PROPERTIES OF ELEKTRO SPARC DEPOSITED STELLITE COATING ON MILD STEEL

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1 PROPERTIES OF ELEKTRO SPARC DEPOSITED STELLITE COATING ON MILD STEEL Wojciech DEPCZYŃSKI, Norbert RADEK, The Centre for Laser Technologies of Metals Faculty of Mechatronics and Machine Design, Kielce University of Technology, Al lecia P.P. 7, Kielce, Poland, EU Abstract Electro-spark deposition (ESD) is a micro-welding process using short-duration, high-current electrical pulses to deposit an electrode material on a metallic substrate. The short duration of the electrical pulse allows an extremely rapid solidification of the deposited material and results in the formation of a surface layer with properties different from those of the base material. The layer produced has good wear and corrosion resistance. This paper presents the results of an concerning the investigation cavitation erosion resistance of surface layers produced by electro-spark deposition. Wettability was also studied using two types of surfaces with different roughness. It has been found that the surface layer produced by ESD has good criteria resistance and that by changing the process parameters, we can control the surface contact angle. Keywords: cavitation; cavitation erosion; electro-spark deposition 1. INTRODUCTION There are many methods for producing surface coatings, for example electroplating or plasma spraying. Very thin deposited layers can be manufactured by vapour deposition method. Various surface treatment techniques have been developed to improve the desired properties of the deposited layers, produced in the substrate material. Some of these methods are expensive and should be used only for unique applications, where the high cost is justified. However, for most applications, there is a need for generation low cost coatings of good properties. The aim of this work is to improve a widely used low cost method of electrospark deposition (ESD). It has been already recognized as an economically effective surface coating [1-6]. ESD is also widely used for its relative low cost equipment required to run this process. The deposition of the coating is achieved by an electrical circuit, which generates sparks between the electrode and the workpiece. Electrical pulses of high frequency and high direct current between the electrode (anode) and workpiece (cathode) releases very hot micro-particles of electrode material, resulting in continuous micro-welding coating on the work-piece surface. Important advantage is that the coating is bonded to the substrate with relatively low heat input. The energy savings effect is based on micro-particles heated region, while the body of the work-piece remains at low temperature. ESD has been known by several other terms such as Spark Hardening (SH), Electric Spark Toughening (EST), and Electro-Spark Alloying (ESA). Electro-Spark Deposited (ESD) coatings have some disadvantages, which can be easily eliminated. For example one of the proposed methods can be laser treatment; where laser beam is used for surface polishing, surface geometry formation, surface sealing or for homogenizing the chemical composition of the coatings deposited [7-9,11]. Research on this technology is being conducted all over the world, and the companies interested in applying it include NASA and the US Navy [10 11]. As electro-spark coatings are reported to be resistant to wear and corrosion, they can be applied, for instance, to: ship propeller components, casting moulds,

2 fuel supply system components, exhaust system components. The electro-spark deposition coating are characterized by non etching structure and staying white after etching. The surface layer is constituted in environment of local high temperature and high pressure. The fundamental value parameters of electro-machining are as follows: shock wave pressure comes from electric spark is (2-7)10 3 GPa, temperature rich (5-40)10 3 Celsius degree value. 2. EXPERIMENT 1.1 Layer fabrication In the experiment, the coatings were electro-spark deposited with using a stellite electrode with a crosssection of 4 x 6 mm (the anode) - onto samples made of carbon steel C45 (the cathode). The equipment used for electro-spark deposited was an EIL-8A model. We based on the previous our work and instructions given by the equipment manufacturer, the following parameters were assumed to be optimal for ESD voltage U = 230 V, capacitor volume C = 300 µf, current intensity I = 2.1 A. The chemical composition of C45 is shown in Table 1. Table 1. Chemical composition of C45 carbon steel Elements C Mn Si P S Content % 0,42 0,50 0,50 0,80 0,10 0,40 0,04 0,04 The chemical composition of stelitte electrode is shown in Table 2. Table 2. Chemical composition of electrode stellite Elements C Cr Mn Co Fe W Content % 7,0 56,0 1,0 25,0 0,6 10, Test methods After ESD coatings, the specimens were sectioned, placed in cold setting acrylic resin (VariDur 10) and prepared for microscopic analysis with a STRUERS automatic polishing machine. The final polishing was performed using a 0.05 μm Al 2 O 3 (METADI) suspension. The preparation of the specimens for the OM and SEM examinations did not include etching. The structure analysis was conducted using a PHILIPS XL30 (LaB 6 ) scanning electron microscope. The chemical composition of the specimens was studied by means of an EDS spectrometer. The microhardness of the layers was investigated using a MATSUZAWA MMT Vickers (HV0.1) hardness tester. Wettability was evaluated as the value of contact angle determined based on the geometric method according to the following formula (1), where h - height of the drop, and r - radius of the drop 1 arccos 1 h r h r 2 2 The study was based on the application of a drop of water on the surface and images of this were captured. The measurement of h and r parameters were analyzed with the computer image analysis system. Determined on the basis of images dimensional proportions drop to calculate the angle. For measuring droplet formation were used micropipettes, and their volume is adjusted to 7 l. The surface roughness of the samples was tested with a profilometer PGM-1C. Cavitation erosion studies were carried out on the (1)

3 device blast shock. The test specimens have the shape of cylinders with a diameter of 20 mm and a height of mm. The samples were fixed in the arms of the rotor in a vertical plane parallel to the axis of the stream of water pumped at a pressure of 0.06 MPa in a continuous manner through a nozzle with a diameter of 10 mm, a distance from the edge of the sample of 1.6 mm. While the sample was rotating, stream heated the water. The water flow was constant and amounted to 1.55 m 3 /hr. Samples were examined for a period of 30 minutes, removed from the handle, defatted in an ultrasonic bath for 10 minutes at 30 C, dried in the oven for 15 minutes at 120 C and weighed, and then re-mounted in the arms of the rotor in their original position relative to the water stream. Four samples were examined with a coating layer and four others sample and the erosion of steel C45. All samples were tested for a total time of 420 min. 3. RESULTS AND DISCUSSION Figure 1 shows SEM image of the cross-sections of the stelitte coated C 45 specimen. In the photograph of the microstructure exhibits a distinct boundary between the coating and the substrate coating and visible pores. Electro coating thickness obtained for the test samples was about 10 to 15 m a) CoK CrK FeK b) Fig.1 Electro-spark coating: a) microstructure and elemental mapping of cross-section, b) linear distribution of elements. Linear analysis (Fig. 1b) showed the erosion coating uneven distribution of the elements in the shell. The shell can be divided into zones containing significant amounts of W, Co and Fe. It was found an increased carbon content in the layer-shaped spark machining. This is an example involving the diffusion displacement

4 of carbon in base material - steel C45 as a result of thermal effects. The Carbon diffuses while the surface layer was formed by ESD technology. The surface roughness of the samples was measured with profilometer PGM-1C, in the range of Ra = m for coated samples and also 0,42 0,46 for C45 m samples. The large roughness is due to imposing of craters, as a result of the substrate erosion and also due to the ridges formed by moving particles of the electrode coating material on the surface (Fig. 2). Fig. 2. SEM microphotography of coating surface formed by ESD Microhardness of the coating was between HV0,1, microhardness ground (substrate) oscillated between 180 HV0,1. As can be seen in the graph (Figure 3) contact angle decreases with time for each test textured surface of the ring. The largest changes occur in the initial phase of placing a drop of the textured surface. Fig. 3. Wettability of coating surface formed by ESD

5 On figure 4 shows the weight loss - damage curves of the samples coated by ESD. Fig. 4. Cavitation damage curves of samples coated with ESD The cavitation damage progress of electro spark coating samples was similar. [12,13] In the initial stage of erosion it removes an oxides of Cr and Co causing concave space on the surface, which became at a later stage of cavitation erosion damage initiators (Fig. 5a). Particles are removed by creating electro spark coating weakened areas which appeared first small craters. The resulting craters have become places of compressive stress concentrations and sources of further crack initiation. The rate of damage in these areas depends on layer thickness and erosion on the amount of internal defects in the formed pores. Further effects of stream water accumulates in cracks and in removed shell fragments (Fig. 5b). After the test period of 300 min at the surface of the test samples can be observed the unfolding of the substrate material. Created cracks on coated material, reaching up to the substructure, caused the removal of grains of ferrite. Later it led to the formation of pits on the surface, by removing entire portions of the coating material and the substrate (Fig. 6). Depth of pitting was observed about m after 420 minutes, erosion studies. a) b) Fig. 5. Effects of cavitational erosion on the surface of samples coated with electro-spark a) loss of Cr and Co oxides, b) craters on the surface.

6 a) b) Fig. 6. Effects of cavitational erosion of samples coated with electro-spark of cross-section. In the initial stage of cavitation damage of steel samples C45 and samples coated with the same electro spark eroded weight loss. During this period, samples of the surface layer are removed by electro spark particles of oxides of Cr and Co, and the surface of samples of C45 steel mesh is formed cracks at the grain boundaries. [14] The different nature of the first stage of cavitation damage results in a further step change in the speed test weight loss, as evidenced by a different angle erosion curves (fig.4). 4. CONCLUSIONS The following are conclusions drawn from the study. 1. The coating layer made on C45 has a higher cavitation erosion resistance as compared to the samples of the substrate material (steel C45). 2. It can be assumed that the coating ESD of greater thickness would increase even greater resistance to cavitation destruction. 3. To receive the layers with a higher resistance to cavitation erosion coating, created ESD shells should have: greater thickness, the single phase composition without internal defects. 4. Investigated electrical discharge coating can be used in the repair and regeneration of turbomachinery for example for filling defects caused by destructive activity of cavitation. LITERATURE [1] Radek, N.: Determining the operational properties of steel beaters after electrospark deposition. Eksploatacja i Niezawodność - Maintenance and Reliability. 2009; 4: [2] Ribalko, A.V.; Sahin, O.: The use of bipolar current pulses in electro spark alloying of metal surfaces. Surface & Coatings Technology. 2003; 168: [3] Chen, Z.; Zhou, Y.: Surface modification of resistance welding electrode by electro-spark deposited composite coatings: Part I. Coating characterization. Surface & Coatings Technology. 2006; 201: [4] Radek, N.; Broncek, J.: Tribological properties of anti-wear carbides coatings. Technolog. 2009; 6: [5] Chang-bin, T.; Dao-xin, L.; Zhan, W.; Yang, G.: Electro-spark alloying using graphite electrode on titanium alloy surface for biomedical applications. Applied Surface Science. 2011; 257: [6] Galinov I.V.; Luban R.B.: Mass transfer trends during electrospark alloying. Surface & Coatings Technology. 1996; 79: [7] Radek, N.; Antoszewski, B.: Influence of laser treatment on the properties of electro-spark deposited coatings. Kovove Materialy-Metallic Materials. 2009; 1:

7 [8] Radek, N.; Bartkowiak, K.: Performance properties of electro-spark deposited carbide-ceramic coatings modified by laser beam. Physics Procedia. 2010; 5: [9] Radek, N.; Bartkowiak, K.: Laser treatment of Cu-Mo electro-spark deposited coatings. Physics Procedia. 2011; 12: U.S. Patent No Method for joining single crystal turbine blade halves [10] Spadło S.: Comparative studies of brush electrodischarge machining with electrodes of alloy steel and tungsten. Advanced Manufacturing Systems and Technology, International Centre for Mechanical Sciences (CISM), Courses and Lectures - No.437, ISBN , Springer Veralag Wien New York, 2002, pp [11] Spadło S.: Theoretical-experimental aspects of electroerosion-mechanical machining. Published by Technical University in Kielce, PL ISSN , 195 pp., Kielce (in polish) [12] Plesset M. S., Chapman R.B.: Collapse of an Initially Spherical Vapour Cavity in the Neighbourhood of a Solid Boundary, Jour. Fluid Mech. (1971) Vol. 47, Part 2, [13] Brennen C.E.: Cavitation and Buble Dynamics, Oxford University Press (1995) [14] Jasionowski R., Podrez Radziszewska M., Depczyński W., Radek N. ;Odporność na korozje Kawitacyjną powłok elektroiskrowych, Inżynieria Materiałowa Nr 4, s , 2011