. The best heat transfer was achieved by using HP with sintered inner structure and force-fit, resulting in nearly full-surface contact

Methodology to Investigate the Transformation Plasticity for Numerical Modelling of Hot Forging Processes

Hot forging is a complex process involving the mutual influence of numerous thermo-mechanical-metallurgical material phenomena. In particular, the strains of transformation-induced plasticity (TRIP) have a significant influence on the distortions and residual stresses of the components. The TRIP strains refer to the anisotropic strains depending on the orientation and significance of the stress conditions during cooling superimposed to the phase transformation. With the use of numerical models, the impact of this effect can be investigated in order to ensure the production of high quality components. However, an experimental determination of the characteristic values of TRIP is challenging, which is why only few corresponding data are available in the literature. Therefore, this paper presents an experimental and numerical methodology as well as the results of studies on the interaction between stresses and phase transformations in the materials AISI 4140 and AISI 52100. The investigations of the TRIP strains are carried out using hollow specimens, which are thermo-mechanically treated in the physical forming simulator Gleeble 3800-GTC. The specimens are austenitised, quenched to test temperature and held there while diffusion controlled phase transformation takes place. The extent of TRIP as a result of different superimposed tensile or compressive loads is determined by means of dilatometry. In addition, the extent of TRIP for diffusionless martensitic phase transformations was investigated by continuous cooling tests under tensile and compressive loads. It was found that the transformation plasticity varies depending on the material, the phase type, the temperature and the tensile or compressive stresses. Subsequently, simulations of the physical experiments using the FE software Simufact. Forming verified the determined phase specific values of TRIP

Experimental investigations on the interactions between the process parameters of hot forming and the resulting residual stresses in the component

In metal forming, the arising residual stresses influence the material behaviour during manufacturing as well as the performance of the final component. In the past, the focus of forming process design was on minimising or eliminating residual stresses. However, residual stresses can also serve to improve the properties of the components through targeted use, for example with regard to distortions or wear behaviour. For this purpose, knowledge of the interactions between the process parameters of the hot forming process and the resulting residual stresses in the final component is required. In this work, the influences of the process parameters are analysed by means of a reference process of hot forming. In this process, cylindrical specimens with eccentric holes are hot-formed, which leads to an inhomogeneous stress distribution in the material as it occurs in an industrial hot forming process. In the reference process, forming temperature, cooling strategy, forming speed, degree of deformation and steel alloys are varied. It is observed that both, process parameters and material properties, have a significant influence on the resulting residual stresses. Mainly responsible for these phenomena are microstructural effects in the material. As a result of forming at temperatures between 1000 °C and 1200 °C, static and dynamic recrystallisation processes occur, which affect the austenite grain size. The austenite grain size as well as the cooling strategy have a significant influence on the microstructure transformation behaviour, which has a decisive effect on the resulting residual stresses. In addition, the cooling strategy determines whether a diffusion-free phase transformation or a diffusion-controlled phase transformation occurs. At high cooling rates, diffusion-free transformation of the austenitic into the martensitic phase takes place, which leads to severe stresses in the crystal lattice. During diffusion-controlled phase transformation, which occurs during air cooling, comparatively lower residual stresses in the range of zero can be observed

Investigation of pressing and ejection performance of friction-reducing powder-compaction tool coatings

Pressing in dies followed by sintering is the most commonly used process for shaping metal powders into components. The mechanical properties (e.g. tensile and fatigue strength) of the final sintered component depend on the green-compact properties resulting from the compaction process. Apart from the powder material used, process-specific factors, such as geometry complexity, compaction pressure and lubrication strategy, have a major impact on the properties of the green compact. The lubrication strategy is also decisive for the economic efficiency of the process as it influences the service life of the tools. Friction-reducing powder-compaction tool coatings (e.g. diamond-like-carbon-based/DLC) provide the potential to positively influence the lubrication conditions during compaction and ejection, thus simultaneously improving product quality and service life. In this study, experimental investigations on the performance of friction-reducing coatings in the die pressing of steel powder (Fe + 0.6 wt% C) with and without admixed lubricant (AncorLube, GKN Hoeganaes) are presented. The results are evaluated by force-displacement measurements, which allows for a more profound analysis of compaction and ejection behaviour. It is shown that the application of the coatings reduces the ejection loads significantly when no admixed lubricant is used, and moderately when lubricant is admixed. However, without lubricant, wear still occurs after a few pressing cycles, so it cannot be completely avoided

Influence of Cooling Parameters on the Surface Layer Structure of Hot Working Tools

The surface layer of hot working tools is subject to alternating thermo-mechanical loads during forging. It experiences a fast increase in temperature on contact with the heated billets, followed by a steep temperature decline during application of the spray-coolant. This can lead to a cyclic surface rehardening of the tool surface layer due to the formation of a martensitic structure, which can either delay or accelerate tool failure. The microstructural changes in the tool surface layer mainly depend on the thermal, mechanical and tribological loads during forging. The influence of these loads is of particular interest to understand the effect of cyclic surface rehardening. The goal of this research is to investigate the influence of cooling parameters on microstructural changes in the tool surface layer during die forging. Mechanical and tribological loads are kept constant while cooling parameters are varied. Three internal thermocouples are applied to forging tools to measure the base tool temperature. To keep the amount of lubricant in each forging cycle at constant levels, cooling and lubrication are separated by use of boron nitride as lubricant, which is applied by electrostatic adherence. For cooling, the duration of water application is varied while maintaining pressure and spray pattern. Fourtools with differenttool base temperatures are investigated and the influence of the thermal loads on the wear behaviour displayed

Investigation on the microstructure of ecap-processed iron-aluminium alloys

The present work deals with adjusting a fine-grained microstructure in iron-rich iron-aluminium alloys using the ECAP-process (Equal Channel Angular Pressing). Due to the limited formability of Fe-Al alloys with increased aluminium content, high forming temperatures and low forming speeds are required. Therefore, tool temperatures above 1100◦C are permanently needed to prevent cooling of the work pieces, which makes the design of the ECAP-process challenging. For the investigation, the Fe-Al work pieces were heated to the respective hot forming temperature in a chamber furnace and then formed in the ECAP tool at a constant punch speed of 5 mm/s. Besides the chemical composition (Fe9Al, Fe28Al and Fe38Al (at.%—Al)), the influences of a subsequent heat treatment and the holding time on the microstructure development were investigated. For this purpose, the average grain size of the microstructure was measured using the AGI (Average Grain Intercept) method and correlated with the aforementioned parameters. The results show that no significant grain refinement could be achieved with the parameters used, which is largely due to the high forming temperature significantly promoting grain growth. The holding times in the examined area do not have any influence on the grain refinement. © 2021 by the authors. Licensee MDPI, Basel, Switzerland

Functionalisation of the Boundary Layer by Deformation-Induced Martensite on Bearing Rings by means of Bulk Metal Forming Processes

During cold forming of metastable austenitic steels, a strength-increasing phase transformation induced by externally superimposed stresses occurs in addition to strain hardening. The effect of deformation-induced martensite formation has so far not been utilized industrially in the area of bulk forming, but could be suitable for the production of highly-loaded components in oxidative atmospheres. The aim of this study is the analysis of local phase transformations in metastable austenitic steels in the boundary layer of bulk formed components. For this purpose, the relationship between the process conditions occurring during bulk metal forming and the resulting martensitic phase fraction was determined. Cylinder compression tests are carried out in which the influence of various process parameters can be investigated. These include forming temperature, true plastic strain and forming speed. In a quantitative measurement by means of a magnetic induction process, local martensite formation is determined and hardness measurements are carried out. The recorded flow stress curves are implemented in a numerical simulation. Furthermore, the influence of different tool surface topographies on the contact conditions of the workpiece-tool system is characterized by means of ring compression tests. With the numerical simulations and experimentally obtained results, a surface hardening process for bearing rings is designed. The relationship between local true plastic strain and deformation-induced martensite development is explained by material flow simulations, taking into account the process route for manufacturing the bearing ring and the varying friction factors