Browsing by Author "Koleva L."

Now showing 1 - 13 of 13
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    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.
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    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.
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    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.
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    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.
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    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.
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    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.
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    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.
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    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.
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    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.
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    Model-based multicriterial optimization of electron beam melting and refining of copper
    (2022-04-04) Koleva E.; Koleva L.; Vassileva V.; Vutova K.; Tsonevska T.
    Experimental results on the quality of electron beam melting and refining of copper samples by implementing the ``method of washers``are analyzed by using an empirical model estimation approach. The influence is studied of the process parameters - melting power and time of refining, on the residual content of impurities, the refining rate, the overall removal efficiency and the material losses. A graphical user interface is designed and developed for calculation, investigation and multicriterial optimization of the quality of the refined copper samples.
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    Modelling the shape of electron beam welding joints by neural networks
    (2018-10-19) Tsonevska T.; Koleva E.; Koleva L.; Mladenov G.
    This article discusses the experimental results from multi-pool electron beam welding, with dynamic positioning of the electron beam (beam splitting) [1], resulting in the formation of two consecutive welding pools. The 12Cr18Ni10Ti stainless steel samples are welded with a change in the process parameters: the distance between the two electron beams (electron beam positions) and the ratio between the two mean electron beam powers, the frequency of the deflection signal, the beam current and the welding velocity. The focusing current is kept at a constant value. The weld cross-sections, experimentally obtained at different process parameters, are used to train, validate and test neural models. The accuracy of prediction of the shapes of the welds (the form of the molten pool) is discussed and compared with that of an estimated regression model.
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    Multicriterial optimization strategies for electron beam welding processes
    (2022-04-04) Koleva E.; Koleva L.; Trushnikov D.; Kolev G.; Petrova Z.
    The electron beam welding process is analyzed of high-strength steel type 15Cr5Mo samples with deflection oscillations along the zone of interaction. The geometries of the molten and the heat affected zones are studied in order to investigate the influence of the deflection oscillations parameters and to improve the seam quality. A robust engineering approach is implemented for the case of production conditions considering the errors in the process parameter settings. The problem of quality improvement through fulfilling pre-set technological and quality requirements is solved by scalarization of the vector criteria by implementing a reference point strategy. Other multicriterial optimization strategies are also proposed.
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    Processing diagram for powder bed additive manufacturing
    (2023-01-01) Koleva E.; Kolev G.; Koleva L.
    This paper presents the developed software for processing parameter simulation and optimization of powder bed additive manufacturing by concentrated energy beams. The simulations are realized by implementation of a normalized processing diagram and analytical model simulating heating of powder layers by a moving linear heat source. Their implementation is demonstrated for electron beam or laser additive manufacturing of industrial alloys. The developed software and the processing diagram provide a useful reference and methodology to support the selection of appropriate processing parameters during the early development stages of concrete powder bed additive manufacturing technology, building production digital twins or development of integrated process management system.