Browsing by Author "Koleva E."
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Item Application of dynamic beam positioning for creating specified structures and properties of welded joints in electron-beam welding(2020-05-01) Olshanskaya T.; Belenkiy V.; Fedoseeva E.; Koleva E.; Trushnikov D.The application of electron beam sweep makes it possible to carry out multifocal and multi-beam welding, as well as combine the welding process with local heating or subsequent heat treatment, which is important when preparing products from thermally-hardened materials. This paper presents a method of electron beam welding (EBW) with dynamic beam positioning and its experimental-calculation results regarding the formation of structures and properties of heat-resistant steel welded joints (grade of steel 20Cr3MoWV). The application of electron beam oscillations in welding makes it possible to change the shape and dimensions of welding pool. It also affects the crystallization and formation of a primary structure. It has been established that EBW with dynamic beam positioning increases the weld metal residence time and the thermal effect zone above the critical A3 point, increases cooling time and considerably reduces instantaneous cooling rates as compared to welding without beam sweep. Also, the difference between cooling rates in the depth of a welded joint considerably reduces the degree of structural non-uniformity. A bainitic-martensitic structure is formed in the weld metal and the thermal effect zone throughout the whole depth of fusion. As a result of this structure, the level of mechanical properties of a welded joint produced from EBW with dynamic electron beam positioning approaches that of parent metal to a greater extent than in the case of welding by a static beam. As a consequence, welding of heat-resistant steels reduces the degree of non-uniformity of mechanical properties in the depth of welded joints, as well as decreases the level of hardening of a welded joint in relation to parent metal.Item Application programming interface for electron beam welding of Inconel 718 thin plates(2024-01-01) Koleva L.; Koleva E.In this paper the simulation of electron beam welding of Inconel 718 thin plates by a moving linear heat source is considered. Neural network models are developed and used for the description of the dependence of the molten pool geometry characteristics on the process parameters - electron beam power, welding velocity, and the plate thickness. Neural network models, based on a multi-layered feedforward neural network, trained with Levenberg-Marquardt error backpropagation algorithm are compared with estimated regression models. The neural networks are trained, tested and validated using a set of experimental data. The resulting models are implemented for developing of an application programming interface for electron beam welding of Inconel 718 thin plates, which is used for prediction, investigation and graphical optimization of the molten pool geometry characteristics of the obtained electron beam welds.Item Automation of the electron-beam welding process(2016-04-07) Koleva E.; Dzharov V.; Kardjiev M.; Mladenov G.In this work, the automatic control is considered of the vacuum and cooling systems of the located in the IE-BAS equipment for electron-beam welding, evaporation and surface modification. A project was elaborated for the control and management based on the development of an engineering support system using existing and additional technical means of automation. Optimization of the indicators, which are critical for the duration of reaching the working regime and stopping the operation of the installation, can be made using experimentally obtained transient characteristics. The automation of the available equipment aimed at improving its efficiency and the repeatability of the obtained results, as well as at stabilizing the process parameters, should be integrated in an Engineering Support System which, besides the operator supervision, consists of several subsystems for equipment control, data acquisition, information analysis, system management and decision-making support.Item Comparative study of the sidewall shape and proximity effect of the AR-N 7520 negative electron beam resist(2022-04-04) Kostic I.; Vutova K.; Koleva E.; Bencurova A.; Konecnikova A.; Andok R.This paper deals with the performance of the AR-N7520 (Allresist) negative electron beam resist (EB resist) which was selected as an etch mask for the fabrication of gratings on GaAs substrates. The developed resist sidewall shape is crucial for this purpose. The required near-to-vertical sidewall shape can be achieved by optimizing the electron beam lithography (EBL) process based on experimental investigations and computer simulations. The sidewall shape dependence on the EBL parameters (exposure dose, resist pattern, etc.) and the proximity effect are studied.Item Computer-aided electron-beam deflection control system(2020-06-02) Koleva E.; Koleva L.; Mladenov G.; Trushnikov D.; Andonov A.The electron beam processes - 3D selective melting (3D printing), welding, etc. are complex processes with many processing variables, which require integration of interfaces for control of the processes on different levels through hierarchical equipment models, functional data and operation control models. We developed a user interface software application for controlling the electron-beam motion along several types of trajectories and a free complex shape trajectory by controlling the deflection system parameters. Several types of motion patterns with different velocities are realized, together with the possibility of choosing a free trajectory by defining the electron-beam path by a graphical image.Item Current-driven ion-acoustic and potential-relaxation instabilities excited in plasma plume during electron beam welding(2014-01-01) Trushnikov D.; Mladenov G.; Belenkiy V.; Koleva E.; Varushkin S.Many papers have sought correlations between the parameters of secondary particles generated above the beam/work piece interaction zone, dynamics of processes in the keyhole, and technological processes. Low- and high-frequency oscillations of the current, collected by plasma have been observed above the welding zone during electron beam welding. Low-frequency oscillations of secondary signals are related to capillary instabilities of the keyhole, however; the physical mechanisms responsible for the high-frequency oscillations (>10 kHz) of the collected current are not fully understood. This paper shows that peak frequencies in the spectra of the collected high-frequency signal are dependent on the reciprocal distance between the welding zone and collector electrode. From the relationship between current harmonics frequency and distance of the collector/welding zone, it can be estimated that the draft velocity of electrons or phase velocity of excited waves is about 1600 m/s. The dispersion relation with the properties of ion-acoustic waves is related to electron temperature 10 000 K, ion temperature 2 400 K and plasma density 1016 m-3, which is analogues to the parameters of potential-relaxation instabilities, observed in similar conditions. The estimated critical density of the transported current for creating the anomalous resistance state of plasma is of the order of 3 A·m-2, i.e. 8 mA for a 3-10 cm2 collector electrode. Thus, it is assumed that the observed high-frequency oscillations of the current collected by the positive collector electrode are caused by relaxation processes in the plasma plume above the welding zone, and not a direct demonstration of oscillations in the keyhole. © 2014 Author(s).Item Data mining techniques for quality improvement of electron beam welding process(2024-01-01) Kolev G.; Asenova-Robinzonova A.; Koleva L.; Koleva E.Besides the fulfilment of the technological requirements for the geometry of the obtained welded joints by electron beam welding, there is a necessity to avoid the conditions, which more probably will lead to defect appearance. It is assumed that the appearance of defects is more probable under some regime conditions. For the modelling of the dependence of bivariate quality characteristics (such as the defect appearance) on the process parameters two different modelling approaches are applied and compared - logistic regression and neural networks. The implemented model-based approaches are compared and applied for the prediction of the defect appearance, depending on the variation of the electron beam process parameters.Item Dependence of PMMA electron beam resist sidewall shape on exposure dose and resist thickness(2021-11-02) Andok R.; Vutova K.; Koleva E.; Bencurova A.; Kostic I.The influence of electron beam lithography parameters (such as electron energy, resist thickness, the exposure dose) on the resist sidewall shape (profile) was studied for the PMMA (polymethyl-methacrylate) positive resist. The profile of the positive tone resist PMMA was investigated depending on varying exposure doses for the resist thicknesses 600 and 1300 nm, and electron energy 30 keV. Simulation results based on measurements along the resist profile depth are presented and discussed. The results obtained contribute to the knowledge on electron scattering in resist/substrate in electron beam lithography for the case of field emission cathode and Gaussian intensity distribution, and to the development and approval of models for the prediction and precise control of resist profiles in thick PMMA layers for 3D proximity effect simulation, the bilayer resist system, and the lift-off method.Item Development of model-based simulation and optimization software for electron beam melting and refining(2023-01-01) Koleva E.; Koleva L.; Tsonevska T.In this paper the development of a software for electron beam melting and refining (EBMR) of different materials is presented. It has the following functional characteristics: simulation of the temperature distributions in the cast ingots, investigation and optimization of the form of the liquid pool during the process of EBMR and estimation of the inclusion refining rates, depending on the EBMR process parameters. The temperature distributions in the cast ingots are simulated by the implementation of quasi steady-state two-dimensional heat model and three-dimensional time-dependent thermal model for different refined metals. Empirical models are estimated, integrated in the developed software and used for the investigation of the molten pool geometry parameters, the content of impurities after the electron beam refining process, refining rate, overall inclusion removal efficiency and the material losses. The influence of EBMR process parameters such as electron beam power, casting velocity, refining time, and electron beam radius are considered. Multicriterial optimization software functions can be implemented for solving concrete technological requirements for each practical use-case.Item EBT 2018-Preface(2018-10-19) Mladenov G.; Koleva E.Item Effects of process parameters on electron beam melting technogenic materials for obtaining rare metals(2018-10-19) Vutova K.; Vassileva V.; Koleva E.; Stefanova V.; Amalnerkar D.In this paper, effects of electron beam melting process parameters (e-beam power, refining time, etc.) and the number of melting operations for fulfilling the requirements concerning the composition and the structure of metals obtained after e-beam processing are studied. Results obtained at electron beam melting of molybdenum and tungsten technogenic materials are presented and discussed. The refining efficiency is evaluated and effective technological regimes (process parameters) for production of metals which meet specific requirements for high-purity and good-quality structure after electron beam melting of Mo and W technogenic materials are proposed.Item Electron beam characterization by a tomographic approach(2016-04-07) Koleva E.; Mladenov G.; Todorov D.; Koleva L.; Kardjiev M.In this paper, experimental data are analyzed for the integral current density distribution when the electron beam parameters are varied, namely, focusing current, beam current, venelt voltage and the distance to the measuring device. The 3D beam radial current density distribution is reconstructed by implementing a tomographic approach. The characterization of the electron beam is considered in connection with the estimation of the following parameters: the radial and the angular beam distribution standard deviations, the position of the beam focus and the beam emittance.Item Electron beam deflection control system of a welding and surface modification installation(2018-04-05) Koleva E.; Dzharov V.; Gerasimov V.; Tsvetkov K.; Mladenov G.In the present work, we examined the patterns of the electron beam motion when controlling the transverse with respect to the axis of the beam homogeneous magnetic field created by the coils of the deflection system the electron gun. During electron beam processes, the beam motion is determined the process type (welding, surface modification, etc.), the technological mode, the design dimensions of the electron gun and the shape of the processed samples. The electron beam motion is defined by the cumulative action of two cosine-like control signals generated by a functional generator. The signal control is related to changing the amplitudes, frequencies and phases (phase differences) of the generated voltages. We realized the motion control by applying a graphical user interface developed by us and an Arduino Uno programmable microcontroller. The signals generated were calibrated using experimental data from the available functional generator. The free and precise motion on arbitrary trajectories determines the possible applications of an electron beam process to carrying out various scientific research tasks in material processing.Item ELECTRON BEAM LITHOGRAPHY PROCESS QUALITY IMPROVEMENT BY ROBUST ENGINEERING APPROACH(2024-01-01) Koleva L.; Asenova-Robinzonova A.; Koleva E.The model-based robust approach is successfully applied to different industrial processes for improving their quality performance characteristics. The Robust engineering approach is implemented in cases of heteroscedasticity of the performance characteristics in presence of errors in the process parameters or noise factors, which is typical for the production processes. Higher quality and reproducibility of the quality characteristics can be achieved both for existing production processes or for technological process design at the production planning and development stages. In this paper the model-based robust engineering approach is implemented for the quality improvement of polymethyl methacrylate (PMMA) resist profiles, obtained by electron beam lithography process. Specific technological requirements concerning the developed profile structures, their variances (reproducibility) in production conditions and robustness toward the errors in the process parameters, are considered. Different multicriteria optimization approaches were applying and compared, aiming to simultaneously fulfil the requirements for the geometric characteristics of the positive resist PMMA.Item Empirical modeling of high-intensity electron beam interaction with materials(2018-04-05) Koleva E.; Tsonevska T.; Mladenov G.The paper proposes an empirical modeling approach to the prediction followed by optimization of the exact shape of the cross-section of a welded seam, as obtained by electron beam welding. The approach takes into account the electron beam welding process parameters, namely, electron beam power, welding speed, and distances from the magnetic lens of the electron gun to the focus position of the beam and to the surface of the samples treated. The results are verified by comparison with experimental results for type 1H18NT stainless steel samples. The ranges considered of the beam power and the welding speed are 4.2 - 8.4 kW and 3.333 - 13.333 mm/s, respectively.Item Enhancing critical thinking skills through the use of ICT in academic writing classes(2021-03-08) Todorova M.; Koleva E.The widespread expansion of electronic information technologies in education leads to continuous processes of modernization and sometimes a complete change in the content, methods, organization of training, the learning space, and the goals of the educational system. The ever-increasing amount of information needs to be understood, processed, and restructured. Comprehensive educational reform is underway, including a change in human thinking. This paper focuses on enhancing critical thinking of Ph.D. engineering students through ICT use in Academic writing classes. A model has been proposed and elaborated.Item Experimental design sequential generation and overall D-efficiency criterion for electron beam grafting of corn starch(2018-10-19) Koleva E.; Koleva L.; Braşoveanu M.; Nemţanu M.R.An experimental investigation of the electron beam irradiation induced grafting of acrylamide onto corn starch was performed. In this way the synthesis of water-soluble copolymers having flocculation abilities is realized. The acrylamide monomer provides a polar graft side chain which turns it into a hydrophilic copolymer. The robust engineering approach was implemented for the estimation of models for the means and the variances of the investigated quality parameters in production conditions, aiming the assurance of low toxicity of the synthesised copolymer, process economic efficiency, copolymer efficiency in the flocculation process and its good solubility in water. In order to improve the accuracy of the estimated models, performing new experiments was planned, based on the sequentially generated D-optimal designs. An approach for the generation of such designs for several quality parameters is presented.Item Heat distribution simulation in electron-beam surface modification of 316L stainless steel samples(2020-06-02) Koleva E.; Tsonevska T.; Koleva L.; Vutova K.; Mladenov G.; Naplatanova M.; Trushnikov D.; Varushkin S.A time-dependent heat model is implemented for the simulation of the heat distribution in 316L stainless steel samples during electron-beam irradiation. The influence is studied of the electro-beam power and processing time on the temperature field produced in the samples. The simulation results are compared with real experimental data of electron-beam surface modification of 0.5-mm-thick 316L stainless steel samples using the ELIT-60 installation in the Institute of Electronics BAS. The simulation and the experimental results show good agreement.Item IEC 62264 standard-based manufacturing operations management resource modelling for electron beam welding(2023-01-01) Vasilev P.; Koleva E.The development and deployment of a Manufacturing Operations Management (MOM) system for electron beam welding aims the integration and organization of the accumulated knowledge for the process, and the application of this knowledge for the improvement of the modelling and control capabilities, their efficiency, adaptability, flexibility and re-configurability, as well as integration with other technological processes. The paper presents the Unified Modelling Language (UML) information modelling of electron beam welding installation as a part of an IEC 62264 standard-based manufacturing operations management system development.Item Integrated control system for electron beam processes(2018-04-05) Koleva L.; Koleva E.; Batchkova I.; Mladenov G.The ISO/IEC 62264 standard is widely used for integration of the business systems of a manufacturer with the corresponding manufacturing control systems based on hierarchical equipment models, functional data and manufacturing operations activity models. In order to achieve the integration of control systems, formal object communication models must be developed, together with manufacturing operations activity models, which coordinate the integration between different levels of control. In this article, the development of integrated control system for electron beam welding process is presented as part of a fully integrated control system of an electron beam plant, including also other additional processes: surface modification, electron beam evaporation, selective melting and electron beam diagnostics.