Removal of Unwanted Layer of Material Deposited on the Die Surface During Electric Discharge Machining

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Removal of Unwanted Layer of Material Deposited on the Die Surface during

Electric Discharge Machining

Prof. Satishkumar V Tawade Department of Mechanical Engineering, Navsahyadri Group of Institutes,

Savitribai Phule Pune University, Pune, India.

International Journal of Research In Mechanical Engineering Volume 3, Issue 2, March-April, 2015, pp. 24-31

ISSN Online: 2347-5188 Print: 2347-8772, DOA : 09032015 iaster 2015, www.iaster.com

ABSTRACT Electrical discharge machining (EDM) is a well-established machining option for manufacturing geometrically complex or hard material parts that are extremely difficult-to-machine by conventional machining processes. A pulse discharge occurs in a small gap between the work piece and the electrode and removes the unwanted material from the parent metal through melting and vaporizing. EDM has been an important manufacturing process for the tool, mould, and dies industries for several decades. Due to recast layer produced during EDM machining, surface defects, such as cracks, micro craters, lead to a decreased surface integrity, probably resulting in a short die life. Powder Mixed Electric Discharge Machining (PMEDM) significantly affects the performance of EDM process. Process parameters, namely peak current, pulse-on time, pulse-off time, concentration & size of electrically conductive powder reduces the insulating strength of the dielectric fluid and increase the spark gap between the tool and the work piece and reduces the recast layer. Mirror like finishing can be obtained by PMEDM. Aluminum powder gives better results in PMEDM than the other. Keywords: Electrical Discharge Machining (EDM), PMEDM, Surface Roughness, Recast Layer, Aluminum Powder. I. INTRODUCTION It is found that the white layer is quite hard and that non- etch able. The white layer is so densely infiltrated with carbon that it has a separate, distinct structure, totally distinguishable from the parent material. The defects within it, such as voids, cracks, induced stresses etc. cause an overall deterioration of the components mechanical properties. Among the surface defects, cracking is the most significant since it leads to a reduction in the material resistance to fatigue and corrosion. The existence of cracks in the machined surface will lower the life of the mould. Observation of the machined surface, and the sample sections, reveals that the surface cracks are often micro-cracks. The high magnification microscope shows that cracks exist in the white layer; initiating at its surface, and travelling down perpendicularly towards the parent material. In the vast majority of cases the cracks terminate within the white layer, or just on the interface of the white layer and the parent material. Only rarely do the cracks penetrate the entire white layer thickness to extend into the parent material. In EDM machining process the white layer is formed over the work piece of skd 61 which is hot steel used for making die. This white layer makes some unwanted cracks, micro craters, lead to a decreased surface integrity, probably

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resulting in a short die life. Manually removal of white layer is very difficult. By mixing fine grain sized aluminum powder in the EDM process itself has reduced the unwanted formation of white layer [4]. II. HEAT AFFECTED ZONE In EDM (Electrical Discharge Machining) process as the sparks are generated the material begins to melt and vaporize with small crater, thus reducing the area of work piece material into gas bubbles. During the each cycle period of EDM crater gets larger, its increasing surface area begins to sink heat away from the spark gap until the vaporization temperature can no longer be sustained. The melting process continues but a pulse interval time period is required to flush away the eroded material. During the off time period when the current is switched off, melting ceases instantly and all molten material break away in the form of small spherical bubbles is drawn back by the surface tension and resolidified back on to the cooler layer of cut material. This thin layer of resolidified material is called recast layer or white layer. Immediately below the recast layer is the area termed as heat affected zone (HAZ). This area is partially affected by the elevated temperature of the spark gap. Within this area, the material did not approach temperature large to melt, but reaches a temperature high to change its temper, reducing its hardness. While machining the merging steel with EDM process a recast layer is formed. These results in premature part failure and shorten the life of parts fine surface finish. The thickness of the recast layer formed on the work piece and the level of thermal damage suffered by the electrode can be determined by analyzing the growth of the plasma channel during sparking. The EDM generates heat affected zone in the machining zone. The researchers measured the micro hardness around the micro- EDM whole cross sections. They concluded that HAZ in micro EDM comprises of low hardness instead of white layer. III. WHITE LAYER The recast layer is referred as white layer since it is difficult to etch and its appearance under optical microscope is white. Beneath the recast layer, a heat affected zone is formed due to the rapid heating and quenching cycles during EDM. It is commonly believed that the white layer formed during machining of steels is caused primarily by a thermally induced phase transformation resulting from rapid heating and quenching. A white layer is a featureless layer that typically forms on machined steel surfaces and appears white when observed under an optical microscope after standard metallographic preparation. There have been many studies about white layers generated in various manufacturing processes such as hard turning, electric discharge machining, reaming, grinding as well as service parts such as locomotive rails and bearings. Various characteristics of the white layer have been reported. It is observed not only in ferrous metals, but also in non-ferrous metals such as titanium and brass. However, the underlying mechanisms that give rise to the white layers are not fully understood. Three key mechanisms responsible for white layer formation in various manufacturing processes are as: Phase transformation due to rapid heating and quenching, Fine grain structure formed due to severe plastic deformation and reaction of the surface with the environment. The very high temperature up to 40,000 K has significant impact on the process-induced surface integrity including surface topography, microstructure change, residual stress, micro hardness and element distributions. In machining of steels in particular, two mechanisms, thermal and mechanical effects, are considered to be the major causes of white layer formation. Although the potential role of mechanical deformation on white layer formation in machining has been acknowledged by researchers, it is commonly assumed in the literature that the white layer is formed when the work piece surface temperature exceeds the nominal phase transformation temperature as austenitization temperature in the equilibrium FeC phase diagram. In the EDM Process, the estimated discharge point temperature is thousands degrees (C) in order to


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rapidly melt machined material at this charge point. The locally generated high-temperature sparks cause the surrounding dielectric fluid to evaporate rapidly and its volume to expand. The high pressure generated by this inertial enclosure effect quickly removes molten metal from the surface of machined material. But, the molten metal on the surface of machined materials are not completely flushed away with the surrounding dielectric fluid during this process. The residual molten material re-solidifies on the machined surface to form a rapidly solidified layer. Thus the rapidly solidified layer produces a huge change in both the surface topography and surface metallurgy of machined material. The state of subsurface characteristics occurs in the rapidly solidified layer and is generally in the form of micro cracks, change in hardness, residual stress, metallurgical transformations, and heat affected zones (HAZ). The rapidly solidified layer also has different micro structural and metallographic characteristic than the base material. EDM on ferrous metals results in surface changes, with the formation of a re-solidified layer, usually known as the recast layer, which varies in thickness. The recast layer undergoes complex structural changes associated with extremely high cooling rate. It is found that the white layer is quite hard and that non-etch able. The white layer is composed mainly of martensite and retained austenite, with some dissolved carbide. The white layer is so densely infiltrated with carbon that it has a separate, distinct structure, totally distinguishable from the parent material. In order to remove the recast layer, manufacturers will normally perform a finishing operation such as the lapping process. By polishing the surface with abrasive grains (e.g. silicon carbide, alumina, or diamond) in the presence of a lubricant, the white recast layer can be removed and a lustrous, mirror-like finish can be achieved. By minimizing the white layer in EDM process we can reduce the machining time and cost of final product with the quality [2]. IV. REMOVAL OF WHITE LAYER A. Ultrasonic Vibration Introduction of ultrasonic vibration to the electrode is one of the methods used to expand the application of EDM and to improve the machining performance. The study of the effects on ultrasonic vibration of the electrode on EDM has been undertaken since mid-1980s. The higher efficiency gained by the employment of ultrasonic vibration is mainly attributed to the improvement in dielectric circulation which facilitates the debris removal and the creation of a large pressure change between the electrode and the work piece, as an enhancement of molten metal ejection from the surface of the work piece. The pulse discharge is produced by the relative motion between the tool and work piece simplifying the equipment and reducing its cost. It is easy to produce a combined technology which benefits from the virtues of ultrasonic machining and EDM [6]. B. Dry EDM Dry electrical discharge machining (EDM) is a technology that has the potential to replace conventional liquid based EDM, owing to its low tool electrode wear, thin recast layer, and environmental friendliness. In dry EDM, tool electrode is formed to be thin walled pipe. High-pressure gas or air is supplied through the pipe. The role of the gas is to remove the debris from the gap and to cool the inter electrode gap. The technique was developed to decrease the pollution caused by the use of liquid dielectric which leads to production of vapor during machining and the cost to manage the waste. Yu et al. investigated the capability of the technique in machining cemented carbide material and compared the machining characteristics between oil EDM milling and oil die sinking EDM. They found that for machining the same shape, oil die sinking EDM shows shorter machining time. But because oil die sinking requires time for producing Electrodes, dry EDM should be more useful in actual production.


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The information given in this paper is interesting and they are reproduced here for better clarity. According to the results, work removal rate of dry EDM milling is about six times larger than that of oil EDM milling, and electrode wear ratio one-third lower. It is shown that the EDM method with the shortest machining time was oil die sinking EDM, dry EDM milling was second, and oil EDM milling third. The lowest electrode wear ratio machining was dry EDM milling. C. Cryogenic Treatment of Tool Cryogenic is the science of study of material at low temperature at which the properties of materials significantly change. Cryogenics processing is the treatment of the materials at very low temperature. This technique has been proven to be efficient in improving the physical and mechanical properties of the materials such as metals, plastics and composites. It improves the wear, abrasion, erosion and corrosion resistivity, durability and stabilizes the strength characteristics of various materials. Darwin investigated that deep cryogenic treatment (DCT) is a one-time permanent process, carried out on steel components in such a way that the material is slowly cooled down to the cryogenic temperature, after which it is held at that temperature for a specified period of time and is heated back to room temperature at a slow rate followed by low temperature tempering. The DCT has a lot of benefits. It not only gives dimensional stability to the material, but also improves wear resistance, strength and hardness of the materials. Cryogenic refines and stabilizes the crystal lattice structure and distribute carbon particles throughout the material resulting in a stronger and hence more durable material [5]. D. EDM in Water Water as dielectric is an alternative to hydrocarbon oil. The approach is taken to promote a better health and safe environment while working with EDM. This is because Hydrocarbon oil such as kerosene will decompose and release harmful vapor (CO and CH4). Research over the last 25 years has involved the use of pure water and water with additives. a. EDM in Pure Water Machining in distilled water resulted in a higher material removal rate and a lower wear ratio than in kerosene when a high pulse energy range was used. With distilled water, the machining accuracy was poor but the surface finish was better. The best machining rates have been achieved with the tap water and machining in water has the possibility of achieving zero electrode wear when using copper tools with negative polarities. The erosion process in water-based media consequently possesses higher thermal stability and much higher power input can be achieved especially under critical conditions, allowing much greater increases in the removal rate. A considerable difference between conventional oil based dielectrics and aqueous media is specific boiling energy of aqueous media is some eight times higher and boiling phenomena occur at a lower temperature level. The use of an oil dielectric increases the carbon content in the white layer and appears as iron carbides in columnar structures while machining in water causes a decarburization. Stresses are found to be increasing rapidly with respect to depth, attaining to its maximum value around the yield strength and then fall rapidly to compressive residual stresses in the core of the material since the stresses within plastically deformed layers are equilibrated with elastic stresses. The potential of electrically conductive chemical vapor deposited diamond as an electrode for micro-electrical discharge machining in oil and water. While doing a comparative study on the surface integrity of plastic mold steel, Ekmekci et al. found that the amount of retained austenite phase and the intensity of micro cracks have found to be much less in the white layer of the samples machined in de-ionized water. The proposed control achieves an optimum and stable


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operation using tap water as dielectric fluid to prevent the generation of undesired impulses and keep the distance between the electrode and the work piece within the optimum stable range. When kerosene as dielectric, it was observed that carburization and sharp crack propagation along the grain boundary occurred after the heat treatment. However, using deionizer water as dielectric the specimen after heat treatment underwent oxidation and showed no crack propagation behavior. b. EDM in Water with Additives A highly concentrated aqueous glycerin solution has an advantage as compared to hydrocarbon dielectrics when working with long pulse durations and high pulse duty factors and discharge currents, i.e. in the roughing range with high open-circuit voltages and positive polarity tool electrode. Some researchers have studied the feasibility of adding organic compound such as ethylene glycol, polyethylene glycol 200, polyethylene glycol 400, polyethylene glycol 600, dextrose and sucrose to improve the performance of demonized water. The surface of titanium has been modified after EDM using dielectric of urea solution in water. The nitrogen element decomposed from the dielectric that contained urea, migrated to the work piece forming a TiN hard layer which resulting in good wear resistance of the machined surface after EDM.

V. PMEDM Fine abrasive powder is mixed into the dielectric fluid. The hybrid material removal process is called powder mixed EDM (PMEDM) where it works steadily at low pulse energy and it significantly affects the performance of EDM process. Electrically conductive powder reduces the insulating strength of the dielectric fluid and increase the spark gap between the tool and the work piece. EDM process becomes more stable and improves machining efficiency, material removal rate. However, most studies were conducted to evaluate the surface finish since the process can provide mirror surface finish which is a challenging issue in EDM. The characteristics of the powder such as the size, type and concentration influence the dielectric performance. As debris in a spark gap usually consists of metal and carbon particles, which will drastically lower the breakdown strength of dielectric, gap debris evidently would facilitate ignition process and increases gap size. Absence of the debris can result in arcing due to a lack of precise feeding mechanism with extremely high position resolution, which occurs frequently in the early stages of the ignition process particularly. Moreover, the amount of debris matters. While the absence of debris does not help improve sparking frequency, too much debris is generally believed to be the dominant cause of spark concentration i.e. arcing that leads to an unstable and inefficient process. In fact, gap debris is of somewhat help. Gap debris reportedly is comparatively a most crucial factor to the stability of machining process, which demands evenly disperse discharge locations that mainly depend upon debris concentration and distribution, bubbles, de-ionization, and surface irregularities. Nevertheless their concurrent presence poses great difficulty to segregate the effects attributed to each factor most present control systems thus cannot directly regulate discharge location. Gap debris in this regard can significantly control discharge transitivity, gap size, breakdown strength, and de-ionization. The remaining core issue is how to decide the function of gap debris. Theoretically, the function of debris will be to a great extent controlled by the characteristics of the additives if they are added in suitable particle size, particle concentration, particle density, thermal conductivity, electrical resistivity, melting point, evaporation point, specific and latent heat, etc. The addition of particles alters the material removal mechanism in the EDM process. It is noted that the addition of powders lead to an increase in gap size that subsequently resulted in a reduction in electrical discharge power density and in gas explosive pressure for a single power pulse. The modified material removal mechanism for addition of powders during normal single electrical discharge time is the combined effect of mechanical thrust


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driven by the gas explosion mainly from the working fluid evaporation with the striking impact by the suspended particle. The materials removed by the grinding effect of suspended particles within the interspaced are negligible. It is worth noting that the weaker gas explosion in the interspaced after powder addition might lead to a reduction in the material removal rate during normal single electrical discharge process. To enhance the machining efficiency of the whole EDM process, the particles striking effect and the discharge transitivity, therefore, play a decisive role. Especially, the latter decides the sparking frequency that governs the entire material removal rate, while the first has minor cutting effect contributing mainly to the improvement of the surface finish. In PMEDM, electrically charged conductive powders are added in plasma channel and leads to decreasing the insulating strength of the dielectric fluid, so the servo controller of EDM machine for stabilizing discharging condition, increases the gap distance as compared to traditional EDM. The enlarged and widened discharge channel reduces the electrical density on the machining spot and thus generates shallow craters and lower surface roughness. On the other hand with increasing the thermal conductivity of powders, more heat is dissipated from electrodes gap through dielectric fluid, consequently the level of thermal energy in the gap distance is decreased. PMEDM has different machining mechanism from conventional EDM. In this process, an appropriate kind of powder is mixed into the dielectric fluid when a voltage of 80320 V is applied to both the electrodes; an electric field in the range of 105107 V/m is created. The spark gap is filled up with additive particles and the gap distance between tool and work piece increased from 2550 m to 50150 m. The powder particles get energized and behave in zigzag fashion. The grains come close to each other under the sparking area and gather in clusters. The interlocking between the different powder particles takes place due to the variation in their shape and size. They arrange themselves in the form of chain at different places under the sparking area. The chain formation helps in bridging the gap between both the electrodes. Due to bridging effect, the gap voltage and insulating strength of the dielectric fluid decreases. The easy short circuit takes place, which causes early explosion in the gap, as a result, the series discharge starts under the electrode area. Due to increase in frequency of discharging, the faster sparking within a discharge takes place which causes faster erosion from the work piece surface at the same time, the added powder modifies the plasma channel. The plasma channel becomes enlarged and widened. The electrical density decreases, hence sparking is uniformly distributed among the powder particles. As a result, even and more uniform distribution of discharge takes place, which causes uniform erosion on the work piece, this results in improvement of surface finish. For improving the surface quality of SKD-11 machined part, applied four kinds of powders, including SiC, Cu, Cr and Al. They found that addition of aluminum powder into the dielectric fluid considerably decreases the thickness of recast layer on the work surface [1].

Fig.1. Principle of Powder Mixed EDM


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VI. SELECTION OF SUITABLE METAL POWDER Low electrical resistivity creates a high spark gap and high thermal conductivity takes more heat away. Non magnetism can be used magnetic filter for separate out debris. The low density of the Al powder corresponds with low explosive impact upon the melted zone, generating fine grinding effects. The characteristic of the following curves shows that the Aluminum powder is comparatively suitable for the machining [3].

Fig.2.The Dependence of the Recast Layer of the EDM Components on Particle Concentration of Al, Cr, and Sic Powders, and on Discharge Current

VII. EXPERIMENTAL ANALYSIS

After the conducting research on the minimization of recast layer in the EDM process we are concluding that the by using powder additives in the dielectric solution recast layer can be significantly reduced. The electric discharge machining was conducted. So this method is verified directly on the EDM machining process. This solution was used as dielectric fluid for conducting the experiments. A small stirring system and a dielectric circulation pump were used in machining tank to ensure uniform distribution of powder particles in dielectric circulation system. A special fixture was made in machine shop and used to hold the work piece in machining tank. A rectangular piece of magnet was placed in machining tank to collect the debris produced during experimentation. Firstly the experiment is performed without using the powder means as the conventional process. The readings of the measurement of the initial process are taken and then by adding the powder in the dielectric, machining on the work piece is performed. During the performance of work the concentration of the powder is varied in the dielectric and observed the machined surface for measurement of surface roughness value. Initially 2gm per liter concentration of the powder is mixed in dielectric and then the concentration is increased by the 2gm per liter and the corresponding results are taken.

VIII. CONCLUSION By performing the successful experiment the results are obtained. It is found during the experiments that the particle size, particle density, particle concentration, electrical resistivity, and thermal conductivity of the powders are the relatively important powder characteristics affecting the surface quality in the EDM process. It is revealed that there is a decrease in the recast layers when adding the particles with concentrations ranging from 0 to 6 gm/lit to the dielectric oil. It is noticed that the particle


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concentration 2gm/lit is less effective in decreasing the recast layer. It thus produces a thicker recast layer compared to both 4 and 6gm/lit that generally generates thinner recast layers. However, from the machining point of view, particle concentration 4gm/lit is found to be the optimal choice in improving the machining efficiency and reducing the tool wear rate, by enhancing the process stability most. The experimental results here may further indicate that the thickest recast layers produced by introducing particle concentration 4gm/lit is the result of strongest accumulated heating effects due to its greater enhancement of discharge transitivity during EDM process. In other words, the thinner recast layer produced by the particle concentration of 4gm/lit and 6gm/lit is due to their weaker accumulated heating effects because of their having higher possibility of abnormal discharging. Aluminum powder produces the best surface finish. The introduction of foreign particles is proved to reduce the recast layer of EDM components, the particle concentration of 2gm/lit to 4gm/lit is found to be the least effective in decreasing the recast layer .By using powder additives in the dielectric solution recast layer can be significantly reduced. REFERENCES [1] EDM Technology and Strategy Development for The Manufacturing of Complex Parts in SiSiC,

S. Clijsters, K. Liu, D. Reynaerts, B. Lauwers; Journal of Materials Processing Technology 210 (2010) 631641;November 2009.

[2] White Layer Formation due to Phase Transformation in Orthogonal Machining of AISI 1045 Annealed Steel, Sangil Hana, Shreyes N. Melkote a,, Michael S. Haluska b, Thomas R. Watkins; November 2007.

[3] Investigation into some Surface Characteristics of Electrical Discharge Machined SKD-11 Using Powder-Suspension Dielectric Oil, Tzeng Yih-fong, Chen Fu-chen; Journal of Materials Processing Technology 170 (2005) 3853912005;june2005.

[4] Advancement in Electric Discharge Machining on Metal Matrix Composite Materials in Recent Times: A Review, Manish Vishwakarma, Vishal Parashar, V.K.Khare; International Journal of Scientific and Research Publications, Volume 2, Issue 3, March 2012 1 ISSN 2250-3153

[5] A Review on Current Research Trends In Electrical Discharge Machining EDM , Norliana Mohd Abbas, Darius G. Solomon, Md. Fuad Bahari; International Journal of Machine Tools & Manufacture 47 (2007) 12141228; November 2006.

[6] Current research trends in variants of Electrical Discharge Machining: A review, Anand Pandey; Anand Pandey et. al. / International Journal of Engineering Science and Technology Vol. 2(6), 2010, 2172-2191;2009.

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Removal of Unwanted Layer of Material Deposited on the Die Surface During Electric Discharge Machining