E-Journal Mathematical Modeling and Computer Simulation

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1 SURFACE MODIFFICATION OF TOOL STEELS BY ELECTRICAL DISCHARGE TREATMENT IN ELECTROLYTE Veselin Paunov Department of Physical Metallurgy and Heat Aggregates, University of Chemical Technology and Metallurgy, 8 Kliment Ohridski Blvd., 1756, Sofia, Bulgaria veskopaunov@yahoo.com Abstract: Some treatments of metals and alloys in electrolyte give opportunities to modify their surfaces. The surface layers after electrical discharge treatment in electrolyte have different structure in comparison with the metal matrix, determining different properties such as higher hardness, strength and corrosion resistance. In this study altering of the surface of tool steels after electrical discharge treatment in electrolyte is investigated. Different process conditions are compared and resultant microstructures are investigated by optical microscopy and XRD. Results show improved surface properties and change in microstructure and chemical composition of the modified surfaces. Keywords: Electrical discharge treatment, Microstructure, Surface treatment INTRODUCTION The idea of these investigations is based on the possibilities of Electrical Discharge Machining (EDM) to modify the treated metallic surfaces along with the erosion. The high energy and high rate of the process gives as result change in microstructure and formation of very hard and difficult to etch layer on the workpiece surface. Normally the process is used for manufacturing dies, moulds and aerospace components. Nowadays the researchers adapt this technique for hardening of the surface. Few authors are researching corrosion behavior of the machined surfaces [1,2,3]. The basic principal of EDM is conversion of electrical energy into heat through series of discrete electrical discharges (sparks) between electrode and workpiece immersed in a dielectric fluid (fig.1) [4]. In the beginning voltage is supplied to both electrodes. Before current can flow, the open gap voltage increase until it has created an ionization path through the dielectric. Once the current starts to flow, voltage drops and stabilizes at working gap level (fig.2). When the current supply is turned off, the plasma channel breaks down. This causes a sudden reduction in the temperature allowing the circulating dielectric fluid to implode the plasma channel and flush the molten material from the pole surface in the form of microscopic debris [5]

2 Figure 1. Basic scheme of Electrical discharge machining [4] Figure 2. Profile of a single EDM pulse, [5]. The machined surface can be alloyed by using powders of different compounds in dielectric liquid. The particles in the dielectric get energized and act as a conductor (fig.3). They disperse the spark energy over the larger surface of the workpiece and attain refined surface roughness and increase material removal rate (MRR) [6,7,8,9]. The application of powder in the dielectric provide bigger gap distance, which make sparking more stable and gives a chance to dielectric to flush better the electrodes affecting surface quality [9,10]. Application of water glycerin mixture show to be more efficient compared to pure water [11, 12]. Figure 3. Principle of powder mixed EDM

In the process of electrical discharge machining the surface of material is locally melted and resolidifys very fast. This leads to formation of layer on the surface with different properties.

3 In the process of electrical discharge machining the surface of material is locally melted and resolidifys very fast. This leads to formation of layer on the surface with different properties. It is known as the white surface layer or recast layer. The chemical etchants did not succeed to review structure in this layer. For scientists and engineers this layer is interesting with its high hardness, wear resistance, and corrosion resistance [1,3]. The layer is formed from two parts (fig.4) outer layer and heat affected zone. The outer part of the layer is formed when melted from the spark material which hasn t been expelled from the surface, resolidifys back on the workpiece surface. It poses higher hardness than the base material. Some of the electrode material is observed on the surface of the work piece after examination with XRD. Also the concentration of the carbon on the surface is increased. The next layer is not melted. It changes under the influence of the top layer temperature and is known as heat affected zone or annealed zone. Figure 4. Schematic section of a spark machined surface: 1 - white layer, 2 - heat affected zone,3 - unaffected material MATERIALS AND METHODOLOGY The process of surface modification by electrical discharge treatment in electrolyte has been developed as an alternative of EDM process for surface modification. Materials used in this study are HS and 90CrSi5 tool steels. The surface modification by electrical discharge treatment in electrolyte goes by a high energy thermal process in a very small volume on the metallic surface, involving melting, vaporisation, activation and alloying in electrical discharges and after that cooling of this surface with high rate in an electrolyte. The high energy process put together with the nonequilibrium phase transformations in the metallic system causes considerable modifications of the metallic surface and obtaining of layers with finecrystalline and nanocrystalline structure. The metallic surface after electrical discharge treatment in electrolyte has a different structure in comparison with the metal matrix which determines different properties. It is observed remarkable increasing of hardness, strength and corrosion resistance related to the nonequilibrium phase transformations and the obtained finecrystalline microstructure. The investigations show that obtained on tools layers have higher hardness, wear resistance, tribocorrosion resistance and corrosion resistance, which give better performance, considerable increasing of working life and wide opportunities for industrial application [13-15]. The samples are with cylindrical shape and diameter of 4 and 6 mm. The types of

4 used electrolytes are chosen on the base of literature review and some experimental experience and have the following compositions: 1. Н 2 О + glycerin + В 4 С + Na 2 CO Н 2 О + glycerin + B + Na 2 CO Н 2 О + urea The electrolyte is brought in a motion by an electromagnetic propeller 1 (fig.5). The workpiece 2 is put on holder 7 and is rotated into electrolyte by a motor 6. After passing of electric current with determinate characteristics through the electrolyte between the workpiece 2 and electrode 3 starts an active sparking on the workpiece surface. The sparking characteristics depend on different factors such as parameters of the electric current, type and composition of the electrolyte, movement of the workpiece and electrolyte. The electric current is controlled by the power supply 5. The time of treatment of the workpiece in electrolyte is between 2 to 4 minutes. Figure 5. Scheme of experimental equipment for electrical discharge treatment in electrolyte: 1 - electromagnetic stirrer; 2 - workpiece; 3 - electrode; 4 - container; 5 - power supply; 6 rotating device; 7 - holder; 8 - measuring devices; 9 propeller RESULTS AND DISCUSSION Layers on the metallic surface can be observed after treatment with voltage higher than 150 V. By lower voltage the obtained recast white layer on HS steel is inhomogeneous and local deposited on the metal surface. The increasing of treatment time in this case shows an insignificant influence on the process. In figure 6 are shown optical micrographs of a high-speed tool steel surface microstructure after electrical discharge treatment without rotation at 180 V

treatment without rotation at 180 V for 2 min, magnification x800 The experiments with rotation of the

By application of higher voltage it is possible to obtain layers with homogeneous surface and uniform

treatment with rotation of the workpiece: a - 200 V for 2 min, b - 220 V for 2min, magnification x800 b The

They are compared by microhardness (fig. 8 and 9).

5 Figure 6. Optical micrographs of a high-speed tool steel HS surface microstructure after electrical discharge treatment without rotation at 180 V for 2 min, magnification x800 The experiments with rotation of the samples show a homogeneous surface without defects. By application of higher voltage it is possible to obtain layers with homogeneous surface and uniform thickness - figure 7. a Figure 7. Optical micrographs of a high-speed tool steel HS surface microstructure after electrical discharge treatment with rotation of the workpiece: a V for 2 min, b V for 2min, magnification x800 b The treatment in different electrolytes shows layers with different characteristics. They are compared by microhardness (fig. 8 and 9). The microhardness of the modified surfaces is higher for all samples compared to base material (fig.8). For samples treated in electrolyte contained В 4 С is observed higher microhardness compared to those treated in electrolyte contained B (fig.9). Figure 8. Microhardness comparison of the samples treated in electrolyte

6 containing B and В 4 С. Figure 9. Comparison of recast layer microhardness with microhardness of untransformed part of the sample The XRD investigation of modified surfaces of HS tool steel show presence of the carbides which are typically for the high-speed steel structure and presence of Me 2 B (fig.10 ) which is obtained by a diffusion process of B from the electrolyte. The light microscopy and XRD investigation of modified surfaces of 90CrSi5 tool steel show presence of martensite, austenite and phases which are formed after diffusion of elements from the electrolyte such as Fe 2 B and FeB (fig.11). The primary structure of the sample is after annealing which prove a quench hardening in the electrolyte of the modified surface and mass transfer of boron and its reactive diffusion in the metallic surface. a b Figure 10. XRD analysis of HS steel surface: a - before electrical discharge modification in electrolyte; b - after electrical discharge modification in electrolyte

7 a b Figure 11. XRD analysis of 90CrSi5 steel surface: a - before electrical discharge modification in electrolyte; b - after electrical discharge modification in electrolyte CONCLUSIONS After electrical discharge treatment in electrolyte on the surface of tool steel is obtained a layer with microstructure and phase composition different from the base material. The changes in chemical composition are proved with light microscopy, XRD investigation and microhardness testing. The higher hardness obtained after the surface modification corresponds with the structure changes. REFERENCES 1. Uno, Y., Okada, A. and Cetin, S., Surface modification of EDMed surface with powder mixed fluid, 2 nd international conference on design and production of dies and molds, Kusadasi- Turkey, Roger N. Johnson, Robust coatings for corrosion and wear: the electrospark deposition process, Battelle, Tri- service conference on corrosion,usa, November N.Hassiotis, I.Laropoulos, G. Petropoulos, A.Koutsomichalis, N.M.Vaxevanidis, Corrosion behavior of EDMed surfaces of tool steel, Proceedings of the 3 rd international conference on manufacturing engineering, 1-3 October, Singh, S.and Bhardwaj, A., Review to EDM by using water and powder- mixed dielectric fluid, JMMCE,USA,Vol.10 (2), 2011, pp , 5. Newman, K.H.Ho,S.T., State of the art electrical discharge machining (EDM), IJMTM, Vol.43, 2003,pp Chow, H. M., Yang, L. D., Lin, C. T., Chen, Y. F., The use of SiC powder in water as dielectric for micro-slit EDM machining. Journal of materials processing technology, Vol. 195, 2008, pp

8 7. Mohri, N., Saito, N., Narumiya, H., Kawatsu, Y., Ozaki, K. T. and Kobayashi, K., Finish Machining by EDM Using Powder Suspended Working Fluids, Journal of Japan Soc. of Electrical-Machining Engrs, Vol. 49, 1992, pp Mori, N., Saito, N., Narumiya, H., Kawatsu, Y., Ootake, H., Tsunekawa Y., Takawashi, T. and Kobayashi, K., Study on Finishing of Large Area Workpiece by EDM, Journal of the Japan Soc. for Prec. Eng., Vol. 53 (1), 1987, 9. Nurumiya, H., Mohri, N., Saito, N., Otake, H., Tsunekawa, Y., Takawashi, T., Kobayashi, K., EDM by Powder Suspended Working Fluid, Proceed. of the ISEM Kobayshi, K., The Present and Future Developments of EDM and ECM, Proceed. of the ISEM- 11, Konig, W., Klocke, F. and Sparrer, M., EDM-Sinking using Water based Dielectrics and Electropolishing A New Manufacturing Sequence in Toolmaking, Proceed. of the ISEM-11, Dunnebacke, G., High Performance EDM Using a Water-based Dielectric, Proceed. of the ISEM-10, Krastev, D, Stefanov, B., Yordanov, B. Angelova, D. Electrical Discharge Surface Treatment in Electrolyte of High Speed Steel, Proceedings of 6 th International Congress on Machines, Technologies and Materials, Sofia, Bulgaria, February 2009, Krastev, D., Yordanov, B. About the Surface Hardening of Tool Steels by Electrical Discharge Treatment in Electrolyte, Solid State Phenomena, 2010, 159, Krastev, D. Improvement of Corrosion Resistance of Steels by Surface Modification. In: Corrosion Resistance, Shih, H, Ed., InTech, 2012,

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