CHAPTER -6 ANALYSIS AND DISCUSSION OF RESULTS

Transcription

1 CHAPTER -6 ANALYSIS AND DISCUSSION OF RESULTS This chapter presents the analysis and discussion of the results obtained from the experimental work conducted in Chapter-5. The effect of cryogenic treated brass wire, effect of process parameters on the performance characteristics and wire breakage in WEDM have also been discussed in this chapter. 6.1 EFFECT OF CRYOGENIC TREATMENT ON BRASS WIRE (PHASE-I) The experiments conducted in phase I are related with the effect of deep and shallow cryogenically treated wire on MRR, SR and WWR in WEDM, The Minitab software (15.1.0) was used to analyze the results with general full factorial experimental design strategy Material Removal Rate (MRR) The experimental data for MRR of the workpiece with untreated and deep cryogenically treated wire s are presented in Table 5.12 (Chapter-5). Based on this data, the main effect plot for MRR is shown in Figure 6.1. It can be seen from Figure 6.1a that MRR value obtained with untreated wire is low as compared to deep and shallow cryogenic treated wire. The deep cryogenic treated wire is slightly better as compared to shallow cryogenically treated wire. It is observed from Figure 6.1b that an increase in pulse width increases the MRR slowly owing to decrease in insulating strength of de-ionized water and stabilization of discharge in the plasma channel. Beyond the pulse width of 0.8 µs, there is sharp increase of MRR, as the enhanced conductivity increases the discharging frequency. The carriers (ions and electrons) move more vigorously towards their respective s resulting in increased avalanche of ions and electrons. This causes more discharge per unit time on the workpiece surface. The faster erosion from the workpiece surface owing to faster frequency within the discharge takes place and hence higher MRR is obtained. It is further observed from the Figure 6.1c that the wire tension has little effect on MRR. The analysis of variance (ANOVA) for the material removal rate (MRR) data is shown in Table 6.1. It is observed that the type of wire, pulse width, wire tension, the interaction between type 90

2 of wire and pulse width and the interaction between pulse width and wire tension have significant effect ( at confidence level of 95%) on MRR. Main Effects Plot for MRR T ype of w ire(a ) Pulse w idth(b) 0 a b Mean (mm3/min) c DCT W E SCT W E W ire t ension(c) UT W E DCTWE- Deep cryogenic treated wire SCTWE- Shallow cryogenic treated wire UTWE- Untreated wire Figure 6.1 Main Effects Plot for MRR Table 6.1 Analysis of Variance for MRR Source DF Seq SS Adj SS Adj MS F P A * B * C * A*B * A*C B*C * Error Total Type of wire, B Pulse width, C Wire tension *Significant at 95% level The interaction plot () for MRR is shown in Figure 6.2. It can be observed that maximum MRR was obtained at high value of pulse width with deep cryogenic treated wire. This could be attributed to the fact that high conductivity of deep and shallow cryogenic treated wire caused more erosive effect on the workpiece surface. The deep and shallow cryogenic treated wire promotes more MRR at low value of wire tension as compared to untreated wire. 91

3 T y pe o f wir e ( A ) Interaction Plot for MRR (mm3 /min) Ty pe of w ire(a ) DC TW E SC TW E U TW E P ulse width( B ) Wir e te nsio n( C ) Pulse w idth(b) DCTWE- Deep cryogenic treated wire SCTWE- Shallow cryogenic treated wire UTWE- Untreated wire Surface Roughness (SR) Figure 6.2 Interaction Plot for MRR The experimental results of the effect of different parameters on surface roughness of the workpiece with untreated, shallow and deep cryogenic treated wire s are presented in Table 5.12 (Chapter-5). Figure 6.3 illustrates the effect of process parameters, namely type of wire, pulse width and wire tension on the workpiece surface roughness (SR). It is observed from Figure 6.3a that an untreated wire produces high SR value. SR value decreases with shallow and deep cryogenic treated wire s. The machining with deep cryogenic treated wire gives the lowest SR. Figure 6.3b also indicates that an increase in pulse width increases SR linearly. Maximum value of SR is observed at 1.2µs of pulse width. Further, SR value is maximum at middle value of wire tension (0.2daN), whereas the improvement in surface roughness can be seen at higher values of wire tension (Fig. 6.3c). The analysis of variance (ANOVA) for the SR data is shown in Table 6.2. The type of wire, pulse width, wire tension, the interaction between type of wire and wire tension, the interaction between pulse width and wire tension are the significant factors at 95% confidence level. 92

4 Main Effects Plot for SR(µm) a T ype of w ire(a ) b Pulse w idth(b) Mean c DCT W E 1.3 SCT W E W ire t ension(c) 2.0 UT W E DCTWE- Deep cryogenically treated wire SCTWE- Shallow cryogenic treated wire UTWE- Untreated wire Figure 6.3 Main Effects Plot for Surface Roughness Table 6.2 Analysis of Variance for SR Source DF Seq SS Adj SS Adj MS F P A * B * C * A*B A*C * B*C * Error Total Type of wire, B Pulse width, C Wire tension *Significant at 95% level The interaction plot (Data means) for SR is shown in Figure 6.. The minimum SR is obtained with deep cryogenic treated wire and at low value of pulse width. It is expected that at low pulse width, an ionization and de-ionization of dielectric fluid is improved. The discharge is uniformly distributed across the surface and machining shall be stable. The results are in line with findings of Aoyama, 2001 and Aoyama et al., The stable machining ensures the uniform and flat craters on the work surface by shallow and deep cryogenic treated wire s. Deep and shallow cryogenic treated wires promote better SR at high values of wire tension. 93

5 Strong interaction is observed between pulse width and wire tension for the improvement of SR. T y p e o f w ir e ( A ) Inte r actio n P lot for S R (µm) Data M eans T y p e o f w ire(a ) D C T W E S C T W E U T W E P u lse w id th ( B ) W ir e t e n s io n ( C ) P u lse w id th (B ) DCTWE- Deep cryogenic treated wire SCTWE- Shallow cryogenic treated wire UTWE- Untreated wire Figure 6. Interaction Plot for SR Wire Wear Ratio (WWR) The main effects plot for WWR is depicted in Figure 6.5, which shows the effect of process parameters, namely type of wire (untreated and cryogenic treated wire s), pulse width and wire tension on wire wear ratio (WWR) in WEDM. It can be seen from Figure 6.5a that the WWR value obtained with untreated wire is high; whereas the shallow and deep cryogenic treated wire shows less WWR. The increase in pulse width (Fig. 6.5b) increases the WWR, whereas WWR decreases slightly with the increase in wire tension (Fig. 6.5c). The analysis of variance (ANOVA) for the WWR data is shown in Table 6.3. Type of wire, pulse width, wire tension, the interaction between type of wire and wire tension and the interaction between pulse width and wire tension are the significant factors at 95% confidence level. The interaction plot () for WWR is shown in Figure 6.6. It can be observed from the Figure that deep cryogenically treated wire with low value of pulse width gives minimum value of WWR. 9

6 Main Effects Plot for WWR a Type of wire(a) b Pulse width(b) Mean c DCTWE SCTWE W ire tension(c) UTWE DCTWE- Deep cryogenic treated wire SCTWE- Shallow cryogenic treated wire UTWE- Untreated wire Figure 6.5 Main Effects Plot for WWR Table 6.3 ANOVA for WWR Source DF Seq SS Adj SS Adj MS F P A * B * C * A*B A*C * B*C * Error Total Type of wire, B Pulse width, C Wire tension *Significant at 95% level The discharge is uniformly distributed across the surface. The stable machining ensures the uniform and flat craters on the wire s and less WWR. Deep and shallow cryogenic treated wire promotes better WWR at high values of wire tension. Strong interaction is observed between pulse width and wire tension for the improvement of WWR. 95

7 T y pe o f wir e ( A ) Interaction Plot for WWR P ulse width( B ) Ty pe of w ire(a ) DC TW E SC TW E U TW E Pulse w idth(b) W ir e te nsion( C ) Figure 6.6 Interaction Plot for WWR On the basis of above mentioned experimental work, following hypothesis has been made; When the ferrous material is machined with cryogenically treated brass wire, 1) MRR gets increased 2) SR gets improved 3) WWR gets improved The justification of above mentioned hypothesis for individual response characteristic is given as: Material Removal Rate (MRR) The review of literature reveals the erosion mechanism in WEDM, which not only depends on the selection of process parameters, but also significantly affected by the wire s with different electrical conductivities. This observation can be verified from the experimental results shown in SEM photographs (Fig. 6.7, Fig. 6.8 and Fig. 6.9) of workpieces machined by untreated, shallow and deep cryogenically treated wire s taken at pulse width of 1.2µs and wire tension of 2 dan. The machined workpiece with untreated brass wire exhibit severe ripples. The shape of the craters is uneven and they are distinct, bigger in size and deep with high density 96

8 of global appendages. SEM photographs of workpieces machined by shallow and deep cryogenic treated wire are shown in Figure 6.8 and Figure 6.9 respectively, which clearly exhibits that craters are even, shallow and well defined. Fine, shallow and almost flat craters can be seen in Figure 6.9. The possible reason behind this may be explained as follows: There is significant increase in conductivity of brass wire after deep and shallow cryogenic treatments (Table 5.). Electrical resistance is produced when the movement of free electrons is resisted due to their continuous collision with positive ions during the directional movement. The presence of any gas or diffusive distribution holes in the material decreases the heat conductivity coefficient. Flaws in the materials causes decrease in heat conductivity coefficient. When the ordered structure is formed in the alloy, the average free distance of electrons is increased. The heat conductivity coefficient will be more in the ordered structure than that in the disordered material (Zhisheng et al., 2003). When brass or any metal is formed into wires, the material develops residual stresses and the crystal molecules comprising the wire are trapped in a random pattern. This haphazard placement causes obstacles for electrons and this obstruction can slow down the flow of electrons. As the cryogenic temperature work at atomic level, it can be argued that at cryogenic temperatures, these crystals align in uniform and compact structure through the removal of kinetic energy. When the brass wire is returned to ambient temperature after cryogenic treatment cycle, new consistent, compact pattern is maintained. It is also reported that (Aoyama et al., 1999), the electrical conductivity is in proportion to the heat conductivity which would intuitively expect to correspond to an increased material removal rate under an effective electric-discharge. SEM examination reveals some micro-cavities in the untreated brass wire section (Fig. 6.10a). These micro cavities are reduced after the deep and shallow cryogenic treatments (Fig (b) & (c)). The brass wire develops residual stresses during its manufacture and crystal molecules develops non uniform pattern. As the temperature decreases in cryogenic treatment chamber, the atomic bond becomes weak and the crystal structure of brass reverts to its original state. Any residual stress gets eliminated. Therefore, more uniform and compact pattern is maintained. The wire strength might have been enhanced owing to 97

9 the fine grains and less micro-cavities. Further, the micro cavities present in the wire before cryogenic treatment makes the electron movement suffers. This adversely affects the material capacity of heat transmission and electrical conductivity. However, the quantity of the micro cavities in brass wire after Deep craters Figure 6.7 SEM (X 0) Photograph of Workpiece Surface Machined by Untreated Wire Electrode (at Pulse Width - 1.2µs and Wire Tension 2 dan) Uniform craters Figure 6.8 SEM (X 0) Photograph of Workpiece Surface Machined by Shallow Cryogenic Treated Wire Electrode (at Pulse Width -1.2µs and Wire Tension 2 dan) 98

10 Even and uniform distribution of craters deep and shallow cryogenic treatment is clearly less than that before cryogenic treatment. The heat transmission and electrical conductivity is increased and soundness of wire is obviously increased. A good conductivity wire allow for applying high energy to the process. Therefore, higher current will be delivered to wire by voltage generator of the WEDM. Owing to increase in frequency of discharge, the faster sparking in discharge channel takes place. It may be assumed that chain of discharges takes place within single discharge and several discharging pulses takes place within single pulse. Owing to the bridging effect, the insulating strength of the dielectric decreases. Early explosion of the plasma channel takes place owing to short circuiting. The series of discharge starts along the wire surface area. The increase in frequency of discharges causes faster erosion from the workpiece surface. As the plasma channel becomes enlarged and widened, sparking becomes even and uniform. Therefore material removal rate and surface finish both gets improved. Surface Roughness (SR) Figure 6.9 SEM (X 0) Photograph of Workpiece Surface Machined by Deep Cryogenic Treated Wire Electrode (at Pulse Width -1.2µs and Wire Tension 2 dan ) The effect of untreated, shallow and deep cryogenically treated wire on SR is evident from the SEM photographs shown in Figures 6.11a, 6.12a and 6.13a respectively. These photographs were taken at pulse width of 0.µs and wire tension 99

11 Micro cavities Figure 6.10a Figure 6.10b Figure 6.10c Figure 6.10 SEM Images of Brass Wire; (a) Before Cryogenic Treatment, (b) After Shallow Cryogenic Treatment, (c) After Deep Cryogenic Treatment of 0.6daN. Figure 6.11a shows the work piece surface machined by untreated wire. The machined surface exhibits non-uniform craters. The craters are uneven, distinct and deep. The Figures 6.12a and 6.13a clearly exhibits that craters are even, shallow and well defined overlapped. The explanation for the possible reason for improvement in SR is explained as follows: The machining with more conductive cryogenic treated wire s makes the discharge passage enlarged and widened. The generated debris from the spark gap is easily evacuated. The discharge is uniformly distributed in the discharge channel in the gap, which results into reduction in the relative difference in electric field between micro-peaks. The smaller and shallow craters are produced due to discharge of microcurrent at each potential discharge point. The surface quality of WEDM is associated 100

12 with the material removal per discharge, which is determined by the pulse energy per discharge. The value of surface roughness increases as the pulse width increases. The increased pulse width results into longer discharge time, which leads to higher discharge energy. The machined surface gets deteriorated owing to increased diameter and larger depth of craters. Increase in wire tension causes slight increase in surface roughness but further increase causes improvement in surface roughness due to decrease in cut width. Energy dispersive spectroscopy (EDS) was used to identify the elements on the machined surface. EDS analysis (Fig.6.11b) reveals that some amount of wire material elements (Cu and Zn) gets deposited on the workpiece surface. The transfer of Cu and Zn elements occurs during sparking, which were deposited on the machined workpiece surface in addition to C and Fe. Shock impulses cause the welding of the detached elements on the worpiece. Figure 6.12b and 6.13b reveals that Zn is eliminated, where as Cu remains present on the machined surface. The possible reason may be owing to more discharge energy by cryogenically treated wire. The boils off the Zn, which is flushed with dielectric fluid during machining. There was clearly less Cu accreting on the surface machined by shallow and deep cryogenically treated wire. The machined surface properties are affected by the transportation of tool material elements on to the workpiece. a Deep and uneven craters b Figure 6.11 SEM (X 0) (a) and EDS (b) Photograph of Workpiece Surface Machined by Untreated Wire (Pulse Width- 0. µs and Wire Tension -0.6daN ) 101

13 a Shallow and even craters b Figure 6.12 (a) SEM (X 0) and( b) EDS Photograph of Workpiece Surface Machined by Shallow Cryogenic Treated Wire (Pulse Width- 0. µs and Wire Tension -0.6daN) a b Smaller and Shallow craters Figure 6.13 (a) SEM (X 0) and( b) EDS Photograph of Workpiece Surface Machined by Deep Cryogenic Treated Wire (Pulse Width- 0. µs and Wire Tension- 0.6daN) Wire Wear Ratio (WWR) Figure 6.1a shows the SEM photographs of wire surface machined by untreated wire in WEDM at pulse width of 1.2 µs and wire tension of 2 dan. The surface of untreated wire after WEDM shows deep and bigger craters. SEM photograph of wire surface machined by shallow and deep cryogenic treated wire in WEDM at pulse width of 1.2 µs and wire tension of 1.3daN are shown in Figure 6.1b and 6.1c respectively. Fine, shallow and almost flat craters can be seen in these Figures. The possible reason behind this may be explained as follows: 102

14 The higher current will be delivered to cryogenically treated wire by voltage generator of the WEDM machine as a good conducive wire delivers high energy to the process. Owing to increase in frequency of discharge, the faster sparking in discharge channel takes place, which causes faster erosion from the wire and workpiece surface. As the plasma channel becomes enlarged and widened, sparking becomes even and uniform. Less WWR of cryogenically treated wires as compared to untreated wires can be attributed to the fact that cryogenic treated wires have high conductivity, which enables the quicker propagation of heat along the surface resulting in less deep craters than untreated wires. Therefore WWR gets improved. The surface of wire after WEDM was also analyzed with EDS as shown in Figure 6.15b, Figure 6.15c, Figure 6.15d. The Figures suggest that some material of work piece surface (Fe) get deposited on the wire surface. This means that during normal sparking, transfer of workpiece material to wire material takes place. But in case of deep and shallow cryogenic treated wire s, transfer of Fe element to wire is not visible. The Fe particles are not migrated to wire because high energy quickly explodes the plasma channel, which flushes the Fe particle debris. a b c Fig. 6.1 SEM Photograph of (a) Untreated, (b) Shallow Cryogenic Treated and (c) Deep Cryogenic Treated Wire Machined by WEDM at Pulse Width- 1.2 µs and Wire Tension -1.3daN. 103

15 a b c Fig EDS Photograph of (a) Untreated, (b) Shallow Cryogenic Treated and (c) Deep Cryogenic Treated Wire Electrode Machined by WEDM at Pulse Width- 1.2 µs and Wire Tension -1.3 dan. 6.2 EFFECT OF PROCESS PARAMETERS ON PERFORMANCE CHARACTERISTICS IN WEDM (PHASE-II) The results of Taguchi based experiments of Phase- II have been discussed in this section. The effect of individual process parameters and their interactions on the performance characteristics have been studied in this phase. From the experimental data (Chapter-5), the mean value and the S/N ratio of response characteristics for each parameter at different levels have been calculated. The main effects curves for mean and S/N ratio have been plotted to study the effect of individual parameter on the response characteristics. The analysis of variance (ANOVA) has been performed to identify the significant factors and their effect on response characteristics. The optimal setting of process parameters has been done by analyzing the response curves and ANOVA Tables Group-I (L 18 OA) The effect of individual process parameters (type of wire (deep cryogenically and untreated wire ), pulse width, time between two pulses, wire tension, servo reference mean voltage) on the selected response characteristics (MRR, SR, WWR) has been discussed in this section. 10

16 Effect on Material Removal Rate (MRR) The average values of MRR and S/N ratio for each parameter at levels 1, 2 and 3 are calculated from Table 5.13 and are given in Table C1 and Table C2 respectively (Appendix-III). The analysis of main effects plot for means, main effects plot for S/N ratio and interaction plot for mean and S/N ratios at 95% confidence level is shown in Figure 6.16, Figure 6.17, Figure 6.18 and Figure 6.19 respectively. It can be observed from these Figures that type of wire (A), pulse width (B), time between two pulses(c) and wire tension (D) have quite considerable effect as compared to servo reference mean voltage(e). A dominant effect of pulse width on MRR is revealed. It can be observed from Figure 6.16a and Figure 6.17a that more MRR is observed from deep cryogenic treated wire. Enhanced MRR with deep cryogenic treated wire than untreated wire is influenced by additional energy expended to the process, which is provided by more electron emission due to high conductive deep cryogenic treated wire. Another observation from the present experiment is that with the increase in pulse width, the MRR improves (Fig. 6.16b and Fig. 6.17b). This is because pulse width has large impact on the input energy. Increase in pulse width increases the discharge current. With increase in discharge current, the ions and electrons move towards their respective s more vigorously, resulting in avalanche of ions and electrons. Machining speed becomes faster owing to increase in Main Effects Plot for Means (MRR) 60 T y p e of wir e ( A ) P ulse width ( B ) T im e b etwe en two p ulses ( C ) a b c Mean of Means UTWE d DCTWE Wir e te nsion( D) S er v o r efe r e nce m ea n v o ltage ( E ) e DCTWE- Deep cryogenic treated wire UTWE- Untreated wire Figure 6.16 Main Effects Plot for Means for MRR 105

17 Main Effects Plot for SN ratios (MRR) 3 a T y pe of wir e ( A ) b P ulse width ( B ) T im e b etwe en two p ulses ( C ) c 32 Mean of SN ratios UTWE DCTWE Wir e te nsio n( D) S er v o r efe r e nce m ea n v o ltage ( E ) d Signal-to-noise: Larger is better e DCTWE- Deep cryogenic treated wire UTWE- Untreated wire Figure 6.17 Main Effects Plot for SN Ratios (MRR) discharge current. This causes more discharge per unit time. Faster erosion from the workpiece takes place due to faster frequency within the discharge. Time between two pulses (Fig. 6.16c and Fig. 6.17c) is another factor that shows the variation in mean and S/N ratio. With the increase in time between two pulses, MRR decreases. As the time between two pulses becomes short, it leads to more number of discharges, which increases the MRR. Wire tension also play significant role in WEDM as revealed from Figure 6.16d and Figure 6.17d. Within considerable range (0.6 dan to 1.3daN) an increase in wire tension decreases the MRR. The reason for decrease in MRR is mainly attributed to wire vibration amplitude, which decreases the cut width and subsequently material removal is decreased. Servo reference voltage (E) seems to have negligible effect on MRR. The reason may be that servo reference mean voltage is related with discharge wait time. Wider discharge gap is produced due to increase in servo reference mean voltage. The discharge conditions become stable due to decrease in discharge cycles and machining conditions become stable. The interactions (Fig and Fig. 6.19) between type of wire (A) and time between two pulses(c) influence the mean MRR significantly. The lower value of time between two pulses with deep cryogenically treated wire gives more MRR. 106

18 Interaction Plot for Means (MRR) T ype of wire(a ) UT WE D CT WE 55 T ype of w ir e(a ) T ime between two pulses (C) UTWE DCTWE T ime between two pulses (C) DCTWE- Deep cryogenic treated wire UTWE- Untreated wire Figure 6.18 Interaction Plot for Means (MRR) Selection of optimal levels The analysis of results leads to the conclusion that second level of parameter A2 (Type of wire), third level of parameter B3 (Pulse width), first level of parameter C1 (Time between two pulses), first level of parameter D1 (Wire tension) play UTWE T ype of wir e(a ) DCTWE Signal-to-noise: Larger is better Interaction Plot for SN ratios (MRR) T ime between two pulses (C) T ype of wire(a ) UT WE D CT WE T ime between two pulses (C) DCTWE- Deep cryogenic treated wire UTWE- Untreated wire Figure 6.19 Interaction Plot for SN ratios (MRR) 107

19 significant role for maximization of MRR. Factor E (Servo reference voltage) is having least significant effect for increasing MRR. It can be further noticed that theses parameters corresponds to the highest value of S/N ratio. The analysis of variance (ANOVA) of mean and analysis of variance of S/N ratio on data was performed to identify the significant factors in order to establish the relative significance of parameters. Table 6. and Table 6.5 presents the ANOVA results for mean and S/N ratio respectively. Table 6. Analysis of Variance of Means (MMR) Source DF Seq SS Adj SS Adj MS F P A * B * C * D * E A*C * Residual Error Total Type of wire, B- Pulse width, C- Time between two pulses, D- wire tension, E- Servo reference mean voltage *- significant at 95% confidence level Table 6.5 Analysis of Variance of S/N Ratio (MMR) Source DF Seq SS AdjSS Adj MS F P A * B * C * D * E * A*C * Residual Error Total Type of wire, B- Pulse width, C- Time between two pulses, D- wire tension, E- Servo reference mean voltage *- significant at 95% confidence level Type of wire (A), Pulse width (B), Time between two pulses (C), Wire tension (D) and the interaction between Type of wire (A) and Time between two pulses (C) are found to have significant effect on MRR. Model adequacy The adequacy of model for MRR (Fig. 6.20) is checked by means of normal probability plot, plots of histogram, plots of residuals verses fitted value and plots of 108

20 residuals verses order. The histogram shows that the residuals are normally distributed. The residuals appear to follow a straight line in normal probability plot. R esidual P lots for M eans 99 No rm al P ro b ab ilit y P lo t V ersu s Fit s Percent 90 Residual Residua l F itte d V a lue 60.8 H ist o g ram V ersu s O rd er Frequency Residual Residua l O bse r va tion O r de r Figure 6.20 Residual Plots for Means No evidence of non normality, skewness or unidentified variables exists. It appears that residuals are randomly scattered about zero. No evidence exists that error terms are correlated with each other. Present discussion shows the adequacy of predictive model Effect on Surface Roughness (SR) The average values of SR and S/N ratio for each parameter at levels 1, 2 and 3 are calculated from Table 5.13 and are given in Table C3 and Table C respectively (Appendix-III). The analysis of main effects plot for means, main effects of plots for S/N ratio and interaction plot of means and S/N ratio at 95% confidence level is shown in Figure 6.21, Figure 6.22, Figure 6.23 and Figure 6.2 respectively. It can be observed from Figure 6.21 and Figure 6.22 that the type of wire (A), Pulse width (B), time between two pulses(c), have quite considerable effect as compared to wire tension (D) and servo reference voltage (E). It can be observed from Figure 6.21a that the improvement in surface roughness is more in deep cryogenic treated as compared to untreated wire. The reason for improvement in SR is that the deep cryogenically treated wire modifies the plasma channel. The plasma channel becomes enlarged and widened. The sparking is uniformly distributed along 109

21 Main Effects Plot for Means (SR) T y p e o f wir e ( A ) P ulse wid th ( B ) T im e b e twe e n two p ulse s ( C ) a b c Mean of Means UTWE d DCTWE Wir e tensio n( D) S e r v o r e fe r e nce m e an v o lta ge ( E) e DCTWE- Deep cryogenic treated wire UTWE- Untreated wire Figure 6.21 Main Effects Plot for means (SR) the surface of wire, hence electric density of sparks decreases. This leads to uniform erosion on the workpiece surface. The results are in line with finding of Sachet et al. (200). It can be observed from Figure 6.21b and Figure 6.22b that the surface roughness increases with increase in pulse width. Main Effects Plot for SN ratios (SR) -7-8 T y p e o f wir e ( A ) P ulse wid th ( B ) T im e b e twe e n two p ulse s ( C ) a b c Mean of SN ratios UTWE d DCTWE Wir e tensio n( D) S e r v o r e fe r ence m e an v o lta ge ( E ) S ignal-to-noise: S maller is better e DCTWE- Deep cryogenic treated wire UTWE- Untreated wire Figure 6.22 Main Effects Plot for SN ratios (SR) 110

22 It may be attributed to the fact that a short pulse width delivers less energy in the gap between wire and workpiece, which causes less vaporization on the work piece surface. Time between two pulses is another factor that shows the variation in main effect of plot for mean and S/N ratio (Fig. 6.21c and Fig. 6.22c). Increase in time between two pulses decreases the surface roughness value as cutting speed is decreased due to more time taken by pulse energy to revitalize. As the time between two pulses becomes short, it leads to more number of discharges. Insufficient time between two pulses results in abnormal arc discharge and damage the workpiece surface. It can be further observed from Figure 6.21d and Figure 6.22d that the wire tension and servo reference voltage does not have pronounced effect on SR owing to stable machining. So, these parameters do not contribute any significant effect on surface roughness. Since surface roughness is smaller the better type characteristics, therefore smaller mean values and their corresponding S/N vales are considered for significant parameters. The interaction between type of wire and pulse width (Fig and Fig. 6.2) affects the SR significantly. These Figures suggest the more improvement in SR at middle value of pulse width with deep cryogenically treated wire owing to series discharge effect. Interaction Plot for Means (SR) T ype of wir e(a ) UT W E D CT W E T ype of wire(a ) P ulse width (B) UTWE DCTWE P ulse width (B) DCTWE- Deep cryogenic treated wire UTWE- Untreated wire Figure 6.23 Interaction Plot for Means (SR) 111

23 Interaction Plot for S N ratios (SR) T ype of wire(a ) UTWE DCTWE P ulse w idth (B) T ype of wir e(a ) UT W E D CT W E P ulse width (B) DCTWE- Deep cryogenic treated wire UTWE- Untreated wire S ignal-to-noise: S maller is better Selection of optimal levels Figure 6.2 Interaction Plot for SN ratios (SR) The analysis of results leads to conclusion that second level of parameter A (A2), First level of parameter B (B1), third level of parameter C (C3) are significant factors. Factor D and Factor F is having least significant effect for minimizing SR. It can be further noticed that theses parameters corresponds to highest value of SN ratio. Table 6.6 Analysis of Variance for Means (SR) Source DF Seq SS AdjSS Adj MS F P A * B * C * D E A*B * Residual error Total Type of wire, B- Pulse width, C- Time between two pulses, D- wire tension, E- Servo reference mean voltage *- significant at 95% confidence level The analysis of variance (ANOVA) of mean and S/N ratio on the data was performed to identify the significant factors in order to establish the relative significance of parameters. Table 6.6 and Table 6.7 presents the ANOVA results for the mean and 112

24 Table 6.7 Analysis of Variance for SN ratios (SR) Source DF Seq SS AdjSS Adj MS F P A * B * C * D E A*B * Residual error Total Type of wire, B- Pulse width, C- Time between two pulses, D- wire tension, E- Servo reference mean voltage *- significant at 95% confidence level S/N ratio respectively. The type of wire (A), pulse width (B), time between two pulses (C) and the interaction between Type of wire (A) and pulse width are found to have significant effect (significant at 95% confidence interval) on SR. Model adequacy The adequacy of model for SR is checked by means of normal probability plot, plots of histogram, plots of residuals verses fitted value, plots of residuals verses order. All the four plots are illustrated in Figure The residuals appear to follow a straight line in Figure 6.25a. In Figure 6.25b it appears that residuals are randomly scattered about zero. Residual seems to be normally distributed in Figure 6.25c. R esidual P lots for M eans Percent a No rm al P ro b ab ilit y P lo t Residual b V ersu s Fit s R e sidua l F itte d V alue H ist o g ram 0.30 V ersu s O rd er Frequency c Residual d R e sidua l O bse r v a tion O r de r Figure 6.25 Residual Plots for Means 113

25 It appears from Figure 6.25d that residuals are randomly scattered about zero. No evidence exists that error terms are correlated with each other. Present discussion shows the adequacy of predictive model Effect on Wire Wear Ratio (WWR) The average value of WWR and their S/N ratio for each parameter at levels 1, 2, 3 are calculated from Table 5.13 and are given in Table C5 and Table C6 (Appendix- III) respectively. The analysis of main effects plot for means, main effects plots for S/N ratio and interaction plot of means and S/N ratio at 95% confidence level is shown in Figure 6.26, Figure 6.27, Figure 6.28 and Figure 6.29 respectively. It can be observed from Figure 6.26(a) and Figure 6.27 (a) that deep cryogenic treated wire lowers the WWR. The high value of WWR with untreated wire could be due to the reason that low conductive wire causes poor dielectric conductivity. Due to abnormal electric discharges, more material is eroded form the wire surface. The high conductive wire s generate stable discharge due to distribution of even energy in the gap and series discharge effect. This results in small amount of material removal from the wire in a unit time that lowers the WWR. Therefore, craters on wire are broader and shallow. Pulse width (Fig. 6.26b and Fig. 6.27b) shows the variation in mean and S/N ratio. Significant increase in Main Effects Plot for Means (WWR) T y p e o f wir e( A ) P ulse wid th ( B ) T im e b e twe e n two pulse s ( C ) a b c Mean of Means UT WE d D CT WE Wir e te nsio n( D) S e r v o r e fe r ence m e an v o lta ge ( E ) e DCTWE- Deep cryogenic treated wire UTWE- Untreated wire Figure 6.26Main Effects Plot for Means (WWR) 11

26 Mean of SN ratios a d UTWE 0.6 T y pe of wir e ( A ) 1.3 Main Effects Plot for SN ratios (WWR) DCTWE Wir e te nsio n( D) S er v o r efe r e nce m ea n v o ltage ( E ) 20 S ignal-to-noise: S maller is better b e P ulse width ( B ) T im e b etwe en two p ulses ( C ) 35 c DCTWE- Deep cryogenic treated wire UTWE- Untreated wire Figure 6.27 Main Effects Plot for SN ratios (WWR) WWW is observed with the increase in pulse width. Higher energy is expended in the gap with the increase in pulse width, which significantly erodes the wire and workpiece. It is also observed from Figure 6.26c and Figure 6.27c that time between two pulses influence the WWR. As the time between two pulses becomes short it leads to more number of discharges, which subsequently affect the WWR. It is revealed that wire tension (Fig. 6.26(d), Fig. 6.27(d)) and servo reference mean voltage (Fig (e) and Fig. 6.27(e)) play insignificant role in WWR. Uniform wear is observed as discharge conditions become more stable within all three levels of wire tension and servo reference voltage. The interaction between pulse width and time between two pulses (Fig and Fig. 6.29) affects the WWR significantly. It can be seen from Figure.28 and Figure 6.29 that at high value of time between two pulses low WWR is observed owing to its higher S/N ratio. This may be attributed to the fact that small energy is dispended to the discharge gap at pulse width of 0. µs and time between two pulses of 16 µs, which significantly reduces the WWR. Higher WWR is observed at time between two pulses of 10 µs and pulse width of 1.2 µs owing to more energy delivered to the gap. Selection of optimal levels The analysis of results leads to conclusion that second level of parameter A 115

27 (A2), First level of parameter B (B1), third level of parameter C (C3) are significant factors. Factor D and Factor F is having least significant effect for minimizing wire Interaction Plot for Means (WWR) P ulse width (B) P ulse width (B) T ime between two pulses (C) T ime between two pulses (C) Figure 6.28 Interaction Plot for Means (WWR) Interaction Plot for SN ratios (WWR) P ulse width (B) P ulse width (B) T ime between two pulses (C) T ime between two pulses (C) S ignal-to-noise: S maller is better Figure 6.29 Interaction Plot for SN ratios (WWR) 116

28 wear ratio. It can be further noticed that theses parameters corresponds to highest value of S/N ratio. The analysis of variance (ANOVA) of mean and S/N ratio on data was performed to identify the significant factors in order to establish the relative significance of parameters. Table 6.8 and Table 6.9 presents the ANOVA results for mean and S/N ratio. Type of wire (A), pulse width (B), time between two pulses (C), the interaction between pulse width and time between two pulses are found to have significant effect (significant at 95% confidence interval) on WWR. Wire tension and Servo reference voltage are insignificant for change in WWR. Table 6.8 Analysis of Variance for Means (WWR) Source DF Seq SS AdjSS Adj MS F P A * B * C * D E B*C * Residual error Total Type of wire, B- Pulse width, C- Time between two pulses, D- wire tension, E- Servo reference mean voltage *- significant at 95% confidence level Table 6.9 Analysis of Variance for SN ratios (WWR) Source DF Seq SS AdjSS Adj MS F P A * B * C * D E B*C * Residual error Total Type of wire, B- Pulse width, C- Time between two pulses, D- wire tension, E- Servo reference mean voltage *- significant at 95% confidence level Model adequacy The adequacy of model for WWR is checked by means of normal probability plot, plots of histogram, plots of residuals verses fitted value, plots of residuals verses 117

29 order. All the four plots are illustrated in Figure For the WWR data, it appears that residuals are normally distributed and randomly scattered about zero. No evidence exists that error terms are correlated with each other, which confirms the adequacy of predictive model. Residual Plots for Means Percent a No rmal Pro babilit y Plo t Residual 0.00 Residual b Versus Fit s 0.0 Fitted Value 0.06 Hist o gram Versus Order Frequency c Residual d Residual Obser vation Or der Figure 6.30 Residual Plots for Means Group-II (L 18 OA) The effect of individual process parameters ( type of wire (untreated and shallow cryogenically treated), pulse width, time between two pulses, wire tension, servo reference mean voltage) of Group- II (Chapter-5) of Phase- II on the selected response characteristics (MRR, SR,WWR) has been discussed in this section Effect on Material Removal Rate (MRR) The average values of MRR and S/N ratio for each parameter at levels 1, 2 and 3 are calculated from Table 5.1 and are given in Table C7 and Table C8 respectively (Appendix-III). The analysis of main effects plot for means, main effect of plots for S/N ratio and interaction plot for means and S/N ratio at 95% confidence level is shown in Figure 6.31, Figure 6.32, Figure 6.33 and figure 6.3 respectively. It can be observed from Figure 6.31 and Figure 6.32 that the process parameters, namely type of wire (A), pulse width (B), time between two pulses(c), wire tension (D) have quite 118

30 considerable effect as compared to servo reference voltage (E). It can be observed from Figure 6.31a and Figure 6.32a that more MRR is observed from shallow cryogenically treated wire. This is owing to the fact that high conductive wires give more energy to the process as additional conductivity promotes more electron emission. The temperature of the wire drops due to less resistance. Since joule heating is important for EDM wires, so most of the joule heating is dissipated during the machining with high conductive wires. Pulse width is another factor which shows the variation on mean and S/N data. Machining speed becomes faster due to increase in discharge current. The discharge will last longer as the pulse width is increased (Figure 6.31b, Figure 6.32b). It is further observed from Figure 6.31c and Figure 6.32c that the time between two pulses influences the MRR. Wire tension also play significant role in WEDM as revealed from Figure 6.31d and Figure 6.32d. Within considerable range (0.6 dan to 1.3 dan) an increase in wire tension decreases the MRR. The reason for decrease in MRR is mainly attributed to the stabilization of vibration amplitude, which decreases the cut width and subsequently material removal is decreased. Servo reference voltage (E) seems to have negligible effect on MRR. The reason has already explained in section ( ). The interactions (Fig and Fig. 6.3) between type of wire (A) and time between two pulses(c) influence the mean MRR significantly. Main Effects Plot for Means (MRR) T y p e of wir e ( A ) P ulse width ( B ) T im e b etwe en two p ulses ( C ) a b c 0 Mean of Means UTWE SCTWE Wir e te nsion( D) S er v o r efe r e nce m ea n v o ltage ( E ) d e SCTWE- Shallow cryogenic treated wire UTWE- Untreated wire Figure 6.31 Main Effects Plot for Means (MRR) 119

31 Main Effects Plot for SN ratios (MRR) 3 32 a T y pe of wir e ( A ) b P ulse width ( B ) T im e b etwe en two p ulses ( C ) c Mean of SN ratios d UTWE SCTWE Wir e te nsio n( D) S er v o r efe r e nce m ea n v o ltage ( E ) e SCTWE- Shallow cryogenic treated wire UTWE- Untreated wire Signal-to-noise: Larger is better Figure 6.32 Main Effects Plot for SN Ratios Interaction Plot for Means (MRR) T ype of wire(a ) UT WE SCT WE Type of wire(a ) T ime between two pulses (C) UTWE SCTWE Tim e between two pulses (C) SCTWE- Shallow cryogenic treated wire UTW- Untreated wire Figure 6.33 Interaction Plot for Means (MRR) Selection of optimal levels The analysis of results leads to the conclusion that second level of parameter A2(Type of wire), third level of parameter B3 (Pulse width), first level of parameter C1(Time between two pulses), first level of parameter D1 (Wire tension) play 120

32 significant role for the maximization of MRR. Factor E (Servo reference voltage) is having least significant effect for increasing MRR. It can be further noticed that these parameters corresponds to highest value of S/N ratio UTWE T ype of wir e(a ) SCTWE Signal-to-noise: Larger is better Interaction Plot for SN ratios (MRR) T ime between two pulses (C) Figure 6.3 Interaction Plot for SN Ratios (MRR) T ype of wire(a ) UT WE SCT WE T ime between two pulses (C) SCTWE- Shallow cryogenic treated wire UTWE- Untreated wire The analysis of variance (ANOVA) of mean and analysis of variance of S/N ratio on data was performed to identify the significant factors in order to establish the relative significance of parameters. Table 6.10 and Table 6.11 presents the ANOVA results for MRR and S/N ratio respectively. Type of wire (A), Pulse width (B), Time between two pulses (C), Wire tension (D) and the interaction between Type of wire (A) and Time between two pulses (C) are found to have significant effect on MRR. Table 6.10 Analysis of Variance for Means (MRR) Source DF Seq SS AdjSS Adj MS F P A * B * C * D * E A*C * Residual error Total Type of wire, B- Pulse width, C- Time between two pulses, D- wire tension, E- Servo reference mean voltage *- significant at 95% confidence level 121

33 Table 6.11 Analysis of Variance for SN Ratios Source DF Seq SS AdjSS Adj MS F P A * B * C * D * E * A*C * Residual error Total Type of wire, B- Pulse width, C- Time between two pulses, D- wire tension, E- Servo reference mean voltage *- significant at 95% confidence level Model adequacy As shown in Figure 6.35, the adequacy of model for MRR is checked by means of normal probability plot, plots of histogram, plots of residuals verses fitted value and plots of residuals verses order. The adequacy of model is confirmed form the fact that the residuals are normally distributed and there is no evidence of skewness or unidentified variables. R esidual Plots for Means 99 No rmal P ro b ab ilit y P lo t Versu s Fit s 90 2 Percent 10 Residual Residual Fitted V alue 60 Hist o g ram Versu s Ord er Frequency Residual Residual O bser vation O rder Figure 6.35 Residual Plots for Means Effect on Surface Roughness (SR) The average values of SR and S/N ratio for each parameter at levels 1, 2 and 3 122

34 are calculated from Table 5.1 and are given in Table C9 and Table C10 respectively (Appendix-III). The analysis of the main effects plot for means, main effects plots for S/N ratio and interaction plot of means and S/N ratio at 95% confidence level is shown in Figure 6.36, Figure 6.37, Figure 6.38 and Figure 6.39 respectively. It can be observed from Figure 6.36 and Fig that type of wire (A), Pulse width (B), time between two pulses(c), have quite considerable effect as compared to wire tension (D) and servo reference voltage (E). Improvement in surface roughness is observed with shallow cryogenic treated wire as compared with untreated wire (Figure 6.36a and Figure 6.37a). Shallow cryogenic treatment enhances the conductivity of wire. The probable reason for improvement of SR has already been explained ( ). Moreover, due to high conductivity of shallow wire more discharge energy is delivered to process and wire keeps good mechanical properties at high energy due to high possible pre-load, which improves the surface finish and accuracy. The results are in line with finding of (Sachet et al. 200). It can be observed from Figure 6.36b and Figure 6.37b that surface roughness increases with increase in pulse width. A short pulse width delivers less energy in the gap between wire and. Main Effects Plot for Means (SR) T y pe of wir e ( A ) P ulse width ( B ) T im e b etwe en two p ulses ( C ) a b c Mean of Means (µm) UTWE SCTWE Wir e te nsio n( D) S er v o r efe r e nce m ea n v o ltage ( E ) d e SCTWE- Shallow cryogenic treated wire UTWE- Untreated wire Figure 6.36 Main Effects Plot for Means 123

35 Main Effects Plot for SN ratios (SR) -7 T y p e of wir e ( A ) P ulse wid th ( B ) T im e b etwe en two p ulses ( C ) a b c Mean of SN ratios UTWE SCTWE Wir e te nsion( D) S e r v o r e fe r ence m e an v o lta ge ( E ) d e SCTWE- Shallow cryogenic treated wire UTWE- Untreated wire Signal-to-noise: Smaller is better Figure 6.37 Main Effects plot for SN Ratios (SR) workpiece, which causes less vaporization on the work piece surface. Time between two pulses is another factor that shows the variation in main effect of plot for mean and S/N ratio (Figure 6.36C and Figure 6.37C). As the time between two pulses becomes short, it leads to more number of discharges. Insufficient time between two pulses results in abnormal arc discharge and damage to the workpiece surface. It can be further observed from Figure 6.36d and Figure 6.37d that wire tension and servo reference voltage does not have pronounced effect on surface roughness. These parameters do not contribute any significant effect on surface roughness. Since surface roughness is smaller the better type characteristics, therefore smaller mean values and their corresponding S/N vales are considered. The interaction between type of wire and pulse width (Figure 6.38 and Figure 6.39) affects the SR significantly. Selection of optimal levels The analysis of results leads to conclusion that second level of parameter A (A2), first level of parameter B (B1), third level of parameter C (C1) are significant factors. Factor D and Factor F are having least significant effect for minimizing the SR. It can be further noticed that theses parameters corresponds to highest value of S/N ratio. 12

36 T ype of w ir e(a ) Interaction Plot for Means (SR) T ype of wire(a ) UT WE SCT WE P ulse width (B) P ulse width (B) 2.00 SCTWE- Shallow cryogenic treated wire UTWE- Untreated wire 2.00 UTWE SCTWE Figure 6.38 Interactions Plot for Means (SR) Interaction Plot for S N ratios (SR) T ype of wir e(a ) UT W E SCT WE T ype of wire(a ) UTWE SCTWE Signal-to-noise: Smaller is better P ulse w idth (B) P ulse width (B) SCTWE- Shallow cryogenic treated wire UTWE- Untreated wire Figure 6.39 Interaction Plot for SN Ratios (SR) The analysis of variance (ANOVA) of mean and S/N ratio on data was performed to identify the significant factors. Table 6.12 and Table 6.13 presents the ANOVA results for mean and S/N ratio. Type of wire (A), pulse width (B), time 125

37 between two pulses (C), and the interaction between Type of wire (A) and pulse width are found to have significant effect (significant at 95% confidence interval) on SR. Table 6.12 Analysis of Variance of Means Source DF Seq SS Adj SS Adj MS F P A * B * C * D E A*B * Residual Error Total Type of wire, B- Pulse width, C- Time between two pulses, D- wire tension, E- Servo reference mean voltage *- significant at 95% confidence level Table 6.13 Analysis of variance of S/N Ratio Source DF Seq SS AdjSS Adj MS F P A * B * C * D E A*B * Residual Error Total Type of wire, B- Pulse width, C- Time between two pulses, D- wire tension, E- Servo reference mean voltage *- significant at 95% confidence level Model adequacy The adequacy of model for SR for is checked by means of normal probability plot, plots of histogram, plots of residuals verses fitted value, plots of residuals verses order. All the four plots are illustrated in Figure 6.0. Residuals are normally distributed and no evidence of skewness or outliers exists. No evidence exists that error terms are correlated with each other, which confirm the adequacy of model Effect on Wire Wear Ratio (WWR) The average value of WWR and their S/N ratio for each parameter at levels 1, 2,3 are calculated from Table 5.1 and are given in Table C11 and Table C12 respectively (Appendix-III). The analysis of the main effects plot for means, main effect plots 126

38 Residual Plots for Means a No rmal Pro babilit y Plo t 0.1 a Versus Fits Percent 10 Residual Residual Fitted Value Frequency a Hist o gram Residual Versus Order a Residual Obser vation Or der Figure 6.0 Residual Plots for Means for S/N ratio and interaction plot for means and S/N ratio at 95% confidence level is shown in Figure 6.1, Figure 6.2, Figure 6.3 and Figure 6. respectively. It can be observed from Figure 6.1a and Figure 6.2a that less WWR is observed from shallow cryogenic treated wire as compared to untreated wire. It can be can be attributed to the fact that shallow cryogenic treated wires have high conductivity, which enables the quicker propagation of heat along the surface resulting in less deep craters than untreated wires. Increased conductivity results in series discharge effect and even discharge energy in the gap. Pulse width (Fig. 6.1b and Fig. 6.2b) shows the variation in mean and S/N ratio. Significant increase in WWR ratio is observed for means and S/N ratio with the increase in pulse width. More pulse energy is expended in the gap with the increase in pulse width, which significantly erodes the wire and workpiece. It is also observed from Figure 6.1c and Figure 6.2c that time between two pulses influence the WWR. It is revealed that wire tension (Fig. 6.1d, Fig. 6.2d) and servo reference mean voltage (Fig. 6.1e, Fig. 6.2e) play insignificant role in WWR. Uniform wear is observed as discharge conditions become more stable within all three levels of wire tension and servo reference voltage. The interaction between pulse width and time between two pulses (Fig. 6.3 and Fig. 6.) affects the WWR significantly. It can be seen that at high value of time between two 127

39 Main Effects Plot for Means (WWR) T y p e o f wir e( A ) P ulse wid th ( B ) T im e b e twe e n two pulse s ( C ) a b c Mean of Means UTWE SCTWE Wir e te nsio n( D) S e r v o r e fe r ence m e an v o lta ge ( E ) d e SCTWE- Shallow cryogenic treated wire UTWE- Untreated wire Figure 6.1 Main Effects Plot for Means (WWR) Main Effects Plot for SN ratios (WWR) T y pe of wir e ( A ) P ulse width ( B ) T im e b etwe en two p ulses ( C ) a b c Mean of SN ratios d UTWE SCTWE Wir e te nsio n( D) S er v o r efe r e nce m ea n v o ltage ( E ) Signal-to-noise: Smaller is better e SCTWE- Shallow cryogenic treated wire UTWE- Untreated wire Figure 6.2 Main Effects Plot for SN Ratios (WWR) pulses, low WWR is observed due to its higher SN ratio. This may be attributed to the fact that small energy is dispended to the discharge gap at pulse width of 0.µs and time between two pulses of 16 µs, which significantly reduces the WWR. Higher WWR is observed at time between two pulses of 10 µs and pulse width of 1.2 µs. 128

40 P ulse w idth ( B ) Interaction Plot for Means (W W R ) Data Me ans T ime betw een tw o pulses ( C ) P ulse w idth ( B ) T ime betw een tw o pulses (C ) Figure 6.3 Interaction Plot for Means (WWR) P ulse w idth (B) Interaction Plot for SN ratios (WWR) T ime between two pulses (C) P ulse width (B) T ime between two pulses (C) Signal-to-noise: Smaller is better Figure 6. Interaction Plot for SN Ratio (WWR) Selection of optimal levels The analysis of results leads to conclusion that second level of parameter A (A2), First level of parameter B (B1), third level of parameter C (C3) are significant factors. Factor D and Factor F are having least significant effect for minimizing wire 129

41 wear ratio. It can be further noticed that theses parameters corresponds to highest value of S/N ratio. The analysis of variance (ANOVA) of mean and S/N ratio on data was performed to identify the significant factors in order to establish the relative significance of parameters. Table 6.1 and Table 6.15presents the ANOVA results for mean and S/N ratio. Type of wire (A), pulse width (B), time between two pulses (C), and the interaction between pulse width and time between two pulses are found to have significant effect (significant at 95% confidence interval) on WWR. Wire tension and servo reference mean voltage are insignificant for change in WWR. Table 6.1 Analysis of Variance of Means Source DF Seq SS Adj SS Adj MS F P A * B * C * D E B*C * Residual Error Total Type of wire, B- Pulse width, C- Time between two pulses, D- wire tension, E- Servo reference mean voltage *- significant at 95% confidence level Table 6.15 Analysis of Variance of S/N Ratio Source DF Seq SS AdjSS Adj MS F P A * B * C * D E B*C * Residual Error Total Model adequacy The adequacy of model for WWR is checked by means of normal probability plot, plots of histogram, plots of residuals verses fitted value, plots of residuals verses order. All the four plots are illustrated in Figure 6.5. From the graphs, it appears that residuals are randomly scattered about zero and their distribution is normal, which confirm the adequacy of model. 130

42 Residual Plots 99 No rmal Pro babilit y Plo t Versus Fit s Percent Residual Residual Fitted Value Hist o gram Versus Order Frequency Residual Residual Obser vation Or der Figure 6.5 Residual Plots for Means 6.3 WIRE BREAKAGE The wire breakage is caused by the flaws created by sparks and these flaws attack the wire cross-section during machining, which results into failure of the wire. The foremost reason for wire breakage on the copper alloy wire is the elevated temperature of the wire (Zhang et al., 1989). The possible causes for wire weakening and breakage are summarized as: 1. Local temperature during sparking process. 2. Too high mechanical load 3. Propagation of cracks until final failure Tomlin (1996) in his research investigated the wire breakage with fracture mechanics approach. Luo (1999) further supported the theory of Tomlin for sudden and unexpected nature of wire failure. The flaws created in the wire are owing to higher temperature in the gap that propagates and leads to wire breakage. The locally higher temperature is presumed to be caused by the frequent occurrence of discharges at the same spot (Schacht, 200a). Various types of wire failure are discriminated, like ductile fracture with reduction in wire diameter, fracture without reduction of the wire diameter etc. (Schacht et al., 200c). 131

43 Figure 6.6 Typical View of Wire Rupture without Reduction in Diameter Figure.7 Typical view of wire rupture with reduction in diameter Figure.6 illustrates the most frequently occurring rupture in the WEDM with untreated wire. The wire failure during WEDM is initiated by the cracks, which propagates the failure. Fracture toughness of the material is important in this case. The wire rupture in untreated brass wire as shown in Figure.7 is caused by the mechanical tension on the wire. The wire rupture was only observed in untreated brass wire (Fig. 5.12a and Fig. 5.16a) but it was unnoticed in cryogenic treated brass within the experimental conditions. The possible reason is explained as; Results have shown that there is decrease in hardness and tensile strength of brass wire when treated cryogenically (Table 5.3). The decreases in hardness and tensile strength of cryogenically treated brass wires may be due to the diminishing of residual stresses, which are developed when the brass is formed into wires. Fracture toughness of the materials resists the failure due to cracks. It has been reported (Kern, 2007a) that the fracture toughness often increases as the tensile strength of given material decreases. Experimental results indicate higher facture toughness value of deep and shallow cryogenic treated brass wire as compared to untreated brass wire. It is expected that wire breakage shall be less in case of cryogenically treated wires. Also, analysis report of the X-ray diffraction (XRD) to calculate the grain size is shown in Table 5.5. The X-ray diffraction analysis of the brass wire reveals that grain size after deep and shallow cryogenic treatment decreases from 76 to 0 nm and 76 to 7nm respectively, which is approximately half from its original size. This 132