Dutova O.S. et al. 2018 Vol. 20 No. 1

OBRABOTKAMETALLOV Vol. 20 No. 1 2018 66 MATERIAL SCIENCE References 1. Fomin V.M., Zasypkin I.M., eds. Generatsiya nizkotemperaturnoi plazmy i plazmennye tekhnologii: problemy i perspektivy [Generation of low-temperature plasma and plasma technologies: problems and prospects]. Novosibirsk, Nauka Publ., 2004. 464 p. Modeling of the Thermal and Structural States of Hollow Cathode of Vacuum Plasmatron Olga Dutova 1, a,* , Andrey Shishkin 1, b , Vladimir Cherednichenko 2, c 1 Kutateladze Institute of Thermophysic SB RAS, 1, Ac. Lavrentieva ave., Novosibirsk, 630090, Russian Federation 2 Novosibirsk State Technical University, 20 Prospekt K. Marksa, Novosibirsk, 630073, Russian Federation a http://orcid.org/ 0000-0001-5366-8010, odutova@ngs.ru , b http://orcid.org/0000-0002-5431-0077 , andrshi@itp.nsc.ru , c http://orcid.org/0000-0002-4844-5928 , bm@skbterm.ru Obrabotka metallov (tekhnologiya, oborudovanie, instrumenty) = Metal Working and Material Science. 2018 vol. 20 no. 1 pp. 55–68 ISSN: 1994-6309 (print) / 2541-819X (online) DOI: 10.17212/1994-6309-2018-20.1-55-68 Obrabotka metallov - Metal Working and Material Science Journal homepage: http://journals.nstu.ru/obrabotka_metallov ARTICLE INFO Article history : Received: 14 November 2017 Revised: 12 January 2018 Accepted: 08 February 2018 Available online: 15 March 2018 Keywords : Plasmatron Cathode Temperature field Recrystallization Grain size ABSTRACT Introduction. Arc plasmatrons are widely used in various fields of science and technology. The resource of continuous work of electrodes determines the efficiency of plasmatron and is one of its most important technologi- cal characteristics. A theoretical and experimental study of physical and mechanical processes in cathode material is focused on increasing the duration of its running time and is a relevant objective. The purpose of the work is the creation of physical and mathematical models and numerical study of thermal and recrystallization processes occur- ring in the hollow cathode of a vacuum plasmatron under the influence of an electric arc. Experimental Technique . To study the temperature field of a cathode under the action of an electric arc, the Fourier differential equation with an internal heat source, Laplace equation for the electric potential, and Ohm’s law equation were solved jointly. When the plasmatron is operating, new nuclei are formed and grow in the cathode. Three interrelated phenomena are most important for recrystallization: material heating, new grain nucleation and growth. The distribution of crys- talline grain size over the cathode volume was calculated based on the temperature field data and activation model parameters of grain nucleation and growth for tungsten. The proposed mathematical models allow simulating the various modes of hollow cathode operation, evaluating the change in the structure of a material during its heating, and can be used to study and improve the performance characteristics of the hollow cathodes of vacuum plasmatrons. Results and Discussion . The obtained solutions showed that the high heating rates and rapid output to stationary mode characterize the cathode heating. It should be noted there is a sharp change in temperature along the cathode length in the area of the active zone (heating surface). The temperature distribution shows the considerable axial and radial temperature gradients, which can lead to large thermal stresses in the cathode. Calculation showed when the superheating over the temperature of recrystallization starts decreasing, the grain size increases. This is due to the fact that when the superheating grows, the nucleation rate outstrips the rate of grain growth, and the grain size decreases. For the investigated flux density values, the size of the primary recrystallized grain, average along the cathode length, is in the range of 3.7–14 μm. The time required to obtain a single-crystal wall of the hollow cathode due to collective and/or secondary recrystallization is 1–32 hours. As a result, complete recrystallization of the grain in the cross-section of tungsten cathode can occur in one cycle of plasmatron operation. This means that the electro- physical and thermal characteristics of the cathode change significantly during its operation. Also the grain size has a significant effect on the resistance to the destructive effects of thermal stresses. For citation: Dutova O.S., Shishkin A.V., Cherednichenko V.S. Modeling of the thermal and structural states of hollow cathode of vacuum plasmatron. Obrabotka metallov (tekhnologiya, oborudovanie, instrumenty) = Metal Working and Material Science , 2018, vol. 20, no. 1, рр. 55–68. doi: 10.17212/1994-6309-2018-20.1-55-68. (In Russian). ______ * Corresponding author Dutova Olga S. , Ph.D. (Physics and Mathematics), Scientific associate Kutateladze Institute of Thermophysic SB RAS, 1, Ac. Lavrentieva ave., 630090, Novosibirsk, Russian Federation Tel: 8 (383) 333-10-96, e-mail: odutova@ngs.ru