Browsing by Author "Vassileva V."
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Item Behaviour of Impurities during Electron Beam Melting of Copper Technogenic Material(2022-02-01) Vutova K.; Stefanova V.; Vassileva V.; Kadiyski M.The current study presents the electron beam melting (EBM) efficiency of copper techno-genic material with high impurity content (Se, Te, Pb, Bi, Sn, As, Sb, Zn, Ni, Ag, etc.) by means of thermodynamic analysis and experimental tests. On the basis of the calculated values of Gibbs free energy and the physical state of the impurity (liquid and gaseous), a thermodynamic assessment of the possible chemical interactions occurring in the Cu-Cu2O-Mex system in vacuum in the temperature range 1460–1800 K was made. The impact of the kinetic parameters (temperature and refining time) on the behaviour and the degree of removal of impurities was evaluated. Chemical and metallographic analysis of the obtained ingots is also discussed.Item Effect of electron beam method on processing of titanium technogenic material(2019-06-01) Vutova K.; Vassileva V.; Stefanova V.; Amalnerkar D.; Tanaka T.This study reveals the efficiency of the electron beam processing of titanium technogenic material with a high level of impurities and the quality of the obtained metal in correlation to process parameters which are discussed. The influence of the beam power and melting time on the composition variation, morphologies, hardness of metal samples and mass losses is investigated. Based on the different technological parameters, the removal efficiency of impurities is also discussed, and the corresponding experiments are carried out in order to make a comparison. Different thermal process conditions are realized during the single-melt operation. Chemical and metallographic analyses are performed, and the results are discussed. The hardness of the titanium decreases by prolonging the time of the electron beam processing. A maximal overall removal efficiency of 99.975% is seen at 5.5 kW beam power for a 40 min melting time and the best purification of Ti (99.996%) is achieved.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 melting efficiency at multiple hafnium e-beam processing(2024-01-01) Markov M.; Stefanova V.; Vutova K.; Vassileva V.; Tanaka T.; Kakugawa K.The method of electron beam melting and vacuum refining has clear advantages over other metallurgical methods since it enables manufacturing of refractory and chemically active metals. This study focuses on the efficiency of removing impurities from technogenic hafnium under multiple electron beam melting. Assessments are performed on the efficiency of double and triple e-beam melting processing of refractory metal hafnium. The influence of different e-beam melting technological modes on the refining effectiveness is investigated. A highest hafnium purity of 99.2% was achieved after double and triple e-beam refinements of the investigated materials, with the highest process efficiency reaching 61.58% and 51.07%, respectively.Item Investigation of tantalum recycling by electron beam melting(2016-11-21) Vutova K.; Vassileva V.; Koleva E.; Munirathnam N.; Amalnerkar D.P.; Tanaka T.Investigations are carried out and obtained experimental and theoretical data for tantalum scrap recycling by electron beam melting (EBM) is presented in this paper. Different thermal treatment process conditions are realized and results are discussed. A chemical analysis is performed and refining mechanisms for electron beam (EB) refining of Ta are discussed. For the performed experiments the best purification of Ta (99.96) is obtained at 21.6 kW beam power for a melting time of 3 min. A statistical approach is applied for estimation of the material losses and the liquid pool characteristics based on experimentally-obtained data. The aim is to improve the EBM and choosing optimal process conditions, depending on the concrete characteristic requirements. Model-based quality optimization of electron beam melting and refining (EBMR) processes of Ta is considered related to the optimization of the molten pool parameters, connected to the occurring refining processes, and to minimal material losses. Optimization of the process of EBM of Ta is based on overall criteria, giving compromised solutions, depending on the requirements concerning the quality of the performed products. The accumulated data, the obtained results, and the optimization statistical approach allow us to formulate requirements on the process parameters.Item 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.Item Modelling of the mechanical properties of steel 316L after EB treatment(2018-10-19) Dzharov V.; Koleva E.; Vassileva V.; Yordanova R.; Yankova S.Investigation of the variation of the mechanical properties of 316L steel after electron beam surface modification is performed. The samples were processed at an installation in IE BAS - ELIT - 60. The irradiated plates were with thickness of 0.5 mm. The experiments were carried out with a change in the beam power in the range of 0.6 to 0.9 kW and different irradiation times between 1 to 120 seconds. Regression analysis was performed on the obtained results.Item Recycling of Technogenic CoCrMo Alloy by Electron Beam Melting(2022-06-01) Vutova K.; Stefanova V.; Vassileva V.; Atanasova-Vladimirova S.In the current work, the possibility of the recycling of technogenic CoCrMo material by electron beam melting is investigated. The influence of thermodynamic and kinetic parameters (temperature and melting time) on the behavior of the main components of the alloy (Co, Cr, and Mo) and other elements (Fe, Mn, Si, W, and Nb) present in it, and on the microstructure of the ingots obtained after e-beam processing is studied. The vapor pressure of the alloy is determined taking into account the activities of the main alloy components (Co, Cr, and Mo). The relative volatility of the metal elements present in the alloy was also evaluated. An assessment of the influence of the temperature and the retention time on the degree of elements removal from CoCrMo technogenic material was made. The results obtained show that the highest degree of refining is achieved at 1860 K and a residence time of 20 min. The conducted EDS analysis of the more characteristic phases observed on the SEM images of the samples shows distinct micro-segregation in the matrix composition.Item Study of the Possibility of Recycling of Technogenic Hafnium during Electron Beam Refining(2022-12-01) Vutova K.; Stefanova V.; Markov M.; Vassileva V.The possibility of removing metallic (such as Zr, Fe, Cr, and Zn) impurities and non-metallic (such as [O] and C) impurities from technogenic hafnium through single and double refining in the conditions of electron beam melting (EBM) has been studied. The influence of thermodynamic and kinetic parameters on the degree of removal of these impurities from the base metal under vacuum conditions and within a temperature interval of 2500 K to 3100 K is defined. The relative volatility of metal impurities and the stability of the oxides and carbides present in the base metal are evaluated. The possibility for complete removal of Fe, Cr, Zn, [O], and C during EBM is shown. In the case of double refining, at a temperature of 2700 K for 20 min, the maximum degree of removal of Zr is 46.8%, the achieved highest hafnium purity is 99.004%, and the overall effectiveness of the refining of hafnium from impurities is 53%. There is a correlation between the degree of removal of Zr and the micro-hardness of the Hf ingots obtained after EBM. The weight losses vary in the ranges of 1.5–5.8% and 1–8% under the studied single and double refining processes, respectively.Item Study on Hardness of Heat-Treated CoCrMo Alloy Recycled by Electron Beam Melting(2023-04-01) Vutova K.; Stefanova V.; Markov M.; Vassileva V.The hardness of heat (thermally) treated CoCrMo ingots, recycled by electron beam melting and refining (EBMR) of a technogenic CoCrMo material (waste from the dental technology) under different process conditions (temperature and residence time) is examined. The heat treatment consists of two-step heating up to temperatures of 423 K and 1343 K and retention times of 40 and 60 min, respectively. The influence of various loads (0.98 N, 1.96 N, 2.94 N, 4.9 N, and 9.8 N) on the hardness of the CoCrMo alloy, recycled by EBMR, before and after heat treatment is studied. It has been found that regardless of the EBMR process conditions, the obtained samples after heat treatment have similar hardness values (between 494.2 HV and 505.9 HV) and they are significantly lower than the hardness of the specimens before the heat treatment. The highest hardness (600 HV) is measured in the alloy recycled at 1845 K refining temperature for 20 min. This is due to the smaller crystal structure of the resulting alloy and the higher cobalt content. The results obtained show that the heat treatment leads to considerable changes in the microstructure of the CoCrMo ingots recycled by EBMR. With the increase of the e-beam refining temperature, after the heat treatment, the grains’ size increases and the grains’ shape indicates an incomplete phase transition from γ-fcc to ε-hcp phase. This leads to a slight increase in the hardness of the alloy.Item Thermodynamic analysis of the processes during electron beam melting and refining of copper(2022-04-04) Stefanova V.; Vutova K.; Vassileva V.In the paper, results are presented and discussed obtained by thermodynamic analysis of the refining processes of copper aimed at removing metal impurities and oxygen via electron beam melting (EBM) in vacuum. Thermodynamic evaluations of the possible chemical interactions between the base metal (Cu) and the metal impurities present in Cu that can proceed during the refining of copper are made based on the calculated values of the Gibbs free energy and equilibrium constants taking into account the physical state of copper and of the metal impurities during EBM.