23 CHAPTER 2 CASE STUDY-I COMPARATIVE ANALYSIS OF EDM FOR INCONEL 718 AND 625 2.1 INTRODUCTION Inconel 625 and 718 super alloys are extremely versatile austenitic nickel based super alloys with excellent strength and good ductility at very high temperature. Due to the improved mechanical properties of nickel-based super alloy sheets, they are extensively used in aerospace applications, gas turbines, rocket motors, spacecraft, nuclear reactors, pumps, and tooling. However, Inconel 718 and 625 is a well known difficult-to-cut material. Its low thermal conductivity and specific heat result in high cutting temperature. In addition, chips are easy to weld on the tool which form build up edge (BUE). As a result of it, cutting tool wears rapidly during machining. Moreover, poor machinability of Inconel 718 when machining it using the traditional mechanical cutting process leads to high tooling cost. EDM process becomes a natural choice for machining nickel based super alloys. Electrical Discharge Machining (EDM) is one of the most successful, profitable, and extensively used non conventional machining process for high degree of dimensional accuracy and economical cost of production of any conductive material irrespective of its hardness. Electrical discharge machining (EDM) have been explored to machine this alloy by using some cylindrical copper and brass electrodes. Based on the literature survey, some researchers, analyzed the process parameters with performance measures while machining Inconel 718
24 during die sinking EDM. In EDM process, material removal and its mechanism has been one of the main concerns for several years. Since the development of this process, researchers have explained the material removal mechanism by developing different methods and mathematical models by considering relationship between pulse conditions and material removal. Traditionally, EDM is used to produce any component in any electrical conductive material with low MRR. In order to increase MRR, some researchers, used multi-channel electrodes, hallow electrodes, and bunched electrode on EDM. They are used for hole drilling to improve the performance on material removal rate, electrode wear and surface integrity for the EDM parameters while machining Inconel 718 super alloy. Typical applications need standard design requirement and close form tolerances in manufactured components. The data regarding cylindricity, circularity, perpendicularity and parallelism of the holes made by EDM are very few or non-available. Recently, only a few researchers optimized the parameters for micro-edm drilling of Inconel 718 super alloy on hole taper ratio and hole dilation by using Grey relational analysis. From the literatures, it is observed that no credible works were conducted on measuring cylindricity and circularity by using CMM on machining new and advanced material, such as Inconel 718 and 625 nickelbased super alloy in EDM process. Thus, this experimental work is attempted to evaluate the form tolerance in EDM of Inconel 718 and 625. Taguchi technique was used to develop Design of Experiments (DoE) to reduce the number of trials.. Additionally, ANOVA is used to find the significant parameter.
25 2.2 DESIGN OF EXPERIMENTS The application of design-of-experiments (DoE) requires careful planning, prudent layout of the experiments, and expert analysis of results. Taguchi has standardized methods for each of these DoE application steps. This approach in finding factors that affect a product in a DoE can dramatically reduce the number of trials required to gather necessary data. Thus, DoE using Taguchi approach has become a much more attractive tool to practicing engineers and scientists. In this case study, a total of four parameters namely peak current, pulse on time, pulse off time, and flushing pressure were chosen for the controlling factor and each parameter was designed to have four levels denoted by 1, 2, 3 and 4 as shown in the Table 2.1. Table 2.1 Machining parameters and their levels Parameter Unit Level 1 Level 2 Level 3 Level 4 A Peak current Amps 6 9 12 15 B Pulse on time µs 200 400 600 800 C Pulse off time µs 10 20 30 40 D Flu.pressure kg/cm 2 0 0.25 0.5 0.75 2.2.1 Running Experiment The chemical composition of Inconel 718 and 625 super alloys are shown in Table A 3.1 in Appendix 3. The hardness values of the Inconel 718 and 625 super alloys is shown in Table A 3.2 in Appendix 3. The experiments were conducted by using a die sinking SPARKONIX Electric Discharge machine with a capacity of 15 Amps as maximum current rating. The die sinking EDM setup is shown in Figure 2.1. The work pieces, Inconel
26 718 and 625 super alloy, which is in the form of disc and plate, are shown in Figures 2.2 and 2.3. The work piece and electrode were connected with +ve and ve polarity. The electrodes as shown in Figure 2.4 to Figure 2.6 were prepared by using CNC lathe (circular) and CNC milling (square and hexagonal). Kerosene was used as dielectric fluid with pressure of 0-0.75 kg/cm², and side flushing technique was used to conduct all the experiments. The weight of the electrode and work piece were measured before machining and after machining for each trial run, by using digital weighing balance, with an accuracy of 0.001 grams. The Material Removal Rate (MRR) was calculated using the formula given below Weight of workpiece material removed MRR (g / min) (2.1) Time given below The Electrode Wear Rate (EWR) was calculated using the formula Weight of electrode material removed EWR (g / min) (2.2) Time Three trials were taken for each set of parameters and the average roughness values were obtained. The form tolerances namely cylindricity, circularity, perpendicularity, and parallelism are measured on electrodes and work pieces (before and after machining) by using Co-ordinate Measuring Machine (CMM) CHECKMASTER.
27 Figure 2.1 EDM experimental setup Figure 2.2 Inconel 718 workpiece
28 Figure 2.3 Inconel 625 workpiece Figure 2.4 Circular Electrodes
29 Figure 2.5 Square Electrodes Figure 2.6 Hexagon Electrodes 2. 3 RESULTS AND DISCUSSIONS The 16 experimental runs were conducted in 3 trials, and the average values of MRR, EWR, cylindricity, circularity, perpendicularity, and parallelism for Inconel 625 and Inconel 718 are listed in Tables A 1.1 in Appendix 1 and Table A 2.1 in Appendix 2 respectively.
30 2.3.1 Analysis of Variance (ANOVA) - Inconel -718 and 625 Tables A 4.1 in Appendix 4 and Table A 5.1 in Appendix 5 show the calculated F-values and P-values of the analysis of variance (ANOVA) for MRR, EWR and form tolerance namely cylindricity, circularity, perpendicularity and parallelism respectively to determine the relative significances of different control factors for electrical discharge machining of Inconel 718 and 625. It is observed that pulse on time (Ton) and peak current have significant effect on MRR and EWR. The confidence interval of MRR, EWR, cylindricity, circularity, perpendicularity, and parallelism for machining of Inconel 718 are 99.3 %, 94.89%, 94.19%, 93.64%, 92.61%, and 94.65% respectively. The confidence interval of MRR, EWR, cylindricity, circularity, perpendicularity, and parallelism for machining of Inconel 625 are 95.49%, 96.21%, 98.36%, 91.64%, 94.91%, and 98.55% respectively. The values of prob. > F in Table 6.6 and 6.7 for the term of models are less then 0.05 (i.e. = 0.05 or 95% confidence) which indicates that the parameters corresponding to these values are considered to be statistically significant to the other parameters. 2.3.2 The Effect of Current on MRR and form Tolerance Index The variations of MRR, perpendicularity index, and parallelism index with respect to peak current while machining with square electrodes are shown in Figures 2.7, 2.8, and 2.9. The variations of circularity index, cylindricity index, and perpendicularity index with respect to peak current while machining with circular electrodes are shown in Figures 2.10, 2.11, and 2.12. The variations of parallelism index with respect to peak current while machining with hexagonal electrode is shown in Figure 2.13. As shown in Figure 2.7, the MRR is increased with peak current. This result may be attributed to the relative increase in discharge energy and impulse force as the peak current increased.
31 In general, during EDM process, the dimensions of work piece depend upon the electrode dimensions. Similarly, the form tolerances of work piece also depend upon the form tolerances of electrode. Moreover, these tolerance value are again affected by the machining time due to the reason that as the EDM machining progresses, the electrode wears out in nonuniform manner. Therefore, it is attempted to study the performances based on form tolerance index. The form tolerance index is separately calculated for each form tolerance considered using the expression given below Form tolerance index Form tolerance of workpiece Form tolerance of electrode *Machining time In all the Figures from Figure 2.8 to Figure 2.13 and from Figure 2.15 to Figure 2.20 the form tolerance index values of Inconel 718 are higher than those of to Inconel 625. This is due to the reason that the Inconel 718 has higher thermal conductivity values when compared to Inconel 625. The circular electrodes which produce uniform spark density on its circumference give better form tolerances where as hexagonal and square electrode gives poor form tolerances. It is also due to the reason that the edges of square and hexagonal profiled electrode are with higher density spark interaction which produced irregular surfaces and also affected the form tolerances. As electrode advances into a work piece, the sparking area changes and sparking is not only takes place at the bottom of the electrode but also at lateral face. Consequently, the higher current tends to increase the EWR and it in turn affects the form tolerance indices of the square, circular, and hexagonal electrode.
32 Influece of peak Current on MRR for various electrode (INCONEL 718) Peak Current Amps Figure 2.7 Influence of peak current on MRR Influence of Peak Current on Perpendicularity Index Peak Current Amps Figure 2.8 Influence of peak current on perpendicularity index (Square electrode)
33 Influence of peak current on parallelism Index Peak Current amps Figure 2.9 Influence of peak current on parallelism index (Square electrode) Influence of Peak Current on circularity Index Peak Current Amps Figure 2.10 Influence of peak current on circularity index electrode) (Circular
34 Influence of current on cylindricity Index Peak Current amps Figure 2.11 Influence of peak current on cylindricity index (Circular electrode) Influence of peak current on perpendicularity Index Peak Current Amps Figure 2.12 Influence of peak current on perpendicularity index (Circular electrode)
35 Influence of Peak Current on Parallelism Index Peak current Amps Figure 2.13 Influence of peak current on parallelism index (Hexagonal electrode) Influence of Ton on MRR for various electrode (Inconel 718) Ton micro secs Figure 2.14 Influence of T on on MRR
36 Influence of Ton onperpendicularity Index Ton micro secs Figure 2.15 Influence of T on on perpendicularity index (Square electrode) Influence of Ton on parallelism Index Ton microsecs Figure 2.16 Influence of T on on parallelism index (Square electrode)
37 Influence of Ton on Circularity Index Ton micro secs Figure 2.17 Influence of T on on circularity index (Circular electrode) Influence of Ton on Cylindricity Index Ton microsecs Figure 2.18 Influence of T on on cylindricity index (Circular electrode)
38 Influence of Ton on perpendicularity Index Ton micro secs Figure 2.19 Influence of T on on perpendicularity index (Circular electrode) Influence of Ton on parallelism Index Ton micro secs Figure 2.20 Influence of T on on parallelism index (Hexagonal electrode)
39 2.3.3 The Effect of Pulse on Time on the MRR and form Tolerance Index The variations of MRR, perpendicularity index, and parallelism index with respect to pulse on time while machining with square electrodes is shown in Figures 2.14, 2.15, and 2.16. The variations of circularity index, cylindricity index, and perpendicularity index with respect to pulse on time while machining with circular electrodes is shown in Figures 2.17, 2.18, and 2.19. The variations of parallelism index with respect to pulse on time while machining with hexagonal electrode is shown in Figure 2.20. As shown in Figure 2.14, the longer pulse on- time results in higher MRR up to half way in the beginning but then starts decreasing with further increase in pulse on-time. This event has been attributed to the increase of input energy in high pulse on time, which results in more chopping on the gap between the work piece and tool electrode, creating a short circuit which decreases the performance of electrical spark erosion. Due to this reason, MRR increased when T on increased, and then MRR reduced on further increase in T on values. Consequently, the longer and wider craters are formed as machined surface further increasing pulse on time values. And also when increasing pulse on time there is a great quantity of molten and floating metal suspended in the electrical discharge gap during EDM and resulting in deterioration of both the electrode and work piece surface. This phenomenon dominates the behavior of the electrode wear. The wear of tool material leads to deterioration in both the size and the shape of the machined hole in EDM drilling, producing a hole with larger roundness error. As a result, the form tolerances are increased when T on increases. On the other hand, perpendicularity index, parallelism index, cylindricity index, and circularity index rapidly decreases with increase in pulse on time
40 duration in the beginning but then starts increasing slowly and further increase with increase in pulse on time as shown in Figure from 2.15 to 2.20. 2.4 SUMMARY OF RESULTS Analysis of form tolerances in Electrical Discharge Machining (EDM) of Inconel 718 and Inconel 625 were performed. An experimental plan of Taghuchi L 16 array table was employed to carry out the experimental work. The effect of machining parameters on the performance characteristics in the EDM process of Inconel 718 and 625 were analyzed based on the ANOVA and second order polynomial graphs for various performances measures yield the following conclusions: 1. The result of ANOVA used to find the most significant parameter for MRR, EWR, cylindricity, circularity perpendicularity and parallelism for machined Inconel 718 and 625 by using square, circular and hexagonal electrodes. For all the performance measures the R 2 values was above 92.5 % (P>0.05, ie = 0.05) confidence interval. It is found that the pulse on time is the most significant parameter rather than current that affects the MRR, EWR and form tolerances. 2. The value of MRR increases with increasing discharge current. Then, it affects the form tolerance index. 3. The value of MRR first increases with an increase pulse on time up to 400 µs, and then decrease with a further increase in the pulse on time. 4. The value of cylindricity index, circularity index, perpendicularity index, and parallelism index first decreases with an increase in pulse on time up to 350 µs, and then increase with a further increase in the pulse on time.