Research Paper STUDY OF MATERIAL REMOVAL RATE OF DIFFERENT TOOL MATERIALS DURING EDM OF H11 STEEL AT REVERSE POLARITY Nibu Mathew 1*, Dinesh Kumar 2, Naveen Beri 3, Anil Kumar 3 Address for Correspondence 1* M. Tech student, Deptt. of Mechanical Engineering, S.S.C.E.T. Badhani, Pathankot, Punjab, India. 2 Deptt. of Mechanical Engineering, S.S.C.E.T. Badhani, Pathankot, Punjab, India. 3 Department of Mechanical Engineering, Beant College of Engineering & Technology, Gurdaspur, (Pb.) India. ABSTRACT Electric Discharge Machining (EDM) is a non-traditional machining process that involved a momentary spark discharges through the fluid due to the potential difference between the electrode and the workpiece. In EDM, improper choose of the electrode material may cause poor machining rate (or) performance. This is due to material removal rate characteristics. Less material removal rate (MRR) needs more time for machining process and become waste which is not good for production. Hence in this study, the effect of input parameters of EDM process ie. electrode type, peak current, gap voltage, and duty cycle on material removal rate of H11 steel are experimentally investigated. A L 18 Taguchi s standard orthogonal array is used for experimental design. Conventional Copper (Cu) and powder metallurgy (PM) copper tungsten (CuW) are used as tool materials. It was found that conventional copper tool electrode gives maximum MRR and best parametric setting for maximum MRR was found at, Cu (99% Cu) tool electrode, 9 ampere current, 50 volts gap voltage and 0.92 duty cycle. ie. A 1 B 2 C 2 D 3. KEYWORDS: Electric discharge machining, Material removal rate, Powder metallurgy, Taguchi method. 1. INTRODUCTION Electrical Discharge Machine (EDM) is now become the most important established technologies in modern industries since many complex 3D shapes can be machined using a simple shaped tool electrode. Electrical discharge machine (EDM) is an important non-traditional manufacturing method, developed in the late 1940s and has been accepted worldwide as a usual processing manufacture of forming tools to produce plastics moldings, die castings, forging dies and etc. New developments in the field of material science have led to new engineering metallic materials, composite materials, and high tech ceramics, having good mechanical properties and thermal characteristics as well as sufficient electrical conductivity so that they can readily be machined by spark erosion. At the present time, Electrical discharge machine (EDM) is a wide spread technique used in industry for high precision machining of all types of conductive materials such as: metals, metallic alloys, graphite, or even some ceramic materials, of whatsoever hardness. Electrical discharge machine (EDM) technology is increasingly being used in tool, die and mould making industries, for machining of heat treated tool steels and advanced materials (super alloys, ceramics, and metal matrix composites) requiring high precision, complex shapes and high surface finish. Traditional machining technique is often based on the material removal using tool material harder than the work material and is unable to machine them economically. An electrical discharge machining (EDM) is based on the eroding effect of an electric spark on both the electrodes used. Electrical discharge machining (EDM) actually is a process of utilizing the removal phenomenon of electrical-discharge in dielectric. Therefore, the electrode plays an important role, which affects the material removal rate. Beri N. et.al. [1] Studied electric discharge machining of AISI D2 steel with copper tungsten (30% Cu and 70%W) electrode and conventional copper electrode by using kerosene. The input factor studied are current, duty cycle, and flushing pressure and output parameters are material removal rate and surface roughness. They concluded that conventional copper electrode gives high MRR and copper tungsten made through powder metallurgy gives higher surface finish. Amri Lajis, M. et.al. [2] reported that effect of tungsten carbide ceramic on graphite electrode by analyzing the machining characteristics. The input parameters selected are peak current, voltage, pulse duration and interval time. The output parameters include Material removal rate, Electrode wear rate, and surface roughness. The taguchi method is used to formulate the experimental layout and reveals that peak current significantly affects the electrode wear rate and surface roughness, while pulse duration mainly affects the material removal rate. Singh, R. et.al. [3] Investigated the effect of cryogenic treatment for enhancing life of EDM tool while machining titanium alloys using taguchi technique. The output parameters selected are tool wear rate (TWR) and surface roughness. They concluded that with the help of cryogenic treatment, the machining parameters like TWR and surface roughness improved by 58.778% and 8% respectively. Kumar, V. et.al. [4] studied the performance of EDM of hastalloy with powder metallurgy tool electrode by taking current and voltage as input parameters and MRR, TWR, % wear rate, surface roughness as output parameters. They concluded that maximum MRR is at the average value of voltage within selected range of process input parameters. The tool wear rate is found minimum for minimum value of current and voltage and surface roughness is minimum for average value of current and voltage. Singh, P. et.al. [5] analyzed the effect of aluminium powder mixed in the dielectric fluid of EDM on machining characteristics of hastalloy by considering concentration of aluminium powder and grain size of powder as input parameters and MRR, TWR, %wear rate and surface roughness as output parameters. They concluded that addition of aluminium powder in dielectric fluid increases MRR, decrease TWR and improves surface finish of hastalloy. Beri N. et.al [6] experimentally investigated AISI D2 steel in kerosene with CuW (25% Cu and 75% W) powder metallurgy electrode. An L 18 orthogonal array was used to identify the best process parameters (Viz. electrode material, duty cycle, flushing pressure, and current etc.) for MRR, SR, and surface hardness. Grey relational analysis was used to solve the EDM
process with multiple performance characteristics which was obtained through grey relation grade. They concluded that EDM process performance can be improved efficiently through this approach and they also concluded that copper tungsten PM electrode gives better multi objective performance than conventional copper electrode. Kumar, D. et.al. [7] investigated the effect of hastalloy steel on sintered tool electrode of copper chromium (Cu-Cr) with reverse polarity in standard EDM oil. The input parameters analyzed are current, voltage, duty cycle, pulse on time, and flushing pressure with MRR, TWR, SR, %wear rate as output parameters. While during experiment current and voltage are varied by keeping other parameters constant on average value. They concluded that MRR and TWR increases with increase in current and voltage but surface roughness and %wear rate decreases with the increase in current and at low value of voltage. Sharma, S. et.al. [8] studied the effect of aluminium powder on machining hastalloy using EDM with reverse polarity by evaluating MRR, TWR, %wear rate, SR. The input parameters selected for the study are concentration and grain size of aluminium powder. They found that with the increase in the concentration of aluminium powder, surface roughness and %wear rate decreases but TWR and MRR increases. Singh, P. et.al. [9] analyzed the effect of electrical parameters such as peak current, gap voltage, pulse on time and duty cycle on MRR, TWR, %wear ratio and surface roughness. The experimental investigations are carried out using copper tool electrode and hastalloy as work material. They concluded that high peak current is desirable factor to yield more material erosion rate but has less effect on surface finish of newly machined surfaces. The wear ratio decreases at high peak currents and for smaller gap voltage. High pulse on time is undesirable factor for material erosion rate but has favorable effect on surface finish. Patel, V.D. et.al. [10] experimentally investigated parameters affecting surface roughness along with structural analysis of surfaces with respect to material removal parameters. The conducted their experiment on mild steel with copper, brass and graphite as tool electrodes with kerosene oil as dielectric fluid. They concluded that MRR increases with increase in discharge current for all three electrodes, but in case of brass and copper it decreases after some limit, due to pulse energy increases as the current increases. They used Scanning Electron Microscope (SEM) and optical microscope to understand the mode of Heat Affected Zone (HAZ). They also observed that molten mass has been removed from surface as ligaments and sheets but in some cases it is removed as chunks, which being in molten state stuck to surface. Dewangan, S. et.al. [11] studied the effect of machining parameter such as discharge current (I p ), pulse on time (T on ), and die of tool using AISI P20 tool steel material on U shaped copper tool electrode with internal flushing was used. The output parameters studied are MRR, TWR, and overcut. They concluded that MRR is directly proportional to discharge current (I P ) and this is because an increase in pulse current produces strong spark which produces the higher temperature causing more material to melt and erode from the work piece. They also found that pulse on time has less effect and tool die has no significant effect on MRR. At the same time, TWR is mostly influenced by pulse on time, then discharge current followed by diameter of the tool. This is because when discharge current increases the pulse energy increases and then more heat energy is produced in the tool and work piece interface leads to increase in melting and evaporation of the electrode. Overcut is mostly affected by discharge current, then diameter of tool followed by pulse on time. This is because I p is responsible for production of spark between tool and work piece. Kumar, D. et.al. [12] investigated the effect of hastalloy steel on sintered tool electrode of copper tungsten (CuW) with straight and reverse polarity. In standard EDM oil. The input parameters analyzed are polarity, tool electrode material, peak current, pulse on time, duty cycle and gap voltage on overcut by using ANOVA and scattered graph. They found that powder metallurgy tool electrode with reverse polarity gives minimum overcut and minimum value of current gives better results for overcut, which is approximately same as that of diameter of tool electrode. Ghewade, D.V. et.al. [13] reported the effect of cutting of Inconel 718 material using EDM with copper electrode tool by taking peak current, gap voltage, duty cycle and pulse on time as input parameters and MRR, EWR, radial overcut (ROC) and half taper angle as output parameters by using taguchi method. They concluded that MRR is mainly affected by peak current (I P ) and gap voltage (V g ), EWR is influenced by pulse on time (T on ) and duty cycle. They also found that peak current and duty cycle (t) have maximum effect on the radial overcut and half taper angle is mainly affected by pulse on time and duty cycle. Sharma, S. et.al. [14] studied the effect graphite powder on machining conventional EDM by evaluating the machining performance in terms of tool wear rate. The input parameters selected for the study are concentration of graphite powder, polarity, electrode type, peak current, pulse on time, duty cycle, gap voltage and retract distance. They found that with the addition of the powder particles in the dielectric fluid and the use of cold treated electrode decreases the tool wear rate. Purohit, R. et.al. [15] reported the effect of input parameters viz. rotating speed of electrode, hole diameter and grain size of SiC on output parameters that is the tool wear rate, MRR, and radial overcut obtained during EDM of 7075 Al-10 wt, % SiC composites by using hollow tube brass electrode of 15 mm diameter. They concluded that MRR increases in rotation spped of the electrode because of the improved debris removal at high speed and further increase of MRR takes place with the decrease in grain size of the SiC particulates. They also noticed that TWR increases with decrease in grain size. This is because of the increase in MRR with decrease in grain size. TWR further decreases with increase in hole diameter of the electrode. Radial overcut is mainly influenced with increase in rotation speed of the electrode, because of the increase in MRR at high rotation speed. Kocher, G. et.al. [16] investigated the effect of discharge current and surface roughness by using D3 tool steel material with copper, copper tungsten and
graphite as electrodes. They observed that copper tungsten is giving good quality surface as compared to copper and graphite electrodes. They also categorized various tools mainly graphite for roughing, copper electrode for semi finishing and copper tungsten for best finishing process. Kubade, P.R. et.al. [17] investigated the influence of EDM parameters on EWR, MRR and radial overcut (ROC) while machining AISI D3 material. The main input parameters selected for the study are pulse on time (T on ), peak current, duty factor and gap voltage. They found that MRR is mainly influenced by peak current where other factors have very less effect on MRR. EWR is mainly influenced by peak current and pulse on time. Peak current has most influence on ROC followed by duty cycle and pulse on time. Gap voltage has very less influence on ROC. In the present research work the main attention is to study the effect of variation in EDM parameters during machining of H11 hot work tool steel using conventional copper and powder metallurgy electrode. Kumar, K.M. and Hariharan, P. [18] investigated the machining characteristics of Austempered Ductile Iron (ADI) using EDM process with copper as electrode. The selected input parameters for the study are peak current, pulse on time, pulse off time and tool geometry by analyzing machining response such as the MRR, TWR, SR and taper angle. They concluded that tool geometry is the least influential parameters but discharge current, pulse on time and austempering temperature are found most influential parameters on each performance measure. 2. DESIGN OF EXPERIMENTS Design of experiment is the primary step before starting the experimental work. Design of experiments (DOE) is used to study the effect of multiple variable simultaneously, which is a powerful statistical technique introduced by R.A.Fisher in England in 1920 s. Table 2.1 shows selected input machining parameters with their designation. Table 2.1: Input machining parameters with their designation Table 2.2: Assigned values of input machining parameters at different levels and their designation The next step is to select an appropriate array keeping in mind that the total DOF for the orthogonal array should be greater than (or) at least equal to those for the process parameters. L 18 (3 3 x 2 1 ) orthogonal array is selected for the present study. This orthogonal array has 4 columns and 18 rows. One machining parameter is assigned to each column. Total 18 rows give the parametric combination for each set of experiment. The experimental combination of the machinery parameters using the L 18 orthogonal array are presented in Table 2.3. Table2.3: Design Matrix of L 18 (2 1 x 3 3 ) orthogonal array 3. EXPERIMENTAL PROCEDURE Experiments are performed on SMART ZNC EDM, Electronica Machine Tools, India (Pune). In this machine, Z axis can be programmed to follow an NC code which is fed through the control panel and is also servo controlled. The gap voltage between the tool and workpiece electrodes is responsible for servo control feedback. During machining, a constant gap distance will maintain and tool is fed in to the workpiece. In this machine gap distance cannot be independently controlled and X and Y axis are manually controlled. All three axis have an accuracy of 5µm. Machining can be programmed to occur up to a fixed depth of cut through an NC code. Stop watch is used to note down the machining time. Before starting the experiment, work piece was perfectly cleaned with pressurized air jet so that dust (or) unwanted particles can be removed. At first work piece and tool electrode was weighed on a precision weighing machine and workpiece mounted on the T- slot table of the EDM machine. Ensure that workpiece is properly held on the fixture assembly and clamp it properly. A micrometer of range 0 to 25 mm; with a least count of 0.01 mm made by Mitutoyo, Japan was used to measure the diameter of electrodes. Then the tool was clamped on the V-block tool holder of the EDM machine, and its alignment was checked with the help of tri square. The tool holder has a facility to make adjustments for the alignment of the electrode with respect to the workpiece. The depth of cut was set at 0.50 mm for each cut. The gap between tool and workpiece was automatically maintained with the help of beep sound. Flushing should be done, since during machining both tool and workpiece should be immersed in dielectric fluid. Then machining should be started, as the desired depth is reached, machining operation stops automatically. However, the actual depth of cut achieved may not be 0.50 mm because electrode wears out during the machining operation and the decrease in the length of the electrode also
gets added in the depth of cut. Hence in order to calculate the MRR, the loss in the weight of the work piece was taken as the criteria, which gives more accurate results. The input values of discharge current, pulse on time, duty cycle and gap voltage was set by using the hand held keyboard for each experiment. The values were noted as per the design of experiment trial conditions using Taguchi method (Table 2). The stop watch was also stopped at that very instant and time of cut (t in seconds) was noted. The drain valve of the tank was opened to flow out the dielectric to the storage tank. A total of 18 experiments were performed (as per table 3) and at the each end of the experiment workpiece was weighed to find out final weight, so that MRR can be calculated. Table 3.1: Chemical composition of H11 (wt %) 3.1. Workpiece material For the present study H11 hot work tool steel is selected as workpiece material. Workpiece of size 65 X 30 X 7 mm is used here. H11 is basically 5% chromium hot work steel. The 1.5% molybdenum imparts very high hardenability to this grade and makes difficult to machine by traditional machining methods. It has good resistance to softening at elevated temperatures, but its outstanding characteristics are high toughness. A slight modification of this grade has been widely used for aircraft and structural applications requiring good ductility and notch strength at high strength levels. The chemical composition of H11 is shown in Table 3.1. 3.2. Electrode Material The electrode material selected for the present investigation is conventional copper (Cu) and copper tungsten (75% Cu and 25% W) electrodes. CuW electrodes are made through powder metallurgy technique by mixing weight of copper and tungsten in blenders. The mixture is then compacted in a die to form green compact. This compact is sintered in a sintering furnace to the temperature of 1600 0 c and sintering time is kept between 3 to 6 hrs. The diameter and length of electrode are kept constant at 8.00 mm and 90 mm respectively. 4. RESULTS AND DISCUSSIONS The results obtained after experimentation on H11 steel with conventional copper tool electrode (Cu) and powder metallurgy copper-tungsten tool electrode CuW (75%Cu and 25%W) are shown in below. The experimental plans for EDM process were based on Taguchi method and for analyzing data; analysis of variance (ANOVA) is performed using Minitab 15.1.1 software For MRR the requirement is to maximize it so the criteria selected using the software is larger is better. An ANOVA (Analysis Of Variance) Table 4.1 is used to summarize the experimental results. The table concludes information of analysis of variance and case statistics for further interpretation. After the ANOVA procedure, further analysis was performed in graphic plots. This will help to observe the variation against important input parameters. Table 4.1. Analysis of Variance for SN ratio for MRR (Larger is better) cycle are the most influencing factor for MRR. Interaction of electrode type and peak current is also influencing MRR. During the process of electrical discharge machining, the influence of various input machining parameter has considerable effect on MRR, as shown in main effects plots for SN ratios of MRR in Figure 4.1. Table 4.2. Response Table for SN ratio for MRR (Larger is Better) Figure 4.1. Main effects plots for SN ratios (MRR) It is clear from the Fig 4.1 that MRR is maximum at the 1 st level of the electrode type, 2 nd level of peak current, 2 nd level of voltage and 3 rd level of duty cycle. Main effects plots for SN ratio suggest these levels of the parameter are best levels for maximum MRR as shown in Table 4.3. Table 4.3. Best levels of input parameters at maximum MRR The ANOVA and Response table for MRR clearly indicates that voltage, Electrode type are relatively less influencing factor for MRR and current, duty The mechanism of material removal of EDM process in mostly widely established principle is the conversion of electrical energy in to thermal energy. During the process of machining the sparks are produced between workpiece and tool electrode. Thus each spark produces a tiny crater in the material
along the cutting path by melting and vaporization, thus eroding the workpiece to the shape of the tool electrode. In the present work the result shows that the effect of MRR is more when conducting the experiment with conventional tool electrode as compare to powder metallurgy tool electrode (CuW). This is because adding of more copper (Cu) powder, making the resin inside the tungsten powder, reducing the bonding strength. Thus during machining, powder easily drops out of the electrode and accumulates on the surface of the workpiece, disturbing the stability and efficiency of machining. Since during negative polarity positive charged ions flows towards workpiece and electrons flows towards electrode. Hence heavy mass ions towards the workpiece strike with less momentum which erodes more material from workpiece resulting is more MRR. MRR is very small at low value of peak current. It is due to the reason that at low current, a small amount of heat is produced out of which some heat is absorbed by machine components, dielectric fluid in the tank surroundings environment etc and left heat is utilized to melt and vaporize workpiece material. But as the current increased, more intermittent arc discharge occurring with higher energy. Due to this a lot of heat is generated and a substantial amount of heat is used to melt and vaporize the workpiece material. This heat increases the MRR. Further increase in current, the MRR will decreases. This because bonding between the copper decreases as a result of copper easily drops of the electrode and accumulated on the surface of workpiece, disturbing the stability and efficiency of machining. With regard to gap voltage, MRR increases when the gap voltage increased. But further increase of gap voltage, MRR decreases. One of the reason for this could be the higher amount of debris formation and higher flushing required to increased spark energy at higher voltages. The increase in spark energy is dominated by the reduction in spark efficiency as voltage is increased. This leads to a reduction in MRR. Increase in duty cycle means increase in pulse on time and decrease in pulse off time. It is observed that increase in duty cycle leads to increase in MRR. It is due to the reason that with an increase in pulse on time, total machining time and hence total current utilization time increases. Increase in pulse on time retains the spark for more time in spark gap. This means more time the heat is available to melt and vaporize the work material. The interaction plot of MRR at different electrode, peak current, voltage and duty cycle is shown in Figure 4.2. From the interaction plot the following observations are drawn. 1. MRR increases with the increase of current (up to 9 amp) for copper electrode and further increase in current, MRR decreases. 2. For copper tungsten powder metallurgy tool MRR increases with the increase in peak current. 3. With conventional copper tool electrode, it is observed that MRR increases with the increase of gap voltage from 40 volt to 50 volt and on further increase of gap voltage value MRR decreases. 4. For powder metallurgy copper tungsten (75% Cu and 25%W) tool electrode it is observed that MRR increases with the increase in gap voltage up to 50 volt and after that MRR decreases with the increase of gap voltage. Fig.4.2. Interaction plot for SN ratios (MRR) 5. With conventional copper tool electrode it is observed that MRR increases continuously with increase of duty cycle. 6. For powder metallurgy CuW tool electrode, it is observed that MRR increases with the increase of duty cycle. 5. CONCLUSIONS Following conclusions can be drawn from the analysis of the results. From the experimental results based on Taguchi method it was found that at reverse polarity with conventional copper tool electrode, MRR increases as compared to powder metallurgy tool electrode. MRR increases with increase in current and gap voltage to some predetermined value and after that MRR decreases. But MRR increases with the increase in duty cycle. Best parametric setting for maximum MRR is with copper (99% copper) tool electrode,
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