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Performance of dielectric media (conventional EDM oil and distilled water) during machining of Inconel 825 super alloy
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 18 (2019) 2679–2687
www.materialstoday.com/proceedings
ICMPC-2019
Performance of dielectric media (conventional EDM oil and distilled water) during machining of Inconel 825 super alloy Santosh Kumar Sahua, Thrinadh Jadamb*, Saurav Dattab a
b
Department of Mechanical Engineering VSSUT, Burla, Sambalpur, Odisha-768018, INDIA Department of Mechanical EngineeringNational Institute of Technology, Rourkela 769008, Odisha, INDIA
Abstract In the present research work, performance of dielectric media (conventional EDM oil and distilled water) is studied in the context of machinability of Inconel 825 during electro-discharge machining. In this work, machinability is evaluated in purview of material removal efficiency, tool wear rate, and surface integrity of the machined specimen. Study of surface integrity includes morphological study as well as analysis of topographical measures like surface roughness, surface crack density, white layer thickness, foreign material migration, metallurgical characterization along with phase alteration and precipitation of intermetallic compounds and finally, micro-hardness tests. Experimental results suggest that as compared to EDM oil, distilled water (used as dielectric media) adversely affects the efficiency towards material removal, and promotes formation of rough surface. An important observation is that when EDM oil is used as dielectric media, carbide formation is attributed to the machined surface due to pyrolysis of hydro-carbon. While in case of distilled water, its decomposition causes oxide formation at the machined surface instead of carbides. Moreover, it is observed that peak discharge current imposes positive effect on both surface crack density, and white layer thickness. Microhardness depth profile confirms presence of hardened layer up to certain depth beneath the machined surface followed by gradual truncation, as distance is increased towards interior of the parent material. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019 Keywords: Dielectric media; Inconel 825; electro-discharge machining; surface integrity
* Corresponding author. Tel.: +91 661 246 2524 E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019
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1. Research background Inconel 825 is a nickel based super alloy which finds huge applications in aerospace engineering. This alloy belongs to the category of ‘difficult-to-cut’ material while machined through traditional routes (turning, milling, drilling etc.). Therefore, non-traditional machining, especially Electro-Discharge Machining (EDM), is attempted to machine this super alloy. However, low thermal conductivity of this alloy limits EDM performance. Kuppan et al. [1] observed peak current, duty factor and tool rotational speed strongly influenced material removal efficiency; whereas, depth-averaged surface roughness was highly influenced by peak current and pulse-on duration for electro-discharge drilling of Inconel 718. Ay et al. [2] optimized µ-EDM (drilling) process parameters to achieve satisfactory machining performance of Inconel 718 in purview of hole taper ratio as well as hole dilation. Wang et al. [3] reported improved machining rate and relative electrode wear rate achieved in a high current density electrodischarge milling of Inconel 718. The authors also observed that the cracks developed in the recast layer penetrated through bulk of the parent material and propagated along the grain boundary. Dhanabalan et al. [4] examined effects of EDM parameters including peak current, pulse-on duration and pulse-off duration on form tolerances of Inconel 718 and 625 end parts. Torres et al. [5] reported dependence of machining rate, tool wear and surface roughness on current intensity, duty cycle, pulse time and polarity during EDM operation of Inconel 600 super alloy. Aggarwal et al. [6] developed response surface models to predict cutting rate and surface roughness during wire-EDM of Inconel 718 work material. Torres et al. [7] examined effects of EDM parameters along with graphite electrode and its polarity on material removal rate, electrode wear and surface roughness during machining of Inconel 600. As compared to traditional copper electrode, the benefits of using graphite electrode with negative polarity were delineated therein. Rao and Venkaiah [8] experimentally observed the influence of wire-EDM parameters on micro-hardness, surface roughness and depth of recast layer of the machined Inconel 690 specimen. As compared to bulk material, microhardness of the machined surface was found relatively less. It was also concluded that micro-hardness and thickness of the recast layer were inversely related to the variation of process input factors. Unune and Mali [9] reported improved material removal efficiency and kerf width achieved during µ-WEDM with low-frequency workpiece vibration assistance. Improved process performance was due to efficient flushing condition and lesser extent of toolworkpiece adhesion resulted by the assisting vibration. Shen et al. [10] reported potential benefits of high-speed dry EDM (milling) using air as dielectric for machining of Inconel 718. As compared to traditional EDM with liquid dielectric, the dry-EDM achieved superior surface finish, lesser depth of recast layer as well as crack density. Shabgard et al. [11] investigated the effect of pulse current and pulse duration on surface and sub-surface microhardness as well as crack density during WEDM of Inconel 617. It was concluded that increased values of input parameters resulted in decreased surface micro-hardness, whilst, sub-surface micro-hardness was found increased by increasing pulse duration. Moreover, crack density was found increasing with increased input parameters then it assumed a deceasing trend. Goyal [12] compared performances of zinc coated wires (normal and cryogenically treated) in purview of material removal efficiency and surface roughness for WEDM of Inconel 625. Pulse-on time, tool electrode and current density were found the most significant factors for their influence on process performance. Ahmed et al. [13] proposed a compound process named as hybrid electro-discharge and arc machining to achieve superior process performance of Inconel 718 whilst compared to conventional EDM process. Bassoli et al. [14] explained the role of debris in the ignition of discharges during EDM drilling of small holes on Inconel 718 work material. Apart from electrical parameters, non-electrical parameters including type of dielectric media, flushing pressure, tool material, tool bottom profile also influence EDM performance. Dielectric media firstly acts as a semiconductor between electrode and workpiece to promote a controlled spark gap ionization condition. Secondly, it also acts as a flushing agent to wash and remove the eroded debris from the spark gap area. Dielectric media provides cooling effect to the tool electrode as well as workpiece. Hence, proper selection of dielectric media is very important. Wong et al. [15] examined flushing rates on the types and distribution of recast layers in EDMed AISI 01 tool steel. An optimal flushing flow rate was determined to ensure minimum crack density as well as recast layer thickness. The effects of quenching property and capability of the dielectric media towards debris removal on formation of recast layer were also studied. Ozgedik and Cogun [16] investigated the effects of WEDM parameters including flushing condition (injection, suction and static) of kerosene dielectric media on tool wear, machining efficiency, and surface roughness of the EDMed part. Ji et al. [17] examined performance of different emulsions as dielectrics on surface roughness and material removal rate during electro-discharge mailing of SiC ceramic. Li et al. [18]
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reported improved EDM performance in terms of material removal rate and tool wear ratio using bunched-electrode due to efficient flushing whist compared to conventional EDM. Ndaliman et al. [19] achieved improved microhardness of EDMed Ti-6Al-4V specimen by proper adjustment of process parameters with Cu-TaC electrode and water as dielectric media. Ndaliman et al. [20] compared performance of dielectric media (urea and distilled water) in purview of surface roughness and micro-hardness of electro-discharge machined titanium alloy. It was concluded that superior surface finish was achieved in case of distilled water as dielectric; whilst, urea was found beneficial for improved microhardness. Yeh et al. [21] reported benefits of using pyrophosphate powder added in pure water as dielectric media for achieving superior cutting speed and surface roughness of WEDMed polycrystalline silicon ingot. Zhang et al. [22] investigated effects of variation of dielectric media viz. gaseous (air, oxygen) and liquid (deionized water, kerosene, water-in-oil emulsion) on crater dimensions and material removal efficiency during EDM process. Tang and Du [23] recommended use of tap water as dielectric media for improved material removal efficiency, tool wear rate and surface finish of the EDMed Ti-6Al-4V specimen. Fattahi and Baseri [24] examined effects of different gaseous dielectric (air, nitrogen, mixture of argon/air) on EDM performance of M35 work material. The process performance was evaluated in terms of material removal efficiency, surface roughness and radial overcut. The study concluded argon/air mixture as the best dielectric. Valaki and Rathod [25] conducted a feasibility study of using vegetable oil based dielectric media to achieve favorable material removal rate, electro wear rate and relative wear ratio of EDMed P20þcold-worked plastic injection mold steel specimen. The proposed dielectric was found superior in performance whilst compared to conventional hydrocarbon based dielectric (kerosene). Dhakar and Dvivedi [26] reported application potential of mixture of two phase (liquid and air) as dielectric media for improved material removal rate, tool wear rate and surface roughness during EDM of high speed steel. Valaki and Rathod [27] recommended use of waste vegetable oil as potential alternative dielectric media promote sustainable EDM process with enhanced performance. Muttamara and Kanchanomai [28] examined the effects of presence of carbon in the dielectric media on characteristics of recast layer generated during EDM process. Guo et al. [29] investigated the influence of discharge parameters and conductivity of the dielectric media on formation of crater during EDM process. The study demonstrated application potential of water-based dielectric media for improved EDM performance. Valaki et al. [30] examined application potential of Jatropha curcas oil based bio-dielectric towards improving sustainability of the EDM process. Authors found that as compared to kerosene, use of bio-dielectric ensured greater material removal efficiency, lower surface roughness and surface hardness. Ng et al. [31] studied effects of biodiesel (canola and sunflower biodiesel) dielectric media on EDM performance of bulk metallic glass as well as titanium alloy. It was found the biodiesel outperformed over conventional dielectric by ensuring higher material removal rate and reduced tool wear. Gov [32] examined the effect of temperature of the dielectric media on material removal efficiency, tool wear rate, overcut, hole-taper, surface finish and recast layer thickness during electro-discharge drilling of DIN 1.2379 tool steel. It was found that decrease in dielectric temperature lead to improved process performance. Objectives of the present work are stated below. 1. 2. 3. 4. 5. 6.
To investigate effects of peak discharge current on material removal rate, tool wear rate, surface roughness, surface crack density, and white layer thickness. To study surface morphology of EDMed specimen. Study of metallurgical characteristics (phases, precipitates etc.) of the machined surface. Elemental analysis of the machined surface. To study variation of microhardness of the machined specimen with respect distance starting from edge towards white layer/HAZ and finally unaffected parent material. To study performance of distilled water (as dielectric media) during EDM of Inconel 825 when compared to that of conventional EDM oil.
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2. Material and methods 3 Inconel 825 square flat plates 50 50 5mm are used as workpiece. The chemical composition and properties of Inconel 825 are provided in [33] and [34], respectively. EDM experiments are carried out in Die-sinking EDM setup (Excetek ED30, Taiwan) with a copper tool electrode of 20mm diameter. Commercially available EDM oil (KYROS FERROLACe 3M, 110, 140), and distilled water, separately is used as dielectric media. Experiments are conducted with varied peak discharge current: Ip=15A, 25A, 35A, and 45A. Other than peak discharge current, gap voltage (~230V), pulse-on time (~2000µs), pulse-off time (~200µs), polarity (~positive), gap distance (~50µm), and depth of cut (~0.75mm) are kept constant for each experimental run.
Fig. 1. Effect of peak current on (a) MRR; (b) TWR; (c) Ra.
After experimentation, the following responses are obtained: Material Removal Rate (MRR), Tool Wear Rate (TWR), surface roughness (Ra), Surface Crack Density (SCD), and White Layer Thickness (WLT). Surface
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roughness measurements are carried out in a stylus type profilometer, Talysurf (Model: Taylor Hobson, Surtronic 3+). Surface morphology including surface cracks, and white layer are observed through scanning electron microscopy using setup (Model: Jeol JSM-6480LV, and Jeol JSM-6390LV; Country: Japan). The same setup is explored for elemental analysis. Phase analysis is performed in XRD Texture Measurement Machine [Model No: D8 ADVANCE with DAVINCI design, Make: BRUKER, Country: Germany]. Microhardness tests are carried out in Vicker’s Microhardness tester [Model No. LECO LM 810], using load of 25gf. 3. Results and discussion Effect of peak current on material removal rate is shown in Fig. 1a. Increase in peak current results in increased discharge energy supplied. Hence, impulsive force acting on the workpiece is high. This enhances formation of large and deep craters; and consequently high material removal efficiency. It is observed that as compared to EDM oil, machining efficiency is inferior if distilled water is used as dielectric media. Increase in peak current results in higher degree of material erosion from the tool electrode. Therefore, with increase in peak current, gradual increase in tool wear rate is observed when distilled water is used as dielectric media. On the contrary, when EDM oil is used as dielectric media, pyrolysis of same promotes carbon deposition at the tool surface. This affects the value of tool wear rate (Fig. 1b). Increase in discharge energy causes high material removal rate which deteriorates surface finish. Hence, an approximate increasing trend of Ra is observed with increase in peak current. It is further noticed that distilled water offers poor surface finish as compared to EDM oil, as dielectric media (Fig. 1c).
Fig. 2. XRD analysis of the EDMed specimen obtained using dielectric media (a) EDM oil; (b) distilled water.
XRD analysis reveals residuals of carbide precipitates at the EDMed surface when EDM oil is used as dielectric media (Fig. 2a). Such carbide precipitation is caused by migration of carbon atoms during dielectric (EDM oil) cracking. EDM oil is basically a hydrocarbon. During spark discharge, pyrolysis of hydrocarbon promotes transfer of carbon atoms at the work surface. They further react with work material and thus variety of carbides are formed: Ni3(Al, Ti)C, AlNi3C0.5, Al75Ni10Fe15. Apart from carbides, presence of Cu0.81Ni0.19 is also detected. Wear of copper tool electrode promotes transfer of copper atoms at the work surface; which further reacts with work material. On the other hand, XRD analysis made on the EDMed surface obtained by using distilled water (as dielectric media) exhibits presence of FeNi, Cu0.81Ni0.19, and (Fe, Ni) (Fig. 2b). No carbide is found therein. Inferior surface morphology is obtained for the EDMed surface, irrespective of the dielectric media used. The disappointing surface morphology is characterized by globule of debris, melted material deposition, pockmarks and surface cracks (Fig. 3). EDM process develops huge thermal stresses; when induced stress exceeds ultimate tensile
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strength of the work surface, cracking takes place. Severity of surface cracking is denoted by surface crack density. It is the ratio of total crack length to the surface area. Increase in peak current enhances discharge energy; and hence, results in evolution of thermal stresses of high magnitude. Therefore, surface crack density increases with increase in peak discharge current (Fig. 4a). EDS analysis detects significant transfer of carbon at the machined surface due to decomposition of EDM oil (Fig. 5a). In contrast to that, residue of oxygen is detected at the machined surface when distilled water is used as dielectric media (Fig. 5b).
Fig. 3. Surface morphology of the EDMed specimen obtained by using distilled water as dielectric media.
Fig. 4. Effect of peak current on (a) surface crack density, and (b) white layer thickness: Dielectric media: distilled water.
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Fig. 5. EDS results of the EDMed surface obtained at Ip=25A using dielectric media (a) EDM oil; (b) distilled water.
Fig. 6. Effect of peak current on white layer thickness: Dielectric media: distilled water.
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Fig. 7. Microhardness depth profile: Specimens EDMed at Ip=25A.
During spark discharge, a portion of the work material melts and partially evaporates. Impulsive force of electrons removes material from the workpiece which is called debris. Due to the dielectric flushing circulation pressure, debris is carried away from the machining zone. However, dielectric media is not efficient enough to remove entire debris. Therefore, residuals of debris, present in the inter-electrode gap, re-solidify over the machined surface. Thus, white layer is developed. Increase in peak discharge current enhances discharge energy. More material is eroded from the workpiece. Consequently, white layer appears progressively thicker with increase in peak discharge current (Fig. 4b and Fig. 6). Microhardness distribution (depth profile) is provided in Fig. 7. It is observed that microhardness distribution follows gradual truncation with increase in distance (along the transverse section of the EDMed specimen) starting from edge of the machined surface towards interior of the bulk material. Near the edge of the machined surface, microhardness value is significantly more. This is due to the fact that the edge zone comprises white layer enriched with carbides when EDM oil is used as dielectric media. Deposition of such carbides contributes to higher microhardness value at this zone. On the contrary, the EDMed specimen obtained by using distilled water (as dielectric media) corresponds to lower microhardness value near the edge of the machined surface while compared to that of EDM oil. This is due to absence of hard carbides at this zone. Lower microhardness value at this zone may be due to presence of porous white layer. 4. Conclusions As compared to conventional EDM oil, distilled water (used as dielectric media) lowers material removal efficiency, and produces rough surface finish during EDM of Inconel 825.Increase in peak current increases material removal rate, tool wear rate, surface roughness, surface crack density, and white layer thickness. Pyrolysis of EDM oil causes immigration of carbon atoms at the EDMed surface. When distilled water is used as dielectric media, decomposition of water causes transfer of oxygen at the machined surface.When EDM oil is used as dielectric media, residuals of carbides are detected at the machined surface. Whilst in case of distilled water, so such carbides are formed.Poor surface morphology of the EDMed specimen is obtained which includes globules of debris, malted material deposition, pockmarks, surface cracks and white layer.EDM operation generates a hardened layer (up to certain depth) beneath the machined surface while EDM oil is used as dielectric media.
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ScienceDirect Materials Today: Proceedings 18 (2019) 2679–2687
www.materialstoday.com/proceedings
ICMPC-2019
Performance of dielectric media (conventional EDM oil and distilled water) during machining of Inconel 825 super alloy Santosh Kumar Sahua, Thrinadh Jadamb*, Saurav Dattab a
b
Department of Mechanical Engineering VSSUT, Burla, Sambalpur, Odisha-768018, INDIA Department of Mechanical EngineeringNational Institute of Technology, Rourkela 769008, Odisha, INDIA
Abstract In the present research work, performance of dielectric media (conventional EDM oil and distilled water) is studied in the context of machinability of Inconel 825 during electro-discharge machining. In this work, machinability is evaluated in purview of material removal efficiency, tool wear rate, and surface integrity of the machined specimen. Study of surface integrity includes morphological study as well as analysis of topographical measures like surface roughness, surface crack density, white layer thickness, foreign material migration, metallurgical characterization along with phase alteration and precipitation of intermetallic compounds and finally, micro-hardness tests. Experimental results suggest that as compared to EDM oil, distilled water (used as dielectric media) adversely affects the efficiency towards material removal, and promotes formation of rough surface. An important observation is that when EDM oil is used as dielectric media, carbide formation is attributed to the machined surface due to pyrolysis of hydro-carbon. While in case of distilled water, its decomposition causes oxide formation at the machined surface instead of carbides. Moreover, it is observed that peak discharge current imposes positive effect on both surface crack density, and white layer thickness. Microhardness depth profile confirms presence of hardened layer up to certain depth beneath the machined surface followed by gradual truncation, as distance is increased towards interior of the parent material. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019 Keywords: Dielectric media; Inconel 825; electro-discharge machining; surface integrity
* Corresponding author. Tel.: +91 661 246 2524 E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019
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1. Research background Inconel 825 is a nickel based super alloy which finds huge applications in aerospace engineering. This alloy belongs to the category of ‘difficult-to-cut’ material while machined through traditional routes (turning, milling, drilling etc.). Therefore, non-traditional machining, especially Electro-Discharge Machining (EDM), is attempted to machine this super alloy. However, low thermal conductivity of this alloy limits EDM performance. Kuppan et al. [1] observed peak current, duty factor and tool rotational speed strongly influenced material removal efficiency; whereas, depth-averaged surface roughness was highly influenced by peak current and pulse-on duration for electro-discharge drilling of Inconel 718. Ay et al. [2] optimized µ-EDM (drilling) process parameters to achieve satisfactory machining performance of Inconel 718 in purview of hole taper ratio as well as hole dilation. Wang et al. [3] reported improved machining rate and relative electrode wear rate achieved in a high current density electrodischarge milling of Inconel 718. The authors also observed that the cracks developed in the recast layer penetrated through bulk of the parent material and propagated along the grain boundary. Dhanabalan et al. [4] examined effects of EDM parameters including peak current, pulse-on duration and pulse-off duration on form tolerances of Inconel 718 and 625 end parts. Torres et al. [5] reported dependence of machining rate, tool wear and surface roughness on current intensity, duty cycle, pulse time and polarity during EDM operation of Inconel 600 super alloy. Aggarwal et al. [6] developed response surface models to predict cutting rate and surface roughness during wire-EDM of Inconel 718 work material. Torres et al. [7] examined effects of EDM parameters along with graphite electrode and its polarity on material removal rate, electrode wear and surface roughness during machining of Inconel 600. As compared to traditional copper electrode, the benefits of using graphite electrode with negative polarity were delineated therein. Rao and Venkaiah [8] experimentally observed the influence of wire-EDM parameters on micro-hardness, surface roughness and depth of recast layer of the machined Inconel 690 specimen. As compared to bulk material, microhardness of the machined surface was found relatively less. It was also concluded that micro-hardness and thickness of the recast layer were inversely related to the variation of process input factors. Unune and Mali [9] reported improved material removal efficiency and kerf width achieved during µ-WEDM with low-frequency workpiece vibration assistance. Improved process performance was due to efficient flushing condition and lesser extent of toolworkpiece adhesion resulted by the assisting vibration. Shen et al. [10] reported potential benefits of high-speed dry EDM (milling) using air as dielectric for machining of Inconel 718. As compared to traditional EDM with liquid dielectric, the dry-EDM achieved superior surface finish, lesser depth of recast layer as well as crack density. Shabgard et al. [11] investigated the effect of pulse current and pulse duration on surface and sub-surface microhardness as well as crack density during WEDM of Inconel 617. It was concluded that increased values of input parameters resulted in decreased surface micro-hardness, whilst, sub-surface micro-hardness was found increased by increasing pulse duration. Moreover, crack density was found increasing with increased input parameters then it assumed a deceasing trend. Goyal [12] compared performances of zinc coated wires (normal and cryogenically treated) in purview of material removal efficiency and surface roughness for WEDM of Inconel 625. Pulse-on time, tool electrode and current density were found the most significant factors for their influence on process performance. Ahmed et al. [13] proposed a compound process named as hybrid electro-discharge and arc machining to achieve superior process performance of Inconel 718 whilst compared to conventional EDM process. Bassoli et al. [14] explained the role of debris in the ignition of discharges during EDM drilling of small holes on Inconel 718 work material. Apart from electrical parameters, non-electrical parameters including type of dielectric media, flushing pressure, tool material, tool bottom profile also influence EDM performance. Dielectric media firstly acts as a semiconductor between electrode and workpiece to promote a controlled spark gap ionization condition. Secondly, it also acts as a flushing agent to wash and remove the eroded debris from the spark gap area. Dielectric media provides cooling effect to the tool electrode as well as workpiece. Hence, proper selection of dielectric media is very important. Wong et al. [15] examined flushing rates on the types and distribution of recast layers in EDMed AISI 01 tool steel. An optimal flushing flow rate was determined to ensure minimum crack density as well as recast layer thickness. The effects of quenching property and capability of the dielectric media towards debris removal on formation of recast layer were also studied. Ozgedik and Cogun [16] investigated the effects of WEDM parameters including flushing condition (injection, suction and static) of kerosene dielectric media on tool wear, machining efficiency, and surface roughness of the EDMed part. Ji et al. [17] examined performance of different emulsions as dielectrics on surface roughness and material removal rate during electro-discharge mailing of SiC ceramic. Li et al. [18]
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reported improved EDM performance in terms of material removal rate and tool wear ratio using bunched-electrode due to efficient flushing whist compared to conventional EDM. Ndaliman et al. [19] achieved improved microhardness of EDMed Ti-6Al-4V specimen by proper adjustment of process parameters with Cu-TaC electrode and water as dielectric media. Ndaliman et al. [20] compared performance of dielectric media (urea and distilled water) in purview of surface roughness and micro-hardness of electro-discharge machined titanium alloy. It was concluded that superior surface finish was achieved in case of distilled water as dielectric; whilst, urea was found beneficial for improved microhardness. Yeh et al. [21] reported benefits of using pyrophosphate powder added in pure water as dielectric media for achieving superior cutting speed and surface roughness of WEDMed polycrystalline silicon ingot. Zhang et al. [22] investigated effects of variation of dielectric media viz. gaseous (air, oxygen) and liquid (deionized water, kerosene, water-in-oil emulsion) on crater dimensions and material removal efficiency during EDM process. Tang and Du [23] recommended use of tap water as dielectric media for improved material removal efficiency, tool wear rate and surface finish of the EDMed Ti-6Al-4V specimen. Fattahi and Baseri [24] examined effects of different gaseous dielectric (air, nitrogen, mixture of argon/air) on EDM performance of M35 work material. The process performance was evaluated in terms of material removal efficiency, surface roughness and radial overcut. The study concluded argon/air mixture as the best dielectric. Valaki and Rathod [25] conducted a feasibility study of using vegetable oil based dielectric media to achieve favorable material removal rate, electro wear rate and relative wear ratio of EDMed P20þcold-worked plastic injection mold steel specimen. The proposed dielectric was found superior in performance whilst compared to conventional hydrocarbon based dielectric (kerosene). Dhakar and Dvivedi [26] reported application potential of mixture of two phase (liquid and air) as dielectric media for improved material removal rate, tool wear rate and surface roughness during EDM of high speed steel. Valaki and Rathod [27] recommended use of waste vegetable oil as potential alternative dielectric media promote sustainable EDM process with enhanced performance. Muttamara and Kanchanomai [28] examined the effects of presence of carbon in the dielectric media on characteristics of recast layer generated during EDM process. Guo et al. [29] investigated the influence of discharge parameters and conductivity of the dielectric media on formation of crater during EDM process. The study demonstrated application potential of water-based dielectric media for improved EDM performance. Valaki et al. [30] examined application potential of Jatropha curcas oil based bio-dielectric towards improving sustainability of the EDM process. Authors found that as compared to kerosene, use of bio-dielectric ensured greater material removal efficiency, lower surface roughness and surface hardness. Ng et al. [31] studied effects of biodiesel (canola and sunflower biodiesel) dielectric media on EDM performance of bulk metallic glass as well as titanium alloy. It was found the biodiesel outperformed over conventional dielectric by ensuring higher material removal rate and reduced tool wear. Gov [32] examined the effect of temperature of the dielectric media on material removal efficiency, tool wear rate, overcut, hole-taper, surface finish and recast layer thickness during electro-discharge drilling of DIN 1.2379 tool steel. It was found that decrease in dielectric temperature lead to improved process performance. Objectives of the present work are stated below. 1. 2. 3. 4. 5. 6.
To investigate effects of peak discharge current on material removal rate, tool wear rate, surface roughness, surface crack density, and white layer thickness. To study surface morphology of EDMed specimen. Study of metallurgical characteristics (phases, precipitates etc.) of the machined surface. Elemental analysis of the machined surface. To study variation of microhardness of the machined specimen with respect distance starting from edge towards white layer/HAZ and finally unaffected parent material. To study performance of distilled water (as dielectric media) during EDM of Inconel 825 when compared to that of conventional EDM oil.
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2. Material and methods 3 Inconel 825 square flat plates 50 50 5mm are used as workpiece. The chemical composition and properties of Inconel 825 are provided in [33] and [34], respectively. EDM experiments are carried out in Die-sinking EDM setup (Excetek ED30, Taiwan) with a copper tool electrode of 20mm diameter. Commercially available EDM oil (KYROS FERROLACe 3M, 110, 140), and distilled water, separately is used as dielectric media. Experiments are conducted with varied peak discharge current: Ip=15A, 25A, 35A, and 45A. Other than peak discharge current, gap voltage (~230V), pulse-on time (~2000µs), pulse-off time (~200µs), polarity (~positive), gap distance (~50µm), and depth of cut (~0.75mm) are kept constant for each experimental run.
Fig. 1. Effect of peak current on (a) MRR; (b) TWR; (c) Ra.
After experimentation, the following responses are obtained: Material Removal Rate (MRR), Tool Wear Rate (TWR), surface roughness (Ra), Surface Crack Density (SCD), and White Layer Thickness (WLT). Surface
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roughness measurements are carried out in a stylus type profilometer, Talysurf (Model: Taylor Hobson, Surtronic 3+). Surface morphology including surface cracks, and white layer are observed through scanning electron microscopy using setup (Model: Jeol JSM-6480LV, and Jeol JSM-6390LV; Country: Japan). The same setup is explored for elemental analysis. Phase analysis is performed in XRD Texture Measurement Machine [Model No: D8 ADVANCE with DAVINCI design, Make: BRUKER, Country: Germany]. Microhardness tests are carried out in Vicker’s Microhardness tester [Model No. LECO LM 810], using load of 25gf. 3. Results and discussion Effect of peak current on material removal rate is shown in Fig. 1a. Increase in peak current results in increased discharge energy supplied. Hence, impulsive force acting on the workpiece is high. This enhances formation of large and deep craters; and consequently high material removal efficiency. It is observed that as compared to EDM oil, machining efficiency is inferior if distilled water is used as dielectric media. Increase in peak current results in higher degree of material erosion from the tool electrode. Therefore, with increase in peak current, gradual increase in tool wear rate is observed when distilled water is used as dielectric media. On the contrary, when EDM oil is used as dielectric media, pyrolysis of same promotes carbon deposition at the tool surface. This affects the value of tool wear rate (Fig. 1b). Increase in discharge energy causes high material removal rate which deteriorates surface finish. Hence, an approximate increasing trend of Ra is observed with increase in peak current. It is further noticed that distilled water offers poor surface finish as compared to EDM oil, as dielectric media (Fig. 1c).
Fig. 2. XRD analysis of the EDMed specimen obtained using dielectric media (a) EDM oil; (b) distilled water.
XRD analysis reveals residuals of carbide precipitates at the EDMed surface when EDM oil is used as dielectric media (Fig. 2a). Such carbide precipitation is caused by migration of carbon atoms during dielectric (EDM oil) cracking. EDM oil is basically a hydrocarbon. During spark discharge, pyrolysis of hydrocarbon promotes transfer of carbon atoms at the work surface. They further react with work material and thus variety of carbides are formed: Ni3(Al, Ti)C, AlNi3C0.5, Al75Ni10Fe15. Apart from carbides, presence of Cu0.81Ni0.19 is also detected. Wear of copper tool electrode promotes transfer of copper atoms at the work surface; which further reacts with work material. On the other hand, XRD analysis made on the EDMed surface obtained by using distilled water (as dielectric media) exhibits presence of FeNi, Cu0.81Ni0.19, and (Fe, Ni) (Fig. 2b). No carbide is found therein. Inferior surface morphology is obtained for the EDMed surface, irrespective of the dielectric media used. The disappointing surface morphology is characterized by globule of debris, melted material deposition, pockmarks and surface cracks (Fig. 3). EDM process develops huge thermal stresses; when induced stress exceeds ultimate tensile
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strength of the work surface, cracking takes place. Severity of surface cracking is denoted by surface crack density. It is the ratio of total crack length to the surface area. Increase in peak current enhances discharge energy; and hence, results in evolution of thermal stresses of high magnitude. Therefore, surface crack density increases with increase in peak discharge current (Fig. 4a). EDS analysis detects significant transfer of carbon at the machined surface due to decomposition of EDM oil (Fig. 5a). In contrast to that, residue of oxygen is detected at the machined surface when distilled water is used as dielectric media (Fig. 5b).
Fig. 3. Surface morphology of the EDMed specimen obtained by using distilled water as dielectric media.
Fig. 4. Effect of peak current on (a) surface crack density, and (b) white layer thickness: Dielectric media: distilled water.
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Fig. 5. EDS results of the EDMed surface obtained at Ip=25A using dielectric media (a) EDM oil; (b) distilled water.
Fig. 6. Effect of peak current on white layer thickness: Dielectric media: distilled water.
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Fig. 7. Microhardness depth profile: Specimens EDMed at Ip=25A.
During spark discharge, a portion of the work material melts and partially evaporates. Impulsive force of electrons removes material from the workpiece which is called debris. Due to the dielectric flushing circulation pressure, debris is carried away from the machining zone. However, dielectric media is not efficient enough to remove entire debris. Therefore, residuals of debris, present in the inter-electrode gap, re-solidify over the machined surface. Thus, white layer is developed. Increase in peak discharge current enhances discharge energy. More material is eroded from the workpiece. Consequently, white layer appears progressively thicker with increase in peak discharge current (Fig. 4b and Fig. 6). Microhardness distribution (depth profile) is provided in Fig. 7. It is observed that microhardness distribution follows gradual truncation with increase in distance (along the transverse section of the EDMed specimen) starting from edge of the machined surface towards interior of the bulk material. Near the edge of the machined surface, microhardness value is significantly more. This is due to the fact that the edge zone comprises white layer enriched with carbides when EDM oil is used as dielectric media. Deposition of such carbides contributes to higher microhardness value at this zone. On the contrary, the EDMed specimen obtained by using distilled water (as dielectric media) corresponds to lower microhardness value near the edge of the machined surface while compared to that of EDM oil. This is due to absence of hard carbides at this zone. Lower microhardness value at this zone may be due to presence of porous white layer. 4. Conclusions As compared to conventional EDM oil, distilled water (used as dielectric media) lowers material removal efficiency, and produces rough surface finish during EDM of Inconel 825.Increase in peak current increases material removal rate, tool wear rate, surface roughness, surface crack density, and white layer thickness. Pyrolysis of EDM oil causes immigration of carbon atoms at the EDMed surface. When distilled water is used as dielectric media, decomposition of water causes transfer of oxygen at the machined surface.When EDM oil is used as dielectric media, residuals of carbides are detected at the machined surface. Whilst in case of distilled water, so such carbides are formed.Poor surface morphology of the EDMed specimen is obtained which includes globules of debris, malted material deposition, pockmarks, surface cracks and white layer.EDM operation generates a hardened layer (up to certain depth) beneath the machined surface while EDM oil is used as dielectric media.
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