EXPERIMENTAL INVESTIGATION ON METALLURGICAL CHARACTERISTICS AND SURFACE ROUGHNESS OF INCONEL 825 USI

International Journal of Mechanical and Production Engineering Research and Development (IJMPERD) ISSN (P): 2249–6890; ISSN (E): 2249–8001 Vol. 10, Issue 3, Jun 2020, 2879-2890 © TJPRC Pvt. Ltd.

EXPERIMENTAL INVESTIGATION ON METALLURGICAL CHARACTERISTICS AND SURFACE ROUGHNESS OF INCONEL 825 USING MICRO ELECTRICAL DISCHARGE MACHINING WITH CRYOGENICALLY TREATED BRASS ELECTRODE N. SULAKSHNA1 & V. CHITTARANJAN DAS2 1 2

Research scholar, Department of Mechanical Engineering, Acharya Nagarjuna Univ, Andra Pradesh, India

Professor, Department of Mechanical Engineering, RVR &JC College of Engineering, Andra Pradesh, India

ABSTRACT Efficiency and surface superiority would extensively affect the performance of micro electrical discharge machining process (µEDM). Thus the machining performance would be better by improving the material removal rate and surface quality. In this investigation, cryogenic LN2 cooling was introduced to the conventional µEDM setup for developing a modern process of cryogenically cooled µEDM process (CµEDM). The positive outcomes of this method were projected by selecting discharge current (I p) and pulse on duration (Ton) for concluding the effects of the machining performance

analysis and a scanning electron microscope (SEM) for evaluation of the effects cryogenically cooled µEDM process.

The experimental result shows 60 to 80% of improvement in material removal rate and 30 to 48% in average roughness values. Hence it is suggested that cryogenically cooled µEDM facilitates improvement in productivity and surface quality.

KEYWORDS: Cryogenic, µEDM, MRR, Surface, Micro Structural Analysis & Topography

Original Article

includes material removal rate (MRR) and surface integrity. Surface quality was also analyzed by micro structural

Received: May 28, 2020; Accepted: Jun 18, 2020; Published: Jul 09, 2020; Paper Id.: IJMPERDJUN2020272

1. INTRODUCTION The drilling of micro holes was a challenging approach due to the hardness of material, aspect ratio and complex angles. Micro holes were drilled by several technologies such as the conventional drilling process, laser drilling method and micro electrical discharge drilling process (µEDM). When it comes with high precision and good quality surfaces of a micro drilled hole, µEDM drilling process is identified as a reliable, cost effective and efficient process [1]. It is an electro thermal machining process in which both the work piece and electrodes are submerged in a dielectric medium. A series of repetitive electrical sparks generated between them generates thermal energy which melts and causes vaporisation of the material [2]. It machines all the electrically conductive materials neglecting their physical properties [3]. The amount of material removed by the µEDM process affects the productivity. In addition, the quality of the surface gets deteriorated by the presence of craters and the recast layer caused due to inefficient cooling by the dielectric medium [4]. Therefore, if the material removal rate and surface quality can be improved effectively, the µEDM process appears to be a promising technology for attaining high quality holes with high productivity.

Previous researchers have attempted the enhancement of µEDM process by using the various techniques such as, cryogenic assisted EDM processes, ultrasonic vibration and magnetic field. Considerable studies have

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focused on the utilization of cryogenics for promoting engineering techniques, but the focus has been only on the traditional manufacturing operations includes turning [5–8], grinding [9–11] milling [12,13] and grinding [14] etc. Their experimental results show the application of cryogenics as the most promising approach that can be regarded as the best alternative for surface finishing Operations. Some of the researchers have investigated the cryogenic assistance only in macro EDM process, who point out the statements are discussed below. Srivatsava et al [15,16] have evaluated the performance of cryogenically cooled electrode in EDM process and also adopted the ultrasonic vibration to the process. They found that a considerable reduction in the tool wear rate with the surface obtained free from micro cracks and surface irregularities. Abdul kareem et al [17,18] has reduced the electrode wear ratio upto 27% by adopting cryogenic cooling in the EDM process. They reported that cryogenic cooling helps in adequate transfer of heat from the tool electrode material improving the electrical and thermal conductivities. Vinoth and Pradeep [19] have investigated the better surface integrity obtained using cryogenic cooled EDM process.

Researchers have attempted adaptation of ultrasonic vibrations to the machining zone for improvement of the material removal rate. The application of ultrasonic vibration enables sufficient evacuation of debris from the machined area and reduces short circuiting and arcing. This stabilizes the machining process concerning the improved material removal rate and surface finish [20–23]. However the design of ultrasonic vibration setup for machining area of µEDM is complicated within the dielectric medium. It is also not suitable for various work piece dimensions. Application of a magnetic field to the µEDM machining process attracts researchers due to its remarkable outcomes. The application of magnetic force perpendicular to the direction of electrode produces a magnetic force while electrode rotational speed produces a centrifugal force. Combination of both produces a resultant force which flushes away the debris from the spark gap and stabilise the machining process.

Yeo et al [24] have investigated the application of a permanent magnet on the µEDM drilling process. They observed achievement of 26% higher hole depth and also found no considerable improvement surface roughness. Jafferson et al [25] combined the ultrasonic vibration and magnetic field in µEDM milling process. They observed that effect of magnetic field improves MRR more than ultrasonic vibration and also suggested that combination of both deteriorate the performance. However, the placement of magnets to the machining zone was complicated. This due to the confinement of plasma channel depends on the intensity of magnetic field which affects the machining performance. Therefore the construction of a magnetic setup combined with µEDM for various work pieces was cost effective with real challenge though complicated.

To the utmost extent of the author’s observation, there is no investigation found in the application of cryogenic LN2 cooling in the µEDM process. These cryogenic processes often ease integration with the µEDM instead of using the application of ultrasonic vibrations and magnetic field by the past researchers. The improvement in the performance characteristics of µEDM process plays a vital role in the manufacturing Industry. Hence in this investigation, cryogenic coolant was applied to the machining zone for developing an inventive process known as cryogenically cooled µEDM drilling. Experiments have been conducted at a higher and lower energy discharge level to explore the effects of performance measures such as material removal rate (MRR) and surface quality. The benefits of cryogenic cooling evaluated by micro structure analysis and surface morphology were also assessed by scanning electron microscope.

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Figure 1: Schematic Diagram of Cryogenic setup with conventional µEDM Machine

2. MATERIALS AND METHODS This hybrid machine was used for carrying out the experiments for this investigation and the exploration of the performance of cryogenically cooled µEDM drilling on Inconel 825. Distilled water was used as a dielectric medium considering its impact on environmental safety. Based on the pilot study and machine manufacturer recommendation, the discharge current and pulse on duration were chosen. The discharge current and pulse on duration over and above the levels cited in the manuscript do not significantly influence in the µEDM drilling process. Feed rate of 4 µm/s offers improved quality of micro holes. A constant feed rate of 4µm/s at 1000 rpm of tool rotational speed and a cryogenic pressure of 120 kg/cm2 is maintained. Figure. 2. Presents the experimental setup of the cryogenically cooled µEDM process. Table 1 shows the experimental conditions for conducting the experiments.

Table 1: Experimental Conditions Working Conditions Descriptions Workpiece Inconel 825 Electrode Tubular brass electrode Cryogenic flushing pressure 120kg/cm2 Current 10,15, 20 A Pulse on time 10, 15,20 µs Dielectric fluid De ionized water Feed rate 4 µm/s Performance measures which included material removal rate (MRR) and surface quality were adopted for evaluating the outcomes of input process parameters on the cryogenically cooled µEDM and the conventional µEDM process. The work piece samples were weighed using weighing balance with 0.01 g precision, resolution before and after the experiment for calculation of MRR.

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Figure 2: Experimental setup of cryogenically cooled µEDM machine. 𝑚𝑔 𝑊𝑜𝑟𝑘 𝑝𝑖𝑒𝑐𝑒 𝑏𝑒𝑓𝑜𝑟𝑒 𝑚𝑎𝑐ℎ𝑖𝑛𝑖𝑛𝑔 − 𝑊𝑜𝑟𝑘 𝑝𝑖𝑒𝑐𝑒 𝑎𝑓𝑡𝑒𝑟 𝑚𝑎𝑐ℎ𝑖𝑛𝑖𝑛𝑔 𝑀𝑅𝑅( ) = 𝑠 𝑇𝑖𝑚𝑒 𝑇𝑎𝑘𝑒𝑛 The work material was cross sectioned and the average roughness values were obtained using non contact surface roughness tester (Talysurf CCI LITE) working based on coherence correlation interferomentry with a resolution of 0.1 Aº.

The work piece material was Inconel 825 which is largely employed in the automobile companies, particularly in the gasoline injector nozzles. The quality aspects of the micro holes determine the effective atomization and spatial distribution of fuel and reflected in the effective combustion of air-fuel mixture. The work piece dimension of 10x10x10 mm was chosen for experimental runs. Table 2 shows the chemical constitution of Inconel 825. The surface of the work piece was finely milled and ground to ensure parallelism.

Table 2: Chemical composition of Inconel 825 Element C Wt% 0.05%

Cr 21.5%

Mn 1.00%

Ni 42%

Fe 28.5%

Cu 2.25%

S 0.03%

Si 0.5%

A tubular brass was selected as electrode material with an outer diameter of 500 µm and inner diameter of 200 µm to facilitate through electrode flushing, which improves flushing efficiency. Electrode taper was avoided through tool tip dressing. Deionised water was used as the dielectric fluid throughout the experiment.

3. RESULTS AND DISCUSSIONS 3.1 Effects of Cryogenically Cooled µEDM on MRR Figure 3. Shows a comparison of MRR for the cryogenically cooled µEDM and conventional µEDM under two different operating conditions (10 A discharge current, 10 µs pulse on duration and 20 A discharge current, 20 µs pulse on duration). The MRR obtained by the cryogenically cooled µEDM was found to be higher than that of the conventional µEDM not only for a lower operating condition (Ip:15 A, Ton:15 µs) but also at a higher operating condition (Ip:20 A, Ton:20 µs). The material removal mechanism caused by melting and vaporisation was small at a lower discharge level, due to the dependence of the Impact Factor (JCC): 8.8746

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expansion size of plasma zone upon the discharge level. The impulsive force developed in the dielectric medium was also smaller and the machining efficiency was decreased, resulting in a lower MRR. The cryogenically cooled µEDM showed an improved MRR due to the cryogenic LN2 improves the electrical conductivity of material [17]. Hence there is larger transfer of electrons to the materials, which leads to a large amount of thermal energy transferred to the work piece. The electrical and thermal conductivity of the materials have significant effect on the performance reported by Urso and Merla [27]. Secondly, there is a reduction in the ductility of material caused by cryogenic LN 2 cooling, since the energy required for initiating the material removal in cryogenically cooled µEDM drilling process is lower than the µEDM drilling process.

Figure 3: Comparison of material removal rate obtained by conventional and cryogenically cooled µEDM. In contrast, at the higher operating condition of 20 A discharge current and 20 µs pulse on duration, the size of the plasma channel was comparatively high due to the prolonged pulse duration. There was larger melting here and vaporisation occurred on account of the expansion of plasma channel which increases the hardness of the material by grain refinement favouring large quantity of material removal and improving the MRR. The experimental results showed a 60 to 80% MRR improvement in the cryogenically cooled µEDM compared to the conventional µEDM. Hence the conclusion: cryogenically cooled µEDM process could improve the machining efficiency of µEDM drilling process.

3.2 Effects Of Cryogenically Cooled µEDM on Surface Roughness Figure 4. shows the comparison of average roughness values obtained by the cryogenically cooled µEDM with the conventional µEDM. Surface roughness values in cryogenically cooled µEDM are found to be lower than the conventional µEDM in both lower and higher operating conditions. At lower operating conditions (Ip: 15A, Ton:15µs) the energy discharged to work piece is small. Here the melting and vaporisation of material restricted depends on the size of the plasma channel. The craters produced at lower energy level were very tiny and shallow, resulting in getting lower roughness value.

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Figure 4: Comparison of surface roughness obtained by conventional and cryogenically cooled µEDM As the cryogenic LN2 cooling was supplied to the machining zone, a uniform cooling was occurred in the machining zone. Re-melting of debris, adhere on the surface was limited, due to sudden cooling forming into a solid debris flushed away by dielectrics. The recast layer and the heat affected zone observed on the cryogenically cooled µEDM were smaller than with the conventional µEDM, which enhances drilled surface quality. At the higher operating conditions (Ip: 20 A, Ton: 20 µs) the energy level discharged to the work piece was greater, creating larger and deeper craters on the machined surface.

Hence the surface obtained had greater roughness value. The size of the plasma channel was large and a large amount of material melted and vaporised. As the cryogenic cooling was offered to the work piece, the expansion of plasma channel was restricted to concentration on the machining zone. Hence there is no depression was produced on the machined surface through a complete vapourisation. Moreover, the enhanced cooling restricted in the deposition of a recast layer on the produced surface. Therefore the surface obtained with cryogenically cooled µEDM was better with a lower roughness value [28].

Figure. 5. shows the roughness profile traced for the cryogenically cooled µEDM and conventional µEDM processes, for the high discharge energy level of operating conditions (Ip:20 A,Ton:20 µs). The plasma channel was larger and deeper with a higher impulsive force. It occurs due to the higher discharge current and pulse on duration causes melting and vaporizing the material. As a result, increasing the surface crack with poor surface finish was observed in the conventional µEDM process. Whereas in the cryogenically cooled µEDM process, the cryogenic LN 2 supply restrict the uneven expansion of plasma channel and causes uniform erosion on the machined surface. It restrained the propagation crack on the surface and produced an enhanced surface.

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Figure 5: Surface roughness profiles for conventional and Cryogenically Cooled µEDM, (Ip:20A,Ton:20 µs) Micro structural analysis carried out on the cryogenically cooled µEDM and the conventional µEDM specimens is shown in Figure. 6. The grain size shows no change in the size at the entry micro structure of the cryogenically cooled µEDM drilled samples. The micro structure shows finer grains along the periphery which is due to the application of cryogenic LN2 cooling and had resulted in higher hardness. However, the hardness remains unchanged as moved further away from the drilled surface is shown in Figure. 7. Cryogenic cooling of µEDM was found to enhance the hardness of the material, which shows improvement in the material property.

Figure 6: Micro structural analysis of Drilled Surface (Ip:20A,Ton:20 µs) www.tjprc.org

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Figure 7: Micro hardness values obtained by conventional and cryogenically cooled µEDM (Ip:20A,Ton:20 µs) Figure. 8. cross sectional view of the microstructure taken in the longitudinal direction of the µEDM and the cryogenic cooled µEDM drilled hole. The conventional µEDM process shows negligible re-cast layer at the surface.

However, the cryogenic cooled µEDM process has not shown any remarkable recast later. In this field, a clear formation of the white re-cast layer could be seen at the edge of the conventional µEDM drilling process. The cryogenic sample showed no changes and the grains remained the same at the periphery and the inner core.

Figure 8. Microstructure of the cross sectional view conventional and cryogenically cooled µEDM (Ip:20A,Ton:20 µs) Figure. 9 shows the comparison of micrographs obtained through the conventional and the cryogenically cooled µEDM at high discharge energy levels (Ip: 20A, Ton: 20 µs). Figure. (9a) shows micrograph obtained at the entry of the hole Figure. (9b) by high magnification on the entry of the hole while Figure. (9c) shows that obtained at the exit of the hole. The micrographs of the drilled holes showed the hole quality as fine. The wall surface shows a large amount of recast layer on the conventional µEDM. As a result of uneven cooling and poor flushing, disabling effective removal of debris and allowing the deposition of melted material on the surface forming the recast layer. The formation of the recast layer deteriorates the strength of the machined surface, while cryogenic flushing helps sudden cooling of the debris, which is flushed easily. Thus the formation of the recast layer was almost negligible in the cryogenically cooled µEDM. The micrographs obtained at the exit of the drilled holes, show burr free formation on the surface machined through cryogenically cooled µEDM, while the conventional µEDM shows a poor surface quality on the machine surface with a remarkable burr formation and a recast layer.

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Figure 9: Drilled surfaces obtained using conventional and cryogenically cooled µEDM, (Ip:20 A,Ton:20 µs).

Figure.10 shows the micrographs of the polished and etched surfaces of the conventional and the cryogenically cooled µEDM drilled hole at entry surface with a high energy discharge level (Ip: 20A, Ton: 20 µs). Thick white layer formed on the conventional µEDM compared to cryogenically cooled µEDM. This can be made clear by the evidence that high discharge energy enlarges the size of plasma channel and causes in extensive melting and vaporization of the material.

The flushing provided by the conventional µEDM was inefficient to flush away the melted material, which got deposited on the parent material and forming a white layer on the machined surface, while the cryogenic cooling facilitates the effective removal of debris and reduces the formation of the white layer. Thus the micrograph shown by cryogenically cooled µEDM was free from a white layer formation.

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Figure 10: White layer obtained using conventional and cryogenically cooled µEDM, (Ip:20 A,Ton:20 µs).

Figure 11: Machined surface obtained by conventional and cryogenically cooled µEDM, (Ip:20 A,Ton:20 µs).

Figure. 11 presents the cross sectional view of the machined surfaces obtained by the conventional µEDM and cryogenically cooled µEDM. The region of the machined surface observed in the conventional µEDM was uneven with many of globules and poke marks. Moreover, it accumulated with complex surface crack density. The cryogenically cooled µEDM suppresses the propagation of crack and attachment of globules on the machined surface to provide improved surface quality.

4. CONCLUSIONS Cryogenically cooled setup coupled with a conventional µEDM machine can enhances the performance measures such as material removal rate (MRR) and average roughness value (Ra). Cryogenic cooling could enhance the properties of the work piece, enabling significant improvement in the machining characteristics. In this experimental investigation, the machining characteristics such as MRR and Ra values were analysed and the following conclusions are arrived at. 

The effect of cryogenically cooled µEDM was analyzed on MRR and Ra under low (Ip: 10 A, Ton: 10 µs) and high (Ip: 20 A, Ton: 20 µs) discharge energy levels. MRR improved by 60 to 80% and Ra by 30 to 48%, which is

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due to improvement in the electrical conductivity of the material. A micro hardness analysis confirms the enhancement of hardness through cryogenic cooling. 

Surface profile traces shows in the surface roughness value and uniform distribution of peaks and valleys in cryogenically cooled µEDM compared to the conventional µEDM, due to the uniform cooling rate through the application of cryogenics.



The formation of a recast layer and crack density were more patent in conventional



µEDM than in the cryogenically cooled µEDM. The cryogenic cooled µEDM suppresses the propagation of crack and the surface quality was relatively high on the machine surface.

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