The electrospark deposition is a process for surfacing of hard metal alloys, e.g. carbides and stellites, on the surfaces of new or old machine elements. In this process, a high current is conducted through an oscillating electrode and a substrate for a very short period of time. In the paper, the process is described and the thickness of deposited layer, chemical composition, dilution rate and the layer roughness are determined.Nanošenje električnom iskrom postupak je za navarivanje tvrdih metalnih slitina, npr. karbida i stelita, na površinu novih ili starih elemenata strojeva. U tom postupku visoka struja provodi se kroz oscilirajuću elektrodu i supstrat na vrlo kratko vrijeme. U članku opisan je postupak i određeni su debljina nanesenog sloja, kemijski sastav, stupanj miješanja i hrapavost sloja

A. Lešnjak

J. Tušek

L. Kosec

T. Muhič

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17METALURGIJA 51 (2012) 1, 17-20J. TUŠEK, L. KOSEC, A. LEŠNJAK, T. MUHIČ ELECTROSPARK DEPOSITION FOR DIE REPAIRReceived - Primljeno: 2011-01-12Accepted – Prihvaćeno: 2011-03-10Original Scientifi c Paper – Izvorni znanstveni radISSN 0543-5846METABK 51(1) 17-20 (2012)UDC – UDK UDC – UDK 621.791.725:621.793:621.791.92:669.018=111The electrospark deposition is a process for surfacing of hard metal alloys, e.g. carbides and stellites, on the surfaces of new or old machine elements. In this process, a high current is conducted through an oscillating electrode and a substrate for a very short period of time. In the paper, the process is described and the thickness of deposited layer, chemical composition, dilution rate and the layer roughness are determined. Key words: welding, electrospark deposition, repair of dies Nanošenje električnom iskrom za popravak alata. Nanošenje električnom iskrom postupak je za navarivanje tvrdih metalnih slitina, npr. karbida i stelita, na površinu novih ili starih elemenata strojeva. U tom postupku visoka struja provodi se kroz oscilirajuću elektrodu i supstrat na vrlo kratko vrijeme. U članku opisan je postupak i određeni su debljina nanesenog sloja, kemijski sastav, stupanj miješanja i hrapavost sloja.Ključne riječi: zavarivanje, nanošenje električnom iskrom, popravak alataINTRODUCTIONIn the everyday industrial manufacturing practice, there are many products for which it is essential that they show high abrasion, corrosion and/or thermal re-sistance. Such products are gas turbines, casting dies, cutting tools, pumps, compressors, and similar. In such equipment, usually only their individual parts or indi-vidual surfaces are exposed to strong dynamic, mechan-ical, thermal, abrasive, corrosion, and other similar loads. Consequently, it is very useful to protect such surfaces or parts with a suitable cladding. Today a number of methods for the deposition of hard claddings on substrates are known. They can be classifi ed with respect to different criteria, one of them being the aggregate state of the material deposited. Known proc-esses are cladding (by rolling and by explosion), surfac-ing (gas, arc, plasma, laser surfacing), spraying (gas, plasma, laser spraying), and evaporating. One of the numerous modes of depositing hard clad-dings on new, damaged, or worn-out products is the electrospark deposition. This process is characterised by the deposition of primarily carbides and other hard alloys on the substrate in very thin layers. The paper aims to show some characteristics of elec-trospark deposition. The process has not been studied thoroughly yet, although it is used in practice. In the literature there are very few publications dealing with J. Tušek, Faculty of Mechanical Engineering, Ljubljana, Slovenia, L. Ko-sec, Faculty of Natural Sciences and Engineering, Ljubljana, Slovenia, T. Muhič, TKC d.o.o., Ljubljana, Slovenia, A. Lešnjak, Q-techna d.o.o. Ljubljana, Sloveniaelectrospark deposition from the scientifi c or engineer-ing point of view [1−5]. The issues not studied yet are primarily the material transfer from the electrode to the workpiece, the infl u-ence of shielding gases on the process of material transfer and properties of the deposited layers, an analysis of the joint formed by the substrate and the deposit, the burn-off of elements, the effi ciency of supplied energy, the infl u-ence of the amount of energy input, and so forth. EXPERIMENTAL PROCEDUREIn the experimental work, a classical unit enabling manual electrospark deposition was used. The unit con-sisted of an electronically controlled source of electric pulses and an electrode holder with a vibrating mecha-nism. The source was of the capacitor type. The parameters of electrospark deposition, i.e. the welding current, the type of fi ller material, and the type of shielding gas, were varied. The substrates used were a hot working tool steel, designated H13 in accordance with ASTM, and a high-alloy austenitic steel. Three dif-ferent types of fi ller material were used (WC, TiC, stel-lite 6). The fi ller materials used were electrodes in the form of uncoated bars with a diameter of 2,4 mm and a length of 100 mm. The deposition process was carried out without a shielding gas, with pure argon (99,99 %), and with pure helium, respectively.EKSPERIMENTAL RESULTSIn the study of compatibility of hard layers and tool steels and of a proper deposition technique, it was found 18  METALURGIJA 51 (2012) 1, 17-20J. TUŠEK et al.: ELECTROSPARK DEPOSITION FOR DIE REPAIRthat the basic requirement, i.e. suitable hardness, was comparatively easy to fulfi l. It is more diffi cult, however, to ensure an appropriate surface homogeneity without cracks, and particularly adherence to the substrate taking into account high thermal and mechanical loads. Droplet sizeThe surface roughness of the deposit and other me-chanical properties of the surface are infl uenced by the number, shape and size of the droplets transferred. The approximate droplet size can be calculated from the known weight of the droplet transferred per unit time. Figure 1 shows the infl uence of electric energy in the electrospark deposition on the average number of drop-lets. The average droplet diameter is around 80 µm and their average mass is around 40 µg (WC). Tungsten car-bide (WC) was used as fi ller material, austenitic stain-less steel (SS) and tool steel (CS) as substrates, and Ar and He as shielding gases. The electric pulse energy has the most important infl uence on the droplet diameter.Rate of dilutionThe fi ller material transferred from the electrode to the small molten area at the workpiece is diluted by the substrate. This dilution decreases considerably with each new material transfer; therefore, the deposit thick-ness is limited to approximately 30 µm. One of very important fi ndings is that the deposit thickness cannot be increased indefi nitely. The most important is the fi rst contact of the electrode with the substrate (fi rst layer). Further contacts (other layer) with the electrode have a negligible infl uence on the dilution; the energy input in the fi rst layer is therefore important. Roughness of the deposited surfaceDeposit roughness was evaluated as well. It was found that the surface roughness depended on the type of the substrate, the material deposited, and the deposi-tion parameters. It was also found that roughness in-creased with an increase in energy input, i.e., higher pulse intensity. It was also found that in a substrate of austenitic stainless steel, the roughness was quite pro-portional to pulse intensity. In case of a substrate being a low-carbon steel plate, it was much more diffi cult to determine a correlation between pulse intensity and roughness. The dependence of roughness on the deposit material, the substrate, and the pulse energy is shown in Figure 2. Addition of a shielding gasIn the investigation conducted, however, the entire process of the material transfer from the electrode to the work piece surface was shielded against the atmosphere by using two inert gases. The respective shielding gases used were He and Ar. The gas was supplied through a nozzle from one side. The fl ow rate of Ar was approxi-mately 6 l/min and that of He 10 l/min. The deposit structures differed considerably. The deposits were free of cracks. There was, however, some porosity, but less than in the case where no shielding gas was used. Figure 3 shows the infl uence of the shielding gas and the pulse energy on the roughness of the deposited layer. When the two shielding gases used were com-pared, it was found that the pores in the deposit obtained with He showed a smaller diameter than those in the deposit obtained with Ar. There was less porosity if heat treatment, i.e., quenching and tempering, was applied after the deposition. This indicates that degasifi cation occurred. The latter was stronger with He as the shield-Figure 1  Average diameter of droplets as a function of electric pulse energyFigure 2  Surface roughness of the deposited layer as a function of electric pulse energyFigure 3  Infl uence of shield gas and electric pulse energy on surface roughness of deposited layer19METALURGIJA 51 (2012) 1, 17-20J. TUŠEK et al.: ELECTROSPARK DEPOSITION FOR DIE REPAIRing gas. It is a fact that He is easier to degasify from the deposited layer than Ar. One of the basic fi ndings is that the deposit made in the shielding atmosphere is much more homogeneous. The manufacturer of the device has not prescribed the use of a shielding gas, but the study showed that in such a case the resulting deposit was in-homogeneous. This is shown in Figure 4a. Type of fi ller materialA comparison of the carbide deposits, i.e. of WC and the stellite 6 deposit, is interesting as well. With stellite 6 it was found that it was easier to deposit in Ar than in He. In this case it was found that the deposit showed almost no porosity, regardless of the fact that stellite 6 was deposited using Ar. The macro specimens of the WC deposit produced without shielding gas, of the WC obtained using He, and the stellite 6 deposit obtained using Ar are shown in Figure 4. The macrographs clear-ly show that the deposition should be carried out using a shielding gas and that stellite 6 is more suitable than tungsten carbide. Thickness of deposited layerThe thickness of the WC and stellite 6 deposits ranged up to 30 µm whereas that of the TiC deposits ranged up to 20 µm. The deposit thicknesses obtained with He were greater than those obtained with Ar by approximately 15 to 20 %. The case was quite different, however, if the deposition was carried out without a shielding gas. In this case maximum thicknesses up to 12 µm were obtained regardless of the type of deposit. Deposit analysisThe only substrate used for the chemical analyses of the deposits made was the H13 tool steel. It was not heat-treated before deposition. For the analysis, TiC and WC layers were deposited on the substrate using Ar as the shielding gas. After deposition, the test pieces were heat-treated, i.e. quenched and tempered. They were ad-ditionally prepared before testing. For a through-thick-ness analysis of the deposited layer, however, the test pieces were cut. The cut surface was then ground, pol-ished, and etched. The through-thickness composition of the deposit is shown in Table 1, not including carbon, whereas Figure 5 shows this for the WC deposit and Figure 6 for the TiC deposit, both obtained using Ar as shielding gas. Table 1 and Figures 5 and 6 indicate that the W and Ti contents decreased from the fi nal layer of deposit to the substrate.The same is true of the Co content in the deposit produced with stellite 6. As to the content of Fe, which is the main element of the substrate, the fi ndings were quite the opposite. The results obtained are logical and were nothing to be surprised about. The thickness of the deposit obtained using Ar ranged between 22 and 30 µm, while that obtained using He ranged between 10 and 20 µm. There were pores in the layers, which were more numerous in the deposits obtained using Ar than in those obtained using He.Table 1  Chemical composition of the deposited layers obtained with diff erent fi ller materials, diff erent shielding gases, and at diff erent locations.Deposit type Shielding gas Location of measurementsChemical compositions / wt %Cr Fe W Ti Co Mo Ni SiTiC Ar under surface 2,7 30,9 − 50,4 − 5,9 10,1 −TiC Ar middle of layer 3,3 40,7 − 41,0 − 5,8 9,2 −TiC He under surface 4,1 59,2 − 29,2 − 3,5 4,0 −TiC He middle of layer 4,0 56,8 − 28,6 − 5,4 5,2 −WC He under surface 2,7 40,7 54,0 2,6 − − − −WC He middle of layer 2,7 41,7 51,9 3,7 − − − −stellite 6 Ar middle of layer 24,8 31,6 − − 38,7 − − 4,9stellite 6 Ar middle of layer 24,8 31,6 − − 38,7 − − 4,9Figure 4  Macrographs of layers deposited: WC layer using no shield gas (a), WC layer using Ar (b), stellite layer using Ar (c)Figure 5  WC layer deposited using Ar (a) and line analysis of elements (b)20  METALURGIJA 51 (2012) 1, 17-20J. TUŠEK et al.: ELECTROSPARK DEPOSITION FOR DIE REPAIRIn the TiC deposit produced using Ar, more Ti, i.e. around 40−50 % wt, was found than in the TiC deposit obtained using He (around 12−29 wt %). Similarly as with the WC deposit, in the layers of the TiC deposit, elements from the substrate, i.e. Fe and Cr, were found, which indicates interdiffusion between the elements of the deposit and those of the substrate. Ni (up to 10 wt %) and Mo (3−6 wt %) were found in the layers, too. They originated from the electrode material. The TiC layers deposited using Ar and He show approxi-mately the same thickness around 10 to 20 µm.In the stellite 6 deposit, Co, Cr, and Si were found. They were due to the elements from the deposit. Fe originated from the substrate. This indicated interdiffu-sion between the stellite 6 deposit and the substrate. The boundary between the layer and the substrate was repre-sented by a line along which diffusion occurred. It was assessed that the zone next to the line was 1,5 to 4,0 µm wide. It was mainly in this zone that the elements from both the fi ller material and the substrate were found.The chemical analysis of the deposits showed that the W content ranged between 4 and 18 wt %, while the Ti content ranged between 12 and 55 wt %. The W and Ti contents depended strongly on the deposition param-eters, i.e. on the deposit thickness. A study of dilution in test pieces showed that the dilution was practically neg-ligible. It was thus found that the type of shielding gas did not affect the rate of dilution. The differences oc-curred only in the deposit thickness, while the thickness of the layer in which dilution occurred was below 5 µm everywhere.CONCLUSIONSThe electrospark process of depositing hard layers is one of the possibilities to improve the mechanical prop-erties of surfaces, to re-establish worn-out surfaces, and thus to extend the life of tools or other machine ele-ments. It is a rather simple process and permits repair of tools on site. The results obtained lead to the following conclusions:1. In electrospark deposition, the use of inert shielding gases is a must. The inert shielding gas should pro-tect the electrode tip and the molten surface on the workpiece.2. The burn-off of Ti is the strongest of all the elements; therefore, Ti carbide should be deposited in vacuum or in an effi cient shield of Ar or He.3. WC, TiC, stellite 6, and other materials may be used as fi ller material. The highest-quality deposit (with respect to porosity) is obtained when using stellite 6 as the fi ller material.4. An average droplet diameter when using WC as a fi ller material is around 80 µm and average droplet mass is around 40 µg.5. On the average, around 100 droplets per second are transferred from the electrode to the workpiece.6. Surface roughness of the deposit ranges between 0,8 and 5,5 µm.7. The thickness of deposits ranges up to 50 µm and is strongly dependent on the welding current.8. The width of the zone of dilution of the deposit with the substrate is around 4 µm.9. On the macro level, the temperature of workpiece surface ranges below 100 °C.It should be underlined that the process is highly useful in practice although it has not been scientifi cally studied much until now.REFERENCES[1]  P.Z.Wang, J.X. Pan, Y.Zhou, J.X. Qu, H.S. Shau: Accele-rated electrospark deposition and the wear behavior of coatings, Journal of materials engineering and performan-ce, 6 (1997), 780[2]  T. Burakowski, T. Wierzcho: Surface Engineering of Me-tals: Principles, Equipment, Technologies, (1999)[3]  R. N. Johnson, G.L. Sheldon: Advances in the electro-spark deposition coating process, Jornal of vacuum science and technology A, 4 (1986), 2740[4]  N.C. Welsh, P.E. Watts: Spark hardening of cutting tools, austenite formation and edge erosion, Journal of the iron and steel institute, (1961), 31-37[5]  D. Heard, J. Milligan, M. Brochu: Investigation of the electro-spark deposition of a nano-structured eutectic alu-minum-silicon coating, Jornal of advanced and speciality materials, (2009), 2535-2541Note:  The responsible for English language is Marko Orešković, Ljubljana, SloveniaFigure 6  TiC layer deposited using Ar (a) and line analysis of elements (b)

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